Can shocks timed to action potentials in low-gradient regions improve bothinternal and out-of-hospital defibrillation?

During the first minute of fibrillation, circulating wavefronts excite new fibrillation action potentials almost immediately following termination of the preceding action potential. The extension of refractoriness hypothesis states that a successful defibrillating shock must produce a uniform postsh ock refractoriness of a specific optimal duration throughout the ventricle, which blocks these wavefronts and terminates fibrillation. We hypothesized that, if shocks are appropriately timed early in the fibrillation action p otential in low-voltage-gradient regions, postshock refractoriness will alr eady be long and the shock need not be strong enough to further extend it. This will result in a lower defibrillation threshold (DFT). This hypothesis was tested in the isolated rabbit heart model. Shocks were synchronized to monophasic action potentials recorded from a low-intensity region. An up/d own protocol was used. I-50 for early shocks was 17% lower than that for la te shocks (31% decrease in E-50). Standard deviation of I-50 was reduced fr om 32% for late shocks to 18% for early shocks. Therefore, shock synchroniz ation improves both DFT and intersubject variability during early fibrillat ion. As fibrillation duration increases, action potential frequency decreas es and periods of diastole occur. Because of these ischemic changes, it is uncertain whether shock timing can produce similar improvements in defibril lation under out-of-hospital conditions.