Factors affecting overdrive suppression of idioventricular pacemakers and associated potassium shifts

A characterization of the factors controlling overdrive suppression of idioventricular pacemakers was investigated in canine hearts with complete atrioventricular block perfused in vitro. The following results were obtained: 1) overdrive suppression increases as a function of driving rate in a sigmoidal fashion; 2) overdrive suppression is maximal after 3 min; 3) the pause is a function of spontaneous rate prior to overdrive; 4) overdrive causes an initial net K loss; 5) overdrive is followed by a transient net K grain; 6) increasing [K]o does not affect K loss; 7) net K loss with drive is less in pre-driven hearts; 8) net K uptake after overdrive is little affected by ventricular activity; 9) acetylcholine does not alter ventricular K balance; and 10) paired stimulation increases overdrive suppression and K loss whether or not each stimulus is followed by a contraction. The following conclusions are drawn. The factors controlling overdrive suppression include the spontaneous rate prior to drive, the rate, duration and pattern of drive, an increase in [K]o during the drive and a decrease in [K]o after drive, but not on acetylcholine-mediated inhibition.