Determinants of intracardiac current in defibrillation. Experimental studies in dogs

Abstract

Defibrillation success depends upon the amount and distribution of intracardiac current (IC), because only a portion of total current flow between the paddles actually traverses the heart. We evaluated two determinants of IC: paddle size and shock energy. Studies were performed in 11 anesthetized dogs using a damped-sine wave form defibrillator. Intramyocardial electrode pairs were implemented subepicardially to subendocardially across the left and right ventricular free walls and in the interventricular septum. IC was quantitated by oscilloscopic measurements of the electric potential gradient (V/m) developed across the implanted electrodes during synchronized shocks of constant energy. External defibrillating paddle electrodes ('paddles') were applied to the left and right lateral chest walls. An initial group of 6 dogs was given shocks with 8.5-cm-diameter or 13-cm-diameter paddles to assess the effect of standard vs large paddles on IC. Mean IC values were substantially greater using 13-cm paddles (p<0.01, paired t test). We then studied a second group of five dogs to determine more precisely the paddle size that produced the highest IC. A series of constant energy shocks using paddles of various sizes (8.5, 10, 12, 14, 16 cm in diameter) were given. In each dog, as paddle diameter increased from 8.5 cm to 16 cm, the IC increased, reached a maximum and then decreased. The maximum IC occurred at an average paddle size of 12 cm. To assess the effect of shock energy level on IC, we gave shocks of variable energy using 8.5-cm-diameter paddles. Each fourfold increase in delivered energy resulted in a doubling of IC. We conclude that defibrillating paddle diameter and delivered erergy are important determinants of IC flow.