The diagnosis of myocardial Infarction in RBBB
RBBB may occasionally produce false positive signs for MI, see above. However, because the abnormality of depolarization in RBBB primarily affects the terminal portion of the QRS complex, it does not usually confound the diagnosis of MI.

Criteria for the diagnosis of myocardial infarction in LBBB
Sgarbossa reported recent criteria (shown below) which performed well in the diagnosis of impending MI in LBBB. When tested in the clinical setting in patients who presented with acute cardiactype pain or acute CHF, or who were resuscitated from cardiac arrest, Shlipak et al. found that these criteria were very insensitive (3%). They thought that the clinical utility of ECG criteria for the diagnosis of MI in these patients with LBBB was very limited, and they suggested that the decision to treat was best made using nonelectrocardiographic criteria.

• Criteria for impending MI in LBBB:
  1. ST elevation > 1 mm in leads with dominant R waves [odds ratio 25.2]
  2. ST elevation > 5 mm in leads with dominant S waves [odds ratio 4.3]
  3. ST depression > 1 mm in V1, V2 OR V3 [odds ratio 6.0]

• Criteria for completed MI in LBBB:
  1. Sign of Cabrera (notching > 50 msec duration in the ascending limb of the S wave in V3 or V4)
  2. Pathologic Q waves (> 30 msec in leads I, AVL, V6, or leads II or AVF)
  3. RV1 > 3 mm with QV6.

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