Objectives

We investigated the dose–effect relationship between wide changes in gravity from 0 to 2.0 Gz (Δ0.5 Gz) and cerebral blood flow (CBF), to test our hypothesis that CBF has a linear relationship with levels of gravity.

Subjects and methods

Ten healthy seated men were exposed to 0, 0.5, 1.0, 1.5, and 2.0 Gz for 21 min, 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 (MAP_Heart), heart rate, stroke volume, cardiac output and respiratory conditions were obtained for the last 6 min at each gravity level. Then, MAP in the middle cerebral artery (MAP_MCA), reflecting cerebral perfusion pressure, was estimated.

Results

Steady-state CBFV decreased stepwise from 0.5 to 2.0 Gz. Steady-state heart rate, stroke volume, estimated MAP_MCA and end-tidal carbon dioxide pressure (ETCO₂) also changed stepwise from hypogravity to hypergravity. On the other hand, steady-state MAP_Heart and cardiac output did not change significantly. Steady-state CBFV positively and linearly correlated with estimated MAP_MCA and ETCO₂ in most subjects.

Conclusion

The present study demonstrated stepwise gravity-induced changes in steady-state CBFV from 0.5 to 2.0 Gz despite unchanged steady-state MAP_Heart. The combined effects of reduced MAP_MCA and ETCO₂ likely led to stepwise decreases in CBFV. We caution that a mild increase in gravity from 0 to 2.0 Gz reduces CBF, even if arterial blood pressure at the heart level is maintained.