What happens during a medical emergency on a plane?
A defined procedure that is considerably more capable than passengers expect — cabin crew are trained, aircraft carry equipment, and ground-based medical advice is available by radio at any point in a flight.
The sequence:
Cabin crew respond first. All are trained in first aid, resuscitation and use of a defibrillator, and recurrently assessed.
A call for medical professionals on board, which is the familiar announcement. Around one flight in several hundred to a thousand has a medical event, and a qualified passenger is available surprisingly often.
Ground-based medical support. Airlines subscribe to services staffed by emergency physicians, contacted from the flight deck. They, not the crew or the volunteer, advise on treatment and on whether to divert — which removes an enormous burden from whoever has stepped forward.
Equipment. Aircraft carry a first aid kit, an emergency medical kit with drugs and equipment for use by a professional, oxygen, and an automated external defibrillator on most commercial aircraft.
A diversion decision, made by the captain weighing the medical advice, the distance to suitable airports, weather, fuel and the aircraft's landing weight. Diversions are expensive and are not taken lightly, and are made when advised.
On volunteering. Good Samaritan protections exist in many jurisdictions, and several airlines indemnify assisting professionals; you are acting under the direction of the ground medical service rather than alone. You may be asked for identification.
What is most common: fainting and light-headedness, which is by far the largest category and usually resolves with lying flat and oxygen; gastrointestinal symptoms; respiratory difficulty; and cardiac symptoms. Deaths in flight are rare.
Why altitude matters. The cabin is pressurised to an equivalent of several thousand feet, so blood oxygen saturation is lower than at sea level, dehydration is greater, and gas in body cavities expands — which is why some conditions are aggravated aloft and why fitness-to-fly advice exists after surgery, for severe anaemia and for some lung and cardiac conditions.
If you have a condition, carry medication in hand luggage with documentation, and check the airline's medical clearance requirements before travelling.