INDIVIDUAL-DIFFERENCES OF CEREBROVASCULAR RESPONSES TO GRAVITATIONAL STRESS - PREDICTION OF ORTHOSTATIC INTOLERANCE

This experiment was designed to investigate the individual differences in the cerebrovascular responses to orthostatic stress. Seven male vo lunteers were exposed to head-up tilt (HUT) at 60 degrees for 15 min a nd to lower body negative pressure (LBNP) at 30 mmHg for 25 min. We me asured the flow velocity of the middle cerebral artery and the quantit y of the oxygenated and deoxygenated hemoglobin (oxy-Hb and deoxy-Hb) in the brain. Based upon oxy-Hb change during exposure to HUT, we clas sified the subjects into two groups: 1) the ''good responder'' group i n which the oxy-Hb gradually increased from the 5th to 10th minute of HUT; 2) the ''bad responder'' group in which the oxy-Hb stabilized at lower levels after the initial decrease. The oxy-Hb changes between th e two groups were significantly different during exposure to LBNP. Dur ing exposure to HUT and LBNP, no significant difference was observed i n blood pressure and heart rate between the two groups. Our results su ggest that there might be an individual difference in the cerebrovascu lar responses to orthostatic stress.