HUMAN SPACEFLIGHT COUNTERMEASURES: CEREBRAL HEMODYNAMICS DURING SHORT-ARM HUMAN CENTRIFUGATION AND THE ROLE OF PERIPHERAL FLUID SHIFTS AND JUGULAR VENOUS CONGESTION

Abstract

Spaceflight induces physiological deconditioning, despite extensive exercise countermeasures. Furthermore, post-flight orthostatic intolerance (OI) and visual impairment and intracranial pressure (VIIP) syndrome; possibly related to venous congestion (VC) suggest more holistic countermeasures, such as artificial gravity (AG) short-arm human centrifugation (SAHC) are warranted. SAHC may reduce OI and VIIP incidence through headward fluid shift/pressure reversal; and thus beneficial effects on cerebral hemodynamics. Therefore, this thesis examined how peripheral fluid shifts, induced by various orthostatic challenges, affect central, peripheral, and cerebral hemodynamics, including indices of VC, in healthy participants. Study one induced 5 min bilateral lower-limb venous cuff occlusion (VCO) up to 120mmHg in nine participants. VCO produced significant increments in leg volume (41-158mL;p<0.05); however, HR and cerebral tissue oxygenation (cTSI) were unchanged, suggesting an inadequate orthostatic challenge. Study two involved twenty participants experiencing 10 min SAHC to 2.4+Gz footlevel with increased g-gradients; specifically, moving rotational axis Position (RAP), independent of g-level, towards the body. Increasing SAHC g-gradients reduced the HR response and increased g-tolerance (χ2(1, n=20)=8.57;p=0.003), similar to decreasing g-level. Heart-level-RAP did not improve cTSI as hypothesised, possibly due to VC. Thus, study three used 5 min head-down tilt (HDT) to -24° and -40mmHg lower body negative pressure (LBNP) on sixteen participants to evaluate VC during headward fluid shifts HDT induced significant VC below -12° p<0.05), abolished with -40mmHg LBNP (107±11 vs. 1±5mm2;p<0.05), and increased cerebral blood flow when LBNP-peripheral and HDT-headward fluid shifts balanced (-0.71±0.56 vs. 4.12±1.36cm.s-1;p=0.013). This thesis suggests that VCO is a poor model of ravitational fluid shifting but large g-gradient SAHC, via heart-level-RAP, is effective; and crucially, more tolerable at higher g-levels. VC may play an important role in crogravity fluid shift related OI and VIIP, which -6° bed rest studies fail to induce. Thus, AG with optimal RAP and VC amelioration, may be a tolerable and effective spaceflight countermeasure.