Criteria for diagnosis of ACS in LBBB
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acute coronary syndrome left bundle branch block diagnosis Sgarbossa modified Sgarbossa ACC guideline
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left bundle branch block AND acute myocardial infarction
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https://www.ahajournals.org/doi/abs/10.1161/JAHA.121.023275
ACS diagnosis when LBBB is present
LBBB causes expected secondary ST-T changes, so usual STEMI ST-elevation thresholds cannot be used. New or presumed new LBBB alone is not a STEMI equivalent. Assess the clinical syndrome, serial ECGs, high-sensitivity troponin kinetics, and use Sgarbossa-based criteria to identify acute coronary occlusion.
1. Original Sgarbossa criteria
In a patient with suspected ACS and LBBB, the following are positive:
| ECG feature | Points |
|---|---|
| Concordant ST elevation ≥1 mm in any lead with a predominantly positive QRS | 5 |
| Concordant ST depression ≥1 mm in V1-V3 | 3 |
| Discordant ST elevation ≥5 mm in a lead with a predominantly negative QRS | 2 |
- A score ≥3 is highly specific for acute MI/acute coronary occlusion.
- Concordant changes are the most convincing findings.
- The isolated absolute 5-mm discordant elevation rule has low sensitivity and is less useful in large-voltage LBBB complexes.
Rosen’s Emergency Medicine, pp. 1008-1009; The Washington Manual of Medical Therapeutics, p. 148.
2. Modified Sgarbossa criteria, preferred for occlusion MI
Positive if any one of the following is present:
- Concordant ST elevation ≥1 mm in any lead
- Concordant ST depression ≥1 mm in V1-V3
- Excessively discordant ST elevation, defined as ST elevation relative to the preceding S-wave depth:
[ \frac{\text{ST elevation}}{\text{S-wave amplitude}} \geq 0.25 ] In practice, ST elevation at the J point is at least 25% of the depth of the S wave in a lead with a negative QRS.
The modified criterion replaces the original fixed “≥5 mm discordant STE” rule and improves sensitivity while preserving good specificity. Rosen’s Emergency Medicine, p. 1008.
3. Diagnosis of ACS is not ECG-only
A patient may have NSTEMI or unstable angina despite not meeting Sgarbossa criteria. Diagnose ACS from:
- Symptoms/signs consistent with myocardial ischemia
- Dynamic rise and/or fall of cardiac troponin, with at least one value above the assay’s 99th-percentile upper reference limit, for MI
- Serial ECG changes or comparison with prior ECGs
- New regional wall-motion abnormality on echocardiography when the diagnosis remains uncertain
- Coronary angiographic evidence when indicated
Practical approach
- Obtain serial ECGs and compare with a previous ECG.
- Perform serial high-sensitivity troponins.
- Use urgent bedside echocardiography if uncertainty persists.
- Treat as an acute coronary occlusion requiring immediate reperfusion/catheterization when modified Sgarbossa-positive or when there is persistent ischemic pain, cardiogenic shock, acute pulmonary edema, malignant arrhythmia, or ongoing hemodynamic instability despite a nondiagnostic ECG.
Expected LBBB “appropriate discordance” means ST segments/T waves usually point opposite the main QRS direction. Concordance or disproportionate discordance is abnormal and concerning for infarction.
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