Swerdlow C D, Olson W H, O'Connor M E, Gallik D M, Malkin R A, Laks M
Division of Cardiology, Cedars-Sinai Medical Center, Los Angeles, CA, USA.
Circulation. 1999 May 18;99(19):2559-64. doi: 10.1161/01.cir.99.19.2559.
The national standard for safe 60-Hz intracardiac leakage current under a single-fault condition is 50 microA. 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.
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 microA); (2) continuous capture at cycle length 282+/-88 ms (minimum value, 32 microA); and (3) VF persisting after AC termination (minimum value, 49 microA). 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 microA 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 microA) and unipolar AC (84+/-27 to 278+/-226 microA).
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 microA may cause VT or VF. The safety standard for leakage current lasting >/=5 seconds should be </=20 microA. This standard should be based on the continuous-capture threshold.
单故障条件下60赫兹心内泄漏电流的国家标准是50微安。该标准旨在保护患者免受低于感觉阈值水平的交流电（AC）影响，但封闭胸腔人体中交流电的最低不安全水平尚不清楚。为了确定该值，我们使用可编程60赫兹交流电源对40例植入式心脏复律除颤器患者进行了研究。
我们以递增强度施加5秒的交流电测试期，直至诱发心室颤动（VF）或达到1毫安。测试了两条电流路径：双极路径，在右心室起搏导管的尖端和环状电极之间；单极路径，从尖端到远端电极。随着交流电强度增加，我们观察到3种反应的特征序列：（1）间歇性心室夺获，QRS形态与通过电极起搏时相同（最小值，20微安）；（2）在周期长度282±88毫秒时持续夺获（最小值，32微安）；（3）交流电终止后VF持续存在（最小值，49微安）。持续夺获导致在交流电持续期间搏动性动脉压丧失和心血管虚脱（平均动脉压，32±8毫米汞柱），心电图无交流电刺激证据。因此，临床表现为低血压性室性心动过速（VT）。双极交流电时，9例患者（22%）的持续夺获阈值≤50微安；单极交流电时，5例患者（12%）的持续夺获阈值≤50微安。所有患者在双极交流电（68±18至216±238微安）和单极交流电（84±27至278±226微安）的较宽范围内均表现出持续夺获。
泄漏电流在低于VF阈值的水平时会导致心血管虚脱。心电图上既无感觉也不可见的无声交流电刺激表现为低血压性VT。对于有心内电极的患者，小于或等于当前50微安标准的泄漏电流可能会导致VT或VF。持续时间≥5秒的泄漏电流安全标准应≤20微安。该标准应基于持续夺获阈值。
