Doppler measurement of cerebral and lower limb flow during a lower body negative pressure test for predicting orthostatic intolerance

Objective. To quantify the cardiovascular response to an orthostatic test for predicting orthostatic intolerance. Methods. Cerebral and lower limb arterial flow and resistance were assessed by Doppler ultrasonography during lower body negative pressure (7 minutes each at -25 and -45 mm Hg). Cardiovascular deconditioning was induced by 42 days in head-down tilt at -6 degrees (7 subjects) and 6-month spaceflights (10 cosmonauts). Results. Orthostatic intolerance during a stand test was observed in 57% of the head-down-tilt subjects and 83% of the cosmonauts. At the lower body negative pressure after head-down tilt and during or after flight, the average cerebral flow velocity and vascular resistance did not change significantly from before head-down tilt and before flight. Conversely, there was a lack of lower limb arterial vasoconstriction (-24% from before head-down tilt and -43% from before flight; P < .01), and the cerebra 1-femoral flow ratio increased less (-27% from before head-down tilt and -52% from before flight; P < .01). The lack of vasoconstriction was more pronounced in intolerant subjects (-25% from before head-down tilt and -48% from before flight) compared with tolerant subjects (-22% from before head-down tilt and -14% from before flight; P > .01). Also, the lack of a cerebral-femoral flow ratio increase was more pronounced in intolerant subjects (-49% from before head-down tilt and -55% from before flight; P < .01) than in tolerant subjects (-1% from before head-down tilt and -33% from before flight; P < .01). The cerebral flow deficit at the lower body negative pressure before head-down tilt was greater than 8% in 3 of the 4 intolerant head-down-tilt subjects and less than 8% in the 3 tolerant subjects. The 3 cosmonauts who were intolerant after flight had a preflight cerebral flow deficit greater than 8%, whereas the tolerant cosmonaut had a cerebral flow deficit less than 8%. Conclusion. Lack of lower limb vasoconstriction and a lower cerebral-femoral flow ratio during lower body negative pressure in disadapted subjects were associated with orthostatic intolerance. A cerebral flow deficit during lower body negative pressure before disadaptation allowed measurement of the predisposition of the subjects to become intolerant.