Recent studies suggest that low energy and current levels are required for direct ventricular defibrillation in man. During open-heart surgery numerous variables may affect the outcome of a given shock. Following apparent complete surgical repair, some hearts fail to defibrillate, defibrillate only after multiple high energy shocks, or resume abnormal post-defibrillation rhythms. The defibrillation threshold for human hearts with various pathologic conditions has not been determined previously during cardiac surgery. We propose to study the threshold for defibrillation following cardiac surgery of human hearts with congenital or acquired diseases. Shocks will be initiated at 1 joule (watt-second) levels, increasing in increments until defibrillation is accomplished. In addition, the study will determine which historical, anesthetic, biochemical, surgical, and other factors can be correlated with successful shocks. Direct determinations of anesthetic, norepinephrine, epinephrine, calcium, magnesium, sodium and potassium levels will be made. A knowledge of the relative importance of each of the factors in the type of cardiac pathology to be investigated in this study would be most beneficial to future open-heart surgical patients by providing optimum conditions for defibrillation with the lowest possible energy.