EFFECTS OF MONOPHASIC AND BIPHASIC SHOCKS ON ACTION-POTENTIALS DURINGVENTRICULAR-FIBRILLATION IN DOGS

This study determined the response of action potentials during ventric ular fibrillation (VF) to timed monophasic and biphasic shocks. A floa ting glass microelectrode was used to record intracellularly from the anterior right ventricle in 10 open-chest dogs. After 10 seconds of el ectrically induced VF, 5-millisecond monophasic and 2.5/2.5-millisecon d biphasic shocks or 16-millisecond monophasic and 8/8-millisecond bip hasic shocks were given via mesh electrodes on either side of the micr oelectrode. Monophasic and biphasic truncated exponential shocks of 5 V/cm were given with coupling intervals timed from the beginning of a VF action potential to the shock ranging from 50 to 70 milliseconds in 5-millisecond increments. Each coupling interval for each waveform wa s tested during a different VF episode.The interval between successive activations during VF was 86+/-15 milliseconds (mean+/-SD). The refra ctory period during VF was 61+/-5 milliseconds for 5-millisecond monop hasic shocks and 66+/-6 milliseconds for 2.5/2.5-millisecond biphasic shocks (P<.05). At each coupling interval, action potential duration a t 50% repolarization (APD50) was significantly prolonged by the shocks compared with the mean preshock APD50 (P<.05). APD50 duration increas ed significantly with increases in the coupling interval (P<.05) for b oth monophasic and biphasic waveforms. For all coupling intervals toge ther, APD50 prolongation as a percent of the mean preshock APD50 was 1 70+/-55%, 192+/-45%, 151+/-44%, and 175+/-45% for 5- and 16-millisecon d monophasic and 2.5/2.5- and 8/8-millisecond biphasic waveforms, resp ectively. This APD50 prolongation was greater for monophasic than biph asic shocks and was greater for longer than shorter waveforms (P<.05). Thus, during VF, (1) the refractory period for 5-V/cm truncated expon ential waveforms lasting 5 milliseconds is approximately 75% of the VF activation interval; (2) the refractory period is shorter for monopha sic than for comparable biphasic waveforms; (3) both monophasic and bi phasic 5-V/cm shock fields cause prolongation of action potential dura tion; (4) prolongation of action potential duration increases as the c oupling interval increases; and (5) prolongation of action potential d uration is greater for monophasic shocks and for longer shock waveform s.