Left Bundle Branch Block (LBBB) and STEMI criteria
Left bundle branch block is manifested on the ECG with wide QRS with deep slurred QS complex in V1 and slurred R wave in V6. ST segment and T wave vectors are discordant to the QRS vector, meaning that when the QRS positive as in V6, the ST segment will be depressed and T wave will be inverted. This is known as secondary ST – T abnormality. If the ST segment and T wave are concordant to the QRS, it indicates a primary myocardial abnormality. An elevated ST segment and an upright T wave in V6 with a positive QRS is taken as an evidence of myocardial infarction / ischemia in the presence of left bundle branch block (LBBB). Other evidences of myocardial infarction in the presence of LBBB are typical coved ST elevation, prominent R waves in V1 and Q waves in V6. Usually the initial r wave in V1 and initial q wave in V6 represents initial left to right activation of the interventricular septum. In LBBB, the initial left to right activation of the septum is absent.
In the 2004 guidelines for the management of ST segment elevation myocardial infarction (STEMI), new or presumably new LBBB was considered as equivalent to STEMI. Many cases in which catheterization laboratory was activated and emergency angiogram done showed no acute coronary artery occlusion. Hence the 2013 guideline removed this recommendation. But this could lead to denial of reperfusion therapy in those who actually have an acute coronary occlusion with LBBB on ECG. In this situation, Sgarbossa electrocardiographic criteria is one of the most validated tool to help in the diagnosis of STEMI with LBBB . A Sgarbossa score of three or more has 98% specificity for acute myocardial infarction with angiographically confirmed acute coronary occlusion. Using Sgarbossa score might prevent denial of reperfusion therapy while avoiding unnecessary cath lab activation.
Other criteria described are Selvester 10% RS criteria and Smith 25% S-wave criteria. All these have concordant ST segment elevation in common with Sgarbossa criteria. Sgarbossa criteria for discordant ST segment elevation is 5 mm (500 microvolt) or more. Selvester 10% RS criteria is ST segment elevation which is 10% or more of |S|-|R| plus STEMI limits (ST elevation required for the given lead). Smith 25% S-wave criteria is ST elevation 25% or more of the S-wave amplitude. Another new criteria for computerized interpretation is based on ST segment elevation in relation to the QRS area in the presence of LBBB .
- Cai Q, Mehta N, Sgarbossa EB, Pinski SL, Wagner GS, Califf RM, Barbagelata A. The left bundle-branch block puzzle in the 2013 ST-elevation myocardial infarction guideline: from falsely declaring emergency to denying reperfusion in a high-risk population. Are the Sgarbossa Criteria ready for prime time? Am Heart J. 2013 Sep;166(3):409-13.
- Gregg RE, Helfenbein ED, Babaeizadeh S. New ST-segment elevation myocardial infarction criteria for left bundle branch block based on QRS area. J Electrocardiol. 2013 Nov-Dec;46(6):528-34.