Linux has no real-time capabilities and guarantees! Maybe with some crazy patch. But recommending general Linux kernel for medical applications is dangerous. It can not even play and capture audio in RT!!
I hear this argument a lot, but the reality is that most devices that need realtime support will either be using the rt patched kernel, or be using application specific mcu's that handle real-time constraints while the Linux system just acts as the main control hub and orchestrator.
There's nothing "crazy" about the preempt-rt patch and coding your application to use the scheduler policies/priorities. Like anything else worth having in open source, it takes practice to learn or money for consultants.