The postponed compensatory pause as a manifestation of positive feedback in atrioventricular conduction.

A patient with hypertensive cardiovascular disease was found to have unusual varieties of premature atrial and ventricular contractions. If the premature atrial contraction resulted in a greatly prolonged P-R interval, such that the increment in P-R exceeded the decrement in the preceding R-P, the next sinus P wave, occurring after a normal P-P interval, was found to be blocked. Also, numerous interpolated ventricular extrasystoles were observed in which the postextrasystolic P-R intervals were markedly prolonged and in which the compensatory pauses were postponed for one or two beats. We also present data from one dog in which a premature atrial activation produced a chain reaction such that complete A-V block occurred three beats later. We propose that the chain reaction which evoked the delayed block in the dog and the postponed compensatory pauses in the patient reflects the operation of a positive feedback mechanism in A-V conduction. Positive feedback is initiated by an extremely long P-R, which results in a very short R-P before the next cycle. This then leads to a still longer P-R, which then elicits a still shorter R-P. Block ultimately supervenes when the atrial activation wave arrives at the A-V junction during its effective refractory period.