MI Diagnosis in LBBB or paced rhythm

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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 [1]) 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.[2]

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

During right ventricular pacing the ECG also shows left bundle brach block and the above rules also apply for the diagnosis of myocardial infarction during pacing.[3][4]

Examples

References

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  1. Error fetching PMID 8559200: [LBTB]
  2. Error fetching PMID 15992631: [Wong]
  3. Error fetching PMID 18651461: [3]
  4. Error fetching PMID 15127382: [4]

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