Brief exposure to -2 Gz reduces cerebral blood flow velocity during subsequent +2 Gz acceleration.

: In order to determine the implication of the cerebral vasoconstriction occurring under -Gz acceleration in the mechanism of the push-pull effect, four healthy male non-pilots were submitted to a control centrifugation at +2 Gz, and then to an experimental run with identical +2 Gz plateau, but preceded by -2 Gz exposure. Cerebral blood flow velocity (CBFV), pulsatility index, and resistance index (RI) were continually measured with a transcranial Doppler instrument. The decrease in blood pressure and in CBFV was more important during the experimental run, when the change in RI was not different. We concluded that the cerebral vasoconstriction occurring under -2 Gz exposure seems not to be a major contributor in the mechanism of the push-pull effect appearing during subsequent +2 Gz acceleration.