Table of Contents
Distinguishing STEMI vs. Left Bundle Branch Block (LBBB)
Diagnosing ST-elevation myocardial infarction (STEMI) in the presence of left bundle branch block (LBBB) is challenging because LBBB causes secondary ST-T abnormalities that may mimic or obscure acute ischemia.
Historically, clinicians relied on the Sgarbossa criteria, which have excellent specificity but poor sensitivity. Newer methods such as the Smith-modified Sgarbossa rule drastically improve diagnostic accuracy.
This page summarizes:
- ECG changes in LBBB
- Classic Sgarbossa criteria
- Smith-modified Sgarbossa criteria
- Additional principles for identifying “proportional” ST elevation
- Practical guidance and images
Primary References:
Why LBBB Mimics or Obscures STEMI
LBBB causes abnormal ventricular conduction, producing:
- Wide QRS (≥120 ms)
- Deep S waves in V1–V3
- Tall R waves in I, aVL, V5, V6
- Secondary ST-segment discordance (ST elevation opposite the QRS)
- Secondary T-wave inversions
Because discordant ST elevation is a normal feature of LBBB, STEMI cannot be diagnosed using standard 1–2 mm ST elevation criteria.
A new or presumed-new LBBB is NOT, by itself, a STEMI equivalent (AHA 2013 update).
I. Classic Sgarbossa Criteria
The original Sgarbossa criteria identify STEMI in LBBB using concordant (same-direction) or “excessively discordant” ST changes.
| Criterion | Points | Description |
|---|---|---|
| Concordant ST elevation ≥ 1 mm | 5 | ST elevation in the same direction as the QRS complex |
| Concordant ST depression ≥ 1 mm in V1–V3 | 3 | Indicates posterior MI |
| Discordant ST elevation ≥ 5 mm | 2 | ST elevation opposite the QRS |
Interpretation:
- Score ≥ 3 points → Very specific (~98%) for MI
- Sensitivity is low (~20%)
- Best used to “rule in” acute coronary occlusion
Reference: Sgarbossa EB, NEJM 1996
II. Smith-Modified Sgarbossa Criteria (mSgarbossa)
Dr. Stephen Smith introduced a modification to the third Sgarbossa criterion to improve sensitivity while preserving specificity.
The Modified Rule:
- ST-segment elevation or depression that is discordant with the QRS AND has an ST/S ratio ≤ –0.25 (“25% rule”).
- In other words: ST elevation must be more than 25% of the depth of the preceding S wave to be considered abnormal.
This replaces the outdated “≥5 mm” discordant ST elevation rule.
Diagnostic performance:
- Sensitivity: ~70–84%
- Specificity: ~90–99%
- MUCH more accurate than classic Sgarbossa for identifying OMI (occlusion MI)
Reference: Smith SW et al., Ann Emerg Med 2012
III. Additional Criteria and Principles
A. Proportionality is Key
In normal LBBB, discordant ST elevation is proportionate to QRS amplitude. Excessive ST deviation relative to QRS voltage suggests acute STEMI.
Guiding principles:
- Discordant ST elevation is normal only if small (usually < 20–25% of S wave).
- ST elevation that is disproportionate to the QRS is abnormal.
- Concordant ST changes are always abnormal.
B. Concordance Is High-Risk
Any concordant ST elevation or depression is strongly predictive of acute coronary occlusion.
C. Serial ECGs Improve Accuracy
LBBB does not prevent evolution of ischemic changes. Look for:
- Increasing ST elevation
- Dynamic proportionality changes
- Development of concordant depression or elevation
IV. Comparison Table: STEMI vs LBBB Mimic
| Feature | Suggests STEMI | Suggests LBBB (non-ischemic) |
|---|---|---|
| ST elevation | Concordant, or disproportionate (>25%) | Discordant but proportionate (<25%) |
| ST depression V1–V3 | Concordant depression → Posterior MI | Discordant depression common |
| QRS morphology | May be wide but ischemic pattern evolves | Classic LBBB pattern stable |
| T waves | Hyperacute or symmetric | Secondary inversions typical |
| Reciprocal changes | Present | Usually absent |
| Serial ECG changes | Dynamic | Stable over time |
| Sgarbossa/Smith criteria | Often positive | Negative |
V. Applying Sgarbossa and Smith Criteria
A. Concordant ST Elevation (≥1 mm)
- Most specific sign
- Diagnostically powerful even if only in one lead
B. Concordant ST Depression (V1–V3)
- Indicates posterior MI
- Highly specific
C. Excessively Discordant ST Elevation (Smith Rule)
- ST elevation ≥ 25% of S wave depth
- Must measure the S wave peak-to-nadir amplitude
- Works in any lead where QRS is negative
VI. Practical Approach for Clinicians
- Step 1: Confirm LBBB pattern
- Step 2: Look for concordant ST elevation/depression
- Step 3: Apply 25% rule in discordant leads
- Step 4: Assess for reciprocal changes
- Step 5: Compare with prior ECGs
- Step 6: If any Sgarbossa-positive feature is present → treat as OMI
- Step 7: When uncertain, repeat ECGs & integrate clinical picture
VII. Summary Table: Diagnostic Criteria
| Criterion | Cutoff | Meaning |
|---|---|---|
| Concordant ST Elevation | ≥ 1 mm | Strongly suggests acute MI |
| Concordant ST Depression (V1–V3) | ≥ 1 mm | Posterior MI |
| Smith-Modified Discordant ST Elevation | ST/S ratio ≤ –0.25 | High suspicion for MI |
| Classic Discordant Elevation (OLD) | ≥ 5 mm | Low sensitivity; outdated |
| QRS Duration | ≥ 120 ms | Required for diagnosing LBBB |
VIII. Sample Images
LBBB with No MI
Image to be added soon.
LBBB with Occlusion MI (anterior)
Image to be added soon.
Key Takeaways
- LBBB makes STEMI diagnosis difficult because of secondary ST-T abnormalities.
- Concordant ST changes are the most important indicator of acute MI.
- Smith-modified Sgarbossa provides the most accurate modern rule for detecting occlusion MI in LBBB.
- Use proportionality (25% rule) rather than fixed ST elevation cutoffs.
- Compare with prior ECGs and obtain serial tracings whenever possible.
- Positive Sgarbossa or modified criteria → treat as acute coronary occlusion until proven otherwise.