Ischemic ventricular fibrillation: The importance of being spontaneous

Although the energy level required to defibrillate normal myocardium is low and constant, as determined from studies of induced ventricular fibrillation, little is known of the specific energy requirements in regionally ischemic hearts for spontaneous or induced ventricular fibrillation. In this study the lowest energy threshold for defibrillation was determined in 10 open chest dogs with reversible 10 minute coronary occlusions at various sites for each of 44 events of ventricular fibrillation, using apical and superior vena caval electrodes with a generator providing variable output of 1 to 30 watt seconds. The ischemic mass, quantitated from postmortem angiographic and planimetric data, was 52 ± 9 percent (mean ± standard deviation) of the left ventricle in dogs with induced ventricular fibrillation (Group I), 52 ± 12 percent in dogs with spontaneous ventricular fibrillation after occlusion (Group II) and 54 ± 9 percent in dogs with spontaneous ventricular fibrillation after reperfusion (Group III). Defibrillation thresholds in watt seconds were 9 ± 7 in Group I (n = 12), 19 ± 10 in Group II (n = 13) and 18 ± 10 in Group III (n = 19). (Group I versus Groups II and III, probability [p]<0.025). In nonischemic hearts, the defibrillation threshold was 3 ± 2 (n = 32) (p <0.001 compared with values in Group I, II or III). Thus, despite similar masses of ischemia, twice as much energy was required for defibrillation of spontaneous ventricular fibrillation (whether after occlusion or reperfusion) as for induced ventricular fibrillation, suggesting that these conditions are caused by different metabolic or pathologic derangements. Such differences should be considered in assessing interventions such as drug therapy designed to inhibit the repetitive ventricular response and in design of implantable defibrillators.