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 f or predicting orthostatic intolerance. Methods. Cerebral and lower limb art erial flow and resistance were assessed by Doppler ultrasonography during l ower body negative pressure (7 minutes each at -25 and -45 mm Hg). Cardiova scular deconditioning was induced by 42 days in head-down tilt at -6 degree s (7 subjects) and 6-month spaceflights (10 cosmonauts). Results. Orthostat ic intolerance during a stand test was observed in 57% of the head-down-til t subjects and 83% of the cosmonauts. At the lower body negative pressure a fter head-down tilt and during or after flight, the average cerebral flow v elocity and vascular resistance did not change significantly from before he ad-down tilt and before flight. Conversely, there was a lack of lower limb arterial vasoconstriction (-24% from before head-down tilt and -43% from be fore 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 toler ant 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 h ead-down tilt and -33% from before flight; P < .01). The cerebral flow defi cit at the lower body negative pressure before head-down tilt was greater t han 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 toleran t cosmonaut had a cerebral flow deficit less than 8%. Conclusion. Lack of l ower limb vasoconstriction and a lower cerebral-femoral flow ratio during l ower body negative pressure in disadapted subjects were associated with ort hostatic intolerance. A cerebral flow deficit during lower body negative pr essure before disadaptation allowed measurement of the predisposition of th e subjects to become intolerant.