Functional characterization of atrial pacemaker activity

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Abstract
With use of Holter monitoring, 24-h heart rate and rhythm traces were compared in intact, conscious dogs, both before and after surgical excision of the sino-atrial node (SAN). Mean 24-h quietly resting heart rates were not different, whether under SAN or subsidiary atrial pacemaker (SAP) control for the first few days or weeks after surgery. The 24-h heart rates under SAP control progressively declined with time and remained significantly lower. Under resting conditions, propranolol failed to significantly influence 24-h resting heart rates in either SAN- or SAP-controlled animals. While junctional rhythms commonly (but not invariably) occurred immediately after SAN excision, P waves with essentially normal configurations reappeared within a few minutes or hours and became stabilized within a few minutes or hours and became stabilized within a few days or weeks. Electrophysiological mapping of the right atrium after stabilization of the P wave revealed the earliest site of activation to be along the sulcus (crista) terminalis or at the junction of the inferior vena cava and the inferior right atrium. Direct electrical stimulation of the vagi induced asystole or profound bradycardia; sympathetic stimulation elicited acceleration, increased contractility and shifting P waves. Left stellate stimulation generally resulted in disappearance of the P wave and the dominance of a junctional pacemaker. These alterations are comparable or even exaggerated in comparison with identical stimulations in the presence of an intact SAN. SAP are intrinsically more automatic than atrioventricular (AV) junctional pacemakers; they are capable of dominance in the conscious canine model after surgical removal of the SAN. They are directly responsive to autononic regulation and are capable of maintaining highly effective cardiac performance in the alert, conscious animal.