Diagnosing STEMI in the Presence of LBBB

Know what a normal LBBB “looks” like

  • QRS duration of > 120 ms
  • Negative QRS Complex in V1
  • 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 leads with a negative QRS 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 Criteria

Smith Modification



 Excessive Discordance in Patient with a Paced Rhythm

Paced Rhythm_STEMIThere’s obvious discordant ST elevation in the inferior leads (>50%). Although the computer cropped the S wave in V1, there’s probably excessive discordance and a “straight” ST segment indicating a probable right ventricular infarction.


 Excessive Discordance in a LBBB

80 yo male cp EKG 1 excessive discordanceAgain, obvious ST elevation this time noted in the anterior leads. Clearly excessively discordant and ST/S almost more than 50%. This was not appreciated the transporting Paramedic or ER Physician!

Time for the podcast!





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1 thought on “Diagnosing STEMI in the Presence of LBBB”

  1. I’m so very glad to see this coming to life! Keep it up and keep giving providers another solid venue for practice discussion!

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