Introduction 

In this article, we look at a condition called bradycardia (braid-ee-car-dia), which is defined as when an individual displays an abnormally low heart rate.^1,2 For most people, this signifies that the body is struggling to function or is under physiological stress from illness or lack of resources, and is therefore a cause for concern. In athletes, however, a low resting heart rate is not always a signifier of poor health; actually, quite the opposite—this article discusses why. 

What is Bradycardia? 

Bradycardia is a medical condition characterized by a slower-than-normal heart rate. In adults, a typical resting heart rate ranges between 60 and 100 beats per minute (bpm). Bradycardia is generally defined as a resting heart rate of fewer than 60 bpm. 

Symptoms of bradycardia can include fatigue, dizziness, weakness, shortness of breath, or even fainting. In some cases, particularly when bradycardia is associated with other heart conditions, it may require medical intervention.^1,2

Physiological bradycardia, which is often seen in athletes, results from the heart's adaptation to regular exercise, leading to increased efficiency and a higher stroke volume. On the other hand, pathological bradycardia can be caused by various factors, including heart disease, certain medications, or imbalances in electrolytes.

Why do athletes have bradycardia? 

Athletes can develop bradycardia due to the body's adaptation to regular, intense exercise, often termed physiological bradycardia. Some mechanisms that have been proposed to explain this include: 

Increased Stroke Volume of the Heart

• Through consistent training, especially endurance exercises like running, swimming, or cycling, an athlete's heart becomes more efficient. The heart muscle strengthens and can pump more blood with each beat, known as increased stroke volume. Because each heartbeat delivers more oxygenated blood, the heart doesn't need to beat as frequently to meet the body's demands, leading to a lower resting heart rate.³

Enhanced Parasympathetic Tone

• The autonomic nervous system, which controls involuntary bodily functions (things you don’t have to think about, like digestion, for example), plays a key role in regulating heart rate. Athletes typically experience an increase in parasympathetic nervous system activity (often referred to as the "rest and digest" system) and a decrease in sympathetic nervous system activity (the "fight or flight" system). This shift promotes relaxation and slows the heart rate, contributing to bradycardia.^4,5

Improved Cardiac Efficiency

• Regular physical training improves the heart's overall efficiency. Not only does the heart pump more blood per beat, but the circulatory system also becomes more efficient at delivering oxygen to the muscles and removing waste products like carbon dioxide. This efficiency allows the heart to maintain a lower rate while still supplying the body with sufficient blood flow during rest.^4,5

Increased Vagal Tone

• Vagal tone refers to the activity of the vagus nerve, which helps regulate the heart's activity. Athletes tend to have increased vagal tone, which further reduces the resting heart rate. This adaptation helps the body recover more effectively after exercise and contributes to the athlete's overall cardiovascular health.⁶

Reduction in Heart Rate Variability

• Although athletes have a lower resting heart rate, their heart rate variability (the variation in time between each heartbeat) is often higher. This is a sign of a healthy heart that can quickly adjust its rate to meet changing demands, whether that means slowing down at rest or speeding up during exercise.^4,5

Training-Induced Changes

• Long-term, intense training leads to structural changes in the heart, such as increased left ventricular size, known as "athlete's heart." These changes are part of the heart's adaptation to handle the increased demands of endurance training, and they contribute to the slower resting heart rate seen in athletes.

In summary, bradycardia in athletes is generally a sign of a well-conditioned heart that has adapted to the demands of regular, intense physical activity. It is usually considered a healthy and normal adaptation.

When does bradycardia become a concern for athletes? 

While bradycardia is often a normal and healthy adaptation in athletes, there are certain situations where it may be cause for concern. Athletes should be aware of the following warning signs and conditions:

Presence of Symptoms

• Dizziness or Lightheadedness: If bradycardia is accompanied by frequent dizziness or lightheadedness, especially during normal activities or exercise, it could indicate that the heart is not pumping enough blood to meet the body's needs
• Fatigue: Unexplained fatigue, especially if it interferes with training or daily activities, may suggest that the slow heart rate is not allowing adequate blood flow and oxygen delivery
• Fainting (Syncope): Fainting or near-fainting episodes can be a serious sign that the brain is not receiving enough blood due to a slow heart rate
• Shortness of Breath: Difficulty breathing or shortness of breath, particularly during activities that previously did not cause such symptoms, could indicate that bradycardia is affecting heart function

Abnormal ECG Findings

• An electrocardiogram (ECG) reading can reveal the nuances in the heart’s activity. Routine screenings or evaluations may reveal abnormalities on an electrocardiogram (ECG) beyond just a slow heart rate, such as irregular heart rhythms (arrhythmias) or heart block. These conditions may require further investigation and treatment. They are not usually the sign of an adaptation and are more of a concern. 

Inadequate Heart Rate Increase During Exercise

• Normally, the heart rate should increase appropriately during exercise to meet the body’s increased demand for oxygen. If an athlete’s heart rate fails to rise adequately during exertion, this could be a sign that the bradycardia is limiting their physical performance and may need medical evaluation.

Symptoms During Recovery

• If an athlete experiences symptoms like chest pain, severe fatigue, or shortness of breath during the recovery phase after exercise, it could indicate that the bradycardia is affecting their heart's ability to return to a normal rate post-exercise.

Family History of Heart Conditions

• Athletes with a family history of heart disease, sudden cardiac death, or other cardiovascular conditions should be more vigilant. Bradycardia in the context of a family history of heart issues may warrant closer monitoring and more thorough evaluations.

Underlying Medical Conditions

• If an athlete has other medical conditions, such as congenital heart disease, electrolyte imbalances, or thyroid problems, bradycardia could be a sign of a more serious underlying issue that needs attention. The athlete, their coaching team, and their medical team should all be well aware of any existing health conditions and how this may impact the individual’s heart functioning. 

Use of Certain Medications

• Some medications, like beta-blockers or certain heart medications, can contribute to bradycardia. If an athlete is on these medications and develops symptoms or an unusually low heart rate, they should consult with a healthcare provider.

Inconsistent Heart Rate Recovery

• An abnormally slow recovery of the heart rate after exercise (heart rate not returning to normal or near-normal levels within a reasonable time) can be a warning sign that bradycardia is more than just a benign adaptation.

When should an athlete seek medical advice about bradycardia?

Athletes experiencing any of the above symptoms or concerns should seek medical advice. A healthcare provider can perform a thorough evaluation, which might include a detailed history, physical examination, ECG, stress test, and possibly a Holter monitor to assess heart function over 24 hours or more. 

It can be difficult as an athlete to determine what symptoms are problematic and indicative of a more serious problem, or whether an individual is just pushing themselves very hard physically and therefore is facing physical stress. It is always worth being safe and getting checked by medical professionals if you have any suspicions or concerns. Regular monitoring can also be effective at determining trends or sudden changes in resting, active, and recovery heart rates.  

Summary

In summary, while bradycardia is often a normal and healthy adaptation in athletes, it becomes a concern when associated with dire physical symptoms, abnormal ECG findings, or inadequate heart response during exercise. Athletes should monitor their heart health closely and consult a healthcare professional if they experience any worrying signs.