Effect of novel short-arm human centrifugation-induced gravitational gradients upon cardiovascular responses, cerebral perfusion and g-tolerance.

KEY POINTS: The aim of this study was to determine the effect of rotational axis position (RAP and thus g-gradient) during short-arm human centrifugation (SAHC) upon cardiovascular responses, cerebral perfusion and g-tolerance. In 10 male and 10 female participants, 10 min passive SAHC runs were performed with the RAP above the head (P1), at the apex of the head (P2), or at heart level (P3), with foot-level Gz at 1.0 g, 1.7 g and 2.4 g. We hypothesized that movement of the RAP from above the head (the conventional position) towards the heart might reduce central hypovolaemia, limit cardiovascular responses, aid cerebral perfusion, and thus promote g-tolerance. Moving the RAP footward towards the heart decreased the cerebral tissue saturation index, calf circumference and heart rate responses to SAHC, thereby promoting g-tolerance. Our results also suggest that RAP, and thus g-gradient, warrants further investigation as it may support use as a holistic spaceflight countermeasure. ABSTRACT: Artificial gravity (AG) through short-arm human centrifugation (SAHC) has been proposed as a holistic spaceflight countermeasure. Movement of the rotational axis position (RAP) from above the head towards the heart may reduce central hypovolaemia, aid cerebral perfusion, and thus promote g-tolerance. This study determined the effect of RAP upon cardiovascular responses, peripheral blood displacement (i.e. central hypovolaemia), cerebral perfusion and g-tolerance, and their inter-relationships. Twenty (10 male) healthy participants (26.2 ± 4.0 years) underwent nine (following a familiarization run) randomized 10 min passive SAHC runs with RAP set above the head (P1), at the apex of the head (P2), or at heart level (P3) with foot-level Gz at 1.0 g, 1.7 g and 2.4 g. Cerebral tissue saturation index (cTSI, cerebral perfusion surrogate), calf circumference (CC, central hypovolaemia), heart rate (HR) and digital heart-level mean arterial blood pressure (MAP) were continuously recorded, in addition to incidence of pre-syncopal symptoms (PSS). ΔCC and ΔHR increases were attenuated from P1 to P3 (ΔCC: 5.46 ± 0.54 mm to 2.23 ± 0.42 mm; ΔHR: 50 ± 4 bpm to 8 ± 2 bpm, P < 0.05). In addition, ΔcTSI decrements were also attenuated (ΔcTSI: -2.85 ± 0.48% to -0.95 ± 0.34%, P < 0.05) and PSS incidence lower in P3 than P1 (P < 0.05). A positive linear relationship was observed between ΔCC and ΔHR with increasing +Gz, and a negative relationship between ΔCC and ΔcTSI, both independent of RAP. Our data suggest that movement of RAP towards the heart (reduced g-gradient), independent of foot-level Gz, leads to improved g-tolerance. Further investigations are required to assess the effect of differential baroreceptor feedback (i.e. aortic-carotid g-gradient).

Galanin and Adrenomedullin Plasma Responses During Artificial Gravity on a Human Short-Arm Centrifuge.

Galanin and adrenomedullin plasma responses to head-up tilt and lower body negative pressure have been studied previously. However, to what extent short-arm human centrifugation (SAHC) affects these responses is not known. In this study, we assessed how the application of variable gradients of accelerations (ΔGz ) via shifting of the rotation axis during centrifugation affects selected hormonal responses. Specifically, we tested the hypothesis, that cardiovascular modulating hormones such as galanin and adrenomedullin will be higher in non-finishers (participants in whom at least one of the pre-defined criteria for presyncope was fulfilled) when compared to finishers (participants who completed the entire protocol in both sessions) during SAHC exposure. Twenty healthy subjects (10 women and 10 men) were exposed to two g-levels [1 Gz and 2.4 Gz at the feet (Gz_Feet)] in two positions (axis of rotation placed above the head and axis of rotation placed at the heart level). Elevated baseline levels of galanin appeared to predict orthostatic tolerance (p = 0.054) and seemed to support good orthostatic tolerance during 1 Gz_Feet SAHC (p = 0.034). In finishers, 2.4 Gz_Feet SAHC was associated with increased galanin levels after centrifugation (p = 0.007). For adrenomedullin, the hypothesized increases were observed after centrifugation at 1 Gz_Feet (p = 0.031), but not at 2.4 Gz_Feet, suggesting that other central mechanisms than local distribution of adrenomedullin predominate when coping with central hypovolemia induced by SAHC (p > 0.14). In conclusion, baseline galanin levels could potentially be used to predict development of presyncope in subjects. Furthermore, galanin levels increase during elevated levels of central hypovolemia and galanin responses appear to be important for coping with such challenges. Adrenomedullin release depends on degree of central hypovolemia induced fluid shifts and a subject's ability to cope with such challenges. Our results suggest that the gradient of acceleration (ΔGz ) is an innovative approach to quantify the grade of central hypovolemia and to assess neurohormonal responses in those that can tolerate (finishers) or not tolerate (non-finishers) artificial gravity (AG). As AG is being considered as a preventing tool for spaceflight induced deconditioning in future missions, understanding effects of AG on hormonal responses in subjects who develop presyncope is important.

Intracranial and Intraocular Pressure During Various Degrees of Head-Down Tilt.

BACKGROUND: More than half of astronauts develop ophthalmic changes during long-duration spaceflight consistent with an abnormal intraocular and intracranial pressure (IOP, ICP) difference. The aim of our study was to assess IOP and ICP during head-down tilt (HDT) and the additive or attenuating effects of 1% CO2 and lower body negative pressure (LBNP). METHODS: In Experiment I, IOP and ICP were measured in nine healthy subjects after 3.5 h HDT in five conditions: -6°, -12°, and -18° HDT, -12° with 1% CO2, and -12° with -20 mmHg LBNP. In Experiment II, IOP was measured in 16 healthy subjects after 5 min tilt at +12°, 0°, -6°, -12°, -18°, and -24°, with and without -40 mmHg LBNP. RESULTS: ICP was only found to increase from supine baseline during -18° HDT (9.2 ± 0.9 and 14.4 ± 1 mmHg, respectively), whereas IOP increased from 15.7 ± 0.3 mmHg at 0° to 17.9 ± 0.4 mmHg during -12° HDT and from 15.3 ± 0.4 mmHg at 0° to 18.7 ± 0.4 mmHg during -18° HDT. The addition of -20 mmHg LBNP or 1% CO2 had no further effects on ICP or IOP. However, the use of -40 mmHg LBNP during HDT lowered IOP back to baseline values, except at -24° HDT. DISCUSSION: A small, posterior intraocular-intracranial pressure difference (IOP > ICP) is maintained during HDT, and a sustained or further decreased difference may lead to structural changes in the eye in real and simulated microgravity.Marshall-Goebel K, Mulder E, Bershad E, Laing C, Eklund A, Malm J, Stern C, Rittweger J. Intracranial and intraocular pressure during various degrees of head-down tilt. Aerosp Med Hum Perform. 2017; 88(1):10-16.

Effect of postural changes on cardiovascular parameters across gender.

INTRODUCTION: We investigated the effect of postural changes on various cardiovascular parameters across gender. Twenty-eight healthy subjects (16 male, 12 female) were observed at rest (supine) and subjected to 3 interventions; head-down tilt (HDT), HDT with lower body negative pressure (HDT+ LBNP at -30 mm Hg), and head-up tilt (HUT), each for 10 minutes separated by a 10 minutes recovery period. METHODS: Measurements were recorded for heart rate (HR), standard deviation of the normal-to-normal intervals, root mean square of successive differences between the normal-to-normal intervals, heart rate variability-low frequency (LFRRI), heart rate variability-high frequency (HFRRI), low frequency/high frequency ratio (LFRRI/HFRRI), systolic blood pressure (SBP), mean arterial pressure (MAP), diastolic blood pressure (DBP), total peripheral resistance index (TPRI), stroke index (SI), cardiac index (CI), index of contractility (IC), left ventricular work index, and left ventricular ejection time. RESULTS: Across all cardiovascular parameters, there was a significant main effect of the intervention applied but there was no significant main effect of gender across all parameters. CONCLUSIONS: The results suggest that there are no specific gender differences in regards to the measured variables under the conditions of this study. Furthermore, these results suggest that in healthy subjects, there appears to be evidence that LBNP partially elicits similar cardiovascular responses to HUT, which supports the use of LBNP as an intervention to counteract the effects of central hypovolemia.