Implementation of short-arm human centrifugation with an altered rotational axis position

Exposure to microgravity causes a head-ward fluid shift that current spaceflight countermeasures fail to fully ameliorate. Artificial gravity (AG) through short-arm human centrifugation (SAHC) has been proposed as a possible countermeasure. Whilst the responses to increasing g-levels have been well characterized, the effects of a shift in rotational axis position (RAP), independent of g-level (at the feet), are not known. We hypothesized that a shift in RAP to in-body rotation (IBR) at heart-level, with a comparable g-level at the feet, would have a beneficial effect on g-tolerance through a supported cerebral perfusion from head-ward arterial pressure increases and a changed gravitational gradient. To test our hypothesis we performed a study with twenty (10 male; 10 female) healthy participants (mean ± SE; age: 26.2 ± 0.9 yrs and BMI: 22.9 ± 0.4 kg.m-2) who underwent passive short-arm centrifugation, in which profiles were randomized, for periods of 10 min five times per day on two separate days, two months apart. Each centrifuge profile consisted of one of three centers of rotation (either above the head - P1; apex of the head - P2 or IBR at heart-level - P3) at one of three different g-levels (either 1.0 gz - G1; 1.7 gz - G2 or 2.4 gz at the feet - G3). Cerebral perfusion (cerebral near-infrared spectroscopy for tissue saturation index, cTSI), central volume loss (strain gauge plethysmography for calf circumference, CC) and cardiovascular response for heart rate (HR) were all continuously measured. G-tolerance to each position was obtained by recording the number of participant centrifuge runs with pre-syncopal symptoms (PSS). Preliminary data suggest that centrifugation about the heart (P3) was more tolerable with a larger proportion of participants not experiencing PSS despite being exposed to the same g-level at the feet in other positions (P1 and P2). Currently within the literature, magnitude of centrifugation as an index of g-stress is quantified by g-level experienced at the feet only. However, within this present study we show that g-level alone is an insufficient method to classify degrees of artificial gravity (AG). The differences in tolerance to both long- and short-arm human centrifugation (L- and SAHC) are thought to arise due to differences in their respective gravitational gradient. Results of this study clearly demonstrate that placement of the rotational axis during centrifugation deserves more attention than previously thought.