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

OBJECTIVES

The purpose of this study was to determine whether the polarity of the first phase of a biphasic shock affects the defibrillation threshold.

BACKGROUND

The polarity of a monophasic shock has been shown to affect the defibrillation threshold.

METHODS

A 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 a 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 a 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 a mean of 6.3 +/- 2.9 J with the former configuration; in three patients the defibrillation threshold was lower by a mean of 6.7 +/- 2.5 J with the latter configuration; and in nine patients it was the same. Using the standard cathodal configuration, a defibrillation threshold < or = 10 J was obtained in approximately 70% of patients, and a subcutaneous patch was not required in any patient.

CONCLUSIONS

The polarity of the first phase of a biphasic shock used with a single transvenous lead does not affect the defibrillation threshold.