ECG criteria
- Find the Q wave — the first negative deflection of the QRS complex before the R wave
- Evaluate I, II, aVL, aVF, V4–V6 using the general criterion: Q duration and depth
- Pathological Q in these leads: duration ≥ 0.03 s and depth ≥ 1 mm, or a QS complex, in two or more contiguous leads (AHA/ACCF/HRS 2009 criterion)
- Evaluate lead III separately: the Q wave there is frequently physiological; interpret it taking into account the cardiac axis, breathing test, and adjacent lead II
- Evaluate V2–V3 separately: a Q wave with a duration of ≥ 0.02 s or a QS complex is pathological; confirm the finding by correct electrode placement and the clinical context
- Do not count V1 separately as equivalent to V2–V3 — isolated QS in V1 can be a normal variant
- Additionally (auxiliary historical criterion): Q/R ratio ≥ 25% in a lead — applied only when a measurable R is present, not as an independent replacement for the main criterion
- Verify that the abnormality is present in two or more contiguous leads of the same wall rather than in isolation, and correlate it with the clinical picture: symptoms, ST-T dynamics, medical history
What it looks like on ECG
The Q wave is the first negative wave of the QRS complex preceding the R wave. It may be absent (e.g., normally V1–V2 often show an rS complex without Q), narrow and shallow (normal variant, septal q), or wide and deep (pathological Q).
The AHA/ACCF/HRS 2009 criterion applies to the main list of leads: a pathological Q is defined as a duration of ≥ 0.03 s combined with a depth of ≥ 1 mm, or a QS complex, present in two contiguous leads from I, II, aVL, aVF, V4–V6. Lead III is not included in this standard list: the Q wave there is frequently physiological and must be interpreted separately, taking into account the cardiac axis, breathing test, and the adjacent lead II. Some classic textbooks use an older threshold of width ≥ 0.04 s — this is historically a different criterion, and when interpreting, one should keep in mind which source is being used. Recalibration reminder: at a recording speed of 25 mm/s, 1 mm horizontally = 0.04 s, meaning 0.03 s corresponds to 0.75 mm.
A separate rule applies to V2–V3: a Q wave with a duration of ≥ 0.02 s or a QS complex is pathological; the depth criterion for Q in these leads is not applied. The finding must be confirmed by correct chest electrode placement and the clinical context, since normally Q is typically absent in V1–V3 (rS complex).
| Parameter | Normal (I, II, aVL, aVF, V4–V6) | Abnormal |
|---|---|---|
| Width | usually < 0.03 s | ≥ 0.03 s (AHA 2009) or ≥ 0.04 s (classic criteria) |
| Depth | usually small, auxiliary < 25% of R | ≥ 1 mm with corresponding width, or QS |
A normal "septal" q wave is usually visible in I, aVL, V5–V6 — it is short (less than 0.04 s) and low in amplitude, reflecting left-to-right interventricular septal depolarization. In III and aVF, an isolated Q wave may occur in healthy individuals due to a vertical heart position, especially during inspiration when it decreases or disappears.
The appearance of Q in V2–V3 (duration ≥ 0.02 s or QS) is a pathological electrocardiographic pattern according to standard criteria, but by itself it does not prove infarction or necrosis: correct chest electrode placement must be verified and non-infarct causes (cardiomyopathies, conduction disturbances, altered heart position) must be excluded. An isolated QS in V1 without changes in V2–V3 may be a normal variant.
Mechanism
Ventricular depolarization begins with the interventricular septum, whose vector is directed from left to right and slightly anteriorly. In leads "looking" at the left ventricle from the left (I, aVL, V5–V6), this initial vector is directed away from the electrode — a small q wave is recorded. In right precordial leads (V1–V2), the initial vector is directed toward the electrode — normally there is no Q wave there.
A pathological Q wave reflects an alteration in the resulting ventricular depolarization vectors. Classically, it is associated with the loss of electrically active myocardium (post-infarction scar) — in this case, the lead "sees" the depolarization of the opposite, undamaged wall, which is recorded as a deep and wide Q or QS. However, this same pattern also occurs without necrosis: in hypertrophic cardiomyopathy, left bundle branch block, WPW syndrome, altered heart position, and a number of other conditions — therefore, Q is not specifically diagnostic of infarction alone.
Causes
- Normal variant — septal q in I, aVL, V5–V6, III/aVF
- Prior myocardial infarction or post-infarction scar
- Acute coronary syndrome with necrosis development (not only transmural and not only with ST elevation)
- Hypertrophic cardiomyopathy (deep narrow Q waves in inferolateral leads)
- Left bundle branch block (alteration of septal depolarization sequence)
- WPW syndrome (pseudo-infarction Q waves due to the delta wave)
- Left ventricular hypertrophy with pronounced septum
- Infiltrative myocardial diseases (amyloidosis, sarcoidosis)
- Pneumothorax, marked pulmonary emphysema (cardiac rotation)
- Dextrocardia and incorrect electrode placement
Differential diagnosis
- Pseudo-infarction Q in WPW — there is a delta wave and a short PR interval, the Q wave reflects the initial part of pre-excitation rather than necrosis
- Q in HCM — usually narrow and very deep in inferior and lateral leads, accompanied by characteristic hypertrophy on echocardiography
- Normal Q in III, aVF — decreases or disappears on deep inspiration, unlike a persistent pathological Q
- QS in V1 as a normal variant — isolated QS only in V1 without changes in V2–V3 is not pathological
- Q due to LBBB — wide QRS with typical bundle branch block morphology should raise suspicion primarily for block rather than infarction; Sgarbossa criteria are used for infarction in the presence of LBBB
Clinical significance and management
A pathological Q or QS is an electrocardiographic sign that may result from a prior infarction or scar, but it does not reliably determine transmurality, timing, or volume of necrosis: a Q wave can appear in non-occlusive and non-transmural infarction, and a transmural infarction is not necessarily accompanied by a Q wave. The presence of Q by itself does not determine the acuity of the process — the ST segment and T wave are evaluated for this purpose, and new Q/QS waves in acute coronary syndrome are significant only in combination with clinical presentation and other ECG changes, rather than as an independent marker of age.
The physician needs to: correlate the localization of Q with the cardiac walls (II, III, aVF — inferior; V1–V4 — septal/anterior; I, aVL, V5–V6 — lateral); check for acute ST-T changes in the same leads; clarify medical history (pain, prior MI, echocardiographic data on hypokinesis zones). In case of symptoms or suspicion of acute coronary syndrome (pain, dyspnea, autonomic manifestations, ST-T dynamics) — emergency routing, repeat ECG, serial high-sensitivity troponin, and treatment according to the ACS protocol. Planned outpatient work-up (echocardiography, stress tests, coronary angiography as indicated) is acceptable only in a stable condition without signs of an acute event.
Common interpretation errors
- Mistaking a normal septal q in III, aVF for a sign of inferior infarction without evaluating the breathing test and adjacent leads
- Failing to notice an isolated Q in V2–V3 — a Q with duration ≥ 0.02 s or QS there requires attention and checking electrode position
- Confusing a QS in V1 (potential normal variant) with pathology without evaluating V2–V3
- Failing to account for LBBB or WPW as causes of pseudo-infarction Q waves
- Evaluating Q in isolation without reference to contiguous leads — the criterion requires involvement of two or more leads of the same wall
- Interpreting a new pathological Q as a definitive diagnosis of necrosis without considering symptoms, ST-T dynamics, and troponin
1. Which criterion corresponds to a pathological Q wave in leads I, II, aVL, aVF, V4–V6 according to AHA/ACCF/HRS 2009?
The main AHA/ACCF/HRS 2009 criterion combines a duration of ≥ 0.03 s and depth of ≥ 1 mm (or QS) with the requirement of two contiguous leads; at 25 mm/s, 1 mm horizontally = 0.04 s.
2. In which leads do a Q wave with duration ≥ 0.02 s or a QS complex have pathological significance according to AHA/ACCF/HRS 2009?
Normally, there should be no Q wave at all in V2–V3, so a Q wave with a duration of ≥ 0.02 s or a QS wave there is pathological, although it requires verification of electrode placement and clinical correlation.
3. What explains the normal septal q wave in leads I, aVL, V5–V6?
The initial vector of septal depolarization is directed from left to right, which is recorded as a small q wave in lateral leads.
Frequently asked questions
Which Q wave is considered pathological?
In leads I, II, aVL, aVF, V4–V6, according to the 2009 AHA/ACCF/HRS criteria, a duration of ≥ 0.03 s combined with a depth of ≥ 1 mm, or a QS complex in two contiguous leads; some classic textbooks use a width threshold of ≥ 0.04 s. Lead III is evaluated separately, as a Q wave there is frequently physiological. A separate rule applies to V2–V3: a Q wave with a duration of ≥ 0.02 s or a QS complex is pathological. A Q/R ratio of ≥ 25% is an additional historical criterion, not a standalone replacement for the main one.
Is a Q wave on the ECG dangerous?
A pathological Q wave is an electrocardiographic sign that may indicate a prior myocardial infarction or scar, but it is not direct proof of necrosis and does not date the event. It requires assessment of cardiac contractile function and clinical correlation. Acute danger is determined by concomitant ST-T changes and the clinical picture, rather than the mere presence of a Q wave.
Can a Q wave be a normal variant?
Yes, a short and shallow septal q wave in I, aVL, V5–V6, as well as a small Q wave in III and aVF, is a common variant of normal, reflecting normal depolarization of the interventricular septum.
How to distinguish a normal Q wave from a pathological one?
A normal septal q wave is short (less than 0.04 s) and low in amplitude, typically in I, aVL, V5–V6 or III/aVF, with attenuation on inspiration in the inferior leads. A pathological Q wave meets the 2009 AHA criteria (≥ 0.03 s and ≥ 1 mm, or QS, in two contiguous leads) or has a duration of ≥ 0.02 s or a QS morphology in V2–V3; it is often accompanied by other signs of scarring.
Does a pathological Q wave disappear over time?
In most patients, the Q wave persists on the ECG for a long time, but in some patients, especially after timely reperfusion, the amplitude and width of the Q wave may decrease over time or the wave may disappear.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger A. Clinical Electrocardiography
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
- 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.