What are the expected ECG (electrocardiogram) changes in a patient with a suspected STEMI (ST-Elevation Myocardial Infarction)?

Expected ECG Changes in STEMI

ST-segment elevation in at least 2 contiguous leads is the hallmark ECG finding in STEMI, with specific threshold criteria varying by lead location and patient gender. 1, 2

Diagnostic Criteria for STEMI

ST-Segment Elevation Thresholds

• Men:
  • ≥2 mm (0.2 mV) in leads V2-V3
  • ≥1 mm (0.1 mV) in other contiguous chest or limb leads
• Women:
  • ≥1.5 mm (0.15 mV) in leads V2-V3
  • ≥1 mm (0.1 mV) in other contiguous chest or limb leads 1, 2

Lead-Specific Considerations

• Posterior MI: ST elevation ≥0.5 mm in posterior leads V7-V9 1, 2
• Right Ventricular MI: ST elevation ≥0.1 mV in right precordial leads V3R-V4R 1, 2

Additional ECG Findings in STEMI

Early Changes

• Hyperacute T waves (tall, peaked T waves) - often the earliest sign of acute ischemia 2, 3
• ST depression in reciprocal leads - helps confirm STEMI diagnosis 2

Evolving Changes

• Development of pathological Q waves (≥0.03 sec duration and ≥0.1 mV depth) 2
• T-wave inversion following ST-segment elevation 1

Location-Specific ECG Patterns

Anterior STEMI

• ST elevation in V1-V6, I, aVL
• Reciprocal ST depression in inferior leads (II, III, aVF)
• Caused by left anterior descending artery occlusion 4

Inferior STEMI

• ST elevation in leads II, III, aVF
• Reciprocal ST depression in leads I, aVL
• Usually caused by right coronary artery occlusion (85-90%) or left circumflex occlusion (10-15%) 4, 5
• Right ventricular involvement: Additional ST elevation in V1 or right-sided leads V3R-V6R 2

Lateral STEMI

• ST elevation in leads I, aVL, V5-V6
• Usually caused by left circumflex or diagonal branch occlusion 2

Posterior STEMI

• ST depression in V1-V3 (reciprocal changes)
• Tall R waves in V1-V2
• ST elevation in posterior leads V7-V9 if recorded 2, 4

Special Considerations

STEMI Equivalents

• Left main or multivessel disease: ST elevation in aVR with widespread ST depression in ≥8 leads 3
• Posterior MI: ST depression in V1-V3 with tall R waves 2, 3

Bundle Branch Block

• New left bundle branch block (LBBB) is no longer considered diagnostic of STEMI in isolation 1
• In LBBB, consider STEMI with:
  • ST elevation ≥1 mm concordant with QRS complex
  • ST depression ≥1 mm in leads V1-V3
  • ST elevation ≥5 mm discordant with QRS complex 1

Clinical Pitfalls

1. Delayed Presentation: Serial ECGs are crucial as approximately 15% of STEMIs are not evident on the first ECG 6

2. Atypical Presentations: Up to 30% of STEMI patients present without chest pain, particularly women and elderly patients 2, 7

3. Left Circumflex Occlusions: Often have minimal ST changes on standard 12-lead ECG, leading to missed diagnoses 4

4. False Positives: Conditions that can mimic STEMI include pericarditis, early repolarization, left ventricular hypertrophy, and Brugada syndrome 2

5. False Negatives: Consider additional ECG leads (right-sided, posterior) when clinical suspicion is high but standard ECG is non-diagnostic 1, 4

Remember that time is critical in STEMI management - a 12-lead ECG should be obtained and interpreted within 10 minutes of first medical contact to identify patients requiring immediate reperfusion therapy 1.