Abstract
We investigated the dose-effect relationship between wide changes in gravity from 0 to 2.0Gz (Δ0.5Gz) and cerebral blood flow (CBF), to test our hypothesis that CBF has a linear relationship with levels of gravity. Ten healthy seated men were exposed to 0, 0.5, 1.0, 1.5, and 2.0Gz for 21min, by using a tilt chair and a short-arm human centrifuge. Steady-state CBF velocity (CBFV) in the middle cerebral artery by transcranial Doppler ultrasonography, mean arterial pressure (MAP) at the heart level (MAPHeart), heart rate, stroke volume, cardiac output and respiratory conditions were obtained for the last 6min at each gravity level. Then, MAP in the middle cerebral artery (MAPMCA), reflecting cerebral perfusion pressure, was estimated. Steady-state CBFV decreased stepwise from 0.5 to 2.0Gz. Steady-state heart rate, stroke volume, estimated MAPMCA and end-tidal carbon dioxide pressure (ETCO2) also changed stepwise from hypogravity to hypergravity. On the other hand, steady-state MAPHeart and cardiac output did not change significantly. Steady-state CBFV positively and linearly correlated with estimated MAPMCA and ETCO2 in most subjects. The present study demonstrated stepwise gravity-induced changes in steady-state CBFV from 0.5 to 2.0Gz despite unchanged steady-state MAPHeart. The combined effects of reduced MAPMCA and ETCO2 likely led to stepwise decreases in CBFV. We caution that a mild increase in gravity from 0 to 2.0Gz reduces CBF, even if arterial blood pressure at the heart level is maintained.
Highlights
The human body, especially in a head-to-foot direction, is often exposed to changes in gravity in daily life, such as with postural change (0–1 Gz), on a roller coaster, and during landing of an airplane (\1.3 Gz)
Objectives We investigated the dose–effect relationship between wide changes in gravity from 0 to 2.0 Gz (D0.5 Gz) and cerebral blood flow (CBF), to test our hypothesis that CBF has a linear relationship with levels of gravity
The combined effects of reduced MAP in the middle cerebral artery (MAPMCA) and ETCO2 likely led to stepwise decreases in CBF velocity (CBFV)
Summary
The human body, especially in a head-to-foot direction, is often exposed to changes in gravity in daily life, such as with postural change (0–1 Gz), on a roller coaster (transient 0–4 Gz due to acceleration), and during landing of an airplane (\1.3 Gz). Gravity-induced decreases in cerebral blood flow (CBF) directly relate to (pre)syncope [3]. Many groups have investigated the effects of mild passive increases in gravity (0–0.5, or 0–0.8 Gz) on CBF using the head-up-tilt test [4, 5]. We previously examined the effects of hypergravity (1.5 Gz) on cerebral circulation using centrifugation [1]. These previous studies reported that CBF decreases with increases in gravity [1, 4, 5], suggesting a dose–effect relationship between gravity and CBF. No study has investigated the relationship between a wide range of changes in gravity and CBF
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