The increase in JV, PV, and FV cross-sectional area during spaceflight confirmed that there was venous blood pooling in the cephalic, splanchnic, and pelvic regions. Further investigation is needed to determine the consequences of this fluid stagnation on the brain, eye, splanchnic, and pelvic organ morphology and or function.

This review summarizes our current understanding of cerebral blood flow regulation with exposure to microgravity, outlines potential mechanisms associated with post-flight orthostatic intolerance, and proposes future directions for research and linkages with cerebrovascular disorders found in the general population. It encompasses research from cellular mechanisms (e.g. hind limb suspension: tissue, animal studies) to whole body analysis with respect to understanding human responses using space analogue studies (bed rest, parabolic flight) as well as data collected before, during, and after spaceflight. Recent evidence indicates that cerebrovascular autoregulation may be impaired in some astronauts leading to increased susceptibility to syncope upon return to a gravitational environment. The proposed review not only provides insights into the mechanisms of post-flight orthostatic intolerance, but also increases our understanding of the mechanisms associated with pathophysiological conditions (e.g. unexplained syncope) with clinical applications in relation to postural hypotension or intradialytic hypotension.

The experiment demonstrated that, using the volume capture method of echography, untrained astronauts were able to capture enough echographic data to display vessel images of good quality for analysis. The increase in both CC and FA IMT during the flight suggest an adaptation to microgravity and to the confined environment of spaceflight which deserves further investigation.

Zuj KA, Arbeille P, Shoemaker JK, Blaber AP, Greaves DK, Xu D, Hughson RL. Impaired cerebrovascular autoregulation and reduced CO2 reactivity after long duration spaceflight. Am J Physiol Heart Circ Physiol 302: H2592-H2598, 2012. First published April 26, 2012 doi:10.1152/ajpheart.00029.2012.-Long duration habitation on the International Space Station (ISS) is associated with chronic elevations in arterial blood pressure in the brain compared with normal upright posture on Earth and elevated inspired CO2. Although results from short-duration spaceflights suggested possibly improved cerebrovascular autoregulation, animal models provided evidence of structural and functional changes in cerebral vessels that might negatively impact autoregulation with longer periods in microgravity. Seven astronauts (1 woman) spent 147 Ϯ 49 days on ISS. Preflight testing (30 -60 days before launch) was compared with postflight testing on landing day (n ϭ 4) or the morning 1 (n ϭ 2) or 2 days (n ϭ 1) after return to Earth. Arterial blood pressure at the level of the middle cerebral artery (BPMCA) and expired CO2 were monitored along with transcranial Doppler ultrasound assessment of middle cerebral artery (MCA) blood flow velocity (CBFV). Cerebrovascular resistance index was calculated as (CVRi ϭ BPMCA/CBFV). Cerebrovascular autoregulation and CO2 reactivity were assessed in a supine position from an autoregressive moving average (ARMA) model of data obtained during a test where two breaths of 10% CO2 were given four times during a 5-min period. CBFV and Doppler pulsatility index were reduced during Ϫ20 mmHg lower body negative pressure, with no differences pre-to postflight. The postflight indicator of dynamic autoregulation from the ARMA model revealed reduced gain for the CVRi response to BPMCA (P ϭ 0.017). The postflight responses to CO2 were reduced for CBFV (P ϭ 0.056) and CVRi (P ϭ 0.047). These results indicate that long duration missions on the ISS impaired dynamic cerebrovascular autoregulation and reduced cerebrovascular CO2 reactivity.transcranial Doppler ultrasound; cerebral blood flow; cerebrovascular resistance THE MICROGRAVITY environment, which causes cephalic fluid shifts with increased arterial pressure at the level of the brain relative to normal daily life on Earth (17), might cause alterations in cerebrovascular structure and function. The impact of microgravity on human cerebrovascular function has primarily been examined during and after short-duration spaceflights (2,5,8,20) with only a few measurements of cerebral blood flow velocity (CBFV) during or after long-duration flights (2, 33). During spaceflight, only modest changes in CBFV have been reported (2, 5) with small increases in cerebrovascular resistance after months in space that were speculated to reflect increased sympathetic vasoconstriction (2). Postflight CBFV in the supine posture was unchanged or slightly elevated from preflight (8,20). Postflight measurements of dynamic cerebrovascular autoregulatory indexes, reflecting vascular smooth muscl...

The observed change in CA and FA stiffness indices suggest that spaceflight results in an increase in arterial stiffness. That these changes were not strongly related to measurements of IMT suggests the possibility of different mechanisms contributing to the observed results.Arbeille P, Provost R, Zuj K. Carotid and femoral arterial wall distensibility during long-duration spaceflight. Aerosp Med Hum Perform. 2017; 88(10):924-930.

This study tested the hypothesis that intermittent compression of the lower limb would increase blood flow during exercise and postexercise recovery. Data were collected from 12 healthy individuals (8 men) who performed 3 min of standing plantar flexion exercise. The following three conditions were tested: no applied compression (NoComp), compression during the exercise period only (ExComp), and compression during 2 min of standing postexercise recovery. Doppler ultrasound was used to determine superficial femoral artery (SFA) blood flow responses. Mean arterial pressure (MAP) and cardiac stroke volume (SV) were assessed using finger photoplethysmography, with vascular conductance (VC) calculated as VC = SFA flow/MAP. Compared with the NoComp condition, compression resulted in increased MAP during exercise [+3.5 ± 4.1 mmHg (mean ± SD)] but not during postexercise recovery (+1.6 ± 5.9 mmHg). SV increased with compression during both exercise (+4.8 ± 5.1 ml) and recovery (+8.0 ± 6.6 ml) compared with NoComp. There was a greater increase in SFA flow with compression during exercise (+52.1 ± 57.2 ml/min) and during recovery (+58.6 ± 56.7 ml/min). VC immediately following exercise was also significantly greater in the ExComp condition compared with the NoComp condition (+0.57 ± 0.42 ml·min·mmHg), suggesting the observed increase in blood flow during exercise was in part because of changes in VC. Results from this study support the hypothesis that intermittent compression applied during exercise and recovery from exercise results in increased limb blood flow, potentially contributing to changes in exercise performance and recovery. NEW & NOTEWORTHY Blood flow to working skeletal muscle is achieved in part through the rhythmic actions of the skeletal muscle pump. This study demonstrated that the application of intermittent pneumatic compression during the diastolic phase of the cardiac cycle, to mimic the mechanical actions of the muscle pump, accentuates muscle blood flow during exercise and elevates blood flow during the postexercise recovery period. Intermittent compression during and after exercise might have implications for exercise performance and recovery.

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