PHYSIOLOGICAL MONITORING OF FLUID SHIFTS DURING ORTHOSTATIC TILT

Abstract

BACKGROUND While the etiology of the Spaceflight Associated Neuro-ocular Syndrome (SANS) is currently unknown, headward fluid shifts due to the removal of all gravitational vectors in space is hypothesized to be a major contributing factor. A countermeasure (CM) that can successfully redistribute body fluids similar to the upright position on Earth may thus be important for the prevention of SANS. Our SPACE-CENT Study (Studying the Physiological and Anatomical Effects of Centrifugation and Head-Down Tilt) seeks to investigate the cerebral, ocular and vestibular effects of 60-day exposure to strict 6o head-down tilt bed rest as a spaceflight analog with and without daily centrifugation. METHODS This study is part of the ongoing AGBRESA study at the DLR :envihab facility. As one component of our SPACE-CENT project, near-infrared spectroscopy (NIRS) derived total hemoglobin concentrations ([HbT]) from 4 body locations (head, chest, thigh, and calf) was measured before, during and after the international-standard orthostatic tilt test: tilt from supine to +80o, maintain for 15 min, then increase lower-body negative pressure (LBNP) by +10 mmHg every 3 minutes until subjects exhibit pre-syncope signs via a simultaneous drop in blood pressure and heart rate. RESULTS As of October 2019, the entire protocol has been completed on n=12 subjects (4 females), 5 days before bed rest onset and on R+0 following 60 days of -6o HDT bed rest. Significant shifts in [HbT]—proportional to blood volume—were observed from the head towards the feet during tilt to +80o in all subjects (Figure 1). All subjects had tests terminated sooner on R+0 (462±346 sec) than on BDC-5 (1105±464 sec), although the inter-subject variability was large (range=125-1751 sec). Interestingly, a subset of subjects (n=3) exhibited an initial increase in cerebral [HbT] at the onset of +80o, whereas the remainder showed no change or a decrease in cerebral [HbT] following tilt onset, which was predictive of longer orthostatic trials (t=2.46, p=0.022). All subjects exhibited an accelerating decrease in cerebral [HbT] that preceded test termination by 59±54 sec. CONCLUSIONS This represents preliminary findings from the first campaign of the study (12 of a planned 24 subjects). As such, final conclusions are not yet possible. However, the consistency of findings across subjects suggests that NIRS-based measures of fluid shifts along the body axis—and particularly out of the head—can predict the onset of pre-syncope by up to nearly a minute. Such data could be combined with the simultaneously acquired European Space Agency measurements of blood pressure to improve predictions of orthostatic tolerance.