Changes in microvascular fluid filtration capacity during 120 days of 6° head-down tilt

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Abstract
We used venous congestion strain gauge plethysmography (VCP) to measure the changes in fluid filtration capacity ( K f), isovolumetric venous pressure (Pvi), and blood flow in six volunteers before, on the 118th day (D118) of head-down tilt (HDT), and 2 days after remobilization (Post). We hypothesized that 120 days of HDT cause significant micro- and macrovascular changes. We observed a significant increase in K f from 3.6 ± 0.4 × 10−3 to 5.7 ± 0.9 × 10−3ml · min−1 · 100 ml−1 · mmHg−1 (+51.4%; P < 0.003), which returned to pretilt values (4.0 + 0.4 × 10−3ml · min−1 · 100 ml−1 · mmHg−1) after remobilization. Similarly, Pvi increased from 13.4 ± 2.1 mmHg to 28.9 ± 2.8 mmHg (+105.8%; P < 0.001) at D118 and was not significantly different at Post (12.4 ± 2.6 mmHg). Blood flow decreased significantly from 2.3 ± 0.3 to 1.3 ± 0.2 ml · min−1 · 100 ml tissue−1 at D118 and was found elevated to 3.4 ± 0.7 ml · min−1 · 100 ml tissue−1at Post. We believe that the increased K f is caused by a higher microvascular water permeability. Because this may result in edema formation, it could contribute to the alterations in fluid homeostasis after exposure to microgravity.