Pharmacology for Anaesthesia and Intensive Care

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Drug interaction

Interactions occur when one drug modifies the action of another. This interaction

may either increase or decrease the second drug’s action. Sometimes these inter-

actions result in unwanted effects, but some interactions are beneficial and can be

exploited therapeutically.

Drug interaction can be described as physicochemical, relating to the properties

of the drug or its pharmaceutical preparation, pharmacokinetic due to alterations in

the way the body handles the drug or pharmacodynamic where the activity of one

drug is affected. The chance of a significant interaction increases markedly with the

number of drugs used and the effects of any interaction are often exaggerated in the

presence of disease or coexisting morbidity.

About one in six inpatient drug charts contain a significant drug interaction, one-

third of which are potentially serious. An uncomplicated general anaesthetic for a

relatively routine case may use ten or more different agents that may interact with

one another or, more commonly, with the patient’s concurrent medication.

Pharmaceutical

These interactions occur because of a chemical or physical incompatibility between

the preparations being used. Sodium bicarbonate and calcium will precipitate out

of solution as calcium carbonate when co-administered in the same giving set. How-

ever, one agent may inactivate another without such an overt indication to the

observer; insulin may be denatured if prepared in solutions of dextrose and may,

therefore, lose its pharmacological effect. Drugs also may react with the giving set

or syringe and therefore need special equipment for delivery, such as a glass syringe

for paraldehyde administration. Glyceryl trinitrate is absorbed by polyvinyl chloride;

therefore, special polyethylene administration sets are preferred.

Pharmacokinetic

Absorption

Inthe case of drugs given orally, this occurs either as a result of one drug bind-

ing another in the lumen of the gastrointestinal tract or by altering the function

of the gastrointestinal tract as a whole. Charcoal can adsorb drugs in the stomach,

preventing absorption through the gastrointestinal tract (charcoal is activated by

steam to cause fissuring, thereby greatly increasing the surface area for adsorption).