Ventilatory response to hypercapnia is increased after 4\xa0h of head down bed rest

K R Murray,Heather Edgell,S Wasef

doi:10.1038/s41598-021-81837-w

Abstract

Head-down bed rest (HDBR) has previously been shown to alter cerebrovascular and autonomic control. Previous work found that sustained HDBR (≥ 20 days) attenuates the hypercapnic ventilatory response (HCVR); however, little is known about shorter-term effects of HDBR nor the influence of HDBR on the hypoxic ventilatory response (HVR). We investigated the effect of 4-h HDBR on HCVR and HVR and hypothesized attenuated ventilatory responses due to greater carotid and brain blood flow. Cardiorespiratory responses of young men (n = 11) and women (n = 3) to 5% CO2 or 10% O2 before and after 4-h HDBR were examined. HDBR resulted in lower HR, lower cardiac output index, lower common carotid artery flow, higher SpO2, and higher pulse wave velocity. After HDBR, tidal volume and ventilation responses to 5% CO2 were enhanced (all P < 0.05), yet no other changes in cardiorespiratory variables were evident. There was no influence of HDBR on the cardiorespiratory responses to hypoxia (all P > 0.05). Short-duration HDBR does not alter the HVR, yet enhances the HCVR, which we hypothesize is a consequence of cephalic CO2 accumulation from cerebral congestion.

Highlights

Head-down bed rest (HDBR) has previously been shown to alter cerebrovascular and autonomic control
After 4-h HDBR, there were no significant changes in the responses of mean arterial pressure (MAP) (P = 0.23), heart rate (HR) (P = 0.19), SVi (P = 0.83), Qi (P = 0.50), or TPRi (P = 0.15) to hypercapnia (Fig. 2)
After 4-h HDBR, there were no significant changes in the responses of middle cerebral artery (MCA) mean velocity (P = 0.94), Cerebrovascular resistance index (CVRi) (P = 0.74), Resistance index (RI) (P = 0.69), pulsatility index (PI) (P = 0.51), or common carotid artery (CCA) blood flow (P = 0.15) to hypercapnia (Fig. 3)

Summary

Introduction

Head-down bed rest (HDBR) has previously been shown to alter cerebrovascular and autonomic control. We investigated the effect of 4-h HDBR on HCVR and HVR and hypothesized attenuated ventilatory responses due to greater carotid and brain blood flow. Longer duration HDBR studies have found lower C O2 chemosensitivity after 20 or 120 days of H DBR10,11 These interactions could be important to note from a clinical perspective as use of intraabdominal CO2 insufflations during surgery in 30–40° head-down posture (i.e., Trendelenburg position) increases arterial PCO2 within 1–2 h12–14, likely stimulating central chemoreceptors, and influencing autonomic c ontrol[15]. Since hypercapnia induced hyperventilation increases the partial pressure of arterial oxygen, and alterations in carotid flow have been shown to alter peripheral chemoreflex s ensitivity[9,16,20,21,22], our secondary objective was to assess the hypoxic ventilatory response (HVR) in response to 4-h HDBR. We hypothesized that 4-h HDBR would attenuate both HCVR and HVR

Objectives

Methods

Results

Discussion

Conclusion