Clinical Overview

Fragmented QRS (fQRS) is a QRS-morphology finding, not a separate rhythm — it describes extra spikes, notches, or additional R waves inside the QRS complex, beyond the single smooth deflection a normal QRS produces. For a narrow QRS (duration under 120 ms), the defining criteria are various RSR′ patterns, notching in the R wave or the S wave, or the presence of more than one R′ wave (fragmentation), present in at least two contiguous leads corresponding to a major coronary artery territory, without a typical bundle branch block pattern (Das et al., Circulation, 2006; Blondeel et al., Reviews in Cardiovascular Medicine, 2025). A separate criterion applies when the QRS is wide (120 ms or more, as with bundle branch block or a paced rhythm): more than two notches in the R wave or the S wave in at least two contiguous leads (Blondeel et al., 2025). This page focuses on the narrow-QRS finding, since fQRS is specifically defined as occurring in the absence of typical bundle branch block morphology — the two are distinguished in the differential below rather than treated as related findings.

Mechanistically, fQRS reflects inhomogeneous, non-uniform ventricular depolarization: the electrical wavefront takes a fragmented, discontinuous path through myocardium interrupted by scar, fibrosis, or otherwise non-conductive tissue, rather than propagating smoothly through healthy muscle (Das et al., 2006; Jain et al., Current Cardiology Reviews, 2014). Because the wavefront has to detour around dead or fibrotic segments instead of moving through them directly, the depolarization vector recorded by the overlying lead breaks into multiple small deflections instead of one smooth complex [CLINICAL REVIEW NEEDED: needs an in-window (2021+) source supporting this specific wavefront-detour/vector-shift mechanism] (Supreeth & Francis, Indian Pacing and Electrophysiology Journal, 2020).

fQRS is a risk marker, not a diagnosis in itself, and its significance depends heavily on the clinical context it appears in. In patients with known or suspected coronary artery disease, narrow-QRS fQRS detects a prior myocardial infarction with markedly higher sensitivity than a classic pathological Q wave — 85.6% versus 36.3% against imaging-confirmed scar in one series, rising to 91.4% sensitivity when the two markers are combined [CLINICAL REVIEW NEEDED: needs an in-window (2021+) source corroborating this specific sensitivity comparison] (Das et al., 2006). In acute myocardial infarction, its presence is associated with substantially higher in-hospital risk of life-threatening ventricular arrhythmia (pooled odds ratio 2.96 for the combined VT/VF endpoint, 3.59 for VT alone; VF alone was not independently significant, OR 1.84, rising to OR 2.45 after excluding a high-risk-of-bias study; meta-analysis of 14 studies, 4,566 patients) (Attachaipanich et al., Indian Heart Journal, 2025). In heart failure, fQRS carries a similarly elevated risk of ventricular arrhythmia (roughly 1.5-fold) and all-cause mortality (roughly 1.7-fold) (meta-analysis of 10 studies, 3,885 patients) (Engstrom et al., Annals of Noninvasive Electrocardiology, 2022). Across cardiac-MRI correlation studies in several cardiomyopathies, fQRS tracks with the presence and extent of underlying myocardial fibrosis, reinforcing that it functions as a surface-ECG proxy for structural scar rather than an isolated electrical curiosity (Bazoukis et al., World Journal of Cardiology, 2022; Silvetti et al., Frontiers in Cardiovascular Medicine, 2023). Isolated fQRS confined to a single lead, most often V1, in an otherwise structurally normal heart — commonly seen in athletes — has not been linked to adverse events in the follow-up periods studied so far, though the authors of that review note that longer-term monitoring is still needed before treating it as reliably benign [CLINICAL REVIEW NEEDED: long-term outcome data for isolated single-lead fQRS in structurally normal hearts remains limited] (Wiradinata & Qhabibi, Egyptian Heart Journal, 2026).

fQRS itself produces no symptoms — it is an ECG morphology finding rather than a rhythm disturbance, so it is frequently noticed incidentally on a routine tracing. Any symptoms a patient reports trace back to the condition producing the fragmentation rather than to the fQRS pattern itself: a patient with fQRS from an old myocardial infarction may be entirely asymptomatic or report only a remote history of chest pain, while a patient with fQRS in the setting of active heart failure or cardiomyopathy more typically reports exertional dyspnea, fatigue, or palpitations tied to the elevated arrhythmia risk the finding flags.

Causes and risk factors include: prior myocardial infarction with resulting scar, the most extensively studied cause [CLINICAL REVIEW NEEDED: needs an in-window (2021+) source corroborating this etiology point] (Das et al., 2006); non-ischemic and hypertrophic cardiomyopathy, where fQRS correlates with the degree of myocardial fibrosis on cardiac MRI (Bazoukis et al., 2022; Silvetti et al., 2023); heart failure of either ischemic or non-ischemic origin, where its presence flags elevated arrhythmic risk (Engstrom et al., 2022); and, at the lower-concern end, an isolated single-lead pattern (particularly in V1) that becomes more common with athletic training and increasing age without a documented adverse correlation to date (Wiradinata & Qhabibi, 2026).

Interpretation Guide

Key Features:

• Rate: not a defining feature — fQRS is a QRS-morphology finding superimposed on whatever the underlying rate happens to be
• Rhythm: not a defining feature — this finding describes QRS morphology, not the rhythm’s origin or regularity
• P waves: within normal limits; unaffected by the QRS fragmentation itself
• PR interval: within normal limits (0.12-0.20 s); the narrow-QRS fQRS criteria presume a normal QRS duration and AV conduction context rather than describing the PR interval directly
• QRS complex: the defining feature — for a narrow QRS (under 120 ms), an additional R wave (R′), multiple R′ waves, or notching within the R wave or S wave, present in at least two contiguous leads corresponding to a single coronary artery territory, in the absence of a typical bundle branch block pattern; a separate wide-QRS criterion (120 ms or more, as with bundle branch block or a paced rhythm) requires more than two notches in the R or S wave in at least two contiguous leads instead (Das et al., 2006; Blondeel et al., 2025)
• ST segment: typically within normal limits for an isolated, chronic fQRS finding; new or evolving ST-segment change accompanying new fQRS points toward an acute ischemic process rather than an old, stable scar
• T waves: typically within normal limits for a chronic finding; concurrent T-wave inversion raises suspicion for an acute or evolving process rather than a stable, old pattern
• QT interval: not independently affected by fQRS itself
• Other findings: fQRS confined to a single lead, most often V1, is common in athletes and has not been associated with adverse events in the follow-up periods studied so far; fQRS spanning two or more adjacent leads is the pattern the literature associates with underlying myocardial scar and warrants further evaluation rather than being dismissed as a normal variant (Wiradinata & Qhabibi, 2026)

Key Leads

• No single lead defines this finding — like a pathological Q wave, which territory shows fQRS depends on which myocardial region is affected, and the pattern is read by contiguous lead groupings rather than any one lead in isolation (Das et al., 2006; Blondeel et al., 2025).
• Leads II, III, aVF – the inferior lead group
• Leads V1-V4 – the anteroseptal lead group; also the group where an isolated, single-lead fQRS (typically V1 alone) is most often a benign, athlete-associated variant rather than a scar marker (Wiradinata & Qhabibi, 2026)
• Leads I, aVL, V5-V6 – the lateral lead group

Differential Diagnosis

• Abnormal Q Wave / Pathologic Q Wave (AQW) — both findings mark prior myocardial injury and can appear on the same strip, but reflect different electrical mechanisms: a pathological Q wave means the lead records no depolarization forces at all from an electrically dead segment, producing a single deep or wide initial negative deflection, while fQRS reflects a viable but disorganized conduction path around scar, producing extra spikes or notches within an otherwise normal QRS [CLINICAL REVIEW NEEDED: needs an in-window (2021+) source corroborating this mechanistic description] (Das et al., 2006)
• Bundle Branch Block (BBB) — a typical bundle branch block reflects a global delay in one ventricle’s activation through its own conduction bundle, producing a QRS duration of 120 ms or more with a stereotyped morphology (rsR′ in the right precordial leads for a right bundle branch block, a broad monophasic R wave in the lateral leads for a left bundle branch block); the narrow-QRS fQRS definition is specifically defined as occurring in the absence of that typical pattern, so a widened QRS meeting classic BBB morphology should be read as BBB rather than fQRS (Das et al., 2006; Blondeel et al., 2025)
• Right Bundle Branch Block (RBBB) — RBBB’s classic rsR′ pattern in V1 can superficially resemble the RSR′ pattern described in fQRS criteria; the distinguishing features are QRS duration (RBBB is 120 ms or more, with a wide terminal S wave in leads I and V6, versus under 120 ms for narrow-QRS fQRS) and overall morphology, since the fQRS definition specifically excludes tracings meeting typical BBB criteria (Blondeel et al., 2025)
• Anterior Myocardial Infarction (AnMI) — fQRS confined to V1-V4 together with a supporting Q-wave or ST-segment pattern and an ischemic clinical picture supports the more specific anterior-infarction label rather than an isolated, non-territory-specific fQRS finding

Treatment Brief

fQRS found on a routine strip is a risk marker, not itself an emergency — the appropriate response depends on the clinical context it appears in, not the pattern alone.

• Confirm lead placement and repeat the strip if the pattern is new or unexpected, since technical factors can occasionally mimic true fragmentation.
• Compare against a prior ECG when one is available — an unchanged fQRS pattern in an asymptomatic patient with a known prior cardiac history is far less concerning than a new finding.
• Note whether fQRS is confined to a single lead (most consistent with a benign variant, particularly in an athletic or otherwise healthy patient) or spans two or more contiguous leads corresponding to a coronary territory (more consistent with underlying scar) (Wiradinata & Qhabibi, 2026).
• In a patient with known coronary artery disease, cardiomyopathy, or heart failure, flag new or extensive fQRS for provider awareness given its association with elevated ventricular arrhythmia and mortality risk in these populations, and keep the patient on continuous rhythm monitoring per unit protocol (Attachaipanich et al., 2025; Engstrom et al., 2022).
• fQRS accompanied by new ST-segment or T-wave change, or by symptoms such as chest pain, dyspnea, or palpitations, warrants prompt provider notification rather than routine documentation alone.