Gender differences in heart rate before and after autonomic blockade: Evidence against an intrinsic gender effect

OBJECTIVES: To document gender differences in heart rate in healthy young adult men and women, and examine the degree to which autonomic tone and other variables may be associated with the gender differences in heart rate. DESIGNS: Cohort study. SETTINGS: Clinical Research Center of a tertiary care medical center. PATIENTS: A volunteer sample of 20 healthy men and 23 healthy women between ages 21 and 39 years. INTERVENTIONS: Subjects were each studied three times: during the menstrual, follicular, and luteal phases of the menstrual cycle in women; and 5 to 10 days apart in men. Electrocardiograms (ECGs) were obtained at baseline and following double autonomic blockade with propranolol 0.2 mg/kg and atropine 0.04 mg/kg. Maximum exercise capacity was determined by bicycle ergometry. MAIN OUTCOME MEASURES: Sinus cycle length at baseline and following double autonomic blockade, before and after correction for confounding variables. RESULTS: Men had longer sinus cycle length both at baseline and after double autonomic blockade (971 ± 88 ms versus 916 ± 115 ms, P < 0.02, and 645 ± 41 ms versus 594 ± 57 ms, P < 0.0001). Sinus cycle length in women was longer than during the menstrual than luteal phase but this difference could not account for the gender difference in sinus cycle length. Men also had a greater maximum exercise capacity than women (1295 ± 167 kpm/min versus 857 ± 227 kpm/min; P < 0.0001). By analysis of covariance, maximum exercise capacity was the most significant predictor of sinus cycle length (P < 0.0003 at baseline, and P < 0.001 post blockade) and gender did not have a significant effect. The relationship of maximum exercise capacity to sinus cycle length was blunted but not abolished by autonomic blockade. CONCLUSIONS: Sinus cycle length is longer in men than women. This difference appears to be associated with a gender difference in exercise capacity rather than intrinsic gender related properties of the sinus node or differences in autonomic tone. In addition, exercise induced bradycardia is mediated by both autonomic and nonautonomic factors in both genders.