Effect of first-phase polarity of biphasic shocks on defibrillation threshold with single transvenous lead system

Objectives. The purpose of this study was to determine whether the polarity of the first phase of biphasic shock affects the defibrillation threshold. Background. The polarity of monophasic shock has been shown to affect the defibrillation threshold. Methods. transvenous defibrillation lead with distal and proximal shocking electrodes was used in this study. In 15 consecutive patients, the defibrillation threshold was determined twice using step-down protocol, in random order: with the distal coil as the anode for the initial phase (anodal biphasic shock) and with the polarity reversed (cathodal biphasic shock). The power to detect 5.0-J difference in this study is 0.96. These patients were 61 ± 11 years old (mean ± SD), and the mean left ventricular ejection fraction was 0.32 ± 0.10. Results. Mean defibrillation threshold using anodal biphasic shocks was 9.9 ± 4.8 J, compared with 9.5 ± 4.2 J using cathodal biphasic shocks (p = 0.8). In three patients the defibrillation threshold was lower by mean of 6.3 ± 2.9 J with the former configuration; in three patients the defibrillation threshold was lower by mean of 6.7 ± 2.5 J with the latter configuration; and in nine patients it was the same. Using the standard cathodal configuration, defibrillation threshold less-than-or-equals, slant10 J was obtained in to 70% of patients, and subcutaneous patch was not required in any patient. Conclusions. The polarity of the first phase of biphasic shock used with single transvenous lead does not affect the defibrillation threshold.