Myocardial Necrosis from Direct Current Countershock

The effect of varying both paddle electrode size and the time interval between direct current countershock on myocardial necrosis was studied. Forty-two dogs were divided into seven groups of six dogs each. All dogs were given ten consecutive, 240 watt-second countershocks (delivered energy into a 50 ohm load). Three groups were shocked with paddle electrode diameters of 8.0 cm (standard electrodes), two groups with paddle electrode diameters of 12.8 cm (large electrodes), and two groups with paddle electrode diameters of 4.3 cm (small electrodes). The time intervals between discharges in the groups shocked with the standard electrodes were 15 seconds, one minute, and three minutes. The time interval between discharges in the groups shocked with small and large electrodes was 15 seconds and three minutes. Myocardial necrosis was quantitated by precordial electrocardiographic mapping recorded minutes after, and by gross and microscopic examination of the hearts four days after direct current countershock. When the time interval between discharges was shorter, myocardial necrosis was greater. When the time interval between discharges was constant, more necrosis was produced with smaller-sized paddle electrodes. It is concluded that large paddle electrodes should be used for delivering direct current countershocks, and that during elective cardioversion, consecutive discharges should be delivered at time intervals greater than three minutes.