STEMI Mimics: Part 2, Left Bundle Branch Block

STEMI Mimics: Part 2, Left Bundle Branch Block Posted By:
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Left bundle branch block (LBBB) is the second most common STEMI mimic. The ECG pattern includes an elevated ST segment at baseline, making it impossible to use the standard STEMI criteria for ST-segment elevation. This is further complicated because an LBBB itself is a risk factor for cardiac death and any new LBBB can be the presenting ECG pattern in a proximal left anterior descending coronary artery (LAD) occlusion. Thus, a new LBBB ECG pattern in patients with a compatible clinical scenario is considered a STEMI equivalent.

LBBBs are characterized by a QRS duration greater than 120 ms with features suggestive of depolarization from the right to the left ventricle. Additionally, there will be a dominant Q or predominant S in V1-V3 and a broad, dominant R wave in the lateral leads (I, aVL, V5, V6). It has traditionally been taught that an acute myocardial infarction could not be diagnosed in the presence of a LBBB. However, in 1996, Sgarbossa described some ECG changes seen in patients with LBBB and developed "the Sgarbossa criteria." The criteria include:

  1. Concordant ST elevation ≥1 mm in any single lead
  2. Concordant ST depression ≥1 mm in at least 1 of leads V1-V3
  3. Proportionally excessive discordant ST elevation in at least 1 lead, as defined by a ratio of ST elevation at the J-point relative to the depth of the S wave (ST/S ratio) of ≥25% (this has replaced the original third criterion of ST elevation, which was an absolute number [≥5 mm])

Cabrera's sign (notching at 40 ms in the upslope of the S wave in leads V3 and V4) and Chapman's sign (a notch in the upslope of the R wave in lead I, aVL, or V6) have also been used to diagnose acute myocardial infarction in the setting of an LBBB, but both have poor sensitivity. Examining the T wave in leads V5 to V6 can be helpful.

ECG noting LBBB

Image courtesy of Benjamin Taylor.
Patient was initially diagnosed with STEMI. Cardiac catheter negative. Later, diagnosed with hyperkalemia and LBBB. Note the predominant S in V1 (blue arrows) and a broad, dominant R wave in the lateral leads (red arrows).
References
  • Sgarbossa EB, at al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. GUSTO-1 (Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries) Investigators. New Engl J Med. 1996;334:481.

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Filed under: Cardiometabolic

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