Pharmacokinetics and pharmacodynamics

Abstract

Pharmacokinetics and pharmacodynamics are vast and complex topics with medical textbooks dedicating entire chapters to explain the details. Due to this, only some of the many relevant concepts have been explained. Compartment models have not been discussed despite their importance to the anaesthetist. These two concepts have been described as mathematical expressions of the relationship between drug dose and concentration for pharmacokinetics, and drug concentration and effect for pharmacodynamics. Concentration provides the link between pharmacokinetics and pharmacodynamics and will enable rational dosing. Pharmacokinetics consists of four linked processes namely absorption, distribution, biotransformation and excretion. Clinical pharmacokinetics including bioavailability, bioequivalence, first-order and zero-order kinetics as well as context-sensitive half-life have been discussed. A basic explanation of drug-receptor interactions, dose-response relationships and therapeutic index is given for pharmacodynamics. The purpose of understanding these concepts is to enable doctors to simultaneously achieve the desired therapeutic effects while minimising adverse effects. This is particularly relevant to the practising anaesthetist where multiple agents are required to produce optimal anaesthesia.