How do you differentiate between a recent and old ST-Elevation Myocardial Infarction (STEMI) on a 12-lead Electrocardiogram (ECG)?

Differentiating Recent vs. Old STEMI on 12-Lead ECG

The key to distinguishing recent from old STEMI on ECG is the evolutionary pattern: acute STEMI shows ST-segment elevation with hyperacute T-waves and preserved R-waves, while old MI demonstrates pathologic Q-waves, normalized or inverted T-waves, and persistent but diminished ST elevation (if present), often with loss of R-wave amplitude.

Clinical Context is Essential

While the question focuses on ECG findings alone, clinical correlation with symptoms and timing is mandatory because ECG changes evolve dynamically over hours to days 1. The 2025 ACC/AHA guidelines emphasize that a working diagnosis of STEMI requires both symptoms consistent with myocardial ischemia (persistent chest pain) AND electrocardiographic signs 1. Serial ECGs are critical—11% of patients ultimately diagnosed with STEMI had an initial nondiagnostic ECG, with 72.4% showing diagnostic changes within 90 minutes 1.

ECG Features of Acute/Recent STEMI

Hyperacute Phase (Minutes to Hours)

• Hyperacute (peaked) T-waves appear before ST elevation develops 2
• ST-segment elevation measured at the J-point in ≥2 contiguous leads 1, 2:
  • ≥2.5 mm in men <40 years in V2-V3
  • ≥2 mm in men ≥40 years in V2-V3
  • ≥1.5 mm in women in V2-V3
  • ≥1 mm in all other leads 1, 3, 2
• Preserved or increased R-wave amplitude 1
• Reciprocal ST-depression in opposite leads (e.g., inferior STEMI with depression in I, aVL) 1

Evolving Phase (Hours to Days)

• Progressive ST elevation with convex upward morphology 1
• T-wave inversion begins as ST segments start to normalize 1
• Q-waves start developing (typically after 6-12 hours) 1
• R-wave amplitude begins decreasing 1

ECG Features of Old/Healed MI

Chronic Phase (Weeks to Permanent)

• Pathologic Q-waves (≥0.04 seconds wide or >25% of R-wave height) persist indefinitely 1
• Loss of R-wave amplitude or absent R-waves in affected leads 1
• Normalized or persistently inverted T-waves (symmetric, deep) 1
• ST segments return to baseline or show minimal persistent elevation (<1-2 mm) 1
• Possible ventricular aneurysm pattern: persistent ST elevation (weeks to months after MI) with deep Q-waves and inverted T-waves 1

Key Distinguishing Algorithm

Step 1: Assess ST-Segment Morphology

• Acute: Convex upward (tombstone) ST elevation with sharp angles 1
• Old: Minimal or absent ST elevation; if present, usually <1-2 mm with rounded contours 1

Step 2: Evaluate T-Wave Configuration

• Acute: Hyperacute (tall, peaked) or upright T-waves in leads with ST elevation 2
• Old: Inverted or flattened T-waves (symmetric inversion suggests prior MI) 1

Step 3: Examine Q-Waves

• Acute: Absent initially, or small/developing Q-waves 1
• Old: Deep, wide pathologic Q-waves (≥0.04 sec or >25% R-wave) 1

Step 4: Check R-Wave Progression

• Acute: Preserved or increased R-wave amplitude 1
• Old: Diminished or absent R-waves (poor R-wave progression in anterior MI) 1

Step 5: Compare with Prior ECGs

• Critical step: The 2025 ACC/AHA guidelines mandate comparing nondiagnostic ECGs to prior tracings 1
• New changes indicate acute process; stable findings suggest old MI 1

Critical Pitfalls to Avoid

Dynamic Nature of ECG Changes

Serial ECGs are essential because 15.4% of STEMIs are missed on the first prehospital ECG, with diagnosis made on second (9.2%) or third (6.2%) tracings within 25 minutes 4. Never rely on a single ECG to exclude acute STEMI 1, 4.

STEMI Mimics

Multiple conditions can produce ST elevation mimicking acute MI 5, 6:

• Left ventricular hypertrophy with strain pattern
• Early repolarization (common in young patients)
• Pericarditis (diffuse ST elevation with PR depression)
• Takotsubo cardiomyopathy
• Ventricular aneurysm (persistent ST elevation from old MI)

The European Society of Cardiology notes that experienced cardiologists show wide variation (sensitivity 50-100%, specificity 73-97%) in differentiating STEMI from non-ischemic ST elevation 5.

Special Considerations

Posterior MI: ST depression in V1-V3 with positive terminal T-waves may represent posterior STEMI equivalent—confirm with ≥0.5 mm ST elevation in posterior leads V7-V9 1, 3, 2.

Right ventricular involvement: In inferior STEMI, obtain right-sided leads (V3R, V4R) to identify RV infarction (ST elevation >0.5 mm) 1, 3, 2.

Left bundle branch block: The 2013 ACCF/AHA guidelines removed new LBBB as a STEMI equivalent because it occurs infrequently and interferes with ST-segment analysis 3, 2. LBBB should not be considered diagnostic in isolation 3, 2.

Time-Sensitive Decision Making

If ST elevation is present with compatible symptoms, assume acute STEMI and activate reperfusion protocols immediately—do not delay for biomarkers 1. The goal is ECG acquisition and interpretation within 10 minutes of first medical contact 1. Complete ST normalization after nitroglycerin suggests coronary spasm requiring early angiography within 24 hours, not necessarily exclusion of MI 1, 3.