Cardiovascular Collapse Caused by Electrocardiographically Silent 60-Hz Intracardiac Leakage Current

Abstract

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.