HIGH-VOLTAGE SHOCK-INDUCED CELLULAR ELECTROPHYSIOLOGICAL EFFECTS - TRANSIENT REFRACTORINESS AND BIMODAL CHANGES IN ACTION-POTENTIAL DURATION

The cellular electrophysiological effects of defibrillation shocks on the myocardium during ventricular fibrillation are not clear. The pres ent study investigated the effects of high voltage shocks on membrane potentials of isolated guinea pig and pig papillary muscles during rap id activations simulating ventricular fibrillation. High voltage shack s induced an action potential with a prolonged duration, followed by a transient refractory state. Subsequent action potentials following th is refractory state had shortened durations. The duration of the trans ient refractory state varied in proportion to shock intensity and stim ulation rate, whether the shock was biphasic or monophasic. Shock indu ced prolonged depolarization was not a consistent finding and mainly o bserved with slow stimulation rates. In conclusion, high voltage shock s induce bimodal changes of the action potential duration associated w ith a transient refractory state during rapid activation. The rate dep endency of this refractory state suggests that the duration of the sho ck induced refractory state may be longer in the fibrillating than the normal beating heart, and may contribute to successful defibrillation .