The height and LnLF power cover the most of sex differences during orthostasis
Authors: Medviedieva Anastasiia , Shekh Vira 
Published: May 2022

The sex differences in autonomic regulation of cardiovascular and respiratory system, as one of the key physiological mechanisms of disease, till now remain underexplored. We hypothesized that cardiovascular and respiratory systems share the same autonomic mechanisms, represented by heart rate variability (HRV) indexes, opposing the effect of orthostasis, which are partially different between sexes.

The 147 participants, 78 men and 69 women were recruited from the student population of local University for the research study. The low and high frequency heart rate variability (LF and HF HRV) was computed from ECG recording in the sitting position for 5 minutes of resting and slow breathing (6 breaths per minute) stages. The sitting position was used according to requirement of spirometry system. The tidal volume (TV) and respiratory rate (RR) were recorded for one minute at the end of each stage. Systolic and diastolic blood pressure (SBP and DBP) were measured with automatic digital sphygmomanometer (OMRON EVOLV, Japan). Two‐way repeated measures MANOVA and MANCOVA (SPSS 22) were used to test for effects of orthostasis and sex on HR, SBP, DBP, HRV variables, TV, RR, and MV. Multiple stepwise regression analysis was used to determine predictors of cardiovascular and respiratory variables. Written informed consent was obtained from each participant.

In the current study we found significant main effects of orthostasis on HR, DBP, LnLF, LnHF (P<0.001 for each), and significant main effects of sex on HR, SBP, LnLF, TV, and MV (for HR P=0.004, for LnLF P=0.006, for the rest P<0.001). Follow the significant sex by orthostasis interaction we found that RR increased only for men upon standing but not for women.

Multiple stepwise linear regression analysis revealed that when adjusting for potentially confounding variables, the change of cardiovagal‐mediated LnHF power (Figure 1A) was an independent predictor of HR and RR response to orthostasis (Table 1, Models 1 and 2). Women was an independent predictor of dRR, while Women by dLnLF interaction determined dHR, showing that LnLF, indicating according to our hypothesis, baroreflex‐mediated sympathoinhibition, contribute to HR response in women (Fig. 1B).

The BMI by LnLF power interaction was a negative predictor of resting RR (Table 1, Model 3). It is suggested that baroreflex‐mediated sympathoinhibition contribute to respiratory central pattern generator particularly in people with increased BMI. The same interaction was a positive predictor of TV (Table 1, Model 4) indicating that increased time of respiratory cycle accompines greater TV.

The comparisons of the similar models with and without dummy variables indicating “Women”, revealed that the dummy variable replaced “Height” or “LnLF”, suggesting that differences in both independent variables relate to mechanisms controlling cardiorespiratory system specific to female gender.

In conclusion, cardiovagal‐mediated LnHF HRV power controls both HR and RR and determines their response to orthostasis. The differences in height and LnLF power cover the most of the sex difference in mechanisms controlling cardiovascular and respiratory variables and their response to orthostasis.