ABSTRACT
The frequency of adverse drug interactions increases disproportionately with the increase in the number of drugs given to patients. It was shown that 40% of patients given 16 drugs experienced an adverse drug interaction, compared with 5% of patients given fewer than 6 drugs. The magnitude of the drug interaction problem increases substantially in anesthetised patients because of: [i] the increased use of multiple drugs in the preoperative and intraoperative periods; and [ii] the growing population of geriatric patients who, in addition to having diminished drug metabolising capacity, are often prescribed multiple medications for concomitant medical illness. Drug interactions with volatile and intravenous anesthetics can be divided into those that are pharmacokinetic and pharmacodynamic in nature. Pharmacokinetic interactions occur when the absorption, distribution, metabolism or excretion of a drug is altered by the coadministration of a second drug. Pharmacodynamic interactions involve a change in the pharmacological effect of a drug as a result of the action of second drug at receptor sites. An interaction between drugs, if not recognised and corrected, can result in decreased drug efficacy or serious toxicity. It is essential that careful scrutiny of the patient's drug history is an integral part of the preoperative assessment. The importance of drug interactions in anesthesia has become increasingly evident for several reasons, including the increased use of multiple drugs in clinical practice and the current medicolegal climate. Anesthetised patients represent a population with a relatively high risk of drug interactions. Patients who are not receiving any drug therapy before admission are likely to receive not less than 6 to 8 drugs during anesthesia for a simple surgical procedure like laparoscopic cholecystectomy. Nevertheless, patients who present for anesthesia frequently receive a surprising number of different drugs. May et al found in the 1970s that patients received an average of 7.9 drugs during their stay in a general hospital, and Cluff et al found that patients given an antibiotic received on average 13 additional drugs. The number of drugs received by comparable patients today is unlikely to be less. Such multiple drug-taking is not confined to hospital patients. Several surveys have shown that, at any one time, 20% of elderly people in the community are taking 3 or more prescribed drugs, and this proportion rises to over 50% for patients referred to a geriatric service. In a review of adverse drug reactions on a medical ward, 22% were found to be the result of drug-drug interactions. The Boston Collaborative Surveillance Drug Program reported that 6.5% [234 of 3600] of adverse reactions were attributable of drug- durg interactions. This survey, which involved 9900 patients and 83200 drug exposures, only monitored patients on medical wards. The potential drug interactions in surgical patients have also been identified. A total of 1825 surgical patients were reviewed and at least 1 drug interaction was found in 17% of patients. The frequency of adverse drug interactions increases disproportionately with the increase in the number of drugs given to patients. It was shown that 40% of patients given 16 drugs experienced an adverse drug interaction, compared with only 5% of patients given fewer than 6 drugs. The incidence of significant drug interactions with general anesthetics is still largely unknown but is probably clinically significant. A complete drug history and clear, accurate and comprehensive drug prescribing and recording documents are essential if such interactions are to be avoided during and after anesthesia. The purpose of this review is to focus on the potentially important interactions with volatile and intravenous anesthetics. Other interactions that might occur between drugs given during anesthesia [e.g. antibiotics and neuromuscular blocking drugs] were considered outside the scope of this review