How do I identify a ST‑segment‑elevation myocardial infarction (STEMI)?

How to Identify STEMI

A 12-lead ECG must be obtained and interpreted within 10 minutes of first medical contact in any patient with chest discomfort or symptoms suggestive of myocardial infarction, with STEMI diagnosed when ST-segment elevation meets specific voltage thresholds in 2 or more anatomically contiguous leads. 1, 2

Immediate ECG Acquisition and Timing

• Obtain a 12-lead ECG within 10 minutes of emergency department arrival or first medical contact for all patients with chest discomfort or anginal equivalent symptoms 1
• Initiate continuous ECG monitoring immediately to detect life-threatening arrhythmias 1
• If the initial ECG is non-diagnostic but clinical suspicion remains high, perform serial ECGs at 5-10 minute intervals or use continuous 12-lead ST-segment monitoring 1
• Serial ECGs are critical because 11% of STEMI patients have an initially non-diagnostic ECG, with 72.4% showing diagnostic changes within 90 minutes 3

Standard ST-Segment Elevation Criteria

The diagnosis requires ST-elevation measured at the J-point in ≥2 anatomically contiguous leads with the following voltage thresholds: 2, 3

Lead-Specific Voltage Thresholds

For leads V2-V3:

• Men ≥40 years: ≥2.0 mm (0.2 mV) 2
• Men <40 years: ≥2.5 mm (0.25 mV) 2
• Women: ≥1.5 mm (0.15 mV) 2

For all other leads:

• ≥1.0 mm (0.1 mV) in any age or sex 2, 3

Contiguous Lead Groupings

The elevation must appear in 2 or more anatomically contiguous leads to reduce false positives from normal variants 2:

• Anterior: V1-V6 1
• Inferior: II, III, aVF 1
• Lateral: I, aVL, V5-V6 1
• Septal: V1-V2 1

Special Lead Considerations for Hidden Infarctions

Right Ventricular Infarction

• In patients with inferior STEMI (ST-elevation in II, III, aVF), obtain right-sided ECG leads V3R and V4R 1
• ST-elevation ≥1.0 mm in V3R-V4R indicates right ventricular involvement 2, 3

Posterior Myocardial Infarction

• ST-depression in leads V1-V3 with positive terminal T-waves suggests posterior MI (ST-elevation equivalent) 1, 3
• Confirm by recording posterior leads V7-V9 looking for ST-elevation ≥0.5 mm (0.05 mV) 2, 3

STEMI Equivalents and High-Risk Patterns

Left Bundle Branch Block (LBBB)

• LBBB makes ECG diagnosis difficult but not impossible 1
• Concordant ST-elevation (in leads with positive QRS deflections) is the best indicator of acute occlusion 1
• In patients with LBBB and ongoing ischemic symptoms, consider emergency coronary angiography rather than waiting for definitive ECG criteria 1
• A previous ECG showing old LBBB is helpful; new or presumed new LBBB with ischemic symptoms warrants immediate reperfusion therapy 1

Left Main or Proximal Multivessel Disease

• ST-elevation in aVR with widespread ST-depression (≥8 leads) indicates left main or proximal LAD occlusion 3, 4
• This pattern requires immediate catheterization laboratory activation regardless of troponin results 4

Subtle Occlusion MI (OMI) Without Classic STEMI Criteria

• Hyperacute T-waves (tall, peaked T-waves disproportionate to R-wave amplitude) may precede frank ST-elevation 2, 3
• High T-wave to R-wave ratio in V2-V4 with diminished R-wave amplitude suggests acute LAD occlusion 3
• Reciprocal ST-depression in opposite leads strengthens the diagnosis of acute occlusion 3

Clinical Context Integration

Symptoms Requiring ECG Evaluation

• Typical presentation: Chest pain/pressure radiating to neck, jaw, or left arm 1
• Atypical presentations (30% of STEMI patients): nausea/vomiting, dyspnea, fatigue, palpitations, syncope 1
• Atypical presentations are more common in women, diabetics, and elderly patients 1

Physical Examination Priorities

• Perform a brief focused examination to assess hemodynamic stability, heart failure signs, and mechanical complications 1
• Complete a limited neurological examination before fibrinolytic therapy to identify stroke contraindications 1
• Assess for aortic dissection features (pulse deficits, blood pressure differentials) as this mimics STEMI but contraindicates reperfusion 1

Biomarkers: When and When Not to Wait

Critical principle: For patients with ST-elevation meeting STEMI criteria and ischemic symptoms, initiate reperfusion therapy immediately without waiting for biomarker results. 1

• Cardiac troponins are the optimal biomarkers but should not delay reperfusion therapy 1
• Laboratory examinations should be ordered immediately but therapeutic decisions proceed based on ECG findings 1
• Point-of-care troponin testing may help in ambiguous cases (e.g., LBBB of uncertain age) but should not delay definitive management 1

Common Pitfalls and STEMI Mimics

Conditions That Mimic STEMI

• Early repolarization: Normal variant with ST-elevation, but lacks hyperacute T-waves and reciprocal changes 5
• Left ventricular hypertrophy: Can cause ST-elevation in precordial leads 1
• Ventricular pacing: Prevents ST-segment interpretation; consider reprogramming if patient is not pacemaker-dependent 1
• Pericarditis, myocarditis, Takotsubo cardiomyopathy: Can all produce ST-elevation 6, 7

Key Distinguishing Features

• STEMI typically shows convex upward ST-elevation with reciprocal depression 3
• Pericarditis shows concave upward ST-elevation without reciprocal changes 6
• Clinical correlation is mandatory: ECG findings must align with ischemic symptoms and timing 3

Ancillary Imaging When Diagnosis Is Unclear

• Portable echocardiography is reasonable when STEMI diagnosis is confounded by LBBB, pacing, or suspected posterior MI 1
• Use chest X-ray, echocardiography, CT, or MRI to differentiate STEMI from aortic dissection when this distinction is unclear 1
• Imaging studies should not delay reperfusion therapy unless aortic dissection is suspected 1

Time-Critical Action Points

• Door-to-needle time: ≤30 minutes for fibrinolytic therapy 1
• Door-to-balloon time: ≤90 minutes for primary PCI 1
• When STEMI is identified, activate reperfusion protocols immediately without waiting for biomarkers or additional testing 1, 3