Cerebral artery blood flow velocity changes following rapid release of lower body negative pressure.

BACKGROUND

Circulatory changes occur during exposure to Lower Body Negative Pressure (LBNP). These changes may have some similarities to exposure to moderately and slowly increased G-loads in a relaxed subject without anti-G suit.

HYPOTHESIS

Changes will also occur in cerebral blood circulation during a rapid release of LBNP.

METHODS

Transcranial Doppler ultrasound (TCD) was used to measure middle cerebral artery blood flow velocity (CBFV) in 14 human subjects following rapid release of a ramped lower body negative pressure (LBNP) (0.33 mm Hg.s) to presyncope (mean peak negative pressure of -124 mm Hg).

RESULTS

The mean CBFV decreased to an average of 60% (p < 0.05) of the baseline value at peak LBNP. Mean CBFV was still decreased to 65% and 84% of the baseline value (p < 0.05) at the third heart beat and 30 s, respectively, after pressure release. The systolic CBFV decreased similarly to 57% (p < 0.05) of baseline during peak LBNP, and was still 63% (p < 0.05) at the third heart beat after pressure release. Heart rate increased by a mean of 51% (p < 0.001) and systolic heart level blood pressure decreased by 28% (p < 0.001) during peak negative pressure. Both heart rate and blood pressure returned to baseline levels within 30 s after pressure release.

CONCLUSIONS

Following a presyncopal LBNP, the CBFV is not fully restored up to 30 s after the release of the negative pressure. This delayed returning of cerebral circulation following orthostatic stress may have some similarities to what occurs after the release of a gradual onset G-load in a relaxed subject without anti-G suit.