Know what a normal LBBB “looks” like:

1) QRS duration greater than 120 ms
2) Negative QRS Complex in V1
3) Positive QRS Complex in lateral leads (I, aVL, V5-V6)

LBBB causes a repolarization abnormality:

Consider a “repol” abnormality when there is a “general pattern of ST discordance”, meaning the ST segment opposite the QRS in nearly every lead (can be caused by LVH, LBBB, WPW, etc.).

In a LBBB there is normally ST elevation in some leads at baseline.
2013 AHA STEMI Guidelines:

“New or presumably new LBBB has been considered a STEMI equivalent. Most cases of LBBB at time of presentation, however, are “not known to be old” because of prior electrocardiogram (ECG) is not available for comparison. New or presumably new LBBB at presentation occurs infrequently, may interfere with ST-elevation analysis, and should not be considered diagnostic of acute myocardial infarction (MI) in isolation”.

New or presumed new LBBB does not predict an MI.

MI occurs at similar frequencies between patients with a new LBBB, an old LBBB, and patients without a LBBB.

Patients with a LBBB frequently have an unequivocal STEMI diagnosis go unrecognized because clinicians aren’t familiar with how to diagnose an MI in this setting.

Criteria for diagnosing STEMI in a LBBB

Standard Sgarbossa Criteria
1) ST-segment elevation ≥1 mm concordant with the QRS complex in any lead (5 points)
2) ST-segment depression ≥1 mm in lead V1, V2, or V3 (3 points)
3) ST-segment elevation ≥5 mm discordant with the QRS complex in any lead (2 points)

Smith Modified Sgarbossa

• ≥ 1 lead with ≥1 mm of concordant ST elevation
• ≥ 1 lead of V1-V3 with ≥ 1 mm of concordant ST depression
• ≥ 1 lead anywhere with ≥ 1 mm STE and proportionally excessive discordant STE, as defined by ≥ 25% of the depth of the preceding S-wave.