Key result
Upright posture enhanced the CO2 chemoreflex ventilatory response in both sexes (P≤0.028), while only men exhibited an augmented mean arterial pressure response (P≤0.017).
Why the study?
Do men and women have different chemoreflex and brain blood flow responses to upright posture and hypercapnia?
Observational (n=27)
Do men and women have different chemoreflex and brain blood flow responses to upright posture and hypercapnia?
p-value: p=≤0.028
Men and women exhibit different ventilatory and blood pressure responses to upright tilt and hypercapnia, suggesting sexually dimorphic chemoreflex enhancement during orthostatic stress.
Findings in small physiological study leave open chemoreflex contribution to sex differences in orthostatic intolerance.
Orthostatic intolerance is more common in women than men, and some studies have found that women in the early follicular (EF) phase of the menstrual cycle experience the greatest feelings of lightheadedness. Chemoreflex function while supine or upright was investigated to determine the potential contribution of ventilatory control to these phenomena. Men (n = 13) and women (n = 14) were tested while supine and 70° upright (head-up tilt [HUT]) and given: (1) normoxia or (2) hypercapnia (5% CO2). Women were tested during the EF phase (days 2–5) and the midluteal phase (ML; days 18–24). During HUT, all groups reduced cerebrovascular resistance index (men: 1.45 ± 0.08 to 1.42 ± 0.07 mmHg/(cm·sec), EF: 1.38 ± 0.11 to 1.26 ± 0.10 mmHg/(cm·sec), ML: 1.25 ± 0.07 to 1.09 ± 0.07 mmHg/(cm·sec); P ≤ 0.019); however, only men increased ventilation (men: 11.99 ± 0.65 to 13.24 ± 0.83 L/min; P < 0.01). In response to hypercapnia in the supine position, men had a smaller increase of diastolic middle cerebral artery velocity compared to women in the ML phase (men: +9.1 ± 2.0 cm/sec, ML: +15.7 ± 3.1 cm/sec, P = 0.039). During hypercapnia in HUT (compared to hypercapnia while supine), all groups had an augmented increase of ventilation (men: +7.46 ± 1.34 vs. +5.84 ± 1.09 L/min, EF: +6.71 ± 0.83 vs. +5.48 ± 0.66 L/min, ML: +7.99 ± 1.13 vs. +5.65 ± 0.81 L/min; P ≤ 0.028), suggesting that all groups experienced augmentation of the CO2 chemoreflex; however, only men had an augmented increase of mean arterial pressure (+0.10 ± 0.58 to +4.71 ± 0.87 mmHg; P ≤ 0.017). Our results indicate that men have different ventilatory responses to upright tilt compared to women, and that the CO2 chemoreflex response is enhanced in upright posture in both sexes. Furthermore, sexually dimorphic blood pressure responses to this chemoreflex enhancement are evident.
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Hazlett et al. (2018) reported an observational. Head-up tilt and hypercapnia (5% CO2) vs. Supine position and normoxia was evaluated on Change in ventilation during hypercapnia in head-up tilt compared to supine (p=≤0.028). Upright posture enhanced the CO2 chemoreflex ventilatory response in both sexes (P≤0.028), while only men exhibited an augmented mean arterial pressure response (P≤0.017).
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