What are the ECG (electrocardiogram) changes in myocardial infarction (MI)?

ECG Changes in Myocardial Infarction

The ECG is a critical diagnostic tool for myocardial infarction, with specific changes including ST-segment elevation, ST-segment depression, T-wave inversion, and pathological Q waves that vary based on infarct location and timing. 1

Acute Myocardial Ischemia ECG Changes

ST-Segment Elevation

• New ST elevation at the J point in two contiguous leads with the following cut-points:
  • 0.1 mV in all leads except V2-V3
  • In leads V2-V3: 0.2 mV in men ≥40 years; 0.25 mV in men <40 years; 0.15 mV in women 1
• Prolonged ST elevation (>20 min), especially with reciprocal ST depression, typically indicates acute coronary occlusion 1

ST-Segment Depression and T-Wave Changes

• New horizontal or down-sloping ST depression ≥0.05 mV in two contiguous leads
• T-wave inversion ≥0.1 mV in two contiguous leads with prominent R wave or R/S ratio >1 1
• ST depression in leads V1-V3 may indicate posterior wall MI (inferobasal), especially when terminal T wave is positive 1

Q Waves (Indicating Prior MI)

• Any Q wave in leads V2-V3 ≥0.02 sec or QS complex
• Q wave ≥0.03 sec and ≥0.1 mV deep or QS complex in leads I, II, aVL, aVF, or V1-V6 in any two contiguous leads
• R wave ≥0.04 sec in V1-V2 and R/S ≥1 with concordant positive T wave (in absence of conduction defect) 1

Location-Specific ECG Changes

Anterior MI

• ST elevation in leads V1-V4
• Often associated with LAD occlusion
• May show reciprocal ST depression in inferior leads (II, III, aVF)

Inferior MI

• ST elevation in leads II, III, aVF
• Often associated with right coronary artery occlusion
• May have reciprocal ST depression in leads I and aVL 2

Lateral MI

• ST elevation in leads I, aVL, V5-V6
• Often associated with circumflex artery occlusion

Posterior MI

• Often missed on standard 12-lead ECG
• ST depression in leads V1-V3 (mirror image of posterior ST elevation)
• Requires posterior leads (V7-V9) for direct visualization
• Cut-point of 0.05 mV ST elevation in V7-V9 (0.1 mV in men <40 years) 1

Right Ventricular MI

• Often accompanies inferior MI
• Requires right-sided leads (V3R, V4R)
• ST elevation ≥0.05 mV in V3R/V4R (≥0.1 mV in men <30 years) 1

Evolution of ECG Changes in MI

1. Hyperacute phase (minutes to hours):

   • Tall, peaked T waves
   • ST-segment elevation
   • Possible loss of R wave amplitude

2. Acute phase (hours to days):

   • Persistent ST-segment elevation
   • Development of Q waves
   • T wave inversion begins

3. Subacute phase (days to weeks):

   • Resolution of ST-segment elevation
   • Deepening T wave inversion
   • Established Q waves

4. Chronic phase (weeks to permanent):

   • Persistent Q waves
   • T waves may normalize
   • ST segments return to baseline

Clinical Considerations and Pitfalls

Diagnostic Challenges

• ECG alone is often insufficient to diagnose MI, as ST deviation can occur in other conditions 1:
  • Acute pericarditis
  • Left ventricular hypertrophy
  • Left bundle branch block
  • Brugada syndrome
  • Stress cardiomyopathy
  • Early repolarization patterns

Important Clinical Practices

• Obtain ECG promptly (within 10 minutes) of presentation 1
• Compare with prior ECGs when available 1
• Perform serial ECGs at 15-30 minute intervals in symptomatic patients with initially non-diagnostic ECGs 1
• Consider additional leads for suspected circumflex occlusion or right ventricular involvement 1
• Remember that a normal ECG does not exclude ACS (1-6% of patients with normal ECG later proven to have MI) 3

Prognostic Implications

• More profound ST-segment shift or T-wave inversion involving multiple leads/territories indicates greater myocardial ischemia and worse prognosis 1
• ST depression on presenting ECG portends highest risk of death at 6 months, with degree of depression correlating with outcome 1
• Patients with confounding ECG patterns (bundle-branch block, paced rhythm, LV hypertrophy) have highest risk for death 1

Special Considerations

Left Bundle Branch Block

• Diagnosis of MI is more difficult with LBBB
• Concordant ST-segment elevation or comparison with previous ECG may help identify acute MI 1

Dynamic Changes

• Dynamic changes in ECG waveforms during acute ischemic episodes often require multiple ECGs 1
• Pseudo-normalization of previously inverted T waves during chest pain may indicate acute ischemia 1

Remember that the ECG is a critical tool in MI diagnosis but should be interpreted in conjunction with clinical presentation and cardiac biomarkers for optimal patient management.