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

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.