EFFECT OF SHOCK POLARITY ON VENTRICULAR DEFIBRILLATION THRESHOLD USING A TRANSVENOUS LEAD SYSTEM

Objectives. The purpose of this study was to determine whether the polarity of a monophasic shock used with a transvenous lead system affects the defibrillation threshold. Background. The ability to implant an automatic defibrillator depends on achieving an adequate defibrillation threshold. Methods. A transvenous defibrillation lead with distal and proximal shocking electrodes was used in this study. In 29 consecutive patients, the defibrillation threshold, using a step down protocol was determined twice in random order: 1) with the distal coil as the anode, and 2) with the polarity reversed. Only the 20 patients in whom an adequate defibrillation threshold could be obtained with the transvenous lead alone were included in this study. These patients were 61 +/- 14 years old (mean +/- SD) and had a mean ejection fraction of 28 +/- 12%. Results. The mean defibrillation threshold was 11.5 +/- 5.0 J with the distal coil as the anode versus 16.9 +/- 7.7 J with the distal coil as the cathode (p = 0.04). The defibrillation threshold was lower by a mean of 9 +/- 7 J with the former configuration in 14 patients and was lower by a mean of 7 +/- 6 J with the latter configuration in 3 patients; in 3 patients it was the same with both configurations. Use of a subcutaneous patch was avoided in five patients by utilizing the distal electrode as the anode. Conclusions. Defibrillation thresholds with monophasic shocks are similar to 30% lower with the distal electrode as the anode. The use of anodal shocks may obviate the need for a subcutaneous patch and allow more frequent implantation of a transvenous lead system.