What does the presence of Q waves in leads 2 and 3 on an electrocardiogram (ECG) signify?

Q Waves in Leads II and III: Diagnostic Significance

Q waves in leads II and III on an ECG most commonly signify an inferior wall myocardial infarction, particularly when they meet specific criteria for pathological Q waves. 1

Diagnostic Criteria for Pathological Q Waves

Pathological Q waves in leads II and III should meet the following criteria:

• Duration ≥0.03 seconds (30 ms)
• Depth ≥0.1 mV (1 mm)
• Present in at least two contiguous leads of the same lead grouping 1

It's important to note that not all Q waves are pathological. Normal variants include:

• Q wave <0.03 sec and <25% of the R wave amplitude in lead III (when frontal QRS axis is between 30° and 0°) 1
• Isolated Q waves in lead III may be normal, especially without repolarization abnormalities in other inferior leads 1

Clinical Significance

When pathological Q waves appear in leads II and III:

1. Inferior Wall MI: This is the most common interpretation, especially when accompanied by:

   • Q waves in aVF (completing the inferior lead group)
   • ST-segment or T-wave changes in the same leads 1

2. Timing Considerations:

   • Acute phase: Q waves appearing <6 hours from symptom onset in inferior MI are not associated with worse outcomes, unlike in anterior MI 2
   • Chronic phase: Persistent Q waves generally indicate completed myocardial necrosis 3

3. Prognostic Implications:

   • In inferior MI, the presence of Q waves on admission ECG is not independently associated with adverse prognosis 2
   • Q waves in lead II specifically may indicate more extensive inferior wall involvement 4

Differential Diagnosis

Q waves in leads II and III may also be seen in:

1. Left Anterior Hemiblock with Inferior MI:

   • When left anterior hemiblock is present, a Q wave in lead II is an important sign suggesting associated inferior infarction 5
   • In this scenario, part of the inferior wall is typically spared, resulting in initial r waves in leads III and aVF 5

2. Non-infarction Causes:

   • Pre-excitation syndromes
   • Cardiomyopathies (hypertrophic, dilated, stress, cardiac amyloidosis)
   • Myocarditis
   • Acute cor pulmonale
   • Hyperkalemia 1, 6

Evaluation Approach

When Q waves are identified in leads II and III:

1. Compare with prior ECGs if available to determine if these are new findings 1

2. Assess for associated ECG changes:

   • ST-segment elevation or depression
   • T-wave inversions
   • Reciprocal changes in other leads 1

3. Consider additional leads:

   • Right-sided leads (V3R-V4R) to assess for right ventricular involvement
   • Posterior leads (V7-V9) to assess for posterior wall involvement 1

4. Cardiac biomarkers to confirm myocardial injury

5. Imaging studies:

   • Echocardiography to assess wall motion abnormalities
   • Cardiac MRI if clinical suspicion remains high despite normal echocardiogram 6

Clinical Pearls and Pitfalls

• Q waves in leads II, III, and aVF may transiently attenuate during exercise in patients with anterior wall ischemia due to left main or LAD stenosis 7

• The specificity of ECG diagnosis for MI is greatest when Q waves occur in several leads or lead groupings 1

• Misinterpreting normal variants as pathological Q waves can lead to unnecessary further investigation 6

• Proper lead placement is crucial, as misplacement can result in false positives 6

• When evaluating Q waves in leads II and III, always consider the clinical context, associated ECG findings, and patient demographics 6