Transthoracic defibrillation potential gradients in a closed chest porcine model of prolonged spontaneous and electrically induced ventricular fibrillation

Abstract

Objective: The purpose of this study was to measure the local electrical field or potential gradient, measured with a catheter-based system, required to terminate long duration electrically or ischaemically induced ventricular fibrillation (VF). We hypothesized that prolonged ischaemic VF would be more difficult to terminate when compared to electrically induced VF of similar duration.

Methods: Thirty anesthetized and instrumented swine were randomized to electrically induced VF or spontaneous, ischaemically induced VF, produced by balloon occlusion of the left anterior descending coronary artery. After 7 min of VF, chest compressions were initiated and rescue shocks were attempted 1 min later. The potential gradient for each shock was measured and the mean values required for defibrillation compared for the VF groups.

Results: The number of shocks and the shock strength required for termination of VF were not significantly different for the groups. The potential gradient of the first successful defibrillating shock was significantly greater in the spontaneous, occlusion-induced VF group (12.80+/-2.82 V/cm vs 9.60+/-2.48 V/cm, p=0.002). The number of refibrillations was greater in the ischaemic group than in the non-ischaemic electrical group (6+/-4 vs 1+/-1, p<0.001). The number of animals requiring a shock at 360J was 2.5 times greater for the ischaemic group.

Conclusions: Defibrillation of prolonged VF produced by acute myocardial ischaemia requires a significantly greater potential gradient to terminate than prolonged VF induced by electrical stimulation of the right ventricular endocardium. The VF duration used in this study approximates that occurring in victims of out-of-hospital cardiac arrest. Our findings may be of clinical importance in the management of such patients.

Figure 1. Shock strength for first successful defibrillation

Shock strength required for successful defibrillation are shown for the electrically induced and ischaemically induced VF all. One-third of VF episodes in the ischemic group required maximum output for termination of VF.

Figure 2. Probability of success curves for electrical and spontaneous VF

The curve for ischaemically induced VF is shifted rightward and the mean potential gradient was significantly greater in the occlusion induced group.