Vagal third-degree atrioventricular block in a highly trained endurance athlete

The electrocardiogram (ECG) characteristics of athletes have recently become a topic of intensive study to accurately identify dangerous and benign alterations in this special population. These subjects develop a high-vagal tone and consequently sinus bradycardia and different degrees of atrioventricular (AV)-block, but it is difficult to determine how much is too much, and the indications for a pacemaker must be clearly defined, as it could ruin their careers. A 39-year-old male smoker that performed high-endurance mountain biking (up to 200 km/day, 5 times/week, for the last 3 years) had 45 bpm asymptomatic bradycardia in a routine examination. The ECG showed a sinus rate of 100 bpm with a complete heart block (CHB) and a nodal escape with a narrow QRS of 45 bpm (Panel A). His ECG was normal. The subject was referred to us for possible pacemaker implantation. Cycle ergometry was performed and CHB disappeared as soon as he started to exercise (without a significant change in the sinus rate), initially presenting with second degree AV-block and then becoming first degree AV-block (PR 260 ms). The test was stopped