Effect of lower body negative pressure on cerebral circulation.

This experiment was performed to study the effects of lower body negative pressure (LBNP) on cerebral circulation. Cerebral hemodynamics were monitored continuously and noninvasively in eight subjects at 30 mmHg of LBNP for 25 minutes by following items: 1) the carotid Doppler flowmeter which measures the carotid blood flow, 2) the transcranial Doppler sonography which measures the flow velocity of the middle cerebral artery, 3) the near infrared spectrophotometer which measures the oxygenated and deoxygenated hemoglobin in the brain. The mean flow velocity of the middle cerebral artery and the blood flow of the common carotid artery significantly decreased during LBNP, even though the mean arterial blood pressure was well maintained. The oxygenated hemoglobin was significantly increased during LBNP, while the deoxygenated hemoglobin was not changed significantly. Our results suggest that exposure to moderate LBNP (30 mmHg) decreased the cerebral blood flow with the vasodilation on the arterial side of the brain.