As a doctor training in Anaesthesia and Intensive Care, and about to sit exams in physiology, I thought I'd clarify a few things and outline some basics that might be informative to some. I'll break it down into what I think are probable mechanisms. Unfortunately, athletes can be at a disavantage for a number of reasons (which on the face of it doesn't make sense, and proposes an alternate theory for the benefit of sitting on the couch drinking beer).
1. Pump problems. There is a condition called HOCM (hypertrophic obstructive cardiomyopathy), which usually affects older people with chronic hypertension. A left ventricle under sustained increases in afterload will remodel to become bigger and thicker (due to the Law of Laplace). A dilated left ventricle is susceptible to outflow tract obstruction and therefore a sudden cessation of aortic (and coronary) flow. This mechanism has been proposed in the sudden death seen in otherwise healthy individuals.
2. Wiring problems. Athletes have typically low resting heart rates. This can lead to susceptibility for arrythmias as the heart muscle cells (myocytes) spend longer time in diastole (relaxation and passive filling of the atria/ventricles). During a longer diastole, the myocytes have a longer relative refractory period, where they are in a potentially excitable state, and may generate an action potential and propagate an arrythmia. If this happens in ventricular miuscle, then pumping is impaired and outflow is compromised. Other conditions that may lead to susceptability for problems include long QT syndrome.
I get my share of ectopic beats, and they feel wierd (feels like a sinking feeling in your chest then a sudden rush as the ventricle ejects a larger stroke volume). If memory serves me correctly, Stuart O'Grady used to have runs of SVT (not sure why or if he had it fixed).
Other previously suggested mechanisms are also likely. A pulmonary embolus could be considered, and the causes are vairied.
The sudden 'collapse' suggests an abrubt cessation of cerebral flow (and therefore cardiac output), most likely due to obstruction or arrythmia. The traditional 'heart attack', where coronary arteries are occluded by throumbus or embolues seems less likely. The lucky thing, if one can draw a positive from this, was that it was witnessed, and he was treated promptly.
As for the 'induced coma', this is a common treatment modality for out-of-hospital-cardiac-arrest. In our hosptial, we admit patients to ICU for a peroid of controlled hypothermia. We cool them for 12h, then rewarm, wake, and assess neurological function. Cooling is supposed to improve neurological outcome (think of stories of near-drownings in icy rivers).
Whether his circumstances have anything to do with doping is impossible and unreasonable to conclude at this time. I feel for his family, having been the bearer of bad news all too often. I can only imangine the agony of not knowing what will happen. I hope he pulls through ok.