Determinants of Intracardiac Current in Defibrillation

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 implanted 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 six 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,