Intrinsic sinus and atrioventricular node electrophysiologic adaptations in endurance athletes

Objectives In the present study, we evaluated sinus and atrioventricular (AV) node electrophysiology of endurance athletes and untrained individuals before and after autonomic pharmacologic blockade. Background Endurance athletes present a higher prevalence of sinus bradycardia and AV conduction abnormalities, as compared with untrained individuals. Previous data from our laboratory suggest that nonautonomic factors may be responsible for the longer AV node refractory period found in well-trained athletes. Methods Six aerobically trained male athletes and six healthy male individuals with similar ages and normal rest electrocardiograms were studied. Maximal oxygen uptake (image2max) was measured by cardiopulmonary testing. The sinus cycle length (SCL), AV conduction intervals, sinus node recovery time (SNRT), Wenckebach cycle (WC) and anterograde effective refractory period (ERP) of the AV node were evaluated by invasive electrophysiologic studies at baseline, after intravenous atropine (0.04 mg/kg) and after addition of intravenous propranolol (0.2 mg/kg). Results Athletes had a significantly higher image2max as compared with untrained individuals. The SCL was longer in athletes at baseline, after atropine and after the addition of propranolol for double-autonomic blockade. The mean maximal SNRT/SCL was longer in athletes after atropine and after propranolol. The WC and anterograde ERP of the AV node were longer in athletes at baseline, after atropine and after propranolol. Conclusions Under double-pharmacologic blockade, we demonstrated that sinus automaticity and AV node conduction changes of endurance athletes are related to intrinsic physiology and not to autonomic influences.