Colloid volume expanders. Problems, pitfalls and possibilities.

Colloid solutions have been developed and used over the past 70 years as expanders of the intravascular space, based on an understanding of Starling's law. Increasing osmotic pressure with colloidal products has remained an attractive theoretical premise for volume resuscitation. Indeed, colloids have been shown to increase osmotic pressure in clinical practice; however, the effects are short-lived. Lower molecular weight colloids exert a larger initial osmotic effect, but are rapidly cleared from the circulation. Larger molecules exert a smaller osmotic pressure that is sustained longer. The main drawback to colloid therapy lies in pathological states with endothelial injury and capillary leak, precisely the clinical scenario where colloids are commonly given. The colloid solution may leak into the interstitium and remain there exerting an osmotic gradient, pulling additional water into the interstitium. There are 4 general types of colloid products available for clinical use. Albumin is the predominant plasma protein and remains the standard against which other colloids are compared. Albumin, pooled from human donors, is in short supply and remains expensive. Dextrans have been used to prevent deep venous thrombosis and to lower blood viscosity during surgery. Hetastarch has been widely used as a plasma volume expander. It provides equivalent plasma volume expansion to albumin, but has been shown to alter clotting parameters in studies (prolonging the activated partial thromboplastin time and prothrombin time). Although severe coagulopathies have been reported in sporadic cases, hetastarch has not been shown to increase postoperative bleeding compared with albumin therapy, even in large doses (3 L/day). Despite some theoretical advantages compared with crystalloid therapy, colloid administration has not been shown to decrease the risk of acute lung injury or to improve survival. Specific indications for colloid products include hypoproteinaemic or malnourished states, patients who require plasma volume expansion who are unable to tolerate larger amounts of fluid, orthopaedic and reconstructive procedures requiring prevention of thrombus formation and leukapheresis.