Significance of the sinus-node recovery time.

The phenomenon of postpacing depression of cardiac pacemakers was utilized to evaluate the sinus-node function in 56 patients by analyzing the sinus-node recovery time (SRT), that is, the interval between the last paced P wave and the following sinus P wave. Corrected SRT (CSRT) is defined as the recovery interval in excess of the sinus cycle (SRT -sinus cycle length). The SRT was measured following sinusnode suppression by (1) isolated premature beats (PABs) and (2) atrial pacing (AP) at rates of 100 to 140/min for periods of 2 to 5 min at each level. Twentyeight patients had normal heart rates (group A), and 28 patients had sinus bradyeardia (SB; group B). Ten of the 28 patients with SB were restudied after receiving atropine (2 mg intravenously). The CSRT with PABs was similar in both group A and group B patients and remained essentially unchanged after atropine despite a decrease in sinus cycle length. The phenomenon of interpolated PABs was demonstrated in seven of the 56 patients. In 27 of the 28 patients with normal heart rates (group A), the CSRT with AP ranged from 110 to 525 msec and was essentially independent of the rate and duration of AP. In the remaining one patient of group A, despite a normal heart rate, the CSRT was prolonged (1810 msec) and directly dependent