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In the isolated rabbit heart the ventricular response during irregular atrial pacing and atrial fibrillation was studied. Irregular pacing was performed by an algorithm that generated a population of random stimuli within a chosen range of intervals. During incremental regular pacing atrioventricular (AV) Wenckebach conduction occurred at an atrial cycle length of 164 +/- 24 ms (n = 11). During irregular atrial pacing with a variation of 20, 60, and 100 ms second-degree AV block developed at average cycle lengths of 171, 182, and 198 ms, respectively. In contrast, the pacing interval resulting in stable 2:1 AV block was shortened from 131 ms to 120 and 112 ms during irregular pacing with 20- and 60-ms variation. During pacing with 100-ms variation stable 2:1 AV block no longer occurred. Addition of acetylcholine (0.25-0.75 X 10(-6) M) increased the maximal degree of stable AV block during regular pacing. During irregular pacing, however, stable AV block disappeared. Instead, during second-degree AV block a monophasic inverse relationship (r = -1.5 +/- 1.7) between atrial and ventricular rates appeared as a result of an increased occurrence of concealed conduction in the AV junction. Also during electrically induced atrial fibrillation an inverse relationship was found between the atrial and ventricular rate. We conclude that the ventricular rate during atrial fibrillation is not only determined by the properties of the AV node but also by the rate and irregularity of the fibrillatory process.

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