Women have a greater cardiac vagal withdrawal to heat stress compared to men

ABSTRACT

The heated environment shifts the sympatho-vagal balance toward sympathetic predominance and vagal withdrawal. Women’s heart is more reliant on vagal autonomic control, while men’s heart is more dependent on sympathetic control. However, sex differences in cardiovascular autonomic responses to heat stress remain unknown. We aimed to investigate the cardiovascular autonomic regulation under heat stress between sexes. Thirty-two young participants (27 ± 4 years old; 16 women) were enrolled in a single visit, resting for 30min at baseline (thermal reference condition TC; ∼24°C) and 30min under a heated environment (HOT; ∼38°C). Blood pressure (BP), skin temperature, electrocardiogram, and respiratory oscillations were continuously recorded. The heart rate variability (HRV) was assessed by spectral analysis (low-frequency [LFnu; sympathetic and vagal] and high-frequency [HFnu; vagal]), and symbolic analysis (0 V% [sympathetic] and 2UV%, and 2LV% [vagal]). The spontaneous baroreflex sensitivity (BRS) was calculated by the gain between BP and R–R within the LF band (αLF). The estimated maximal aerobic capacity and body surface area were employed as covariates in sex comparisons. The effects of HOT were the following: 1) Women have a greater cardiac vagal withdrawal to heat stress compared to men; 2) Sex differences on cardiac autonomic response to heat stress exist after controlling for the effect of estimated physical fitness and body surface area. Therefore, heat stress provokes a higher vagal withdrawal to the heart in women compared to men. It could be attributed to sex per se since significant differences between men and women were not modified after covariate analysis.

KEYWORDS:

• Sexual dimorphism
• women
• heat stress
• heart rate variability
• baroreflex control

Abbreviations

αLF: baroreflex sensitive index obtained by LF components

∆: Delta

0V: Patterns with no variations of R-R interval

2LV: Patterns with two unlike variations of R-R interval

2UV: Patterns with two like variations of R-R interval

(A_D/m): surface area-to-mass

ANCOVA: Analysis of covariance

ANOVA: Analysis of variance

BM: Body mass

BMI: Body mass index

BP: Blood pressure

BPV: Blood pressure variability

BRS: Spontaneous baroreflex sensitivity

HF_R-R: High-frequency power (0.15–0.4 Hz)

HFnu: HF normalized unity

HI: Heat Index

HOT: Heated environment

HRV: Heart rate variability

LF/HF: Sympatho-vagal balance

LF_R-R: Low-frequency power (0.04–0.15 Hz)

LFnu: LF normalized unity

SBP LF: Low-frequency component of the systolic blood pressure variability

SBP: Systolic blood pressure

SUP: Supine position

SW: Sweat rate

TC: Thermal reference condition

TP_R-R: Total power of R–R interval

Tskin: Skin temperature

Tsub: Sublingual temperature

USG: Urine-specific gravity

VLF_R-R: Very-low-frequency band below 0.04 Hz

VO_2max: Maximal oxygen uptake

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), Programa de Fomento à Pesquisa na UFF (FOPESQ-2020), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Italian Ministry of Health, and the APC central fund of the University of Milan.