Have you told the cardiologist that you are an endurance athlete? this will give him a lot of clues. For example, many athletes have resting heart rates of less than 60, it has been known for some to be 25. As your heart evolves for your activity, it simply becomes much more efficient at its job, so it doesn't need to pump as often. It is also very common for an ekg in an athlete to look different from an average persons. For example the ST segment, the pause between ventricles contracting and relaxing can be longer, and the T wave can be very different because you will have a larger left ventricle taking longer to repolarize. If your cardiologist IS aware of your running, then he may be spotting something a bit unusual. The problem with athletes is that they rarely notice a problem due to the way the heart has adapted. When you hear of athletes dying of heart failure, most are due to problems that existed before they began their training. I read a report just last week where a cardiologist stated young men/women should be tested for heart problems before they get into the athletics arena of life.
The key thing here is that your cardio knows you do a lot of endurance running.
Ed describes it well. The EKG will produce an abnormal result if the heart rate is below 60 or above 100. A doctor will do a differential diagnoses to rule out anything that could ALSO contribute to the condition. A slow heart rate is sometimes normal and can be a sign of being very fit. Healthy young adults and athletes often have heart rates of less than 60 beats a minute.
Inaddition to an athletes' heart there could be a problem with the heart’s electrical system . It means that the heart's natural pacemaker is not working right or that the electrical pathways of the heart are disrupted. In severe forms of bradycardia, the heart beats so slowly that it does not pump enough blood to meet the body's needs. This can be life-threatening.
It would be malpratice to conclude the bradycardia is due to physical fitness without ruling out other possibilities that could contribute or be the dominant cause.
Actually, I should have clarified. I was taking a physical exam for possible entrance into the military. The doctor that reviewed my EKG was not even present that day....the results were sent off for review. My guess is that he has no idea that I am an athlete. I will see if I can make sure something is noted on the file next time.