I hate to be a buzz-kill, but I feel using an Arduino for a (any) medical purpose is irresponsible. As cool as it is, it is really just a toy and is not designed for or certified for potential life and death situations. If it were, it would cost $30K, not $30. You may indeed get issues with something like the Arduino, once in a while. I can tell you, I am not eager to be subject to that 'once'. My advice is not to proceed with this unless you and your entire design team have some extraordinary insurance.
Just a question, so that I can avoid it, which hospital will this be used in?
BTW, that battery is woefully inadequate.