Assessment of sinus node function in patients with the sick sinus syndrome.

SUMMARY Thirty-one patients with symptomatic sinus node dysfunction were evaluated with electrocardiograms, Holter monitor recordings, exercise, isoproterenol infusions, atropine administration, Valsalva maneuvers, carotid sinus massage, and overdrive pacing. Four basic clinical subsets were recognized: (1) carotid sinus hypersensitivity (2) bradyeardia-tachyeardia syndrome, (3) episodic sinus arrest, and (4) persistent symptomatic sinus bradyeardia. The study group demonstrated a normal heart rate response to exercise and isoproterenol infusion (%A = +95 exercise, +144 isoproterenol) in the face of diminished responsiveness to atropine administration (%A = +23). Marked carotid sinus hypersensitivity was demonstrated in eight patients, and four patients demonstrated slight abnormalities during performance of Valsalva maneuvers. Significant suppression of sinus node dysfunction was observed following atrial overdrive in the study group (postpacing pause = 3087 464 msec) as compared to patients without significant sinus node function (postpacing pause = 1073 63 msec) (P < 0.001). In patients with intact V-A conduction, ventricular overdrive also resulted in sinus node suppression (postpacing pause = 1901 357 msec). There was a marked decrease in the degree of sinus node depression following atropine administration. Ten of 31 patients demonstrated various degrees of A-V block following atrial pacing at rates less than 100 beats/min. It is concluded that the present methods of evaluation of sinus function, especially sinus node recovery time following overdrive pacing, may prove of value in the investigation of patients with syncope of unknown etiology.