Endurance trained athletes show more positive tilt tests and more arrhythmias than non-trained people. The mechanisms remain unclear, several hypotheses exist including structural and autonomic cardiac adaptations. Training bradycardia (heart rate (HR)<50bpm) may be related to several of these factors. However, to our knowledge, this link have never been explored yet.

Untrained non bradycardic subjects (UTNB, n=16, HR=66,1±5,9bpm) were compared with trained non bradycardic athletes (TNB, n=13, HR=61,3±3,3bpm) to specify the influence of endurance training. Also TNB were compared to trained bradycardic athletes (TB, n=17, HR=43,8±3,1bpm) to specify the influence of bradycardia. All subjects were healthy men between 18 and 36 years old. They performed incremental maximal exercise test, resting echocardiography, 24h ECG Holter monitoring, and 45 min of an 80° tilt test.

Training history did not differ between TNB and TB (8,3±2,8 vs 8,8±3,1 hours/week since 8,1±4,2 vs 6,4±4,6 years). VO2max was 43,5± 5,9 ml/min/kg in UTNB, 67,9±6,2 in TNB (p<0,001) and 67,8±5,5 in TB (NS with TNB). TNB showed higher left ventricular end-diastolic dimension index (LVEDDI) (28,3±1,7 vs 25,9±1,5mm/m², p<0,001) and LV end-systolic dimension index (18,0±1,5 vs 16,6±1,3mm/m², p<0,05) than UTNB. TB have also a higher LVEDDI than TNB (30,1±1,6 vs 28,3±1,7mm/m², p<0,01). Holter results did not differ except for resting HR. Tilt test positivity rates were 44%, 62% and 53% respectively for UTNB, TNB and TB. VASIS I syncope occurred in 57% in UTNB, 63% in TNB but only in 33% in TB. VASIS II syncope occurred in 14% in UTNB, 25% in TNB and 56% in TB.

From our results, training bradycardia does not seem to be an additional risk factor for syncope or arrhythmias in endurance athletes. However VASIS II reactions are more frequent in bradycardic athletes who experienced syncope.