Action of Driving Stimuli from Intrinsic and Extrinsic Sources on in Situ Cardiac Pacemaker Tissues

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Abstract
The interactions between pacemakers, and the effects on pacemakers, of terminating imposed driving were studied in the in situ heart of anesthetized dogs. Following atrial fibrillation or termination of a fast drive imposed through an artificial pacemaker, pacemaker action in intrinsic pacemakers is suppressed. Pacemakers tend to accelerate and compete with imposed drives which exceed control rates by only a small percentage (10 to 15%). Arrhythmias may result if imposed drive is slower than or identical with intrinsic pacemaker rate. Post-drive depression of pacemakers and the resulting deceleration of the heart is followed normally by an overshoot or supranormal acceleration. The magnitudes and durations of depression and late acceleration are proportional, within limits, to the rate and duration of drive. Atrioventricular and ectopic atrial pacemakers are much more readily depressed than is the sinoatrial pacemaker. Furthermore, beats of ectopic origin are much more likely to occur while subsidiary pacemakers are recovering from post-drive depression. Augmentation of depression by Prostigmin, its diminution by atropine, and the potentiation of late acceleration by cocaine and its absence after reserpine or guanethidine pretreatment, indicate that acetylcholine and catecholamines are liberated by driving stimuli. Placement of the pacemaker over the sinoatrial node, or near to regions where nerve terminals are concentrated, results in the greatest post-drive effects. The fact that propagated action potentials cause depressions and accelerations subject to drug block or potentiation indicates that mediators are also released in the course of propagated activity. Since atropine does not completely block post-drive depression, it is thought that a potassium ion shift may be involved.