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©2013 UpToDate ® Print Email Typical left bundle branch block Electrocardiogram in typical complete LBBB. The asynchronous activation of the two ventricles increases the QRS duration (0.14 sec). The abnormal initial vector results in loss of normal septal forces as manifested by absence of physiologic q waves in leads I, aVL, and V6. The late activation of the left ventricle prolongs the dominant leftward progression of the middle and later depolarization forces, leading to a positive and widened R wave in the lateral leads. Both the ST segment and T wave vectors are opposite in direction from the QRS, a "secondary" repolarization abnormality. Courtesy of Ary Goldberger, MD.