Francois,
In general, arrythmias are no more common in athletes than the general population, with the exception of sinus bradycardia (i.e. slow heartbeat). This is more common in endurance athletes than the general population, however, it appears to be a normal adaptation to training and is not dangerous so long as it is asymptomatic (i.e., you aren't passing out in your breakfast).
I have read that t wave reversal, peaking T waves and exercise reversible changes in T waves can all be considered normal EKG findings in athletes. You can look up the article at your local university library: Zehender M. ECG variants and cardiac arrhythmias in athletes: clinical relevance and prognostic importance. Am Heart J. 01-Jun-1990; 119(6): 1378-91.
I have also read that PVC's are equally common in athletes and nonathletes, and are not a problem until you start seeing several in a row. then you get your defib :^)
Atrial fib is a different story. I remember reading this is more common in athletes than nonathletes, but I do not remember where or why. Afib is common in enlarged hearts in general, whether or not this is true in the athletic heart I do not know. Theoretically, short bouts of it are not disasterous, but if left unchecked it is possible to form clots in the atria, which can be dislodged, shot out of a ventricle, and give you a stroke, infarct a piece of lung, etc. You would be wise to consult a cardiologist with experience with athletes on this front.
Please remember that I am just a med student, so don't construe any of this as medical advice! I only know what I have been taught, and what I have had time to read, which often times does not have much relation to the way things happen in the real world.
Philbert
Dr. Philip Skiba
Scientific Training for Endurance Athletes now available on Amazon!