The Penthrox inhaler is a hand-held inhaler used for self-administration of methoxyflurane for pain relief. It is manufactured and marketed by Medical Developments International (Melbourne, Victoria, Australia). It is sometimes called a "green whistle."
The Penthrox inhaler is indicated for use by children and adults for the self-administration of methoxyflurane for analgesia in emergency and remote settings. A non-opioid alternative to morphine, this device is also easier to use than nitrous oxide.
Due to the risk of organ (especially kidney) toxicity, methoxyflurane is contraindicated in patients with pre-existing kidney disease or diabetes mellitus, and is not recommended to be administered in conjunction with tetracyclines or other potentially nephrotoxic or enzyme-inducing drugs.
The maximum recommended dose is 6 millilitres per day or 15 millilitres per week because of the risk of cumulative dose-related nephrotoxicity, and the inhaler should not be used on consecutive days.
This portable, disposable, single-use inhaler device, along with a single 3 milliliter brown glass vial of methoxyflurane is provided in doctor's kits that allows conscious hemodynamically stable patients (including children over the age of 5 years) to self-administer the drug, under supervision. Each 3 milliliter dose lasts approximately 30 min.
Despite the potential for renal impairment when used at anesthetic doses, no significant adverse effects have been reported in the literature when it is used at the lower doses (up to 6 millilitres) used for producing analgesia and sedation.
As of 2010, methoxyflurane was listed under the Australian Pharmaceutical Benefits Scheme for the initial management of pain due to acute trauma, as well as for brief painful procedures such as changing of wound dressings or for patient transport.
Methoxyflurane has been extensively used since the 1970s in Australia as an emergency analgesic for short-term use, mostly by the Australian and New Zealand Defence Forces, and the Australian ambulance services.
In 1968, Robert Wexler of Abbott Laboratories developed the Analgizer, a disposable inhaler that allowed the self-administration of methoxyflurane vapor in air for analgesia. The Analgizer consisted of a polyethylene cylinder 5 inches long and 1 inch in diameter with a 1 inch long mouthpiece. The device contained a rolled wick of polypropylene felt which held 15 milliliters of methoxyflurane. Because of the simplicity of the Analgizer and the pharmacological characteristics of methoxyflurane, it was easy for patients to self-administer the drug and rapidly achieve a level of conscious analgesia which could be maintained and adjusted as necessary over a period of time lasting from a few minutes to several hours. The 15 milliliter supply of methoxyflurane would typically last for two to three hours, during which time the user would often be partly amnesic to the sense of pain; the device could be refilled if necessary. The Analgizer was found to be safe, effective, and simple to administer in obstetric patients during childbirth, as well as for patients with bone fractures and joint dislocations, and for dressing changes on burn patients. When used for labor analgesia, the Analgizer allows labor to progress normally and with no apparent adverse effect on Apgar scores. All vital signs remain normal in obstetric patients, newborns, and injured patients. The Analgizer was widely utilized for analgesia and sedation until the early 1970s, in a manner that foreshadowed the patient-controlled analgesia infusion pumps of today. The Analgizer inhaler was withdrawn in 1974, but use of methoxyflurane as a sedative and analgesic continues in Australia and New Zealand in the form of the Penthrox inhaler.
- Babl F, Barnett P, Palmer G, Oakley E, Davidson A (2007). "A pilot study of inhaled methoxyflurane for procedural analgesia in children (subscription required)". Pediatric Anesthesia. 17 (2): 148–53. doi:10.1111/j.1460-9592.2006.02037.x. PMID 17238886.
- Grindlay J, Babl FE (2009). "Efficacy and safety of methoxyflurane analgesia in the emergency department and prehospital setting". Emergency Medicine Australasia. 21 (1): 4–11. doi:10.1111/j.1742-6723.2009.01153.x. PMID 19254307.
- Babl FE, Jamison SR, Spicer M, Bernard S (2006). "Inhaled methoxyflurane as a prehospital analgesic in children (subscription required)". Emergency Medicine Australasia. 18 (4): 404–10. doi:10.1111/j.1742-6723.2006.00874.x. PMID 16842312.
- McLennan JV (2007). "Is methoxyflurane a suitable battlefield analgesic?" (PDF). Journal of the Royal Army Medical Corps. 153 (2): 111–3. doi:10.1136/jramc-153-02-08. PMID 17896540. Archived from the original (PDF) on 2011-07-15. Retrieved 2010-11-21.
- National Prescribing Service (2010). "Methoxyflurane (Penthrox) for analgesia (doctor's bag listing)" (PDF). NPS RADAR. Canberra, Australia: National Prescribing Service, Department of Health and Ageing. Retrieved 2010-11-21.[permanent dead link]
- Medical Developments International Pty. Ltd. (2009). "PENTHROX (methoxyflurane) Inhalation: Product Information" (PDF). Springvale, Victoria, Australia: Medical Developments International Limited. Retrieved 2010-11-21.
- Rossi, S, ed. (2009). Australian Medicines Handbook (10th ed.). Adelaide: Australian Medicines Handbook Pty, Ltd. ISBN 978-0-9757919-9-8.
- Buntine P, Thom O, Babl F, Bailey M, Bernard S (2007). "Prehospital analgesia in adults using inhaled methoxyflurane (subscription required)". Emergency Medicine Australasia. 19 (6): 509–14. doi:10.1111/j.1742-6723.2007.01017.x. PMID 18021102.
- Johnston S, Wilkes GJ, Thompson JA, Ziman M, Brightwell R (2010). "Inhaled methoxyflurane and intranasal fentanyl for prehospital management of visceral pain in an Australian ambulance service (subscription required)". Emergency Medicine Journal. 28 (1): 57–63. doi:10.1136/emj.2009.078717. PMID 20466829.
- Wexler RE (1968). "Analgizer: Inhaler for supervised self-administration of inhalation anesthesia". Abbott Park, Illinois: Abbott Laboratories. Retrieved 2010-11-21.
- Romagnoli A, Busque L, Power DJ (1970). "The "analgizer" in a general hospital: a preliminary report" (PDF). Canadian Journal of Anesthesia. 17 (3): 275–8. doi:10.1007/BF03004607. PMID 5512851. Retrieved 2010-11-21.
- Packer KJ, Titel JH (1969). "Methoxyflurane analgesia for burns dressings: experience with the Analgizer (subscription required)". British Journal of Anaesthesia. 41 (12): 1080–5. doi:10.1093/bja/41.12.1080. PMID 4903969.
- Major V, Rosen M, Mushin WW (1966). "Methoxyflurane as an obstetric analgesic: a comparison with trichloroethylene". BMJ. 2 (5529): 1554–61. doi:10.1136/bmj.2.5529.1554. PMC . PMID 5926260.
- Dragon A, Goldstein I (1967). "Methoxyflurane: preliminary report on analgesic and mood modifying properties in dentistry (subscription required)". Journal of the American Dental Association. 75 (5): 1176–81. PMID 5233333.
- Firn S (1972). "Methoxyflurane analgesia for burns dressings and other painful ward procedures in children (subscription required)". British Journal of Anaesthesia. 44 (5): 517–22. doi:10.1093/bja/44.5.517. PMID 5044082.
- Josephson CA, Schwartz W (1974). "The Cardiff Inhaler and Penthrane. A method of sedation analgesia in routine dentistry". Journal of the Dental Association of South Africa. 29 (2): 77–80. PMID 4534883.