Chemoreflex function and brain blood flow during upright posture in men and women.

Christopher Hazlett,Heather Edgell

doi:10.14814/phy2.13571

Abstract

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.

Highlights

Orthostatic intolerance is more prevalent in young women compared to men (Convertino 1998; Ganzeboom et al 2003; Waters et al 2005)
We hypothesized that: (1) during upright tilt, women would have lower mean arterial pressure (MAP), lower brain blood velocity, and lower respiratory rate compared to men; (2) women in the midluteal phase (ML) phase would have greater cerebrovascular reactivity to CO2 compared to early follicular phase (EF); (3) CO2 chemoreflex function would be enhanced in the upright posture compared to supine in both sexes; and (4) women during the EF phase would have enhanced CO2 chemoreflex activity during tilt compared to the ML phase and men
Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society

Summary

Introduction

Orthostatic intolerance is more prevalent in young women compared to men (Convertino 1998; Ganzeboom et al 2003; Waters et al 2005). Peltonen et al (2015) found no difference between the sexes when investigating the brain blood flow response to hypercapnia in men versus women in the EF phase of the menstrual cycle (low estrogen and low progesterone). We hypothesized that: (1) during upright tilt, women would have lower MAP, lower brain blood velocity, and lower respiratory rate compared to men ( contributing to lower orthostatic tolerance); (2) women in the ML phase would have greater cerebrovascular reactivity to CO2 compared to EF; (3) CO2 chemoreflex function would be enhanced in the upright posture compared to supine in both sexes; and (4) women during the EF phase would have enhanced CO2 chemoreflex activity during tilt compared to the ML phase and men

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