Effect of Sex and Prehypertension on Blood Pressure and HRV Responses to Slow Breathing

Abstract

The slow breathing reduces blood pressure in hypertensive patients but not in normotensive subjects. In general, young women have lower blood pressure compared to age-matched men. We hypothesized that slow breathing reduces sex differences and differences between normotensive and prehypertensive subjects in blood pressure and autonomic regulation. A total of 293 men and women were recruited from V.N. Karazin Kharkiv National University student population. The subjects were subdivided into normotensives and prehypertensives according to JNC-7 classification. The resting stage was followed by two slow breathing stages (10 and 6 breaths per minute) for 5 min each. ECG was continuously recorded and high and low frequency (HF and LF) HRV power were computed (CardioLab CS, XAI-Medica, Ukraine). The systolic blood pressure (SBP) and diastolic blood pressure (DBP) were recorded by automatic digital sphygmomanometer (OMRON EVOLV, Japan) at the end of each stage. Three-way repeated measures MANOVA (SPSS 22) was used to test for effects of slow breathing, sex, and prehypertension on HR, SBP, DBP, and HRV variables. The written informed consent was obtained from each participant. The slowing respiratory rate to 10 and 6 breaths per minute decreased SBP in prehypertensive men and women (Fig. 1B). Due to the opposite tendency, SBP was higher in normotensive men at 6 breaths per minute compared with 10 breaths per minute (Fig. 1A). The DBP was decreased only in prehypertensive women at both breathing rate (not shown). The slowing respiratory rate to 10 breaths per minute increased LnHF HRV power indicating cardiovagal outflow, and to 6 breaths per minute increased LnLF HRV power indicating sympathoinhibitory arm of baroreflex according to our hypothesis presented at EB2018 conference. At rest, LnHF power was lower in normotensive men than in women, indicating sex differences in cardiovagal outflow (not shown). Because of LnHF power increased at both breathing rates in normotensive men, the sex difference was no longer significant. The resting LnLF power was higher in prehypertensive men than in women (Fig. 2B). Unlike women, men decreased LnLF power at 10 breaths per minute (Fig. 2A and 2B), indicating decrease activity of sympathoinhibitory arm of baroreflex at the time of increased parasympathetic activity. As a result, the sex difference in LnLF power was no longer significant during slow breathing. At 10 breaths per minute LnHF power was higher in normotensive than in prehypertensive women, indicating higher reserves of cardiovagal outflow in this group. The LnLF power was also higher in normotensive than in prehypertensive women at rest and at 10 breaths per minute (6.94±0.09 vs. 6.50±0.12 Lnms2 and 6.85±0.11 vs. 6.38±0.13 Lnms2, P=0.003 and P=0.007, respectively). At 6 breaths per minute, this difference was no longer significant (P=0.195). According to our hypothesis, an equalizing of LnLF power indicates a similar activity of sympathoinhibitory arm of baroreflex, that can contribute to the decrease of SBP and DBP in prehypertensive women. In conclusion, slowing respiratory rate to 10 and 6 breaths per minute decreased SBP in prehypertensive men and women, and DBP only in prehypertensive women. The slow breathing reduced differences in autonomic nervous system activity between genders as well as between normotensives and prehypertensives.