Effect of shock strength on survival and acute cardiac damage induced by open-thorax defibrillation of dogs.

The safety of open thorax defibrillation with single damped sine-wave shocks and 6-cm-diameter electrodes was evaluated in healthy anesthetized dogs. Twenty-one dogs were allotted to 6 groups: Group A were nonshocked controls and groups B through F were given single shocks of 4-, 7-, 12-, 19-, or 32-fold, respectively, greater than a defibrillation threshold dose (30 mA/g of heart). Immediate postshock death resulted in group F dogs; group A through E dogs survived and were killed after 2 days. The incidence and severity of cardiac morphologic damage increased with shock strength (mild damage occurred in 1 of 3 dogs in group C and in 3 of 4 dogs in group D and severe damage occurred in 2 of 3 dogs in group E). The cardiac lesions were characterized grossly and microscopically. In dogs that died immediately after shocking, damage was apparent as pale circular zones of edema and myofibrillar degeneration in the ventricular free walls beneath the electrode placement sites on the cardiac surface. In the dogs that survived 2 days, the defibrillator-induced areas of myocardial necrosis and calcification were concentrated in arc or ringlike patterns beneath the periphery of the electrode placement sites. All dogs that were studied 2 days after shocking had mild fibrinous pericarditis. Postshock electrocardiographic changes were not good indicators of cardiac damage because the mild epicardial inflammatory reaction associated with the surgical procedure produced large ST and T wave changes which masked any changes associated with myocardial necrosis induced by the electric shocks. It was concluded that a substantial safety margin exists between the required defibrillation threshold shock dose and the large shocks required to produce marked cardiac damage or death in healthy dogs.