Role of the glycocalyx in fluid management: Small things matter.

Intravenous fluid therapy and perception of volume effects are often misunderstood. The pharmacokinetical difference between colloids and crystalloids depends on the condition of the vascular permeability barrier. Its functioning is still largely based on Starling's principle from 1896, realising that transport of fluid to and from the interstitial space follows the balance between opposing oncotic and hydrostatic pressures. In the past decade, the endothelial glycocalyx, located on the luminal side of healthy vasculature, has increasingly been taken into consideration around models of transvascular fluid filtration. While crystalloids can freely pass through the glycocalyx, colloids are held back in the vasculature by this structure. This is reflected by a markedly higher intravascular persistence of isooncotic colloids (80-100%) versus crystalloids (around 20%), at least as long as the glycocalyx is intact. Protecting this structure in surgical practice means limiting the surgical trauma and avoiding intravascular hypervolemia.