A baseline LBBB on EKG makes detection of myocardial infarction (MI) more difficult but not completely impossible. In 1996, Sgarbossa defined three EKG criteria which, if present in the setting of a known LBBB, increased the likihood of MI. Since the publication of these criteria, there has been some debate regarding their sensitivity and specificity. In a recent meta-analysis by Tabas et al, he concludes that two of the original three Sgarbossa criteria are indeed clinically useful:
- 1 mm of ST elevation that is concordant with the direction of the major QRS
- 1 mm of ST depression in V1 to V3
If either is present in setting of LBBB, positive likelihood ratio of MI is 7.9, specificity 98% and sensitivity 20%.
Source
Tabas et al. "Electrocardiographic Criteria for Detecting Acute Myocardial Infarction in Patients With Left Bundle Branch Block: A Meta-analysis."
Ann Emerg Med. 2008; 52: 329-336.