Electrical defibrillation thresholds with transventricular simple-capacitor discharge under conditions of ischemia by...

The threshold for cardiac defibrillation is defined as that intensity of electrical stimulation that results in a 50% probability of success. Using transventricular simple-capacitor discharge, peak current defibrillatory thresholds were determined under conditions of relative normothermia with ligation of the left descending coronary artery. The average value over 151 successful shocks in 15 dogs was 81.1 mA/g of heart (SD = 29.3). This value was compared by means of the unpaired Student's t-test with the average obtained in a control series, without occlusion, but keeping the same general procedure and experimental conditions (89.5 mA/g, SD = 32.8, 346 defibrillations, 20 dogs). The difference (t = 2.82) was significant at a level P less than 1%. In many cases, there were spontaneous refibrillations after successful discharges, or cardiac arrests which called for mechanical pacing. The latter, in turn, easily led to a new fibrillation, especially when contractility was impaired. When the analysis of variance was applied (Snedecor f-test), we found that the threshold value was stable with respect to time (f = 0.35) during the average experimental period (124 min, SD = 55). We concluded that, on the average, peak current defibrillatory threshold decreased because of myocardial ischemia; however, during the recovery stage, the ventricles showed a high level of instability. Both phenomena (lower threshold and unstable recovery) were probably related to alterations in cellular excitability.