Comparative effects of overdrive on sinus and subsidiary pacemaker function.

Recent evidence has suggested a difference in response to overdrive pacing dependent on the location of the pacemaker within the A-V conduction system. To test this hypothesis, the effects of overdrive pacing were evaluated in five anesthetized dogs with experimentally induced A-V block and in seven patients with advanced A-V block. In the animals, sinoatrial node recovery times were studied over wide ranges of rates (130 to 210 beats per minute) and durations (30 to 180 seconds) of atrial pacing. All sinus node recovery times were less than 600 msec. with a mean maximum pause of 0.540+/-0.043 seconds (M.+/-S.E.M.). In contrast, after ventricular pacing (rates 90 to 150 beats per minute; durations 30 to 180 seconds), subsidiary pacemaker recovery times were significantly greater (p less than 0.025) with a mean maximum recovery time of 28.4+/-8.3 seconds. In the seven patients studied, all sinus node recovery times were less than 1,400 msecs. with a mean maximum pause of 0.954+/-0.051 seconds. As seen with the experimental animals, a significantly longer (p less than 0.025 mean maximum subsidiary pacemaker recovery time of 3.55+/-0.92 seconds was observed. The present studies in both experimental animals and in man without evidence of sinus node dysfunction showed that sinus node recovery time was independent of both rate and duration of atrial overdrive pacing. In contrast, subsidiary pacemaker recovery time was correlated with both rate and duration of ventricular overdrive pacing. In both experimental protocols, subsidiary pacemaker recovery time was shown to exceed sinus node recovery time at all rates and at all durations of pacing. Postrecovery sinus node acceleration was consistently observed after atrial overdrive pacing. In contrast, postrecovery subsidiary pacemaker "depression" characterized ventricular overdrive pacing. It is concluded that subsidiary pacemakers are significantly more susceptible to overdrive suppression than the sinoatrial node, a feature of substantial clinical significance.