How do you identify a myocardial infarction (MI) on an electrocardiogram (ECG)?

Identifying Myocardial Infarction on ECG

The most reliable ECG findings for identifying myocardial infarction (MI) include ST-segment elevation or depression, pathological Q waves, and T-wave changes in at least two contiguous leads, with specific criteria for different lead groups. 1

Acute MI ECG Findings

ST-Segment Changes

• ST-segment elevation ≥0.1 mV in two or more contiguous leads (≥0.2 mV in leads V1-V3) is the hallmark of acute STEMI 1
• ST depression in leads V1-V3 may indicate posterior wall MI, especially when the terminal T wave is positive (ST elevation equivalent) 1
• More profound ST-segment shift involving multiple leads/territories correlates with greater myocardial ischemia and worse prognosis 1
• Prolonged new ST-segment elevation (>20 min), particularly when associated with reciprocal ST depression, usually reflects acute coronary occlusion 1

T-Wave Changes

• Hyperacute T waves (tall, symmetrical T waves) may be an early sign of MI, preceding ST-segment elevation 1
• T-wave inversion in leads with Q waves increases the likelihood of MI 1
• Pseudo-normalization of previously inverted T waves during chest pain may indicate acute ischemia 1

Q-Wave Formation

• Pathological Q waves (≥0.03 sec and ≥0.1 mV deep or QS complex) in at least two contiguous leads suggest myocardial necrosis 1
• Q waves in leads V2-V3 ≥0.02 sec or QS complex in these leads are significant 1
• R wave ≥0.04 sec in V1-V2 with R/S ratio ≥1 and positive T wave may indicate posterior MI 1

Special Lead Considerations

Posterior MI

• Use posterior leads (V7-V9) at the fifth intercostal space when suspecting left circumflex artery occlusion 1
• ST elevation ≥0.05 mV in V7-V9 is significant (≥0.1 mV in men <40 years) 1
• ST depression in V1-V3 with positive terminal T waves may indicate posterior MI 1

Right Ventricular MI

• Record right precordial leads V3R and V4R when suspecting right ventricular involvement with inferior MI 1
• ST elevation ≥0.05 mV in these leads (≥0.1 mV in men <30 years) supports the diagnosis 1

Lateral MI

• ST depression in lateral leads (I, aVL, V5, V6) carries particularly poor prognosis 2
• Pay special attention to lead aVL for subtle signs of inferior MI 3

ECG Interpretation Challenges

Bundle Branch Block

• In LBBB, concordant ST-segment elevation or a previous ECG may help diagnose acute MI 1
• In RBBB, new ST elevation or Q waves should raise suspicion for MI despite common ST-T abnormalities in V1-V3 1

Normal Variants vs. Pathological Findings

• A QS complex in lead V1 is normal 1
• Q wave <0.03 sec and <25% of R-wave amplitude in lead III is normal if frontal QRS axis is between 30° and 0° 1
• Q wave may be normal in aVL if frontal QRS axis is between 60° and 90° 1
• Septal Q waves (<0.03 sec and <25% of R-wave amplitude in leads I, aVL, aVF, V4-V6) are normal 1

Common Pitfalls in ECG Diagnosis

False Positives

• Early repolarization pattern 1, 4
• Left bundle branch block 1
• Pre-excitation syndromes 1
• Brugada syndrome 1, 4
• Pericarditis or myocarditis 1, 4
• Left ventricular hypertrophy 1, 4
• Metabolic disturbances (e.g., hyperkalemia) 1

False Negatives

• Prior MI with Q waves and/or persistent ST elevation 1
• Paced rhythm 1
• Left bundle branch block 1

Practical Approach to ECG Interpretation

• Compare with previous ECGs when available 1, 4
• Obtain serial ECGs at 15-30 min intervals in symptomatic patients with initially non-diagnostic ECGs 1, 4
• Consider continuous 12-lead ECG monitoring in high-risk patients 1
• Record additional leads (posterior, right ventricular) when standard leads are non-diagnostic but clinical suspicion remains high 1
• Interpret ECG findings in the context of clinical presentation and cardiac biomarkers 4, 3

Evolution of ECG Changes in MI

• Hyperacute T waves may appear within minutes of coronary occlusion 1
• ST-segment elevation typically develops within hours 1
• Q waves may develop within hours to days, but not all MIs evolve to Q-wave MI 5
• T-wave inversion may persist for weeks to months after the acute event 1

Remember that while the ECG is a valuable diagnostic tool, it has limitations. The initial ECG may be non-diagnostic in a significant proportion of patients with acute MI 6, and ST-segment shifts are not always reliable predictors of subsequent Q-wave evolution 5. Therefore, serial ECGs and correlation with clinical findings and cardiac biomarkers are essential for accurate diagnosis.