ECG criteria
- For the classic definition: QRS <120 ms. When QRS ≥120 ms, with bundle branch block, or pacing, separate modified criteria are used rather than outright excluding fragmentation.
- In leads with an R-wave type complex: an additional R' wave (splitting, more than one R peak) — for example, RSR', RSR'S', or ≥2 notches on the ascending or descending limb of the R wave
- In leads with rS, QS, or QR type complexes: a notch on the ascending or descending limb of the S wave, or ≥2 notches in the nadir of the S wave
- The change must be present in two or more contiguous leads corresponding to the same vascular/anatomic territory
- The notches are not related to an artifact or a normal variant (e.g., J-point elevation in early repolarization)
- The morphology is stable across consecutive complexes — this increases reliability and helps rule out artifacts, but is not an independent mandatory criterion for classic fQRS
What it looks like on ECG
fQRS is described as any variant of an additional wave or notch within the QRS complex. For a narrow QRS (<120 ms), several classic morphological variants are distinguished: additional R′ waves (RSR′, RSR′S′), ≥2 notches on the ascending or descending limb of R or S, and notching in the nadir of S in two contiguous leads of the same territory. Importantly, with a narrow QRS, this does not represent complete bundle branch block.
When QRS ≥120 ms, in complete left or right bundle branch block, or ventricular pacing, validated universal criteria for all variants of wide QRS do not exist, and routine interpretation is limited. Notches are less specific as they can be part of the bundle branch block itself.
A finding is considered significant if it is repeated in two or more contiguous leads of the same territory. One of the possible topographical schemes:
| Territory | Leads |
|---|---|
| Inferior | II, III, aVF |
| Septal | V1–V2 |
| Anterior | V3–V4 |
| Lateral | I, aVL, V5, V6 |
In studies, fQRS often utilizes anterior V1–V5 and lateral I, aVL, V6 leads. The presence of fragmentation in two contiguous leads remains decisive. A single notch in one lead has low diagnostic value — evaluation in the context of neighboring leads and the clinical picture is required.
Mechanism
The underlying mechanism of fQRS is non-uniform (inhomogeneous) propagation of the depolarization wave through the ventricles. Areas of fibrosis, post-myocardial infarction scar, or diffuse myocardial replacement by connective tissue create zones of delayed and asynchronous conduction. The activation wavefront bypasses these areas at varying speeds, resulting in a total depolarization vector that does not form a single smooth wave, but breaks down into several short-duration peaks — which is recorded as notching. fQRS may reflect activation heterogeneity in scar or fibrosis; the number and localization of fragmented leads do not allow for reliable quantitative estimation of scar volume.
Causes
- Previous myocardial infarction (scar) — the most studied cause; fQRS correlates with scar localization on contrast-enhanced MRI
- Coronary artery disease without an overt ECG scar (occult microfocal fibrosis)
- Hypertrophic cardiomyopathy
- Dilated cardiomyopathy
- Arrhythmogenic right ventricular cardiomyopathy
- Cardiac sarcoidosis
- Chagas disease
- Normal variant in some healthy individuals without structural heart disease (requires cautious interpretation)
Differential diagnosis
Incomplete right bundle branch block — rsr′/rsR′ in V1 or V2, QRS 110–119 ms, and a wide terminal S in I, aVL, V5–V6. RSR′ with a narrower QRS (<110 ms) may be a normal variant, but this is not synonymous with incomplete RBBB and is not a standard definition of fQRS.
Epsilon wave — a low-amplitude signal after the QRS in V1–V3, located outside the main complex rather than within it; classically described in ARVC, this sign has low sensitivity and is not an independent diagnostic criterion.
Early repolarization syndrome — J-point or initial ST-segment elevation ≥1 mV in ≥2 contiguous inferior and/or lateral leads, with notching or slurring of the terminal part of QRS near the J-point, rather than fragmentation within the main body of the complex; when clinically suspected, structural pathology is evaluated separately.
Crochetage sign — a notch on the peak of the R wave in inferior leads in atrial septal defect, a more specific and localized phenomenon.
Recording artifact — baseline wander, 50 Hz interference produce irregular, beat-to-beat non-reproducible small spikes, unlike the stable morphology of fQRS.
Clinical significance and management
fQRS is considered a non-specific marker associated with myocardial fibrosis and, in certain populations, with an increased risk of ventricular tachyarrhythmias and sudden cardiac death, especially in patients with prior myocardial infarction, cardiomyopathies, and reduced left ventricular ejection fraction. fQRS may be an independent statistical risk marker in individual studies, but it is not an independent validated criterion for ICD implantation and does not replace recommended risk stratification. In an isolated finding in an asymptomatic individual without structural heart disease, specific actions are not required.
When fQRS is detected, the physician should evaluate the finding in the clinical context: history of coronary artery disease, symptoms, echocardiographic data (ejection fraction, hypokinesis zones). If structural pathology is suspected, echocardiography and, if necessary, cardiac MRI with contrast to assess fibrosis are indicated. In patients with established structural heart disease and reduced ejection fraction, fQRS may be described as an additional exploratory or supportive marker, but ICD decisions are made according to validated criteria based on disease type, LVEF, symptoms, documented arrhythmias, MRI data, and genetic factors, if applicable.
Common interpretation errors
- Diagnosing fQRS based on a single lead without confirmation in a contiguous lead of the same territory
- Confusing fQRS with incomplete right bundle branch block or a normal rsr′ variant in V1–V2 in a healthy individual
- Disregarding QRS duration — classic criteria are applied for narrow QRS (<120 ms), whereas no validated universal criteria exist for wide QRS (≥120 ms) or bundle branch block
- Mistaking an artifact or interference for true notching without assessing beat-to-beat reproducibility
- Drawing conclusions on prognosis and management from a single fQRS finding without considering the clinical picture, disease, and cardiac structure
- Overestimating the number of fragmented leads as a direct quantitative measure of scar volume
1. Which of the following features is mandatory for diagnosing classic fQRS?
The classic diagnostic criterion for fQRS requires reproducible notching in two or more contiguous leads of the same territory with QRS <120 ms. Modified criteria are used for wide QRS.
2. What mechanism underlies the appearance of QRS notching?
Areas of fibrosis or scar create zones of uneven conduction, causing the depolarization vector to split into multiple peaks instead of a single smooth wave.
3. Which ECG condition is differentiated from fQRS by the localization of the additional signal outside the main QRS complex?
An epsilon wave is recorded as a low-amplitude signal after the end of the QRS, outside the main complex, rather than as a notch within the complex itself, unlike fQRS.
Frequently asked questions
How does fQRS differ from incomplete right bundle branch block?
Incomplete RBBB is defined as rsr′/rsR′ in V1 or V2 with a QRS of 110–119 ms and a wide terminal S in I, aVL, V5–V6. RSR′ with a narrower QRS (<110 ms) may be a normal variant. fQRS is a broader concept: additional notches can appear in any leads of any territory and are more frequently associated with myocardial scar or fibrosis.
Is QRS notching dangerous?
The finding itself is not immediately life-threatening, but in patients with coronary artery disease or cardiomyopathy, it serves as a non-specific marker associated with an increased risk of ventricular arrhythmias. In healthy individuals without structural heart disease, fQRS may have no clinical significance. The risk is assessed comprehensively, taking a combination of factors into account.
In which leads is fQRS most commonly sought?
The anatomical regions are evaluated: inferior (II, III, aVF), septal (V1–V2), anterior (V3–V4), and lateral (I, aVL, V5, V6). In studies, fQRS is often assessed in the anterior (V1–V5) and lateral (I, aVL, V6) leads. A finding is considered significant when present in two or more contiguous leads of the same anatomical region.
Is additional evaluation necessary when fQRS is found?
For an isolated finding in an asymptomatic patient without risk factors, no special workup is required. If there is a history of coronary artery disease, symptoms, or suspected cardiomyopathy, echocardiography and, when indicated, contrast-enhanced cardiac MRI are warranted.
Can fQRS be observed in the presence of a wide QRS?
Classical fQRS criteria were developed for QRS <120 ms. For QRS ≥120 ms, complete bundle branch block, or ventricular pacing, there are no validated universal criteria for all variants of wide QRS, and routine interpretation is limited. Notching may be part of the block itself and is less specific.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Das M.K., Khan B., Jacob S. et al. Significance of a fragmented QRS complex versus a Q wave in patients with coronary artery disease. Circulation, 2006
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice, 6th ed.
- Orlov V.N. Guide to Electrocardiography
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
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.