Widespread distribution and rate differentiation of the atrial pacemaker complex

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Abstract
A multicentric rather than unifocal origin of the atrial depolarization wave was found in dogs. Three to five pacemakers, or origin points, were distributed over a 30-40 mm area compared to the 11 mm sinus node. Of the sites, 2 or 3 could excite simultaneously, or 1 specific site would dominate excitation. Each separate origin point dominated excitation within a specific range of heart rates, and on reaching either the upper or lower limits of this range, a new focus abruptly dominated and initiated the atrial wave front. Apparently, there is a widely distributed atrial pacemaker complex. The spatial distribution of this system exceeded the dimensions of the canine sinus node by a factor of 3-4 times. The pacemaker centers, although distributed, were consistently located at specific positions along the superior vena caval-right atrial junction. Each separate pacemaker site appeared functionally differentiated to generate a specific range of heart rates. In addition to the sinus node, there are other specialized atrial pacemaker centers. This specialization, including the differentiated response and coordination, is conferred by focal receptor characteristics and their inputs.