How to Diagnose STEMI and NSTEMI on ECG
The key to diagnosing STEMI on ECG is identifying ST-segment elevation in at least 2 contiguous leads, while NSTEMI typically shows ST-segment depression, T-wave inversion, or non-specific changes without meeting STEMI criteria.
STEMI Diagnosis on ECG
ST-Elevation Criteria
- ST-segment elevation must be present in at least 2 contiguous leads 1:
- V1-V3: ≥1 mm elevation
- V2-V3: ≥2 mm elevation in men, ≥1.5 mm in women
- V7-V9: ≥0.5 mm elevation
- V3R-V4R: ≥0.1 mV elevation
- Other leads: ≥1 mm elevation
STEMI Equivalents
New left bundle branch block (LBBB) with specific criteria 1:
- ST elevation ≥1 mm concordant with QRS complex
- ST depression ≥1 mm in leads V1-V3
- ST elevation ≥5 mm discordant with QRS complex
Posterior MI (often missed) 1:
- ST depression in leads V1-V3
- Requires posterior lead ECG (V7-V9) to confirm ST elevation
Hyperacute T waves - tall, peaked T waves may be the earliest sign of ischemia before ST elevation appears 1
NSTEMI Diagnosis on ECG
Key ECG Findings
- ST-segment depression (seen in approximately 31% of cases) 2
- T-wave inversion (seen in approximately 12% of cases) 2
- ST-segment depression combined with T-wave inversions (16% of cases) 2
- Normal ECG or non-specific changes (approximately 41% of cases) 2
Important Considerations
- NSTEMI is diagnosed when cardiac biomarkers (troponin) are elevated without ST-segment elevation meeting STEMI criteria 3
- Approximately 30% of NSTEMIs are associated with complete coronary occlusion despite not showing ST elevation 4
- High-sensitivity troponin measurements are the preferred test to evaluate for NSTEMI 2
Diagnostic Algorithm
Obtain 12-lead ECG within 10 minutes of first medical contact for any patient with suspected ACS 3
Evaluate for STEMI criteria:
- ST-segment elevation in ≥2 contiguous leads meeting threshold criteria
- STEMI equivalents (posterior MI, new LBBB with specific criteria)
If STEMI criteria present:
If STEMI criteria absent but ACS suspected:
- Check for ST depression, T-wave inversion, or non-specific changes
- Obtain serial ECGs (every 15-30 minutes if symptoms persist)
- Order cardiac biomarkers (preferably high-sensitivity troponin)
- Diagnose NSTEMI if cardiac biomarkers are elevated 3
Special Considerations
Reciprocal Changes
- Reciprocal ST depression in leads opposite to the area of infarction can help confirm true STEMI 1
- More pronounced reciprocal changes often correlate with larger infarct size 1
Right Ventricular Involvement
- Should be suspected in inferior STEMI with ST elevation in V1 or right-sided leads (V3R-V6R) 1
- Requires right-sided ECG leads to confirm
Challenging Presentations
- Up to 15% of STEMIs are not evident on the first ECG - obtain serial ECGs 1
- Up to 30% of STEMI patients present without chest pain, particularly women and elderly patients 1
- Left circumflex occlusions often have minimal ST changes on standard 12-lead ECG 1
STEMI Mimics
- Conditions that can mimic STEMI include 1:
- Pericarditis (diffuse ST elevation, PR depression)
- Early repolarization (concave ST elevation, prominent J-point)
- Left ventricular hypertrophy
- Brugada syndrome
- Acute pulmonary embolism
Key Pitfalls to Avoid
Failing to obtain serial ECGs when initial ECG is non-diagnostic but symptoms persist
Missing posterior MI by not recognizing ST depression in V1-V3 as a potential posterior STEMI equivalent
Overlooking subtle STEMI in patients with atypical presentations (women, elderly, diabetics)
Delaying treatment while waiting for cardiac biomarkers in clear STEMI cases
Not recognizing high-risk NSTEMI patients who may benefit from urgent angiography despite lack of ST elevation 5, 4
By following this systematic approach to ECG interpretation, clinicians can rapidly and accurately diagnose STEMI and NSTEMI, leading to appropriate and timely treatment decisions that improve patient outcomes.