Assessment of orthostatic fluid shifts with strain gauge plethysmography

We evaluated the use of strain gauge plethysmography (SGP) for the assessment of orthostatic fluid shifts during head up tilt (HUT). Subjects wore a parachute harness fixed to the tilt table to avoid muscle tension in the lower limbs during HUT. 22 Healthy subjects (9 women) were tilted for 5 minutes. Calf volume changes as measured by SGP, surface EMG, heart rate and blood pressure were measured continuously. Ten subjects underwent a second tilt test during which circulation in one leg was occluded with a pressure cuff at 250 mmHg. During HUT with occlusion, calf volume increased in the non-occluded leg by 1.9+/-0.3% (mean +/- SEM) and 0.2+/-0.2% in the occluded leg (p<0.001). During HUT without occlusion a significant correlation (r = 0.9) was found between measurements of the left and right leg with a mean difference of 0.03+/-0.1%. HUT did not cause significant changes of surface EMG. An unexpected gender effect was found: calf volume increased significantly more in men than in women. Men were significantly taller, but the hemodynamic response to HUT did not differ between both sexes. The gender effect on orthostatic increases of calf volume remained significant after adjustment for heart-to-calf distance. SGP during HUT with a parachute harness is a new, promising method to assess orthostatic fluid shifts. The gender differences in orthostatic pooling in the calf may be explained by a higher calf compliance in men together with a greater hydrostatic pressure due to a greater height in men.