**6. Heart rate variability and menopause**

Postmenopausal women have greater sympathetic and less parasympathetic activity than premenopausal women (Brockbank et al., 2000; Earnest et al., 2010). Moreover, Mercuro et al.'s study (2000) reveals the harmful effects of hypoestrogenism on the autonomic modulation of the HR, while other studies have demonstrated numerous evidences that endogenous hormones (estrogen and progesterone) contribute to a cardioprotective phenotype in women (Vitale et al., 2009).

Parasympathetic modulation shifts to a lower range with normal aging. Although parasympathetic modulation is generally higher in women than men, aging reduces the difference between genders, with changes in HRV beginning approximately at menopause (Earnest et al., 2010). Boettger (2010) examined changes in cardiovascular autonomic parameters obtained from short-term recordings over time. The data he collected indicated a lifelong shift in autonomic balance toward sympathetic predominance, starting at the age of 30 years.

Zuttin (2009) evaluated and compared autonomic modulation of the HR at rest in healthy young, premenopausal and postmenopausal women leading a sedentary lifestyle, to verify cardiovascular adjustment in response to postural changes. This investigation involved 113 healthy sedentary women, who were divided into a young group (YG) with an average age of 23±3.4 years (n=40), a premenopausal group (PreMG) aged 36±3.1 years (n= 39), and a postmenopausal group (PostMG) with an average age of 55±4.5 years (n=34).

In the supine position, it was found that the YG presented significantly higher values of the HF index in absolute units (ms2) and lower LF values (ms2) and ratio than the PostMG. In addition, the YG and PostMG showed a statistical difference in all the evaluated indices (p<0.05), while no difference was found between the PreMG and PostMG groups (p>0.05). In a comparison of the YG vs. PreMG and YG vs. PostMG groups in the sitting position, the YG presented significantly higher values for the ratio (p<0.05).

With regard to the effect of postural adjustment on the autonomic HR modulation, a comparison of the indices obtained in the supine and sitting positions revealed significant differences (p<0.05) in all the indices. On the other hand, the PreMG groups showed a difference in the LF/HF index (p<0.05), while the PostMG group showed no significant difference (p<0.05).

Having calculated the regression coefficients, it was found that the straight line of the adjusted regression indicates that, as the age of the subjects increases, it is possible to estimate the reduction of the HF index (ms2). The parameters indicate mainly a reduction of the postural change in parasympathetic modulation. With aging, the adjustment capacity diminishes, as indicated by the delta between the supine and sitting positions.
