Endurance athletes often have low resting heart rates as a result of intense training, but a new study published in Circulation suggests that a predisposition to prominent bradycardia may be encoded in their genes.

In the study, André La Gerche, MBBS, PhD, from the Department of Medicine at The University of Melbourne in Parkville, Australia, and colleagues used a heart rate-associated polygenic risk score to assess genetic susceptibility to bradycardia. They found that a low score was more common among endurance athletes than that of the general population.

“This raises the possibility that a tendency of low heart rate in athletes may be an inherited trait rather than a fully acquired phenomenon,” he said.

La Gerche said studies show bradycardia, defined in the study as a heart rate ≤ 40 beats/min, and sinus pauses of 2-3 seconds are present in a significant proportion of endurance athletes and are well tolerated in this population.

“Our data suggest that both fitness and genetic variation contribute to sinus node function in endurance athletes,” he said, adding that the heart rate-associated polygenic risk score differed between athletes and nonathletes.

The finding, La Gerche noted, suggests genetics may be a determinant of athleticism. “This turns previous theories on their head where the assumption was that athletes merely train harder.”

Low Polygenic Risk Score Linked to Low Heart Rates

Bradycardia refers to a resting heart rate < 60 beats/min, noted La Gerche. “Our study team, however, was interested in investigating very low heart rates that may be considered a red flag,” he said. “Since most clinicians regard a heart rate lower than 40 bpm [beats/min] as a ‘grey zone’ of uncertain significance and uncertain long-term outcome, our study defined bradycardia as a resting heart rate lower than 40 bpm.”

In a cohort of 465 endurance athletes, the researchers found that athletes with bradycardia were younger and fitter and, compared with those without bradycardia, showed greater athletic cardiac remodeling. Mean heart rate-associated polygenic risk score was notably lower in all athletes and athletes with bradycardia compared with nonathletes and athletes without bradycardia.

“[Athletes with bradycardia] also showed more profound heart changes that we find in well-trained athletes, such as larger hearts with good function,” La Gerche said.

Genetic vs Pathological Bradycardia

Fengwei Zou, MD, who was not involved with the study, noted that this genetic predisposition to bradycardia independently predicted lower heart rates even after accounting for age, sex, fitness level, and cardiac remodeling.

“Genetic bradycardia differs from pathological bradycardia in that it is typically lifelong, asymptomatic, and compatible with superior exercise capacity,” said Zou, a fellow in clinical cardiac electrophysiology at Montefiore Einstein, Bronx, New York. “Rather than reflecting disease of the heart’s electrical system, it represents an inherited tendency toward slower electrical activity that may actually favor endurance performance.”

However, he added, nongenetic bradycardia in the general population is more often acquired, associated with symptoms, and linked to progressive disease in the heart’s electrical system. “The study supports the concept that some athletes are not only trained to have low heart rates but are also genetically predisposed to them,” Zou noted.

In nonathletic populations, though, bradycardia can have adverse effects, which can lead to reduced blood flow to vital organs and may cause symptoms such as dizziness, fainting, weakness, or lethargy. “In contrast, this study demonstrates that bradycardia in endurance athletes is usually a benign physiological adaptation rather than a pathological condition,” he said. “Even very low heart rates, pauses of 2-3 seconds, and atrioventricular block were common in this cohort and were not associated with increased rates of syncope, dangerous arrhythmias, or adverse outcomes over a 5-year follow-up.”

This research raises the possibility that being born with a lower heart rate is in itself associated with greater fitness and the likelihood of being an athlete, according to La Gerche. “This unexpected finding is certain to provoke further research and inquiry,” he said.

La Gerche and Zou reported no relevant financial disclosures.

Martta Kelly is a medical journalist who lives in the New York metropolitan area.