When I was in the military, I went through a string where I flew a lot and there were medical mishaps. After one episode where the pax required CPR and died prior to landing, I contacted the JAG lawyer who looked into it... apparently you can be sued in any state that the plane took off, landed in, or flew over. Which means that if the plane crosses over a state without a good samaritan law, you can be sued (and theoretically loose).

In my personal experience, I approach inflight medical issues very conservatively. For example, on an ANC-SEA flight, shortly after reaching altitude, a pax complained of chest pain, shortness of breath, was diaphoretic, and his pulse felt a bit thready. I had the captain turn around and land in ANC (on UA, so no intermediate landing site).

The medical kits on planes are pretty poor. I've run into BP cuffs that are calibrated to sea level (so the needle was less than 0 at altitude). I've also seen O2 tanks with old fashioned non-rebreathers that couldn't deliver more than 4 L/M.

From my experiences, I personally believe that if you are conservative you will have minimal liability. I document everything I did and saw on the airline's reporting paperwork and jot notes for myself just in case. At a minimum, I insist that paramedics meet us on arrival and take the pax somewhere more appropriate for further diagnosis and treatment.