When I had a day in the Emergency department of our local hospital last year, they covered me with various bits of machinery (a couple of which, like the character in the novel that I’m reading at the moment, This Book Will Change Your Life, I still had clinging to me when I got home). I now discover that one of the little machines is called an pulse oximeter. It’s that feller that sits on your finger to keeps track of your pulse and makes an electronic beeping noise (similar to the one your computer makes if it’s overheating) if your pulse starts to climb too high.
Apparently these are readily available for anyone who might like to go through their day checking their pulse. The retailers suggest people like pilots or those involved in high activity sports - or even joggers, I suppose - to check out their heart rate. They tell us that the item is so durable it can be dropped on a concrete floor and show no damage, rather like your average insect falling from a great height.
I think they’d be useful for opera singers as well, especially tenors and sopranos like Anna Leese (who's always turning up in these posts). All those high notes tend to make singers a little light-headed, so keeping a check on their pulse would be ideal, in case they felt they were about to pass out. The only problem would be ensuring that the electronic beeping was in the same key as the orchestra.