Premature atrial stimulation during regular atrial pacing: a new approach to the study of the sinus node

Using an induced premature atrial depolarization after atrial pacing for 8 beats to scan the postpacing cycle, sinus node (SN) response was studied in 23 patients and the results were compared with those obtained using the Strauss method. Late extrastimuli resulted in compensatory return cycles (zone I), increasing prematurity gave rise to less-than-compensatory return cycles (zone II or "plateau") and interpolation or echo responses occurred at shorter coupling intervals (zone III). The sinoatrial (SA) conduction time was defined as the difference between return cycle and postreturn cycle lengths that fell in the latter portion of zone II. The SA conduction time was similar to those derived from the Strauss method (r = 0.91, n = 17, p less than 0.01) and remained similar at the 2 pacing cycles, 172 +/- 52 ms (mean +/- standard deviation) at 739 +/- 71 ms (cycle 1) and 170 +/- 60 ms at 596 +/- 57 ms (cycle 2). In 4 patients, the atrial pacing method could unmask zone II. The SN refractory period was defined as the longest coupling interval at which zone III was apparent. It was compared at cycles 1 and 2 in 14 patients. There was a significant increase in the SN refractory period with faster pacing rates, from 406 +/- 104 ms (cycle 1) to 462 +/- 112 ms (cycle 2) (p less than 0.05). The atrial pacing method provides an alternative evaluation of SA conduction and may permit the study of drug effects at identical basic rates.