INCREASE IN DEFIBRILLATION THRESHOLD IN NON-THORACOTOMY IMPLANTABLE DEFIBRILLATORS USING A BIPHASIC WAVE-FORM

We have previously reported a chronic increase in defibrillation threshold in a non-thoracotomy implantable cardioverter-defibrillator (ICD) system using monophasic waveforms. To determine if this phenomenon is related to the lead system or the waveform used, we studied the chronic defibrillation threshold in consecutive patients receiving an ICD capable of delivering biphasic waveforms with the same lead system previously evaluated. Twenty-five patients received an ICD with biphasic shock waveform and have been followed for 4 to 15 months. All have undergone defibrillation threshold measurements using the identical testing protocol with biphasic waveforms at implant and at 2 months. Coronary artery disease was present in 15, idiopathic dilated cardiomyopathy in 9, and valvular heart disease in 1. The presenting arrhythmia was ventricular fibrillation in 11, ventricular tachycardia in 10, and syncope with inducible ventricular tachycardia in 4. The configuration of the shocking electrodes was randomized; the lead-only configuration was used in 14 patients (56%), and a subcutaneous patch was used in the remaining patients. Mean defibrillation threshold using a step-down technique was 9.8 +/- 1.0 J at implant, 13.2 +/- 1.6 J at 2 months, and 12.4 +/- 1.5 J at 6 months (p = 0.01 by analysis of variance). There was no change in clinical status, cardiac size, radiographic lead position, or impedance between implant and 2 months. These findings suggest the increase in defibrillation threshold in this ICD system is not related to the type of waveform used, but rather is a feature of non-thoracotomy as opposed to epicardial electrodes.