What is the evidence that prolonged endurance execise damages the conduction system of the heart.

In 1985, RJ Northcutt studied 20 males endurance athletes that were recruited from a Scottish Harrier's club. All were  older than 45 years and ran for 25 miles per week  or more. Stress testing and 48 hour Holter type monitoring were done.. He found that 9 had heart  rates less than 35 beats per minutes,6 had a prolonged PR interval, 4 had Mobitz type ii heart block and three with complete heart block one of whom had a pacemaker implanted. With exercise all of the various blocks disappeared.

Northcott with co-author Stuart Hood (1,2) studied 19 of the 20 subjects  twelve years later.Two of the group had pacemaker implantation in the interval, one for complete heart block with atrial fibrillation and the other for asystolic intervals up to 15 seconds. None of the others had any "bradycardic problems". 7 of the 20 in 1985 has systolic pauses greater than 2 seconds and 5 of those had none on follow-up. The other two had decreased the intensity of their running. ( not clear from the article if those 2 had pauses or not) Quoting the authors: "Our finding nonetheless suggest that clinically significant bradycardia symptoms are a real but rare potential complication of lifetime endurance exercise."

Baldesberger et al  (3) studied 62 former professional cyclists who had long since (over thirty years) retired from active competition.Two had pacemaker implantation. None had complete RBBB or LBBB and two had EKG pattern of left anterior hemiblock.None had second degree heart block. Six had "sinus node disease which the authors defined as heart rate less than 40 beats per minute.

Andersen's 2013 article (4) is often quoted in support of the argument that there is an increased risk of atrial fibrillation in endurance athletes but it also has data on the risk of  "bradyarrhythmias".The authors studied the records of over 52 thousand participants in a 90 km cross country race in Sweden  (the Vasaloppet). They then compared those who raced more (five or more races) with those who only did one race and compared the faster skiers with the slower. The  Hazard ratio (HR) for atrial fibrillation was 1.2 (0.93-155) while the HR for bradyarrhythmias was 1.85 (0.97--3.54). When comparing those who did more races with those who did only  one they reported a HR for AF of 1.29 (1.04-1.61 and a HR for bradyarrhythmias 2.10 (1.28-3.47).The bradyarrrythmis were mainly type ii heart block but the ICD s codes used did not enable the investigators to separate type 1 and 2 second degree block, an important distinction as Wenckeback is usually considered much less  serious that type 2 second degree block. No mention was made on any one requiring a pacemaker. The rhythm outcome endpoints were obtained from hospital records those race finishers hospitalized with rhythm disturbances.

Comment.This is a very "coarse grain" study.All that was known about the subjects was the race numbers and times ,ICD codes of those hospitalized with arrhythmias, and their age, education and occupational status. How much they exercised outside of this race and other pertinent health factors that could influence the outcome ( BP,diabetes,obesity,height,smoking history,alcohol use) were not known and the various hazard ratios were not all statistically significant. Incidentally,  the HR for atrial fibrillation were much lower than the five times increased risk often quoted for AF in several case control studies.



1.Northcote R. et al. Electrocardiographic findings in male veteran endurance athletes. Br Heart J. 1989, 61: 155-160

2.Hood S and Northcote,R Cardiac Assessment of veteran endurance athletes;a 12 year follow up study. Br J Sport Med, 1999, 33: 239-243

3.Baldesberger S,  et al Sinus Node disease and arrhythmias in the long term followup of former professional cyclists. Eur Hear J. 200829.71-78

4.Andersen K. Risk of arrhythmias in 52,755 long-distance cross country skiers: A cohort study.
Eur Heart J 2013 Dec 34(47) 3624-3631