Dynamic cerebral autoregulation after mild dehydration to simulate microgravity effects.

INTRODUCTION: During and immediately following a 2-wk spaceflight, dynamic cerebral autoregulation is improved, leading to stable cerebral blood flow velocity (CBFV). However, the factors inducing this improved cerebral autoregulation remain unclear. Although mild reduction of plasma volume (one of the key adaptations to spaceflight) may lead to improved cerebral autoregulation, there have been no reports estimating the alteration of dynamic cerebral autoregulation during mild dehydration. We, therefore, investigated the effect of mild intravascular dehydration on dynamic cerebral autoregulation.

METHODS: Furosemide, 0.2 mg x kg(-1), was administered to 14 men. Dynamic cerebral autoregulation was estimated before and approximately 45 min after furosemide administration using spectral and transfer function analyses between mean blood pressure (MBP) variability and mean CBFV variability. We then assessed the degree of dehydration as plasma volume changes, and/or central venous pressure (CVP).

RESULTS: Furosemide administration caused an approximate 10% reduction in plasma volume, with CVP decreasing by 1.2 mmHg. Steady state MBP and mean CBFV did not change. However, transfer function gain in the low-frequency range significantly decreased, implying a decrease in the magnitude of transfer from MBP oscillations to mean CBFV fluctuations.

DISCUSSION: The present degree of dehydration was similar to the alteration observed after short-term spaceflight, and enhanced dynamic cerebral autoregulation in the low-frequency range with unchanged steady state mean CBFV. Consequently, these results on cerebral circulation were consistent with those of a previous spaceflight study. Thus, it is suggested that mild intravascular dehydration partly explains the improved dynamic cerebral autoregulation observed during and immediately after spaceflight.