Depends on the problem. Faking it long enough actually helps. Patients can't know when I'm panicking about whether or not I can actually save them. I've been a nurse long enough that very few things surprise me anymore. When things are really crazy, you don't have time to freak out. All you have is the job in front of you. It's kind of zen that way.
I know that feeling. "I know I got food poisoning from that taco truck a few months ago, but I'm really hungry, and it smells so good, and I have about 5 minutes before I have to get back to work..."