Medlink
A lot of airlines use medlink and the service is very good but the chances of having a medical professional on the aircraft are very high.
Both the medical diversions that I have had to carry out resulted in an immediate diversion on the advice of a doctor who was one of the passengers and I had no time to call medlink due to the urgency of the situation and the proximity of diversion options. ( one heart attack one stroke ).
Interestingly the cabin crew made the correct diagnosis before the doctor had the chance to examin the ill person.