Experimental studies on sick sinus syndrome: relationship of extent of right atrial lesions to subsidiary pacemaker shift and its function.

An area where the sinus node was located in 36 dogs was defined as the "SN-area." Five to 7 days after cauterization of the SN-area, a subsidiary pacemaker remained in the SN-area in seven (group S) and shifted to outside the SN-area in 14 dogs (group E). The SN-area in group S had escaped complete destruction, but that in group E was destroyed completely. Heart rate in group S and group E was 154.0 +/- 27.1 and 107.6 +/- 16.8 beats/min (p less than 0.001), respectively. Maximum duration of asystole after overdrive (maxDAO) in group S and group E was 656.7 +/- 260.2 and 2470.2 +/- 2271.6 msec (p less than 0.01), respectively. Propranolol (0.2 mg/kg intravenously) decreased heart rate in group S and group E to 88.3 +/- 7.9 and 71.7 +/- 8.7% (p less than 0.001), and prolonged maxDAO to 140.1 +/- 28.1 and 492.7 +/- 591.4% (p less than 0.025), respectively. It is concluded that the rate and stability of subsidiary pacemakers after cauterization of the sinus node have a close relationship with their location in the right atrium, which is dependent on the extent of lesions in the SN-area. Sympathetic tone played a significant role in maintaining these functions in group E.