Femoral flow response to lower body negative pressure: an orthostatic tolerance test

The objective was to check, during a lower body negative pressure (LBNP) test, new vascular parameters for the detection of orthostatic intolerance induced by head-down-tilt (HDT) and spaceflight.

The lower-limb volume flow and vascular resistance were evaluated by Doppler ultrasound. The HDT population consisted of two groups: control [6 subjects resting in a HDT (-6 degrees) position for 28 d] and countermeasure (6 subjects also in HDT for 28 d, but with repeated LBNP and exercise). The LBNP orthostatic test (four steps: -20, -30, -40, -50 mm Hg, of 3 min each) was performed before, during, and after the HDT. For the 14-d spaceflight (Antares) the cosmonaut underwent the LBNP test (10 min at -25 mm Hg and 10 min at -45 mm Hg), at preflight (3 times), inflight (day 11), and postflight (twice).

HDT--As the LBNP pressure decreased, the femoral blood flow decreased and the lower-limb vascular resistances increased in both HDT groups. In the control group the femoral flow was less reduced, at each of the 4 levels of depressure (p < 0.01). The amplitude of the leg vascular resistances was reduced at -40 mm Hg, and at -50 mm Hg, on HDT day 15 in both groups (before LBNP, after 1 week's exercise for the countermeasure group), and on post-HDT day 1 (p < 0.01) only in the control group. The femoral vascular resistance response had completely recovered in the countermeasure group on post-HDT day 1. During the post-HDT tilt table test, all 6 controls had a drop in blood pressure of 20 mm Hg (4 subjects) or 10 mm Hg (2 subjects); 3 had pre-syncopal symptoms. The HDT countermeasure subjects had neither any clinical signs of orthostatic intolerance nor any blood pressure drop. 14d-Spaceflight--During the flight, the cosmonaut did not use any countermeasures (exercise, LBNP). On flight day +11, and on postflight day 3, the femoral vascular resistance response to LBNP was decreased as observed in the control group on HDT day +15 and post HDT. On postflight day 7, the femoral response had completely recovered. The middle cerebral flow response to the various pre-, in-, and postflight LBNP tests consisted of a slight decrease of the cerebral flow together with resistances of comparable amplitude (-10 to -20%) to those measured during the same LBNP test in the HDT control group.

The femoral hemodynamics are much more disturbed than the cerebral ones in vascular deconditioning. The assessment of the lower limb vascular reactivity will be of interest in predicting orthostatic intolerance, and checking the efficiency of counter-measures.