Overview
This lecture covers the clinical use of methoxyflurane, focusing on its indications, mechanism of action, dosing guidelines, contraindications, side effects, pharmacokinetics, and important safety considerations.
Mechanism of Action
- Methoxyflurane is an inhaled analgesic originally developed as a general anaesthetic.
- The exact mechanism of action is not clear.
Indications and Contraindications
- Indicated for moderate to severe pain, especially if opiate administration is delayed.
- Contraindications: known severe allergy, personal or family history of malignant hyperthermia, known renal impairment, or methoxyflurane given in the past week.
Cautions and Special Populations
- Use caution for patients aged 75 or older (especially if frail) and those with pre-eclampsia.
- Avoid using in confined spaces unless adequate ventilation is possible.
- Safe for use in labour, but discuss with the lead maternity carer if there is foetal distress.
- Safe during breastfeeding, but patient should stop and seek further advice if needed.
- Not contraindicated for patients on dialysis or with kidney stones/renal colic.
Dosage and Administration
- Adults/adolescents ≥12 years: max 6 ml (two doses).
- Children <12 years: max 3 ml (one dose).
- Self-administered via a handheld inhaler using a charcoal filter.
- Instruct patient to exhale through the inhaler; do not supplement with oxygen via inhaler.
- Place unused inhaler in a closed bag for possible reuse by the same patient.
Adverse Effects and Interactions
- Common side effects: sedation and feeling light-headed.
- Analgesic and sedative effects increase with opiates, benzodiazepines, or alcohol.
Pharmacokinetics, Preparation, and Monitoring
- Onset: 12 minutes; Duration: 25 minutes after stopping.
- Supplied as a 3 ml bottle with plastic inhaler.
- About 20% is exhaled; the rest is metabolised by the liver, producing fluoride (risk for renal impairment).
- Always follow the 5Rs (right medicine, dose, patient, time, route), 4Cs (clarity, concentration, colour, container), and Expiry date checks.
- Thorough documentation and informed consent required.
Key Terms & Definitions
- Anaesthetic — Medication reducing sensation; general anaesthetics affect the whole body.
- Renal impairment — Reduced or lost kidney function.
- Malignant hyperthermia — Rare, life-threatening reaction to certain anaesthetics causing severe hyperthermia and muscle breakdown.
- Dialysis — Blood filtering procedure for renal failure.
- LMC (Lead Maternity Carer) — Healthcare provider overseeing maternity care.
Action Items / Next Steps
- Review Medicines Management policy for methoxyflurane responsibilities.
- Complete accompanying quiz and skills sheet on methoxyflurane administration.
- Ensure clear understanding of contraindications, safe dosing, and monitoring protocols.