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

Electrocardiogram (ECG) characteristics of athletes have recently become a topic of intensive study. Athletes are well known to have low-heart rates, but it is difficult to determine how low is too low for pacemaker indication. A 39-year-old asymptomatic male highly trained endurance athlete underwent a routine ECG, showing a sinus rate of 100 bpm with complete heart block (CHB) and nodal escape at 45 bpm (Panel A). Cycle ergometry was performed and complete atrioventricular (AV) block immediately disappeared at ∼12 metabolic equivalents. We suggested deconditioning and did not implant a pacemaker. After mild deconditioning, he continued to be asymptomatic and a Holter register showed first-degree AV block during sleep as the only conduction defect. His basal ECG was normal (Panel B). Junctional CHB with a sinus rate of 100 bpm secondary to a high-parasympathetic tone is rare in athletes. We believe this subject had localized hyperresponsiveness of the AV node to vagal tone with a relatively predominant sympathetic influence at the sinoatrial node. This speculation is supported by the immediate improvement in AV conduction without changes in the sinus rate. A careful case-by-case assessment is required when deciding about pacemaker implantation, considering the site of block, the pathophysiology and the symptoms. The full-length version of this report can be viewed at: http://www.escardio.org/Guidelines-&-Education/E-learning/Clinical-cases/Electrophysiology/EP-Case-Reports.