In a patient presumed to be having an acute myocardial infarction (AMI), finding a left bundle branch block (LBBB) or continued ventricular paced beats on the electrocardiogram has historically challenged the clinician. This is because LBBB can mask or mimic the classic changes of AMI. This can result in both false positive and false negative 12 lead EKG interpretations for AMI in this population. Elena B. Sgarbossa et al tested EKG criteria for the diagnosis of AMI in patients with left bundle-branch block and confirmed AMI by cardiac enzymes in patients enrolled in the GUSTO-1 trial. They found that ST-segment deviation can be useful in the diagnosis of AMI in the presence of LBBB when specific criteria was applied to the tracing.
The ST changes that were significant for AMI are: (only one needs to be present)
1. ST elevation > or = 1 mm and concordant with QRS.
2. ST depression > or = 1 mm in V1, V2 or V3.
3. ST elevation > or = 5 mm and discordant with QRS.
Elena.B.Sgarbossa et al; New England Journal of Medicine, Volume 334; Number 8, February 22, 1996.