How can early repolarization be distinguished from acute ST‑segment elevation myocardial infarction using ECG morphology, clinical presentation, and cardiac biomarkers?

Differentiating Early Repolarization from ST-Elevation Myocardial Infarction

The most reliable way to distinguish early repolarization from acute STEMI is to identify reciprocal ST depression in electrically opposite leads—present in 83-100% of STEMI cases but absent in early repolarization—combined with assessment of ST-segment morphology, terminal QRS characteristics, and clinical context. 1

Key Discriminating ECG Features

Reciprocal ST Depression (Most Specific Finding)

• In anterior ST elevation: ST depression ≥0.025 mV in lead II occurs in 40% of STEMI but in 0% of early repolarization cases 1
• In inferior ST elevation: ST depression ≥0.025 mV in lead I is present in 83% of STEMI but in 0% of early repolarization cases 1
• Reciprocal changes indicate a larger area of myocardium at risk and confirm acute coronary occlusion rather than a benign variant 2

ST-Segment Morphology

• Early repolarization: Upward concave ST segments with rapid upsloping morphology in 95% of cases 3, 4
• STEMI: Convex (upward bowing) or horizontal ST elevation occurs in 22% of STEMI versus only 9% of early repolarization 1
• The rapidly ascending ST-segment pattern is consistently benign, while horizontal or descending ST segments suggest ischemia 3

Terminal QRS Complex Characteristics

• Early repolarization: Terminal QRS notching or slurring at the end of the QRS complex, with the J-point measured at the peak of the notch or onset of slur 3, 4
• STEMI: Terminal QRS distortion is present in 40% of STEMI cases but only 7% of early repolarization 1
• J-wave notching in early repolarization appears as a low-frequency deflection at the QRS terminus, distinct from the injury current of STEMI 3

Quantitative ECG Measurements for Anterior ST Elevation

Use this validated equation to differentiate subtle anterior STEMI from early repolarization: 5

(1.196 × ST elevation 60ms after J-point in V3) + (0.059 × QTc) - (0.326 × R-wave amplitude in V4)

• If >23.4: Predicts STEMI (sensitivity 86%, specificity 91%, positive likelihood ratio 9.2)
• If ≤23.4: Predicts early repolarization 5

Key components:

• R-wave amplitude in V4: Lower in STEMI, higher in early repolarization 5
• QTc interval: Significantly longer in STEMI versus early repolarization 5
• ST elevation 60ms after J-point in V3: Greater in STEMI 5

PR Segment Depression

• Chest-lead PR depression: Uncommon in STEMI (12%) but present in 38% of non-ischemic ST elevation including pericarditis 1
• PR depression in aVR is associated with non-ischemic diagnosis 1

T-Wave Characteristics

• Early repolarization: Tall, peaked, slightly asymmetrical T waves with large amplitude, concordant with ST elevation 6, 4
• STEMI: T-wave amplitude does not significantly differ, but the T/R ratio is higher due to lower R-wave amplitude 5

Distribution and Lead Patterns

Early Repolarization Distribution

• Most commonly in precordial leads V3-V4, but can occur in lateral (V5-V6, I, aVL) or inferior (II, III, aVF) leads 3
• Diffuse or widespread ST elevation across multiple territories 4
• Reciprocal ST depression in aVR only 6
• Prevalence 50-80% in highly trained athletes, 1-10% in general population 3

STEMI Distribution

• Localized to anatomically contiguous leads corresponding to coronary artery territories 3
• Anterior STEMI: V1-V4 with reciprocal depression in inferior leads 2
• Inferior STEMI: II, III, aVF with reciprocal depression in I, aVL 2
• Posterior STEMI: ST depression V1-V3 with upright terminal T waves (requires posterior leads V7-V9 for confirmation) 2

Clinical Context and Dynamic Changes

Patient Demographics

• Early repolarization: Male predominance, younger than 50 years, often athletes, all races equally 6, 3
• STEMI: Any age, presence of cardiac risk factors and ischemic symptoms 2

Response to Physiologic Maneuvers

• Early repolarization: ST elevation normalizes with exercise or isoproterenol administration 6
• STEMI: ST elevation persists or worsens with ongoing ischemia 2

Serial ECG Changes

• Early repolarization: Stable pattern on serial ECGs, no evolution over time 6
• STEMI: Dynamic changes with evolution—hyperacute T waves → ST elevation → T-wave inversion → Q-wave development 2
• Obtain serial ECGs at 15-30 minute intervals if initial presentation is unclear 2, 1

Cardiac Biomarkers

• Troponin elevation confirms myocardial necrosis in STEMI but is absent in early repolarization 7
• Critical caveat: Do not wait for troponin results to initiate reperfusion therapy if ECG shows STEMI 2
• Troponins may not rise for 3-6 hours after symptom onset—serial measurements are essential 8

Accompanying ECG Features

Early Repolarization Pattern

• Vertical QRS axis 6
• Shorter and depressed PR interval 6
• Abrupt precordial transition with counterclockwise rotation 6
• Presence of U waves 6
• Sinus bradycardia 6

STEMI Pattern

• Hyperacute T waves as earliest sign, preceding ST elevation 2
• Increased R-wave amplitude and width in leads with ST elevation 2
• Development of pathological Q waves (≥0.03 sec, ≥0.1 mV deep in ≥2 contiguous leads) 2

Common Pitfalls to Avoid

• Do not dismiss widespread ST elevation as "just early repolarization" without checking for reciprocal changes—absence of reciprocal depression strongly favors early repolarization 1
• Beware of posterior STEMI mimicking early repolarization—ST depression in V1-V3 with upright T waves requires posterior leads V7-V9 2
• Athletes with early repolarization may have horizontal ST segments—this does not indicate high risk in asymptomatic individuals (risk only 1 in 3000) 3
• Early repolarization in African/Caribbean athletes may include ST elevation with T-wave inversion in V2-V4—this is a physiological variant 3
• Compare with prior ECGs when available—this dramatically improves diagnostic accuracy 2, 8

Immediate Management Algorithm

1. Assess for reciprocal ST depression in electrically opposite leads—if present, activate STEMI protocol immediately 1
2. Evaluate ST-segment morphology—concave upsloping favors early repolarization; convex or horizontal favors STEMI 3, 1
3. Check for terminal QRS notching/slurring—if present with tall T waves and no reciprocal changes, consider early repolarization 3, 4
4. For anterior ST elevation, calculate the validated equation using R-wave V4, ST elevation 60ms after J-point in V3, and QTc 5
5. Obtain serial ECGs at 15-30 minute intervals if diagnosis remains uncertain 2, 1
6. Measure cardiac biomarkers immediately but do not delay reperfusion therapy if STEMI is suspected 2, 7
7. Consider additional leads—posterior V7-V9 for suspected circumflex occlusion, right-sided V3R-V4R for inferior MI with possible RV involvement 2, 7