Cardiovascular echographic and Doppler parameters for the assessment of orthostatic intolerance

To quantify the hemodynamic changes associated with orthostatic intolerance.

The aortic flow, the cerebral and lower limb arterial flow and resistance, the flow redistribution between these two areas, and the femoral vein distension were assessed during two orthostatic tests (tilt-up and LBNP:lower body negative pressure) by echography and doppler. Eight subjects stayed 4 days in HDT (head down tilt) in order to induce orthostatic intolerance, and 1 month later, 4 days in confinement. Tilt and LBNP were performed pre- and post-HDT and confinement.

Orthostatic intolerance was significantly more frequent after HDT (63%) than after confinement (25%). At rest, the stroke volume was significantly decreased after HDT (-16%, P<0.01) but not after confinement. Cardiac and cerebral hemodynamic responses to tilt-up and LBNP were similar pre- and post-HDT or confinement. Conversely, during both tilt and LBNP the femoral resistances increased less (P<0.002), and the femoral flow reduced less (P<0.001) after HDT than before HDT or after confinement. The cerebral to femoral flow ratio increased less after HDT than before (P<0.002) but remained unchanged before and after confinement. This ratio was significantly more disturbed on the tilt test non-finisher subjects. The femoral superficial vein was more distended during post-HDT LBNP than pre-HDT or after confinement (IP<0.01). In conclusion, 4 days in HDT were enough to alter the lower limb arterial and venous response to tilt-up and LBNP and reduced the flow redistribution in favor of the brain in all subjects. Confinement did not alter significantly the hemodynamic response to orthostatic tests.

The femoral resistance index, and the cerebral to femoral flow ratio measured by doppler during LBNP were the best predictors for orthostatic intolerance.