Assessment of orthostatic fluid shifts with strain gauge plethysmography

In the present study we evaluated the use of SGP (strain gauge plethysmography) for the assessment of orthostatic fluid shifts during HUT (head-up tilting). Subjects wore a parachute harness fixed to the tilt table to avoid muscle tension in the lower limbs during HUT. Twenty-two healthy subjects (nine women) were tilted for 5 min. Changes in calf volume, as measured by SGP, surface EMG (electromyography), 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 in the non-occluded leg increased by 1.9±0.3% (mean±S.E.M.) 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 in the left and right leg with a mean difference of 0.03±0.1%. HUT did not cause significant changes in surface EMG measurements. An unexpected gender effect was observed: calf volume increased significantly more in men than in women. Men were significantly taller, but the haemodynamic response to HUT did not differ between both genders. The gender effect on orthostatic increases in 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.