Effect of right ventricular pacing on ventricular rhythm during atrial fibrillation.

In 13 patients with atrial fibrillation, the effect of right ventricular pacing at various rates on spontaneous RR intervals was studied. Five hundred consecutive RR intervals were recorded and measured before and during varying right ventricular pacing rates. As anticipated, all RR intervals longer than the right ventricular pacing intervals were abolished. However, RR intervals shorter than the right ventricular pacing intervals were also eliminated. It is difficult to explain the elimination of RR intervals shorter than the pacing intervals with the accepted concepts concerning the mechanisms governing the rate and rhythm of the ventricular response to atrial fibrillation. An alternative explanation may be that during atrial fibrillation the atrioventricular node behaves as a nonprotected pacemaker that is electrotonically modulated by the chaotic atrial electrical activity. The result is a random ventricular rhythm. With right ventricular pacing, the automatic focus is depolarized by the retrogradely concealed conducted ventricular impulses, the short RR intervals are not generated as a consequence and the rhythm becomes pacemaker dependent.