PLASMA EXPANDERS – INTRODUCTION
A drug may be classified by the chemical type of the active ingredient or by the way it is used to treat a particular condition. Each drug can be classified into one or more drug classes.
Plasma expanders are agents that have relatively high molecular weight and boost the plasma volume by increasing the osmotic pressure.
Typically, colloids are used to expand the plasma volume, although combinations of hypertonic crystalloid and colloid have recently been used. The currently available colloids vary in their physico-chemical, pharmaco-dynamic and pharmaco-kinetic properties.
PLASMA EXPANDERS – INDICATION
Plasma expanders are used to restore the circulating volume of a hypovolaemic patient. They are used to treat patients who have suffered hemorrhage or shock. Shock occurs due to reduced blood volume (usually due to hemorrhage) and it is necessary to get the blood volume back to normal as quickly as possible.
PLASMA EXPANDERS – INFORMATION
Plasma expanders are used to restore the circulating volume of a hypovolaemic patient. Typically, colloids are used to expand the plasma volume, although combinations of hypertonic crystalloid and colloid have recently been used. The currently available colloids vary in their physico-chemical, pharmaco-dynamic and pharmaco-kinetic properties. In particular, they differ in molecular weight, which partly determines their duration of action, and in their ability to expand the plasma volume. Dextran, hydroxyethyl starch and hypertonic colloid solutions improve oxygen flux within the microcirculation. Despite their benefits, the use of dextran and high molecular weight starches is limited by their negative impact on coagulation. In addition, these macro-molecules may also induce acute renal failure in susceptible patients. Current research focuses on the development of artificial oxygen-carriers as plasma expanders. These substances, which include modified stromal-free haemoglobin and perfluorocarbon emulsions, are undergoing clinical trials.
The plasma expanders most widely used are isotonic crystalloids and synthetic colloids. Albumin, due to its cost and the traceability requirements inherent to all blood-derived products, is rarely prescribed as first-line treatment. The synthetic colloids most widely used are hydroxyethyl starch (HES) solutions and gelatins. In many countries, dextrans are no longer marketed because of their adverse effects, especially anaphylactic reactions.
It should be noted that nonionic 5% or 10% dextrose solution is not a plasma expander, as the volume remaining in the vascular compartment after intravenous infusion is insignificant because of rapid diffusion to all compartments of the body. These solutions behave like free water and infusion of large volumes induces a risk of hyponatremia and water intoxication with potentially serious cerebral consequences.
The low cost and few apparent adverse effects of crystalloids justify their widespread use despite a plasma expansion property of the order of 20%. Synthetic colloids provide a plasma expansion property of close to 100% but with a risk of anaphylaxis, renal failure and clotting disorders.
The plasma expansion property of a solution theoretically has direct metabolic effects. A product with a high expansion property corrects blood volume more effectively, limiting the risks of tissue hypoperfusion responsible for lactic acidosis. The expected benefit of a colloid should therefore be logically greater than that of a crystalloid, but this superiority has never been demonstrated.
This absence of clear-cut superiority of a particular plasma expander can be explained by the following elements:
- Despite the apparent safety of crystalloids, large volumes must be infused when these compounds are used alone, with a risk of interstitial fluid overload that can be harmful during the perioperative period.
- Furthermore, the plasma expansion property of a crystalloid can be 20% higher in certain conditions,especially in the case of moderate hypovolemia.
- Finally, despite the toxic effects related to massive use of colloids, compliance with the maximum recommended dose and the development of low molar substitution HES limit the risks of adverse effects.