ECG Basic – Q Wave
Appcardio ECG Academy · ECG Fundamentals
Q Wave on the ECG
Learn how to identify the Q wave, distinguish normal septal q waves from abnormal Q-wave patterns, and interpret their significance in the context of myocardial infarction and other conditions.
Learning objectives
1. What is a Q wave?
The Q wave is the first negative deflection of the QRS complex, occurring before the first positive deflection (the R wave). A negative deflection that follows an R wave is called an S wave, not a Q wave. If the entire QRS complex is negative, it is described as a QS complex.
Figure 1. Original schematic showing the naming convention for Q, R, and S deflections.
Core rule: Q is defined by its position in the sequence, not by how deep or wide it looks. The first downward deflection before an R wave is a Q wave.
Why can a normal Q wave occur?
Early ventricular activation normally includes left-to-right depolarization of the interventricular septum. This initial electrical vector can move away from electrodes facing the left lateral surface of the heart, producing a small initial negative deflection in selected leads.
2. Normal septal Q waves and lead distribution
Small septal q waves may be seen in leads that view the left side of the heart, particularly I, aVL, V5, and V6. They reflect normal early septal activation and should not automatically be interpreted as myocardial scar.
Commonly seen
I, aVL, V5, V6
Small initial q waves can be a normal finding in these lateral leads.
Interpret with caution
III and aVR
The supplied LITFL teaching article notes that deeper Q waves may occur as normal variants in III and aVR. Axis and the overall ECG pattern matter.
Right precordial leads: The source article describes Q waves in V1–V3 as abnormal under usual circumstances. However, do not use a single-lead observation in isolation: QS morphology, lead placement, conduction patterns, and the full clinical context must be assessed.
3. When is a Q wave pathological?
Pathological Q waves can indicate loss of electrically active myocardium and may be associated with prior myocardial infarction. Their interpretation is criterion-dependent. The original LITFL lesson gives a practical set of warning features; contemporary consensus criteria provide a more specific framework for ECG evidence of prior infarction.
Practical warning features from the supplied source
- Q-wave duration greater than 40 ms (1 mm at standard paper speed of 25 mm/s).
- Q-wave depth greater than 2 mm.
- Q-wave depth greater than 25% of the QRS complex depth/amplitude as described in the source.
- Q waves present in leads V1–V3.
Important distinction: These are the simplified LITFL teaching features and should not be treated as a single interchangeable rule or as a stand-alone diagnosis of MI. In particular, a Q wave in V1–V3 requires morphology- and lead-specific assessment.
Contemporary criteria: Fifth Universal Definition of MI (2026)
The 2026 Fifth Universal Definition lists pathological Q waves as an ECG manifestation of myocardial ischemia or infarction when Q-wave duration is at least 40 ms and/or depth is at least 25% of the R-wave amplitude in two contiguous leads. Apply the full definition and consider confounding conditions; the ECG finding alone does not establish an acute MI diagnosis.
| Framework | Criterion highlighted | How to use it |
|---|---|---|
| Supplied LITFL teaching article (2020) | >40 ms; >2 mm deep; >25% of QRS depth; or present in V1–V3 | Useful screening prompts for learners; assess the whole tracing and avoid treating each item as independently diagnostic. |
| Fifth Universal Definition of MI (2026) | Duration ≥40 ms and/or depth ≥25% of R-wave amplitude in two contiguous leads | Contemporary consensus description; integrate with clinical evidence and the complete MI definition. |
How to measure on standard ECG paper
Figure 2. At 25 mm/s, one small horizontal box (1 mm) represents 40 ms. Schematic measurement aid.
At 25 mm/s, each small horizontal box represents 0.04 seconds (40 ms), and each large box represents 0.20 seconds (200 ms). Confirm the ECG paper speed before measuring.
4. Clinical interpretation: Q waves are not synonymous with acute MI
Pathological Q waves are commonly associated with current or previous myocardial infarction, but their presence does not independently determine whether an infarction is acute, recent, or remote. Acute MI diagnosis requires the clinical context and the applicable universal-definition criteria, including acute myocardial injury demonstrated by a rise and/or fall in cardiac troponin with at least one value above the 99th percentile upper reference limit, plus evidence of acute ischemia.
Patterns shown in the supplied LITFL source
| Lead territory / pattern | Additional ECG feature in source example | Source interpretation |
|---|---|---|
| Inferior Q waves (II, III, aVF) | ST-segment elevation | Example labelled acute MI. |
| Inferior Q waves (II, III, aVF) | T-wave inversion | Example labelled previous MI. |
| Lateral Q waves (I, aVL) | ST-segment elevation | Example labelled acute MI. |
| Anterior Q waves (V1–V4) | ST-segment elevation | Example labelled acute MI. |
| Anterior Q waves (V1–V4) | T-wave inversion | Example labelled recent MI. |
These are teaching examples from the source, not a reliable clock for infarct age. ST elevation, T-wave inversion, Q-wave evolution, symptoms, biomarkers, serial ECGs, and imaging must be integrated. A Q wave may persist after the acute event, and Q-wave formation is not required for MI.
5. Differential diagnosis and common pitfalls
The supplied article identifies several non-infarction explanations for Q waves or Q-wave-like patterns:
- Cardiomyopathy: including hypertrophic cardiomyopathy and infiltrative myocardial disease.
- Cardiac rotation: marked clockwise or counter-clockwise rotation can alter the apparent lead pattern.
- Lead placement errors: incorrect limb-lead placement can create misleading Q-wave patterns.
- Myocardial infarction: current or previous infarction remains an important cause when the pattern and clinical evidence support it.
Additional contemporary interpretation caveats include conduction abnormalities and other myocardial disorders. The 2018 Fourth Universal Definition specifically lists conditions such as pre-excitation, amyloidosis, left bundle branch block, left anterior fascicular block, ventricular hypertrophy, myocarditis, and acute cor pulmonale as potential confounders. The 2026 Fifth Universal Definition should be consulted for the current full framework.
Practical check: If a new Q-wave pattern is unexpected, verify electrode placement, compare with prior ECGs, repeat the tracing when appropriate, and correlate with symptoms, biomarkers, and cardiac imaging.
6. Loss of normal septal Q waves
The supplied LITFL article notes that absent small septal q waves in V5–V6 may be abnormal and are most commonly associated in that article with left bundle branch block (LBBB). Interpret this finding in the context of QRS morphology and conduction, rather than using absent lateral q waves as an isolated diagnosis.
7. A systematic approach to Q-wave assessment
Confirm the tracing. Check calibration, paper speed, artifact, and correct limb and chest lead placement.
Identify the morphology. Determine whether the initial negative deflection is a Q wave, whether the complex is QS, and which leads are involved.
Measure and map. Measure duration and depth, assess the Q-to-R amplitude relationship, and determine whether contiguous leads are involved.
Look for context. Review ST segments, T waves, QRS duration and morphology, axis, and any evidence of conduction disease or hypertrophy.
Correlate clinically. Compare prior ECGs and integrate symptoms, serial high-sensitivity troponin, echocardiography, and other imaging when indicated.
8. Knowledge check
A negative deflection occurs after a positive R wave in a QRS complex. How is this deflection named?
Key takeaways
- A Q wave is the first negative deflection before an R wave; a negative deflection after R is an S wave.
- Small septal q waves are often normal in I, aVL, V5, and V6 because of normal septal activation.
- Q-wave duration, depth, relative amplitude, lead distribution, and contiguous-lead involvement all matter.
- Pathological Q waves can support a diagnosis of prior MI, but do not by themselves establish acute MI or determine infarct timing.
- Cardiomyopathy, conduction abnormalities, cardiac rotation, and lead misplacement can confound interpretation.
- Always integrate the ECG with clinical presentation, serial biomarkers, prior tracings, and imaging when appropriate.
References
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