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:

• Sgarbossa EB et al., NEJM 1996
• Smith SW et al., Ann Emerg Med 2012
• LITFL LBBB ECG Library
• ACC/AHA STEMI Guidelines

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).

The original Sgarbossa criteria identify STEMI in LBBB using concordant (same-direction) or “excessively discordant” ST changes.

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

Dr. Stephen Smith introduced a modification to the third Sgarbossa criterion to improve sensitivity while preserving specificity.

• 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This replaces the outdated “≥5 mm” discordant ST elevation rule.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

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.

Any concordant ST elevation or depression is strongly predictive of acute coronary occlusion.

LBBB does not prevent evolution of ischemic changes. Look for:

• Increasing ST elevation
• Dynamic proportionality changes
• Development of concordant depression or elevation

• Most specific sign
• Diagnostically powerful even if only in one lead

• Indicates posterior MI
• Highly specific

• ST elevation ≥ 25% of S wave depth
• Must measure the S wave peak-to-nadir amplitude
• Works in any lead where QRS is negative

• 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

Image to be added soon.

Image to be added soon.

• 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.