Effects of Atrial Premature Systoles on Sinus Rhythm in the Rabbit

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Abstract
Although it is well known that atrial premature depolarizations (APDs) frequently lead to lengthening of the next cycle (returning cycle), it is usually not appreciated that shortening of the returning cycle (RC) can also occur. Moreover, the mechanism for these alterations is not clearly understood. APDs were therefore electrically induced in rabbit sinoatrial (SA) preparations, and four basic patterns of alteration in the SA nodal rhythm were observed. The type of change depended very much on the timing of the APD in the cycle. The atrial events, more often than not, failed to reflect the underlying SA nodal events. Thus, lengthened atrial RCs occurred in spite of shortened SA nodal RCs. This and other discrepancies indicate that the extracellular records of atrial activity do not faithfully reflect the events within the SA node and that information gathered from them should be interpreted with caution. Timing of the APD in the cycle, antegrade and retrograde SA conduction time, SA nodal action potential characteristics, and shifts in pacemaker sites are important in the determination of the altered SA nodal and atrial responses to APDs.