Ui. Balldin et al., CEREBRAL-ARTERY BLOOD-FLOW VELOCITY CHANGES FOLLOWING RAPID RELEASE OF LOWER-BODY NEGATIVE-PRESSURE, Aviation, space, and environmental medicine, 67(1), 1996, pp. 19-22
Background: Circulatory changes occur during exposure to Lower Body Ne
gative Pressure (LBNP). These changes may have some similarities to ex
posure to moderately and slowly increased C-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: Transcrania
l Doppler ultrasound (TCD) was used to measure middle cerebral artery
bled flow velocity (CBFV) in 14 human subjects following rapid release
of a ramped lower body negative pressure (LBNP) (0.33 mm Hg . s) to p
resyncope (mean peak negative pressure of -124 mm Hg). Results: The me
an CBFV decreased to an average of 60% (p < 0.05) of the baseline valu
e at peak LBNP. Mean CBFV was still decreased to 65% and 84% of the ba
seline 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 d
ecreased 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 pre
ssure. This delayed returning of cerebral circulation following orthos
tatic stress may have some similarities to what occurs after the relea
se of a gradual onset G-load in a relaxed subject without anti-G suit.