Abstract
Head-down bed rest (HDBR) is commonly considered as ground-based analog to spaceflight and simulates the headward fluid shift and cardiovascular deconditioning associated with spaceflight. We investigated in healthy volunteers whether HDBR, with or without countermeasures, affect cerebral autoregulation (CA). Twelve men (at selection: 34 ± 7 years; 176 ± 7 cm; 70 ± 7 kg) underwent three interventions of a 21-day HDBR: a control condition without countermeasure (CON), a condition with resistance vibration exercise (RVE) comprising of squats, single leg heel, and bilateral heel raises and a condition using also RVE associated with nutritional supplementation (NeX). Cerebral blood flow velocity was assessed using transcranial Doppler ultrasonography. CA was evaluated by transfer function analysis and by the autoregulatory index (Mxa) in order to determine the relationship between mean cerebral blood flow velocity and mean arterial blood pressure. In RVE condition, coherence was increased after HDBR. In CON condition, Mxa index was significantly reduced after HDBR. In contrast, in RVE and NeX conditions, Mxa were increased after HBDR. Our results indicate that HDBR without countermeasures may improve dynamic CA, but this adaptation may be dampened with RVE. Furthermore, nutritional supplementation did not enhance or worsen the negative effects of RVE. These findings should be carefully considered and could not be applied in spaceflight. Indeed, the subjects spent their time in supine position during bed rest, unlike the astronauts who perform normal daily activities.
Highlights
Cerebral autoregulation (CA) allows the maintenance of constant cerebral blood flow (CBF) despite changes in blood pressure by adapting the vascular tone and cerebral vessel diameter (Nelson et al, 2014)
There were no time effects for systolic blood pressure (SBP) (P = 0.83), diastolic blood pressure (DBP) (P = 0.47), and middle cerebral arterial blood flow velocity (MCAv) (P = 0.43), whereas significant time effects were observed for heart rate (HR) (P = 0.02)
In CON condition, we noted no significant modifications either in SBP (P = 0.93) or DBP (P = 0.85) or MCAv (P = 0.25), whereas HR increased at R + 1 compared to BDC-3 (P < 0.01)
Summary
Cerebral autoregulation (CA) allows the maintenance of constant cerebral blood flow (CBF) despite changes in blood pressure by adapting the vascular tone and cerebral vessel diameter (Nelson et al, 2014). Several mechanisms may be involved in CA These include metabolic influences (PaCO2, PaO2) and autonomic innervations (Paulson et al, 1990; Blaber et al, 2013; Zhang and Hargens, 2018). Available data indicate controversial CA adaptation after exposure to microgravity. CA could be either impaired (Zhang et al, 1997) or improved (Jeong et al, 2014) when subjects spent a few weeks of bed rest. A preserved or even enhanced CA was observed after short duration (Iwasaki et al, 2007) or long duration (Herault et al, 2000; Arbeille et al, 2001; Kotovskaia and Fomina, 2010) spaceflight
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