Delayed conversion to sinus rhythm after direct-current countershock

Elective cardioversion for supraventricular arrhythmias has been performed in 203 patients during a 30-month period beginning in May, 1972. In five instances conversion to normal sinus rhythm occurred well after the electric shock had been delivered. The interval ranged from 4 to 105 seconds after DC countershock. In one patient, atrial fibrillation clearly persisted for eight seconds after DC countershock, however, immediately after countershock the ventricular rate became regular and, in addition to the fibrillatory activity, apparent sinus P-waves became visible and the same regular rate persisted after disappearance of fibrillatory waves. This raised the possibility of atrial dissociation. Our experience suggests that if apparent failure of conversion is noted after DC countershock, a repeat shock at a higher level should not be administered immediately but only after observing the rhythm for up to two minutes. Other possible mechanisms of delayed conversion are discussed.