Incomplete interpolation caused by sinoatrial pacemaker shift. A report of two cases.

Shortened return cycles after premature atrial stimulation (PAS) are commonly referred to as sinoatrial entrance block and exit delay at the sinoatrial junction or sinus-node reentries. In the 2 reported cases PAS at critical coupling intervals was followed by shortened return cycles characterized by a changed high right electrogram (and surface P waves in 1 case) and a normal sequence of atrial activation with unaltered intraatrial conduction. These changes lasted for some beats and a concomitant shorter or longer atrial cycle length was observed. Electrophysiological events furnish indirect evidence of sinoatrial pacemaker shift as a cause of incomplete interpolation in man.