What does the presence of inferior Q waves on an electrocardiogram (ECG) indicate?

Inferior Q Waves on ECG

Inferior Q waves on an electrocardiogram indicate prior myocardial infarction (MI) of the inferior wall of the left ventricle when they meet pathologic criteria, though normal variants and other conditions must be excluded. 1

Pathologic Q Wave Criteria in Inferior Leads

Pathologic Q waves in the inferior leads (II, III, aVF) are defined as:

• Duration ≥0.03 seconds (≥30 ms) AND depth ≥0.1 mV (≥1 mm) in two or more contiguous inferior leads 2, 1
• Alternatively, a Q/R ratio ≥0.25 (Q wave depth at least 25% of R wave amplitude) in two or more contiguous leads 2
• QS complexes (complete absence of R wave) also qualify as pathologic 2

The specificity for MI diagnosis is greatest when Q waves occur in multiple leads within the inferior lead grouping (II, III, aVF) 2, 1

Clinical Significance

Pathologic inferior Q waves indicate:

• Transmural myocardial necrosis from prior inferior wall MI, typically involving the inferior or inferoposterior left ventricular wall 1, 3
• Larger infarct size (typically >6.2% of left ventricular mass) 1
• The inferior wall is supplied by either the right coronary artery (most common) or left circumflex artery 4

Prognostic Implications

Interestingly, the prognostic significance differs by location:

• Inferior MI with Q waves does NOT carry the same adverse prognosis as anterior MI with Q waves 5
• In anterior MI, admission Q waves are associated with higher mortality, heart failure, and peak creatine kinase 5
• In inferior MI, Q waves on admission are NOT independently associated with adverse outcomes 5

Critical Normal Variants and Pitfalls

Before diagnosing pathologic inferior Q waves, exclude these normal variants:

Lead III Specific Considerations

• Q waves in lead III alone may be normal if the frontal QRS axis is between -30° and 0° (leftward axis deviation) 2, 1
• Q waves <0.03 seconds and <25% of R wave amplitude in lead III are considered normal with this axis 1
• The ECG pattern "Q3qF" (Q wave in III with small q in aVF) should be excluded from pathologic criteria 6

Lead aVL Considerations

• Q waves may be normal in aVL if the frontal QRS axis is between 60° and 90° (rightward axis) 2, 1

Positional Changes

• Deep inspiration does NOT reliably differentiate positional from pathologic Q waves in inferior leads 3
• Changes in heart position due to body habitus or diaphragm position can create pseudo-pathologic Q waves 3

Conditions That Mimic Inferior MI

Non-ischemic causes of inferior Q waves include:

• Hypertrophic cardiomyopathy (HCM) 2
• Arrhythmogenic right ventricular cardiomyopathy (ARVC) 2
• Cardiac amyloidosis and infiltrative diseases 2, 1
• Accessory pathways (pre-excitation syndromes like WPW) 2, 1
• Left ventricular hypertrophy 1, 4
• Bundle branch blocks 1, 4

Diagnostic Approach

When inferior Q waves are identified:

1. Verify technical factors: Ensure proper lead placement and exclude lead transposition 2

2. Apply strict criteria: Confirm Q waves meet pathologic definitions in at least two contiguous inferior leads 2, 1

3. Assess QRS axis: Determine if axis deviation explains Q waves in lead III or aVL 2, 1

4. Look for supporting evidence:

   • Minor Q waves (0.02-0.03 sec, 0.1 mV deep) become more significant when accompanied by inverted T waves in the same lead group 2, 1
   • ST-T wave abnormalities in the same leads increase diagnostic accuracy for MI 1

5. Consider posterior involvement:

   • Inferior Q waves may accompany posterior MI 4, 3
   • Look for tall R waves in V1-V2 (posterior Q wave equivalent) 4
   • Consider recording posterior leads (V7-V9) if posterior MI suspected 4

6. Obtain echocardiography: This is the minimum evaluation to assess for:

   • Regional wall motion abnormalities consistent with prior inferior MI 2
   • Alternative diagnoses (cardiomyopathy, infiltrative disease) 2, 1

7. Cardiac MRI if needed: Consider based on echocardiographic findings or when diagnosis remains uncertain 2

Additional Context

Q waves may evolve over time:

• Approximately 65% of patients show diminution in Q wave area over months to years following inferior MI 7
• In 14% of cases, ECGs may normalize completely with no residual evidence of prior infarction 7
• This natural history does not negate the original diagnosis but explains why some prior MIs may not show persistent Q waves 7