MI Diagnosis in LBBB or paced rhythm

In case of a left bundelbranch block (LBBB), infarct diagnostics based on the ECG is difficult. The baseline ST segments and T waves tend to be shifted in a discordant direction with LBBB, which can mask or mimic acute myocardial infarction. However, serial ECGs may show a moving ST segment during ischemia secondary to dynamic supply versus demand characteristics. A new LBBB is always pathologocal and can be a sign of myocardial infarction. The criteria (Sgarbossa LBTB) that can be used in case of a LBBB and suspicion of infarction are:

• ST elevation > 1mm in leads with a positive QRS complex (concordance in ST deviation) (score 5)
• ST depression > 1 mm in V1-V3 (concordance in ST deviation) (score 3)
• ST elevation > 5 mm in leads with a negative QRS complex (inappropriate discordance in ST deviation) (score 2)

At a score-sum of 3, these criteria have a specificity of 90% for detecting a myocardial infarction.

Examples

• 
  Acute myocardial infarction in in a patient with a pacemaker and LBBB. Concordant ST elevation in V5-V6 are clearly visible. There is discordant ST segment elevation > 5 mm in lead V3.
• 
  The same patient as in the first example 2 months before the myocardial infarction. Normal LBBB pattern.
• 
  Acute MI in a patient with LBBB

References

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1. LBTB pmid=11265742

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