Premature Atrial Stimulation as a Key to the Understanding of Sinoatrial Conduction in Man

Since recording potentials directly from the sinoatrial node (SAN) is not yet possible, the electrophysiologic evaluation of this structure in intact human subjects must be accomplished with indirect technics. Two technics have been used to study SAN function in man: premature atrial stimulation (PAS) and rapid atrial pacing. Recent discussions of data using these technics have emphasized their role in determining SAN automaticity, but their role in evaluating conduction from atrium to SAN or SAN to atrium has not been fully explored. Using the technic of PAS, we have studied five patients with sinus bradycardia and symptoms of dizziness or syncope. Our analysis of the results obtained from these studies discloses the unique ability of this technic to evaluate conduction into and out of the SAN. An atrial premature depolarization (APD) elicited late in atrial diastole is followed by a compensatory pause (nonreset of the SAN pacemaker). An APD elicited earlier in atrial diastole is followed by a pause that is less than compensatory (SAN reset). From these responses estimates of sinoatrial conduction time were made. In one patient reset was never seen, suggesting markedly prolonged sinoatrial conduction. With these results in mind, the literature was reviewed and an alternate interpretation posed for existing data. PAS is not only a means of determining SAN automaticity, but also a very useful means of unmasking sinoatrial conduction abnormalities.