This discussion was recorded on March 29, 2024. The transcript has been edited for clarity.
Robert D. Glatter, MD: Joining me today to discuss the use of methoxyflurane (Penthrox), an inhaled nonopioid analgesic for the relief of acute pain, is Dr. William Kenneth (Ken) Milne, an emergency physician at Strathroy Middlesex General Hospital in Ontario, Canada, and the founder of the well-known podcast The Skeptics’ Guide to Emergency Medicine (SGEM).
Also joining me is Dr. Sergey Motov, an emergency physician and research director at Maimonides Medical Center in Brooklyn, New York, and an expert in pain management. I want to welcome both of you and thank you for joining me.
RAMPED Trial: Evaluating the Efficacy of Methoxyflurane
Dr. Glatter: Ken, your recent post on Twitter [now X] regarding the utility of Penthrox in the RAMPED trial really caught my attention. While the trial was from 2021, it really is relevant regarding the prehospital management of pain in the practice of emergency medicine, and certainly in-hospital practice. I was hoping you could review the study design but also get into the rationale behind the use of this novel agent.
William Kenneth (Ken) Milne, MD, MSc: Sure. I’d be happy to kick this episode off with talking about a study that was published in 2020 in Academic Emergency Medicine. It was an Australian study by Brichko et al., and they were doing a randomized controlled trial looking at methoxyflurane vs standard care.
They selected out a population of adults, which they defined as 18-75 years of age. They were in the prehospital setting and they had a pain score of greater than 8. They gave the participants methoxyflurane, which is also called the “green whistle.” They had the subjects take that for their prehospital pain, and they compared that with whatever your standard analgesic in the prehospital setting would be.
Their primary outcome was how many patients had at least 50% reduction in their pain score within 30 minutes. They recruited about 120 people, and they found that there was no statistical difference in the primary outcome between methoxyflurane and standard care. Again, that primary outcome was a reduction in pain score by greater than 50% at 30 minutes, and there wasn’t a statistical difference between the two.
There are obviously limits to any study, and it was a convenience sample. This was an unmasked trial, so people knew if they were getting this green whistle, which is popular in Australia. People would be familiar with this device, and they didn’t compare it with a sham or placebo group.
Pharmacology of Penthrox: Its Role and Mechanism of Action
Dr. Glatter: The primary outcome wasn’t met, but certainly secondary outcomes were. There was, again, a relatively small number of patients in this trial. That said, there was significant pain relief. I think there are issues with the trial, as with any trial limitations.
Getting to the pharmacology of Penthrox, can you describe this inhaled anesthetic and how we use it, specifically its role at the subanesthetic doses?
Sergey M. Motov, MD: Methoxyflurane is embedded in the green whistle package, and that whole contraption is called Penthrox. It’s an inhaled volatile fluorinated hydrocarbon anesthetic that was predominantly used, I’d say 40, 50 years ago, for general anesthesia and slowly but surely fell out of favor due to the fact that, when used for prolonged duration or in supratherapeutic doses, there were cases of severe or even fatal nephrotoxicity and hepatotoxicity.
In the late ‘70s and early ‘80s, all the fluranes came on board that are slightly different as general anesthetics, and methoxyflurane started slowly falling out of favor. Because of this paucity and then a subsequent slightly greater number of cases of nephrotoxicity and hepatotoxicity, [the US Food and Drug Administration] FDA made a decision to pull the drug off the market in 2005. FDA successfully accomplished its mission and since then has pretty much banned the use of inhaled methoxyflurane in any shape, form, or color in the United States.
Going back to the green whistle, it has been used in Australia probably for about 50-60 years, and has been used in Europe for probably 10-20 years. Ken can attest that it has been used in Canada for at least a decade and the track record is phenomenal.
We are using subanesthetic, even supratherapeutic doses that, based on available literature, has no incidence of this fatal hepatotoxicity or nephrotoxicity. We’re talking about 10 million doses administered worldwide, except in the United States. There are 40-plus randomized clinical trials with over 30,000 patients enrolled that prove efficacy and safety.
That’s where we are right now, in a conundrum. We have a great deal of data all over the world, except in the United States, that push for the use of this noninvasive, patient-controlled nonopioid inhaled anesthetic. We just don’t have the access in North America, with the exception of Canada.