Transcapillary fluid balance in pre-eclampsia.

The fluid transport between the plasma and interstitial fluid compartment is governed by the Starling forces, i.e. the capillary pressure (Pc), interstitial fluid hydrostatic pressure (Pi) and colloid osmotic pressure in plasma (COPp) and interstitial fluid (COPi). Interstitial fluid was collected from subcutaneous tissue on the thorax and ankle by implanted wicks and Pi was measured using the 'wick-in-needle' technique. In pre-eclampsia, COPp is reduced due to hypoproteinaemia and this predisposes towards loss of fluid from the vascular compartment. An important oedema-preventing mechanism is reduction of COPi, which serves as a homeostatic buffer against increased capillary filtration. This mechanism works in moderate, but not in severe pre-eclampsia. A higher COPi was found both at the thorax (8.3 vs 7.0 mmHg) and ankle (5.9 vs 3.9 mmHg) in the group with severe pre-eclampsia compared with the group moderate pre-eclampsia, in spite of a significant reduction in COPp (15.5 vs 19.9 mmHg). These findings suggest that an increased microvascular permeability of plasma proteins to subcutaneous tissue contributes to COPp reduction in severe pre-eclampsia.