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

We used venous congestion strain gauge plethysmography (VCP) to measure the changes in fluid filtration capacity (K-f), isovolumetric venous pressure (Pv(i)), 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 x 10(-3) to 5.7 +/- 0.9 x 10(-3) ml.min(-1).100 ml(-1).mmHg(-1) (+51.4%; P < 0.003), which returned to pretilt values (4.0 +/- 0.4 x 10(-3) ml.min(-1).100 ml(-1).mmHg(-1)) after remobilization. Similarly, Pv(i) 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 nil tissue(-1) at D118 and was found elevated to 3.4 +/- 0.7 ml.min(-1).100 nil tissue(-1) at 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.