REGIONAL CONTROL OF ATRIAL-FIBRILLATION BY RAPID PACING IN CONSCIOUS DOGS

Background. In five chronically instrumented conscious dogs, we studied the effects of rapid pacing on sustained electrically induced atrial fibrillation. Methods and Results. Twenty-three unipolar atrial electrograms were recorded simultaneously from the bundle of Bachmann and the lateral wall of the right and left atria. During sustained atrial fibrillation, the surface electrocardiogram showed continuous irregularities of the baseline without P or F waves as well as an irregular ventricular rhythm with narrow QRS complexes. The atrial electrograms showed rapid irregular activity with a median cycle length of 85 +/- 8 msec and a range (P5-95) of 33 +/- 18 msec. Overdrive pacing of atrial fibrillation was performed using symmetric biphasic rectangular stimuli (2-msec duration, sixfold that of threshold) applied to a pair of stimulating electrodes at the left atrial appendage. Stimulation was started at pacing intervals of about 10 msec longer than the local median fibrillation interval and subsequently shortened in steps of 1 msec. At a critical pacing interval slightly shorter than the median fibrillation interval, the atrium around the pacing site was suddenly captured by the electrical stimuli. The area of local capture had a diameter of 6.1 +/- 1.6 cm. The time window of capture was 12 +/- 4 msec. Conclusions. These observations show that during electrically induced atrial fibrillation in chronically instrumented conscious dogs, a short excitable gap is present, permitting regional control of the fibrillatory process by rapid pacing.