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 sinusnode 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.