Obstetric Anaesthetists' Association
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Invention of Entonox

Dr Mike Tunstall    describes the invention of Entonox:

When I moved to Oxford early in 1962, on the second leg of my joint appointment with Portsmouth, BOC continued to supply me with 48 cu.ft size (F) cylinders, painted grey, to allow me to go on using premixed N2O/O2 on patients. Click here to read my early correspondence with BOC. I even toted these cylinders to a couple of dental practices in Oxford to drive Halothane via a Goldman vaporiser with a view to developing a simple portable machine.

However while at Oxford Dr. Epstein (physicist to the Nuffield Dept of Anaesthetics) and Dr. Cole, an anaesthetist, found that after cooling the cylinders in the laboratory the subsequent output of the mixture was not stable even after re-warming. I was not involved with what they did, but I was told that the mixture could not be regarded as safe. It seemed that it had no future! That was that, until I got to Aberdeen in December 1962 as a consultant.

In the Aberdeen Maternity Hospital not only was I allowed to do what I wanted but I was expected to do it! The late Sir Dugald Baird told me that if one was appointed to a teaching hospital one was morally obliged to undertake research as well as teaching.

I was determined to find out exactly what happened when cylinders of premixed N2O/O2 were cooled and then re-warmed, and to see if I could sort out a solution for getting a consistent mixture during clinical use if the cylinders had previously been exposed to cold. After many experiments including x-raying the cylinders for fluid levels the solution was found to be very simple. Donald’s ice-cream factory cooled my cylinders down to minus 25°C for me. The effect of cooling on pre-mixed gas mixtures for obstetric analgesia was published in the BMJ in 1963.

A long chain of events, including the results of the MRC trials, attendance on their subcommittee etc. led to the approval of 50% N2O in oxygen by unsupervised midwives in 1965.

My preliminary communication to the Lancet was published in 1961. While according to dictionary definition I was the inventor of a premixed N2O/O2 gas mixture, subsequently called Entonox, BOC did not involve me with the patent. I was too unsophisticated for the matter even to enter my mind at that time. It is worth noting that BOC avoided listing the first published paper on premixed gas (in the Lancet) in their promotional booklet, published in September 1970, called the “Entonox Digest”. The digest was a 35 page summary of articles, correspondence and papers. There were extracts of various publications in the medical literature on the clinical uses of Entonox.

No organisation likes sharing a patent. Once BOC had secured their position they were happy to keep on supplying me with full cylinders when I got to Aberdeen so that I could continue my investigations, even though premixed gas seemed to have been condemned in Oxford. But it worked out very well in the long run. It was the beginning of a most wonderfully satisfying working life in Aberdeen.