Higher capillary filtration rate in the calves of endurance-trained subjects during orthostatic stress.

The effect of endurance training on the rate of transcapillary filtration during orthostasis was studied in the human calf. Two groups of sports students with markedly different aerobic capacities performed an orthostatic tilt table test (25 min supine, 10 min upright, 10 min supine). The following parameters were measured: heart rate, brachial and peripheral blood pressure (Finapres), calf volume changes (impedance), and calf blood flow (venous-occlusion-technique). The two groups did not differ in maximal calf circumference, body height, or weight. No syncope occurred, and heart rate and blood pressure responses to upright tilt were similar in both groups. However, the capillary filtration rate revealed much higher values in the trained group: 0.086 vs. 0.036 ml.min-1.100 ml-1. The estimated additional fluid accumulation in the interstitial space in trained subjects may be as high as 260 ml within the first 20 min of orthostasis and may play a role in often reported late syncopes, depending on the preexisting fluid state.