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.

Examples

References

  1. Sgarbossa EB. Value of the ECG in suspected acute myocardial infarction with left bundle branch block. J Electrocardiol. 2000;33 Suppl:87-92. DOI:10.1054/jelc.2000.20324 | PubMed ID:11265742 | HubMed [LBTB]
  2. Wong CK, French JK, Aylward PE, Stewart RA, Gao W, Armstrong PW, Van De Werf FJ, Simes RJ, Raffel OC, Granger CB, Califf RM, White HD, and HERO-2 Trial Investigators.. Patients with prolonged ischemic chest pain and presumed-new left bundle branch block have heterogeneous outcomes depending on the presence of ST-segment changes. J Am Coll Cardiol. 2005 Jul 5;46(1):29-38. DOI:10.1016/j.jacc.2005.02.084 | PubMed ID:15992631 | HubMed [Wong]

All Medline abstracts: PubMed | HubMed