Sinoatrial response to premature atrial stimulation during atrial pacing in aged patients with and without sinus node dysfunction.

Sinus node function was analysed in 15 aged patients, including 8 with sick sinus syndrome and 7 without apparent sinus node dysfunction, by rapid artial pacing and premature atrial stimulation during sinus rhythm and 5 beats of atrial pacing. Three patterns of sinoatrial response were identified by premature atrial stimulation during atrial pacing. Type I response (3 cases) represented approximately constant return cycles and type II (2 cases) represented a markedly prolonged return cycles following premature stimulation. Type III response was characterized by a progressive shortening of return cycles as the prematurity of atrial extrastimuli was increased. Underlying mechanisms responsible for these responses were discussed in relation to the sinus node automaticity and sinoatrial conduction.