(Circulation. 1999;99:2559-2564.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
Cardiovascular Collapse Caused by Electrocardiographically Silent 60-Hz Intracardiac Leakage Current
Implications for Electrical Safety
From the Division of Cardiology, Cedars-Sinai Medical Center, Los Angeles, Calif (C.D.S., D.M.G.); Medtronic Inc, Minneapolis, Minn (W.H.O., M.E.O.); the Joint Department of Biomedical Engineering at The University of Memphis and the University of Tennessee-Memphis, Memphis, Tenn (R.A.M.); and the Division of Cardiology, Harbor-UCLA Medical Center, Torrance, Calif (M.L.).
Correspondence to Charles D. Swerdlow, MD, 8635 W Third St, Suite 1190 W, Los Angeles, CA 90048. E-mail swerdlow{at}ucla.edu
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Abstract |
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Background—The national standard for safe 60-Hz intracardiac leakage current under a single-fault condition is 50 µA. This standard is intended to protect patients from alternating current (AC) at levels below the threshold for sensation, but the minimum unsafe level for AC in closed-chest humans is not known. To determine this value, we studied 40 patients at testing of implantable cardioverter-defibrillators using a programmable source of 60-Hz AC.
Methods and Results—We applied AC for 5-second test periods in
increasing strengths until ventricular fibrillation (VF)
was induced or 1 mA was reached. Two current paths were tested:
bipolar, between tip and ring electrodes of a right
ventricular pacing catheter, and unipolar, from tip to a
remote electrode. We observed a characteristic sequence of 3 responses
as AC was increased: (1) intermittent ventricular capture
with QRS morphology identical to pacing through the electrodes (minimum
value, 20 µA); (2) continuous capture at cycle length 282±88 ms
(minimum value, 32 µA); and (3) VF persisting after AC termination
(minimum value, 49 µA). Continuous capture caused loss of pulsatile
arterial pressure and cardiovascular
collapse (mean arterial pressure, 32±8 mm Hg) for
the duration of AC with no ECG evidence of AC stimulation. Thus, the
clinical picture was that of hypotensive ventricular
tachycardia (VT). The continuous-capture threshold was 
50
µA in 9 patients (22%) for bipolar AC and in 5 (12%) for unipolar
AC. All patients showed continuous capture over a wide range for both
bipolar AC (68±18 to 216±238 µA) and unipolar AC (84±27 to
278±226 µA).
Conclusions—Leakage current causes cardiovascular
collapse at levels below the VF threshold. Stimulation by silent AC
that is neither felt nor visible on the ECG presents as hypotensive
VT. In patients with intracardiac electrodes, leakage current less than
or equal to the present standard of 50 µA may cause VT or VF. The
safety standard for leakage current lasting 
5 seconds should be 20
µA. This standard should be based on the continuous-capture
threshold.
Key Words: electrical stimulation • fibrillation • tachyarrhythmias
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Introduction |
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Power-line–operated electromedical equipment, connected to patients for monitoring or therapeutic purposes, may permit accidental flow (leakage) of weak alternating current (AC) through patients to ground. An intracardiac catheter may provide a low-resistance path to ground through the patient's heart and thereby place the patient at risk for electrically induced ventricular tachycardia (VT) or ventricular fibrillation (VF).
In 1993, the American national standard for leakage current through the heart under a single-fault condition in mains-operated electrical equipment was increased from 10 to 50 µA,1 the value of the European standard since 1988.2 Both the 10-µA standard3 4 and the 50-µA standard2 were based on estimates of the risk of AC-induced VF. However, AC may cause cardiovascular collapse at levels that are below the VF threshold.5 6 7 8 9 This adverse response to AC was not considered in the selection of either safety standard. Furthermore, safe levels of AC have not been determined in closed-chest humans.
The 10-µA standard was adopted in 1967 to ensure patient safety during cardiac catheterization10 and pacemaker11 procedures. The annual number of invasive cardiac procedures in the United States has increased from <60 000 when the 10-µA standard was adopted to >3 million today. The potential number of adverse outcomes from leakage current has increased correspondingly.
Electromedical devices contain electrical isolation circuits and insulation to limit leakage current. Manufacturers continue to comply with the original 10-µA standard, but they may realize substantial cost savings by equipment designs that comply only with the newer 50-µA standard.12 However, the American Heart Association continues to recommend the 10-µA standard.13 14 15 We therefore determined the minimum unsafe levels for AC in closed-chest humans.
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Methods |
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Patients
We studied 40 patients under propofol anesthesia at postoperative testing of transvenous implantable defibrillators. All patients gave written informed consent according to a protocol approved by the Human Subjects Committee of Cedars–Sinai Medical Center. Table 1
shows clinical
characteristics of the study patients. At the time of study, all
antiarrhythmic drugs had been discontinued for 5 half-lives, except for
digoxin (3 patients) and ß-blockers (7 patients). The 3 patients in
whom atrial fibrillation was the indication for a dual-chamber
implantable defibrillator had neither spontaneous nor inducible VT
or VF.
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Study Procedure
Radial arterial pressure was recorded
continuously. We gave AC through a standard 6F temporary pacing
catheter with a distal electrode area of 15
mm2 and ring-electrode area of 7
mm2 (Explorer, Boston Scientific). It was
inserted via the right femoral vein and positioned with its tip near
the right ventricular apex. AC was delivered from a
battery-operated, custom-built, constant-current source (Medtronic
model 2917 60-Hz fibrillator, IDE No. G970011) that permitted us to
program the duration and strength of AC. We measured the
root-mean-square value of delivered AC using a digital ammeter (model
87 True RMS Multimeter, Fluke Inc). Fixed-duration AC was applied in
increasing strengths until sustained VT or VF was induced or the
maximum output of 999 µA was reached. Sustained VT was defined as VT
requiring termination by cardioversion or pacing. Sustained VF was
defined as VF requiring defibrillation. We recorded the 12-lead ECG
and right ventricular electrogram on optical disk using a
multichannel, electrophysiology data–acquisition system (Cardiolab 3.1
or 4.0, Prucka, Inc). We also recorded single-channel ECGs on a
monitor designed for intraoperative or intensive care unit use (model
M1094A or M1094B, Hewlett Packard) and on the monitor of an external
defibrillator (model PD1200, Zoll Medical). The latter 2 monitors meet
American Heart Association guidelines for 60-Hz notch
filters4 on ECG equipment; the former does not.
Threshold Measurement
The initial programmed strength of AC was 40 µA. We increased
it by 10-µA increments up to 200 µA, by 50 µA between 250 and 600
µA, and by 100 µA thereafter. A characteristic sequence of 3
responses occurred as AC was increased (Figure 1): (1) intermittent
ventricular capture with QRS morphology identical to pacing
through the electrodes; (2) continuous ventricular capture
with the same QRS morphology; and (3) initial continuous capture
progressing to sustained VT or VF that persisted after termination of
AC. We defined continuous capture as a consecutive series of stimulated
QRS complexes without intervening spontaneous QRS complexes for the
duration of AC.
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AC was increased if intermittent capture was not stimulated by a level of 40 µA. If intermittent capture was stimulated, this threshold procedure was performed: AC was decreased by 2 levels and was applied up to 5 times at this and the next-higher level unless intermittent capture was induced. This process was iterated until 5 consecutive AC applications at a given level did not result in intermittent capture. The weakest AC strength that stimulated any QRS complexes was defined as the intermittent capture threshold. We then repeated the threshold process until we determined the continuous-capture threshold and the VF threshold or until we reached the maximum output of the fibrillator. If AC induced sustained VT, the sustained-VT threshold was accepted as an end point, and the VF threshold was not determined. We defined the sustained-VT/VF threshold as the weakest current that induced either sustained VT or VF. No threshold procedure required >1 decrease (step down) in AC. There was a 15-second rest period after each application of AC that produced continuous capture and a 4-minute rest after each episode of sustained VT or VF.
Study Protocols
In all patients, we applied AC for 5-second test intervals over
2 current paths in random order: bipolar, between tip and ring
electrodes; and unipolar, from tip to an ECG patch electrode positioned
on the skin near the left or right shoulder. In the first 20 patients,
we tested reproducibility of the continuous-capture and VF thresholds.
Unipolar and bipolar thresholds were determined twice. Randomization
for the first and second trials was done independently. In the final 20
patients, we determined a strength-duration relationship for AC. In
addition to determining thresholds for unipolar and bipolar AC of
5-second duration, we determined thresholds for bipolar AC synchronized
to the QRS complex with durations of 0.5 and 1 second. The order of
testing was determined randomly. We recorded only the capture
threshold and sustained-VT/VF threshold for 0.5- and 1-second
applications because intermittent and continuous capture could not be
distinguished.
Statistical Analysis
We compared bipolar and unipolar thresholds for intermittent
capture, continuous capture, and sustained VT/VF using paired
t tests. We analyzed possible correlations between
selected clinical variables and threshold values. We
analyzed prior myocardial infarction, left
ventricular ejection fraction, and heart failure class to
test the hypothesis that thresholds are lower for patients who have
more advanced heart disease. We performed correlations between
thresholds and the clinical variables of body-surface area, age,
and sex to test the null hypothesis.
To analyze reproducibility, we used the Lin concordance coefficient16 between first and second determinations of each threshold value in the first 20 patients. We used a previously described17 t test based on absolute differences normalized by median value to determine whether continuous-capture threshold or VF threshold was more reproducible.
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Results |
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Table 2
shows that group mean values
for intermittent-capture, continuous-capture, and sustained-VT/VF
thresholds were lower for bipolar AC than for unipolar AC
(P0.001).
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Cardiovascular Collapse Caused by Continuous
Capture
Continuous capture caused loss of pulsatile arterial
pressure and cardiovascular collapse for the duration
of AC (Figure 2). Mean
arterial pressure fell from 80±14 mm Hg before AC to
32±8 mm Hg at the end of AC. Figure 3A is a percentile plot of the
continuous-capture threshold for 5-second applications of AC. Table 2
shows minimum, mean, median, and maximum values. The
ventricular cycle length at the continuous-capture
threshold was not significantly different for unipolar AC versus
bipolar AC (285±92 versus 274±101 ms; P=0.45).
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Relation of Pacing Threshold to Continuous-Capture
Threshold
The pacing threshold for single 0.5-ms, constant-voltage pulses
was not significantly different for unipolar pulses versus bipolar
pulses (1.01±0.89 versus 0.96±0.38 mA; P=0.65), but these
values were higher than the corresponding continuous-capture thresholds
(P<0.001). The ratio of the pacing threshold for single
0.5-ms pulses to the continuous-capture threshold was 13±13 for
unipolar AC and 15±7 for bipolar AC.
ECG Findings at Continuous Capture
At the bipolar continuous-capture threshold, AC interference could
not be detected on any of the 3 monitors in any patient. At the
unipolar continuous-capture threshold, AC interference 0.5 mV was
recorded in 4 patients (10%) on the
electrophysiological recording
system in leads I, aVR, or aVL. Six other patients (15%) had subtle
thickening of the baseline <0.5 mV. With the other 2 monitors, AC
interference could not be detected for either unipolar or bipolar AC
500 µA. Thus, the typical ECG appearance of continuous capture was
that of VT with morphology identical to pacing through the electrode
catheter but no AC interference.
Sustained VT/VF
Sustained VT or VF was induced in 34 patients (85%) for bipolar
AC and in 32 patients (80%) for unipolar AC. Figure 3B is a
percentile plot of sustained-VT/VF thresholds. Table 2 shows
minimum, mean, median, and maximum values. The end-point
arrhythmia (VT or VF) was the same for bipolar and unipolar AC
in all patients. It was VF in 25 patients and sustained monomorphic VT
(cycle length, 274±51 ms) in 9. Both the group mean threshold and the
lowest individual patient threshold were lower for bipolar AC than
unipolar AC. The continuous-capture and VF thresholds were equal for 2
patients with bipolar AC and for 1 with unipolar AC.
Ranges of Intermittent and Continuous Capture
Intermittent capture occurred over a narrow range (14±13 µA for
bipolar AC versus 22±21 µA for unipolar AC; P<0.02).
Continuous capture occurred over a wider range than intermittent
capture (P<0.001). In patients in whom sustained VT/VF was
induced, the range of continuous capture was less for bipolar AC than
for unipolar AC (146±236 versus 193±222 µA; P=0.001). In
those patients in whom sustained VT/VF could not be induced, the
difference between the maximum output of 999 µA and the
continuous-capture threshold (bipolar, 60±7 µA [n=6]; unipolar,
77±18 µA [n=8]) provides a lower limit for this range.
Clinical Correlates of Threshold Values
None of the clinical variables evaluated correlated with the
continuous-capture threshold or the VF threshold for unipolar or
bipolar AC. The patient with the lowest continuous-capture threshold
(32 µA) was a 60-year-old woman who had hypertensive
cardiovascular disease, normal coronary
arteries, and a left ventricular ejection fraction of 60%.
The patient with the lowest VF threshold (49 µA) was a 72-year-old
man who had valvular heart disease and an ejection
fraction of 20%. There were too few patients without spontaneous or
inducible ventricular arrhythmias to provide a
meaningful analysis of this subgroup. Of these patients, the
lowest continuous-capture thresholds occurred in a 73 year-old man who
had coronary artery disease, paroxysmal atrial fibrillation,
and a left ventricular ejection fraction of 58%. These
values were 50 µA for bipolar AC and 60 µA for unipolar AC.
Reproducibility
Figure 4 shows reproducibility of
the continuous-capture and sustained-VT/VF thresholds in the first 20
patients. The continuous-capture threshold was more reproducible than
the sustained-VT/VF threshold for both unipolar AC
(P<0.005) and bipolar AC (P<0.001).
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) measures closeness of points to line of
identity. Concordance coefficients were higher for continuous-capture
thresholds than for sustained-VT/VF thresholds, both for unipolar AC
(
Strength-Duration Relation
In the final 20 patients, there were no significant differences
among the capture threshold for 0.5 second of AC (68±18 µA), the
capture threshold for 1 second of AC (67±18 µA), and the
continuous-capture threshold for 5 seconds of AC (66±17 µA). In
contrast, we induced sustained VT/VF in none of the tested patients
when we gave 0.5 second of AC, in 10 patients when we gave 1 second of
AC (50%), and in 16 patients when we gave 5 seconds of AC (80%)
(P<0.001). In those 10 patients in whom the sustained-VT/VF
threshold for 1 second of AC could be measured, the value was 474±255
versus 217±254 µA for 5 seconds (P<0.001). Figure 5 shows an example. The lowest VF
threshold for 1 second of AC was 140 µA.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The present study is the first report of the effects of AC applied directly to the heart in closed-chest humans. We found that continuous capture by AC causes cardiovascular collapse over a wide range of AC at levels that are below the VF threshold and in some patients, below the present national standard for a single-fault condition of 50 µA. At this 50 µA-level, AC causes no interference on the ECG, and continuous capture is thus indistinguishable from hypotensive VT. Leakage current
50 µA may be
a cause of VT or VF in patients with intracardiac electrodes.
Cardiovascular Collapse Caused by Continuous
Capture
Prior Studies
Green et al5 first described a sequence of 3
responses to increasing AC that corresponds to the sequence of
intermittent capture, continuous capture, and electrically induced VF
observed in the present study. They and other early investigators
were unable to record the ECG during AC because of inadequate
filtering. They recognized continuous capture by its
hemodynamic consequences.5 6 8 9
Subsequent investigators who used filtered ECGs described continuous
capture as "rapid, ineffectual VT"6 and "runs of
ectopic beats."18 In canines, continuous capture for 3
to 5 minutes always resulted in death.8
Mechanism
The continuous-capture threshold for AC is substantially below the
capture threshold for a single pacing stimulus. This observation
suggests that continuous capture at low levels of AC requires a
cumulative or summation effect of subthreshold
stimuli.19 20 21
Significance
As a basis for safety standards, the continuous-capture threshold
is superior to the VF threshold for the following reasons: (1) it
defines the minimum unsafe level of AC lasting 5 seconds; (2)
continuous capture results in cardiovascular collapse
over a wide range of AC below the VF threshold; (3) the continuous
capture threshold can be determined without induction of VF in the vast
majority of patients; (4) the continuous-capture threshold is
independent of the duration of AC, whereas the VF threshold is strongly
dependent on duration; and (5) the continuous-capture threshold is more
reproducible and behaves more like a step function than the VF
threshold.
Factors That Influence Thresholds
Electrode size22 23 and location9 23
have been reported to influence the VF threshold in animal studies. To
hold these factors constant, we placed a temporary electrode near the
right ventricular apex, the most common clinical location.
However, some clinically used electrodes have smaller surface areas
than the electrodes used in the present study. They would be
expected to have correspondingly lower continuous-capture and VF
thresholds.22 23
Our study is the first to compare unipolar and bipolar thresholds for AC. We found that the group mean thresholds for continuous capture and VF were lower for bipolar AC than for unipolar AC. Bipolar AC may produce higher intramyocardial current density8 24 or cause unipolar stimulation from the ring electrode, which has a smaller surface area than the tip electrode. The common clinical path for leakage current is unipolar.
Our study is also the first to examine the strength-duration relationship for AC in humans. For AC durations between 0.5 and 5 seconds, we found that the VF threshold decreases but the capture threshold remains constant. The duration of leakage current may depend on the cause: short for a power-line surge, intermediate for current induced by electromagnetic interference from nearby equipment, or long for a broken ground wire.
Limitations
We determined the minimum unsafe value of AC, not the maximum safe
value. We studied patients under propofol anesthesia;
thresholds might differ in conscious patients. Most patients in this
study had ventricular arrhythmias and structural
heart disease. However, patients with cardiac disease are most likely
to undergo invasive cardiac procedures and thus to be exposed to
intracardiac AC.
Clinical Implications
The principal clinical implication of our study is that AC causes
cardiovascular collapse in closed-chest humans at
levels substantially below the VF threshold.
A second implication is that the physician cannot rely on the ECG to
distinguish continuous capture by AC from spontaneous VT. American
Heart Association guidelines require 60-Hz notch filters on ECG
equipment to suppress AC interference,4 and our data show
that intracardiac AC at 50 µA causes no such interference. Thus, the
clinical presentation of continuous capture by AC 50 µA
is electrocardiographically silent and indistinguishable from
hypotensive VT. Transient continuous capture may be misdiagnosed as
nonsustained VT. This spurious diagnosis may lead to unnecessary
diagnostic procedures, including costly and invasive
electrophysiological studies. Sustained
flow of intracardiac leakage current could present as VT refractory
to cardioversion. In this situation, the patient's survival depends on
rapid interruption of the leakage-current circuit. However, a
responsible physician would probably treat the patient unsuccessfully
with the sequence of electrical cardioversions and antiarrhythmic drugs
recommended for VT. In a patient with an intracardiac catheter, leakage
current should be considered a new mechanism in the differential
diagnosis of VT.
A third implication is that under certain circumstances, routine methods would fail to detect leakage-current–induced VF, resulting in sudden cardiac death.14 AC at 50 µA is below the threshold of cutaneous sensation25 and could thus be conducted through an unsuspecting device operator to an unsuspecting patient.
A fourth implication is that safety standards may consider the duration
of AC and, by implication, its cause. Under transient conditions 1
second, such as power-line surges, the 50-µA standard may be
safe.
A final implication is that the 50-µA standard is insufficient to
protect patients with intracardiac electrodes from VT or VF caused by
leakage current lasting 5 seconds. Our results indicate that the
safety standard for AC lasting 5 seconds must be based on the
continuous-capture threshold. In the present study, the maximum
value that did not cause cardiovascular collapse in any
patient was 20 µA.
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Acknowledgments |
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This work was done during the tenure of a national Grant-in-Aid from the American Heart Association and the Council on Clinical Cardiology to Dr Swerdlow.
Received October 26, 1998; revision received February 8, 1999; accepted February 12, 1999.
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