NEJM recently published a paper looking at the outcomes of medical emergencies on commercial flights:

• 1 in 604 flights had a medical emergency occur
• physician passengers provided assistance in 48% of cases
• the most common problem was syncope/pre syncope, followed by respiratory or gastrointestinal symptoms
• few in-flight emergencies resulted in death or diversion of the aircraft

The full article, including recommendations for physicians who provide medical assistance on a commercial airliner, can be found HERE.