What happens when you are the doctor on the airplane?
I have had several medical emergencies onboard as a pilot. One thing I constantly say to physician friends (my wife is a doctor), is that we (the crew) expect from them to make as fast assessment of the situation as possible.For example in my company we don´t have an online doctor to ask for help, so the volunteer has to take care of the situation. Recognizing the problem as soon as possible and prescribing the necessary action (Is not going to be easy with such limited capabilities, sometimes is just impossible, but is better an overreaction), is important because commercial planes fly at 8 NM/minute, and depending where are you flying ( flying over the ocean or africa) 10 minute delays could mean another 30 extra min to land or even several hours if you have passed a no return point in the middle of the ocean.Don´t hesitate to take control of the situation regarding the patient, stewardesses and other passengers:- Ask for the medikit (only physicians are allowed to use it). I have seen some chief stewardess resisting to bring it, just to avoid having to write the compulsory report.
- If it is necessary to lay the passenger on the ground or other seats, bring him water or any other thing, just give the needed orders and ask for help. 
- Ask the stewardesses to keep other passengers away if they are interfering too much.
- As soon as practicable give a report to the captain and if necessary ask him to land the plane. He will be waiting for it.
- If possible never declare a decease onboard , first you could be wrong!, and it is a bureaucratic mess. Is better to keep trying CPR till the emergency team can take care of the patient. We apply this in Europe although it is not a written rule, I don´t know how it works in USA.
-Remember when in doubt it is better to ask the captain to land. Everybody will lose an hour or two but a live will probably be saved.