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 vector is 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.
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