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The arrhythmogenic effect of static electricity on the dog heart
The arrhythmogenic effect of static electricity on the dog heart*t Richard J. McCarty, M.D., F.A.C.C., Colonel (MC) USA** Stephen P. Glasser, M.D., F.A.C.C.*** El Paso, Texas
The explosive growth of electronic gadgetry in medicine has been accompanied by ever-increasing electrical hazards to patients and staff. The existence of such hazards has been known for years, but only recently has there been attention focused on this problem at the bedside level. An ad hoc committee, appointed by the Intersociety Commission, investigated the problem and made recommendations for electrical safety encompassing the entire electronic environment in critical care areas. 1 Improved manufacturing standards, continuing programs of medical maintenance, and intensive education of personnel involved with electronic equipment were stressed. In this report, 1 the potential hazard of static electricity was mentioned. I t was stated that static charges could induce fatal arrhythmias, ~ but no firm evidence was presented to establish this claim. Nonetheless, the recommendation was made that carpet not be placed in critical care areas in order to minimize static charges. In an effort to substantiate the validity of this recommendation, we undertook an investigation of the cardiac effects of static electricity. Our results demonstrated that cardiac rhythms could be altered in everyday situations and provided evidence which supports the committee's recommendations. From the Cardiology Service, William Beaumont Army Medical Center, El Paso, Texas.
Received for publication March 4, 1976. Reprint requests: Technical Publications Editor, Letterman Army Medical Center, Presidio of San Francisco, Calif. 94129. *Presented at the National Meeting, Association for Advancement in Medical Instrumentation, Washington, D. C., March, 1973. **Current address: Box 601, Letterman Army Medical Center, Presidio of San Francisco, CA 94129. ***Current address: Louisiana State University Medical Center, Shreveport, La. 71130. tThe opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.
496
Material and methods
Seven mongrel dogs, weighing between 8 to 15 kilograms, were anesthetized with pentothal and maintained on continuous fluothane anesthesia with respiratory support.* A No. 5 bipolar pacing catheter was placed in the right ventricle of the dogs through a cutdown over the left jugular vein. Each animal was continuously monitored electrocardiographicalty. Catheter position indicating good endocardial contact was verified by an injury current recorded from the distal electrode. A Medtronics portable pacing unit was used to establish the pacing threshold. All dogs could be paced at less than 1 mamp. (which was calculated to represent approximately one microjoule of delivered energy). Static electrical charges were then generated by shuffling our shoed feet over synthetic or wool carpet. Static charges were discharged from the fingertips and measured through a 50.1 megohm resistor into a Tektronix storagescope (Type 564B, with 3A6 dual trace vertical amp. and 3B3 time base) (Fig. 1). More than one spike, i.e., "double static discharge" (Fig. 2 A, and B), was induced occasionally by incomplete initial contact with the catheter tip. Similar static charges were then applied directly on the external end of the pacemaker wire leading to the distal electrode within the right ventricle. The effect of the current on the heart rhythm was continuously monitored on the surface electrocardiograph (ECG). By repetitive trial, charges were introduced during all phases of the cardiac cycle. This procedure was performed in four normal dogs; and the experimental plan was repeated in three dogs in whom acute myocardial infarction was induced by open chest ligation of the circum*The research described in this report involved animals maintained in animal care facilities fully accredited by the American Association for Accreditation of Laboratory Animal Care.
April, 1977, Vol. 93, No. 4, pp. 496-500
Arrhythmogenic effect of static electricity
secs (Msec)
Fig. 1. Represents the wave form of a typical static discharge with notable nnging in evidence. There were 12.5 KV generated and a spark was noted at the moment of discharge. Calibration factors are 2.5 KV per division vertically and 20 msec. per division horizontally. ,
,
_
. . . . . . . . . . .
~
Fig. 2A. The waveform of a double Static discharge illustrates the time between the first and the second discharges. The first discharge represents approximately 2 KV and the second discharge approximately 1 KV. Neither of these were perceptible to the generating person. The double discharge resulted from incomplete contact when the pacing catheter was first touched followed in 80 msec. by completion of the discharge.
Fig. 2B. A double static discharge. Note two spikes under the arrow 0.08 second apart. The first spike occurs in the relative refractory period and does not pace. The second spike, however, results in a paced beat.
American Heart Journal
497
McCarty and Glasser
measured on a calibrated Simpson meter at 10,000 to 50,000 ohms. The amount of delivered energy through this lower resistance pathway within the dogs was considered identical to that measured through the 50.1 megohm resistor into the storage scope. The effect of static charges on the heart rhythm of the normal dogs. Static discharges
exceeding 2,000 volts uniformly resulted in paced beats, unless delivered in the absolute refractory period (Fig. 3). Ventricular beats were often initiated by discharges below the level of perceptible shock {Fig. 3, dog No. 7). In fact, a single gliding movement of one's foot across the rug generated a sufficient charge to pace the heart. This allowed repetitive pacing at rates nearing 60/min. (Fig. 4). Powerful shocks delivered in the vulnerable period of the cardiac cycle did not produce repetitive tachyarrhythmias (Fig. 5). The effect of static charges on heart rhythm of dogs with acute myocardial infarction. The
Fig. 3. Static discharges above 2 KV (the approximate level of perception to the generating person) uniformly resulted in paced beats unless delivered in the absolute refractory period. Note, however, that a paced beat also occurred in Dog No. 7 when no sensation (NS) was felt by the generating person. This was not an uncommon occurrence. The KV of each impulse illustrated represents an estimate.
flex branch of the left coronary artery. Static charges were also delivered directly into the infarcted area by a metal needle. Results Quantification of electrical energy, The voltage of static charges reached 22,000 volts. The
charges varied with the shoes* worn, type of rug, duration of shuffling, and the d a y - p r e s u m a b l y related to temperature, humidity, etc. The perceptible level of shock to the finger was established at two to three thousand volts. The energy delivered, as measured through the 50.1 megohm resistor, was calculated at 4.3 millijoules (4300 microjoules) for an 8,000 volt discharge. Resistance of the pacing wire and intervening tissues from the right ventricle to the ground lead were *Shoes had to be a type which would create measurable friction; foot coverings such as operating room "booties" did not generate enough energy to quantitate.
498
results obtained in the first two dogs were essentially the same results that were obtained in the normal dog. In the third dogs, however, a static discharge approximating 6,000 volts clearly initiated ventricular fibrillation (VF) (Fig. 6). This appears to have resulted from a double discharge in which current was repetitively delivered after the initial discharge had produced a ventricular premature beat (VPB). This secondary shock occurred within the vulnerable period of the VPB (note the deformity of the terminal part of the T wave) and initiated VF. Electrical countershock returned the rhythm to sinus. At no time before nor immediately after were any spontaneous premature beats evident. Placement of the static charge directly into the area of infarction by a metal needle, resulted in no pacemaker activity whatsoever. Discussion
It has been established that electrical energy of minute proportions (1 microjoule)may result in cardiac pacemaker function/ We established that the single gliding movement of one's foot across the rug generated a sufficient charge to pace the heart and, in event of battery failure, repetitive gliding could conceivably serve as an energy source. Alternating current as small as 20 microamps for periods of 2 seconds, representing 4,400 microjoules of energy has resulted in VF in dogs2 In
April, 1977, Vol. 93, No. 4
Arrhythmogenic effect of static electricity
Fig. 4. The arrows indicate static discharges recorded as spikes on the surface ECG. They occur at approximately 60 times per minute. No pacing occurs when the spikes fall in the refractory period of normally conducted beats. contrast, direct currents of 35 mamp. for only 2 msec. representing 1,000 micro joules, has been the average a m o u n t of energy required to produce VF when delivered during the vulnerable period. 5 In diseased hearts the fibrillatory threshold is significantly lower. 6 Indeed, VF has been initiated by the m i n u t e energy of a b a t t e r y - o p e r a t e d pacem a k e r when delivered within the vulnerable period. 7 In this instance, assuming a m a x i m u m 9 volt b a t t e r y output, the actual energy initiating VF would have been 36 microjoules. It is not surprising therefore, t h a t the energy levels generated by static charges (in the range of several t h o u s a n d microjoules) can result in pacem a k e r function and VF. One might wonder why VF was not produced more regularly when these levels of energy were delivered within the vulnerable period. T h e probable explanation revolves a r o u n d the short d u r a t i o n of peak energy in the decaying wave form of static discharges. This contrasts sharply with the longer square wave form of direct current. One explanation for the occurrence of VF in one dog and not in a n y of the o t h e r dogs, in which similar charges were delivered well within the vulnerable period, relates to the lowered fibrillat o r y threshold of early VPBs. A prime determin a n t of fibrillatory threshold is the t e m p o r a l dispersion of recovery of excitability, which is greater in early p r e m a t u r e beats t h a n in beats originating in fully excitable tissue. 8 F u r t h e r more, it has been established t h a t the effect of p r e m a t u r i t y on the fibrillation threshold is greatest at points n e a r the sight of origin of the p r e m a t u r e response. 9 In our case, the discharge giving rise to the V P B and the subsequent seco n d a r y discharge were delivered at the identical site, accounting for a substantial reduction in the fibrillatory threshold.
American Heart Journal
Fig. 5. Static discharges in the range of 5 to 12 KV, even when delivered in the vulnerable period, did not produce repetitive tachyarrhythmias in any of the normal animals.
Summary Seven dogs, three with surgically-induced acute myocardial infarction, were subjected to the effects of static electricity discharged from our fingertips onto the e x t e r n a l end of a transvenous right ventricular pacemaker. Initiation of ventricular beats occurred regularly when shocks were perceptible to the generating person and, not infrequently, when energy levels were below perception. In one dog with myocardial infarction, ventricular fibrillation was clearly related to the static discharge. T h e d a t a obtained in this s t u d y s u p p o r t the r e c o m m e n d a t i o n s of the Electrical Safety H a z a r d
499
McCarty and Glasser
Fig. 6. Static discharges approximating 6 KV were generated resulting in VPBs. On the third occasion, this was followed by ventricular tachycardia which rapidly degenerated into VF. The explanation we feel lies in the "double discharge" (~eeFigs. 2A and B) which is manifest in the T wave of the paced beat (compare the T wave of the third paced beat with that of the other two). C o m m i t t e e t h a t c r i t i c a l care a r e a s n o t be carp e t e d i n o r d e r t o m i n i m i z e s t a t i c charges. I n a d d i t i o n to low s t a t i c floor c o v e r i n g i n t h e s e areas, o n e c a n r e d u c e s t a t i c c h a r g e b y t h e u s e of f o o t c o v e r i n g s s u c h as t h o s e w o r n i n t h e o p e r a t i n g r o o m . W e f o u n d t h a t n o s t a t i c c h a r g e c o u l d be developed when wearing the expandable operati n g r o o m " b o o t i e s . " S i n c e s t a t i c c h a r g e s were t r a n s m i t t e d t h r o u g h o u r fingertips, p r o b a b l y t h e b e s t m e a n s of p r o t e c t i n g t h e e l e c t r i c a l l y s e n s i t i v e p a t i e n t a g a i n s t t h e h a z a r d of s t a t i c e l e c t r i c i t y is t o i n s t r u c t p e r s o n n e l to w e a r r u b b e r gloves w h e n ever d i r e c t c o n t a c t w i t h t h e e x t e r n a l e n d s of a pacing catheter becomes necessary. Routinely, s u c h e x t e r i o r i z e d p a c e m a k e r e n d s s h o u l d also be covered with a n o n c o n d u c t i v e substance. We would like to thank the personnel of the Department of Medical Research and Development, Veterinary Medicine Service, and Medical Maintentance Branch for their cooperation and enthusiastic support. In particular, Dr. Lee Chenault, V.C,, Mr. Charles Salsman, Mr. Clinton Winstead, and Mr. Richard Costa are to be commended.
500
REFERENCES
1. Electronic equipment in critical care areas, Report of the Intersociety Commission for Heart Disease Resources, Circulation 44:A-237, 1971. 2. Walter, C. H. (ed.): Electrical hazards in hospitals, Washington, D.C., 1970, National Academy of Sciences. 3. Lown, B., Klein, M. D., and Hershberg, P. I.: Coronary and precoronary care, Am. J. Med. 46:705, 1969. 4. Whalen, R. E., Starmer, F., and McIntosh, H. D.: Electrical hazards associated with cardiac pacemaking. Ann. N.Y. Acad. Sci. 11 1:922, 1963-64. 5. Widman, W. D., Eisenberg, L., Levilsky, S., Mauro, A., and Glenn, W. W.: Ventricular fibrillation complicating electrical pacemaking; a comparison of direct current and radiofrequency cardiac pacemaker stimualtion, Surg. Forum 14:260, 1963. 6. Phibbs, C. M., Levy, L. M., and Maclean, L. D.: The influence of temperature and coronary occlusion on the ventricular fibrillation threshold, Surg. Gynecol. Obstet. 109:216, 1959. 7. Bilitch, M., Cosby, R. S., and Cafferky, E. A.: Ventricular fibrillation and competitive pacing, N. Engl. J. Med. 276:598, 1967. 8. Han, J., and Moe, G. K.: Nonuniform recovery of excitability in ventricular muscle, Circ. Res. 14:44, 1964. 9. H a n , J., DeJalon, G., and Moe, G. K.: Fibrillation threshold of Premature ventricular responses, Circ. Res. 18:18, 1966.
April, 1977, Vol. 93, No. 4
The explosive growth of electronic gadgetry in medicine has been accompanied by ever-increasing electrical hazards to patients and staff. The existence of such hazards has been known for years, but only recently has there been attention focused on this problem at the bedside level. An ad hoc committee, appointed by the Intersociety Commission, investigated the problem and made recommendations for electrical safety encompassing the entire electronic environment in critical care areas. 1 Improved manufacturing standards, continuing programs of medical maintenance, and intensive education of personnel involved with electronic equipment were stressed. In this report, 1 the potential hazard of static electricity was mentioned. I t was stated that static charges could induce fatal arrhythmias, ~ but no firm evidence was presented to establish this claim. Nonetheless, the recommendation was made that carpet not be placed in critical care areas in order to minimize static charges. In an effort to substantiate the validity of this recommendation, we undertook an investigation of the cardiac effects of static electricity. Our results demonstrated that cardiac rhythms could be altered in everyday situations and provided evidence which supports the committee's recommendations. From the Cardiology Service, William Beaumont Army Medical Center, El Paso, Texas.
Received for publication March 4, 1976. Reprint requests: Technical Publications Editor, Letterman Army Medical Center, Presidio of San Francisco, Calif. 94129. *Presented at the National Meeting, Association for Advancement in Medical Instrumentation, Washington, D. C., March, 1973. **Current address: Box 601, Letterman Army Medical Center, Presidio of San Francisco, CA 94129. ***Current address: Louisiana State University Medical Center, Shreveport, La. 71130. tThe opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.
496
Material and methods
Seven mongrel dogs, weighing between 8 to 15 kilograms, were anesthetized with pentothal and maintained on continuous fluothane anesthesia with respiratory support.* A No. 5 bipolar pacing catheter was placed in the right ventricle of the dogs through a cutdown over the left jugular vein. Each animal was continuously monitored electrocardiographicalty. Catheter position indicating good endocardial contact was verified by an injury current recorded from the distal electrode. A Medtronics portable pacing unit was used to establish the pacing threshold. All dogs could be paced at less than 1 mamp. (which was calculated to represent approximately one microjoule of delivered energy). Static electrical charges were then generated by shuffling our shoed feet over synthetic or wool carpet. Static charges were discharged from the fingertips and measured through a 50.1 megohm resistor into a Tektronix storagescope (Type 564B, with 3A6 dual trace vertical amp. and 3B3 time base) (Fig. 1). More than one spike, i.e., "double static discharge" (Fig. 2 A, and B), was induced occasionally by incomplete initial contact with the catheter tip. Similar static charges were then applied directly on the external end of the pacemaker wire leading to the distal electrode within the right ventricle. The effect of the current on the heart rhythm was continuously monitored on the surface electrocardiograph (ECG). By repetitive trial, charges were introduced during all phases of the cardiac cycle. This procedure was performed in four normal dogs; and the experimental plan was repeated in three dogs in whom acute myocardial infarction was induced by open chest ligation of the circum*The research described in this report involved animals maintained in animal care facilities fully accredited by the American Association for Accreditation of Laboratory Animal Care.
April, 1977, Vol. 93, No. 4, pp. 496-500
Arrhythmogenic effect of static electricity
secs (Msec)
Fig. 1. Represents the wave form of a typical static discharge with notable nnging in evidence. There were 12.5 KV generated and a spark was noted at the moment of discharge. Calibration factors are 2.5 KV per division vertically and 20 msec. per division horizontally. ,
,
_
. . . . . . . . . . .
~
Fig. 2A. The waveform of a double Static discharge illustrates the time between the first and the second discharges. The first discharge represents approximately 2 KV and the second discharge approximately 1 KV. Neither of these were perceptible to the generating person. The double discharge resulted from incomplete contact when the pacing catheter was first touched followed in 80 msec. by completion of the discharge.
Fig. 2B. A double static discharge. Note two spikes under the arrow 0.08 second apart. The first spike occurs in the relative refractory period and does not pace. The second spike, however, results in a paced beat.
American Heart Journal
497
McCarty and Glasser
measured on a calibrated Simpson meter at 10,000 to 50,000 ohms. The amount of delivered energy through this lower resistance pathway within the dogs was considered identical to that measured through the 50.1 megohm resistor into the storage scope. The effect of static charges on the heart rhythm of the normal dogs. Static discharges
exceeding 2,000 volts uniformly resulted in paced beats, unless delivered in the absolute refractory period (Fig. 3). Ventricular beats were often initiated by discharges below the level of perceptible shock {Fig. 3, dog No. 7). In fact, a single gliding movement of one's foot across the rug generated a sufficient charge to pace the heart. This allowed repetitive pacing at rates nearing 60/min. (Fig. 4). Powerful shocks delivered in the vulnerable period of the cardiac cycle did not produce repetitive tachyarrhythmias (Fig. 5). The effect of static charges on heart rhythm of dogs with acute myocardial infarction. The
Fig. 3. Static discharges above 2 KV (the approximate level of perception to the generating person) uniformly resulted in paced beats unless delivered in the absolute refractory period. Note, however, that a paced beat also occurred in Dog No. 7 when no sensation (NS) was felt by the generating person. This was not an uncommon occurrence. The KV of each impulse illustrated represents an estimate.
flex branch of the left coronary artery. Static charges were also delivered directly into the infarcted area by a metal needle. Results Quantification of electrical energy, The voltage of static charges reached 22,000 volts. The
charges varied with the shoes* worn, type of rug, duration of shuffling, and the d a y - p r e s u m a b l y related to temperature, humidity, etc. The perceptible level of shock to the finger was established at two to three thousand volts. The energy delivered, as measured through the 50.1 megohm resistor, was calculated at 4.3 millijoules (4300 microjoules) for an 8,000 volt discharge. Resistance of the pacing wire and intervening tissues from the right ventricle to the ground lead were *Shoes had to be a type which would create measurable friction; foot coverings such as operating room "booties" did not generate enough energy to quantitate.
498
results obtained in the first two dogs were essentially the same results that were obtained in the normal dog. In the third dogs, however, a static discharge approximating 6,000 volts clearly initiated ventricular fibrillation (VF) (Fig. 6). This appears to have resulted from a double discharge in which current was repetitively delivered after the initial discharge had produced a ventricular premature beat (VPB). This secondary shock occurred within the vulnerable period of the VPB (note the deformity of the terminal part of the T wave) and initiated VF. Electrical countershock returned the rhythm to sinus. At no time before nor immediately after were any spontaneous premature beats evident. Placement of the static charge directly into the area of infarction by a metal needle, resulted in no pacemaker activity whatsoever. Discussion
It has been established that electrical energy of minute proportions (1 microjoule)may result in cardiac pacemaker function/ We established that the single gliding movement of one's foot across the rug generated a sufficient charge to pace the heart and, in event of battery failure, repetitive gliding could conceivably serve as an energy source. Alternating current as small as 20 microamps for periods of 2 seconds, representing 4,400 microjoules of energy has resulted in VF in dogs2 In
April, 1977, Vol. 93, No. 4
Arrhythmogenic effect of static electricity
Fig. 4. The arrows indicate static discharges recorded as spikes on the surface ECG. They occur at approximately 60 times per minute. No pacing occurs when the spikes fall in the refractory period of normally conducted beats. contrast, direct currents of 35 mamp. for only 2 msec. representing 1,000 micro joules, has been the average a m o u n t of energy required to produce VF when delivered during the vulnerable period. 5 In diseased hearts the fibrillatory threshold is significantly lower. 6 Indeed, VF has been initiated by the m i n u t e energy of a b a t t e r y - o p e r a t e d pacem a k e r when delivered within the vulnerable period. 7 In this instance, assuming a m a x i m u m 9 volt b a t t e r y output, the actual energy initiating VF would have been 36 microjoules. It is not surprising therefore, t h a t the energy levels generated by static charges (in the range of several t h o u s a n d microjoules) can result in pacem a k e r function and VF. One might wonder why VF was not produced more regularly when these levels of energy were delivered within the vulnerable period. T h e probable explanation revolves a r o u n d the short d u r a t i o n of peak energy in the decaying wave form of static discharges. This contrasts sharply with the longer square wave form of direct current. One explanation for the occurrence of VF in one dog and not in a n y of the o t h e r dogs, in which similar charges were delivered well within the vulnerable period, relates to the lowered fibrillat o r y threshold of early VPBs. A prime determin a n t of fibrillatory threshold is the t e m p o r a l dispersion of recovery of excitability, which is greater in early p r e m a t u r e beats t h a n in beats originating in fully excitable tissue. 8 F u r t h e r more, it has been established t h a t the effect of p r e m a t u r i t y on the fibrillation threshold is greatest at points n e a r the sight of origin of the p r e m a t u r e response. 9 In our case, the discharge giving rise to the V P B and the subsequent seco n d a r y discharge were delivered at the identical site, accounting for a substantial reduction in the fibrillatory threshold.
American Heart Journal
Fig. 5. Static discharges in the range of 5 to 12 KV, even when delivered in the vulnerable period, did not produce repetitive tachyarrhythmias in any of the normal animals.
Summary Seven dogs, three with surgically-induced acute myocardial infarction, were subjected to the effects of static electricity discharged from our fingertips onto the e x t e r n a l end of a transvenous right ventricular pacemaker. Initiation of ventricular beats occurred regularly when shocks were perceptible to the generating person and, not infrequently, when energy levels were below perception. In one dog with myocardial infarction, ventricular fibrillation was clearly related to the static discharge. T h e d a t a obtained in this s t u d y s u p p o r t the r e c o m m e n d a t i o n s of the Electrical Safety H a z a r d
499
McCarty and Glasser
Fig. 6. Static discharges approximating 6 KV were generated resulting in VPBs. On the third occasion, this was followed by ventricular tachycardia which rapidly degenerated into VF. The explanation we feel lies in the "double discharge" (~eeFigs. 2A and B) which is manifest in the T wave of the paced beat (compare the T wave of the third paced beat with that of the other two). C o m m i t t e e t h a t c r i t i c a l care a r e a s n o t be carp e t e d i n o r d e r t o m i n i m i z e s t a t i c charges. I n a d d i t i o n to low s t a t i c floor c o v e r i n g i n t h e s e areas, o n e c a n r e d u c e s t a t i c c h a r g e b y t h e u s e of f o o t c o v e r i n g s s u c h as t h o s e w o r n i n t h e o p e r a t i n g r o o m . W e f o u n d t h a t n o s t a t i c c h a r g e c o u l d be developed when wearing the expandable operati n g r o o m " b o o t i e s . " S i n c e s t a t i c c h a r g e s were t r a n s m i t t e d t h r o u g h o u r fingertips, p r o b a b l y t h e b e s t m e a n s of p r o t e c t i n g t h e e l e c t r i c a l l y s e n s i t i v e p a t i e n t a g a i n s t t h e h a z a r d of s t a t i c e l e c t r i c i t y is t o i n s t r u c t p e r s o n n e l to w e a r r u b b e r gloves w h e n ever d i r e c t c o n t a c t w i t h t h e e x t e r n a l e n d s of a pacing catheter becomes necessary. Routinely, s u c h e x t e r i o r i z e d p a c e m a k e r e n d s s h o u l d also be covered with a n o n c o n d u c t i v e substance. We would like to thank the personnel of the Department of Medical Research and Development, Veterinary Medicine Service, and Medical Maintentance Branch for their cooperation and enthusiastic support. In particular, Dr. Lee Chenault, V.C,, Mr. Charles Salsman, Mr. Clinton Winstead, and Mr. Richard Costa are to be commended.
500
REFERENCES
1. Electronic equipment in critical care areas, Report of the Intersociety Commission for Heart Disease Resources, Circulation 44:A-237, 1971. 2. Walter, C. H. (ed.): Electrical hazards in hospitals, Washington, D.C., 1970, National Academy of Sciences. 3. Lown, B., Klein, M. D., and Hershberg, P. I.: Coronary and precoronary care, Am. J. Med. 46:705, 1969. 4. Whalen, R. E., Starmer, F., and McIntosh, H. D.: Electrical hazards associated with cardiac pacemaking. Ann. N.Y. Acad. Sci. 11 1:922, 1963-64. 5. Widman, W. D., Eisenberg, L., Levilsky, S., Mauro, A., and Glenn, W. W.: Ventricular fibrillation complicating electrical pacemaking; a comparison of direct current and radiofrequency cardiac pacemaker stimualtion, Surg. Forum 14:260, 1963. 6. Phibbs, C. M., Levy, L. M., and Maclean, L. D.: The influence of temperature and coronary occlusion on the ventricular fibrillation threshold, Surg. Gynecol. Obstet. 109:216, 1959. 7. Bilitch, M., Cosby, R. S., and Cafferky, E. A.: Ventricular fibrillation and competitive pacing, N. Engl. J. Med. 276:598, 1967. 8. Han, J., and Moe, G. K.: Nonuniform recovery of excitability in ventricular muscle, Circ. Res. 14:44, 1964. 9. H a n , J., DeJalon, G., and Moe, G. K.: Fibrillation threshold of Premature ventricular responses, Circ. Res. 18:18, 1966.
April, 1977, Vol. 93, No. 4