Identifying a ST Elevation Myocardial Infarction (STEMI) in an ECG with a Left Bundle Branch Block(LBBB) can be challenging. We do know that the number of patients that come in with LBBB and chest pain, that are having a STEMI is very low (J Am Coll Card2012;60:96-105). It makes it even more important to try and determine if the ECG is ischaemic in these patients.
A clever woman named Sgarbossa developed some rules that allowed us to identify an infarct in the presence of LBBB (NEJM 1996;334:481-487). A points score was assigned to each and it was found that greater than 3 points were needed to diagnose an acute myocardial infarction in LBBB.
The rules included:
- Concordance with ST elevation > 1mm (5 points)
- ST depression in V1 or V2, or V3 of >1 mm ( 3 points)
- Discordance with ST elevation of > 5mm where the QRS is negative (2 points)
The criteria come in, then fall out of favour, but have been shown to have a low sensitivity.
Smith et al (Ann of EM Vol 60, No 6:Dec 2012), improved Sgarbossa’s 3rd rule and used angiographic evidence of occlusion as proof of infarction: resulting in an improved sensitivity and specificity.
All they did was use a ratio of ST/S to replace the >5mm ST elevation in rule 3. The sensitivity increase to 91% and the specificity to 90% and it was more accurate than rules 1 or 2.
Amplitude of ST segment elevation  /  Preceeding S wave depth      <-0.25 In any one lead.
or > 0.25, given they are discordant.
