Pharmacokinetics and anaesthesia

Pharmacokinetics explains what happens to a drug in the body, whereas pharmacodynamics describes the actions produced by the drug on the body. Therefore, the effects of a drug result from a combination of its pharmacokinetic and pharmacodynamic characteristics in that individual. Wherever possible, drug administration should be based on a measured patient response, which will incorporate both of these aspects of its pharmacology. However, such an approach may not always be possible. The response may be masked by other factors (e.g. neuromuscular blockers masking signs of anaesthetic depth) or difficult to quantify precisely (e.g. action of antibiotics or anti-emetics). Under these circumstances, previously established pharmacokinetic and pharmacodynamic data are used to guide administration. This article aims to explain and simplify the principles of pharmacokinetics so that their application to clinical practice can be better understood.