Relation between tonic sympathetic and vagal control of human sinus node function.
BACKGROUND: Clinical conditions, such as heart failure or myocardial infarction are associated with enhanced sympathetic and reduced parasympathetic activity as compared to normal controls. The reciprocal alteration in cardiac autonomic tone likely contributes to the electrical instability of the myocardium. Little information is available on the relation between sympathetic and vagal cardiac control in healthy human subjects. METHODS AND RESULTS: Heart period changes in response to autonomic blockades were measured in 16 young, healthy human subjects. Adrenergic and cholinergic blockades were induced by i.v. propranolol (0.2 mg/kg) and atropine (0.04 mg/kg) in two opposite orders on two occasions; interindividual correlations were performed between the R-R interval responses to propranolol and to atropine obtained under the various blockade conditions, and the magnitude of the responses were compared by a drug x order two factorial ANOVA design. It was found, that previous adrenergic blockade did not reduce the extent of cardioacceleration produced by subsequent cholinergic blockade and that the R-R interval responses to atropine and to subsequently given propranolol did not share significant variance across subjects (r = 0.22, P = 0.234). Also, no interindividual correlation was found between the R-R interval responses to propranolol and to atropine, with the influence of the other, respective, autonomic division already blocked (r = 0.42, P = 0.114). CONCLUSIONS: Under resting conditions, activity levels of cardiac vagal and sympathetic outflows are not related across young, healthy human subjects and peripheral interaction is not manifest between the autonomic divisions.