Sinoatrial node sympathovagal balance and intrinsic heart rate at rest: no difference between young healthy women and men.

There are numerous differences in arrhythmia propensity and cardiac electrophysiology between women and men. One proposed reason is differences in the autonomic nervous system (ANS) influence. We therefore compared the sympathetic and parasympathetic influence (sympathovagal balance) on the sinoatrial node (SAN) at rest in healthy young women (n = 15) and men (n = 15) with a mean age of 24 yr. Pharmacological blockade of the ANS influence on the SAN was induced by sequential bolus injections of atropine (0.04 mg/kg body wt) and propranolol (0.2 mg/kg body wt) during continuous electrocardiographic recordings. The heart rate (HR) at baseline, after atropine, and the intrinsic heart rate (IHR) after adding propranolol were used to calculate the accelerator "m," which is ≥1.00, and decelerator "n," which is ≤1.00 according to the Rosenblueth and Simeone concept and equation: HR = m × n × IHR. On the group level, IHR was median [interquartile range (IQR)] 93.3 (88.4-98.8) beats/min, m was 1.16 (1.13-1.22), n was 0.58 (0.55-0.64), and the sympathovagal balance m × n was 0.70 (0.65-0.76), confirming dominant parasympathetic influence at rest. There were no significant differences between women and men in any of these measures, and thus no fundamental difference in the ANS influence on the main impulse generator of the heart at rest. This result contrasts with the significant differences in electrophysiological measures at rest and their responses to stress tests, as well as to the differences in arrhythmia propensity between women and men on both the atrial and ventricular level of the heart.NEW & NOTEWORTHY Differences in the autonomic nervous system (ANS) activity is one potential explanation for differences in cardiac electrophysiology between women and men. We therefore compared the ANS influence on the sinoatrial node (SAN; main cardiac impulse generator), between women and men at rest. Atropine and propranolol were used to sequentially block the two ANS limbs. There were no statistically significant differences between women and men in the ANS influence on the SAN at rest.