Volatile Inhalational Anaesthetic Pregnancy: There are no or limited data from use in pregnant women and animal studies have shown reproductive toxicity; use during pregnancy only if the benefit outweighs the potential risk. Isoflurane has uterine relaxant effects with a potential risk of uterine bleeding - use the lowest possible concentration in obstetric operations; concentrations up to 0.75% have been shown to be safe for maintenance of anaesthesia for caesarean section. It is not known whether isoflurane/metabolites are excreted in human milk; exercise caution in nursing women.

Isoflurane

Brand names: Forane

Isoflurane is a halogenated ether volatile anaesthetic agent used to induce and maintain general anaesthesia by inhalation, and occasionally for sedation in critical care.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Induction: begin at 0.5%, increasing to 1.5-3.0% (surgical anaesthesia usually produced in 7-10 minutes). Maintenance: 1.0-2.5% inspired concentration in an oxygen/nitrous oxide mixture (an additional 0.5-1.0% may be required when given with oxygen alone)

Route: Inhalation (vapour delivered through a vaporiser specifically calibrated for isoflurane)

Frequency: Titrated continuously to the depth of anaesthesia; MAC (minimum alveolar concentration) varies with age

Volatile halogenated anaesthetic (SPC: ISOFLURANE 100% Inhalation Vapour, Liquid). Average adult MAC in 100% oxygen approximately 1.28% at ~26 years, 1.15% at ~44 years and 1.05% at ~64 years (lower with 70% nitrous oxide, e.g. about 0.50% at 44 years). Elderly patients normally require lower concentrations. Induction: because isoflurane is mildly pungent, precede inhalation with a short-acting barbiturate or other intravenous induction agent to prevent coughing. Caesarean section: 0.5-0.75% in an oxygen/nitrous oxide mixture. NOT recommended as an inhalation induction agent in infants and children (risk of cough, breath-holding, desaturation, increased secretions and laryngospasm). Paediatric maintenance MAC values (100% oxygen): preterm neonate under 32 weeks 1.28%; 32-37 weeks 1.41%; 0-1 month 1.60%; 1-6 months 1.87%; 6-12 months 1.80%; 1-5 years 1.60%. Use only vaporisers giving a predictable, accurate concentration; monitor hypotension and respiratory depression as indicators of anaesthetic depth; administer only in an adequately equipped anaesthetising environment by those familiar with the drug. Verify paediatric use against a children's formulary. Interaction list drawn from the US product label (section 7) plus SPC section 4.4, as the captured UK SPC sections did not include a standalone section 4.5.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Known sensitivity to isoflurane or to other halogenated anaesthetics
Known or suspected genetic susceptibility to malignant hyperthermia

Side effects

Dose-dependent hypotension and respiratory depression
Arrhythmia; cardiac arrest, bradycardia and tachycardia; QT prolongation with torsade de pointes (in exceptional cases fatal)
Malignant hyperthermia; hyperkalaemia; elevated serum creatine kinase and myoglobinuria
Post-operative shivering, nausea, vomiting, ileus, agitation and delirium
Hepatobiliary effects (hepatocellular injury, hepatic necrosis, raised blood bilirubin); anaphylactic and hypersensitivity reactions; laryngospasm and bronchospasm

Interactions

Nitrous oxide and opioids reduce the MAC of isoflurane - adjust dose accordingly (opioids such as fentanyl and analogues may cause a synergistic fall in blood pressure and respiratory rate)
Potentiates the effect of all neuromuscular blocking agents and reduces the dose required (ED95 of succinylcholine, atracurium, pancuronium, rocuronium and vecuronium reduced by approximately 25-40% or more versus nitrous oxide/opioid anaesthesia)
Sensitises the myocardium to the arrhythmogenic effect of exogenous adrenaline (epinephrine) - submucosal adrenaline doses greater than 5 micrograms/kg may produce multiple ventricular arrhythmias
Interacts with dry/desiccated carbon dioxide absorbents to form carbon monoxide (with rare reports of extreme heat, smoke or fire) - do not allow CO2 absorbents to dry out (SPC section 4.4)

Clinical monograph

How it works

Like other volatile agents it produces anaesthesia largely by potentiating inhibitory GABA-A and glycine receptor activity and modulating other ion channels in the central nervous system, with potency expressed as minimum alveolar concentration.

Prescribing in practice

It is a recognised trigger for malignant hyperthermia and must be avoided in susceptible patients; dantrolene and a malignant hyperthermia protocol should be immediately available.
It causes dose-dependent reductions in blood pressure (mainly through vasodilatation) and respiratory depression.
Delivery requires a calibrated, agent-specific vaporiser and anaesthetic gas scavenging.

Monitoring

Monitor end-tidal agent concentration, blood pressure, oxygenation, ventilation, and temperature throughout administration.

Counselling the patient

Drowsiness, nausea, and shivering can occur as the anaesthetic wears off.
Inform the team of any personal or family history of adverse reactions to anaesthetic gases.

Evidence & guidelines

Use reflects long-standing anaesthetic practice; MHRA safety information on malignant hyperthermia and the SPC should be consulted.

Reference: RCoA GPAS 2023; AAGBI MH Guidelines 2020; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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