[hatcravat]

I too think this is a very exciting sub-field. EMTs and the like already have something like this, only better: It's called a pulse oximeter, and it determines both heart rate and blood oxygen saturation by measuring the absorption ratio of two wavelengths of light. Commercial devices clip on your finger and cost about $50 (the whole device is about the size of two fingers).

[eru]

I've seen those. But having even a mediocre version of the same available as software for each smartphone will only improve what the casual user or even first-aider is capable of.

I wonder whether it would make sense to give people first aid instructions via an app? I guess when actually panicking, people would work better talking to an experience person over the phone. At least for the time being.