What ECG changes require STEMI (ST-Elevation Myocardial Infarction) activation in patients presenting with symptoms suggestive of acute coronary syndrome?

ECG Changes Requiring STEMI Activation

STEMI activation should be triggered for new or presumed new ST-elevation ≥1 mm in ≥2 anatomically contiguous leads (measured at the J-point) in all leads except V2-V3, where the threshold is ≥2 mm in men ≥40 years, ≥2.5 mm in men <40 years, and ≥1.5 mm in women regardless of age. 1

Standard STEMI Criteria

The 2025 ACC/AHA/ACEP/NAEMSP/SCAI guidelines provide precise thresholds for STEMI activation: 1

• ST-elevation ≥1 mm in ≥2 contiguous leads for all leads except V2-V3
• In leads V2-V3 specifically:
  • Men ≥40 years: ≥2 mm elevation
  • Men <40 years: ≥2.5 mm elevation
  • Women (all ages): ≥1.5 mm elevation

The ECG must be obtained and interpreted within 10 minutes of first medical contact to identify STEMI patients requiring immediate reperfusion therapy. 1

Additional ECG Patterns Requiring STEMI Activation

Posterior Wall STEMI

Obtain posterior leads (V7-V9) in patients with suspected left circumflex occlusion, particularly when isolated ST-segment depression ≥0.5 mm is present in leads V1-V3. 1 This represents a posterior STEMI equivalent and requires immediate catheterization laboratory activation.

Hyperacute T Waves

Ongoing chest pain with hyperacute T waves in a territorial distribution requires immediate STEMI protocol activation and emergency coronary angiography with primary PCI. 2 These tall, peaked T waves represent the earliest ECG manifestation of acute coronary occlusion and may precede frank ST-elevation by minutes. Serial ECGs every 5-10 minutes should be performed to monitor for evolution to ST-elevation. 2

Wellens Syndrome

Patients with Wellens syndrome (deeply inverted or biphasic T waves in precordial leads) should be considered high-risk NSTE-ACS requiring urgent evaluation and primary PCI within 120 minutes. 3 This pattern indicates critical proximal LAD stenosis and represents a pre-infarction state, though technically classified as NSTE-ACS rather than STEMI. 3

Critical Timing Requirements

Once STEMI is identified, immediate EMS transport to a PCI-capable hospital with early advance notification and cardiac catheterization team activation is mandatory, with a system goal of first medical contact-to-device time ≤90 minutes. 1

Important Caveats and Pitfalls

New or Presumably New LBBB

A new LBBB at presentation should NOT be considered diagnostic of acute MI in isolation; clinical correlation is required. 1 A new LBBB in an asymptomatic patient does not constitute a STEMI equivalent. 1 This represents a significant change from older guidelines that treated new LBBB as automatic STEMI activation criteria.

STEMI Mimics to Consider

ST-segment elevation may be observed in several non-ischemic conditions that should not trigger STEMI activation without appropriate clinical context: 1

• Acute pericarditis (diffuse ST elevation with PR depression)
• Left ventricular hypertrophy with strain pattern
• Brugada syndrome (coved ST elevation in V1-V3)
• Right ventricular pacing
• Takotsubo syndrome
• Early repolarization (benign variant with J-point elevation)

The key differentiating factor is clinical presentation—symptoms consistent with myocardial ischemia must be present. 1, 4

Serial ECGs Are Essential

In patients with suspected ACS where the initial ECG is nondiagnostic of STEMI, serial ECGs should be performed to detect potential ischemic changes, especially when clinical suspicion is high, symptoms persist, or the clinical condition deteriorates. 1 Approximately 5% of patients with normal initial ECGs are ultimately found to have acute MI or unstable angina. 3

Practical Algorithm for STEMI Activation

1. Obtain 12-lead ECG within 10 minutes of patient contact 1
2. Measure ST-elevation at the J-point in all leads 1
3. Apply sex- and age-specific criteria for V2-V3 leads 1
4. If isolated ST-depression in V1-V3, obtain posterior leads V7-V9 1
5. If hyperacute T waves with ongoing symptoms, activate STEMI protocol 2
6. If new LBBB, do NOT activate based on ECG alone—require clinical correlation 1
7. Activate catheterization laboratory immediately when STEMI criteria met with compatible symptoms 1
8. Target door-to-balloon time ≤90 minutes 1, 4

The combination of ECG criteria meeting STEMI thresholds plus symptoms suggestive of myocardial ischemia is required for activation—neither alone is sufficient. 1, 4