Determinants of intracardiac current in defibrillation. Experimental studies in dogs.

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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. Two determinants of IC, paddle size and shock energy, were evaluated. 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 6 dogs was given shocks with 8.5- 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). A 2nd group of 5 dogs was studied 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 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, shocks of variable energy using 8.5-cm-diameter paddles were given. Each 4-fold increase in delivered energy resulted in a doubling of IC. Defibrillating paddle diameter and delivered energy apparently are important determinants of IC flow.