Q Wave

What is a Q wave? Any downward (negative) deflection that comes before an R wave. Q waves represent the normal electrical activation of the wall between the left and right ventricles (the interventricular septum).

• Timing: Abnormal Q waves from heart muscle death occur 2-24 hours after heart attack onset, always with ST elevation
• Pathologic Criteria: Q wave ≥0.04 seconds (1 small box) wide, OR >2mm deep, OR ≥25% depth of R wave suggests infarction
• Lead III Caution: Most incorrect MI diagnoses come from non-diagnostic Q waves in III and aVF—lead III is the most unreliable
• R Wave Progression: Expected normal R wave: V1 0-6mm, V2 >0.2mm, V3 ≥1mm
• Q Wave Evolution: >80% persist 4-5 years post-MI; ~10% become non-diagnostic; ~10% disappear; ~5% ECG returns to normal
• Normal Septal Q Waves: Small Q waves are typically seen in left-sided leads (I, aVL, V5, V6). Under normal circumstances, Q waves should NOT be seen in right-sided leads (V1-3)

Small, narrow Q waves are normal in many leads. The depth of the Q wave is not as important as the width.

Timing and Evolution

• Abnormal Q waves from myocardial necrosis occur as early as 2 hours and as late as 24 hours after onset of clinical symptoms
• Q waves of acute infarction are always associated with abnormal ST elevation
• From 6-12 hours after symptom onset, ST elevation recedes but Q waves become more prominent

Diagnostic Criteria

Q Wave Evolution Over Time

• >80%: Pathologic Q waves persist 4-5 years after acute MI
• ~10%: Q waves become nondiagnostic but still suspicious
• ~10%: Q waves disappear
• ~5%: ECG returns to normal in patients with Q wave infarction

Pathological Q Wave Criteria (LITFL)

Q waves are considered pathological if they meet ANY of these criteria:

• Width: > 40 milliseconds (1 small box / 0.04 seconds) wide
• Depth: > 2mm deep
• Proportion: > 25% of the depth of the QRS complex
• Location: Seen in leads V1-3 (normally Q waves should NOT appear in right-sided leads)

Note: Location determined from ECG findings is not always precise, particularly for anterolateral, anteroseptal, and posterior infarctions.

Inferior Infarction (Bottom Wall)

• Q waves: Abnormal Q waves in leads II, III, and aVF
• Width criteria: Q waves in II, III, aVF >0.03 second; Q in lead III >0.04 second
• Acute: ST elevation in II, III, aVF; more specific if you see mirror-image ST depression in I, aVL, V1, V2 during early hours
• Old: Abnormal Q waves with non-specific ST-T changes in II, III, aVF; lead III most unreliable
• Specificity: Q >0.03 sec in leads II and aVF: 96% specificity, ~50% sensitivity

Anterior Infarction (Front Wall)

• Q waves: Abnormal Q, QS, or QR waves in V2-V4 or V5, or V1-V6 with extensive anterior infarction
• Acute: ST elevation in V2-V4 or V5; mirror-image ST depression may develop in II, III, aVF during early hours
• Old: Abnormal Q waves, QS complexes in V2-V4 or V5; ST usually flat but some abnormality often remains

Anteroseptal or Anteroapical Infarction (Front Wall Near Septum or Apex)

• Q waves: Abnormal Q waves, QS pattern in V1-V3
• Acute: ST elevation in V1-V3 in patients with acute chest pain
• Important: Recent ultrasound and angiography studies showed 92% of patients with ST elevation in V1-V3 (traditionally called "anteroseptal MI") actually had damage to the tip of the heart (apex) with normal septum

Anterolateral Infarction (Front and Side Wall)

• Q waves: Abnormal Q waves in V5, V6, I, aVL (less specific)
• Pattern often reflects: Damage to tip of heart; may also be found in scarring of the septum and thickened heart muscle disease
• Specificity: QS pattern in V4 makes diagnosis more reliable

Right Ventricular Infarction (Right Side of Heart)

• Usually occurs together with inferior infarction
• Diagnostic ECG: ST elevation in V4R and V3R along with ST elevation and developing Q waves in II, III, aVF
• Important timing: ST elevation in V4R goes away within 8 hours of symptom onset—check early!

Posterior Infarction (Back Wall)

• True posterior infarction often occurs with inferior MI
• V1: R wave taller than S wave, R/S ratio >1; T wave (often negative elsewhere) is upright and may be tall; R wave should be >0.04 second
• V2: R wave is tall, normally positive T wave may be tall and peaked
• More reliable if: ST elevated in V7-V9 when ECG also shows acute inferior infarction

Differential Diagnosis: Other conditions that can cause pathological-appearing Q waves without actual infarction:

• Myocarditis (heart muscle inflammation, including Chagas disease and AIDS) may cause abnormal Q waves
• Hypertrophic Cardiomyopathy: Thickened heart muscle disease can cause abnormal Q waves (deep, wide Q waves in II, III, aVF, and V4-V6)
• Infiltrative heart disease: Conditions where abnormal substances deposit in heart muscle (amyloidosis, sarcoidosis)
• Wolff-Parkinson-White Syndrome: Extra electrical pathway creates false Q waves in II, III, aVF that may look like inferior MI (look for short PR interval and delta wave)
• Left Ventricular Hypertrophy (LVH): QS pattern may occur in V1, V2, or V3 and look like MI
• Heart Rotation: Extreme clockwise or counter-clockwise rotation can create unusual Q wave patterns
• Lead Placement Errors: Incorrect electrode placement (e.g., arm leads on legs) can create false Q waves

The absence of small septal Q waves in leads V5-6 should be considered abnormal.

Other causes of absent Q waves in V5-6:

• Left Bundle Branch Block (LBBB): Most common cause
• Left Ventricular Hypertrophy: Thickened heart muscle can mask normal Q waves
• Pre-excitation (WPW): Abnormal early activation pathway changes normal pattern

Step-by-Step Approach to Q Wave Analysis

1. Measure Q wave width: Is it ≥0.04 seconds (1 small box)? Width is more important than depth
2. Check adjacent leads: Are ≥2 neighboring leads involved?
3. Look for ST elevation: Q waves of acute MI are ALWAYS with ST elevation
4. Look for mirror-image changes: ST depression in opposite leads makes diagnosis more reliable in acute phase
5. Lead III needs special attention: Is the Q only in lead III, or does it also appear in II/aVF?
6. Check for poor R wave progression: Is this actually anteroseptal MI or something else (wrong lead placement, COPD, LBBB, LVH, late transition, normal variant)?
7. Rule out mimics: WPW (false Q waves), thickened heart muscle, heart inflammation, LVH (QS in V1-V3)
8. Compare with old ECGs: When available, determine if Q waves are new or old; look for changes over time
9. Clinical correlation: Always combine ECG with symptoms, cardiac blood tests (troponin), and imaging

When to Report "Borderline" or "Clinical Correlation Required"

• Q waves in II, III, aVF with borderline width—especially if only in lead III
• Poor R wave progression without clear ST-T abnormalities
• Q waves when confusing factors present (LBBB, LVH, wrong lead placement)
• Old Q waves with ongoing ST-T abnormalities but no information on when MI occurred (report as "infarction age cannot be determined")

Common Mistakes to Avoid

• Over-reading lead III: Remember, lead III is the least reliable lead; always check II and aVF
• Ignoring lead placement error: Especially important when evaluating poor R wave progression in women
• Missing normal patterns: Very small R waves in V1-V3 in young women; early transition that looks like posterior MI
• Forgetting the timeline: Q waves develop and change over time; ST elevation should be present in acute phase; compare with old ECGs when possible
• Not checking for tall P waves: Supports COPD diagnosis when poor R progression present

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