A left bundle branch block occurs when electrical conduction is unable to course through the left bundle and instead travels down the right bundle. The electrical impulse then reaches the left ventricle from the right ventricle directly through the myocytes, which is a slow process. This results in widening of the QRS complex as described below. Approximately 50% of left bundle branch blocks are associated with structural heart disease, while the other half are idiopathic or due to fibrosis of the cardiac conduction system which occurs during the aging process.
The ECG criteria for a left bundle branch block (LBBB) include:
QRS duration of > 120 ms.
Absence of Q wave in leads I, V5 and V6.
Monomorphic R wave in I, V5 and V6.
ST and T wave displacement opposite to the major deflection of the QRS complex.
Note: Incomplete LBBB is present if the QRS duration is between 100 ms and 119 ms and criteria from numbers 2, 3 and 4 of the above are met.
Traditionally it has been taught that MI cannot be diagnosed via ECG in the presence of a LBBB; however, in 1996 Sgarbossa and colleagues described some ECG changes seen in those with LBBB and concomitant MIs and devised a point scoring system. This is called the Sgarbossa criteria.
ST elevation > 1 mm and in the same direction (concordant) with the QRS complex = 5 points
ST depression > 1 mm in leads V1, V2, or V3 = 3 points
ST elevation > 5 mm and in the opposite direction (discordant) with the QRS = 2 points
A score of 3 points is required to diagnose an acute MI. Criteria No. 3 is under debate as to its usefulness so, in essence, you need to have either criteria 1 or criteria 2.
Of note, Cabrera's sign and Chapman’s sign have been used to diagnose acute MI in the setting of a LBBB.