A good friend of mine is a cardiologist and I asked him about the effectiveness of ECG's a number of years ago.
Basically what he told me is that the ECG is very good at picking up one Kind of heart problem, cardimyopathy if I remember rightly but for every thing else it's a bit fuzzy.
What it boils down to is if you have a Normal ECG it means that at the time of the ECG you were probably OK but it is no indicator of what's going to happen.
When they do a cardio workup on someone the ECG is used in conjuction with all the other tests they do to build up the whole picture.
The JAA uses the ECG as the whole picture.
this link will take you to an incident report that happened on board a flight, the pilot involved had passed his ECG within the previous six months.
http://www.aaiu.ie/AAIUviewitem.asp?...g=ENG&loc=1652