The Influence of Electromagnetic

Transcription

1 The nfluence of Electromagnetic Environment on the Performance of Artificial Cardiac Pacemakers Seymour Furman, M.D., Bryan Parker, Martin Krauthamer, M.D., and Doris J. W. Escher, M.D. T he danger of producing ventricular fibrillation by passage of 60-cycle alternating current (AC) through pacemaker leads attached to a patient s heart has been outlined by Burchell [l], Weinberg [71, Noordjik [6], and Furman [3]. Each of them described patients who had died because alternating current had leaked in minute quantity to the heart via direct wired connection and had produced ventricular fibrillation. Whalen [8] has described the techniques required to insure that no leakage of current can occur into the heart. A parallel problem has been observed somewhat more recently and described by Carleton et al. [2], Lichter et al. [4], and Mansfield [5]. This is the response of implanted cardiac pacemakers to the presence of electromagnetic fields or intracorporeal alternating currents without direct contact with the pacemaker or its lead system. Each of the earlier studies [ 1, 3, 6-83 included only asynchronous pacemakers. With the development of atrial synchronous, ventricular synchronous, and demand pacemakers, an effort has been made to determine the sensitivity of 10 types of pacemakers to the effects of low-frequency alternating current in the range of l cycle per second to 100 kilocycles per second and to the effects of impulses resulting from electrical corona. n each instance the variety of response, i.e., increase in rate, development of erratic rate, change from a triggered to a fixed rate, or cessation of pacemaker function, has been determined. Case 1. As a patient with an implanted Cordis Atricor r. (atrial synchronous pacemaker) lay on a fluoroscope table that was connecte d to an alternating current-operated electrocardiograph, he promptly developed a pacemaker-induced rate of 145 beats per minute. On investigation, the frame of the fluoroscope table was found to be connected to the neutral terminal of the AC line, rather than to ground; also, a potential of 0.5 volts AC existed between the table and From the Cardiothoracic Service. Division of Surgery, and the Cardiology Service, Division of Medicine, Montefiore Hospital and Medical Center, Bronx, N.Y. Supported in part by U.S. Public Health Service Grants HE and HE Presented at the Fourth Annual Meeting of The Society of Thoracic Surgeons, New Orleans, La., Jan , THE ANNALS OF THORACC SURGERY

2 NOTE: Electromagnetic Znfluence on Pacemakers the properly grounded electrocardiograph. This resulted in a 50-microampere, 60-cycle current between the patient's right arm, which touched the table, and his right leg, which was grounded via the electrocardiograph. This small current (below the level of the patient's perception) was sufficient to interfere with pacemaker operation. Case 2. This patient had an implanted Cordis Atricor; he developed an irregular pulse and a sense of cardiac fluttering when standing within three feet of the back of an opened television set (although not in front of the screen with the set properly assembled). He found that his erratic pulse accompanied corona discharges from the high voltage system of the television set. When he stepped back from the set, or to its side, no cardiac effect occurred. METHOD All pacemakers were bench tested to determine the effects of an alternating current generated by a Hewlett-Packard Model 202C low-frequency oscillator applied directly to the electrodes. The maximum voltage was restricted to 3 volts root mean square (RMS) to avoid damage to the new and unused pacemakers. n each instance the sensitivity of response of each pacer was determined and a curve plotted for millivolts against frequency. Three in vivo tests were carried out: (1) An alternating current was passed from the right arm to the right leg via standard pasted ECG electrodes. Cardiac response was observed from the electrocardiogram, or where the ECG was obliterated by the applied current, from the record of the pulse from a finger plethysmograph. n all instances the applied voltage was kept below 3 volts to remain below the threshold of sensation. (2) A %"-long spark generated by a 6-volt automobile coil was moved toward and away from the implanted pacemaker. (3) Three operating electric shavers were brought up to the pacemaker. These were Norelco Models SC7970 and SC7960 and a Ronson CFL. The electrocardiogram was continually monitored in all instances. RESULTS ASYNCHRONOUS PACEMAKERS Cordis Ventricor 111C. These pacemakers were unaffected in vivo by any interfering signal. During bench testing, direct application of 60-cycle alternating current to the pacemaker output terminals induced a rapid emission rate. General Electric Single and Dual Rate Pacemakers. n vivo, alternating current produced no effect; the spark at 9 inches and the razors at 2 inches produced an erratic increase in rate (Fig. 1). Direct application of alternating current to the pacemaker produced only a slight increase in rate. Electrodyne TR-14. No interference effects were noted. Airborne nstrument Laboratory's Radio Frequency Pacemaker. This partially implanted system was subjected to interference near the external pulse generating unit. Only the spark coil produced rate increases to 95 impulses per minute and then only when it was within 6 inches of the pacemaker; however, this range was increased to two feet when the spark coil was placed on a steel dolly. Medtronic 5870C. No interference effects were noted. NONCOMPETTVE (TRGGERED PACEMAKERS) American Optical Co. Demand Pacer-Bench test. Frequencies below 20 cycles per second inactivated the pacemaker. At higher frequencies-including 60-cycle alternating current-the pacemaker lost its sensitivity to R wave activity and reverted to its fixed-rate mode, even when cardiac activity was more rapid. VOL. 6, NO. 1, JULY, 1968 gi

3 FURMAN, PARKER, KRAUTHAMER, AND ESCHER FG. 1. This pacemaker is designed to respond to external stimuli in order to modify its function. Electromagnetic interference increased its rate-in this instance to 150 impulses per second. With increase in rate, pacer output fell. The entire frequency response of the pacemaker was the narrowest of any of the tested units. Cordis Bentricor Z. This pacemaker, similar in mode of operation to the American Optical Co. unit, was readily stopped by all tested interference signals both in vivo and on the bench. n vivo, alternating voltages less than one volt stopped pacemaker output at a wide range of frequencies (Figs. 2, 3). The spark coil and the electric razor both caused cessation of pacer output at a range of up to 9 inches. This unit also ceased to function in close proximity to a fluorescent light tube. Cordis Ectocor (Ventricular Synchronous). Alternating currents applied directly to the pacemaker caused the unit to revert to fixed-rate operation at its automatic rate (70 impulses per minute). With a frequency and amplitude of 40 CORDS VENTRCOR FG. 2. Electromagnetic interference in this highly sensitive demand pacemaker produced inactivity and asystole during the period of interference. With removal of the interfering signal, the pacer returned to its normal sensitivity to cardiac action. 92 THE ANNALS OF THORACC SURGERY

4 NOTE: Electromagnetic Znfluence on Pacemakers ma 7r VENTRCOR N VVO ,^ to 60 cycles and 1 to 1.2 millivolts per second, the unit increased to its maximum synchronous rate (125 per minute) (Fig. 4). n vivo, both maximum synchronous rate and fixed-rate operation were induced with intracorporeal alternating currents. Erratic operation was initiated by a spark at distances less than one foot, and intermittent extra impulses were emitted with an electric razor on the skin over the pacemaker. Medtronic Demand n vivo, this unit was unaffected by any of the tests. On the bench, alternating voltages applied to the electrodes inhibited the unit s output with the frequency response curve shown in Figure 5. When this unit was placed in a saline-filled plastic vessel, 35 volts of 60-cycle alternating current were needed to affect its operation. Cordis Atricor and Atricor Jr. (Atrial Synchronous). This variety of pacemaker was the type implanted in the patients referred to earlier. n vivo tests were applied to patients with both transvenous and myocardial electrodes. Both systems readily produced tachycardia at the maximum synchronous rate (125 for Atricor, 145 for Atricor Jr.). At more intense applied alternating current, both reverted to fixed-rate, asynchronous operation because of amplifier saturation. The spark coil and shavers all caused these units to operate at maximum synchronous rate-in one patient at a distance of 4 feet. CORDS ECTOCOR FG. 4. The ventricular synchronous pacemaker responded to interfering signals by increasing its rate of impulse formation to its maximum synchronous rate, or reverting to fixed stimulation at its automatic rate (70 per minute). VOL. 6, NO., JULY,

5 FURMAN, PARKER, KRAUTHAMER, AND ESCHER FXED RATE REGON - SWTCH OFF REGON (T! FREQUENCY (cycles/sec) FG. 5. Frequency plot of alternating voltages applied to electrodes required to interfere with the operation of three triggered pacemakers. Voltages in the area above the curves disable the pacemaker, either by causing pacer inactivity or its reversion to an unsynchronized fixed-rate mode. The mode of failure is indicated by interrupted or solid lines. Note that the American Optical demand unit ceased function below 20 cycles and reverted to asynchronous operation above this frequency. All three pacemakers returned to normal function with cessation of the interfering stimu 1 i. DSCUSSON Ten different pacer units were tested for their sensitivity to alternating current applied to the human and to signals generated by a spark coil or by the small-brush variety of electric motors. Bench tests were used to find the sensitivity of the pacemaker units to 60-cycle alternating current and to determine the energy frequency range that would produce a response. A wide variety of response to extraneous stimuli was found among pacemakers of different manufacturers. The common asynchronous pacers were all insensitive to interference, except for those two designed to allow external radio frequency control of operation. n analyzing the data, the pacemakers must be divided into triggered and untriggered units. The untriggered or asynchronous (fixedrate) units do not respond to body electrical activity and should, if possible, remain fixed in their output despite any provocation. The triggered units, i.e., demand, atrial, and ventricular synchronous pacers, modify their activity as a result of body electrical activity. Their ability to respond only to appropriate (physiologically generated) electrical stimuli while at the same time rejecting extraneous stimuli is an impor- 94 THE ANNALS OF THORACC SURGERY

6 NOTE: Electromagnetic Znfluence on Pacemakers tant consideration, as is the variety of response. An atrial or ventricular synchronous pacemaker that increases its rate to 120 per minute, the maximum synchronous rate, or that returns to its automatic rate with increasing current input is far less hazardous than a demand pacemaker that perceives 60-cycle alternating current as ventricular contractions and ceases to function. All pacemakers tested reverted to normal function when interference was discontinued. Of great interest was the mode of operation of the American Optical Co. demand pacemaker. t ceased function only at low frequencies, which are rare. At higher frequencies it was almost unaffected except in a narrow frequency range; there it lost its demand function and returned to a nondemand, fixed-rate function. This unit seemed to combine greatest resistance to interference with the most satisfactory variety of response when affected by such interference. SUMMARY Ten cardiac pacemakers, triggered and untriggered, were tested in vivo and on the bench for sensitivity to radio-frequency signals and intracorporeal 60-cycle alternating current. All untriggered units were insensitive in vivo except for those designed to be controlled by radiofrequency signals. Demand and synchronous units exhibited greater sensitivity and greater reversion to inactivity, fixed-rate operation, or maximum synchronous rate in the presence of interference. Two demand units were unaffected by the interfering signals in vivo. REFERENCES 1. Burchell, H. B. Hidden hazards of cardiac pacemakers. (Editorial) Circulation 24:161, Carleton, R. A., Sessions, R. W., and Graettinger, J. S. Environmental influence on implantable cardiac pacemakers. J.A.M.A. 190:938, Furman, S., Schwedel, J. B., Robinson, G., and Hurwitt, E. S. Use of an intracardiac pacemaker in the control of heart block. Surgery 49:98, Lichter,., Borrie, J., and Miller, W. M. Radio-frequency hazards with cardiac pacemakers. Brit. Med. J. 1:1513, Mansfield, P. B. On interference signals and pacemakers. Amer. J. Electronics First Quarter. Page 61, Noordjik, J. A., Oey, F. T.., and Tebra, W. Myocardial electrodes and the danger of ventricular fibrillation. Lancet 1 :975, Weinberg, J., Artley, J. A., Whalen, R. E., and Mcntosh, H. D. Electric shock hazards in cardiac catheterization. Circ. Res. 11: 1004, Whalen, R. E., Starmer, F., and Mcntosh, H. D. Electrical hazards associated with cardiac pacing. Ann. N.Y. Acad. Sci. 11:922, VOL. 6, NO. 1, JULY,