Drug interactions in anaesthetic practice

Abstract

When two or more drugs are administered simultaneously, the pharmacological response may be greater or less than the sum of the effects of the individual drugs. One drug may antagonize or potentiate the effects of the other and there may be also qualitative differences in response. Although some drug interactions increase the toxicity or result in loss of therapeutic effect, others are beneficial. Indeed, modern anaesthetic techniques depend on beneficial drug interactions. A sound combination of drugs helps clinicians to increase both the efficacy and safety of drug treatment. Drugs may interact on a pharmaceutical, pharmacodynamic, or pharmacokinetic basis. Many pharmacodynamic interactions are predictable and can be avoided by the use of common sense. However, it is much more difficult to predict the likelihood of pharmacokinetic and pharmaceutical interactions despite good prior knowledge of pharmacokinetics and chemical properties of individual drugs. Pharmaceutical drug interactions usually occur before the drug is given to the patient and they are caused by chemical (such as acid–base, salt formation, oxidation–reduction, hydrolysis, or epimerization) or physical (such as adsorption/absorption or emulsion breaking) reactions. When drugs have a pharmacokinetic interaction, one drug alters the absorption, distribution, or the elimination of the other drug. Many pharmacokinetic drug interactions are due to inhibition or induction of cytochrome P450 enzymes. Pharmacodynamic drug interactions are caused by drugs having an effect on the same receptors or the same physiological system. This chapter gives anaesthetists an overview of clinically relevant perioperative drug interactions.