[The heart rate alternative sequence during pharmacologic total autonomic blockade in patients with sick sinus syndrome: differential effects in relation to age].

To study the age-related changes of the sinus node function and the variations of influence of autonomic nervous system, pharmacologic total autonomic blockade (TAB) was conducted in 35 patients with symptomatic sinus bradycardia (21 men and 14 women, 50 +/- 21 years, mean +/- SD). Twenty-one patients [Group I, consisting of 14 patients younger than 60 years (group IY) nd 7 patients 60 years or older (group IE)] had a normal intrinsic heart rate (IHRo), and 14 patients had an abnormal IHRo [Group II, consisting of 8 patients younger than 60 years (group IIY) and 5 patients 60 years or older (group IIE)]. The basic cycle length was significantly longer in group II than in group I, suggesting that intrinsic sinus node function was more seriously deteriorated in group II in spite of the compensatory effect of autonomic regulation. In group II it was characteristic that the cycle length (CL) after atropine sulfate administration was longer than the CL of the predicted intrinsic heart rate (IHRp). Otherwise, some group II patients might be regarded as normal by atropine sulfate administration alone. Parasympathetic tone showed a negative correlation with age, and it was most enhanced in group IIY, suggesting that parasympathetic negative chronotropy was stronger in this group. In the course of propranolol administration, prolongation of CL was significantly larger in group IIE than in group IIY, but there was no age-related difference in group I. In group II, beta-adrenergic blockade with propranolol administration showed that sympathetic positive chronotropy was a critical compensatory factor around the upper limit of the CL of IHRp.(ABSTRACT TRUNCATED AT 250 WORDS)