In Michigan anyway, the good Samaritan law does NOT cover medical professionals; they're held fully liable for harm. And in general, once you begin to render aid, you're not allowed to stop until you're relieved by someone more qualified.

At least, this is as well as I can remember the stuff they talked about in First Aid class. Mainly they were talking about that WE would be covered, as people trained only in basic first aid and CPR, but someone asked "what about doctors?"