It is not unusual for athletes to exhibit an “abnormal” ECG due to the effect of exercise training on the anatomy and autonomic control of the heart. Most of the changes are present in athletes who participate in endurance events; they are more common in males than in females, and in individuals of African decent (Corrado et al., 2009, 2010). Up to 80% of highly trained athletes show some evidence of ECG changes. Most of these changes can be considered physiologic adaptations to exercise training and do not necessarily indicate any pathology or concern. However, it is important to understand which ECG abnormalities may be physiologic and which are likely to be pathologic. A joint statement by the European Association of Cardiovascular Prevention and Rehabilitation and the European Society of Cardiology provides the following guidelines (Corrado et al., 2010).
The most common training-induced ECG changes in athletes are sinus bradycardia, first-degree AV block, second-degree AV block (Mobitz type 1 only), incomplete RBBB (QRS duration <0.12 s), early repolarization, and isolated QRS voltage criteria for LVH. It is generally agreed that a high degree of endurance training increases parasympathetic tone and decreases sympathetic control of the heart. These changes are likely responsible for the high prevalence of sinus bradycardia, first-degree AV block, and Mobitz type 1 in athletes. However, more serious conditions, such as Mobitz 2 or third-degree AV block, as well as sick sinus syndrome, are indicative of pathologic changes that need medical attention.
Training-induced increases in wall thickness and chamber size are the likely causes of isolated QRS voltage criteria for LVH and for incomplete RBBB. The increased LV size causes higher QRS amplitude, and the increased RV size causes the conduction delay responsible for the incomplete RBBB. However, other criteria for LVH, such as a strain pattern or enlarged atrial size, are rarely present in athletes; and those changes are considered pathologic. Furthermore, a full RBBB or LBBB is not training induced and should also be considered pathological.
Early repolarization is present in 50% to 80% of highly trained athletes. It is characterized by a J point elevation of at least 0.1 mV (1 mm) from baseline, which is often associated with a slurring or notching of the terminal QRS complex. It is most often found in the left precordial leads. It is generally believed that autonomic influences induced by training cause early repolarization, since a slowing of the heart rate exaggerates the ST-segment elevation. However, ST-segment depression or ST-segment changes indicative of a strain pattern are not common and should be considered pathological.