Cerebral hemodynamics and brain functional activity during lower body negative pressure

Introduction: Exposure to high 1G z acceleration forces on a centrifuge or in an aircraft can severely decrease cerebral blood perfusion and
cause rapid G-induced loss of consciousness. However, milder acceleration may gradually reduce cerebral blood fl ow and affect cognitive function in subtler ways. This study used lower body negative pressure (LBNP)
to mimic 1G z circulatory effects in order to study cerebral hemodynamics and brain function. Methods: Subjects were 15 healthy men, 19-21 yr
of age. They were exposed to LBNP at two levels for 5 min each separated by a 10-min recovery period. The conditions were low (LO), 24.00
kPa ( 230 mmHg) and high (HI), 26.67 kPa ( 250 mmHg). Variables measured before, during, and after LBNP included cerebral blood fl ow velocity (CBFV) in the middle cerebral artery, blood oxygen saturation
(S aO2), heart rate (HR), blood pressure, P300 of event-related EEG potentials, reaction time, and tracking error. Results: LO signifi cantly reduced
CBFV at 4 and 5 min, increased HR, and decreased the amplitude of
P300, but none of the other variables changed from baseline. In contrast,
HI produced signifi cant changes in most variables: CBFV decreased at
2 min and then fell further at 4 and 5 min, HR increased, and S aO2 decreased. Signifi cant neurocognitive changes included increased latency
and reduced amplitude of P300, slower reaction time, and greater tracking error. Conclusion: The higher level of LBNP used here reduced cerebral perfusion suffi ciently to impair neurocognitive function. This model
may be useful for further studies of these and other variables under
closely controlled conditions.