Clinical pharmacokinetics during plasma exchange

Drug removal during plasma exchange (PE) is a complex phenomenon that is de fined by the molecule pharmacokinetic characteristics. Plasma-protein bindi ng and the volume of distribution (Vd) are two kinetic parameters that stro ngly affect the efficiency of drug removal by PE. The effect of PE on drug kinetics has been specifically studied with antivirals, cardiotonic agents, antibiotics, corticosteroids, antalgics, anti-epileptic agents and nonster oidal anti-inflammatory drugs. This effect call be evaluated using differen t parameters: extracorporeal clearance, half-life, amount eliminated, and f raction of the drug removed. The estimated fraction eliminated (F-e) from t he body by PE is the best parameter to evaluate the effectiveness of the ex change procedure; it can account for 0.5-30 per cent. Results reported in t he literature showed that PE most influences drugs with a low Vd, regardles s of the extent of protein binding. We established that, during PE, there i s a linear relationship between F-e and the fraction of the drug in extrace llular fluids. The fraction eliminated during PE is approximately one-seven th of the fraction of the drug in extracellular fluids. We propose to use t his extracellular fraction as a predictive index: when i 20, extraction is low; the amount eliminated becomes consequential only when the index >20. D osage supplementation may be needed to maintain an adequate drug concentrat ion in the body. Practically, for drugs with a low Vd (< 0.3 l/kg), it seem s necessary to adjust the dosage.