Gender-specific differences in the QT interval and the effect of autonomic tone and menstrual cycle in healthy adults.

Gender differences in the corrected QT interval have been noted since Bazett's initial description during the 1920s. The mechanism of this gender difference is unknown, and this study was undertaken to evaluate potential autonomic and menstrual cycle effects on the QT interval. The study population consisted of a healthy volunteer sample of 23 women and 20 men. Twelve-lead electrocardiographic determinations were made at rest and following double autonomic blockade (with atropine and propranolol) during the menstrual, follicular, and luteal phases of the menstrual cycle. Men were studied during 3 separate visits as controls. The corrected QT interval at baseline tended to be longer in women than men (421 +/- 16 ms vs 414 +/- 15 ms: p <0.07). Following double autonomic blockade, the corrected QT interval increased to 439 +/- 11 ms: p <0.001). However, the gender difference in corrected QT interval was unchanged (443 +/- 15 ms vs 437 +/- 12 ms). At baseline, there was no significant difference in the corrected QT interval among the 3 phases of the menstrual cycle (421 +/- 10, 423 +/- 18, and 420 +/- 18 in the menstrual, follicular, and luteal phases, respectively) and the corrected QT interval was longer in women than men at each visit. Following double autonomic blockade, the corrected QT interval in women was shorter in the luteal phase (438 +/- 16 ms) versus the menstrual (446 +/- 15 ms) or the follicular phase (444 +/- 13 ms; p <0.05). However, this difference, which was not present at baseline, does not appear to be responsible for the gender difference in the QT interval at rest. In conclusion, our results confirm that the corrected QT interval tends to be longer in women than men. Differences in autonomic tone and menstrual cycle variability in the corrected QT in women at rest do not appear to be responsible for the gender differences in the QT interval. The mechanism responsible for the longer QT interval in women remains to be defined.

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 918 +/- 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.