title: Sex-related differences in autonomic modulation of heart rate in middle-aged subjects.
abstract: BACKGROUND
  Women have worse outcomes when they experience acute myocardial infarction (MI), but the reasons for this sex-related difference are not well understood. Because cardiovascular neural regulation plays an important role in cardiac mortality, we studied possible sex-related differences in the autonomic modulation of heart rate (HR) in middle-aged subjects without known heart disease.
  
  
  RESULTS
  Baroreflex sensitivity (BRS) and HR variability were studied in randomly selected, age-matched populations of middle-aged women (n = 186; mean age, 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 maneuver was significantly lower in women (8.0 +/- 4.6 ms/mm Hg, n = 152) than in men (10.5 +/- 4.6 ms/mm Hg, n = 151) (P < .001), and the low-frequency component of HR variability measured from ECG recordings also was lower in women (P < .001), whereas the high-frequency component was higher in women than in men (P < .001). The ratio between the low-and high-frequency oscillations also was lower in the women (P < .001). The increase of HR and decrease of high-frequency component of HR variability in response to an upright posture were smaller in magnitude in women than in men (P < .01 for both). After adjustment for differences in the baseline-variables, such as blood pressure, HR, smoking, alcohol consumption, and psychosocial score, the sex-related differences in BRS and HR variability still remained significant (P < .001 for all). Women with estrogen replacement therapy (n = 46) had significantly higher BRS and total HR variance than the age-matched women without hormone treatment (P < .01 for both), and the BRS and HR variability of the women with estrogen therapy did not differ from those of the age-matched men.
  
  
  CONCLUSIONS
  Baroreflex responsiveness is attenuated in middle-aged women compared with men, but the tonic vagal modulation of HR is augmented. Hormone replacement therapy appears to have favorable effects on the cardiovascular autonomic regulation in postmenopausal women.
authors.affiliation: Division of Cardiology, University of Oulu, Finland.
authors.forename: H V
authors.initials: HV
authors.lastname: Huikuri
authors.forename: S M
authors.initials: SM
authors.lastname: Pikkujämsä
authors.forename: K E
authors.initials: KE
authors.lastname: Airaksinen
authors.forename: M J
authors.initials: MJ
authors.lastname: Ikäheimo
authors.forename: A O
authors.initials: AO
authors.lastname: Rantala
authors.forename: H
authors.initials: H
authors.lastname: Kauma
authors.forename: M
authors.initials: M
authors.lastname: Lilja
authors.forename: Y A
authors.initials: YA
authors.lastname: Kesäniemi
mesh_terms: D001341:Autonomic Nervous System / Q000502:physiology*; D017704:Baroreflex / Q000502:physiology*; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011897:Random Allocation; D012737:Sex Factors; D014630:Vagus Nerve / Q000502:physiology