Autonomic effects on the human cardiac conduction system. Evaluation by intracardiac electrocardiography and programmed stimulation techniques.

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Abstract
After right heart catheterization in 20 subjects, aged 30-82 yr, the techniques of intracardiac electrocardiography and programmed stimulation were used to determine variables of normal function of the sinuatrial node, atrioventricular node and intraventricular conduction system. Specific attention was paid to autonomic influences on these variables, which were assessed by comparison of determinations made before and after cardiac autonomic block induced by i.v. atropine, 0.03 mg/kg, and propranolol, 0.15 mg/kg. Sympathetic and larger depressant vagal effects on sinuatrial node function (sinus cycle length and sinus node recovery time after overdrive atrial pacing) were demonstrated. Automic effects on atrioventricular node conduction (AH interval and Wenckebach threshold to incremental right atrial pacing) were small, of similar magnitude (approximately 20% change), and opposing. Only significant vagal effects in increasing the effective and functional refractory periods of the atrioventricular node (pacing at 100 bpm [beats per minute]) were demonstrated (approximately 15 and 12% change, respectively). The atrial effective refractory period, duration of the His bundle electrogram, and the HV interval were unchanged by either drug. Effects on refractory periods of the intraventricular conduction system could not be assessed because exact determinations were limited by refractoriness of the atrioventricular node. The QT interval with pacing at 100 bpm decreased significantly after atropine but was unchanged after propranolol.