Very interesting. There have been a number of top cyclists treated for heart anomalies. One of those I can recall was Navaradauskas but there have been others. I also know a couple of people who've been flagged with an abnormal rhythm during routing tests but later told it was typical of highly trained athletes.Good question. Highly trained athletes can develop unusual rhythms - but they're generally not of concern. Normal heart rhythm is called 'sinus rhythm' which is the normal anatomical way a heartbeat is created and travels through the heart. Athletes can get not only sinus bradycardia (slow heart) but 'heart block' which is an abnormality in the electrical pathway. The most trivial just means the electrical pulse moves slower through one of the junction boxes. If it moves even slower you can get dropped beats. Some also develop something called a bundle branch block which is another non-worrying electrical issue.
On scans the heart can be enlarged, thickened and it can be very hard to differentiate between a diseased heart and a fit one on scans and heart tracings. Sometimes even if you exercise someone hard it's not always apparent - someone could be very fit *and* have early stages of a disease. So it's a very interesting field.
The second part of the answer to your question is that later in life, endurance athletes have higher rates of something called atrial fibrillation - an irregular heart. This is important as it can predispose to a stroke. We're not entirely sure why this happens.
http://www.cyclingnews.com/news/navardauskas-back-to-100-per-cent-after-heart-surgery/