The origins of the sinus node pacemaker complex in man: demonstration of dominant and subsidiary foci.

To test the hypothesis that the human sinus node is capable of demonstrating multiple sites of impulse generation, we assessed spontaneous shifts in the sinus node pacemaker complex, and shifts after overdrive atrial pacing, premature atrial stimulation and carotid sinus massage. A total of 24 patients aged 59 +/- 15 years (mean +/- SD) in whom stable sinus node electrograms were obtained were selected for the study. Ten of the 24 patients had sick sinus syndrome, whereas 14 had no sinus node dysfunction. All 24 patients had atrial pacing at cycle lengths of 1,000 to 300 ms; 9 patients had premature atrial stimulation and 12 had carotid sinus massage. Shifts in the sinus node pacemaker complex occurred spontaneously in 4 (17%) of the 24 patients; after atrial pacing at cycle lengths of 800 to 300 ms (mean 387 +/- 92) in 15 (63%) of 24 patients; after premature atrial stimulation at one or more coupling intervals in 5 (56%) of 9 patients and during carotid sinus massage in 9 (75%) of 12 patients. Shifts in the sinus node pacemaker complex lasted one to six beats and returned to the original site within two to seven beats. Spontaneous shifts in the sinus node pacemaker complex occurred in 3 of 14 patients without sick sinus syndrome and were induced in 6 (60%) of 10 patients with sick sinus syndrome and 11 (79%) of 14 patients without sick sinus syndrome. Shifts in sinus node pacemaker complex were characterized by loss of primary negativity, change in P wave morphology, significant (p less than 0.001) prolongation of sinoatrial interval and sinus cycle length.