The Athlete's ECG: What Is Physiological Adaptation and What Is a Warning Sign

The athlete's ECG remains one of the most challenging areas of clinical electrocardiography. The same tool that can identify potentially fatal diseases is also responsible for a high false-positive rate when interpreted without context. The key is not just recognizing patterns, but understanding what is physiological remodeling and what represents structural or electrical disease.

In this article, we organize the clinical reasoning in a practical, progressive way.

Why does the athlete's ECG still cause so much uncertainty?

The historical problem with the athlete's ECG has always been the balance between sensitivity and specificity.

• If interpreted as in non-athletes → excessive additional testing.
• If overly permissive → risk of underdiagnosis.

Most athletes at risk of sudden death are asymptomatic, and the event is often the first manifestation of the disease. History and physical examination alone have low sensitivity. Adding the ECG significantly increases diagnostic yield.

The challenge, therefore, is not deciding whether to obtain an ECG, but knowing how to interpret it correctly.

The role of the ECG in preparticipation screening

The Italian model showed a significant reduction in sudden death after systematic implementation of the ECG in preparticipation screening. This experience was essential for establishing ECG screening.

The 2017 International Criteria organized findings into three categories:

• Normal
• Borderline
• Abnormal

This division substantially improved specificity without a major loss of sensitivity. Today, they are the international reference standard.

Structural remodeling: expected physiological changes

Intense physical training induces increased ventricular mass, chamber dilation, and conduction changes.

On the ECG, this mainly translates into:

Increased ventricular voltages

Sokolow-Lyon criteria for LVH are frequently positive in athletes and, when isolated, rarely indicate hypertrophic cardiomyopathy.

Likewise, criteria for right ventricular hypertrophy may be found, especially in Black athletes, with no pathological significance when isolated.

The key point: isolated voltage is not disease.

Incomplete right bundle branch block

Incomplete RBBB is extremely common and reflects right ventricular remodeling. A QRS < 120 ms with an rsR’ pattern in V1 is considered a physiological variant.

Autonomic and electrophysiological remodeling

For years, athletic bradycardia was attributed solely to increased vagal tone. More recent evidence suggests intrinsic electrophysiological remodeling of the sinus and AV nodes, mediated by changes in ion channel expression (reduced If and ICa,L currents).

Sinus bradycardia

Rates < 60 bpm are common. Values close to 30 bpm may be seen in endurance athletes, provided that:

• The athlete is asymptomatic
• It reverses with exercise
• It is proportional to the training level

First-degree AV block and Mobitz I

They are frequent at rest and during sleep. They should normalize with exercise. Persistence or a PR > 400 ms requires investigation.

Junctional and ectopic atrial rhythms

They may appear in the setting of more pronounced bradycardia (for example, a resting heart rate < 40 bpm). In general, they are physiological when transient, asymptomatic, and disappear as the heart rate increases during exercise.

Early repolarization

J-point elevation ≥ 1 mm is common, especially in men and Black athletes. The typical benign pattern shows upsloping ST segments in the anterior/lateral leads.

Inferior early repolarization with a horizontal/downsloping ST segment calls for greater caution.

Borderline changes: the gray zone of the athlete's ECG

The International Criteria classify the following as borderline:

• Axis deviation
• Atrial enlargement
• Complete right bundle branch block
• Low QRS voltage in the limb leads (not formally included in 2017, but increasingly discussed)

In isolation, they do not require investigation.

Two or more findings → investigate.

Low QRS voltage

This one is particularly interesting. In a physiologically hypertrophied heart, increased voltage is expected. Low voltage in the limb leads is uncommon and may be a marker of:

• Arrhythmogenic cardiomyopathy
• Nonischemic myocardial scar
• Infiltrative diseases

Recent studies associate isolated low voltage with a higher prevalence of ventricular arrhythmias and fibrosis on CMR.

It is not a finding to be dismissed automatically.

Abnormal changes: the true red flags

These findings should never be attributed to training. Unlike physiological adaptations, they have no plausible explanation in exercise-induced cardiac remodeling and, in many cases, are the only manifestation of a structural heart disease or channelopathy potentially associated with sudden death.

Here, the reasoning must shift: we are no longer deciding whether it is a physiological adaptation, but which disease we need to rule out.

T-wave inversion outside benign contexts

T-wave inversion is probably the most sensitive ECG marker of cardiomyopathy in athletes. When located in the lateral (I, aVL, V5–V6) or inferolateral leads, the probability of underlying myocardial disease increases significantly.

Even with a normal echocardiogram, cardiac magnetic resonance imaging is often needed to rule out:

• Early-stage hypertrophic cardiomyopathy
• Arrhythmogenic cardiomyopathy
• Focal myocardial fibrosis

In addition, T-wave inversion may precede detectable structural changes, which justifies clinical follow-up even when the initial workup is negative.

Pathologic Q waves

They should be defined not only by depth but also by duration (≥ 40 ms) and by their ratio to the subsequent R wave (≥ 25%). In athletes, deep Q waves may occur due to increased voltage, but wide, proportionally large Q waves suggest myocardial scar or cardiomyopathy.

Anatomical context matters: Q waves in the inferior or lateral wall deserve closer attention.

ST depression

ST-segment depression is not a physiological training variant. When present, consider:

• Hypertrophic cardiomyopathy
• Myocardial ischemia (especially in older athletes)
• Infiltrative myocardial disease

It is a low-prevalence finding in healthy athletes and therefore has high predictive value when identified.

Prolonged QTc

QT prolongation should always raise the possibility of congenital or acquired long QT syndrome. In athletes with marked bradycardia, Bazett's correction may overestimate the true value. In these cases, it is prudent to:

• Repeat the ECG at a higher heart rate
• Carefully review the measurement technique
• Investigate a family history of syncope or sudden death

The diagnosis should not rest on a single number, but on the overall clinical and electrocardiographic picture.

Premature ventricular contractions

The mere presence of two PVCs on a resting ECG does not define risk. What really guides the workup is the phenotype of the arrhythmia.

Features that raise suspicion of a structural substrate:

• Morphology not typical of an outflow tract or fascicular origin
• Superior axis with a right bundle branch block pattern
• Complexity (couplets, runs, nonsustained ventricular tachycardia)
• Increase with exercise
• Reproducibility on serial tests

In these cases, the workup should include exercise testing, ambulatory monitoring, and often cardiac magnetic resonance imaging.

In summary, red flags on the athlete's ECG are not just “abnormal” items on a list. They are potential electrical markers of hidden structural disease. Recognizing them requires less interpretive tolerance and more investigative rigor.

Special populations: where the criteria fall short

The criteria were validated mainly in young men (12–35 years).

Children and adolescents

Not all adult adaptations are present. Anterior T-wave inversion should be interpreted with caution after puberty.

Women

• Longer QTc
• Lower voltages
• Lower prevalence of marked bradycardia (very low resting rates, typically < 40 bpm)
• T-wave inversion up to V2 may be physiological, especially in Black women

Master athletes

The leading cause of sudden death is coronary artery disease, not cardiomyopathy. The ECG has low sensitivity for coronary artery disease. Maximal exercise testing and risk stratification are essential.

Conclusion

The athlete's ECG is a powerful tool when interpreted with technical rigor and appropriate clinical context. The International Criteria are the best standard currently available, but they do not eliminate the need for clinical judgment.

The ultimate goal is not to exclude athletes from sport, but to accurately identify those who are truly at risk.