Background - To study the mechanism by which shocks succeed (SDF) or fail (FDF) to defibrillate, global cardiac activation and recovery and their relationship to defibrillation outcome were investigated for shock strengths with approximately equal SDF and FDF outcomes (DFT50). Methods and Results - In 6 isolated pig hearts, dual-camera video imaging was used to record optically from ≈8000 sites on the anterior and posterior ventricular surfaces before and after 10 DFT50 biphasic shocks. The interval between the shock and the last ventricular fibrillation activation preceding the shock (coupling interval, CI) and the time from shock onset to 90% repolarization of the immediate postshock action potential (RT90) were determined at all sites. Of 60 shocks, 31 were SDF. The CI (59±7 versus 52±6 ms) and RT90 (108±19 versus 88±8 ms) were significantly longer for SDF than FDF episodes. Spatial dispersions of CI (36±5 versus 34±3 ms) and RT90 (40±16 versus 40±8 ms) were not significantly different for SDF versus FDF episodes. The first global activation cycle appeared focally on the left ventricular apical epicardium 78±32 ms after the shock. Conclusions - For near-threshold shocks, defibrillation outcome correlates with the electrical state of the heart at the time of the shock and on RT. Global dispersion of RT was similar in both SDF and FDF episodes, suggesting that it is not crucial in determining defibrillation outcome after DFT50 shocks.