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In the isolated right atrium of the rabbit, premature beats were elicited by electrical stimulation. When the activation front of a premature beat did not reach the sinoatrial (S-A) node soon enough to discharge the pacemaker prematurely, the pause following the premature beat was compensatory. How- ever, when the activation front forced the pacemaker to discharge prematurely, the premature beat cycle outlasted the normal spontaneous pause, although it was not completely compensatory. Moreover, several cycles after the prema- ture beat were prolonged. The postextrasystolic pause lasted longer if the premature beat was elicited earlier in the atrial cycle. Several changes in the activity of the S-A node caused the prolonged diastole after the atrial premature beat; the most prominent change was a pacemaker shift that persisted several cycles after the premature beat. We found a progressive decrease of the conduction velocity within the S-A node when the latency between the spon- taneous discharge of the pacemaker and the arrival of the premature impulse from the atrium in the S-A node was shortened. This may explain the occurrence within the S-A node of a shift of the pacemaker in the direction of the ectopic focus. The rhythm of the new pacemaker was slower than that of the original one.

Change of Cardiac Rhythm in the Rabbit after an Atrial Premature Beat