At a Glance
- Represents
- Electrical force lost from scar, or altered septal activation. A Q is the first negative deflection of the QRS.
- Normal (Adult)
- Non-pathologic: <30 ms or <1 mm, outside V2–V3. Pathologic needs both ≥30 ms and ≥1 mm in two contiguous leads; in V2–V3, Q ≥20 ms or QS. Narrow septal q in I, aVL, V5–V6 is normal; a lone Q in III is not pathologic.
- Measure It
- Width from Q onset to baseline (ms); depth from the PR baseline (mm); two contiguous leads in the same group.
- Abnormal → Think
- Prior or evolving infarct in those leads. Mimics first: LBBB, paced rhythm, WPW, hypertrophic cardiomyopathy, lead reversal.
- Red Flags
- New Q waves with chest pain or ST elevation; any Q ≥0.02 s or QS in V2–V3 with symptoms; Q waves absent from the prior ECG.
- Bedside Action
- Compare with the prior ECG; act on symptoms and ST-T change, not the Q alone. Repeat 12-leads; add V4R or V7–V9 when indicated.
Physiology & What It Represents
A Q wave is the first negative deflection of the QRS complex. Here's the mechanism: the normal septum activates from left to right. Its first vector points away from the left-sided leads (I, aVL, V5, V6) and toward V1. So those leads record small, narrow q waves. V1 shows an rS or r pattern.
Dead muscle makes no electrical force. The live wall on the far side keeps sending its force away from the scar. The lead that faces the scar sees that force moving away, so it records a negative deflection. That deflection is a pathologic Q wave. The Q points to the territory of the scar, and it tends to last as long as the scar does.
The Q wave is a structural, late finding. Acute ischemia shows ST-segment and T-wave change first (see the ST Segment & Ischemia page). Use the Q to date and localize an infarct, not to detect one.
Q waves can also show up without infarction, whenever septal activation is altered. Left bundle branch block removes the normal septal q waves. Pacing changes the sequence. A hypertrophied septum and infiltrative disease build deep Q waves. Pre-excitation shifts early activation. Limb lead misplacement can copy several of these patterns.
Key concept: A Q wave is missing force. It comes from scar or from altered septal activation. Width, depth, lead group, QRS morphology and the prior ECG tell you which.
Normal Values & How to Measure
Small, narrow Q waves are normal in several leads. Don't call a Q pathologic until width and depth both pass across a lead group. The thresholds come from the Fourth Universal Definition of Myocardial Infarction, in the next section. Measure at standard paper speed and gain (25 mm/s and 10 mm/mV). Check the calibration mark first.
| Parameter | Adult Normal | Pediatric Note | Where / How to Measure |
|---|---|---|---|
| Q duration | <30 ms (<0.03 s) is non-pathologic outside V2–V3. In V2–V3, ≥20 ms (≥0.02 s) is abnormal | Adult thresholds; age-matched norms apply | From QRS onset to the return to baseline in the widest lead of the group |
| Q depth | <1 mm (<0.1 mV) is non-pathologic at any width. Depth alone is not diagnostic; narrow septal q waves in I, aVL and V4–V6 can be 1 mm or deeper and still be normal | Adult thresholds; age-matched norms apply | From the isoelectric PR (or TP) baseline to the nadir of the Q |
| Isolated Q in lead III | Not pathologic by itself | Same as adult | Check II and aVF; a Q needs two contiguous leads |
| QS in V1 | Can be normal when alone | Right-ventricular dominance in infants changes V1 patterns | Look at V2 and V3 before calling it pathologic |
| Q in V2–V3 | Any Q ≥0.02 s, or any QS, is abnormal | Adult criteria | Measure in V2 and V3 on the same tracing |
- Choose the lead group: I and aVL; V1 to V6; or II, III and aVF. A Q is judged within a group, not in one lead.
- Measure duration: from the start of the Q to its return to baseline in the widest lead, in ms (at 25 mm/s, 40 ms = one small box).
- Measure depth: from the isoelectric PR (or TP) segment to the Q nadir, in mm (at 10 mm/mV, 1 mm = 0.1 mV).
- Check two contiguous leads in the same group before calling a pathologic Q.
- Check the QRS: width, bundle branch block, pacing and pre-excitation change how the septum activates. Compare with the prior ECG.
Measurement pitfall: Depth measured off a shifted or elevated ST segment is wrong. Measure from the PR (or TP) baseline every time.
Pathologic Q Waves
A pathologic Q wave clears the Fourth Universal Definition of Myocardial Infarction thresholds for prior infarction. The test is width and depth, applied across a lead group. One lead doesn't decide it.
ECG Criteria
- V2–V3: any Q wave ≥0.02 s, or any QS complex.
- Two contiguous leads (I and aVL; V1–V6; or II, III and aVF): Q ≥0.03 s and ≥1 mm deep (both required), or a QS complex.
- Posterior equivalent: R wave ≥0.04 s in V1–V2 with R/S ≥1 and a concordant positive T wave, in the absence of a conduction defect.
- Scope: these criteria describe prior infarction in the absence of left ventricular hypertrophy and bundle branch block.
Causes
- Prior infarction: the usual cause. The Q marks scar in the territory of its leads.
- Evolving infarction: a Q can form during the event itself, and it can outlast it.
- Non-infarct patterns: hypertrophic cardiomyopathy, infiltrative disease, bundle branch block, pre-excitation and lead misplacement. Each can build a Q with no infarct. See Differential Diagnosis & Mimics.
Clinical Significance
- A pathologic Q marks necrosis in the leads that show it, old or evolving. On its own, it can't tell you whether the process is acute.
- The Q alone can't date the infarct. Pull the prior ECG and decide whether it's new.
- A Q in a new territory with symptoms could be a new event. Assess the ST-T on the current tracing.
Warning: A Q in lead III with no matching Q in II or aVF fails the two-lead rule. Don't diagnose an infarct from lead III alone.
Infarct Localization by Q Waves
Q waves localize an infarct only roughly. The territories below use the lead groups from the Fourth Universal Definition, which places ECG changes by lead too.
| Territory | Leads With Q or QS | Key Points |
|---|---|---|
| Anteroseptal | V1–V3 | Q or QS in V2–V3 is pathologic at ≥0.02 s |
| Anterolateral | V4–V6, I, aVL | Two contiguous leads in the group meet the width and depth rule |
| Inferior | II, III, aVF | Confirm with II and aVF rather than lead III alone; right-sided leads (V3R, V4R) when inferior features are present |
| Inferoposterior | II, III, aVF with V7–V9 | Posterior leads (V7–V9) are needed when posterior involvement is suspected |
| Posterior | V1–V2 (tall R) | Posterior infarction is diagnosed from a tall R wave, not a Q |
Note: ST-T changes on the current tracing pin down an active infarct better than the Q alone.
Acute vs Old Infarct
The Q can't separate acute from old infarction on its own. The ST segment, the T wave and the prior ECG can.
ECG Criteria
- Acute: new ST elevation (≥1 mm in two contiguous leads, with the V2–V3 thresholds in the ST-T page) or new T-wave change with symptoms. A Q may be present or may develop during the event.
- Old: Q waves with normal or residual ST-T change, no symptoms, and no change from a prior ECG.
- Age unknown: Q waves with no prior ECG and no ST-T change. Report that the infarct age cannot be determined.
Clinical Significance
- A normal ST segment doesn't rule out ischemia. A Q doesn't rule out an acute event either. Weigh symptoms, the ST-T pattern and the troponin result per protocol.
- Q waves that stay after symptoms resolve are usually scar. New Q waves in the same leads with ongoing symptoms need immediate action.
Pearl: Match the Q to the old tracing. If it matches, it's usually old scar. If it's new and the patient has symptoms, it's a possible new event until proven otherwise.
Loss of Septal Q Waves
When septal activation changes, the normal septal q waves vanish from I, V5 and V6. Left bundle branch block (LBBB) is the most common cause.
ECG Criteria
- Absent septal q: no small q in I, V5 or V6 where one is expected. A narrow q may persist in aVL.
- LBBB pattern: QRS ≥0.12 s with a broad notched or slurred R in I, aVL, V5 and V6, no septal q in I, V5 and V6, plus QS or rS in V1–V3 (AHA/ACCF/HRS Part III).
Causes
- LBBB: the most common cause.
- Ventricular paced rhythm: pacing changes the septal sequence. The septal q waves can disappear.
- Other conduction disease: see the Conduction Blocks page.
Clinical Significance
- A missing septal q in LBBB is expected, and it doesn't mean infarction. Read ischemia with the Smith-modified Sgarbossa criteria on the ST-T page.
- A lost septal q with no LBBB and no pacing is a reason to check lead placement and the QRS width.
Differential Diagnosis & Mimics
These patterns can pass for old infarction. Each one has a distinguishing feature. No single lead settles it.
| Feature | Old Infarction | Hypertrophic Cardiomyopathy | WPW Pre-Excitation | Limb Lead Reversal |
|---|---|---|---|---|
| Q pattern | Territory-based; two contiguous leads | Deep, narrow Q in I, aVL, V4–V6 or inferior leads | Negative delta wave in inferior or lateral leads can mimic a Q or QS | Large Q in I and aVL (RA/LA) or II, III and aVF (RA/LL) |
| Associated findings | Prior history; scar-pattern ST-T change | LVH voltage; prominent precordial R waves | Short PR, delta wave, wide QRS | RA/LA: inverted P and QRS in I, negative aVL, II and III swapped. RA/LL: inverted P waves in II, III and aVF; lead I mirrors III |
| What settles it | Prior ECG shows the same Q | Echocardiogram and family or clinical history | Short PR and delta wave on the same tracing | Lead order corrected; P-wave polarity in I and in II, III, aVF, and the Q, change on repeat |
Other Pseudo-Infarct Patterns
- Hypertrophic cardiomyopathy: a pseudoinfarction pattern. Prominent inferolateral Q waves and prominent precordial R waves come from the extra force of the hypertrophic septum.
- Infiltrative disease (amyloidosis): a pseudoinfarction pattern with low QRS voltage, sometimes with normal coronary arteries. The clue is low voltage that doesn't match the wall thickness.
- Left bundle branch block: QS or rS in V1–V3 and a missing septal q in I, V5 and V6. See Loss of Septal Q Waves.
- WPW: a negative delta wave in inferior or lateral leads can mimic a Q or QS. Pre-excitation can also hide the true first vector. A case report showed a pseudo-infarct pre-excitation pattern hiding a real inferior infarct. Read the short PR and the delta wave before you trust the Q.
Artifact & Lead Misplacement
- Limb lead reversal: RA/LA reversal inverts lead I and swaps aVR and aVL. Leads I and aVL can then look like an old lateral infarct. RA/LL reversal inverts lead II, aVF and the P waves in II, III and aVF. The inferior leads can then look like an old inferior infarct.
- Reversed precordial leads: wreck the normal R-wave progression from V1 to V6.
- Electrode level: precordial electrodes set too high or too low shift the R-wave progression. That can mimic an anteroseptal pattern.
Warning: A limb lead reversal can look exactly like an old lateral or inferior infarct. Check lead order and P-wave polarity first, then act on the Q.
Normal Variants
- Some Qs are normal. A lone narrow Q in lead III, a QS in V1 alone, a QS in aVR (whose QRS is normally negative) and small q waves in I, aVL, V5 and V6 can all be normal (see Normal Values).
Bedside Management & Escalation
A Q wave on its own won't change what you do next. Symptoms decide it, along with the ST-T pattern on the current ECG and the prior tracing. STEMI activation runs on local protocols and the current ACC/AHA acute coronary syndrome guideline (2025).
- Symptoms with new ST elevation: go down the STEMI pathway, per local protocol and online medical direction. The criteria live on the ST-T page.
- Old Q, no symptoms, no ST-T change: chart it as prior infarction. The Q alone doesn't trigger activation.
- New Q with symptoms but no ST elevation: compare with the prior ECG, repeat the 12-lead and obtain troponin per protocol. Notify the receiving team.
- Inferior or posterior features: if the protocol asks for them, bring in right-sided (V4R) or posterior (V7–V9) leads.
Follow local protocols and medical direction.
Pediatric & Special Populations
Pediatrics
- These Q-wave thresholds are adult criteria. Read pediatric Q waves against age-matched ECG norms.
- Infants show right-ventricular-dominant patterns and different early vectors. Don't apply adult lead-group thresholds to neonates or infants.
- Congenital heart disease can build abnormal Q waves. In an infant, ALCAPA (anomalous left coronary artery from the pulmonary artery) can show pathologic Q waves in I, aVL and V4–V5 with ST-T change, and it can pass for myocarditis. One reported case saw the Q waves resolve after surgery. Involve pediatric cardiology early.
- For age-based heart rate and vital sign norms, see Pediatric Vital Signs.
Other Populations
- Older adults: silent prior infarction is common. Pull the prior ECGs before you call a Q new.
- Ventricular pacing: altered septal activation changes the Q pattern. Read ischemia with the Sgarbossa-type rules on the ST-T page.
- Bundle branch block: LBBB removes the septal q waves. See Loss of Septal Q Waves.
Monitoring & Transport Considerations
| Issue | What to Do |
|---|---|
| Serial 12-leads | Repeat every 15–30 min during ongoing chest pain. Compare each tracing with the prior one. |
| Prior ECG | Send or transmit the prior 12-lead. A Q that matches an old tracing is usually old scar. |
| Right-sided and posterior leads | Add V4R or V7–V9 when inferior or posterior features appear. |
| Lead order | After moving or re-attaching electrodes, confirm limb lead order and P-wave polarity before reading a Q. |
| Artifact | Shivering and movement can mimic Q waves. Repeat once the patient is still. |
| Transmission | Transmit a new Q with symptoms or ST change per local protocol. |
Step-by-Step Approach & Pitfalls
- Identify the Q: the first negative deflection of the QRS. Measure its width in ms in the widest lead of the group.
- Measure depth in mm from the PR or TP baseline.
- Check the lead group: I and aVL, V1–V6, or II, III and aVF. A pathologic Q needs two contiguous leads.
- Check V2–V3 for any Q ≥0.02 s or any QS complex.
- Check the QRS: LBBB, pacing and pre-excitation change the read. Use the ST-T page for ischemia in those rhythms.
- Check lead order: P-wave polarity in I and aVL (RA/LA) and in II, III and aVF (RA/LL) before you trust a Q. P in lead I should be upright and P in aVR inverted.
- Compare with the prior ECG to decide whether the Q is new.
- Interpret in context: symptoms, ST-T change and troponin per protocol.
Common Pitfalls
- Calling an infarct from lead III alone.
- Calling a new Q old when there's no prior ECG.
- Reading a Q in LBBB or a paced rhythm as infarction, skipping the ST-T rules.
- Missing a limb lead reversal. Check P-wave polarity in lead I (RA/LA) and in II, III and aVF (RA/LL).
- Measuring depth off a shifted baseline.
- Assuming a normal ST segment means no recent event.
ECG Examples
Clinical Pearls
Two-Lead Rule: Two contiguous leads need ≥0.03 s and ≥1 mm for a pathologic Q, or a QS complex. In V2–V3, ≥0.02 s or any QS settles it alone.
Isolated Lead III: A Q in lead III alone is not a diagnosis. Look at II and aVF before you call an inferior infarct.
Lead Reversal: Reversed limb leads can fake a big Q in I and aVL, or in II, III and aVF. Confirm P-wave polarity in I and in II, III and aVF before you act.
Prior ECG: Compare the Q with the old tracing. A match usually means old scar. A new Q with symptoms could be a new event.
Q Alone Is Not Activation: The Q by itself doesn't define STEMI. The call comes from symptoms and the ST-T change on the current ECG.
LBBB and Pacing: A missing septal q and QS in V1–V3 are what you expect in LBBB. Read ischemia with the Sgarbossa-type rules, not the Q alone.
References
- Thygesen K, Alpert JS, Jaffe AS, et al. Fourth Universal Definition of Myocardial Infarction (2018). Circulation. 2018;138(20):e618–e651. https://doi.org/10.1161/CIR.0000000000000617
- Rao SV, O'Donoghue ML, Ruel M, et al. 2025 ACC/AHA/ACEP/NAEMSP/SCAI Guideline for the Management of Patients With Acute Coronary Syndromes. Circulation. 2025;151. https://doi.org/10.1161/CIR.0000000000001309
- Surawicz B, Childers R, Deal BJ, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances (landmark standardization statement). J Am Coll Cardiol. 2009;53(11):976–981. https://doi.org/10.1016/j.jacc.2008.12.013
- Butt MU, Patel RB, Darrat YH, Morales GX, et al. When a Pseudo-Infarct Electrocardiogram (ECG) Pattern in a Posterior Accessory (Wolff-Parkinson-White) Pathway Masks a True Inferior Infarct. Am J Case Rep. 2018;19:685–688. https://doi.org/10.12659/AJCR.909189
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- Sociedade Brasileira de Cardiologia. Guidelines on the Diagnosis and Treatment of Hypertrophic Cardiomyopathy – 2024. Arq Bras Cardiol. 2024;121:e202400415. https://doi.org/10.36660/abc.20240415
- Hypertrophic Cardiomyopathy Phenocopies: Classification, Key Features, and Differential Diagnosis. Biomedicines. 2025;13:3062. https://doi.org/10.3390/biomedicines13123062
- Ommen SR, Ho CY, Asif IM, Balaji S, Burke MA, Day SM, et al. 2024 AHA/ACC/AMSSM/HRS/PACES/SCMR Guideline for the Management of Hypertrophic Cardiomyopathy. Circulation. 2024;149(23):e1239–e1311. https://doi.org/10.1161/CIR.0000000000001250
- Kligfield P, Gettes LS, Bailey JJ, Childers R, Deal BJ, Hancock EW, et al. Recommendations for the standardization and interpretation of the electrocardiogram: part I: the electrocardiogram and its technology. Circulation. 2007;115(10):1306–1324. https://doi.org/10.1161/CIRCULATIONAHA.106.180200
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