ECG criteria
- Sinus bradycardia — asymptomatic HR 30–60 bpm is typical for trained athletes; HR <30 bpm, pauses, symptoms, or inadequate heart rate response to exercise (chronotropic incompetence) require evaluation regardless of training level. Sinus rhythm (P wave preceding every QRS, positive in II)
- Sinus arrhythmia — cyclic variability of PP intervals, usually respiration-related, with preserved sinus P morphology. Wanderer pacemaker rhythm — at least 3 different P morphologies in a single lead with a shift in the atrial/low atrial source.
- First-degree AV block — PQ(PR) interval > 200 ms; isolated moderate prolongation is usually vagal and shortens with exercise. Marked PR prolongation (e.g., around 400 ms or more), persistence of the block during exercise, or symptoms require evaluation of the conduction system.
- Second-degree AV block Mobitz I (Wenckebach) — periodic QRS dropouts with progressive PQ prolongation, typical at night, disappearing during exercise/atropine testing.
- Isolated high QRS voltage: Sokolov–Lyon index (SV1 + RV5 or RV6) ≥ 35 mm; Cornell criterion (RaVL + SV3) > 28 mm in men and > 20 mm in women — without other signs of LVH (no ST depression, T wave inversion, left atrial enlargement, left axis deviation). An isolated voltage criterion in an athlete does not diagnose pathologic LVH and is assessed alongside ST-T, QRS, atrial, and clinical data.
- Early repolarization — J-point elevation typically ≥ 1 mm in two or more contiguous leads (most commonly anterolateral V2–V5), with upwardly concave ST elevation and a notch/slur at the J point; inferolateral pattern with horizontal or downsloping ST is a separate, less favorable finding requiring attention in the presence of clinical risk factors.
- Incomplete right bundle branch block — rSr' or rSR' in V1 with QRS < 120 ms.
- Isolated inverted T wave in V1, III, or aVR — a benign variant in adults; anterior inversion up to V3 is acceptable in athletes under 16; V1–V4 inversion with J-point elevation is acceptable in Black athletes. T-wave inversion beyond these recognized patterns (including V1–V3 in white adults, lateral and inferior leads) requires further evaluation.
What it looks like on ECG
Baseline finding — sinus bradycardia: resting heart rate in endurance athletes (running, cross-country skiing, cycling, swimming) is frequently 40–60 bpm, and below 40 bpm in some. Sinus rhythm: P wave positive in II, preceding each QRS.
Frequently concomitant: sinus arrhythmia (PP variability with preserved sinus P morphology), wandering atrial pacemaker (shift in P morphology), first-degree AV block (PQ(PR) slightly above 200 ms), and second-degree AV block Mobitz I — vagal findings that typically disappear with exercise testing or orthostasis (HR increases, PQ normalizes), although normalization is not mandatory in every case.
In chest leads — high QRS voltage (amplitude criteria for LVH are frequently formally positive), but without "secondary" overload signs: no downsloping ST depression, no T-wave inversion in lateral leads, no left atrial enlargement, and no left axis deviation.
In V1–V2, an isolated rSr'/rsR' with QRS < 120 ms is common — a frequent variant of the athletic ECG; a diagnosis of right ventricular enlargement is not made based on this single finding and is evaluated only in conjunction with other data (clinical picture, echocardiography).
In anterolateral chest leads (V2–V5), early repolarization is frequently visible: J-point elevation (typically ≥ 1 mm in two contiguous leads), upwardly concave ST elevation following a notch or J wave, usually with an upwardly sloping ST continuation.
| Finding | Leads | Interpretation in the athlete |
|---|---|---|
| HR 30–60 bpm, asymptomatic | II | normal (vagotonia); <30 bpm, pauses or symptoms — evaluate further |
| PQ(PR) > 200 ms | II | usually normal if mild and resolves with exercise |
| High R wave | V5–V6 | isolated — normal |
| J elevation and concave ST | V2–V5 | early repolarization, normal |
| rSr' with QRS < 120 ms | V1 | incomplete RBBB, normal |
Mechanism
Underpinned by two processes. First: increased vagal tone and decreased sympathetic influence at rest in the trained athlete slow sinus node automaticity and AV nodal conduction (the bundle of His is rarely affected) — hence bradycardia, first-degree AV block, and Mobitz I, which attenuate with sympathetic activation (exercise, atropine).
Second: physiological myocardial remodeling (increase in ventricular mass and cavities in response to volume and/or resistance training) increases the electrical vector of depolarization — hence high QRS voltage. Right ventricular volume overload in endurance athletes may contribute to delayed terminal ventricular depolarization — hence incomplete right branch block, although a mandatory link to RV enlargement does not exist. The early repolarization pattern relates to myocardial repolarization features and is more frequent in young males and athletes; the exact mechanism is multifactorial and not reducible to high vagal tone alone.
Causes (what triggers this ECG)
- Regular intensive endurance training (long-distance running, cross-country skiing, cycling, triathlon, swimming) — most pronounced changes
- Strength sports — changes are typically less marked (hypertrophy without significant bradycardia)
- Duration and training history — the longer, the more noticeable the adaptation
- Young age, male sex — more pronounced findings
- High baseline vagal tone
Differential diagnosis
- Sick sinus syndrome / bradycardia-tachycardia syndrome — in an athlete, bradycardia disappears with exercise and there are no symptomatic pauses (syncope); in SSS, HR does not increase adequately to exertion
- Hypertrophic cardiomyopathy — HCM typically presents with a pathological Q wave, marked T-wave inversion in anterior/lateral leads, axis deviation, left atrial enlargement; isolated high voltage without these features is typical for an athlete's heart
- Arrhythmogenic right ventricular cardiomyopathy — T-wave inversion in V1–V3 in an adult athlete beyond recognized variants (isolated V1, juvenile pattern, pattern in Black athletes with J-point elevation) and an epsilon wave are reasons to suspect ARVC rather than considering them normal
- Brugada syndrome — Type 1 Brugada pattern (coved ST elevation ≥ 2 mm in V1–V2) does not belong to benign athletic changes and requires cardiological evaluation
- Long or short QT syndrome — requires separate assessment considering sex and correction method; not included in the list of benign athletic findings
- True second-degree Mobitz II AV block and complete AV block — do not disappear with exercise, unlike the vagal Mobitz I seen in athletes
Clinical significance and management
Seattle criteria (international consensus on athletic ECG interpretation) divide findings into three groups: normal (vagal and voltage changes described above — do not require further evaluation in the absence of symptoms), borderline, and abnormal.
Borderline findings — isolated axis deviation, isolated atrial enlargement, isolated complete right bundle branch block, and certain other features individually do not require evaluation; further workup is indicated when two or more borderline features coexist.
Red flags include: T-wave inversion beyond recognized normal variants (isolated V1, III, aVR in adults, adolescent and ethnic patterns — see above), especially in two or more contiguous leads; pathological Q wave — duration ≥ 40 ms or with a Q/R ratio ≥ 0.25 in two or more adjacent leads (with caveats for III and aVR, where a deep Q wave can be present in healthy individuals); left axis deviation less than −30° or right axis deviation greater than +120°; complete left bundle branch block (significantly more suspicious than complete RBBB); frequent, polymorphic, or complex ventricular ectopy, runs of nonsustained ventricular tachycardia (NSVT), or suspicious ventricular complex morphology — a single rare ventricular premature beat by itself is not necessarily pathological; Brugada type 1 pattern; QTc ≥ 470 ms in men and ≥ 480 ms in women (suspicious), QTc ≥ 500 ms (particularly dangerous), QTc < 320 ms (significantly shortened); pre-excitation (delta wave).
QTc must be interpreted taking into account the correction method: Bazett's formula overestimates QTc at tachycardia and underestimates it at bradycardia; in athletes with a heart rate of 40–60 bpm or lower, this can lead to a false-negative result and a missed long QT syndrome. At heart rates near the limits of normal (marked bradycardia or tachycardia), Fridericia's or Framingham's formulas are preferred, or manual checking of the QT interval using the «QT–HR» curve.
The functional diagnostics physician's task is not to miss red flags, as they may underlie sudden cardiac death in young athletes (cardiomyopathies, channelopathies, pre-excitation). For isolated vagal and voltage changes, knowing the medical history (regular training) is sufficient; an exercise test or orthostatic challenge can be used to confirm the vagal nature of bradycardia and first-degree / Mobitz I AV block (disappearance or reduction with sympathetic activation) and to assess chronotropic function. An exercise test does not replace a targeted workup for red flags: T-wave inversion beyond normal limits, pathological Q wave, prolonged QT, pre-excitation, or suspected cardiomyopathy warrant echocardiography, Holter monitoring, exercise stress testing, cardiac MRI, and genetic evaluation as indicated.
Common interpretation errors
- Interpret any bradycardia in a young patient as an «athlete's heart» without clarifying the training history and without assessing symptoms, pauses, and response to exercise
- Attribute high QRS voltage to an athlete's heart without checking for other signs of LVH (ST-T changes, atrial enlargement, axis deviation)
- Miss T-wave inversion beyond recognized normal variants in an adult athlete, considering it a normal variant without regard to age and ethnicity
- Fail to distinguish vagal first-degree / Mobitz I AV block from organic AV block and automatically consider any athlete's bradiarrhythmia physiological
- Consider QTc normal according to Bazett's formula in marked bradycardia, failing to account for the fact that this formula may underestimate the true QTc
- Ignore numerical QTc thresholds and the Brugada pattern against the background of an impressive «athletic» bradycardia picture
1. Which ECG finding in an athlete is considered benign (a normal variant) according to the Seattle Criteria?
Vagal findings (moderate bradycardia, first-degree AV block) that diminish with sympathetic activation are classified as normal athlete's heart changes.
2. Which feature primarily indicates the vagal (functional) nature of Mobitz I AV block in an athlete rather than organic conduction system disease?
Vagal Mobitz I AV nodal block typically occurs at night, is accompanied by a narrow QRS, and decreases with sympathetic stimulation; persistence or worsening of the block during exercise, a wide QRS, and symptoms necessitate evaluation for organic pathology.
3. Why can the QTc calculated using Bazett's formula be misleading in an athlete with marked sinus bradycardia?
At low heart rates, Bazett's formula yields an underestimated QTc value; therefore, in marked bradycardia, Fridericia's or Framingham's formulas are preferred, otherwise long QT syndrome may be missed.
Frequently asked questions
Is bradycardia dangerous in an athlete?
Asymptomatic sinus bradycardia of 30–60 bpm in a trained athlete is usually a physiological adaptation. A heart rate below 30 bpm, pauses, symptoms (syncope, weakness), or inadequate heart rate response to exercise warrant evaluation even in a highly elite athlete.
How to distinguish an athlete's ECG from hypertrophic cardiomyopathy?
An athlete has isolated high QRS voltage: no pathological Q wave, no deep T-wave inversion in anterior/lateral leads, no left atrial enlargement, and no axis deviation. When in doubt — echocardiography or cardiac MRI to assess wall thickness.
What are the Seattle Criteria?
This is an international consensus algorithm for ECG interpretation in athletes, categorizing findings into normal (requiring no workup), borderline (requiring attention if two or more coexist), and abnormal/red flags (requiring further workup to rule out cardiomyopathies, channelopathies, and arrhythmogenic conditions).
Does an athlete with an abnormal ECG need to stop training?
No, if the findings pertain to benign vagal and voltage changes. In the presence of red flags (T-wave inversion beyond normal limits, pathological Q wave, Brugada pattern, etc.), clearance for sports participation is decided after a cardiology evaluation.
Do athlete's heart changes resolve after stopping training?
Vagal findings (bradycardia, first-degree AV block, Mobitz I) usually diminish after several weeks of detraining. True ventricular remodeling (increased mass and cavity dimensions) regresses more slowly and not always completely.
ECG Course
Interpretation from basics to myocardial infarction — learn to read any ECG using a systematic algorithm.
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ECG Strip Simulator
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Related terms
More in the section "Children, Athletes, Pregnant Women"
Sources
- International Recommendations for Electrocardiographic Interpretation in Athletes (Seattle Criteria, 2017)
- Hampton J.R. The ECG in Practice
- Surawicz B., Knilans T.K. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.