Sinoatrial node sympatho-vagal 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) activity. We therefore compared the sympathetic and parasympathetic influence (sympatho-vagal balance) on the sinoatrial node (SAN) at rest in healthy young women (n=15) and men (n=15) with a mean age of 24 years. Pharmacological blockade of the ANS activity on the SAN was induced by sequential bolus injections of atropine (0.04 mg per kg b.w) and propranolol (0.2mg per kg b.w.) during continuous electrocardiographic recordings. The heart rate (HR) at baseline, after atropine, and the intrinsic HR (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 & Simeone concept and equation: HR=m*n*IHR. On the group level IHR was median (IQR) 93.3 (88.4-98.8) beats per min, m was 1.16 (1.13-1.22), n was 0.58 (0.55-0.64), and the sympatho-vagal 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 to 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.