The raspberry pi will be a very capable CPU for a medical monitoring. The issue is the sensor and the power unit. Isolation is very important and the sensor reading needs to be correct.

It's certainly a fair question. I don't work in medical devices, but know people that have, and the problem for medical hardware has two steps: One, build something that won't kill the patient, and then step two is proving that you're correct about step one. Step two is the expensive bit.