Sinoatrial pacemaker shift following atrial stimulation in man.

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Abstract
Indirect evidence of a sinoatrial pacemaker shift after programmed atrial stimulation in man is presented. Following electrically induced beats, time intervals and postextrasystolic morphology of atrial electrogram and P waves were scrutinized in 30 catheterization studies. Applying premature atrial stimulation, a decrease of the interval between the last basic atrial depolarization and the stimulus-produced atrial excitation (curtailed cycle) below a critical interval was followed by a sinoatrial pacemaker shift in three cases. This electrophysiologic event consisted of a concomitant change in shape of high right atrial electrogram and an increase of atrial cycle length. Simultaneous alteration of P waves could be detected in 2/3 patients. Assuming that the pacemaker shift indicates the arrival of ectopic activation in the sinus node, capture of the sinus node by the premature beat could be distinguished from failure to capture. Thus, pacemaker shift can be used for estimating sinoatrial conduction time in addition to present methods using measurement of postextrasystolic atrial intervals. The changes described could be seen both before and after atropine administration. Tracings of a pacemaker shift after cessation of rapid atrial pacing are also presented. In summary, we found a sinoatrial pacemaker shift underlying sinus node response to ectopic atrial activation in man, a phenomenon which contributes to our understanding of indirect assessment of sinoatrial conduction time by the premature stimulation technique.