The Slow Heart Rates Produced by Endurance Exercise Training May Not Be Benign

And May Lead to More Pacemakers

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Endurance athletes, including four-minute milers, (1)  live longer than their sedentary counterparts, but it is not clear if this is due to the exercise alone, inherent differences in those who exercise train, or a combination of both.  A combination seems most likely since athletic achievement is partly inherited, and yet physical activity improves atherosclerotic risk factors, and prolonged exercise training can produce remarkable cardiac adaptations.  These adaptations include enlargement of all four cardiac chambers.(2)  This permits the increase in cardiac stroke volume that is required for success in endurance sports.  The increase in stroke volume also permits resting bradycardia, which can be extreme. 

For example, Jack Lovelock, the New Zealand physician, who won the 1500 meter run at the 1936 Berlin Olympics, was said to have a resting heart rate in the low 30s.(3)  Lovelock once had his heart rate broadcast during an experiment at King’s College Hospital in London.(3)  He died at age 39 when he felt ill at his work at the Hospital for Special Surgery in New York and collapsed in front of a New York subway train on his way home.(4)  (I have always wondered about the cause of his collapse given his marked bradycardia. I doubt complete heart block, that is rare with athletic bradycardia, but did the bradycardia contribute?). The man who finished second in that Olympic race, the American miler, Glenn Cunningham, was likely one of the four endurance athletes reported by the famous Boston cardiologist Paul Dudley White in one of the earliest reports of endurance training bradycardia in 1942.(4)  He reported four athletes, including one identified only as Cunningham, who had heart rates under 40 BPM.(5) 

Exercise-related bradycardia has generally been considered benign, but studies over the last two decades have questioned the health benefits of several of the cardiac adaptations that accompany endurance exercise training.  Lifelong endurance athletes have an increased prevalence of coronary atherosclerosis, more small myocardial scars where the right ventricle inserts into the interventricular septum, increased aortic root size, and an increased prevalence of atrial fibrillation.(2)  These maladaptations are generally observed only in the male athletes. 

When I retired from full-time practice at Hartford Hospital, I called several of my former patients from around the country who had come to see me because they were endurance athletes and had marked bradycardia.  I learned that several had eventually required a pacemaker because of symptoms related to their bradycardia.  This suggested their bradycardia was not necessarily benign.

 There is now more evidence to support that hypothesis.  Sweden's Niclas Svedberg, MD, and colleagues used national health data to determine the incidence of bradycardia and cardiac pacemaker placement in 209,108 Swedes who participated in the famed Vasaloppet cross-country ski races at distances of 30, 45, or 90 km.(6)  Skiers were followed for a median of 8.8 years and were compared with 532,299 non-skiers.  There was no difference in diagnosed bradycardia or pacemaker implantation among women, but bradycardia and pacemaker placement were 19% and 17% more frequent in male skiers and especially more frequent in the best performing athletes and in those competing in the most events.  Sick sinus syndrome was the predominant indication for pacer implantation in male skiers.  Third-degree AV block was the predominant indication in non-skiers.  Despite the more frequent bradycardia and pacemakers, skiers had a 16% lower mortality.

This report is the strongest evidence to date that the bradycardia associated with lifelong endurance exercise is not benign, but questions remain.  Were the pacemakers inserted because the skiers had symptoms or out of caution because of the bradycardia?  We assume the former, but do not know.  Also, what was the physical activity status of skiers who were bradycardic or who received a pacemaker?  Without that information we cannot discern if the changes were permanent and produced by the skiers' former athletic activity or due to ongoing, recent physical activity. 

What are the clinical implications of these results?  Clinicians should be ready to provide reassurance once these results are widely known.  The incidence of events for both groups was low. Only 0.21% of the skiers and 0.24% of the non-skiers were diagnosed with bradycardia; only 0.23% of the skiers and 0.27% of the non-skiers received a pacemaker over 8.8 years of follow-up.  These numbers show a higher incidence in the non-skiers, but the skiers were younger, despite attempts at matching, so the results were adjusted for chronological age to account for risk over their lifetime.  These results also apply primarily to the best skiers and those participating in the most events.  They do not apply to most patients who do not do enough exercise.  Also, most life-long endurance athletes exercise not just for health benefits, but for personal pleasure.  These results show low overall incidence rate for pacer placement and therefore should not dissuade athletes from continuing to be active.  Despite the results, the athletes had lower overall mortality and the pacemakers did not adversely affect their survival. 

But in addition to reassurance, clinicians should ensure that their athletic patients know to seek medical care if they develop symptoms possibly related to bradycardia because  exercise-training induced bradycardia may not always be benign.

(I have also written an editorial on the Svedberg article which will appear in the print version of Circulation and a commentary in PracticeUpdate: https://www.practiceupdate.com/content/association-of-endurance-training-with-long-term-incidences-of-bradycardia-and-pacemaker-implantation/169055/65/2/1.)

1.         Maron BJ, Thompson PD. Longevity in elite athletes: the first 4-min milers. Lancet. 2018;392(10151):913.

2.         Thompson PD, Eijsvogels TMH, Kim JH. Can the Heart Get an Overuse Sports Injury? NEJM Evid. 2023;2(1):EVIDra2200175.

3.         https://www.alamy.com/jack-lovelocks-heart-beats-being-broadcast-from-the-kings-college-hospital-the-olympic-runner-took-part-in-a-scientific-experiment-at-the-hospital-the-heart-testing-machine-record-the-beats-under-normal-conditions-and-then-after-he-had-made-a-series-of-jumps-7-september-1937-image359619554.html 

4.         Thompson PD. D. Bruce Dill Historical lecture. Historical concepts of the athlete's heart. Med Sci Sports Exerc. 2004;36(3):363-370.5.           

5.         White, P. Bradycardia (below rate of 40) in athletes, especially long distance runners. JAMA 120:642, 1942.

6.         Svedberg N, et al. Long-Term Incidence of Bradycardia and Pacemaker Implantations Among Cross-Country Skiers: A Cohort Study.  Circulation. 2024 Aug 5. Online ahead of print.

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