Which leads on a 12-lead Electrocardiogram (EKG) do you check for a ST-Elevation Myocardial Infarction (STEMI)?

Which Leads to Check for STEMI on a 12-Lead ECG

You must systematically evaluate all 12 standard leads plus consider additional leads (V3R, V4R, V7-V9) based on the clinical presentation to avoid missing STEMI or STEMI-equivalents. 1

Standard 12-Lead Evaluation

Anterior Wall STEMI

• Check leads V1-V6 for ST elevation, with specific thresholds: 1
  • ≥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 V1, V4, V5, V6

Inferior Wall STEMI

• Check leads II, III, and aVF for ST elevation ≥1 mm 1
• ST elevation must be present in at least 2 contiguous leads 1

Lateral Wall STEMI

• Check leads I, aVL, V5, and V6 for ST elevation ≥1 mm 1

High Lateral STEMI

• Check leads I and aVL for ST elevation ≥1 mm 1

Critical Additional Leads (Often Missed)

Right Ventricular Infarction

• Obtain right-sided leads V3R and V4R in all patients with inferior STEMI (leads II, III, aVF) 2
• Look for ST elevation >0.5 mm (or >1 mm in men <30 years) 3
• This is crucial because RV infarction predicts high rates of in-hospital complications and alters management (avoid nitrates, give fluids) 2
• Critical pitfall: ST elevation in right-sided leads disappears much faster than in standard leads, so record V3R/V4R immediately 2

Posterior Wall STEMI (Frequently Hidden)

• Check leads V1-V3 for ST depression with positive terminal T-waves (this is an ST elevation equivalent) 2, 1
• Obtain posterior leads V7-V9 to confirm with ST elevation ≥0.5 mm 2, 1
• Approximately 4% of acute MIs show ST elevation isolated to posterior leads V7-V9 that is "hidden" from the standard 12 leads 2
• This qualifies the patient for immediate reperfusion therapy as a STEMI 2

STEMI-Equivalent Patterns (Not Classic ST Elevation)

Lead aVR Elevation Pattern

• ST elevation in aVR with ST depression in ≥8 other leads suggests left main or proximal LAD occlusion 4
• This is a STEMI-equivalent requiring immediate catheterization 4

Hyperacute T-Waves

• Tall, peaked T-waves may precede ST elevation in early presentation 1, 4
• If clinical suspicion is high, perform serial ECGs at 5-10 minute intervals to detect evolving ST elevation 2

De Winter T-Waves

• Upsloping ST depression at the J-point in precordial leads with tall, symmetric T-waves indicates acute LAD occlusion 4

Special Circumstances That Complicate Interpretation

Left Bundle Branch Block (LBBB)

• LBBB alone is NOT a STEMI equivalent and should not trigger reperfusion in isolation 1, 3
• The 2013 ACC/AHA guidelines removed "new LBBB" as a STEMI criterion because it occurs infrequently 1
• Apply Sgarbossa criteria if LBBB is present: 2
  • ST elevation ≥1 mm concordant with QRS (sensitivity 73%, specificity 92%)
  • ST depression ≥1 mm in V1-V3 (sensitivity 25%, specificity 96%)
  • ST elevation ≥5 mm discordant with QRS (sensitivity 19%, specificity 82%)

Ventricular Paced Rhythm

• Use the same Sgarbossa criteria as for LBBB 2

Systematic Approach Algorithm

1. Measure ST elevation at the J-point (where QRS meets ST segment) using standard calibration (1 mm = 0.1 mV) 1

2. Check all standard leads systematically: 1

   • Anterior: V1-V6
   • Inferior: II, III, aVF
   • Lateral: I, aVL, V5, V6

3. If inferior STEMI is present, immediately obtain V3R and V4R 2

4. If ST depression in V1-V3 with positive T-waves, obtain V7-V9 2, 1

5. Look for reciprocal changes (ST depression in leads opposite to ST elevation) to confirm true STEMI 1

6. If initial ECG is non-diagnostic but suspicion remains high, repeat ECG every 5-10 minutes 2

   • Serial ECGs increase STEMI detection from 84.6% (single ECG) to 100% (three ECGs over 25 minutes) 5

Common Pitfalls to Avoid

• Do not delay reperfusion waiting for biomarkers - treatment decisions are based on ECG and clinical presentation 3
• Do not miss posterior MI by failing to recognize ST depression in V1-V3 as an ST elevation equivalent 2
• Do not forget right-sided leads in inferior STEMI - this changes management significantly 2
• Do not diagnose STEMI based on LBBB alone without applying Sgarbossa criteria 1, 3
• Do not rely on a single ECG - if clinical suspicion is high, obtain serial tracings 2, 5