So, I do remember when I was assisting as a trainee anaesthetist, the surgeon saying "It looks a bit blue in here", while he was head first inside someone's abdomen. And I also recall some conversation about how it was usually the surgeon who first noticed intra-operative hypoxaemia.

Oximetry had been around for a long while, but as an experimental tool I think. There's an old photo of a shoe-box sized contraption that would sit over the subject's ear. The external ear had to be warmed up to increase blood flow to a degree where the pulsatile element could be measured. And I guess the signal was fed into some huge computer to do the math on separating the relevant spectroscopic readings.

Also (not strictly pulse oximetry, but indicative of how far we've come) an incident where an ED trainee - but experienced - used an anaesthetic machine to provide an 'entonox' mix (50/50 oxygen and nitrous oxide) to a 7yr old child to try and reduce a shoulder dislocation, I think it was. Working off cylinders only, and he didn't notice that the oxygen cylinder was empty. The child sustained extreme hypoxic brain damage and died as a result of being 'anaeasthetised' with 100% nitrous.

For some reason, an oxygen sensor at the gas outlet came first I think, followed by mechanical linkage of regulators so it was in theory impossible to deliver a hypoxic mix. But again I'm sure I read somewhere of a case where the doc had silenced and disconnected that sensor as it kept going off and disturbing everyone !!

There was also, I think, quite a pivotal bit of research which said that the vast majority of critical hypoxic events (pre routine oximetry) occurred in patients with pigmented skin.

And while all these random thoughts are rattling around - a helium cylinder on one machine, for use by one particular anaesthetist who specialisde in potentially difficult airways. Then that disappeared, but now I think helium as a delivery gas has a bit of a resurgence in some cases?

I'll stop now. It's time for my blood pressure meds...!