767-1 Determinants of Defibrillation Energy Requirements in Heart Failure

A recent study using a monophasic, epicardial defibrillation system in a canine model has raised concern that congestive heart failure may dramatically increase the energy requirements of defibrillation. In order to test the effect of heart failure on the defibrillation efficacy of a transvenous biphasic defibrillation system, 16 dogs were studied; 11 dogs were ventricular paced at 225 ppm for two weeks to induce heart failure, and 5 control dogs remained unpaced. All dogs were tested using an endocardial right ventricular defibrillation lead and a cutaneous patch. Shocks were biphasic with equal leading edge voltages and fixed 65% tilt for each phase. Dose-response defibrillation probability curves were generated for each animal at baseline, after two weeks (at which time pacing was discontinued in the paced group), and then one week later. The ED20, ED50, and ED80 (the 20%, 50%, and 80% effective defibrillation energies, respectively) were determined for each dog at each study.In the paced dogs, the ejection fraction (EF) fell from 55% to 25% after pacing (p<0.0001), and rose to 46% one week after its discontinuation (p=0.0002). The effective refractory period (ERP) prolonged from 152 to 170 msec (p<0.005), then shortened to 164 (p=NS). There was no significant change in EF or ERP in the control dogs between the three studies. The ED20, ED50, and ED80 remained unchanged in both the control and paced groups for all three studies, even with adjustment for dog weight or echocardiographic left ventricular mass. In a multiple regression model, EF did not significantly predict the ED50. However, the ED50 increased with increasing left ventricular (LV) size and decreased with LV wall thinning. Rapid pacing led to LV cavity dilatation and wall thinning; these two morphologic changes had opposing effects on defibrillation energy, so that there was no net change in the ED50 in heart failure.ConclusionThe defibrillation efficacy of a biphasic waveform in a transvenous lead system is not changed by the development of heart failure in the rapid paced canine model. The energy required for defibrillation was predicted by the ventricular size and wall thickness but not EF