Clinical Pharmacodynamics

Abstract

Pharmacokinetics and pharmacodynamics follow the logic of cause and consequence. Receptor-mediated and reversible effects can be distinguished from direct and irreversible effects. Reversible effects are capacity-limited and saturable whereas irreversible effects are limited only by the number of viable targets. In the case of receptor-mediated and reversible effects a threshold and a ceiling concentration can be defined. Antimicrobial drugs with concentration-dependent action are distinguished from drugs with time-dependent action. Concentration-dependent effects are associated with a high ceiling concentration and the target is the high peak. Time-dependent effects are associated with a high threshold concentration and the target is the high trough. During kidney dysfunction, alterations of drug response are usually attributed to pharmacokinetic but rarely to pharmacodynamic changes. Dose adjustment calculations, therefore, tacitly presume that pharmacodynamic parameters remain unchanged while only pharmacokinetic parameters are altered in kidney failure. Kidney dysfunction influences the pharmacokinetic parameters of at least 50% of all essential drugs. Clinicians usually consider pharmacokinetics when kidney disease is found, but pharmacodynamics is as important. Alterations of pharmacodynamic parameters are conceivable but only rarely reported in kidney failure. Sometimes surprising dosing adjustments are needed when pharmacodynamic concepts are brought into the decision process of which dose to choose. Pharmacokinetics and pharmacodynamics should both be considered when any dosing regimen is determined.