MI Diagnosis in LBBB or paced rhythm

Changes in LBBB during ischemia

In case of a left bundle branch block (LBBB), infarct diagnosis 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 pathological 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). This criterium is sensitive, but not specific for ischemia in LBBB. It is however associated with a worse prognosis, when present in LBBB during ischemia.Wong

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

Examples

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  Case 1: 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.
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  Case 1: The same patient as in the first example 2 months before the myocardial infarction. Normal LBBB pattern.
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  Case 2: Acute MI in a patient with LBBB
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  Case 3: Acute MI in a patient with LBBB
• 
  Case 3: Non-ischemic ECG in this patient

References

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1. LBTB pmid=8559200
2. Wong pmid=15992631

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