Pression oncotique et hémodilutionOncotic pressure and haemodilution

The appropriate fluid therapy in neurosurgical patients remains an area of disagreement between neurosurgeons and anaesthesiologists. Fluid restriction has long been practiced in patients with brain pathology, in order to reduce or prevent the formation of cerebral oedema. This grows from a fear that rapid administration of fluids, particularly noncolloidal fluids, can enhance cerebral oedema, although there is a lack of experimental evidence to substantiate this belief. On the other hand, fluid restriction can lead to relative hypovolaemia, causing haemodynamic instability during anaesthesia and influence defavourably cerebral perfusion. The appropriate fluid management of patients with brain pathology requires a careful review of the Starling's law and a clear understanding of osmolality, oncotic pressure (OP) and the nature of the blood-brain barrier (BBB). The Starling equation of ultrafiltration states that the net movement of fluid between the intra- and extravascular compartments is the result of the summated influences of the pressure gradients (hydrostatic pressure, OP, and osmotic pressure) between those compartments and the properties of the barriers (capillary endothelium) that separate them. In most peripheral tissues this barrier is freely permeable to small molecules and ions and net fluid movement depends on intravascular hydrostatic pressure and OP. Under normal circumstances, intraluminal hydrostatic pressure is higher than interstitial pressure, favouring water egress. By contrast, intraluminal OP is higher than interstitial OP, favouring water retention. These forces do not balance exactly, and fluid accumulation is prevented by the lymphatics. If this net movement exceeds the capacity of the lymphatic clearance mechanisms, fluid accumulates, which is the definition of oedema.