[Clinical practice interpretation of oncotic pressure, serum albumin and protein determination and their ability for guiding therapeutics in cases of disturbances of capillary exchanges].

The osmotic pressure of the serum proteins, the colloid osmotic pressure (COP) or oncotic pressure, is one of the four Starling forces determining fluid movements across a capillary. Theoretical and empirical formulas considering plasma protein or albumin concentration have been used. However, such calculations are only approximations, being inadequate especially when protein fractionation pattern is abnormal or when artificial colloids are used. It is preferable to measure COP directly. In healthy standing adults COP is around 25 mmHg. It is influenced by age, position, blood pressure and pH. The interpretation of the COP must take into account, on the one hand the possible variations of the other factors involved in the Starling's equilibrium and, on the other hand the structural and functional differences between systemic and pulmonary circulations. When capillary permeability is not altered, a decreased COP is associated with an increased filtration rate across the capillaries, resulting in oedema in skeletal muscle, skin, subcutaneous tissue, intestine and heart. The lung is relatively insensitive to extravascular fluid shifts caused by lowering of the COP, due to the efficacy of the pulmonary lymph drainage and the high pulmonary interstitial protein concentration. The main determinant of fluid exchange in the lung is then the hydrostatic force, reflected by the pulmonary artery wedge pressure. When capillary permeability is altered, the decreased reflectance coefficient results in a smaller effect of the COP on interstitial oedema formation. Several studies demonstrated that oedema formation in the injured tissues seems unaffected by changes in COP. During fluid resuscitation, COP measurement is useful for assessing the risks of peripheral oedema formation and additional measurements of pulmonary artery wedge pressure for assessing the risks of pulmonary oedema. Therefore COP measurement can serve as a useful guide for selection of colloids or crystalloids in critically ill patients, avoiding expensive albumin administrations.