The revised starling principle and its relevance to perioperative fluid management

Abstract

The Starling Principle states that fluid movements between blood and the tissue are determined by differences in hydrostatic and colloid osmotic pressures between plasma inside the microvessels and fluid outside them. While experimental evidence has established the general validity of Starling Principle, difficulties in interpreting it quantitatively became apparent when measurements of interstitial fluid (ISF) hydrostatic and colloid osmotic pressures became possible. The revised interpretation recognizes that since vessel walls are permeable to macromolecules, a static equilibrium resulting from the balance of pressures cannot be achieved. Colloid osmotic pressure differences between plasma and interstitial fluid depend on low levels of filtration in most tissues. Plasma volume is maintained as a steady state with fluid loss by filtration being roughly matched by fluid gains from lymph. These differences in colloid osmotic pressure that determine blood-tissue fluid exchange are those across the ultrafilter in vessels walls, namely, the glycocalyx on the luminal surface of vascular endothelium. These differences are distinct from those between mean values of plasma and interstitial fluid since most macromolecules do not pass through the intact glycocalyx. Unlike transient changes, steady state fluid transport is nonlinear with changes in microvascular pressure. This nonlinearity predicts differing effects of the dilution of plasma protein depending on mean microvascular pressures, with increased transcapillary filtration when pressures are similar to plasma colloid osmotic pressure but negligible filtration at low pressures. Since pulmonary capillary pressures are low, monitoring plasma colloid osmotic pressure during large crystalloid infusions may be useful in averting pulmonary edema.