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

We have previously reported a chronic increase in defibrillation thres hold 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 chron ic defibrillation threshold in consecutive patients receiving an ICD c apable of delivering biphasic waveforms with the same lead system prev iously evaluated. Twenty-five patients received an ICD with biphasic s hock waveform and have been followed for 4 to 15 months. All have unde rgone defibrillation threshold measurements using the identical testin g protocol with biphasic waveforms at implant and at 2 months. Coronar y artery disease was present in 15, idiopathic dilated cardiomyopathy in 9, and valvular heart disease in 1. The presenting arrhythmia was v entricular fibrillation in 11, ventricular tachycardia in 10, and sync ope with inducible ventricular tachycardia in 4. The configuration of the shocking electrodes was randomized; the lead-only configuration wa s used in 14 patients (56%), and a subcutaneous patch was used in the remaining patients. Mean defibrillation threshold using a step-down te chnique 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 w as no change in clinical status, cardiac size, radiographic lead posit ion, or impedance between implant and 2 months. These findings suggest the increase in defibrillation threshold in this ICD system is not re lated to the type of waveform used, but rather is a feature of non-tho racotomy as opposed to epicardial electrodes.