Effects of cycle length on atrial vulnerability.

The effect of cycle length on atrial vulnerability was studied in 14 patients manifesting reproducible repetitive atrial firing during atrial extra-stimulus (A2) testing. Repetitive atrial firing was defined as the occurrence of two or more premature atrial responses with return cycle (A2-A3) of 250 msec or less and subsequent mean cycle length of 300 msec or less, following A2. The zone of repetitive atrial firing could be defined in terms of its longest and shortest A1-A2 coupling intervals. Each patient was tested at a long cycle length (CL1) (mean 884 msec) and a short cycle length (CL2) (mean 557 msec). CL1 was sinus rhythm and CL2, an atrial paced rhythm. Repetitive atrial firing occurred in two patients at CL1 and in all patients at CL2. Of the former two patients (group 2), the zone of repetitive atrial firing was markedly widened in one at CL2 due to a shortening of atrial functional refractory period (FRP) at CL2. In the other, zone of repetitive atrial firing could not be totally defined due to induction of sustained atrial flutter preventing definition of atrial FRP. The occurrence of repetitive atrial firing at only CL2 in 12 patients (group 1) reflected: 1) a shortening of atrial FRP from 294 +/- 11 msec at CL1 to 242 +/- 10 msec at CL2 (mean +/- SEM; P less than 0.01), allowing delivery of A2 at shorter coupling intervals (9); 2) the new occurrence of repetitive atrial firing at A1-A2 coupling intervals achievable at both cycle lengths (1); or 3) both effects (2). In conclusion, decrease of cycle length potentiated atrial vulnerability. This demonstration implies that atrial pacing could potentiate occurrence of paroxysmal atrial fibrillation or flutter.