This hysterical fear of volunteering medical help in an emergency makes me ill. I'm with Mac. I was fortunate enough to obtain medical training. For me to sit on my hands and watch someone suffer due to my lack of assisting would be a moral lapse of the highest order.
I'm not aware of a single case in the US or on a US airline where a physician responding to an in-flight emergency has been sued for his/her activities. Samaritan laws specifically protect such activity, and also require that such service be provided without expectation of compensation, and the extent that the emergency requires. Non US medical folks seem to have an inordinate fear of this.
I have responded a couple of times, as have others on board. We quickly determine who is best qualified for the matter at hand, do what we can, then return to our bored existence. Now that most of the airlines have linkage to ground-based medical consultation, the whole issue is easier to manage. I hope medical personnel will continue to respond appropriately.