value. It has been estimated that 80 percent of the implanted pacemakers are of this type. The units that sense electrical activity are generally more susceptible to interference. Interference related failures (failure to pulse within design specifications) in demand pacemakers have been attributed to such diverse sources as radar, small electrical appliances, lightning strokes, microwave ovens, and motor vehicles. To identify potential hazards for pacemaker users, rf sources which cause failures should be quantified as to frequency, field intensity, and modulation. Fortunately, many pacemakers have been laboratory tested, and it is now possible to identify some important trends in the thresholds of failure of pacemakers in terms of frequency, field intensity, and modulation. These trends will aid in evaluating the SPS power beam as a potential pacemaker hazard. Table 4 indicates the electrical test conditions employed by various workers in establishing levels of potential hazards for pacemakers. Very early work on highly specific testing has been omitted here in favor of later work which tends to identify important current trends. 4.2 The Effects of Frequency The field intensities or thresholds at which demand pacemakers malfunction increases dramatically with frequency. For example (Figure 5), Mitchell and Hurt (1975) found that the average threshold increased from less than 50 volts/meter at 450 MHz to nearly 600 volts/meter at 3200 MHz even when the pulse rate and width of the test source were maintained constant. This observation is qualitatively supported by some earlier work (Figure 5) of Bonney et al., (1973), which shows that the potentially hazardous field intensity level increases from about 75 volts/meter at 915 MHz to 250 volts/meter at 2810 MHz. One of the conclusions of the study by Bonney is that "a field of 75 volts/meter appears to be acceptable as a nominally safe figure for all units tested at each of the four frequencies and for all modes.” This conclusion is particularly applicable to SPS since one of the four frequencies tested was 2.45 GHz. The tests at 2.45 GHz, however, were conducted with a 120 Hz half-wave rectified modulated waveform. The only evidence that was found indicating a potential for pacemaker failures in microwave fields below 75 volts/meter is in a study by Georgia Institute of Technology which identifies failures in fields as low as 40 volts/meter (Jenkins and Denny, 1976). These tests were conducted in a 3.0 GHz, pulsed interference field using 110 pulses per second and 120 microsecond pulse width. Even then, only 10 percent of the sample was affected at the 75 volt/meter level, with 50 percent of
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