Intravenous Fluids: Redistribution

Crystalloid and colloid solutions are the two primary options for intravenous fluid administration. Crystalloids are solutions that are composed of water and electrolytes. Colloids are composed of large-molecular weight substances, and can be divided into natural colloids (albumin) and synthetic colloids (such as hydroxyethyl starches and glycans). The large molecular size of the substances dissolved in colloid solutions makes these solutions remain intravascular much longer than crystalloid solutions. The intravascular half-life of crystalloid solutions is approximately 20-30 minutes, while the intravascular half-life of most colloids is between 3-6 hours¹.

The ability of the colloid solutions to remain intravascular is due to the oncotic pressure that large molecules, such as albumin, exert. Because crystalloid solutions lack these larger molecules they tend to leave the vascular compartment for the interstitial space relatively quickly. Because of this difference a smaller volume of colloid may be required for intravascular volume expansion compared to crystalloid solutions. It is traditionally taught that 1 mL of colloid solution produces the same intravascular effect as approximately 3 mL of crystalloid solution (1:3 ratio)^{1, 2, 3}. Recent studies have shown that with synthetic colloids this ratio tends to break down in critically ill adults. In several studies the ratio of volume of synthetic colloid to volume of crystalloid infused in ICU patients was closer to 1:1 or 1:1.5³. Even in critically ill patients, however, albumin tends to retain its efficient intravascular volume expanding effects³. This may allow for a smaller volume of albumin to be given than crystalloid or even synthetic colloids. As with all other medications the decision to use crystalloid and colloid solutions needs to be individualized to each patient and circumstance.