Further observations on entrainment of atrial flutter in the dog

To determine whether the first postpacing interval after entrainment was affected by recording and pacing sites, overdrive atrial pacing was undertaken in 13 episodes of atrial flutter with a mean flutter cycle length (FCL) of 140 +/- 8 msec induced in seven dogs. Atrial flutter was induced by means of an anatomic obstacle. Seven recording sites, four in the right atrium and three in the left atrium, and three pacing sites, two in the right atrium and one in the left atrium, were selected. After entrainment from the right atrium at pacing cycle lengths that were 94% of the FCL, the first postpacing interval was not significantly different from the intrinsic FCL at each recording site, but it tended to be shorter than the FCL at the recording sites near pacing sites. For entrainment from the left atrium, the first postpacing interval was longer than the FCL at recording sites in the left atrium (p less than 0.001), but it was not different from the FCL at recording sites in the right atrium. These results are due to differences in placement of recording and pacing electrodes relative to the reentrant circuit. Also we observed that activation sequences involving three appropriately selected recording sites were always identical when paced from two different pacing sites at a single constant pacing cycle length. This new phenomenon may best be explained by postulating reentry as the mechanism for atrial flutter.