__Biophysical_Bases_of_Electrotherapy

ELECTROMAGNETIC WAVES FOR THERAPY 295 when they fall back into their original orbitals, the energy released is radiated as light The light emitted by a of a particular frequency. Because electrons may be kicked out of, and fall back to, burning salt is usually at a different orbitals, the light emitted is a mixture of several specific frequencies. By mixture of waves of different contrast, laser radiation has but a single frequency. frequency. The different The light emitted by a burning salts is also incoherent, meaning that electrons drop frequencies correspond to back into their ground-state orbitals randomly so there is no synchronization of the electrons returning to different radiated electromagnetic waves. By contrast, lasers are devices which force 'ground state' orbitals. electrons to drop back into one particular orbital in an avalanche effect, i.e. almost The term 'monochromatic' simultaneously. The result is that the emitted waves are all synchronized (coherent) literally means 'one colour'. and have the same frequency. In most contexts this means The avalanche effect and resulting coherence of a laser beam is achieved by that each wave has the same bouncing waves back and forth between two reflectors. For example, a helium-neon frequency. laser consists of a cylindrical tube containing helium and neon gas. Each end of the tube has a reflector, one is fully reflecting and the other is partially reflecting so as to allow some light (the laser beam) to escape. The back-and-forth reflection triggers a resonance effect where electrons to drop back into a specific ground-state orbital synchronously and a coherent, monochromatic beam of waves is produced. each wave having the same frequency. To keep the laser operating it is necessary to bump electrons out of their ground-state orbitals and into a higher-energy orbital, ready to drop. For this reason a power supply (a source of energy) is required. Sometimes the energy is provided by an electric current, sometimes by a by a burst of light energy. In the case of a helium- neon laser, a power supply is used to energise a flashlight (rather like a camera flash) which provides rapid-fire bursts of light energy to push electrons into an excited state. In the case of diode lasers, current flow through the diode provides the necessary energy. We can summarize the differences between laser light and light from a common, incandescent light bulb as follows. Light from a normal incandescent source has a spectrum of frequencies and the waves are incoherent. Lasers are beams of coherent waves of identical frequency. There is some clinical evidence that laser ELECTROMAGNETIC WAVES FOR THERAPY 296 beams can be therapeutically beneficial. What has not been established is whether High power lasers are used laser beams have any advantage over simpler (and cheaper) torch beams. No to cut steel sheets several comparisons have yet been reported. centimetres thick. Much lower powers are used in Beam Intensity microsurgery, where focused beams are used to cut tiny The output of a laser can vary from tens of milliwatts to tens of kilowatts, depending on regions of tissue. the type and the physical construction. Lasers used therapeutically have power levels Lasers are often applied with between these two extremes. They are typically of relatively low power and intensity. only a thin film of plastic Intensities are normally in the range 1 mW.cm-2 to 50 mW.cm-2. separating the laser from the The beam diameter of the low power lasers used clinically is about 3 mm (an area of skin surface, so beam about 7 mm2). Thus if the output intensity is, for example, 20 mW.cm-2 and the area is divergence is not important. 7 mm2 = 0.07 cm2, the power of the beam is 20/0.07 mW ≈ 300 mW or 0.3 W. By way of comparison, a torch might have a beam 8 cm in diameter (an area about 50 cm2) and use a 12 W light bulb. As far as visible light output is concerned, the bulb is about 25% efficient (75% of the energy is emitted at infrared frequencies). Hence the power of the visible light-beam is approximately 3 W. The visible-light beam intensity is 3/50 = 0.06 W.cm-2 or 60 mW.cm-2. The intensity of the infrared component is approximately 180 mW.cm-2. A torch beam thus has a similar and, if anything, a higher power and intensity than a clinical laser but is polychromatic. The wave energy is spread over a range of frequencies. Any clinical significance of the polychromatic/monochromatic difference has yet to be established. Beam Divergence Light from a light bulb can be formed into a pencil-like beam (as in a searchlight) by using a parabolic reflector but the beam divergence is larger than that of a laser because of the practical difficulty of producing a perfectly shaped reflector. This difference would be of no clinical significance for beams between a light source and the patient, a distance of only a few centimetres or tens of centimetres.

ELECTROMAGNETIC WAVES FOR THERAPY 297 Beam Diameter Coherence is only possible if waves have identical The beam diameter of the low power lasers used clinically (commonly referred-to as frequencies. If the 'low level lasers') is about 3 mm (an area of about 7 mm2 = 0.07 cm2). A frequencies (and thus, the consequence is that if the area of the skin surface which is to be treated is several wavelengths) are different, cm2, the beam must be scanned over the area. This means that both the average they cannot stay in-phase. intensity and the energy delivered per unit area are reduced. For example, if the area Since coherence is lost to be treated is 5 cm x 5 cm (25 cm2), the reduction in average intensity and energy when lasers are beamed delivered per unit area is 25/0.07 = 3500 times. By contrast, a torch beam would through tissue, whether the illuminate the same area with no reduction in intensity or energy delivered. light source is a laser or superluminous diode Coherence appears irrelevant. The light from a light-globe is incoherent. The radiated waves have different frequencies (a spread of frequencies about some mean) and the waves are not 'in synch' with each other. Synchronization is impossible because the wavelengths are different. The coherence of a laser beam is not likely to be of practical significance as biological tissues are quite inhomogeneous at a microscopic level. This means that waves will be scattered and slowed to varying extents so coherence will be lost. A coherent beam striking the skin surface will be incoherent after traversing a distance through tissue of only a few cell diameters. Although coherence is rapidly lost in biological tissue, the beam remains monochromatic i.e. the waves still have identical frequencies. Producing a coherent beam using diode lasers is technically difficult. Superluminous diodes are easier to manufacture. These are devices which produce monochromatic, laser-like beams which are non-coherent. It should be noted that some diode 'lasers' used in physiotherapy produce relatively incoherent beams and should more correctly be described as 'superluminous diodes'. The lack of coherence in the beam of radiation produced would appear to be of no clinical significance. Laser Light Wavelengths The particular wavelength of radiation emitted by a laser is determined by the physical ELECTROMAGNETIC WAVES FOR THERAPY 298 design; in particular its chemical composition. Thus helium-neon lasers emit red The range of wavelengths light with a wavelength of 632.8 nm. Ruby lasers, which consist of a cylindrical rod of which can be produced by synthetic ruby (a gemstone made of aluminium oxide) emit red light with a wavelength laser action is quite large, of 694.3 nm. from the microwave region Gallium aluminium arsenide (GaAlAs) diodes emit radiation at a frequency of the spectrum to the X-ray determined by the ratio of gallium to aluminium. The particular wavelength can be region. between 650 nm (in the visible, red part of the spectrum) and 1300 nm (in the near Consideration of beam area infrared). and average intensity Two types of lasers are commonly used in physiotherapy: helium-neon lasers, which, indicates that torch-beam as noted above, produce red light of wavelength 632.8 nm and gallium aluminium therapy might be a cheaper arsenide diode lasers, operating at near-infrared wavelengths (normally between 810 and more effective treatment and 850 nm). than laser therapy. Penetration Depth The penetration depth of laser radiation is the same as ordinary electromagnetic radiation of the same frequency. The wave coherence and the monochromatic nature of the laser beam make no difference. Thus the penetration depth of visible light from a helium-neon laser is a mm or so and most of the wave energy is absorbed in the epidermis (figure 11.3). The infrared radiation produced by commercial GaAlAs diodes has greater penetration depth but most of the wave energy is absorbed in the epidermis and dermis. This perhaps explains why laser irradiation has been shown to be of value for treating ulcers and other skin conditions. What has not been shown, and is not likely to be shown, is that laser treatment is any better than shining a torch beam on the area. Similar considerations indicate that laser irradiation is not likely to be of value for treating deeper tissue injuries. The therapeutic benefit and relative cost effectiveness of laser therapy must thus be questioned.

ELECTROMAGNETIC WAVES FOR THERAPY 299 EXERCISES 1 (a) What are the similarities and differences between infrared, ultraviolet and microwave radiation? (b) State the wavelength range and frequency range of each kind of radiation. 2 Figure 11.1 shows a schematic diagram of a mercury vapour lamp. (a) Describe the mechanism whereby ultraviolet radiation is produced in the lamp. (b) Why must the power supply used for the lamp be current limiting? (c) Why must special glass be used for the lamp envelope? 3 Compare the output of UV, visible and infrared radiation of air and water cooled UV lamps and fluorescent tubes (figure 11.2). (a) Why do water-cooled lamps put out a negligible proportion of infrared radiation? (b) Why do fluorescent tubes put out a negligible amount of radiation at wavelengths less than 280 nm? 4 (a) Describe the process of production of infrared radiation by lamps and electric heaters. (b) What effect does the use of a reflector have on the directionality and wavelength of the radiation produced? 5 The filament of a light bulb is at a temperature of 3000 K and its wavelength of maximum emission is 960 nm. If the filament temperature was lowered to 1000 K by decreasing the current what would be the new wavelength of maximum emission? In what part of the electromagnetic spectrum is this wavelength? ELECTROMAGNETIC WAVES FOR THERAPY 300 6 (a) Use figure 11.3 to describe the variation with frequency of the penetration depth of near infrared radiation. (b) Describe the ways in which heat produced by near infrared radiation is transferred to subcutaneous tissue. Which would you expect to be the most efficient transfer mechanism? 7 Compare and contrast the principal effects of infrared and ultraviolet radiation on tissue. How are the differences related to the wavelength of the radiation? 8 (a) What is meant by the term erythema as related to dosage in ultraviolet therapy? (b) Briefly list the characteristics of a first, second, third and fourth-degree erythema reaction to ultraviolet radiation. 9 Figure 11.4 shows a schematic diagram of apparatus used for the production of microwaves. (a) Briefly describe the function of each subsection. (b) Why is a magnetron valve rather than conventional electronic circuitry used in microwave apparatus? (c) What is the relationship between the size of the antenna in figure 11.4 and the wavelength of the microwaves produced? (d) What determines the frequency of the microwave radiation produced by the apparatus? 10 Figure 11.5 shows the beam produced by a point source of radiation positioned at the focus of a parabolic reflector. Draw diagrams to show the effect on the beam shape of: (a) mounting the point source between the focus and the reflector surface (still on the central axis)

ELECTROMAGNETIC WAVES FOR THERAPY 301 (b) mounting the point source on the central axis but further from the reflector than the focus. 11 (a) Explain why parabolic reflectors are not used with microwave diathermy apparatus. (b) What are the most important factors determining the size and shape of the reflectors used with microwave diathermy apparatus? 12 (a) Give a brief explanation (in molecular terms) of why tissues with high dielectric constant and conductivity have low values of penetration depth for microwave radiation (b) Refer to the figures given in table 11.1 and comment on the relative values of penetration depth for microwaves in fat, muscle and bone. Which tissues would be expected to have similar values of penetration depth and why? 13 It has been said that a frequency of 2450 MHz represents a very poor choice for microwave radiation used in therapy because of the unpredictability of dosage. Explain. 14 Using data in table 11.1 determine the thickness of fat required to absorb 50% of the transmitted microwave energy at a frequency of: (a) 1000 MHz (b) 2000 MHz (c) 4000 MHz 15 For microwaves of frequency 2000 MHz (table 9.1) calculate the fraction of energy remaining after travelling through (a) 2 cm fat (b) 2 cm muscle (c) 2 cm bone. In which tissue is the energy absorbed most rapidly? ELECTROMAGNETIC WAVES FOR THERAPY 302 16 Refer to figure 11.6 and explain the origin of the peaks and troughs (maxima and minima) in the heating pattern. 17 Refer to figure 11.6. Draw the corresponding graph of relative rate of heating which would be expected if the reflection coefficient of the fat/muscle interface was: (a) 0.0 (b) 1.0 18 Compare figures 11.7, 11.8 and 11.9 and explain how the microwave wavelength is related to the differences in heat production in each case. 19 H. P. Schwann has shown that from the point of view of reliable dose prediction microwaves with frequencies either below 1000 MHz or above 3000 MHz are preferred. Compare figures 11.6 and 11.9 and say whether high frequencies or low frequencies would be preferred from the point of view of the pattern of heating produced. 20 Compare figures 10.6 and 10.7 with figures 11.10 to 11.13 and explain the differences in heat production in terms of: (a) the wavelength associated with the modality (b) the penetration depth in each tissue. 21 Figure 11.13 shows a beam of microwaves striking an arm or leg. Briefly explain why the beam converges in fatty tissue and muscle. 22 The diagram below shows a uniform microwave beam striking a tissue surface. The fat and muscle layers have only slight curvature. The bone surface is markedly curved. Fatty tissue and bone have low values of dielectric constant and conductivity. The corresponding values for muscle are high.

ELECTROMAGNETIC WAVES FOR THERAPY 303 Complete the diagram to show the refraction effects. Briefly explain what happens to waves at each boundary and why.

DOSAGE AND SAFETY CONSIDERATIONS 304 12 Dosage and Safety Considerations Electrical safety including hazards associated with Having examined the effects of electric current, electric fields, ultrasound and electrical stimulation and the electromagnetic waves on tissue we conclude by looking at two factors of great use of mains powered importance in the clinical situation. These are the assessment of dose and some apparatus are treated specific safety considerations. The two are firmly linked. Under the heading 'safety separately in chapter 13. considerations' we include hazards associated with diathermy and exposure to There have been reports of electromagnetic radiation. Under the heading 'dosage' we concern ourselves with deep tissue injury requiring how to establish safe but therapeutically effective dosage both with diathermic surgical intervention following modalities and those which are more superficial (infrared, ultraviolet and visible incorrect application of 1 MHz radiation, including laser). ultrasound. When considering electrical stimulation of nerve and muscle for therapy a satisfactory statement of 'dosage' includes specification of the waveform used, duration of treatment, position and size of the electrodes used and a description of the response obtained. Since a response is produced immediately, the therapist can adjust the machine controls to obtain precisely the required effect. In this way there is the potential for optimum conditions to be achieved and for the patient to obtain maximum therapeutic benefit at each treatment. With other forms of treatment, using electric or magnetic fields, electromagnetic radiation or ultrasound, the therapist does not have such reliable feedback. For these modalities there is too long a delay between the commencement of treatment and the effects produced. For example, in the case of diathermy the subjective response of the patient - a feeling of warmth - gives only a poor guide to dosage and effect. With efficient diathermic modalities it is possible that by the time a sensation of heat is felt, the deeper tissue temperatures are high enough to produce irreversible tissue damage. This is because temperature receptors are located superficially, where they are needed to detect the kind of damaging temperature elevations which are experienced normally. For this reason particular consideration needs to be given to the question of dosage as applied to diathermy. It is convenient to consider dosage in two parts: the first as applied to infrared, visible DOSAGE AND SAFETY CONSIDERATIONS 305 and ultraviolet radiation, the second as applied to the diathermic modalities. In this book, the more familiar Before discussing how we might reliably and reproducibly estimate dosage we need term 'intensity' is used rather a clear definition of three important quantities. These are: than the more technically * The dose: in other words the total energy supplied to the patient - normally correct 'irradiance'. A dose of 10 J administered expressed in joules (J). in a second or so would * The dose rate: the rate at which energy is supplied. This has units of joules per evoke a marked physiological response. The same dose second. One joule per second (J.s-1) is one watt (W). applied over a 10 minute * The irradiance: the dose rate per unit area of body surface. Normally in units of period would have little effect. joules per square centimetre per second (J.cm-2.s-1) i.e. watts per square centimetre (W.cm-2). When talking about radiation (sound or electromagnetic waves) this quantity is what we call the intensity. The general requirement in specifying dosage is that all three of these quantities be stated, either directly or indirectly. Each gives important facts about the treatment. For example, consider the heating effect of ultrasound. The total amount of heat developed is determined solely by the dose. The temperature increase, however, depends on the dose rate, the time of treatment and the area treated: that is, on all three factors listed above. Since it is the temperature rise rather than heat production as such which determines the physiological response, a knowledge of dose alone is insufficient. DOSAGE: INFRARED AND ULTRAVIOLET RADIATION When we consider the question of dosage with infrared or ultraviolet radiation two problems arise: * The therapeutic effects depend not just on the energy output of the lamp but also on the frequency of the radiation (chapter 11). This is most noticeable with ultraviolet radiation where only narrow ranges of frequency produce the desired reactions.

DOSAGE AND SAFETY CONSIDERATIONS 306 * A given dosage from the same lamp will produce a greater response in some With infrared exposure, the patients than others. Again greater variation is found with ultraviolet radiation. intensity used is normally that which produces a mild, For infrared treatment, specifying the particular type of lamp, the reflector used, the comfortable warmth after 5 patient-to-lamp distance and the time of exposure is an adequate statement of minutes. If this does not dosage. Dose, dose rate and intensity are thus specified indirectly. Generally the come about, the lamp-to- intensity used is that which produces a mild, comfortable warmth after 5 minutes. If patient distance can be this does not come about, the lamp-to-patient distance can be adjusted during the adjusted during treatment. treatment. As a UV lamp ages the With ultraviolet therapy the maximum effects are not produced until long after ultraviolet output diminishes: treatment is complete. For this reason no adjustment of the dose can be made for this reason the average during treatment. A close estimate of the dose requirement is needed beforehand. dose figure must be re- How can this be achieved? A measurement of the total power output of the lamps is determined periodically. insufficient. Even if the output was measured at different frequencies this would take no account of variation in sensitivity of individual patients. A more useful and direct method is to test lamps in terms of the amount of radiation needed to produce a specific biological response in each particular patient. Ultraviolet Therapy and Erythema Dosage For lamps which produce an appreciable output of UV-B radiation (see chapter 11) tests are carried out to determine the amount of radiation needed to produce a first degree erythema. This will vary from patient to patient and even between different skin areas on a particular patient, but an average figure for the lamp will provide a useful starting point in determining the test dose requirements of an individual. Once known, the dose requirements of a particular patient can be specified as multiples of the first degree erythema dose (erythema dosages were described in chapter 11 previously). The dose required to produce a first-degree erythema is determined by exposing small parts of an area similar to that to be treated (usually a few square centimetres) for varying lengths of time. The patient-to-lamp distance is kept constant. Inspection of the exposed areas after 24 hours enables the dosage to be determined. DOSAGE AND SAFETY CONSIDERATIONS 307 Specification of the dosage in this case requires a statement of the particular lamp used, the exposure time and the patient-to-lamp distance. Once the time and distance required for a particular lamp are known the dosage needed to produce any other degree of erythema can be established from table 12.1. E1 refers to a first-degree erythema, E2 to one of second- Erythema reaction E1 E2 E3 E4 degree and so on. The values quoted are experimentally Conversion factor 1 2.5 5 10 determined and represent a consensus of agreement amongst physiotherapists. To obtain the exposure time Table 12.1 required for a second, third or fourth degree erythema the Conversion factors for different degrees of time for first-degree erythema production is multiplied by the appropriate conversion factor. For example if a first-degree erythema. erythema is produced after 6 seconds exposure, table 12.1 indicates that 5 x 6 s = 30 s exposure is required to produce a third-degree erythema. In table 12.1 the lamp-to-patient distance is assumed to be the same. For different distances an inverse-square law is applied (see below) to correct the conversion factor. After a first exposure to ultraviolet radiation, there is thickening of the epidermis. Consequently an increase in exposure time is required to produce the same effect on subsequent occasions. Table 12.2 lists the increase required. Again the conversion factors quoted are experimental results, not theoretical values (which would be extremely difficult to calculate). Table 12.2 Erythema reaction E1 E2 E3 E4 Conversion factor 1.25 1.5 1.75 * Conversion factors for repeated exposure to ultraviolet radiation. * not normally progressed

DOSAGE AND SAFETY CONSIDERATIONS 308 Thus a second exposure to ultraviolet radiation would require a 50% increase in Following marked exposure time in order to reproduce a second degree erythema. The figures shown desquamation it is common are only approximate and may need modification in many cases. practice to reduce the exposure time to its original PUVA Therapy and Dosage (first exposure) value. Patients who always burn in As indicated in chapter 11 previously, UV-A radiation alone does not produce erythema the sun are progressed by except at extremely high dosages. The common use of UV-A, however, is in 0.5 J.cm-2. Those who never combination with a photosensitizing drug, 8-methoxy-psoralen, for the treatment of or rarely burn are progressed psoriasis. by 1 J.cm-2. In psoralen-UVA (PUVA) therapy the drug is administered two hours before UV-A exposure. The drug renders the patient UV-A sensitive and an erythema response is readily evoked. The dosage required to produce a minimal erythema 72 hours after exposure is determined. This is called the minimal phototoxicity dosage (MPD). It is found by exposing test areas of the patient's skin to predetermined dosages of UV-A (for example 0.5, 1, 2, 3 and 4 J.cm-2) and inspecting the test areas 72 hours later. Once the MPD has been determined, treatment can be given with the dosage specified in J.cm-2. The present practice is to use the MPD for the first treatment and to progress the dosage by 0.5 J.cm-2 or 1 J.cm-2 (depending on skin type) on each subsequent treatment. UV-A fluorescent tubes display a significant drop in output intensity, particularly over the first 200 hours of use. For this reason it is essential that the output intensity of the UV-A source be regularly measured. Special meters, calibrated in W.cm-2, are available for this purpose. If the output intensity of the source is known the dosage in J.cm-2 can be calculated using the relationship: Dosage (in J.cm-2) = Intensity (in W.cm-2) x time (in s) DOSAGE AND SAFETY CONSIDERATIONS 309 THE INVERSE SQUARE LAW AND DOSAGE Turning now to the more general aspects of dosage we consider the effect of the distance from the source of radiation to the patient. These considerations apply to both infrared, visible and ultraviolet radiation - and also to microwaves to a more limited extent. Consider a screen with a small square aperture, behind which is placed a point-source of radiation. The arrangement is shown in figure 12.1. Figure 12.1 The law of inverse squares. Radiation produced by the point source spreads uniformly in all directions. The aperture in the screen allows a beam of square cross-section through. The distance from source to screen is s and the sides of the screen aperture have length x. The beam area at the screen is thus x2. At a distance 2s from the source (a further distance s from the screen) the beam area is (2x)2 = 4x2. At a distance 3s from the source the area is (3x)2 = 9x2. In other words, as we progress 1,2,3,4 units of distance from the source the beam area increases to 1,4,9,16 times the original area: it increases in proportion to the square of the distance.

DOSAGE AND SAFETY CONSIDERATIONS 310 What happens to the wave energy which passes through the square aperture? For If the energy of the beam is infrared, ultraviolet and microwave energy there is very little absorption in air over a constant but is spread over distance of a few metres. In other words their penetration depths in air are large. This a larger and larger area with means that the wave energy in the rectangular beam in figure 12.1 is virtually constant. distance from the source then the intensity, which is the If the beam of radiation has an energy E at the aperture then the intensity - the energy energy per unit area, must per unit area - is E/x2. At a distance 2s from the source this energy is spread over an decrease. area 4x2, so the intensity is E/4x2 or one quarter of its value at the screen. At a For example if the intensity is distance 3s the intensity is one-ninth of the value at the screen. 0.1 W.cm-2 at a distance of 1 m from the source, then at a An inevitable conclusion is the law of inverse squares which states that the intensity of distance of 2 m the intensity radiation from a point source varies inversely with the square of the distance from the will be 0.1/4 = 0.025 W.cm. source. Mathematically this is written: I = Io .... (12.1) d2 where I in the intensity at a distance d from the source and Io is the intensity at unit distance. How is the law of inverse squares applied to dosage? Strictly speaking, the inverse square law only holds for point sources of radiation. Sources of infrared and ultraviolet are extended sources, usually mounted in reflectors. The effect of the reflector is to reduce the divergence of the beam, but for the lamps used in physiotherapy departments the effect is not too great and the law provides a rough, rule-of-thumb, but satisfactory basis for calculations. An Example: Consider treatment with a high pressure mercury vapour lamp (a hot quartz lamp; chapter 11). Suppose we know that the minimal erythema dose with a particular lamp is 18 seconds at a distance of 1 metre and we wish to use the lamp at a distance of DOSAGE AND SAFETY CONSIDERATIONS 311 1.5 metres. What exposure time is required at this new distance? The production of the erythema reaction depends on the intensity of radiation and the exposure time. At a distance of 1.5 metres the intensity is reduced by a factor of (1.5)2: thus the exposure time needs to be increased by a factor of (1.5)2 to produce the same effect. The new exposure time is then 18 x (1.5)2 = 41 seconds. This kind of calculation shows how the law of inverse squares, which nominally Using equation 12.1 and the relates intensity to distance is adapted to relate exposure time to distance. fact that dose per unit area is the product of intensity and t = to.d2 .... (12.2) exposure time it is a simple do2 matter to derive equation 12.2 which relates exposure time Here to and do refer to the original exposure time and the original distance at one distance to exposure respectively. t is the new exposure time at the new distance d. time of any other distance. The Effect of the Angle of Incidence What happens if the beam of radiation does not strike the surface of the patient's skin at right-angles? This happens in the circumstances illustrated in figure 12.2. In each case some or all of the radiation has an angle of incidence, θ, which is not zero. When this happens the amount of reflection is increased and the beam is spread over a larger area. Figure 12.2 (a) effect of beam divergence on the angle of incidence, θ. (b) effect of both beam divergence and surface curvature. (continued overleaf)

DOSAGE AND SAFETY CONSIDERATIONS 312 Figure 12.2 (c) effect of beam divergence and angulation of the reflector on the angle of incidence, θ. Reflection is minimal at an angle of incidence of 0o. As the angle is increased, the amount of reflection increases. The relationship between reflectance and angle is a complex one (at the critical angle, reflection is 100%). In each of figures (a), (b) and (c), reflection is increased because the angle of incidence is not always zero. The beam (or part of the beam) is spread over a greater area in figures 12,2(b) and (c). This results in a decrease in the intensity of radiation at the surface. In figure 12.2(b) the area illuminated by the beam is larger because the surface is curved. In figure 12.2(c) the beam is spread over a wider area because of the angulation of the reflector. The effect of angulation is further illustrated in figure 12.3 where, for simplicity, we consider a tiny portion of the beam with width x and square cross-section. Figure 12.3 The effect of angle of incidence on intensity. DOSAGE AND SAFETY CONSIDERATIONS 313 When the beam is incident at right angles the energy is spread over an area x2 so the iifnnottreeannnssiiattyyngiifsleEEco/oxf s2inθ.c/xiWd2e:itnthhcaethtoeisf ,a1nt5hgoel,ecinootsfeθnins=citiyd0e.i9sn7creeadnθudctthehede area irradiated is x2/cosθ and the by a factor of cosθ. For example, intensity is reduced by 3%. Table 12.3 shows the relative intensity (as a fraction of the intensity for θ = 0) for different values of θ. This effect is quite noticeable when using a torch to see one's way on a dark night. angle of incidence relative Pointing the torch downwards gives a circular beam. Shining the torch ahead gives a (degrees) intensity larger area of illumination with an egg shape. Here the area depends on both the distance (through the inverse square law) and the angle of incidence. 0 1.00 15 0.97 In the application of infrared and ultraviolet radiation, the therapist should be aware of 30 0.87 this effect. It is normal practice to keep the beam as near to perpendicular to the 45 0.71 treated surface as possible: thus the situation shown in figure 12.2(c) should be 60 0.50 avoided. 75 0.26 90 0.00 Even if a perpendicular arrangement is used, parts of the treated area near the periphery may receive a lower dosage (figure 12.2 (a and b)). If necessary the lamp should be moved to give additional exposure to these areas. DOSAGE AND DIATHERMIC MODALITIES Table 12.3 Relative intensity for different In the previous chapters we examined the relative rate of heating of different tissues in combination. We saw that the resulting temperature increase depends on a number values of θ, the angle of of factors including the dose rate and time of exposure. In any treatment there will be incidence. threshold values of dose and dose rate which cannot be exceeded without risk of harm to the patient. There will also be minimum values of useful dosage. Below this, heat development See the chapter by Schwan in the tissues will be within the range which the body's temperature regulating in: Licht, S H, Therapeutic mechanism can cope with and there will be an insignificant local rise in temperature. Heat and Cold, (2nd Edition), The maximum safe energy dose exceeds the minimum required to produce an Williams & Wilkins (1968). appreciable effect by less than a factor of ten: thus a knowledge of dosage is of great

DOSAGE AND SAFETY CONSIDERATIONS 314 importance. The meter on the front panel In order to specify dose and dose rate we need to know first the energy produced by of most shortwave diathermy the apparatus and second, the fraction of that energy which is absorbed by the body. machines only indicates a Unfortunately both of these quantities are not always known, as we will see. relative value of power - it gives no indication of the Shortwave Diathermy actual dosage. This is the modality which has been in use for the greatest length of time. It is also the one for which the dosage is least predictable. In the case of the capacitor field technique the energy produced by the apparatus varies with the position and size of the electrodes and the amount and type of tissue in the field. It is possible to simulate the conditions of therapy by placing a 'dummy load' between the electrodes. The load must have the right electrical properties and be correctly positioned to simulate the conditions of therapy. In this way the energy produced can be measured, though not the energy absorbed by the patient. Scott (see Licht (1968)) describes how a series of subjects were tested with apparatus adjusted for a predetermined rate of energy production. The extreme variation in the responses obtained indicates that a knowledge of energy production alone is of little value in establishing dosage. A further complication is that the field spreads as it passes through the body. This results in the area treated being much larger than the electrodes used, and varying with depth. This makes it impossible to predict accurately the heat developed in a particular part of the tissue. With the inductive coil technique of application the situation is just as complex - due to the difficulty in establishing the pattern of induced electric field intensity with the geometries used. For the present, the most reliable estimate of correct dosage is obtained by adjusting the intensity until the patient feels a mild, comfortable warmth in the treated area. This is a relatively safe method of assessing dose as greatest temperature elevation is DOSAGE AND SAFETY CONSIDERATIONS 315 produced in tissues where pain and temperature receptors are abundant. Since Since even minute amounts temperature elevation is less in the more deeply located structures there is little risk of of air can interrupt the flow of overheating them without first producing pain and damage to superficial tissues. The energy, the space between need to rely on physical sensations indicates why shortwave diathermy (or indeed any transducer and skin is filled diathermic modality) is contra-indicated for areas where sensory impairment is by a coupling medium. suspected. Up to 80% of the microwave energy may be reflected at a Ultrasound frequency of 2450 MHz (see chapter 11). In the case of ultrasound therapy, virtually all the energy produced by the generator is transferred to the patient, provided that intimate contact is maintained between the transducer and body surface (chapter 10). Generator-produced power can be read directly from the meter on the front of the apparatus: thus the dose is obtained simply by multiplying the power (in watts: 1 W = 1 J.s-1) by the treatment time (seconds). The irradiance or average intensity (in W.cm- 2) is not so reliably known when the usual massage technique of application is used and the treatment head is moved in small circles over the area to be treated. The average intensity is calculated by dividing the total power by the area treated: it will only be a reliable figure if the therapist is able to expose all parts of the treated area for the same length of time. Ultrasound ranks highest of the diathermic modalities in terms of reliability and reproducibility of dosage. Even so it is difficult to assess the dosage applied to a particular structure or tissue layer within the treated part. This is partly because of the difficulty in estimating the thickness of different tissue layers and, more significantly, because the ultrasound beam intensity (figure 10.4) is nonuniform. Microwaves The power produced by a microwave source can be quite accurately measured. It can usually be read directly from a meter on the front of the apparatus. Unfortunately, at a frequency of 2450 MHz, the proportion of energy actually absorbed depends on a complex way on the thickness of skin and subcutaneous fatty tissue (chapter 11).

DOSAGE AND SAFETY CONSIDERATIONS 316 In view of the practical difficulty in estimating tissue thickness, microwave therapy at The difficulty in establishing 2450 MHz does not permit accurate dosage measurement. microwave dosage is As with shortwave diathermy, the dosage, and consequently the heat development discussed in Lehmann, J F, can only be estimated roughly. The physiotherapist must be guided by a knowledge Therapeutic Heat and Cold of the pattern of heat production, a knowledge of 'normal' dose rates and the (3rd Edition) Williams and subjective reports of the patient. A more quantitative assessment of dose can, at Wilkins (1982): chapter 6). least, be made for lower frequency microwaves, though the proximity of the patient to The scope of this book the source of radiation (less than one wavelength in normal applications) makes the precludes consideration of distribution of energy difficult to calculate. the indications and contra- indications for each modality. IMPLANTS AND CAVITIES Rather we restrict ourselves to the effect of implants and In this and the following sections we consider some of the safety aspects of the cavities, and to some specific treatments we have discussed. safety hazards. When using any of the diathermic modalities in the region of a metallic implant, air or fluid-filled cavity, particular consideration needs to be given to the likely effects of the cavity or implant on the pattern of heat production in nearby tissue. The effects are different for each modality so we will consider each in turn. Shortwave Diathermy We considered in chapters 6 and 7 the way in which different tissues (fat, muscle and bone) modify the field pattern and determine the magnitude of real and displacement current. This in turn determines the pattern of heat production in tissue combinations. The two quantities determining these effects are, as we saw, the dielectric constant and conductivity of the tissues. To determine the effect of a cavity or implant we need to know its depth, shape and size and, most importantly, its electrical properties. * Metals have extremely high conductivities - several thousand times higher than muscle. DOSAGE AND SAFETY CONSIDERATIONS 317 * Air, for all practical purposes, can be considered a perfect insulator. The conductivity can be taken as zero and the dielectric constant as 1. * Body Fluids can be considered equivalent to muscle and other tissues of high water content. The differences in electrical properties are negligible as far as shortwave diathermy is concerned. Figure 12.4 shows the effect of a cylindrical object of high dielectric constant or conductivity on a uniform electric field. This illustrates the focussing effect on the field lines of a metallic implant in tissue. The effect of the orientation of the object on the field is apparent. A greater focussing effect is produced when the long axis of the object is aligned with the field (figure 12.4a). When the length along the field direction is short, as in figure 12.4(b) and (c), the field distortion is less. Figure 12.4 A material of high dielectric constant or conductivity in a uniform electric field. The effect of different orientations.

DOSAGE AND SAFETY CONSIDERATIONS 318 The heat production within a metal implant is very low because the field intensity If the metal was a perfect within the metal is very low - due to the rapid, free movement of charge which results conductor the resistance in accumulation of charge on the surface and termination of field lines. would be zero, the field The field intensity is greatest near the surfaces of the metal perpendicular to the field intensity zero and the heat lines. It is here that maximum heat is produced. The risk, then, is of overheating production zero. tissue adjacent to a metallic implant. For this reason shortwave diathermy is often See the chapter by Scott contraindicated when a metallic implant is present. The effect of metallic implants is in: Licht, S H, Therapeutic discussed more fully by B. O. Scott in Licht, (1968). Heat and Cold, (2nd Edition), Figure 12.4 is also applicable to fluid-filled cavities in fatty tissue or bone. This is Williams & Wilkins (1968). because the dielectric constant and conductivity of body fluids are considerably higher than those of bone or fat. We saw an example of this with blood vessels in fatty tissue in chapter 7. Field lines will be focussed resulting in maximum heat production near the cavity. The field intensity within the cavity will be reduced by charge accumulation at the interface, but this may not be sufficient to prevent overheating. Fluid-filled cavities in muscle or other tissues of high water content will not affect the electric field pattern appreciably. The temperature rise in the cavity will however be greater than in muscle because heat is not transferred efficiently to adjacent tissues or the bloodstream. The effect of an air-filled hollow in tissue was discussed in chapter 7 (see figure 7.14). The field lines bend around the hollow. This results in an increased intensity in the tissue adjacent to the sides of the hollow which are parallel to the field. The effect proves useful when it is desired to selectively heat the surfaces of hollows, such as the sinuses. Ultrasound We saw in chapter 9 that reflection of ultrasound occurs when there is a mismatch of acoustic impedance between two adjacent tissue layers. The impedances of muscle and fatty tissue are similar but that of bone is much higher. There is thus an appreciable reflection of ultrasound at the muscle/bone interface. DOSAGE AND SAFETY CONSIDERATIONS 319 In order to determine the effects of implants or cavities we need to know the acoustic A selective build-up of heat impedances of metals, air and body fluids. could result, however, if the * Metals have an acoustic impedance about thirty times higher than fat or muscle heat cannot be transferred to adjacent tissues or the so there will be significant reflection at a tissue/metal interface. Using the figures bloodstream. in table 10.1 and equation 9.5 we find that the reflection coefficient for a fat/metal Little heat will be produced or muscle/metal interface is about 0.94. Thus about (0.94)2 x 100 or 90% of the in a metal as little energy is ultrasound energy will be reflected. transmitted at the tissue/ * Air has an acoustic impedance which is only a tiny fraction of that of tissue so metal interface and virtually 100% of the energy incident upon a tissue/air interface will be reflected. ultrasound is not absorbed * Body fluids have an acoustic impedance closer to that of water, muscle and fatty very rapidly in metals (the tissue. Fluid-filled cavities will not pose any problems as regards reflection of penetration depth is high). the ultrasound beam. Whenever reflection occurs there will be an increase in the ultrasound intensity adjacent to the reflecting surface and greater heat production in this region (see chapter 10). For a metallic implant in tissue, reflection will be significant and almost all the heat will be produced between the implant and the tissue surface. The excess heating of the tissue layer above the implant may or may not be advantageous depending on the actual location of the tissue to be heated. The additional factor of the effect of shape of the implant on the ultrasound field pattern is discussed by Lehmann in Licht, (1968). The presence of an air-filled cavity in tissue will have a substantial effect on the pattern of heat production. Almost total reflection will occur at the interface and almost all of the heat will be produced in the intervening tissues. This has particular implications for treatment of the chest wall or throat. As we saw, the presence of fluid-filled cavities has little effect on the pattern of heat production with ultrasound. The only factors to be assessed are the likelihood of

DOSAGE AND SAFETY CONSIDERATIONS 320 selective heating within the cavity due to poor heat dissipation, and whether this is The electrical properties of desirable. metals, air and body fluids were considered earlier in Microwaves this chapter. The phenomenon of rapid As we saw in chapter 11, the reflection of microwaves and the rate of absorption are absorption (and consequently determined by the electrical properties (dielectric constant and conductivity) of tissues. great heat production) can be Since metals have a much higher conductivity than any biological tissue, reflection at a demonstrated quite tissue/metallic-implant boundary will be pronounced. The high conductivity of metals convincingly by igniting a also results in rapid absorption of microwaves - penetration depths are extremely piece of steel wool in a small. The result is that pronounced reflection occurs at a tissue/metal boundary and microwave oven. the transmitted wave is absorbed over a very short distance. Microwaves penetrating metallic implants will be absorbed in a fraction of a millimetre with significant heat production. However, metals are good conductors of heat and the energy will be rapidly conducted throughout the metal and spread into the adjacent tissues. Reflection of microwaves at a tissue/metal interface will result in the production of standing waves. The energy reflected and the resulting standing wave pattern will produce a concentration of energy in the tissues adjacent to the metal. There is also the risk of focussing the waves with a curved metal surface (chapter 11) which can result in 'hot spots' being produced in the patient's tissues. The rather poor penetration depth of 2450 MHz microwaves suggests, however, that metallic implants located well below the surface of the body will have little effect on heat production. The effect of an air-filled cavity is similar to that of a metal implant: reflection occurs at the boundary and a standing wave pattern is produced. The implications of this were discussed above. Fluid-filled cavities within muscle and other tissues of high water content will not affect the pattern of heat production, but may undergo a selective rise in temperature if heat is not conducted away efficiently. DOSAGE AND SAFETY CONSIDERATIONS 321 SOME SPECIFIC HAZARDS Sunglasses or goggles Electromagnetic Waves and Safety made of plastic or glass and painted with a filter (often It is known from various studies that certain body tissues are more susceptible than coloured blue) can effectively others to damage from electromagnetic waves. The eyes and reproductive organs block ultraviolet transmission. are most frequently mentioned in this regard. As with all forms of therapy The eyes, not having a covering of skin, are susceptible to damage by ultraviolet the risks associated with UV radiation. In ultraviolet therapy the eyes should always be protected from direct exposure must be calculated irradiation by use of glasses which reduce the visible light intensity and absorb most and weighed against the of the ultraviolet radiation. Sunglasses perform this role quite adequately. The only therapeutic benefit in risk is of UV exposure through the areas not covered by the sunglasses. For this deciding a course of reason, protective goggles, which cover the eyes completely, are preferred. treatment. Exposure of the eyes to a sufficiently high dose of ultraviolet radiation produces photopthalmia - acute inflammatory reactions of the superficial parts of the eye. This is commonly known as snow-blindness (snow reflects a large part of the UV radiation in sunlight). It can be produced by sunlight, electric welding arcs or any other source of ultraviolet radiation. The reactions cause acute pain, beginning after a latency period of a few hours and reaching a maximum in about 48 hours. The effects subside over a period of days. Only very large doses produce permanent damage. It should also be borne in mind when considering prolonged or repeated courses of treatment with ultraviolet radiation that such radiation is carcinogenic. Certain forms of cancer are known to occur more frequently in people exposed to higher levels of ultraviolet radiation. Microwave therapy is contraindicated for treatment of eye conditions. Generally, extreme caution should be exercised when treating nearby structures. The susceptibility of the eyes to damage by microwave radiation is due to two factors (a) reflection and refraction producing 'hot-spots' within the eye cavities and (b) a relatively poor blood supply which limits the eye's ability to conduct heat away.

DOSAGE AND SAFETY CONSIDERATIONS 322 It has been known for some time that sufficiently high intensities of microwave The hazards with microwave radiation can bring about the formation of cataracts in the eye. Experimental work exposure are discussed in using laboratory animals indicates a threshold intensity level for cataract formation a Lehmann, J F, Therapeutic little in excess of 100 mW.cm-2 for prolonged exposure. Heat and Cold (3rd Edition) It is common practice to avoid exposing the reproductive organs to microwave Williams and Wilkins (1982): radiation. The testes are particularly susceptible to stray radiation in therapy. chapter 7). For a detailed description of the hazards of microwave exposure and references to Australian standard AS2772.1 relevant experimental work see S. M. Michaelson in Lehmann (1982). For a more (1998) stipulates the general description of the hazards of both radio-frequency and microwave radiation maximum exposure levels see publications by the World Health Organization (search their website at allowed for radio-frequency www.who.int). These documents discuss the known biological effects of such and microwave frequency radiation and summarize exposure safety limits proposed or in use in different radiation. countries. All practicing physiotherapists should be familiar with the relevant safety standards and their implementation. It should be noted, however, that the exposure limits stipulated apply to the general public but not to the patient receiving treatment, nor the therapist. For example, the maximum exposure level for a therapist using 27 MHz shortwave diathermy apparatus is 1.2 mW.cm-2. For non-occupationally exposed individuals such as secretarial staff and members of the general public the stipulated levels are one fifth of these values. For patient exposure, there is no prescribed limit. It is assumed that the therapist has weighed the therapeutic benefits against the potential hazards and on this basis has prescribed treatment. Ultrasound And Boundary Effects A fundamental characteristic which distinguishes ultrasound from other diathermic modalities is that the ultrasound wave is a mechanical disturbance in a material medium. Particles within the medium oscillate back and forth, undergoing large changes in velocity and acceleration. This gives rise to two phenomena which can result in selective heating at or near a boundary. The processes are called velocity gradient heat production and shear wave production. DOSAGE AND SAFETY CONSIDERATIONS 323 We consider first velocity gradient heat production. Suppose an ultrasound beam is This means that particles on directed so as to strike a tissue boundary at a grazing angle. In other words the one side of the boundary are waves travel almost parallel to the interface. At this angle of incidence little or no wave oscillating back and forth with energy will be transmitted. the wave motion while There will be a thin layer, just each side of the boundary where the velocity changes particles on the other side of from maximum to zero. Thus there is a velocity gradient in this narrow region. If the the boundary are stationary. boundary region is very narrow a very high velocity gradient exists and the adjacent In practical terms this means region are subject to greater stresses than those outside. The higher oscillatory that shear wave production is stresses give rise to greater heat production than occurs in the medium in which the relatively unimportant at soft waves are travelling. Hence boundary layer heat production can be significantly tissue interfaces but is greater than heat production due to normal wave energy absorption in a medium. important at the muscle/bone The thickness of the boundary layer determines the velocity gradient and this in turn interface. depends on the rigidity of each medium. For tissues of similar stiffness, such as muscle and fatty tissue the boundary layer is wide, the velocity gradient is small and boundary layer heat production is minimal. For tissues of quite different stiffness, such as muscle and bone, the velocity gradient is high and the rate of heat production at the interface is much higher than in the bulk of the tissues. Shear waves can be produced when an ultrasound beam strikes a boundary. They are not produced when the wave strikes the boundary at a right angle (zero angle of incidence), nor is production appreciable at grazing angles. Maximum production occurs near the middle of the range. While normal sound waves are a longitudinal wave motion, shear waves are transverse. In other words the particle displacement is at right angles to the direction of propagation. A further point which should be noted is that shear waves can only exist in solids or very viscous liquids and they are absorbed more rapidly than transverse waves. Shear waves are produced when the wave velocity is different in two adjacent tissues. The wave frequencies must be identical so the difference in wave velocity results in a

DOSAGE AND SAFETY CONSIDERATIONS 324 different wavelength in each tissue (by equation 9.1, v = f.λ). Dunn and Frizzel (see This means that regions of compression and rarefaction are separated by different Lehmann, 1982, chapter 9) distances in the adjoining tissues. So at the boundary, a region of compression on calculate that for 1 MHz one side will periodically be aligned with a region of rarefaction on the other. The ultrasound, up to 80% of the resulting pressure differential will cause particles near the boundary to oscillate in a energy transmitted at the direction transverse to the direction of the reflected and transmitted waves. The high muscle/bone interface may stresses produced at the interface result in greater heating than in the bulk of each go into production of shear tissue. waves. When an ultrasound beam in muscle strikes the muscle/bone interface the amount of A pressure of 20 N.cm-2 is 20 energy taken by the shear wave can be large. As the rate of absorption of shear wave x 104 N.m-2 or 20 kPa). This energy is much higher than that of longitudinal waves, heating of the surface region of is about 1/5 of atmospheric the bone is accentuated. pressure. In chapter 10 we saw that in the fat/muscle/bone system, 1 MHz ultrasound produces the greatest rate of heating in the first few mm of bone. The resulting temperature rise in the periosteum places a limit on the rate at which energy can be supplied to the patient. This happens when ultrasound is incident on each tissue boundary at a right angle and shear wave production and velocity gradient effects are negligible. For ultrasound incident upon the bone at other than a right angle, the heating rate of tissue adjacent to the bone will be even greater so the rate at which energy can be supplied to the patient will be further limited. The implication is that in therapy, the risk of producing periosteal pain and tissue damage is enhanced if the ultrasound beam does not strike the bone surface at a right angle. The therapist should position the treatment head as close to parallel to the surface of the underlying bone as possible so as to minimize velocity gradient and shear wave effects. Ultrasound and Cavitation We discussed in chapter 10, the mechanical stresses produced when an ultrasound wave travels through tissue. With 1 MHz ultrasound at an intensity of 2 W.cm-2, regions separated by 0.75 mm differ in pressure by about 20 N.cm-2. The large pressure gradient can result in gaseous cavitation: the formation of tiny bubbles from DOSAGE AND SAFETY CONSIDERATIONS 325 gas dissolved in the tissue fluid in a region of rarefaction (low pressure). The formation of a gas An upper limit to the therapeutic dose rate of ultrasound is set by the threshold bubble can cause damage, intensity for cavitation. Treatment at intensities above this threshold could only tearing apart the tissue. Also, produce the therapeutically undesirable outcome of mechanical tissue damage. In the bubble may collapse practice this threshold is unlikely to be achieved clinically. An intensity of 2 W.cm-2 during the subsequent would produce sufficient heating for temperature elevation to be the limiting factor. compression phase, creating Thus although cavitation is a risk and a therapeutic hazard, it is not likely to be of a minute but intense shock practical significance. wave in the immediate area. Cavitation effects are described more fully by Coakley (Physiotherapy. 64, 166-169, 1978). Treatment of the eyes with ultrasound is generally avoided because of the risk of damage in any tissue which has a poor or restricted blood supply. Cardiac Pacemakers Cardiac pacemakers present a special hazard as far as diathermy is concerned. Two kinds of pacemaker are used: the fixed rate unit which provides a constant frequency train of stimuli to the heart and the more popular noncompetitive units which provide a stimulus frequency based on feedback signals from the heart. Noncompetitive units are more satisfactory medically as the heart rate is adjusted by the oxygen demand of the patient. There are two risks in the application of diathermy: * the risk of selective heating of the unit and tissues in contact with the unit and its wires. Each diathermic modality presents this hazard when used close to the unit. * more importantly, the risk of interfering with pacemaker action. Microwave and shortwave diathermy present the greatest hazard in this regard. The fixed

DOSAGE AND SAFETY CONSIDERATIONS 326 frequency pacemaker is less susceptible as it does not require any feedback See J. F. Lehman in Krusen's signal. Noncompetitive units can change their frequency or cease to function Handbook of Physical completely as a result of currents induced by microwave radiation or the Medicine and Rehabilitation shortwave field. (3rd Edition), Williams and The limited amount of research in this area to date indicates that shortwave and Wilkins (1982) and J. Heath microwave radiation are contraindicated when a pacemaker is present. Some units Modern Medicine (Abstract) are found to cease functioning when brought within a few metres of microwave Vol. 43, June 15, 1975 apparatus. Ultrasound therapy seems safer in this regard but extreme care should be exercised, when using this modality, to avoid the area containing the pacemaker and wires. EXERCISES 1 (a) Define the terms dose, dose rate and irradiance. What are the units of each quantity? (b) Use infrared therapy as an example and explain how each of the above quantities are specified in describing the treatment conditions. 2 (a) List the treatment modalities for which the response of the patient can be used as an immediate guide as to dosage requirements. (b) For each treatment modality not listed in (a) above, explain why the immediate response of the patient can not be used to assess dosage requirements. Distinguish those cases where the immediate response is a poor guide and where an immediate response is not normally produced. 3 Describe the way in which dose requirements are established for (i) ultraviolet therapy and (ii) PUVA treatment in terms of: (a) calibration of the lamp (b) testing an individual patient DOSAGE AND SAFETY CONSIDERATIONS 327 4 A patient about to receive ultraviolet therapy is tested with a particular lamp and found to require 25 seconds exposure at a distance of 0.6 m to produce a first- degree erythema reaction (E1). (a) What exposure time would be required to produce a second-degree erythema reaction (E2) with the same lamp-to-patient distance? (See table 12.1). (b) What exposure time would be required to again produce a second degree erythema reaction with a subsequent treatment? (See table 12.2). (c) What exposure time would be required to produce an E2 reaction at the second treatment if the patient-to-lamp distance is decreased to 0.4 m? 5 A patient is found to require a 35 second exposure from a particular lamp to produce a first-degree erythema reaction. The lamp-to-patient distance is 0.5 m. (a) What exposure time would be required to produce an E1 reaction if the lamp-to-patient distance was changed to 1 m? (b) What exposure time would be required to produce an E3 reaction with the lamp-to-patient distance kept at 0.5 m? (c) What exposure time would be required to produce an E3 reaction with the lamp-to-patient distance reduced to 0.25 m? 6 Equation 12.1 was derived by considering light from a point source passing through a rectangular aperture in a screen (figure 12.1). Show that the same relationship is obtained when we consider light passing through a screen with a circular aperture of diameter x. 7 Using the fact that dosage depends on the product of intensity and exposure time, derive equation 12.2 from equation 12.1. 8 What are the two factors which contribute to lower the dosage when radiation is not incident at right angles to a tissue surface?

DOSAGE AND SAFETY CONSIDERATIONS 328 9 (a) Draw a diagram similar to figure 12.3(b), showing a light beam of square cross-section striking a surface at an angle θ. (b) Use simple trigonometry to show that the square beam consequently illuminates a rectangular area with sides of length x and x/cosθ. (c) Hence show that the light intensity at the surface is Ecosθ/x2, where E is the light energy in the beam. 10 A beam of light strikes a surface at right angles as shown in figure 12.3. The light intensity at the surface is measured as 50 J.m-2. Calculate the light intensity if the surface is tilted so that the angle of incidence is: (a) 15o (b) 30o (c) 45o 11 Briefly summarize the particular problems of assessing dosage in: (a) shortwave diathermy (b) ultrasound diathermy (c) microwave diathermy For which modality is the dosage most reproducible and for which is it least reproducible? 12 (a) Draw a diagram similar to figure 6.19(a) but including a metal implant (say a metal plate on the surface of the bone). (b) Draw the pattern of electric field lines and indicate clearly where the field intensity is increased and where it is decreased. (c) Describe the effect of the implant on heat production in different areas of the tissue. Why would heat production within the metal be low? DOSAGE AND SAFETY CONSIDERATIONS 329 13 Assume that the electric field within a tissue layer is originally uniform and draw diagrams to show the effect of a spherical implant or cavity for the following cases: (a) an air-filled cavity in muscle (b) an air-filled cavity in fatty tissue (c) a fluid-filled cavity in muscle (d) a fluid-filled cavity in fatty tissue (e) a metal implant in muscle In each case state reasons for the effects on the field pattern which you have shown. 14 Briefly describe the effects of shape and orientation of a metal implant in tissue subjected to an electric field (figure 12.4). What are the implications for shortwave diathermy treatment using parallel plate electrodes? 15 Consider the effect of implants and cavities on the distribution of ultrasound energy in tissue. (a) What factors determine the amount of energy reflected at a tissue/implant or tissue/cavity interface? (b) Would you expect significant reflection at the following interfaces? (i) tissue/metal (ii) tissue/air cavity (iii) tissue/body fluid cavity Justify your answers. 16 The pattern of heat production for 1 MHz ultrasound in a fat/muscle tissue combination is shown in figure 10.5. (a) Draw diagrams similar to figure 10.5 to show qualitatively the effect on the

DOSAGE AND SAFETY CONSIDERATIONS 330 pattern of heat production of a metal implant located 4 cm below the tissue surface. (b) Would you predict any difference in the pattern of heat production if an air- filled cavity rather than a metal implant was included in the tissue? Justify your answer. (c) What differences would be expected in the temperature elevation of tissue adjacent to a metal implant as compared to an air-filled cavity? Explain. 17 Briefly describe the effects of the following on the pattern of heat production in tissue exposed to microwaves: (a) an air-filled cavity (b) a metal implant (c) a fluid-filled cavity What differences in temperature elevation would occur with (a) as compared to (b)? Explain. 18 (a) Briefly describe why the eyes must be protected from exposure to (i) microwaves and (ii) ultraviolet radiation in therapy. (b) For ultraviolet therapy adequate shielding of the eyes is achieved by wearing protective glasses. Would similar glasses provide adequate protection from microwaves? Explain. (c) List other specific hazards associated with microwave and ultraviolet radiation. 19 (a) What is meant by the term 'velocity gradient heat production' and under what circumstances is this effect significant? (b) What is a shear wave and under what circumstances is shear wave production significant? (c) What effect will shear wave production and velocity gradient heat production have on the temperature distribution in tissue? DOSAGE AND SAFETY CONSIDERATIONS 331 20 (a) What is meant by the term 'gaseous cavitation'. Does it pose a hazard in ultrasound therapy? (b) Why is gaseous cavitation associated with ultrasound waves and not microwaves or ultraviolet radiation? 21 Cardiac pacemakers present a special hazard for diathermy. What are the risks and how do shortwave, microwave and ultrasound diathermy differ in this regard?

ELECTRICAL SAFETY 332 13 Electrical Safety In Australia, the UK and many other countries the mains Most of the apparatus used in diathermy and electrotherapy is plugged into the mains supply is 240 volts AC at a supply - 240 volts AC with a frequency of 50 Hz. Any apparatus of this kind represents frequency of 50 Hz. Other a potential hazard: the risk of electric shock. In this chapter we consider how a shock countries (including the USA) can occur, its likely effect and methods of shock protection. use a 120 volts, 60 Hz supply. It is convenient to distinguish two kinds of shock mechanism; these are macroshock For the clinician in an and microshock. intensive-care ward, the risk * Macroshock: The familiar mechanism which has posed a risk since the advent of of microshock is an important commercially supplied electricity. Here current flows from the body surface, through consideration. In private the skin and into the body. In order to produce harmful effects a relatively large voltage practice, the potential hazards and current are needed. A high voltage is needed to produce a sufficiently high are less likely to exist. current as the skin offers a high electrical impedance. A high current is needed as current spreads as it flows through deeper tissues and it is the current density (in A.m- 2 or mA.cm-2) which determines the physiological effects. * Microshock: As a result of increasing sophistication in medical technology the patient, in a hospital setting, may be connected to a number of pieces of apparatus some of which provide a direct electrical pathway to the heart (for example a myocardial electrode or a transvenous catheter). A very small current applied directly to the heart via this pathway can be fatal. Only a low voltage is needed as the subcutaneous tissues have a low electrical impedance and the current is localized, resulting in a high current density. How to get a shock To avoid the risk of electric shock, it is necessary to understand how it can occur. In order to produce an electric shock two conditions must be satisfied. Firstly the victim must complete a circuit and secondly the current levels involved must be high enough to produce an adverse reaction. ELECTRICAL SAFETY 333 Suppose a person inadvertently contacts one terminal of a battery. In this situation no shock can occur. A shock current can only flow when the person completes a circuit and current is able to flow from one terminal through the person and ultimately to the opposite terminal of the battery. In order for a current to flow the person must simultaneously contact both terminals of the battery. This is illustrated in figure 13.1. Figure 13.1 A person must 'complete a circuit' for shock to occur. In figure 13.1(a) a shock can not occur, regardless of the size of the battery voltage, as there is no continuous pathway for the current to travel. In figure 13.1(b) current is able to flow from one terminal of the battery, through the person, to the opposite terminal: the circuit is complete and a shock can result if the current flow is large enough. SIZES OF SHOCK CURRENT For shock to occur the current flowing through the person must exceed a certain level. Currents below about 10 mA (0.01 amps) when applied to the whole body via the skin are unlikely to cause an electric shock. This is because the current is distributed through the body so that the amount of electrical energy applied to a particular organ is small. Macroshock only poses a significant risk if the current level exceeds 10 mA.

ELECTRICAL SAFETY 334 By the same token a current in excess of about 100 µA (0.0001 amps) applied directly Shocks are described in to the heart (for example via a myocardial electrode) may be fatal. The microshock terms of current flow and not risk threshold is more than 100 times lower than that of macroshock. voltage. A shock's effect is Table 13.1 shows the effect of macroshock, i.e. when current passes through the skin determined by the amount of and through the body: that is when the shock is not given directly to vital organs. The current which flows through a values quoted refer to mains frequency (50 Hz) AC, since shock via the mains supply particular organ, not the is the greatest hazard in most situations which the physiotherapist will encounter. For voltage which produces it. figures appropriate to DC and other frequencies see Standards Association publication AS/NZS 60479:2002. Table 13.1 Effects of shock current through body. While any amount of current over 10 mA is capable of producing painful to severe shock, currents between 50 and 250 mA are potentially lethal. At values as low as 20 mA breathing becomes laboured, finally ceasing completely even at values below 75 mA: the victim can suffocate due to uncontrollable contraction of the muscles of the thorax and abdomen. ELECTRICAL SAFETY 335 If the current exceeds about 50 mA, ventricular fibrillation of the heart is likely to occur - A fibrillating heart is unable to an uncoordinated twitching of the walls of the heart's ventricles. Once ventricular pump blood so the victim will fibrillation is induced the heart will not spontaneously revert to its normal pattern of die unless first aid (cardiac beating. Normal cardiac rhythm can only be restored by administering a massive massage and artificial current pulse from a cardiac defibrillator. The machine, which should only be respiration) is administered operated by qualified personnel, supplies a short (3-4 ms) current pulse with an until medical help arrives. instantaneous amplitude of up to 40 to 80 amperes. Such high currents forcibly A useful rule-of-thumb is that clamp the heart. When the clamping action ceases the heart is more likely to revert to in most practical situations, its normal pattern of contraction. the deep-tissue impedance For shock currents above about 250 milliamps, the muscular contractions are so is around 200 Ω. severe that the heart is forcibly clamped during the shock. This clamping protects the heart from going into ventricular fibrillation and the chances of survival are improved. From a practical viewpoint, after a person is knocked out by an electrical shock it is impossible to tell how much current passed through the vital organs of his body. Artificial respiration must be applied immediately if breathing has stopped: if no pulse is detectable external cardiac massage should also be applied. An important question is 'how much current will flow if a particular voltage is applied externally i.e. to the skin surface'. This depends more on the skin impedance than on the impedance of deeper tissues. The impedance of deeper tissues depends on their shape and volume, but does not vary a lot. Between the ears, for example, the internal resistance at low frequencies (less the skin resistance) is 100 ohms, while from hand to foot it is close to 500 ohms. The skin impedance varies much more than that of the underlying tissue. For 50 Hz AC it can be lower than 1000 ohms for moist skin to higher than 0.5 megohms for dry skin. The body current flowing when a person contacts the mains supply (240 volts) is calculated from Ohm's law to vary between 0.5 mA when the skin is dry and 240 mA when the skin is moist. If the victim is startled from an initial mild shock, sweating can

ELECTRICAL SAFETY 336 result in a lowering of skin resistance and a rise in current from sub-lethal to lethal Perspiration is an unfortunate levels in a short space of time. This is one reason why it is essential, in an electric accompaniment to pain and shock situation, to terminate the shock current as quickly as is safely possible. fright. This lowers the skin resistance and increases the HOW SHOCK CAN OCCUR: MACROSHOCK shock current. To understand the hazards associated with the use of mains powered apparatus we need a clear picture of the way in which mains electricity is supplied. The very high voltage electricity which is generated at power stations is distributed by cables to electricity substations where step-down transformers reduce the voltage to a lower value. A single, large step-down transformer may be used to supply the 240 volts to many buildings in a residential neighbourhood. Large buildings in a city (for example a hospital) may have their own step-down transformers. Figure 13.2 shows the essential features of the power supply to a building. Figure 13.2 Mains supply to a building (schematic) ELECTRICAL SAFETY 337 One terminal of the stepped-down supply is earthed at the electricity substation. This As we will see later, earthing is called the neutral line. When the substation serves several buildings the neutral the supply affords a simple line is normally also earthed at the fuse box in each building. but efficient means of primary 240 volts AC is thus supplied to the fuse box in a building using two wires, the active protection against shock wire and the neutral wire. The neutral wire is nominally at earth potential (zero volts) hazard situations. and the active wire is at a high potential. The active line connects through a power meter to a switch and fuse or to a circuit breaker. From the fuse box, power wires run to light switches and power outlets. Power outlets have three terminals; an active, a neutral and an earth terminal. The earth terminal is connected to a wire which is physically connected to earth at the building. Figure 13.3 shows the connections of the active, neutral and earth wires to a power outlet socket. Figure 13.3 Wiring convention for an Australian power outlet (viewed from the front). Both the neutral and earth terminals of a power point are normally at earth or ground potential. However, it should not be assumed that the active terminal (on the left in figure 13.3) is the only hazardous one. For example it is quite possible for the active and neutral connections to be inadvertently interchanged when the power point is installed. Mains- powered equipment will still function normally when plugged in to the power point: the fault can only be determined by a specific test. Even when the power point is correctly wired it is possible for the neutral terminal to be above ground potential. This happens when appliances which draw a high current

ELECTRICAL SAFETY 338 are connected to the same circuit. A high current flowing in the neutral line will result For these reasons both the in a potential difference between the power point neutral terminal and the connection active and neutral terminals to earth at the fuse box. This is because the resistance of the neutral cable, while of a power point should be small, is not zero. If the neutral wire has a resistance R and carries a current I, the treated with equal respect potential difference produced is given by Ohm's law as V = I.R. when considering potential In what follows we assume that the power point is correctly wired and consider other shock hazards. hazards associated with the mains supply. In normal operation, when an appliance is plugged into the mains outlet, current flows between the active and neutral terminals. The earth wires does not normally carry any current. The earth connection is only provided as a safety measure. The advantages of a three-terminal mains supply can be seen by inspecting figure 13.4. Figure 13.4 Earthing of mains-powered apparatus casing. The circuitry within the apparatus (represented by an equivalent resistance Re in figure 13.4) is powered from the active and neutral wires. The earth wire is connected to the casing of the apparatus to ensure that there is never any voltage on the casing. The idea is that if the active wire within the apparatus makes accidental contact with the casing a very high current will flow through the earth wire to ground. The low ELECTRICAL SAFETY 339 resistance of the earth wire ensures that the current flow will be large enough to blow For this reason a fuse should the fuse, thus cutting off the active supply. In this way, the casing of the apparatus can never be replaced with a not become 'live' and present an electric shock hazard to anyone touching it. conductor other than a fuse As long as the earth wire and connections remain intact there is no risk of shock from of the same rating. touching the apparatus. Australian Standard AS3100 Some apparatus - electric shavers and hair dryers are examples - is 'double requires that double insulated'. The casing is usually made of a non-conducting plastic and special insulated apparatus has two precautions are taken to ensure that an electric shock is virtually impossible. The distinct and independent advantage here is that no reliance is placed on an earth wire which could come loose layers of insulation. Failure of or break. In fact, no provision at all is made for an earth connection to the apparatus. both insulating layers is The use of the double insulation principle is restricted to small and easily insulated almost impossible without apparatus. Any exposed metal on double insulated apparatus is not connected to complete fracture of the earth but is doubly isolated from the internal electrical circuitry. apparatus' casing. All apparatus which plugs into the mains, then, is macroshock protected, either by double insulation or by earthing. Nevertheless hazards remain in the form of faulty or worn equipment or careless workmanship. Figure 13.5 illustrates how an electric shock can result when apparatus is not earthed - because the earth wire is damaged or disconnected. The shock hazard in figure 13.5 arises when the active terminal short-circuits to the casing of the apparatus. In this case two faults have occurred - a break in the earth connection and a short circuit of the active wire to the casing. Figure 13.5 A person contacts apparatus which is not earthed and has the active wire touching the casing.

ELECTRICAL SAFETY 340 Since the neutral line is earthed at the fuse box and power sub-station, a person The fuses used in typical standing on the ground is effectively connected to the neutral terminal of the mains apparatus must have high supply. To complete the circuit and receive a shock, the person need only touch the ratings so they will not blow in active terminal or something connected to the active terminal. Current flows from the normal operation (where the active terminal through the person to ground and hence to the neutral connection at current flowing between the fuse box or power substation. active and neutral may be Two important things should be noted about the situation illustrated in figure 13.5 measured in amperes). * A shock has occurred because the earth wire is damaged. If the earth connection was intact the fuse in the active line (figure 13.2) would blow and isolate the apparatus from the mains supply. * The fuse in the active line will not protect the person from receiving an electric shock. The fuses used for normal apparatus have a rating of several amperes. The person can receive a lethal shock (see table 13.1) without blowing the fuse. A question which might occur to you is 'do both faults shown in figure 13.5 have to exist in order for a shock to result?' The answer is no. A shock can result when the apparatus is not earthed even though there is no direct physical contact between the active terminal and the casing. This is because the active wire and the case must have a small capacitance associated with them and insulation will not be perfect. Thus it is possible for small currents to leak via the insulation to the casing. With new and well looked-after apparatus the insulation impedance will be high and the maximum leakage current will be very small. Bad design or deteriorating insulation can, however, increase leakage currents to hazardous levels. Only by earthing the casing and providing an extremely low resistance pathway to ground can the risk of shock be minimized. MACROSHOCK PROTECTION From the previous discussion it should be apparent that the fuses in the mains supply serve a protective role only when currents of several amperes are involved. For this to ELECTRICAL SAFETY 341 happen the active wire must short-circuit to the earthed casing. How then can we protect against shock involving much lower currents? There are two commonly used ways - by using a core balance relay or a protected earth-free supply. Core Balance Relays Under normal circumstances the currents flowing in the active and neutral wires are equal. When an electric shock occurs the current in the active wire will be slightly greater than that in the neutral wire. This is because some current flows from the active wire through the victim to ground and through the ground to the neutral connection at the fuse box. Core balance relays are used to detect any imbalance and disconnect the power supply when the imbalance exceeds a predetermined value. The arrangement is shown in figure 13.6. Figure 13.6 Core-balance relay protection. The active and neutral wires both pass through a magnetic core around which a sensing coil is wound. The currents in these wires are in opposite directions and when they are equal no current is induced in the sensing coil. If the currents are unequal a current proportional to the difference in active and neutral current is induced in the sensing coil. The induced current is amplified and used to operate a magnetic relay which disconnects both the active and neutral supply lines.

ELECTRICAL SAFETY 342 Once the core balance relay has been 'tripped', the supply remains disconnected until the circuit breaker is manually reset. The response time of core balance relays is quite short (less than 100 ms) and typical units can be adjusted to trigger on an imbalance of as little as 5 mA. They are available for permanent installation (usually inside the fuse box) and are also supplied as portable units suitable for connecting between power points and appliances. From the foregoing description it should be apparent that these units protect against the 'normal' situation where a shock current flows through a person's body to earth. They will not protect against the more unusual situation where a person inadvertently contacts both the active and neutral lines simultaneously. Earth Free Supplies In the situation shown in figure 13.5 the person receives an electric shock because his hand makes contact with the active line and his feet are in contact with the ground to which the neutral is connected. A question which might occur to you is 'would it be safer if the supply neutral was not earthed?' In this case the earthed person could not complete a circuit by touching the active line and so would not receive a shock. The answer to the question is a qualified 'yes'. Figure 13.7 shows how the normal Figure 13.7 mains supply can be rendered earth- Isolation with a transformer. free by using an isolating transformer. If neither side of the transformer secondary is earthed a person can ELECTRICAL SAFETY 343 touch both earth and one transformer terminal without receiving a shock. At first glance it would seem that a person can only receive a shock if both transformer secondary terminals are contacted simultaneously. Unfortunately this is not the case in practice and the reasons are twofold: * If a piece of apparatus plugged into the power point should develop a short circuit to earth no fuses will blow. The fault can remain unnoticed indefinitely. In the meantime the earth free supply has been converted to an earthed supply and we have no knowledge of which side of the transformer has become 'active' and which 'neutral'. * If faulty or poorly designed apparatus is plugged into the power point the insulation impedance between either supply terminal and earth can be reduced to the extent that the supply is effectively earthed: again with no knowledge of which terminal is at earth potential. The system can be rendered safe by adding an earth leakage detector between the mains earth and the two transformer secondary wires as shown in figure 13.8. In normal operation a negligible Figure 13.8 amount of current flows through Isolation with earth leakage detection. the leakage detector. If, however, apparatus with a short circuit or defective insulation is plugged into

ELECTRICAL SAFETY 344 the power point a current will flow through the detector and activate the alarm. When earth free supplies are Of the two systems the protected earth free supply is somewhat safer than an earthed used an earth leakage supply fitted with a core balance relay. Unfortunately the isolation transformers and detector system is leakage detection circuitry needed are both bulky and expensive. For this reason mandatory. The combination protected earth free supplies are only found in areas of high shock hazard. Core provides a high degree of balance relays which are relatively cheap and easy to install are considered adequate electrical safety. for more general use such as in physiotherapy clinics and the physiotherapy departments of hospitals. Whichever method of protection is used it is important that the system be checked at regular intervals to ensure that the protection mechanisms are operating correctly. MICROSHOCK The use of electronic monitoring and measuring devices in the hospital setting has proved of immense value for patient monitoring and assessment. It has, however, also introduced some special risks of which the modern member of the health care team must be aware. Consider the patient in an intensive care unit. In some cases the patient may have apparatus connected by a direct electrical pathway to the heart. One such situation is illustrated in figure 13.9. Here a very special hazard exists because of the low current needed to cause ventricular fibrillation. Even if all the equipment is earthed the patient can still be electrocuted unless adequate precautions are taken. Figure 13.9 A microshock hazard situation. ELECTRICAL SAFETY 345 The patient, in this situation, is connected to two pieces of apparatus: an For example, if the resistance electrocardiograph (ECG) machine and a blood pressure monitor. For simplicity only of the earth wire is 0.1 Ω, and the earth wires are shown. The patient is connected to earth by two pathways: the a spike of leakage current of electrode connected to the right leg is earthed via the ECG machine and the fluid filled 100 mA flows, a potential catheter is connected to a pressure transducer which is earthed via the blood difference of 10 mV is pressure monitor. produced. If the resistance of The risk of shock arises when a potential difference exists between the earth the tissue is 100 Ω, a current terminals on outlets 2 and 3. If a current I flows along the earth wire connecting the of 100 µA will flow. two outlets a potential difference V will result. V is given by Ohm's law V = I.R where R is the resistance of the earth wire between the outlets. Although R is very small it is not zero. If I is large enough the potential difference produced will be sufficient to electrocute the patient - remember that currents in excess of 100 microamperes or so flowing through the patient's heart may be fatal. Normally, of course, little or no current flows in the earth wire - it is only there to carry leakage current from the apparatus plugged-in. If, however, an appliance with a high leakage current, such as a vacuum cleaner, is plugged into outlet 1 a dangerous situation can result. Vacuum cleaners are notorious for producing large leakage currents (particularly at switch-on) because the motor is continually exposed to dust and moisture which lower the insulation impedance. Visualize the situation where the patient in figure 13.9 is connected as shown and a cleaner, working his way down the corridor, plugs a vacuum cleaner into outlet I (on the corridor outside) and switches it on. The instantaneous leakage current flowing in the earth wire could raise the potential at the earth terminal of outlet 2 to a sufficiently high value (relative to outlet 3) to electrocute the patient. The solution, in this case, is to plug all apparatus around the patient into a single power outlet or to interconnect the earth terminal of each outlet with heavy gauge copper wire. It is also necessary to ensure that the wiring for the power outlets in the patient's room does not connect to the power outlets in adjacent rooms or corridors.

ELECTRICAL SAFETY 346 A further precaution which must be taken is to ensure that any apparatus which is This last criterion would used in the patient's room has been tested for earth leakage and meets the exclude most domestic and appropriate safety standards. industrial vacuum cleaners and many domestic PATIENT TREATMENT AND ELECTRICAL SAFETY appliances. From the foregoing considerations of shock and shock protection it is apparent that there are three levels of risk associated with patient treatment. The greatest risk is to patients coupled to apparatus which may have a direct electrical connection to the heart. A lower level of risk exists when there are no invasive electrical connections; however we should distinguish the patient who is coupled to electromedical apparatus by surface electrodes from the patient who is not electrically connected to any piece of apparatus. The reason is that if a patient is connected by electrodes to, say, an electrocardiograph the potential for a shock to occur is increased by the deliberate electrical connection. In addition the skin resistance has been minimized by cleaning and application of a conductive electrode gel. In this case the voltage needed to produce a fatal shock current is reduced. Protection is afforded at two levels: * by using apparatus which meets appropriate safety standards and * by appropriate protection built into the mains supply. We consider each factor in turn. Protection and the Mains Supply Patient treatment areas in hospitals are distinguished according to the kind of procedures or treatment being used and different safety standards apply to the mains supply in each case. Three types of treatment area are distinguished: * Cardiac protected electrical areas. These are areas which are suitable for carrying out procedures which involve direct electrical connection to the heart. The safety requirements for both the electrical supply and apparatus to be used in such areas are stringent (see SAA Standards AS 3200 and AS 3003). These ELECTRICAL SAFETY 347 are described as 'Type CF' or simply 'cardiac protected' areas. In Australia and If the maximum potential some other countries, these used to be described as 'Class A' treatment areas. difference is kept below 100 * Body protected electrical areas. These are areas which are suitable for mV then the maximum patient carrying out procedures which do not involve direct electrical connection to the current will be below 100 µA heart but which do involve the patient being in direct electrical contact with (assuming a minimum electromedical apparatus. Safety requirements are more stringent than those patient resistance of 1000 applying to areas where no electrical connection between patient and apparatus ohms). is necessary. Such areas are described as 'Type BF' or simply 'body protected' areas. They used to be known as 'class B' treatment areas. * Other patient areas. These are areas which are not specifically suited to 'cardiac type' or 'body type' procedures. Apparatus which is not electrically connected to the patient can be used. Apparatus which is intended to connect electrically to the patient can be used in these areas, but only if the apparatus itself meets stringent safety requirements (equivalent to those of a cardiac protected or body protected treatment area). When direct electrical connection is made to the heart, shock currents as low as 100 µA can be fatal. For this reason cardiac-protected treatment areas are designed to minimize this risk. The earth wiring in these areas is constructed from heavy gauge copper wire so that even when substantial currents (up to 1 ampere) flow in the earth wire the potential difference between different earth terminals is kept below 100 mV. An area which meets this and other requirements (see SAA Standard AS3003) is described as an equipotential earth (EP) area. In addition to the requirement for equipotential earth wiring, cardiac protected areas must also have core-balance relay protection or have a protected earth free supply. Body protected areas are those designed to protect patients who may be connected directly to electromedical apparatus from macroshock currents. It is not necessary for the area to have an equipotential earth system but the supply must have core-balance relay protection or a protected earth free supply.

ELECTRICAL SAFETY 348 Best protection is afforded by a protected earth free supply but such installations are expensive. Core-balance relay protection can be provided economically and gives an adequate level of safety. Body protected areas which have appropriate core-balance protection will have the mains supply disconnected within 60 milliseconds of the active and neutral current imbalance exceeding 10 mA (SAA Standard AS3003). Class CF (cardiac protected) and BF (body protected) treatment areas are normally identified by signs displayed in, or on the doors of, the area. The signs have an identifying symbol and the words 'CARDIAC PROTECTED ELECTRICAL AREA' or 'BODY PROTECTED ELECTRICAL AREA' printed in white letters on a green background. The symbols for these areas are shown in figure 13.10. Figure 13.10 Symbols used to identify different classes of equipment or treatment area. (a) class CF (microshock protected) (b) class BF (body protected). Patient areas which are not designated class A or B have no 'special' safety requirements other than those which apply to commercial, industrial and domestic supplies (SAA Standard AS3000). This means that the area does not provide protection if contact is made (either directly or indirectly) between the active supply wire and earth. It is recommended, though not mandatory, that such areas be provided with core- balance relay protection. The recommendation should be considered seriously since normal protective devices (fuses or circuit breakers) can allow currents of up to 150 times the macroshock hazard level without operating to cut-off the supply. ELECTRICAL SAFETY 349 Protection and Electromedical Apparatus Both kinds of apparatus must meet certain safety standards Electromedical apparatus used for patient treatment falls into one of two categories. specified in terms of the In the first category we have apparatus which does not have a deliberate and direct amount of leakage current contact with the patient, such as an infrared or ultraviolet lamp. In the second category which can flow under different we have apparatus which requires deliberate electrical connection with the patient; for conditions. Apparatus which example, apparatus for delivering interferential or conventional TENS currents. In this has a patient circuit must case the apparatus has a patient circuit. meet additional safety Consider first electromedical apparatus which does not have a patient circuit. In this standards. case the significant risk to the patient is if the patient inadvertently contacts the apparatus casing. If the maximum contact current which can flow is below a specified value (10 µA through a 1000 ohm load) and the earth leakage current is less than 100 µA then the apparatus is designated class CF. This is the safest kind of electromedical apparatus. Other electromedical apparatus must have a maximum casing-contact current below 100 µA: this is considered to offer adequate protection when the patient has no possibility of direct electrical connection to the heart. Class CF equipment can easily be recognized by the 'heart in the square' symbol (figure 13.10a). This is normally displayed on the rear panel of the equipment, near where the power cord enters. When a piece of electromedical apparatus has a patient circuit then the patient circuit itself can be either class CF, BF or B. * A class CF patient circuit is the most safe. If the leakage current to the patient circuit is normally below 10 µA and below 50 µA even when a fault condition exists (when the earth lead is broken or the patient inadvertently contacts the active terminal of the mains supply) the patient circuit is designated class CF. A class CF patient circuit affords microshock protection. * A class BF patient circuit is macroshock protected. The normal leakage current is below 100 µA and the current which can flow when the earth lead is broken is below 500 µA. To comply with class BF specifications the fault current which can

ELECTRICAL SAFETY 350 flow from the patient circuit through the patient to the active terminal of the mains supply (in the event of the patient accidentally contacting the mains active lead, either directly or indirectly) must be below 5 mA. In other words a class B patient circuit has adequate isolation from the mains supply to minimize the risk of macroshock. * A class B patient circuit affords a minimum level of macroshock protection. This kind of patient circuit may have one terminal earthed. Such a circuit must have leakage currents below those needed to represent a macroshock hazard when the apparatus is operating normally or when the earth lead is broken. However, no protection is offered against the situation where the patient inadvertently contacts the mains active lead. Class CF and BF patient circuits are identified by the symbols shown in figure 13.10. The appropriate symbol is prominently displayed immediately adjacent to the patient circuit output sockets of the machine. If no symbol is found, the patient circuit should be assumed to be class B. Protection in Summary It should be apparent, from the foregoing description, that electrical safety is only ensured if: * the equipment meets appropriate safety standards for the treatment procedures involved; * the electrical supply meets appropriate safety standards for the treatment procedures involved. Figure 13.11 shows a flowchart summarizing the requirements for earthed mains- powered apparatus and the class of area in which it can be used. The flowchart is based upon those of Australian Standard AS3200 and figure 5.3 of AS2500. ELECTRICAL SAFETY 351 START Procedure must be carried YES Is there a possibility of an NO Does the equipment have a NO out in a class CF area. intra-cardiac conductor patient circuit? (class CF procedure)? YES All mains powered NO Does the mains supply have equipment must: class CF or class BF * be connected in the protection? same equipotential area. YES * meet relevant safety Use class CF or BF Use class CF, BF or No special standards. patient circuit. B patient circuit. precautions Will individual patient circuits NO (if any) have possibility of intra-cardiac connection? Use class CF, BF or B YES patient circuit. Patient circuit must be class CF. Figure 13.11 Flowchart for the safe application and use of electromedical equipment.

ELECTRICAL SAFETY 352 The strictest safety standards are mandatory when the patient has apparatus connected directly to the heart. In this case the mains supply should be that provided in a Class CF area and electromedical apparatus with a patient circuit should not be used unless either the patient circuit is class CF or there is no possibility of a direct electrical connection with the heart. In this way the risk of microshock is minimized. When there is no direct electrical connection to the heart it is sufficient to protect against the risk of macroshock. This can be achieved either by using equipment with a class CF or BF patient circuit or by treating the patient in a class CF or BF area. If the electrical wiring in a patient treatment area is class CF or BF then patients can be safety treated with apparatus which has a class CF, BF or B patient circuit. If the electrical wiring in a patient treatment area is not class CF or BF then the patient circuit must be class CF or BF. In other words if the mains supply is of the normal household variety then electromedical apparatus should have either a class CF or BF patient circuit. When there is no patient circuit and no possibility of intra-cardiac connection, electromedical equipment may be used on a normal earthed (but unprotected) mains supply. EXERCISES 1 (a) What is meant by the terms 'macroshock' and 'microshock'? (b) Electric shocks are always described in terms of shock current rather than voltage. Why is this so? 2 (a) Consider macroshock and list the factors which will determine the size of shock current when a person accidentally contacts the mains supply. (b) Table 13.1 shows the effect of different sizes of shock current. Explain why shock currents in the range 50-250 mA represent a greater hazard than shock currents above 250 mA. ELECTRICAL SAFETY 353 3 A person accidentally contacts the mains supply (240 volts). Given that the skin resistance may vary from 1000 Ω to 0.5 MΩ calculate the possible range of shock currents which may result. What is the practical significance of this in terms of terminating the shock as quickly as possible? 4 Figure 13.3 shows the wiring convention for a power outlet. Given that the neutral and earth terminals are grounded (earthed) at the fuse box in a building according to figure 13.2, does this mean that the neutral and earth terminals are 'safe'? Explain. 5 (a) Figure 13.2 is a schematic diagram of the mains supply to a building. The neutral line is earthed at the electricity sub-station. Why is it an advantage to also earth the neutral line at the fuse box? (b) The neutral line of the mains supply is normally earthed at the fuse box. Even so, this does not mean that the neutral terminal of a correctly wired power point is at earth potential. Explain. 6 Consider figure 13.4 and explain what would happen if: (a) the active line accidentally makes contact with the apparatus casing (b) the neutral line accidentally makes contact with the apparatus casing. Would this blow the fuse? Could a short-circuit of the neutral line to the casing pose a shock hazard? Explain. 7 (a) What is meant by the term 'double insulated' as applied to electrical apparatus? (b) What is the principal advantage of double insulation for electrical safety? (c) Explain the relative merits of double insulated apparatus and earthed apparatus as far as leakage currents are concerned.

ELECTRICAL SAFETY 354 8 (a) Figure 13.5 shows a person receiving an electric shock because two faults have occurred. What are they? (b) Is it possible for a person to receive a shock from apparatus in which only one of these faults has occurred? Explain. 9 It has been said that fuses are included in the mains supply line only to protect the apparatus. Is it possible for a fatal shock to be delivered without blowing the fuse in the following two cases: (a) when the earth wiring is damaged? (b) when the earth wiring is undamaged? Explain. 10 (a) Describe the principles of operation of a core balance relay (as shown in figure 13.6). (b) The fuses included in the mains supply must have a rating of several amperes and so can not protect against macroshock, yet a core balance relay can have a 'rating' of a few milliamperes but will not disconnect the mains supply in normal operation. Explain. (c) Under what circumstances is a core balance relay unable to protect against macroshock (even involving shock currents of several hundred milliamperes)? 11 Figure 13.7 shows an isolating transformer used to generate an earth-free supply. (a) Could a person receive a shock by contacting any one of the terminals of the power outlet? (b) Under what circumstances could a person receive a shock from an earth- free supply (assuming no faults in the wiring)? (c) What is the principal disadvantage of the simple earth-free supply shown in figure 13.7? ELECTRICAL SAFETY 355 12 (a) Explain why earth-free supplies can only be regarded as safer than a normal earthed supply if an earth leakage detector (figure 13.8) is included in the circuitry. (b) Why is a protected earth-free supply preferable to a core balance relay protected supply for areas of high shock hazard? Are there any disadvantages associated with the installation of protected earth-free supplies? 13 Consider figure 13.9 where the patient is connected to (i) the supply earth of a blood pressure monitor via a transvenous catheter and (ii) the supply earth of an ECG machine via an electrode attached to the right leg. Explain how a shock hazard situation arises as a result of the ECG machine and blood pressure monitor being connected to separate power outlets. 14 A microshock of only 200 µA flowing directly through the heart can be fatal. (a) Given that the resistance of the patient's tissues between the catheter and the electrode applied to the right leg in figure 13.9 is about 1000 Ω, calculate the potential difference needed to produce a fatal shock current. (b) The resistance of the earth wire connecting mains outlets 2 and 3 in figure 13.9 is 4.0 Ω . Calculate the current flowing through the earth wire connecting the outlets which would be sufficient to cause electrocution of the patient. (c) A cleaner plugs a vacuum cleaner into mains outlet 2 (see figure 13.9). The leakage current of the vacuum cleaner is 70 mA. Does this represent a microshock hazard? 15 Consider the microshock hazard situation shown in figure 13.9. (a) How might the microshock hazards be minimized? (b) What are the implications of this situation for the use of electrotherapy apparatus on or near the patient?

ELECTRICAL SAFETY 356 16 (a) What is the difference between a class CF and a class BF treatment area? (b) How are class CF and class BF treatment areas identified? 17 (a) How would you recognize equipment with a class CF or class BF patient circuit? (b) A piece of equipment has a class Z patient circuit. Under what circumstances should the equipment be used for patient treatment? (c) What class of patient circuit should apparatus have if it is to be used in a patient's home? 18 Suppose you have (a) an ultrasound machine and (b) an interferential therapy machine which are to be used with an unprotected mains supply. What electrical safety standards apply to each machine?

APPENDICES 357 APPENDIX 1 Prefixes Used to Specify Multiples and Submultiples of Units. The following table lists some important prefixes used to specify multiples and submultiples of units in the système internationale (SI). Prefix Symbol Multiple SOME EXAMPLES 109 hertz (Hz) = 1 gigahertz (GHz) giga G 109 106 ohms (Ω) = 1 megohm (MΩ) mega M 106 103 joules (J) = 1 kilojoule (kJ) kilo k 103 10-2 metre (m) = 1 centimetre (cm) deci d 10-1 10-3 watt (W) = 1 milliwatt (mW) centi c 10-2 10-6 henry (H) = 1 microhenry (µH) milli m 10-3 10-12 farad (F) = 1 picofarad (pF) micro µ 10-6 nano n 10-9 Note that the prefixes deci- and centi- are considered pico p 10-12 acceptable (due to their common usage) but are not recommended in the SI. APPENDICES 358 APPENDIX 2 Quantities and Units The following tables list the quantities used in this text, their units and their symbols. Table 1 lists quantities which are measured directly in SI base units. The quantities listed in Table 2 have units derived from the base SI units. For a comprehensive listing of quantities, their SI units and their definitions see Quantities and Units in Science by O. Ogrim and A. E. Vaughan, Science Press (1977). TABLE 1: QUANTITIES MEASURED IN Sl BASE UNITS quantity symbol for unit symbol for Some quantities have several quantity* unit alternative acceptable symbols. The ones shown are SI recommended and length l metre m are used in this book. mass m kilogram# kg # notice that the base unit of mass is the time s second s kilogram, not the gram. The kilogram is current I ampere A the only Sl base unit to include a prefix temperature T kelvin# # K in the name. amount of n mole mol ## Although the base Sl unit of temperature substance is the kelvin, the degree Celsius angle θ radian### rad (symbol °C) is an acceptable alternative. Temperatures in degrees Celsius are converted to kelvins by adding 273.15. The size of the degree Celsius is equal to the size of the kelvin. ### Although the base Sl unit of angle is the radian, the degree (symbol o) is an acceptable alternative. One radian is 180/2π degrees (approximately 57o).

APPENDICES 359 TABLE 2: SOME QUANTITIES MEASURED IN Sl DERIVED UNITS # This quantity is more properly termed the relative permittivity. The term name symbol unit dielectric constant is used in this text to conform with common usage in the capacitance C farad F (= C.V-1) literature. charge q coulomb C (= A.s) conductivity S.m-1 (= Ω-1.m-1) ## This quantity is more properly termed current density σ A.m-2 the relative permeability to distinguish density (mass density) i kg.m-3 it from the absolute permeability. dielectric constant# - Since absolute permeability is not electric field strength ρ V.m-1 (= N.C-1) used in this text the term permeability energy joule J (= N.m) is used for simplicity. force ε newton N (= kg.m.s-2) frequency hertz Hz (= s-1) heat energy E joule J (= N.m) intensity (wave) E W.m-2 (= J m-2s-1) impedance (electrical) F ohm Ω impedance (acoustic) f kg m-2 s-1 magnetic field strength Q A.m-1 penetration depth I m permeability# # Z - potential difference Z volt V (= W.A-1) power B watt W (= J.s-1) resistance ohm Ω resistivity δ Ω.m specific heat capacity µ J .kg-1.K-1 velocity V m . s-1 volume rate of heating P W.m-3 ( = J m-3.s-1) wavelength R m ρ c v Pv λ