What are the characteristics of a STEMI (ST-Elevation Myocardial Infarction) ECG (Electrocardiogram)?

STEMI ECG Characteristics

A STEMI ECG is diagnosed by new ST-segment elevation measured at the J-point in at least 2 contiguous leads: ≥2 mm (0.2 mV) in men ≥40 years or ≥1.5 mm (0.15 mV) in women in leads V2-V3, and ≥1 mm (0.1 mV) in all other contiguous leads. 1, 2

Standard Diagnostic Criteria

ST-Segment Elevation Thresholds:

• Leads V2-V3: ≥2.5 mm in men <40 years, ≥2 mm in men ≥40 years, ≥1.5 mm in women 1, 2
• All other leads: ≥1 mm elevation required 1, 2
• Measurement point: At the J-point where QRS complex meets ST segment 1, 2
• Contiguity requirement: Must appear in ≥2 anatomically contiguous leads 1, 2

Contiguous Lead Groupings:

• Anterior: V1-V6 2
• Inferior: II, III, aVF 2
• Lateral: I, aVL, V5-V6 2
• Additional pairs: I and aVL; aVF and III; I and -aVR; -aVR and II 3

Key ECG Features of Acute STEMI

Primary Changes:

• Convex upward ST elevation (tombstone pattern) in affected leads 2
• Hyperacute T-waves may precede ST elevation in very early presentation 1, 2
• Preserved or diminishing R-wave amplitude in acute phase 2

Reciprocal Changes:

• ST depression in opposite leads significantly increases diagnostic accuracy 2, 4
• Reciprocal changes improve positive predictive value to 93-95% 4

STEMI Equivalents and Special Patterns

Posterior MI:

• ST depression in V1-V3 with positive terminal T-waves indicates posterior wall involvement 1, 2
• Confirm with ST elevation ≥0.5 mm in posterior leads V7-V9 2, 5

Right Ventricular Infarction:

• Record right-sided leads V3R and V4R in all inferior STEMIs 2, 5
• ST elevation >0.5 mm in V4R confirms RV involvement 5

Left Main or Proximal LAD Occlusion:

• Multilead ST depression with ST elevation in aVR suggests left main or proximal LAD disease 1

Critical Pitfalls to Avoid

Left Bundle Branch Block (LBBB):

• New or presumably new LBBB is NOT a STEMI equivalent and should not trigger reperfusion in isolation 1, 5
• The 2013 ACCF/AHA guidelines removed LBBB as automatic STEMI equivalent 1, 2
• Use concordant ST elevation (ST elevation in leads with positive QRS) if LBBB present 5

Left Ventricular Hypertrophy (LVH):

• Standard STEMI criteria do not apply to patients with LVH 5
• Compare with prior ECGs to identify baseline repolarization abnormalities 5
• Look for dynamic ST changes on serial ECGs during ongoing symptoms 5

Non-Ischemic ST Elevation Mimics:

• Left ventricular hypertrophy (33% of false positives) 4
• Left bundle branch block (21% of false positives) 4
• Pericarditis, Brugada syndrome, early repolarization 1, 6

Clinical Application Algorithm

Step 1 - Initial Assessment:

• Obtain 12-lead ECG within 10 minutes of presentation 2
• Measure ST elevation at J-point using standard calibration (10 mm/mV) 2
• Identify ≥2 contiguous leads meeting voltage criteria 1, 2

Step 2 - Look for Reciprocal Changes:

• Presence of reciprocal ST depression increases positive predictive value to >90% 4
• Absence of reciprocals does not exclude STEMI 4

Step 3 - Extend Lead Placement When Indicated:

• Inferior STEMI: Add V3R, V4R to detect RV involvement 2, 5
• Suspected posterior MI: Add V7-V9 if ST depression in V1-V3 2, 5

Step 4 - Serial ECGs if Initial Non-Diagnostic:

• 72.4% of STEMIs show diagnostic changes within 90 minutes if initial ECG equivocal 2
• Repeat ECG every 15-30 minutes during ongoing symptoms 2

Step 5 - Consider Echocardiography:

• Use transthoracic echo to identify focal wall motion abnormalities when ECG equivocal 1, 5

When to Activate Reperfusion Despite Atypical ECG

Proceed directly to catheterization if:

• High clinical suspicion with prolonged chest pain >20 minutes unresponsive to nitroglycerin 5
• Dynamic ST changes on serial ECGs during symptoms 5
• Posterior MI pattern (ST depression V1-V3 with positive terminal T-waves) 1, 2
• Hyperacute T-waves in appropriate clinical context before ST elevation develops 1, 2

Do not delay reperfusion for biomarkers when ECG meets STEMI criteria with compatible symptoms 2, 5