Vagal-mediated atrial premature beats: a computer model.

A computer algorithm is described that used experimental data to model the arrhythmogenic interaction of phasic vagal stimuli and atrial ectopic pacemakers. The model consisted of a dominant sinus node and a single ectopic pacemaker center separated by conducting atrial tissue. Its primary operation was to predict the timing and incidence of atrial premature beats resulting from transient escape of ectopic automatic impulses when vagal-induced entrance block of the sinus impulse was simulated near the ectopic focus. These predictions were based on a series of experimentally derived phase-response and corrected recovery time curves, describing the modulation of ectopic pacemaker periodicity by vagal input and overdrive suppression, respectively. Depending on the combination of curves tested, the model predicted premature beats to develop only with critically timed vagal stimuli. The coupling intervals of vagal-induced premature beats were > 300 ms and varied as a function of vagal timing and sinus cycle length. The model suggests therefore that phasic vagal stimuli within the atrium may transiently protect ectopic pacemaker foci from conducted sinus impulses and mediate the genesis of atrial extrasystoles with long coupling intervals.