CPR is a violent process - the rib cage normally prevents the type of manipulation of the heart necessary for CPR to work. Broken ribs are pretty much an essential side effect for adults if done correctly. It’s typically only done on folks who are dead and unresponsive at the time, to try to keep their brain alive until they can get more sophisticated help. The so called ‘golden hour’ (which really is the golden 15 minutes/5 minutes if you look at success rates).

What sort of response time are we talking about from the ISS in a medical emergency? I would be shocked if it was less than a day.

They’re probably better off with a whole lot of drugs, an ultrasound, and a defibrillator. But hey, if they can afford the weight and already have those things, a cpr machine would help keep blood flowing while they try to figure things out.

Ground control has Doctors that can walk them through pretty much anything they might want to do.

It's worse than that. The patient is forced by the compressions to vomit. In zero-G, that means the highly acidic contents of their stomachs are now several forcefully-ejected packets of goo.

Forget the dreaded conductive pencil-lead shavings; that stuff is going to cause problems to hardware and wetware alike!