Pharmacology and physiology of colloids

The importance of an adequate circulating volume in the critically ill is well established. Plasma, albumin, synthetic colloids and crystalloids may all be used for volume expansion but the first two are expensive and crystalloids have to be given in much larger volumes than colloids to achieve the same effect. Synthetic colloids provide a cheaper, safe, effective alternative. There are three classes of synthetic colloid; dextrans, gelatins and hydroxyethyl starches; each is available in several formulations with different properties which affect their initial plasma expanding effects, retention in the circulation and side-effects. There is no ideal colloid but those with low molecular weights such as gelatins are more suitable for rapid, short term volume expansion whilst in states of capillary leak where longer term effects are required hydroxyethyl starches are more effective. Dextrans are as effective as the alternatives but produce more side-effects and the need to pre-treat with hapten-dextran renders them unwieldy in use. Albumin is as persistent as hydroxyethyl starch in the healthy circulation but is retained less well in states of capillary leak. It has no significant advantages over starches and is much more expensive.