Muscle Relaxants: Isoflurane produces sufficient muscle relaxation for some intra-abdominal operations. It is compatible with all commonly used muscle relaxants, the effects of which may be markedly potentiated by isoflurane. The effect is most notable in nondepolarising agents, thus lower doses should be used in the presence of isoflurane. The effect of nondepolarising muscle relaxants can be counteracted by administering neostigmine as this has no effect on the relaxant properties of isoflurane.
Adrenaline: Administration of adrenaline (epinephrine) by any route during isoflurane anaesthesia may cause supraventricular or ventricular arrhythmias. Where adrenaline is used, the amount used should be limited to a maximum of 3 mg/kg of body weight in patients with normal hearts and less in those with rhythm disturbances.
Concurrent use of other β-sympathomimetic drugs eg, amphetamines may predispose to arrhythmias. If possible, interrupt treatment a few days before surgery.
Calcium Antagonists (and Other Vasodilators): Isoflurane can cause marked hypotension in patients receiving concomitant therapy with calcium antagonists, especially those of the dihydropyridine class. Patients receiving chronic therapy with other vasodilators eg, ACE inhibitors (eg, captopril, enalapril, lisinopril) or α1-adrenoceptor antagonists (eg, prazosin), may show unpredictable hypotension with any type of anaesthesia.
Monoamine Oxidase Inhibitors (MAOI): Monoamine oxidase inhibitors have been shown to enhance the effects of general anaesthetics. If possible, patients should stop taking the MAOI drug at least 14 days before anticipated surgery.
Beta-Blockers: Beta-blockers used in the peri-operative period will prevent or reduce any tendency for isoflurane to increase the heart rate. Thus, β-blockade will normally tend to be cardioprotective. Should an increase in heart rate or vasoconstriction be required, appropriate sympathomimetics should be given by the anaesthetist. A full drug history should always be taken by the anaesthetist.
Isoniazid: Isoniazid induces enzymes. Patients receiving isoniazid may be more susceptible to hepatotoxicity from volatile anaesthetics. If possible, cease isoniazid treatment 1 week before surgery.
Opioid Analgesics: Opioid analgesics potentiate the respiratory depressant effect of isoflurane.