Intravenous volume replacement: which fluid and why?

Fluids available for intravenous volume replacement may be either crystalloid or colloid. The fundamental differences between these fluids are their effects on the Starling equation (table 1) which describes fluid flux between the intravascular and interstitial spaces. Starling stated that the rate of fluid movement into or out of a capillary is related to the net hydrostatic pressure minus the net colloid osmotic pressure.1 The Starling equation has been modified to include coefficients which represent the permeability of the capillary membrane to small solutes (Kfc) and its ability to prevent large molecules such as plasma proteins from crossing it (oc).' Colloids may be used to replenish the oncotic strength of the blood, thereby enhancing its water retaining capacity.