Cardiac rhythm following exclusion of the sinoatrial node and most of the right atrium from the remainder of the heart.

In this study two surgical interventions on the right atrium were followed by long-term observation of their effect on the cardiac rhythm. In the first, the sinoatrial (SA) node and a small area of surrounding atrium were excluded from the heart by a circumferential incision. This was followed immediately by an unstable junctional rhythm accompanied by periods of pacemaker arrest. After 14 days a rhythm, indistinguishable from sinus rhythm, became permanently established. On atrial mapping the area of earliest epicardial breakthrough was the low right atrium, indicating that the pacemaker was in this area. In the second procedure, most of the right atrium including the SA node was excluded by an incision from the remainder of the heart, but left in continuity were the left atrium, right atrium in the area about the coronary sinus and inferior vena caval ostium and the atrial septum (areas where the low atrial pacemakers are known to occur), and the ventricles. A permanent junctional rhythm associated with periods of pacemaker arrest was produced. In two dogs permanent atrial fibrillation eventually developed. The area of earliest breakthrough found on epicardial mapping was to the right of the right inferior pulmonary vein, close to the atrial septum. One of the differences between the two interventions was the amount of right atrium remaining between the potential low atrial pacemakers and the atrioventricular (AV) node. It is suggested that summation, a factor needed for the atrial excitation wave to penetrate the AV node, was inadequate in the more extensive intervention. This may explain the failure of a potential low atrial pacemaker to become dominant in the second intervention. The clinical implications of these findings are discussed.