SEX-RELATED DIFFERENCES IN AUTONOMIC MODULATION OF HEART-RATE IN MIDDLE-AGED SUBJECTS

Background Women have worse outcomes when they experience acute myocar dial infarction (MI), but the reasons for this sex-related difference are not well understood. Because cardiovascular neural regulation play s an important role in cardiac mortality, we studied possible sex-rela ted differences in the autonomic modulation of heart rate (HR) in midd le-aged subjects without known heart disease. Methods and Results Baro reflex sensitivity (BRS) and HR variability were studied in randomly s elected, age-matched population of middle-aged women (n = 186; mean ag e, 50 +/- 6 years) and men (n = 188; mean age, 50 +/- 6 years) without hypertension, diabetes, or clinical or echocardiographic evidence of heart disease. BRS measured from the overshoot phase of the Valsalva m aneuver was significantly lower in women (8.0 +/- 4.6 ms/mm Hg, n = 15 2) than in men (10.5 +/- 4.6 ms/mm Hg, n = 151) (P < .001), and the lo w-frequency component of HR variability measured from ECG recordings a lso was lower in women (P < .001), whereas the high-frequency componen t was higher in women than in men (P < .001). The ratio between the lo w- and high-frequency oscillations also was lower in the women (P < .0 01). The increase of HR and decrease of high-frequency component of HR variability in response to an upright posture were smaller in magnitu de in women than in men (P < .01 for both). After adjustment for diffe rences in the baseline variables, such as blood pressure, HR, smoking, alcohol consumption, and psychosocial score, the sex-related differen ces in BRS and HR variability still remained significant (P < .001 for all). Women with estrogen replacement therapy (n = 46) had significan tly higher BRS and total HR variance than the age-matched women withou t hormone treatment (P < 0.1 for both), and the BRA and HR variability of the women with estrogen therapy did not differ from those of the a ge-matched men. Conclusions Baroreflex responsiveness is attenuated in middle-aged women compared with men, but the tonic vagal modulation o f HR is augmented. Hormone replacement therapy appears to have favorab le effects on the cardiovascular autonomic regulation in postmenopausa l women.