Paediatric Respiratory Care A guide for physiotherapists and health professionals by S. Ammani Prasad (auth.),

PAEDIATRIC RESPIRATORY CARE

PAEDIATRIC RESPIRATORY CARE A guide for physiotherapists and health professionals Edited by S. Ammani Prasad Superintendent Physiotherapist, Respiratory Unit, Great Ormond Street Hospital for Children NHS Trust, London, UK and Juliette Hussey Lecturer in Physiotherapy, School of Physiotherapy, Faculty of Health Sciences, Trinity College, Dublin, Republic of Ireland Jo Campling Commissioning Editorial Consultant unlI SPRINGER-SCIENCE+BUSINESS MEDIA, B.V

First edition 1995 © Springer Science+Business Media Dordrecht 1995 Originally published by Chapman & Hall in 1995 Typeset in 10/12 Palatino by Mew Photosetting, Beckenham, Kent ISBN 978-0-412-55000-3 ISBN 978-1-4899-4469-6 (eBook) DOI 10.1007/978-1-4899-4469-6 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Catalog Card Number: 94-72639 ^y Printed on permanent acid-free text paper, manufactured in accordance with the proposed ANSI/NISO Z39.48-1992 and ANSI/NISO Z39.48-1984 (Permanence of Paper).

To Guy and Ella, Garry and Robert with love

CONTENTS List of contributors ix xi Foreword xiii 1 Acknowledgements 13 25 1 Growth and development of the cardiorespiratory system 39 S. Ammani Prasad 56 67 2 Physiology of the cardiorespiratory system 105 Juliette Hussey 122 142 3 Imaging the paediatric chest 151 Christopher D. George and Isky Gordon 159 4 Management of the acutely ill child in respiratory failure 175 Robert C. Tasker 5 Assessment of the child with respiratory disease Juliette Hussey and S. Ammani Prasad 6 Chest physiotherapy techniques and adjuncts to chest physiotherapy S. Ammani Prasad and Juliette Hussey 7 Common neonatal disorders Anne Greenough and Annette Parker 8 Paediatric cardiac surgery Martin Elliott and Juliette Hussey 9 Neurological intensive care Robert C. Tasker and S. Ammani Prasad 10 Respiratory tract disorders Robert Dinwiddie 11 Cystic fibrosis Robert Dinwiddie and S. Ammani Prasad Index

CONTRIBUTORS Dr Robert Dinwiddie MB, FRCP Professor Anne Greenough MD, FRCP, DCH Consultant Paediatrician, Professor of Clinical Respiratory Respiratory Unit, Great Ormond Street Hospital for Physiology, Children NHS Trust, London. Department of Child Health, King's College Hospital, London. Mr Martin Elliott MD, FRCS Consultant Cardiothoracic Surgeon, Mrs Annette Parker MCSP, SRP Great Ormond Street Hospital for Superintendent Physiotherapist, Children NHS Trust, London. Department of Child Health, King's College Hospital, London. Dr Christopher D. George FRCS, FRCR Consultant Radiologist, Dr Robert C. Tasker MA, MB, MRCP Department of Diagnostic Radiology, Consultant in Paediatric Critical Care Epsom District Hospital, Epsom. Medicine, Dr Isky Gordon FRCR Great Ormond Street Hospital for Consultant Radiologist, Children NHS Trust, London. Department of Diagnostic Radiology, Great Ormond Street Hospital for Children NHS Trust, London.

FOREWORD The last decade has witnessed many that this new book is published at this time. advances in our knowledge of and practice The editors have merged the clinical input of health care. This has been accompanied from experts in the most important areas of by an increasing trend towards subspecial- paediatric respiratory disorders with the ization, both in clinical medicine and its principles of physiotherapy as discussed by very experienced paediatric physiothera- allied professions. This has certainly been pists. This amply illustrates both the true in paediatric practice, particularly with importance of the physiological and clinical the current development of paediatric inten- background to sound physiotherapy, and sive care as a speciality in its own right. the importance of the team approach to the Paediatric pulmonology has been established care of sick children. for a somewhat longer period and in many respects is a major stimulus to the develop- Dr Ian G. James ment of paediatric intensive care. Clinical Director Paediatric Intensive Care Unit Physiotherapy is an integral and impor- Great Ormond Street Hospital for tant part of the management of the respira- Children NHS Trust, tory problems that are seen so frequently in sick children, whether or not the primary London problem is pulmonary. It too is becoming more specialized and it is highly appropriate UK

ACKNOWLEDGEMENTS We are very grateful to the contributors who We express our gratitude to medical have taken time from their busy schedules colleagues who have always been available to make this text possible. Weare indebted to provide support and encouragement in all aspects of our work, particularly Robert to our physiotherapy colleagues Lindsey Dinwiddie, Ian James, Robert Tasker, Kathy Hayward and Melanie Andrews, whose Wilkinson, Martin Elliott and Duncan clinical expertise and friendship have Macrae. Special thanks also to Su Madge contributed to this book in many ways. Our and to the staff of the respiratory laboratory thanks are extended to Laura Irwin, at Great Ormond Street. Catherine Dunne and all the physiotherapy staff at Great Ormond Street Hospital for The expertise of Sylvia Prasad and of the Children NHS Trust, whom we have been Department of Medical Illustrations at the hospital is acknowledged. privileged to work with, and to Mrs Jeeva- Mutucumarana for her support. Mention Peter Yung and colleagues at the School must be made of the nursing staff on the of Physiotherapy, Faculty of Health Sciences, Trinity College, Dublin are thanked for their paediatric intensive care unit, the respiratory help and encouragement. unit and the cardiothoracic unit whose dedication and expertise have been an S.A.P. inspiration over many years. J.M.H.

GROWTH AND DEVELOPMENT OF THE 1 CARDIORESPIRATORY SYSTEM s. Ammani Prasad DEVELOPMENT OF THE LUNGS STAGES OF LUNG DEVELOPMENT Fetal lung development begins at approx- Embryonic stage imately three weeks' gestation and is a During the embryonic period the lung arises complex process rarely complete before 34 as an outgrowth of the foregut. The lining weeks' gestation. Classically, four stages of of the whole respiratory system arises from human fetal lung development have been this endodermal bud. As it separates from described (Inselman and Mellins, 1981): the the foregut the respiratory bud grows in a embryonic stage (3rd-5th week), the pseudo- caudal direction, forming a midline tubular glandular stage (6th-16th week), the cannali- structure, the trachea and two lateral lung cular stage (17th-24th week) and the terminal buds. This process is usually complete by 28 sac stage (25th week-term) (Figure 1.1). Pseudo- -- --Postnatal glandular Canalicular - - -• • -25 Terminal sac CII Acinus - - - - -..c2.u. 20 5-9 -lCiIIi15 - - -- Bronchioli (non- Ol B respiratory) .:ccU10 - - - -C Bronchi E z:J 5 0 Adult 5 10 15 20 25 30 35 40 Weeks Figure 1.1 Fetal and postnatal development of the bronchial tree, showing the number of bronchial gener- ations (line A), respiratory bronchioles and terminal sacs (area A-AI), extent of cartilage along the bronchial tree (line B) and the extension of mucous glands (line C) during the four phases of lung development. Reproduced with permission from P.D. Phelan et al. Respiratory Illness in Children, Blackwell Scientific Publications, 1982.

2 Growth and development days. The right lung bud subsequently The pulmonary arteries arise from the divides into three branches and the left into sixth branchial arches (Langman, 1977). On two branches, the major bronchi. The lung the right the dorsal part of the artery, distal buds continue to grow in a caudolateral to the origin of its pulmonary branch, direction penetrating the surrounding disappears leaving the right pulmonary mesoderm, thereby filling the pericardio- artery unconnected with the systemic peritoneal canals. The parietal and visceral circulation. On the left, the pulmonary artery pleura develop from mesoderm. retains connection with the dorsal aorta by means of the ductus arteriosus which Pseudoglandular stage remains open until birth. During this ten week period the airways Cannalicular stage develop through dichotomous branching and become lined with either columnar or This stage involves differentiation of the cuboidal epithelial cells, closely related to the mesenchyme and maturation of the airways. adjacent mesenchyme. The latter The mesenchyme thins and a rich vascular subsequently differentiates from ten weeks circulation develops within it. There is into the surrounding cartilage, muscle, blood considerable proliferation of the capillaries and lymphatic vessels and connective tissue which become closely associated with and (Avery and Fletcher, 1974; Yu, 1986). By the protrude into the airway epithelium, in end of the pseudoglandular period all the particular with the respiratory bronchioles branches of the conducting portion of the and alveolar ducts, thus preparing the airway, from the trachea to the terminal lungs for their future role in gas exchange bronchioles, are established. The conducting (Woods and Daulton, 1958). This distal portion of the airway is known as the portion of the lung, comprising the respir- preacinus. Although these branches may atory bronchioles, alveolar ducts and the increase in size with subsequent lung alveoli themselves, is called the acinus growth, no new branches are formed. (Figure 1.2). Trachea Major Segmental/ Bronchioli Alveoli I bronchi subsegmental Terminal Respiratory Ducts Sac bronchi Airway 31 generations 3 10-15 8-10 1 3 \" \"/ / / / Preacinus Acinus ~I Figure 1.2 Anatomy of the tracheobronchial tree. Preacinus comprises the conducting portion (trachea, bronchi and bronchioli to terminal bronchiolus). Acinus comprises the gas exchanging unit (respiratory bronchioli, alveolar ducts and alveolar sacs). Reproduced with permission from R. Dinwiddie, The Diagnosis and Management of Paediatric Respiratory Disease, Churchill Livingstone, 1990.

Development of the lungs 3 Terminal sac stage is that of the body wall which provides a narrow peripheral segment. Although the This final stage of lung differentiation lasts diaphragm is completely formed by the from 24 weeks until delivery. Some of the eighth week of gestation, modelling terminal bronchioles undergo further continues throughout gestation. transformation into respiratory bronchioles, which then give off further subdivisions or FETAL LUNG LIQUID saccules. Finally the saccules may subdivide to form the alveoli. Although termed alveoli, During lung development fluid-filled spaces in utero these structures are larger and lack appear after the tracheal buds have formed a smooth outline; they are, however, capable and grown laterally into the mesenchyme. of effective gas exchange. These represent primitive air spaces. The fluid these spaces contain is not aspirated With advancing gestation the epithelial amniotic fluid. There is a net outflow of fluid thickness decreases and the cells differ- from the trachea that increases in volume entiate. At birth the distal airways are lined with gestational age (Dawes and Patrick, by flattened epithelium, the intermediate 1985). The site of secretion of this lung fluid airways by cuboidal epithelium and the most has not been established, but it is thought proximal airways by pseudostratified to originate from the epithelial lining. The columnar epithelium. The alveoli are lined volume produced is similar to the functional by pneumocytes. Type I pneumocytes cover residual capacity of a neonate (approxi- more than 95% of the alveolar surface and mately 30 ml/kg body weight). The function their principal function is to enhance gas of this fluid is in helping to determine the exchange within the lung. Type II pneumo- shape and volume of the peripheral lung cytes, up to ten times larger than type I cells, units. Animal studies have shown that are principally responsible for the production chronic drainage of this fluid results in of pulmonary surfactant. pulmonary hypoplasia (Alcorn et al., 1977). In the human fetus laryngeal atresia results THE DIAPHRAGM in lung fluid retention and consequent lung hypertrophy (Wigglesworth et al., 1987). The diaphragm is a dome-shaped septum dividing the abdominal and thoracic cavities. SURFACTANT It is formed by the fusion of four different structures: (i) the septum transversum, (ii) Pulmonary surfactant is a complex mixture the dorsal oesophageal mesentery, (iii) the of lipids and protein, characterized by high pleuroperitoneal membrane, and (iv) the surface activity, which is absorbed as a body wall (Gray and Skandalakis, 1972; monolayer at the alveolar air-liquid interface Moore, 1982; Snell, 1983). The septum to promote alveolar stability on deflation transversum is a mesodermal structure (Morely, 1992). It has also been suggested giving rise to the central tendon of the that surfactant may have a role in pulmonary diaphragm. The medial part of the dia- defence mechanisms and fluid movement phragm originates from the dorsal oeso- (Ballard, 1986). phageal mesentery. The pleuroperitoneal membranes initially provide a relatively large Gas exchange in the lung occurs through segment of the embryonic diaphragm, an air-liquid interface within a spherical closing the pleuroperitoneal cavity. structure of small radius. Such a structure However, their contribution to the would collapse unless the surface tension is developed diaphragm is relatively small, as reduced. It is now well documented that

4 Growth and development pulmonary surfactant prevents alveolar the chest is compressed and this aids the collapse by progressively reducing surface expression of a considerable quantity of tension during expiration. As early as 1959 fluid. Subsequent recoil of the chest wall investigators (Avery and Mead) established produces a small passive inspiration of air. that hyaline membrane disease, or respira- A significant negative pressure of up to 25 tory distress syndrome (RDS), is primarily cmH20 must be generated by the muscles a disease of surfactant deficiency. Surfactant of respiration to encourage alveolar expan- is a product of type II alveolar pneumocytes sion. The volume of the first inspiration is and is stored in characteristic lamellar bodies 12-16 ml, some 30% of which remains which are discharged into the alveolar space following expiration, helping to form the via the cell surface. Small quantities of functional residual capacity (FRC). Until surfactant may be detected in the fetal lung recently it was generally believed that some as early as 23 weeks' gestation, but it is not lung fluid was rapidly removed, perhaps by lymphatic drainage. However, it is now usually found in physiological amounts until apparent that the fluid may be lost slowly, at least 30 weeks (Gluck and Kulovich, 1973). the residual fluid helping to form the Surfactant production is accelerated during surfactant bubbles that maintain alveolar labour and following initiation of respiration stability (Scarpelli, 1976) and establish the (Gluck et al., 1967). FRe. INITIATION OF RESPIRATION POSTNATAL DEVELOPMENT Using antenatal ultrasound scanning fetal At birth the structure and branching pattern breathing movements are seen as para- of the respiratory airways is complete. There doxical motions of the fetal chest and are no major changes in the number of abdomen. These movements appear to be generations or structure of the airways the result of diaphragmatic contraction, postnatally. Lung growth is characterized by retracting the chest wall and distending the the formation of true alveoli, maturation of abdomen. Appropriate amniotic fluid lung structures and the secretion of various volume together with fetal breathing substances within the lung. At birth the movements appear to be essential for normal alveoli are multilocular and in postnatal life lung development. The association between these increase in size and number. Thus in fetal breathing movements and subsequent the newborn human lung there are approxi- neonatal respiratory function in pregnancies mately 150 million alveoli and the adult complicated by reduced amniotic fluid number of 300-400 million is reached by four volume (oligohydramnios) is uncertain (Blott years of age (Hislop et al., 1986). Most of the et al., 1987; Moissinger et al., 1987). alveoli develop in the first year of life and up to three years of age the increase in lung The onset of breathing at birth is initi- size is mainly due to alveolar multiplication, ated by the interaction of a variety of there being little change in alveolar size. stimuli (Walters and Olver, 1978; Yu, 1986). Thereafter the alveoli continue to increase Amongst these temperature change, relative in size and number up to the age of seven hypoxia and hypercapnoea are the strongest. years. From eight years the alveoli increase Other factors include pain, tactile stimuli and in size only, until the chest wall stops the release of catecholamines. For effective growing. The average diameter of an adult gas exchange to occur the fetal lung fluid alveolus is 250-300 Jlm as compared to 150- must be removed and replaced by air. 180 Jlm at two months of age (Dunhill, 1962). During the final stages of a vaginal delivery

Development of the lungs 5 The smaller alveolar size of the infant segmental bronchi. It is present from the predisposes it to alveolar collapse. Until five twelfth week of gestation but increases years of age distal airway growth lags behind that of the proximal airways. This gives rise in total area throughout childhood (Thurl- to a high peripheral airways resistance in the beck, 1975; Sinclair-Smith et al., 1976); thus young. Since resistance to flow is inversely infants have a relatively weak cartilagenous related to the fourth power of the radius support compared to adults. This may (Poiseuille's law) a small decrease in an explain the dynamic compression of the infant's airway diameter (e.g. caused by trachea associated with high expiratory flow inflammation or oedema) significantly rates and increased airway resistance asso- increases the peripheral airway resistance and therefore the work of breathing (Figure ciated with certain disease states (e.g. 1.3). bronchiolitis). In the first few years of infancy the large In adults anatomical collateral ventilatory airways increase in diameter (Hislop and channels allow ventilation distal to an Reid, 1974). Following the final alveolar obstructed airway. These channels may be budding the lung continues to increase in interbronchiolar (channel of Martin), size until physical growth is complete. The bronchiole-alveolar (canals of Lambert) and alveolar surface area therefore increases from interalveolar (pores of Kohn). These path- 4 m2 at birth to 75 m2 in the adult (Hislop et ways have not been identified in neonates al., 1986; Dunhill, 1962). There is a linear and are generally assumed to develop after relationship between the increase in air- infancy. Pores of Kohn have been identified tissue interface and body surface area. between the first and second year of life and Cartilaginous support stabilizes the con- canals of Lambert by six years of age. ducting airways down to the level of the Without collateral ventilatory pathways infants and young children are at increased Adult and infant airway diameters risk of atelectasis and hyperinflation asso- ciated with infection. .. Effect of a 1mm cuff of oedema 15mm The angle of insertion of the diaphragm in the child is almost horizontal, whereas in , Infant Adult I I I I I I ,I I I I .. 25% Lumen area +75% Figure 1.4 Angle of insertion of the diaphragm, showing the more oblique insertion of the adult Figure 1.3 Adult and infant airway diameters, (dotted line) compared to the horizontal insertion showing the effect of lumen size in the presence of in the infant (continuous line). Modified from N.L. a 1 mm cuff of oedema. In the infant there is a 75% Muller and A.C. Bryan, Chest wall mechanics and reduction in the lumen area, significantly increasing respiratory muscles in infants, Pediatric Clinics of airway resistance and therefore the work of North America, 1979. breathing.

6 Growth and development Thoracic configuration Infant Child/adult Sternum Abdomen Thoracic cross section Figure 1.5 Rib cage configuration in the adult (above right) compared to the infant (above left). Rib growth at costochondral junctions and posterior rib angles as a possible explanation to changes in thoracic cross- sectional shape (below left and right). Reproduced with permission from P. Oppenshaw et al., Changes in rib cage geometry during childhood, Thorax, 1984. the adult it is oblique. This results in a ABNORMALITIES OF LUNG GROWTH AND decreased efficiency of contraction. In DEVELOPMENT addition contraction of the diaphragm may tend to distort the rib cage inward (Muller Pulmonary agenesis results from failure of and Bryan, 1979) (Figure 1.4). The con- normal embryological development of the endodermal tracheal bud or from an abnor- figuration of the rib cage of the infant differs mal interaction between the bud and the from that of the adult in that the ribs lie surrounding mesenchyme. Left sided agenesis is more common than right sided horizontally. In the horizontal plane the agenesis and has a better prognosis (Avery rib cage of an infant is circular, whereas that et al., 1981). This condition may be associated of an adult is ellipsoid (Figure 1.5). There- with congenital cardiovascular or thoracic cage abnormalities. In the normal situation fore in the infant anteroposterior and lung development is dependent on the interaction of several hormones including transverse diameter expansion is reduced, glucocorticosteroids (Liggins and Howie, 1973) thyroid hormone, prolactin and oestra- limiting the potential increase in lung diol (Ballard, 1984). In addition normal volume. The development of the adult ovoid pattern is associated with the adoption of an upright posture and occurs at approxi- mately 2-3 years of age (Oppenshaw et al., 1984).

Development of the cardiovascular system 7 lung development is also dependent on an of 3-4%. It is a disease primarily of the adequate thoracic space for growth, normal preterm infant and is a consequence of a fetal breathing movements, appropriate deficiency in surfactant. Male infants and amniotic fluid volume and adequate nutri- infants of diabetic mothers are at an tion. increased risk of developing this condition. Normal postnatal development of the lungs Pulmonary hypoplasia occurs when there may be impaired due to complications are lesions that diminish the intrathoracic resulting from mechanical ventilation that may be required to treat respiratory distress space available for lung growth: congenital syndrome or other neonatal respiratory diaphragmatic hernia (Areechon and Reid, disorders (see Chapter 7, p. 107). 1963), cystic adenomatoid malformation (Yu, 1986), pleural effusions (Barr and Burdi, DEVELOPMENT OF THE CARDIOVASCULAR SYSTEM 1975), enlarged thymus (Balcom et al., 1985) and skeletal dysplasias (Davis and Reid, DEVELOPMENT OF THE HEART 1971). It may also occur secondary to oligo- hydramnios. Oligohydramnios may be a Development of the heart begins on the 18th consequence of premature rupture of the to 19th postconceptual day. Two endothelial placental membranes, fetal renal tract tubes meet in the midline and form a single malformation (e.g. renal agenesis) or severe heart tube lying within the pericardial cavity. This tube is suspended from the dorsal wall intrauterine growth retardation. of the cavity by a dorsal mesocardium. Congenital diaphragmatic hernia is most A process of elongation, dilatation and constriction eventually results in a four- often caused by a failure of the pleuro- chambered structure. The heart begins to peritoneal membranes to close the pericar- contract at 22 days. Initially there are six dioperitoneal canals. This allows abdominal cardiac chambers which are, from the viscera (usually the stomach) to enter the caudal to cranial ends, the sinus veno- thoracic cavity. Consequently the lungs are sus, the atrium, the atrioventricular canal, the ventricle, the bulbus cordis and the compressed and may become hypoplastic. truncus arteriosus (Figure 1.6). The sinus Cystic adenomatoid malformation is a venosus is a dilatation consisting of left hamartoma of the lung characterized by and right sinus horns, each of which receives overgrowth of the terminal bronchioles at the principal venous channels on the ipsi- the expense of the saccular spaces. The lateral side. The atrium is another dilatation disease generally affects one lobe and may which is separated from the sinus venosus be macro or microcystic in nature. The by a sinuatrial opening guarded by a pair abnormal lung tissue displaces the heart and of flimsy venous valves. The atrioven- may compress the normal lung, impairing tricular canal is a narrow segment of tube its normal development. Several fetal leading cranially from the atrium and con- abnormalities may give rise to intrauterine taining a pair of subendocardial cushions. pleural effusions (e.g. congenital heart The primitive ventricle is a dilatation follow- disease). Irrespective of the aetiology of the ing the canal and leading to the bulbus cordis. This structure is also a dilated effusion pulmonary hypoplasia may result chamber and a slight constriction demarcates from compression and displacement of it from the ventricle. The truncus arteriosus normal lung tissue. The value of antenatal pleural shunting for this condition is not fully established (Nicolaides and Azar, 1990). Respiratory distress syndrome is the most common neonatal respiratory disorder, occurring in up to 15% of infants under 2.5 kg with an associated neonatal mortality

8 Growth and development A v Be (a) (b) (c) Figure 1.6 Growth and development of the embryonic heart. Initially there is a linear sequence ofchambers, the truncus arteriosus (TA) lying cranially (a), later (b) the bulbus cordis (BC) bends to lie on the right side of the ventricle (V) and then (c) the bulbus cordis and ventricle move caudally to the atrium (A). is the most cranial part of the heart and is The developmental processes, whereby another narrowed segment. the heart is partitioned into left and right streams, are associated with the appearance The intrapericardial part of the heart of the lung buds and the development of the consists of the future bulboventricular primitive pulmonary circulation. The prin- portion. The atrial portion and the sinus cipal embryonic veins entering the sinus venosus are paired and remain outside the venosus are the cardinal, vitelline and pericardium, in the mesenchyme of the umbilical veins, of which the latter two cross septum transversum. The initially longi- the septum transversum. As the septum tudinal arrangement of the heart chambers transversum is invaded by the developing is of no disadvantage because the embryo liver, the vitelline and umbilical veins are does not possess a double circulation. interrupted and contribute to the hepatic However, a pulmonary circulation must sinusoidal network. Of the posthepatic eventually be incorporated and therefore the channels only the right vitelline vein persists cardiac chambers undergo realignment. This to form the inferior vena cava. The two occurs during the 4th and 5th weeks of vitelline veins become linked and form the gestation, whilst the internal processes of partition of the heart continue into the hepatic portal vein, receiving blood from the second month of gestation. This is the time gut and liver. The right umbilical vein of greatest risk to teratogenic influences. The disappears and oxygenated blood is carried bulboventricular portion of the heart tube by the left umbilical vein. Most of this blood grows much more rapidly than the pericardia! enters the hepatic sinusoids, but 25% is space and because its two ends are fixed diverted past the liver in the ductus venosus. Eventually the left sinus horn of the sinus outside the pericardial cavity, further elonga- tion of the tube cannot be achieved without venosus receives three tributaries: (1) the it bending. The bulboventricular loop is posthepatic segment of the right vitelline shifted to the right, ventrally and caudally. vein (inferior vena cava), (2) the right As a consequence of this bending and torsion, common cardinal vein (superior vena cava) the atrioventricular junction comes to lie on and (3) the attenuated left sinus horn the left side of the pericardial cavity. (coronary sinus).

The atrial chamber is partitioned by two Development of the cardiovascular system 9 septa, the septum primum and septum secundum, which appear in that order. A as the dorsal aorta. The fifth aortic arches small gap remains in utero between the are transient and never well developed. The septum primum and the atrioventricular sixth aortic arches on the right supply cushions, termed the foramen ovale. This branches to the lung buds forming the right structure allows oxygented blood to pass pulmonary artery and on the left the distal from the right side to the left. The two part persists as the ductus arteriosus (Lang- ventricles are separated from each other by man, 1977). a downgrowth from the atrioventricular cushions (Navaratnam, 1975). The pulmonary arteries develop alongside the airways during the psuedoglandular and DEVELOPMENT OF THE GREAT VESSELS cannalicular periods of lung development. With each airway division the arteries branch The major intraembryonic vessels are repre- forming the preacinar vessels. By 16 weeks' sented by dorsal aortas which form as gestation this development is complete. The continuations of the endocardial heart tubes. preacinar arteries give rise to as many as four As a result of rotation and fusion of the heart branches that supply the adjacent lung tubes, the dorsal aortas become arched. Six tissue. After 16 weeks intra-acinar arteries pairs of aortic arches form as a consequence develop alongside the respiratory bronchioles of the development of the pharyngeal and saccules. This vascular development arches. The first and second arch arteries continues during childhood as new alveolar involute. The third arch arteries give rise to ducts and alveoli arise. the common carotid and internal carotid arteries. The fourth arch on the left persists THE FETAL CIRCULAnON The essential features of the fetal circulation are: (i) the umbilical arteries and vein Fetus Newborn Figure 1.7 Changes in the fetal circulation, to circulation in the neonate and the adult. (A, ductus arteriosus; B, foramen ovale)

10 Growth and development supplying and draining the placenta, (ii) a 30 mmHg within the first 24 hours. There is shunt from the right to the left atrium a rapid tenfold increase in pulmonary blood (foramen ovale) and (iii) a shunt from the flow. Pulmonary vascular resistance pulmonary artery to the aorta (ductus continues to fall gradually for up to six weeks arteriosus) (Figure 1.7). The umbilical artery postnatally, eventually reaching adult values initially arises as a branch of the dorsal aorta (9 mmHg). The changes in pulmonary but by term has become a branch of the resistance are induced by local effects and internal iliac artery. Its purpose is to carry sustained by humoral changes. The fetal deoxygenated blood and waste products lungs, fluid filled in utero, rapidly fill with to the placenta. The umbilical vein runs gas and the alveoli expand. As the pulmon- through the liver to the inferior vena cava ary vascular resistance falls the pulmonary carrying oxygenated blood to the fetal heart. arterioles dilate (Cassin et al., 1964). Increas- The foramen ovale streamlines oxygenated ing fetal oxygenation causes pulmonary blood arriving in the right atrium into the vasodilation and increased blood flow left atrium and thence to the left ventricle. (Heymann and Hoffman, 1979). Poorly oxygenated blood from the superior vena cava is directed into the right ventricle. Closure of the ductus arteriosus occurs in The left ventricle pumps oxygenated blood two stages, initial functional closure followed into the aorta and thence to the brain and by permanent closure with connective tissue upper limbs. The right ventricle ejects poorly formation. The increase in postnatal oxygen oxygenated blood towards the pulmonary concentration appears to be the most potent trunk. However, in fetal life a high vascular stimulus for this change. Other stimuli may resistance exists in the pulmonary circulation include acetylcholine and catecholamines and the blood flow is diverted through the (Heymann and Rudolph, 1975). Eventually ductus arteriosus into the aorta. The ductus the ductus fibroses to become the liga- joins the aorta distal to those branches mentum arteriosum attaching the left supplying the brain and upper limbs and its pulmonary artery to the aortic arch. After blood flow is therefore principally to the birth the left atrial pressure increases due to distal organs. increased pulmonary blood flow and the right atrial pressure falls, as a result of POSTNATAL CHANGES cessation of umbilical blood flow and reduced inferior vena caval return. These At birth there are major cardiovascular changes represent a reversal of the fetal changes as gas exchange is transferred from circulation and the foramen ovale is oblit- the placenta to the lungs (Figure 1.7). A few erated some six to eight months after minutes after birth the umbilical arteries delivery by connective tissue growth. cease to pulsate due to touch and cold stimuli. The umbilical vein may remain open ABNORMALITIES OF CARDIOVASCULAR for up to 30 minutes and a substantial volume of blood is thereby returned from the DEVELOPMENT placenta to the baby. With cessation of umbilical blood flow there is a marked Abnormalities of cardiovascular structure decrease in deoxygenated blood returning and function are relatively common in the to the heart and a significant increase in fetus and newborn. Congenital heart disease systemic vascular resistance. After birth and may occur in 7-8 per 1000 live births and is the onset of ventilation, pulmonary vascular more common in stillbirths. There are resistance falls rapidly from 70-75 mmHg to several possible aetiologies. In some cases there appears to be a familial component as certain specific problems may be more

References 11 common in twins and specific families. All B.D. Fletcher), W.B. Saunders, Philadelphia, types of chromosomal abnormalities (e.g. pp. 1-21. Down's syndrome) are associated with Avery, M.E., Fletcher, B.D. and Williams, R.A. congenital heart disease. Several drugs (e.g. (1981) Lung and its Disorders in the Newborn phenytoin and lithium) may be teratogenic Infant, 2nd edn, W.B. Saunders, Philadelphia, particularly when exposure occurs in the pp. 171-2, 290-6. early developmental phases. Infective causes Avery, M.E. and Mead, J. (1959) Surface proper- such as rubella are also well recognized (Best ties in relation to atelectasis and hyaline and Banatvala, 1990). membrane disease. American Journal of Diseases of Childhood, 97, 517. Embryological abnormalities of cardiac Balcom, R.J., Hakanson, D.O., Werner, A. and development fall into six groups. Abnormal- Gordon, L. (1985) Massive thymic hyperplasia ities of the atrial septum may give rise to a in an infant with Beckwith-Wiedemann single atrial chamber or premature closure syndrome. Archives of Pathological Laboratory of the foramen ovale. Failure of normal Medicine, 109, 153-5. development of the endocardial cushions Ballard, P.L. (1984) Combined hormonal treat- give rise to atrioventricular canal defects, e.g. ment and lung maturation. Seminars in failure of the cushions to fuse results in a Perinatology, 8, 283-92. persistent atrioventricular canal. Abnormal- Ballard, P.L. (1986) Lung development, in ities of the interventricular septum will lead Hormones and Lung Maturation, (ed. P.L. to defects of varying sizes affecting the Ballard), Springer Verlag, Berlin, pp. 1-23. membranous septum. The severity of such Barr, M. and Burdi, A.R. (1975) Spontaneous a lesion would depend on its size, but at its pleural effusion in the human fetus. Terato- worst may present as a single common logy, 11, 139-42. ventricle. Failure of normal development of Best, J.M. and Banatvala, J.E. (1990) Rubella, in the truncus is classically associated with Principles of Bacteriology, Virology and Immunity, tetralogy of Fallot. Defects of the semilunar 8th edn, (eds L. Collier and M. Timbury), valves may give rise to stenosis or regurgi- Edward Arnold, London, pp. 501-31. tation. If fusion of the valve leaves is Blott, M., Greenough, A., Nicolaides, K.H. et al. complete valvular atresia results and may be (1987) Fetal breathing movements as predictor secondarily associated with hypoplastic of favourable pregnancy outcome after oligo- development of the musculature. Finally the hydramnios due to membrane rupture in the heart may not be positioned correctly within second trimester. Lancet, 2, 129-31. the thorax; the most frequently seen anomaly Cassin, S., Dawes, G.S., Mott, J.e. et al. (1964) is dextrocardia where the heart is located in The vascular resistance of the fetal and newly the right chest. ventilated lung of the lamb. Journal of Physiology, 171, 61-79. REFERENCES Davis, G. and Reid, L. (1971) Effect of scoliosis on growth of alveoli and pulmonary arteries Alcorn, D., Adamson, T.M., Lambert, T.F. etal. and on right ventricle. Archives of Diseases of (1977) Morphological effects of chronic Childhood, 46, 623-32. tracheal ligation and drainage in the fetal lamb Dawes, G.S. and Patrick, J.E. (1985) Fetal lung. Journal of Anatomy, 123, 649-60. breathing activity, in Pulmonary Development, (ed. G.H. Nelson), Marcel Dekker, New York, Areechon, N. and Reid, L. (1963) Hypoplasia of p.93. lung with congenital diaphragmatic hernia. Dunhill, M.S. (1962) Postnatal growth of the British Medical Journal, 1, 230-3. lung. Thorax, 17, 329-33. Gluck, L. and Kulovich, M.V. (1973) LIS ratios Avery, M.E. and Fletcher, B.D. (1974) Lung in amniotic fluid and abnormal pregnancies. development, in The Lung and its Disorders in American Journal of Obstetrics and Gynecology, the Newborn Infant, (eds M.E. Avery and 115, 539-52. Gluck, L., Motoyama, E.K., Smits, H.L. and Kulovich, M.V. (1967) The biochemical

12 Growth and development development of the surface activity in the Morely, c.J. (1992) Surfactant, in Textbook of mammalian lung 1. Pediatric Research, 1, Neonatology, 2nd edn, (ed. N.C.R. Roberton), 237-46. Churchill Livingstone, Edinburgh, pp. 369-83. Gray, S.W. and Skandalakis, J.E. (1972) The diaphragm, in Embryology for Surgeons: the Muller, N.L. and Bryan, A.c. (1979) Chest wall Embryological Basis for the Treatment of mechanics and respiratory muscles in infants. Congenital Defects, (eds S.W. Gray and J.E. Pediatric Clinics of North America, 26, 503-16. Skandalakis), W.B. Saunders, Philadelphia, pp.359-85. Navaratnam, V. (1975) The Human Heart and Heymann, M.A. and Hoffmann, J.1. (1979) Circulation, Academic Press, London. Pulmonary circulation in the perinatal period, in Neonatal Pulmonary Care, (eds D.W. Nicolaides, K.H. and Azar, G.B. (1990) Thoraco- Thibeault and G.A. Gregory), Addison amniotic shunting. Fetal Diagnosis and Therapy, Wesley, Menlo Park, p. 70. 5, 153-64. Heymann, M.A. and Rudolph, A.M. (1975) Control of the ductus arteriosus. Physiology Oppenshaw, P., Edwards, S. and Helms, P. Reviews, 55, 62-78. (1984) Changes in rib cage geometry during Hislop, A. and Reid, L. (1974) Development of childhood. Thorax, 39, 624-7. the acinus in the human lung. Thorax, 29, 90-4. Scarpelli, E.M. (1976) Fetal pulmonary fluid, in Hislop, A., Wigglesworth, J.S. and Desai, R. Reviews in Perinatal Medicine, (eds E.M. (1986) Alveolar development in the human Scarpelli and E.V. Cosmi), University Park fetus and infant. Early Human Development, 13, Press, Baltimore, pp. 49-107. 1-11. Inselman, L.S. and Mellins, RB. (1981) Growth Sinclair-Smith, c.c., Emery, J.L., Gadson, D., and development of the lung. Journal of Linsdale, F. and Baddeley, J. (1976) Cartilage Pediatrics, 98, 1-15. in children's lungs: a quantitative assessment Langman, J. (1977) Medical Embryology, Williams using the right middle lobe. Thorax, 31, 40. and Wilkins, Baltimore, pp. 201-24, 233-51. Liggins, G.c. and Howie, R (1973) A controlled Snell, RS. (1983) Clinical Embryology for Medical trial of antipartum glucocorticoid treatment for Students, Little, Brown, Boston, pp. 177-94. prevention of the respiratory distress syn- drome in premature infants. Pediatrics, 50, Thurlbeck, W.M. (1975) Postnatal growth and 515-25. development of the lung. American Review of Moissinger, A.C., Fox, H.E., Higgins, A. et al. Respiratory Disease, 111, 803. (1987) Fetal breathing movements are not a reliable predictor of continued lung develop- Walters, D.V. and Olver, RE. (1978) The role of ment in pregnancies complicated by oligo- catecholamines in lung liquid absorption at hydramnios. Lancet, 2, 1297-9. birth. Pediatric Research, 12, 239-42. Moore, K.L. (1982) The developing human, in Clinically Oriented Embryology, (ed. K.L. Wigglesworth, J.S., Desai, R and Hislop, A.A. Moore), W.B. Saunders, Philadelphia, pp. (1987) Fetal lung growth in congenitallaryn- 172-5. geal atresia. Pediatric Pathology, 7, 515-25. Woods, de G.L. and Daulton, A.J. (1958) The ultrastructure of lung tissue for newborn and embryo mice. Journal of Ultrastructure Research, 2, 28-54. Yu, V.Y.H. (1986) Development of the lung, in Respiratory Disorders of the Newborn, (ed. V.Y.H. Yu), Churchill Livingstone, Edin- burgh, pp. 1-17.

PHYSIOLOGY OF THE 2 CARDIORESPIRATORY SYSTEM Juliette Hussey THE RESPIRATORY SYSTEM becomes relatively stiffer and gains a greater amount of outward recoil, Le. the thoracic The primary function of the respiratory compliance decreases. This is due to increas- system is to allow oxygen (02) to move ing calcification of the ribs, connective tissue from the air into the venous blood and changes and growth of the chest wall relative carbon dioxide (C02) to move out. Oxy- to the lungs. Once the upright posture has gen and carbon dioxide pass between air been assumed and the abdomen has grown, and blood by simple diffusion across the abdominal contents shift away from the alveolar-capillary membrane. The basic upper abdomen thus creating a more nega- concepts of cardiorespiratory physiology and tive pressure under the diaphragm, favour- the differences that exist between adults and ing outward recoil of the chest wall. The infants will be discussed in this chapter. chest wall of the adult has a similar compliance to that of the lungs and so is MECHANICS OF VENTILATION better able to oppose the action of the diaphragm. The principal muscle of respira- Chest wall and respiratory muscle function tion is the diaphragm. An adult diaphragm has 55% of slow twitch, high oxidative fibres, During spontaneous respiration a pressure whereas a term infant has only 25% and a gradient develops between the mouth and preterm may only have 10% of these fatigue alveoli. This is achieved by the contraction resistant fibres (Muller and Bryan, 1979). of the respiratory muscles, which increases Inspiration is initiated by the diaphragm and the intrathoracic volume and lowers the intercostal muscles. When an adult pleural pressure to subatmospheric levels. diaphragm contracts the dome descends and Compliance is the volume change per unit aids the elevation of the lower ribs. In the of pressure. Total lung compliance is a upright posture the adult rib cage and function of the lung tissue, the surface diaphragm contribute equal amounts to the characteristics and volume. The chest wall volume change during quiet tidal volume of the infant is cartilaginous, soft and pliable (and therefore facilitates easy passage breathing whereas in the supine position, through the birth canal) and this means that 70% of the volume change is abdominal it is very compliant compared to adult lungs. (Konno and Mead, 1967). Contraction of the The floppy chest wall of the infant is unable intercostal muscles elevates the anterior end to counteract the elastic recoil of the lungs, resulting in a low functional residual capacity of each rib and therefore increases thoracic (FRC). With increasing age the thorax volume during inspiration. The intercostal spaces become tense and stabilize the rib cage during the diaphragmatic descent.

14 Physiology of the cardiorespiratory system In the neonate nearly all the volume change to such an extent that the baby may become is abdominal and the circular rather than apnoeic. During rapid eye movement (REM) oblique lie of the ribs in the horizontal plane sleep it is believed that tonic inhibition of does not allow such increases in lung intercostal muscles occurs, which may allow volume. The accessory muscles of respira- rib cage distortion during inspiration. This tion (sternocleidomastoid and scalene) are may have serious consequences as the not used during quiet tidal volume breathing premature infant spends as much as 50% of but are recruited during times of respiratory sleeping time in the REM sleep state and embarrassment. Expiration is passive and is may during this time be less able to defend facilitated by the elastic properties of the against an increase in inspiratory load. lungs. Clinical implications The respiratory pump and control of ventilation There are many clinical implications in the The respiratory pump is a feedback loop mechanical differences between developing mechanism consisting of sensors (chemo- and mature chest walls. In the range of receptors, lung and airway receptors), con- normal breathing the thorax of the infant is trolling mechanisms (cerebrum, brainstem highly compliant. This becomes functionally and spinal cord) and effectors (respiratory significant in the presence of lung disease, muscles) (Berger et al., 1977). The respiratory when the greater negative pressure and muscles are described later in this chapter. therefore respiratory effort required to inflate the lungs can 'suck' in the chest wall. This The peripheral chemoreceptors are located in the carotid bodies at the bifur- results in less effective gas exchange and a cation of the common carotid arteries and further increase in the work of breathing. they monitor the arterial oxygen tension The very low elastic recoil pressure of the (Pa02) of blood perfusing the brain. A falling Pa02 stimulates the carotid bodies, newborn chest wall is one of the factors signalling the brainstem to increase ventil- which predisposes the infant to lung col- ation. The central chemoreceptor lies in the lapse. In order to achieve an adequate tidal medulla. When arterial carbon dioxide volume the infant has to generate compara- tension (PaC02) rises, the cerebral spinal tively greater pressures than an adult, fluid (CSF) surrounding the chemoreceptors because the contraction of the diaphragm becomes acidic and the increase in hydrogen produces a negative pleural pressure which ion (H +) concentration (pH) also stimulates tends to distort the compliant rib cage. In the brainstem to increase the ventilation. times of respiratory distress the infant may Pulmonary receptors lie in the upper show signs of intercostal, subcostal and airways and they moderate breathing. sternal recession. With severe distress the Sensory input is received by the respiratory rib cage will actually move inward as the abdomen moves outward during inspira- controller in the brainstem. This centre tion, causing a 'see-saw' effect. is divided into the pneumotaxic centre, apneustic centre and the medullary centre. The infant's muscles of respiration will The pneumotaxic centre tunes respiration by fatigue at a relatively quicker rate than an switching inspiration to expiration. The adult's (due to the difference in fibre apneustic centre is responsible for cutting off composition). When confronted with the inspiration but neither is absolutely essen- need to increase their work of breathing to tial. It is the medulla that is the site of the maintain ventilation (e.g. during times of basic respiratory rhythm generator and its respiratory distress) the muscles can fatigue

The respiratory system 15 destruction leads to apnoea (Mitchell, 1963). pulmonary capillary bed. The alveolar- The cerebral cortex connects with the brain- capillary membrane then permits the stem respiratory centres and, together with transfer of gases between the inspired gas the spinal cord, allows voluntary control of and the circulating blood, whilst at the same respiration. The spinal cord finally interprets time restricting the movement of fluid from the central nervous system (eNS) com- the pulmonary vasculature. Not all the gas mands before relating them to the phrenic entering the respiratory system takes part and intercostal nerves supplying the respira- in gas exchange and the volume of this tory muscles. 'wasted' gas is termed dead space. Total dead space within the respiratory system is The respiratory pump of the newborn and known as 'physiological dead space' and this the preterm infant has impaired regulation is then divided into'anatomical dead space' of alveolar ventilation. There is a reduction (gas within the conducting part of the of carbon dioxide responsiveness during respiratory system) and the 'alveolar dead hypoxaemia which is a response opposite to space' (gas which has reached the terminal that found in adults (Rigatto, 1982). Preterm lung units but does not take part in infants often breathe periodically and their exchange). breathing is marked by phases of periodic breathing (Pasterkamp, 1990). The more Airway resistance premature the infant the higher the incid- ence of such breathing patterns. There is an When air flows through a tube, a difference association between periodic breathing and in pressure exists between the ends. A major true apnoea, defined as an absence of component of airflow is airway resistance breathing for 20 seconds or more. The term and this is measured by relating flow to the infant has a higher oxygen demand per unit pressure drop from the alveolus to the mass (V02/kg) than the adult. To meet mouth. Resistance to gas flow along a tube these higher requirements the newborn has depends on whether the flow is laminar or a greater minute ventilation (MV) per unit turbulent. Low gas flow within a straight mass. wide tube is termed laminar, i.e. the gas moves in a series of concentric cylinders with AIRWAYS AND AIR SPACES the innermost moving the fastest. When gas travels at high velocities through tubes there The airways are a series of dividing tubes is a high resistance to flow and the stream which become narrower, shorter and greater lines become disorganized and local eddy in number as they branch from the trachea currents may develop. This is termed turbu- outward to the alveoli. Gas passes through lent flow. In the respiratory system, the these conducting airways from the nose and velocity of airflow increases from small to mouth to the respiratory alveoli where gas large airways (O'Brodovich and Hadad, exchange occurs across the alveolar-capillary 1990) and in terms of type of flow the membrane. situation is complex. True turbulent flow may be seen in the trachea but in general Dead space flow is transitional, i.e. with eddy formation at the branches (West, 1992). Poiseuille's The vital process of gas exchange occurs in law describes the pressure gradient re- the terminal respiratory units. In order for quired to maintain laminar flow through a this to occur efficiently the volume of gas tube: arriving at the alveoli must be comparable to the volume of blood passing through the

16 Physiology of the cardiorespiratory system p=V(81 11 ) P = pressure -===---'<--___ Interbronchiolar 1t r 4 v flow Channel of Martin I length ~--'\\--=::::--_ _ Bronchiol-alveolar r tube radius Canal of Lambert 11 gas viscosity - - - - - - - - - ' r - - PInotreeraolvf eKoolhanr Because resistance is equal to pressure Figure 2.1 Pathways of collateral ventilation. divided by flow, it is the radius of the tube which is the most important determinant of resistance. If the radius is halved the resistance is increased 16-fold. In the infant the small peripheral airways may contribute to up to 50% of the total airway resistance and this proportion does not decrease until the age of approximately five years (Hogg et al., 1970). Interdependence Alveoli Without surfactant smaller alveoli would tend to empty into larger alveoli in accord- After birth there is a dramatic increase in the ance with the Laplace relationship which number of alveoli, from 20 million alveolar relates pressure across a surface (P) to saccules at birth to 300 million by the age of surface tension (1) and the radius of eight years (Dunhill, 1962). Although alve- curvature (r): P=2Tlr. The smaller the olar multiplication is the predominant mech- anism for lung growth, the size increase of radius, the greater the tendency to collapse. individual alveoli is also important. The large However, individual lung units are effec- expansion in alveolar size and number tively connecting bubbles, i.e. the inside wall results in an increase in alveolar surface from of one alveolus is the outside wall of an 2.8 m2 at birth to 32 m2 by the age of eight adjacent alveolus. The interdependence years. Therefore the diffusing capacity of model of the lung demonstrates how any oxygen across the alveolar-capillary mem- decrease in size of one alveolus is stabilized brane increases. The small alveolar size of by adjacent alveoli. Deep breathing tech- the infant is a liability because it predisposes niques and breath holds use the concept of the infant to alveolar collapse. interdependence in order to try and increase alveolar expansion. Collateral ventilation Clinical implications The adult lung develops channels which Adults differ from children in the site of the allow ventilation to an obstructed airway main contribution to the total airway by 'collateral' flow. Three types of path- resistance. The upper airway, particularly way have been described: peres of Kohn the nose, accounts for the major proportion (intra-alveolar), canals of Lambert (bron- of total resistance in the adult. Up to five chiole-alveolar) and channels of Martin years of age the peripheral airway resistance (interbronchiolar) (Figure 2.1) and have been is approximately four times greater than that previously discussed in Chapter 1, p. 5. of the older child/adult. This, together with the late development of collateral ventilation,

The respiratory system 17 probably explains the high incidence of normally at his own comfortable rate and lower obstructive airways disease in young then takes a maximal inspiration followed children. Pores of Kohn and canals of by maximal expiration. The amount of gas Lambert may be found in the infant lung, moved during normal quiet respiration is but are probably not sufficiently large to termed the tidal volume (TV). Functional allow air drift (Wohl and Mead, 1990). The infant and small child is therefore at greater residual capacity (FRC) represents the risk of developing atelectasis and consequent volume of gas remaining in the lungs at the ventilation/perfusion mismatching. How- end of a quiet expiration during tidal ever, despite the several factors which may breathing. Following a maximal inspiration appear as a handicap to respiratory function to total lung capacity (TLC), the gas exhaled in the infant, one distinct gain is that of with a maximal expiration is known as the continuing alveolar development due to the vital capacity (VC). The residual volume (RV) ability of the alveoli to increase in number, is the air that remains in the lung after expand in size and participate in gas exchange following disease induced lung maximal expiration. The FRC (comprising damage. the expiratory reserve volume (ERV) and the LUNG VOLUMES AND LUNG COMPLIANCE RV) serves as a source of oxygen during expiration until the lungs are reinflated with Static volumes of the lungs at various stages the next breath; thus major changes in Pa02 of respiration offer valuable measurements are buffered. which help in the assessment of lung func- tion. These lung volumes are represented in Another useful volume measurement is Figure 2.2 where the subject initially breathes that of the FEV1 which represents the volume of air expired in the first second of I a forced expiration from TLC. Most of these IRV volumes are measured using simple spiro- metric techniques but measurement of FRC TLC and RV requires more sophisticated tech- Figure 2.2 Lung volumes representing normal tidal niques such as the helium dilution method volume breathing followed by maximum inspiration (see Chapter 5, p. 60). The peak expiratory and full expiration. TV = tidal volume, flow rate (PEPR) is a measurement of the FRC = functional residual capacity, TLC = total maximal flow (litres/minute) achieved lung capacity, VC = vital capacity, RV = residual volume, ERV = expiratory reserve volume, during a forced expiration from TLC and is IRV = inspiratory reserve volume. easily measured using a peak flow meter. Closing volume (CV) is the volume at which small airways close. The closing capacity is defined as the sum of the CV and the RV. When the closing capacity exceeds the FRC some lung segments are closed during tidal breathing and as a result the ventilation/perfusion (V/Q) ratio falls. It is the relationship between FRC and the closing capacity that to a large extent determines the matching of ventilation and perfusion. Children under six years of age and adults over 40 have a closing capacity which is greater than PRC when in the supine posi- tion (Mansell et al., 1972). The relatively high

18 Physiology of the cardiorespiratory system closing capacity in the child is due to the and the intrathoracic pressure is greatly reduced elastic recoil of the lung and the increased. The external surface of the high compliance of the thoracic cage. When bronchi are then submitted to a higher the closing capacity exceeds FRC and TV intrathoracic pressure. In a forced expiration some lung segments will be closed during the point at which the extrabronchial and both the inspiratory and expiratory phase of intrabronchial pressures are equal is termed tidal breathing. This represents complete the equal pressure point (EPP). The concept atelectasis of these segments. of the equal pressure point allows the airway to be divided into two segments; the Clinical implications . upstream segment from the EPP to the alveoli and the downstream segment from The very low elastic recoil of the newborn the EPP to the mouth (Figure 2.3). During chest wall raises the risk of lung collapse. a forced expiratory manoeuvre the EPP first The majority of tidal breathing in the infant develops in the trachea and as the takes place in the range of the closing intrathoracic pressure becomes sufficiently capacity. When lung volumes are reduced positive it travels upstream as the airflow below PRC (e.g. postoperatively) and closing increases and then reaches the bronchi. The capacity exceeds FRC, small alveoli and point of equal pressure therefore moves peri- airways in dependent regions of the lung are pherally as expiration progresses. This occurs closed, resulting in atelectasis and impair- because the resistance of the airways rises ment of ventilation. The use of positive as lung volume falls and the pressure within pressure (positive end expiratory pressure the airways falls more rapidly with distance or continuous positive airway pressure) to from the alveoli. The dynamic bronchial com- increase FRC above closing capacity in pression is an indispensable part of the disease states associated with alveolar mechanism of an effective cough (Gaultier, collapse is common practice. As the chest wall of the newborn is very compliant, lung volume in the completely relaxed state is 10-15% of TLC compared to 30-35% in the adult. However, the FRC of Pleural pressure a full term infant can be maintained at a higher volume by a mechanism of expiratory breaking (grunting), where upper airway resistance is increased during expiration thus increasing end expiratory lung volume (Helfaer et al., 1992). Grunting therefore, is produced by adducting the vocal cords, a mechanism premature infants may lack. / The equal pressure point During inspiration the bronchial diameter Figure 2.3 The equal pressure point, showing increases and the elastic forces of the lung compression of the airways at the point where airway pressure equals pleural pressure. Repro- parenchyma produce traction on the exterior duced with permission from J.B. West, Pulmonary Pathophysiology, Williams and Wilkins, 1987. of the bronchial walls. Expiration is by means of passive recoil. During forced expiration the expiratory muscles are active

The respiratory system 19 1982; West, 1992). Forced expiration man- and as a result less discrepancy between oeuvres using the concept of the EPP which the lungs in terms of gravitational venti- moves peripherally during the course of a lation. forced expiratory manoeuvre are used to enhance bronchial clearance from peripheral Bronchospirometric and radiOisotope lung regions (Pryor, 1991). studies of regional ventilation in adults have shown that ventilation is preferentially DISTRIBUTION OF VENTILATION distributed toward dependent lung regions (Zack et al., 1974). However, due to Inspired gas is not evenly distributed differences in chest wall mechanics and throughout the lungs. During normal quiet diaphragmatic function the reverse situation breathing in the upright position the bases is observed in infants where ventilation is of the lungs receive about 50% more ventila- preferentially distributed toward the upper- tion than the apices. The differences in most parts of the lungs. This has been ventilation between areas of the lungs are demonstrated by the use of krypton 81 m due to gravity and to variations in airway ventilation scans (Davies et al., 1985). This size and elastic properties. In the upright observation reinforces the finding that position at FRC the alveoli in the apex of the children with unilateral lung pathology have lung are more distended than those at the a higher arterial oxygen tension when base. This is due to the pleural pressure positioned with their unaffected lung upper- being more negative at the top of the pleural most (Heaf et al., 1983). The effects of cavity. Alveoli at the apex are more gravity on perfusion are however, similar in distended than those at the bases, they are children and adults and this imbalance may less yielding and therefore less air enters the have clinical importance (Bhuyan et al., apices than the bases. 1989). The distribution of ventilation is also Clinical implications related to the resistance of the airways and compliance of the air spaces. The product Children with airway disease will have an of flow resistance and compliance is known increase in airway resistance (caused by as the time constant. Each branch of the lung factors such as oedema and accumulation of has a time constant so the distribution of gas secretions). This will lead to an increased within a lung depends on all its various time time constant, incomplete expansion and constants. In disease there may be local therefore impaired ventilation. Children increases in resistance which lengthen the with unilateral lung pathology (particularly time constant to such an extent that complete critically ill infants) should be nursed with expansion of alveoli does not occur during their affected lung dependent in order to inspiration. achieve optimum ventilation/perfusion matching and therefore maximum Pa02. In the adult there is a considerably greater load (from the abdominal viscera) on the TRANSPORT OF OXYGEN IN THE BLOOD dependent hemidiaphragm when the sub- ject is in the side lying position (Davies et While in utero the lungs are fluid filled and al., 1985). In the infant the difference in gas exchange takes place across the placenta. preload on the dependent as compared to Fetal haemoglobin has a higher affinity for the uppermost diaphragm is much smaller oxygen than adult haemoglobin, i.e. the as the abdomen is smaller and narrower. blood leaving the placenta with a Pa02 of Therefore there is little difference in the 4 kPa is 68% saturated whereas the oxygen contractility between the hemidiaphragms

20 Physiology of the cardiorespiratory system saturation (Sa02) of the mother's blood at The oxyhaemoglobin dissociation curve is this tension is 6.8% lower. Fetal haemo- affected by changes in pH, PaC02 and globin has a high haemoglobin concentration temperature. A decrease in pH, an increase in PaC02 or an increase in temperature which results in an increase in oxygen shifts the curve to the right. Thus for a given carrying capacity (Strang, 1977). When the Pa02 the saturation percentage is less under fetal blood gives up carbon dioxide the acidotic or hypercapnoeic conditions. The change this produces in acidity (pH) fetal oxyhaemoglobin dissociation curve lies increases its oxygen affinity. to the left of that of the adult at a similar pH. Thus at a given Pa02 fetal haemoglobin Oxygen molecules are transported in the contains more oxygen than adult haemo- globin. This ensures that maximal oxygen blood in two ways. The vast majority of reaches fetal tissues, since the fetus in utero oxygen in the blood is combined with has a Pa02 of approximately 4 kPa. Fetal haemoglobin (Hb). When the blood is fully haemoglobin disappears from the circulation saturated 1 g of Hb carries 1.34 ml of oxygen. shortly after birth and at two months of age A small proportion of the oxygen is dis- only 2% of the total haemoglobin is fetal in solved in the plasma and water of the red origin. blood cell. The oxyhaemoglobin dissociation OXYGEN DELIVERY TO THE TISSUES curve (Figure 2.4) shows that Hb is nearly The cardiopulmonary system must transport 95% saturated at a Pa02 of 10.7kPa. The oxygenated blood to the systemic tissues. steep portion of the curve indicates that at The total oxygen delivery to the tissues is this level large amounts of oxygen can be determined by the Pa02, the amount of removed from haemoglobin resulting in only saturated haemoglobin, the left ventricular output and the patency of the vessels. small changes of Pa02' Under normal cir- Therefore three ways in which oxygenation cumstances inspiring 100% oxygen will only can be improved are to increase haemo- increase the amount of oxygen carried by the globin saturation, increase haemoglobin blood by a small amount, because at a Pa02 concentration and to augment cardiac of 13.3 kPa Hb is already 97.5% saturated. output. Cyanosis is one clinical sign of inade- quate oxygenation of systemic tissues. The 100 -- 18 degree of visible cyanosis depends on the amount of unsaturated haemoglobin present 80 .& .... \\ E in the blood perfusing the superficial vessels. 14 00 An anaemic patient may be inadequately c O2 combined oxygenated without appearing cyanotic. The with Hb opposite is true for a patient with 0 polycythaemia. Therefore the most reliable E~ estimate of the oxygen content of the blood ~ 60 E requires sampling and direct measurement. 10 EQ) 1:e1:nll I.0 40 6 u0 0C\\J -0;? _____D_is_so.l1v.e_d _0_2 ~- 2 20 20 40 60 80 100 600 0 P02 mmHg Figure 2.4 Oxygen dissociation curve. The solid line CARBON DIOXIDE TRANSPORT represents oxygen combined with haemoglobin, the broken line represents total blood oxygen content. Arterial carbon dioxide (PaC02) is also a Reproduced from J.B. West, Respiratory Physiology, reflection of lung function and a measure of Williams and Wilkins, 1984.

The respiratory system 21 gas exchange at alveolar level. Any rise in alveolar level. Pulmonary vascular thickness PaC02 is an acute stimulant to breathing in is a function of gestational age and blood the normal child and results in a compen- flow. Children with longstanding pul- satory increase in respiratory rate and tidal volume. Elevated PaC02 levels are monary hypertension, e.g. as a result of indicative of ventilatory failure which can be congenital heart disease, have a significant due to reduced lung function or loss of the increase in smooth muscle in their pul- central control of breathing. Increased monary arteries. Premature infants have less PaC02 causes cerebral vasodilation, whilst well developed vascular smooth muscle reduced PaC02 results in cerebral which regress more quickly after birth than vasoconstriction and a resulting decrease in that of the term infant. Incomplete develop- cerebral blood flow. ment and earlier regression of this smooth muscle produces an earlier drop in pul- THE PULMONARY CIRCULATION monary vascular resistance (PVR) in the preterm infant following birth. The development of the pulmonary vasculature closely follows the development Posture is a major determinant of pul- of the airways. During development there is monary blood volume, which is increased remodelling of the muscular wall of the pul- in the supine position due to a shift of blood monary arterial tree. Muscular arteries reach from dependent portions of the body to the the pleural surface in the adult, but in the central circulation (Hirasuna and Gorin, fetus and the newborn extend only to the level of the terminal bronchiole. During 1981). Pulmonary perfusion is distributed childhood these gradually extend to reach according to the forces of gravity so most blood travels to the dependent regions. The upright lung can schematically be divided into three regions (Figure 2.5). In zone 1 the Zone 1 PA > Pa> Pv Alveolar Zone 2 ~P\\~Pv Arterial / Venous Zone 3 Pa> Pv > PA Figure 2.5 Distribution of blood flow in the lung based on pressures affecting the capillaries. Reproduced with permission from J.B. West et al., Journal of Applied Physiology, 1964.

22 Physiology of the cardiorespiratory system alveolar pressure (PA) is greater than the the initial length of the cardiac muscle fibre. pulmonary artery presure (Pa) and much In the heart the length of the muscle fibre greater than the pulmonary venous pressure is proportional to the end diastolic volume. (PV). Therefore pulmonary vessels collapse This correlation is known as the Frank- and pulmonary blood flow and gas exchange stops. In zone 2 blood flows once Pa exceeds Starling relationship. It is present in infants PA. Flow rates increase linearly as the but is incomplete, with a narrow range over gradient between Pa and PA increases down which stroke volume increases with increas- the lung until pulmonary venous pressure and alveolar pressure are equal. In the third ing end-diastolic pressure. The ability of zone Pa and PV are both greater than PA. heart muscle to generate greater contractility The driving pressure Pa-PV is constant with volume increases is believed to be a throughout this lower dependent zone because gravity produces equal increases in function of the fundamental properties of Pa and PV. The transmural pressures Pa- actin-myosin mechanics. The relationship of pleural pressure (PPL) and PV-PPL are increased in the dependent regions and muscle fibres to adjacent fibres in the heart therefore the vessels are dilated and the flow may also be a contributory factor. There are is increased. This explanation pertains to a model of the upright lung; it is assumed that many muscular bridges between the muscle in infants lying supine the gravity related fibres in the myocardium which at rest are differences in distribution of flow are less mainly perpendicular to the main vector of pronounced. muscle fibre contraction. With stretching from an increase in volume these bridges may become parallel to the predominant fibres and then may contribute more effectively to the overall strength of contraction (Parker and Case, 1979). THE CARDIAC SYSTEM TRANSITIONAL CIRCULATION The cardiac cycle is a sequence of contraction The fetal circulation and the circulatory and relaxation of the heart muscle. At rest adjustments made after birth are fully a preloaded right ventricular pressure of described in Chapter 1. The placenta allows about 40 mmHg exists. The tricuspid valve transport of oxygen from the mother to the closes as isovolumic contraction begins fetus and carbon dioxide transport in the before the pulmonary valve opens. Con- opposite direction. It allows the passage of traction continues after the pulmonary valve nourishment and the excretion of fetal opens and the blood rushes into the pul- waste. Blood enters the placenta from the monary circuit until the pulmonary valve umbilical arteries and returns through the closes. The cardiac muscle then relaxes and umbilical vein. Oxygen is transferred by the pressure falls rapidly. Right ventricular diffusion and the fetal venous Pa02 is pressure approaches zero, the tricuspid always less than the maternal Pa02. Blood valve opens and the right ventricular volume returning to the fetus has a Pa02 of reaccumulates. This cycle is then repeated. 4-4.7kPa (30-35mmHg) and passes through The same occurs on the left side of the heart the liver. By midterm 60% of the umbilical with the mitral and aortic valves, but the venous blood bypasses the hepatic circu- pressures generated are much greater. lation through the ductus venosus to enter the inferior caval vein. Following closure of THE FRANK-STARLING RELATIONSHIP the duct after birth all portal venous blood passes into the capillary system of the liver The energy of contraction is proportional to before entering the inferior vena cava

References 23 (IVC) via the hepatic veins. Therefore blood are present in the fetus. Fetal cardiac output of a relatively high oxygen content enters the is controlled almost completely by changes right atrium from the IVC and one third of in fetal heart rate as the fetal heart has little this blood is directed into the left atrium ability to alter its stroke volume. There is a through the foramen ovale. The highly lag in sympathetic innervation of the heart saturated left atrial blood enters the left compared to that of the parasympathetic ventricle and is pumped into the ascending system. The distribution of blood flow to aorta. fetal tissue is controlled by changes in local vascular resistance. Under conditions of The mature infant usually passes through stress, e.g. fetal growth retardation, fetal the transitional circulation without any cardiac output favours the maintenance of problems. Delay is usually due to hypoxia umbilical flow and support of the myco- and therefore pulmonary vasoconstriction cardium and brain probably due to chemo- and dilation of the duct. A mild hypoxic receptor reflexes (Benson and Freedom, stimulus can stimulate an increase in pul- 1991). monary vascular response. A vicious cycle can ensue if hypoxia remains untreated. REFERENCES Pulmonary artery pressure remains elevated, the duct remains open and there may be Benson, L.N. and Freedom, R.M. (1991) The right to left shunting of blood through the transitional circulation, in Neonatal Heart foramen ovale. The respiratory rate increases Disease, (eds R. Freedom, L.N. Benson and and myocardial function may be impaired. J.F. Smallhorn), Springer Verlag, London, pp. This situation is termed persistent fetal 149-62. circulation (PFC) and is discussed further in Chapter 8. Significant cardiac malform- Berger, A.J., Mitchell, R.A. and Severinghaus, ations, e.g. transposition of the great J.W. (1977) Regulation of respiration. New arteries, pulmonary stenosis and septal England Journal of Medicine, 297, 92-7. defects, may also disturb normal transitional circulation. Bhuyan, U., Peters, A.M., Gordon, I. and Helms, P. (1989) Effect of posture on the CARDIAC OUTPUT distribution of pulmonary ventilation and perfUSion in children and adults. Thorax, 44, In fetal life one third of the cardiac output 480-4. (CO) enters the ascending aorta and two thirds enters the pulmonary artery. Only 8% Davies, H., Kitchman, R., Gordon, I., and of blood entering the pulmonary artery Helms, P. (1985) Regional ventilation in enters the lungs and the remainder passes infancy. New England Journal of Medicine, 313, through the ductus arteriosus to the 1626-8. descending aorta. There is a greater cardiac output per kilogram (kg) of body weight in Dawes, G.S. (1968) Foetal and Neonatal Physiology. the fetus and newborn infant than in the Year Book Medical, Chicago, pp. 141-59. adult. The infant has a resting cardiac output per kg of body weight of 2-3 times that in Dunhill, M.S. (1962) Postnatal growth of the the adult (Dawes, 1968). lung. Thorax, 17, 329. CIRCULATORY CONTROL Gaultier, C (1982) Physiology of respiration: application of functional exploration of the Baroreceptor and chemoreceptor functions lungs in infants and children, in Paediatric Respiratory Disease, 2nd edn, (eds J. Gerbeaux, J. Couvrer and G. Tournier), John Wiley & Sons, New York, pp. 17-59. Heaf, D.P., Helms, P., Gordon, I. and Turner, H.M. (1983) Postural effects on gas exchange in infants. New England Journal ofMedicine, 303, 1505-8. Helfaer, M.A., Nichols, D.G. and Rogers, M.C (1992) Developmental physiology of the

24 Physiology of the cardiorespiratory system respiratory system, in Textbook of Pediatric Children, 5th edn, (ed. V. Chernick), W.B. Intensive Care, 2nd edn, (ed. M.e. Rogers), Saunders, Philadelphia, pp. 3-47. Williams and Wilkins, Baltimore, p. 104. Parker, J.O. and Case, R.B. (1979) Normal left Hirasuna, J.O. and Gorin, A.B. (1981) Effect of ventricular function. Circulation, 60, 4-12. prolonged recumbancy on pulmonary blood Pasterkamp, H. (1990) The history and physical volume in normal humans. Journal of Applied examination, in Kendigs Disorders of the Physiology, 50, 950-5. Respiratory Tract in Children, 5th edn, (ed. V. Hogg, J.e., Williams, J., Richardson, J.B. et aI. Chernick), W.B. Saunders, Philadelphia, pp. (1970) Age as a factor in the distribution of 56-77. lower airway conductance and in the Pryor, J.A. (1991) The forced expiration tech- pathologic anatomy of obstructive lung nique, in Respiratory Care, (ed. J.A. Pryor), disease. New England Journal of Medicine, 282, Churchill Livingstone, Edinburgh, pp. 79-100. 1283-7. Rigatto, H. (1982) Apnea. Pediatric Clinics of North Konno, K. and Mead, J. (1967) Measurement of America, 29, 1105-66. the separate volume changes of rib cage and Strang, L.B. (1977) Oxygen transport in the abdomen during breathing. Journal of Applied blood, in Neonatal Respiration, (ed. L.B. Physiology, 22, 407-22. Strang), Blackwell Scientific Publications, Mansell, A., Bryan, e. and Levison, H. (1972) Oxford, pp. 138-60. Airway closure in children. Journal of Applied West, J.B. (1992) Ventilation, in Pulmonary Physiology, 33, 711. Pathophysiology, 4th edn, (ed. J.B. West), Mitchell, R.A. (1963) Respiratory responses Williams and Wilkins, Baltimore, pp. 3-18. mediated through superficial chemosensitive Wohl, M.E.B. and Mead, J. (1990) Age as a factor areas on the medulla. Journal of Applied in respiratory disease, in Kendigs Disorders of Physiology, 18, 523. the Respiratory Tract in Children, 5th edn, (ed. Muller, N.L. and Bryan, A.e. (1979) Chest wall V. Chernick), W.B. Saunders, Philadelphia, mechanics and respiratory muscles in infants. pp.175-82. Pediatric Clinics of North America, 26, 503-16. Zack, M.B., Pontoppidan, H. and Kazemi, H. O'Brodovich, H.M. and Haddad, G.G. (1990) (1974) The effect of lateral positons on gas The functional basis of respiratory physiology, exchange in pulmonary disease. American in Kendigs Disorders of the Respiratory Tract in Review of Respiratory Diseases, 110, 49-55.

IMAGING THE PAEDIATRIC CHEST 3 Christopher D. George and Isky Gordon INTRODUCTION MODALITIES IN PAEDIATRIC CHEST Despite the advent of new imaging modal- IMAGING ities, such as ultrasound, computed tomo- graphy (CT), magnetic resonance imaging PLAIN CHEST RADIOGRAPHS AND (MRI) and ventilation/perfusion lung scans (V/Q scan), the plain chest radiograph FLUOROSCOPY remains the mainstay of paediatric chest imaging. In most circumstances the clinical Chest radiographs may be taken in the erect history and examination will be augmented posteroanterior (PA) or anteroposterior (AP) by a chest radiograph before a working position or in the supine AP position. In diagnosis is made and treatment or further some circumstances, such as on the neonatal investigations planned. unit (NNU), where patient handling is minimized all films are obtained in the This chapter aims to provide a concise and supine AP projection. Up to the age of three practical introduction to imaging the years any of the projections may be used paediatric chest, emphasizing the import- depending on the policy of the department. ance of the plain chest radiograph but also Over the age of three most units obtain erect indicating where other modalities provide PA films. It is important that within any additional information or allow the same given unit techniques are standardized and information to be acquired with less use films clearly labelled as the appearances of of ionizing radiation. The first section some radiological signs, particularly those provides an overview of imaging modal- of pleural fluid and pneumothorax, are pro- ities currently available, the second reviews foundly different in the erect and supine important radiological signs commonly positions. These changes will be discussed seen in paediatric chest radiographs and in greater detail in the next section. Frontal the final section discusses common paedi- chest radiographs should be obtained in atric chest problems and their radiological inspiration, using a short exposure time and signs. with attention to technical factors so as to minimize the radiation exposure to the The text has not been referenced exten- patient and attendants. sively; however, a number of selected general references suitable for further The lateral chest radiograph necessitates reading are given at the end of the chap- a significantly higher exposure than the ter. frontal and is not required routinely. It is usually obtained during the follow-up of patients with cystic fibrosis, malignant

26 Imaging the paediatric chest disease likely to metastasize to the chest and available conventional tomography is no in the assessment of recurrent chest infec- longer used and bronchography is only tions. A lateral view may also be performed rarely undertaken to demonstrate focal to clarify an abnormality seen on the frontal bronchial narrowing. projection. ULTRASOUND Coned, AP plain radiographs using a high kV technique and filtration to give an Ultrasound is useful for exanumng the optimal exposure are used to demonstrate pleural space for fluid (Figure 3.1). Effusions the anatomy and calibre of the major and empyemas can be located, measured airways. and drained under ultrasound control. Because the ultrasound beam is strongly One of the disadvantages of conventional reflected by the aerated lung, ultrasound is radiographs is that it is difficult to adequately less useful for assessing lung lesions unless demonstrate all soft tissue and bony struc- they are peripheral, lie against the chest wall tures using the same exposure factors. Two and consist of either solid or fluid. The major recent developments have attempted movement and integrity of the hemidia- to overcome these disadvantages. The first phragms can be assessed using ultrasound. is digital chest radiography in which a The disadvantage is that each hemidia- phosphor plate is used for the exposure. The phragm can only be assessed independently plate is then scanned with a laser beam and not in relationship to each other. This which reads the information and stores it in is important in mild hemidiaphragm paresis. digital form. The information can then be Cardiac ultrasound is an extremely accurate reconstructed as the 'chest X-ray' on a non-invasive way of assessing congenital computer screen and manipulated to allow heart disease. optimal visualization of areas of interest. Hard copies of the images can be printed on COMPUTED TOMOGRAPHY (CT) AND a laser imager. The advantages of this MAGNETIC RESONANCE IMAGING (MRI) technique in paediatric radiology are the uniformity of image that can be maintained In many ways these techniques are from day to day, the facility for image complementary and will be discussed manipulation and a reduction in radiation together. Both techniques require the patient dose. to remain still for the duration of the scan and this is particularly important in MRI. The second technique, scanning equal- Neonates and young infants may be ization radiography (SER), uses a beam of examined if asleep after a feed but older X-rays which scans the patient. The expo- infants and children usually require sedation sure is continuously changing according to or general anaesthesia. the tissues within the beam at any given time. This results in a more even exposure CT uses a narrow beam of X-rays to image and a more uniform image. the patient in 'slices'. The thickness of the slice may be varied from 1.5 mm to 1 cm and Fluoroscopy remains a useful technique slices may be taken with or without gaps for assessing diaphragmatic movements and between them depending on the region for detecting changes in airway calibre being examined and the likely pathology. during respiration in conditions like tracheo- Assessment of the mediastinum and of vas- malacia. cular structures is facilitated by using intra- vascular contrast medium. High resolution BRONCHOGRAPHY AND TOMOGRAPHY Since CT and MRI have become widely

Modalities in paediatric chest imaging 27 Figure 3.1 Sagittal ultrasound of a large pleural effusion which is poorly echogenic and appears black (white arrows). The patient's back is seen at the bottom of the image and the liver lies to the right. The effusion surrounds the partly collapsed, triangular lower lobe. computed tomography (HRCT) uses a thin Intravascular contrast medium for MRI is slice thickness and special software to available. demonstrate the lung parenchyma. HRCT is used in the diagnosis of diffuse paren- CT has better spatial resolution and can chymal disease and bronchiectasis. detect fine calcification which affords it an advantage in evaluating mediastinal masses In MRI the patient lies within a strong and lymphadenopathy. Bone structure and magnetic field and is exposed to pulses of in particular cortical changes are best radio frequency energy. This energy is imaged on CT. Currently lung pathology is absorbed by protons within the body. When best evaluated on CT using HRCT if the radio frequency pulses are stopped the necessary. Vascular structures are well protons return to their normal state but as demonstrated on CT if intravascular contrast they do so they release energy, the magnetic is used and ultrafast CT scanners, which resonance signal, which can be detected by enable the entire chest to be scanned in a coils placed around the body. Magnetic matter of seconds, facilitate the investi- resonance signals are different for different tissues and may be altered by disease. gation of congenital vascular and cardiac abnormalities.

28 Imaging the paediatric chest MRI of the chest is made difficult by There is no reliable technique for the cardiac and respiratory movements. These positive diagnosis of aspiration; this includes effects can be minimized by only taking the barium swallow/meal as well as the images at the same point in each cardiac and isotope milk scan. Recurrent aspiration may respiratory cycle, a technique known as be inferred when there is severe gastro- gating. MRI has three major advantages over oesophageal reflux. CT: its superior soft tissue contrast, its ability to acquire images in any plane and RADIONUCLIDE STUDIES the fact that it does not use ionizing radia- tion. Sagittal and coronal images are of Radionuclide studies provide quantifiable immense value in assessing the extent of a functional information which complements mediastinal mass and in deciding whether the anatomical information provided by a paraspinal mass extends into the spinal other imaging modalities. The ventilation/ canal. perfusion scan (V/Q scan) uses krypton (81mKr) gas for ventilation and technetium One of the most exciting branches of MRI (99mTc) labelled macroaggregates for is magnetic resonance angiography (MRA) perfusion. The V/Q scan is the only method which allows blood vessels and the heart to which will provide information on regional be imaged without the need for artery lung function. The radiation burden from the puncture or the injection of any contrast V scan is less than one fifth of a chest media. radiograph while the Q scan has a dose equal to less than 60 seconds of fluoroscopy. ANGIOGRAPHY AND CARDIAC CATHETERIZAnON Most ventilatory disturbances result in a corresponding reduction in perfusion where- Cardiac ultrasound and the advent of MRA as if the pulmonary artery to a region is have reduced the indications for conven- occluded (pulmonary embolus, sequestrated tional angiography and cardiac catheter- segment or pulmonary artery disease) that ization to assess congenital anomalies of the region remains ventilated. Occasionally aorta and pulmonary vessels and congenital other radionuclide studies such as bone arteriovenous malformations. scans or milk scans are indicated in the assessment of chest pathology. BARIUM STUDIES BASIC SIGNS ON THE PLAIN CHEST RADIOGRAPH These studies have a limited but very important role in the assessment of chest CONSOLIDAnON problems, specifically the barium swallow to evaluate extrinsic oesophageal com- Replacement of air in the very distal airways pression by aberrent vessels or masses and alveoli by fluid or solid is called consoli- and the swallow/meal to assess intrin- dation. The cardinal signs of consolidation sic abnormalities such as incoordinated are an area of increased opacity which may swallowing, abnormal oesophageal peristal- have an irregular shape, irregular margins, sis or gastro-oesophageal reflux which can a non-segmental distribution and contains cause aspiration. If a tracheo-oesophageal an air bronchogram (Figure 3.2). The volume fistula is suspected a tube oesophagogram of the affected lung remains unchanged and must be performed in the prone posi- consequently there are no signs of loss of tion. volume. If an area of consolidation abuts

the mediastinum, heart or diaphragm their Basic signs on the plain chest radiograph 29 clear silhouette, which is dependent upon the sharp radiological contrast between The commonest causes of consolidation are normally aerated lung (black) and solid listed in Table 3.1. structures (white), is lost (Figure 3.2). Similarly the presence of air bronchograms within an area of consolidation can be explained by the sharp contrast between air in the medium and large bronchi (black) and the surrounding non-aerated and 'solid' lung (white) (Figures 3.2 and 3.3). A variant of infective consolidation fre- quently seen in infants and children is the 'round pneumonia'. This may mimic a mass lesion radiologically since it has well defined borders; however, the clinical picture points to an infective aetiology. While infection is the commonest cause of consolidation, it is also caused by any pathological process in which the alveoli are filled by fluid or solid. Figure 3.3 AP supine radiograph of a premature neonate with respiratory distress syndrome (RDS). The lungs show generalized opacity due to con- solidation and a prominent air bronchogram. Table 3.1 Common causes of consolidation Pulmonary oedema Cardiogenic Non-cardiogenic Respiratory distress syndrome Aspiration Pulmonary exudate Infection Blood Traumatic contusion Infarction Aspiration Figure 3.2 Right lower lobe consolidation caused Other rare causes by the bacterium Streptococcus pneumoniae. There is Alveolar proteinosis increased opacity in the right lower and mid zones, Alveolar microlithiasis loss of the clear outline of the right hemidiaphragm Lymphoma and a proximal air bronchogram. Sarcoidosis

30 Imaging the paediatric chest COLLAPSE Collapse means loss of lung volume and this may affect a lung, lobe or segment. This is manifest on the radiograph by shift of the normal fissures and crowding of airways in the collapsed lung (Figures 3.4a,b and 3.Sa,b). If the volume loss is large there may also be mediastinal shift towards the affected side, elevation of the ipsilateral hemidia- phragm, ipsilateral rib crowding and alter- ation in hilar positon. The collapsed lobe Figure 3.4 (a) AP radiograph taken in a lordotic (b) projection to show the bandlike opacity of middle lobe collapse. Part of the right heart silhouette is lost where the collapsed lobe abuts the heart. (b) Lateral radiograph showing the collapsed middle lobe and displaced fissures. In addition the lungs show generalized overinflation with some flattening of the diaphragm.

Basic signs on the plain chest radiograph 31 (b) Figure 3.5 (a) Frontal radiograph of a patient with asthma and a left lower lobe collapse caused by a mucus plug. Generalized overinfiation, increased opacity in the left cardiac region and loss of clarity of the outline of the medial left hemidiaphragm. (b) The lateral radiograph shows the collapsed left lower lobe as a wedge-shaped opacity in the lower chest posteriorly. mayor may not cause increased radio- patient. In the erect position the fluid collects opacity and there may be compensatory at the bases and initially causes blunting to hyperinflation of unaffected lobes. If the the costophrenic angles. Larger effusions collapsed lobe abuts on part of the cause a homogeneous opacity with a diaphragm or cardiomediastinal silhouette concave upper border higher laterally than the clear outline of these may be lost on the medially, the meniscus. Very large effusions radiograph as in consolidation (Figures may cause mediastinal shift to the opposite 3.4a,b and 3.Sa,b). Collapse is most often side. due to obstruction of a large airway by foreign body, mucous plug, tumor or In the supine position, often used for extrinsic compression. Less commonly it neonatal and infant radiographs, an effusion occurs secondary to poor ventilation. causes reduced transradiancy (whiter hemithorax) of the affected side and may PLEURAL FLUID collect around the apex of the lung. In larger effusions a peripheral band of soft tissue The radiological appearance of plural fluid density appears between the chest wall and is largely determined by the position of the the lung; on the right this band has a characteristic step at the position of the

32 Imaging the paediatric chest horizontal fissure (Figures 3.1 and 3.6). anteriorly and cause an increased ipsilateral Pleural fluid may collect and loculate within transradiancy (darker hemithorax) and fissures or between the inferior surface of the lung and the diaphragm, the I subpulmonic' increased sharpness of the cardiomediastinal silhouette (Figure 3.7). effusion. Figure 3.6 Supine radiograph showing a pleural Figure 3.7 Supine radiograph showing a effusion. There is reduced transradiancy on the right postoperative right pneumothorax. There is in- and a peripheral band of soft tissue density creased transradiancy of the right hemithorax. The paralleling the chest wall with a 'step' at the position right heart border is very clearly defined and the of the horizontal fissure. right lung edge is visible. PNEUMOTHORAX Figure 3.8 Supine radiograph of a patient with RDS. There is a left tension pneumothorax causing In the erect positon pleural air collects at the flattening of the hemidiaphragm and mediastinal apex causing increased apical transradiancy shift to the right. The pneumothorax is seen (darker apex) and absent lung markings surrounding a consolidated left lung. A needle drain beyond a visible lung edge. In the supine has been inserted. position air collects initially in the anteroinferior chest causing quite different and often subtle signs. These include small slivers of air at the apex, around the heart and between the lung and the diaphragm. Where free air as opposed to aerated lung abuts part of the cardiomediastinal or diaphragmatic silhouette the clarity of that border is especially sharp, this being the opposite of the effect seen in consolidation (see above). A large pneumothorax in neo- nates and infants when supine may collect

Common paediatric chest problems 33 A tension pneumothorax occurs when a sided, situated posteriorly and large. pleural tear acts as a one-way flap valve, Abdominal organs are sited in the chest and allowing air into the pleural space but appear on the radiograph as a cystic/solid preventing egress. The pressure within the mass. The mediastinum is shifted to the hemithorax rises and may remain positive contralateral side and one or both lungs may for much of the respiratory cycle causing be hypoplastic (Figure 3.9). When large the mediastinal shift to the contralateral side and condition carries a high mortality. flattening, or even eversion, of the ipsilateral hemidiaphragm (Figure 3.8). Hiatus hernia COMMON PAEDIATRIC CHEST PROBLEMS A sliding hiatus hernia exists when the lower oesophageal sphincter and part of the CONGENITAL ABNORMALITIES OF THE CHEST stomach are situated in the thorax, above the diaphragm. This condition is usually Congenital diaphragmatic hernia associated with incompetence of the sphincter and may result in feeding Large congenital diaphragmatic hernias problems, gastro-oesophageal reflux and frequently present as neonatal respiratory aspiration. distress although many are now diagnosed antenatally on routine antenatal ultrasound Congenital lobar emphysema examination. Many are associated with other congenital anomalies. Most hernias are left A focal abnormality of a lobar bronchus leads to a ball valve effect causing air trapping and overinflation of the affected lobe. The left upper, right middle and right upper lobes are most frequently affected. Initial radio- graphs in the first few hours of life may show an opaque mass in the region of the affected lobe. As fluid clears the appearances are those of an overinflated lobe with compression of normal surrounding lung and mediastinal shift to the contralateral side (Figure 3.10). Treatment is surgical excision of the affected lobe if the neonate is in respiratory distress; if found in the older infant conservative management is advo- cated. Figure 3.9 A large left diaphragmatic hernia. The Cystic adenomatoid malformation left hemithorax contains the stomach (nasogastric tube) and loops of small bowel. The mediastinum This condition, caused by a disorganized and is shifted to the right . The right lung is airless and usually cystic mass of pulmonary tissue, can opaque because the patient is on an extracorporeal mimic both congenital diaphragmatic hernia membrane oxygenator (ECMO). and congenital lobar emphysema. The hamartomatous mass can affect any lobe, although the middle lobe is rarely affected and in one fifth of cases more than one lobe

34 Imaging the paediatric chest pulmonary interstitial emphysema (PIE) (Figure 3.11), pneumothorax (Figures 3.8 and 3.11), pneumomediastinum and bron- chopulmonary dysplasia (BPD) (Figure 3.12). Pulmonary interstitial emphysema is caused by gas leaking from overdistended alveoli and tracking along bronchovascular sheaths. The radiographic appearance is that of a branching pattern of gas with associated bubbles affecting all or part of the lung (Figure 3.11). Figure 3.10 Congenital lobar emphysema of the left upper lobe. The lower lobe is compressed and the mediastinum is shifted to the right. is affected. The radiograph shows a well defined cystic mass which may be large, compressing adjacent lung and causing mediastinal shift. NEONATAL CHEST PROBLEMS Respiratory distress syndrome (RDS) Immature surfactant produced in premature Figure 3.11 Pulmonary interstitial emphysema infants, infants of diabetic mothers and (PIE) complicating RDS. There is a branching infants who experience perinatal asphyxia pattern of gas with associated small bubbles. fails to reduce the alveolar surface tension Bilateral chest drains and persistent right pneumo- sufficiently to prevent alveolar collapse. This thorax. is the commonest cause of respiratory dis- Bronchopulmonary dysplasia is seen tress in premature neonates and causes exclusively in infants who have been on positive pressure ventilation, usually for tachypnoea, cyanosis, expiratory grunting RDS. The combination of high pressure and chest wall retraction. The radiograph trauma and oxygen toxicity results in lung shows bilateral symmetrical hypoaeration, damage. The lung passes through a number of radiological stages during the evolution small volume lungs, ground glass granul- of BPD. There is a RDS pattern which progresses to almost complete opacification arity of the pulmonary parenchyma and well and then to a coarse pattern of linear defined air bronchograms extending from the hilum into the peripheral lung (Figure 3.3). These neonates frequently require inter- mittent positive pressure ventilation which may give rise to specific complications of

Common paediatric chest problems 35 Figure 3.12 Bronchopulmonary dysplasia. A coarse pattern of linear opacities and cystic lucencies. opacities and cystic lucencies (Figure 3.12). The lack of adequate oxygenation in RDS may result in failure of the ductus arteriosus to close. The consequent left-to-right shunt may progress to frank plethora and heart failure. Meconium aspiration syndrome This is the commonest cause of respiratory Figure 3.13 Meconium aspiration syndrome. Areas of patchy collapse with other areas of overinflation. distress in full or post-term neonates. The The right lung is most affected. aspirated meconium causes a chemical pneumonitis and bronchial obstruction. The radiographic picture is of bilateral diffuse patchy collapse with other areas of overinfla- tion (Figure 3.13). Spontaneous pneumo- thorax, pneumomediastinum and small effusions are common but air bronchograms are rare.

36 Imaging the paediatric chest RESPIRATORY TRACT INFECTIONS perihilar streakiness, coarse patchy parenchymal infiltrates, nodular or reticulo- Viral infections nodular shadowing or diffuse hazy shadowing, most often basal. One important Viral infections generally affect the bronchi pattern to recognize is the diffuse bilateral and peribronchial tissues and this is reflected in the radiological signs: symmetrical granularity of group B haemolytic strepto- parahilar, peribronchial streaky shadowing coccal pneumonia which so closely mimics radiating for a variable distance into the lung periphery, hilar lymphadenopathy, RDS. occasionally reticulonodular shadowing, In infants bacterial infection is more segmental/lobar collapse and generalized overinflation secondary to narrowing of the often seen as lobar or patchy consolidations bronchi (Figure 3.14). Effusions are rare. (Figure 3.2). The organisms are most com- Organisms commonly encountered are the monly Haemophilus injluenzae, Streptococcus respiratory syncytial virus (RSV), influenza pneumoniae, Staphylococcus aureus and and parainfluenza viruses, adenovirus and Mycoplasma pneumoniae. Pleural effusions, rhinovirus. empyemas, abscesses and pneumatocoeles are well recognized complications. The 'round pneumonia' is an area of infec- tive consolidation which transiently has a rounded configuration and mimics a mass lesion. Mycoplasma pneumoniae infection can mimic the radiographic appearances of both bacterial and viral pneumonia. However, one pattern that is highly specific is unilobar reticulonodular infiltration especially if associated with hilar lymphadenopathy and/or a small pleural effusion. Figure 3.14 Viral pneumonia caused by the Tuberculosis respiratory syncytial virus (RSV). There is sym- metrical parahilar, peribronchial streaky shadowing Tuberculosis acquired in infancy is usually and mild hilar lymphadenopathy. manifest by unilateral hilar or paratracheal lymphadenopathy and occasionally the Bacterial and mycoplasma infections primary or Ghon focus is seen as an area In the neonatal period the most common of consolidation in the periphery of the organisms are non-haemolytic streptococci, ipsilateral lung (Figure 3.15). Collapse is Staphylococcus aureus and Escherichia coli. seen, usually due to compression of a Lobar consolidation is rare and more often bronchus by lymph nodes. Bronchopneu- the following signs are seen: radiating monic spread, with widespread areas of consolidation, occurs if either an infected node discharges into a bronchus or when host resistance is very low facilitating spread through the airways. Miliary tuberculosis, with multiple small nodules, is caused by the haematogenous spread that occurs when an infected node discharges into the blood stream. Cavitation is unusual in children.

Common paediatric chest problems 37 with parahilar, peribronchial infiltrates but hilar lymphadenopathy is rare. Plugs of viscid mucus obstruct the airways and cause recurrent segmental or lobar collapse (Figure 3.Sa,b). Pneumomediastinum is a common complication but rarely requires specific treatment; pneumothorax is seen less fre- quently. Figure 3.15 Primary tuberculous infection. Obstruction by foreign bodies Unilateral right hilar lymphadenopathy and an area of consolidation (Chon focus) in the ipsilateral lower Aspirated foreign bodies most commonly zone. lodge in the major bronchi and act like a ball valve causing a distal obstructive emphysema (Figure 3.16a,b). Radiographs are taken in inspiration and expiration to demonstrate the air trapping. Less com- monly the lung distal to the obstruction collapses and may become infected. AIRWAY OBSTRUCTION CYSTIC FIBROSIS Asthma This autosomal recessive condition causes The radiological features are rarely seen excessively thick and viscid mucus. In the before the age of three. In chronic asthma neonatal period bowel obstruction due to there is Qeneralized overinflation of the lunss meconium ileus may draw attention to the condition. In the chest the earliest signs are (a) (b) Figure 3.16 (a) Aspirated foreign body lodged in the left mainstem bronchus. Marked air trapping in the affected lung causing overinflation, increased transradiancy and mediastinal shift. (b) Same patient after bronchoscopic removal of the obstruction.

38 Imaging the paediatric chest smith and Mrs Susan Ranson of the Depart- very similar to those of viral bronchiolitis; ment of Diagnostic Radiology at the Royal overinflation, focal collapse and parahilar, Surrey County Hospital, Guildford, and the peribronchial infiltrates (Figure 3.17). Department of Medical Illustration at the Recurrent infections lead to bronchiectasis, Great Ormond Street Hospital for Children fibrosis and generalized overinflation with NHS Trust, for their help in preparing the segmental areas of collapse. Bronchial illustrations. collaterals are recruited and when these become large haemoptysis may be a FURTHER READING problem. De Bruyn, R (1993) Paediatric chest, in Clinical Figure 3.17 Cystic fibrosis. Overinflation, focal Ultrasound: Abdominal and General Ultrasound, collapse and parahilar, peribronchial infiltrates. vol. 2, (eds D. Cosgrove, H. Meire and K. ACKNOWLEDGEMENTS Dewbury), Churchill Livingstone, London, pp.983-8. We are most grateful to Dr B.J. Loveday at the Royal Surrey County Hospital, Guild- Edelman, RL. and Warach, S. (1993) Magnetic ford, and Dr D.B. Reiff at St George's resonance imaging (part 1). New England Hospital, London, for allowing us to use Journal of Medicine, 328, 708-16. their radiographs as illustrations. We wish to thank Mrs Hazel Cook, Mrs Mary Shoe- Edelman, RL. and Warach, S. (1993) Magnetic resonance imaging (part 2). New England Journal of Medicine, 328, 785-91. Gordon, I., Helms, P. and Fazio, F. (1981) Clinical applications of radionuclide lung scanning. British Journal of Radiology, 54, 576-85. Gordon, I., Matthew, D.J. and Dinwiddie, R (1987) Respiratory system, in Diagnostic Imaging in Paediatrics, (ed. I. Gordon), Chapman & Hall, London, pp. 27-57. Hayden, C.K. and Swischuk, L.E. (eds) (1992) Pediatric Ultrasonography, 2nd edn, Williams and Wilkins, Baltimore. Newman, B. (1993) The pediatric chest. Radiology Clinics of North America, 31, 453-719. Piepsz, A., Gordon, I. and Hahn, K. (1991) Paediatric nuclear medicine. European Journal of Nuclear Medicine, 18, 41-66. Swischuk, L.E. (1989) Imaging of the Newborn, Infant and Young Child, 3rd edn, Williams and Wilkins, Baltimore. Weinberger, E. and Brewer, D.K. (1992) Pediatric body imaging, in Computed Tomography of the Body with Magnetic Resonance Imaging, 3rd edn, (eds A.A. Moss, G. Gamsu and H.K. Genant), W.B. Saunders, Philadelphia, pp. 1267-96.

MANAGEMENT OF THE ACUTELY ILL 4 CHILD IN RESPIRATORY FAILURE Robert C. Tasker INTRODUCTION Table 4.1 Clinical signs of Respiratory Failure Many of the medical problems seen in acutely Respiratory Cardiac ill children have the potential for culminating Tachypnoea Tachycardia in respiratory failure. The approach to manag- Altered depth and pattern Hypertension ing such patients whilst requiring paediatric Bradycardia intensive care has largely 'evolved' over the of breathing (deep, Hypotension past 20 years from an historical disease-orien- shallow, apnoea, irregular) Cardiac arrest tated focus to a meticulous and coordinated Intercostal recession Cerebral multiorgan system of review (Rogers, 1987). Nasal flaring Restlessness Cyanosis Irritability In children, the conditions that most com- Decreased or absent breath Headache monly cause respiratory failure differ with sounds Mental age. In the newborn infant, prematurity, hya- Expiratory grunting line membrane disease, asphyxia and aspira- Wheeze and/or prolonged confusion tion pneumonia are the most common aetio- expiration Papilloedema logies. Under two years of age, bronchopneu- General Seizures monia, bronchiolitis, croup, status asthmat- Fatigue Coma icus, foreign body inhalation and congenital Excessive sweating heart and airway anomalies are important, compared with asthma, accidental poisoning breath sounds, severe intercostal recession, and central nervous system infection, trauma and hypoxia/ischaemia in the over two year accessory muscle use during inspiration, olds. Whilst it is evident that the lung is the a decreased level of consciousness and 'target organ' for many of these conditions, cyanosis, needs emergency assistance (Table when respiratory failure ensues, there may 4.1). However, so do those patients with a be significant vital organ impairment resulting less severe degree of respiratory failure, from reduced arterial blood gas tensions, which may be more easily and more effec- acidosis and hypercarbia. The emphasis of tively treated. Over recent years technolog- this chapter will be an overview of respiratory ical developments in transcutaneous oxygen supportive therapy in the context of managing and carbon dioxide monitoring and pulse the critically ill child. oximetry have been invaluable in enabling RESPIRATORY MONITORING such an early recognition, particularly The infant or child with obvious respiratory because of their sensitivity in detecting distress, consisting of decreased or absent significant hypoxia and hypercarbia not recognizable by clinical examination. Hypoxaemia is defined as a blood oxygen level that is less than normal, which mayor may not be adequate to meet the body's

40 Management of the acutely ill child metabolic demands. Hypoxia occurs when dioxide, whereas the oxygenation that the oxygen requirements of the body's occurs in the pulmonary capillary promotes unloading. A rise in the arterial blood gas tissues are not met by the oxygen being level of carbon dioxide causes a shift of the delivered to the tissues by the circulating oxygen-haemoglobin dissociation curve to blood. In response to hypoxia, the body has the right, facilitating the release of oxygen several mechanisms to increase the systemic to the tissues (Figure 4.1). Carbon dioxide also stimulates the respiratory centres to delivery of oxygen. These include: increased increase ventilation, which results in carbon dioxide excretion and increased oxygen pulmonary gas exchange, increased pul- consumption. Finally, the autonomic effects monary blood flow, improved matching of associated with a high arterial carbon dioxide level, such as increased pulmonary vascular ventilation and pulmonary perfusion, resistance, bronchoconstriction, increased increased haemoglobin affinity for oxygen, cardiac output and increased ventilation, are greater dissociation of oxygen from haemo- in the main due to secondary pH changes. globin in the tissues, increased cardiac Given these potential physiological alterations observable during impending output and tissue perfusion. respiratory failure, respiratory monitoring of Carbon dioxide, the end product of the seriously ill child should systematically include a periodic assessment of respiratory aerobic cellular metabolism, is carried by the effort, a continuous display of arterial blood in a manner similar to oxygen. After oxygen saturation and an intermittent review of gas exchange and the adequacy of diffusing into the blood plasma most of the ventilation (Table 4.2). carbon dioxide enters the red blood cells and The clinical observations recorded should undergoes chemical transformation to include: skin colour, peripheral perfusion, carbonic acid. Ultimately hydrogen and respiratory rate and pattern, whether or not bicarbonate ions are formed. The bicarbonate accessory muscles are being used, breath freely diffuses from the red blood cell into sounds and air entry and the level of con- the plasma. Some of the hydrogen ions sciousness and fatigue. liberated in the red blood cell as a result of hypoxia are bound to and reduce the haemo- MONITORS globin molecule. The presence of reduced haemoglobin promotes the loading of carbon Pulse oximetry is a readily applicable clinical tool which non-invasively measures arterial 100 ... -T_ota1l O_2-- 1-- 22 oxyhaemoglobin (Hb02) (Figure 4.2). This device functions by placing a pulsating f- arterial bed (e.g. finger or earlobe) between a two-wavelength light source and a light 80 ...\\ 18 detector. The degree of change in trans- O2 combined mitted light is proportional to the size of the E arterial pulse change, the wavelengths of c with Hb 14 00 the light used and the Hb02 saturations. Assuming that the pulsatile waveform is 0 ~ entirely due to the passage of arterial blood, C the use of the appropriate light wavelengths ~ ,60 10 .cl!l C:>ii (/) D- 40 u0 I ,p 6 0'\" 0 20 ____D_is_soilv.e_d _O_2 _ - 2 0 20 40 60 80 100 600 P02 mmHg Figure 4.1 Oxygen-haemoglobin dissociation curve. Reproduced from J.B. West, Respiratory Physiology, Williams and Wilkins, 1984.

Respiratory monitoring 41 Table 4.2 Normal values Newborn Up to 3 years 3-6 years Above 6 years Respiratory rate 40-60 20-30 20-30 15-20 (breaths/minute) Arterial blood 7.30-7.40 7.30-7.40 7.35-7.45 7.35-7.45 pH 30-35, 4.0.-4.7 30-35, 4.0-4.7 35-45, 4.7-6.0 35-45, 4.7-6.0 PC02 (mmHg, kPa) P02 60-90, 8.0-12 80-100, 10.7-13.3 80-100, 10.7-13.3 80-100, 10.7-13.3 Cardiovascular Heart rate (beats/minute) 100-200 100-180 70-150 70-150 75-130 90-140 90-140 Systolic blood pressure 45-90 50-80 50-80 (mmHg) 60-90 Diastolic blood pressure (mmHg) 30-60 Figure 4.2 A pulse oximeter with probe.