It is important to remember that because of the loss of normal septal and left ventricular depolarization that occurs when left bundle branch block is present, the electrocardiographic diagnosis of abnormalities that usually affect the initial portions of the QRS complex, such as hypertrophy and a prior myocardial infarction, can often not be made. This is in marked contrast to the situation in right bundle branch block in which septal and left ventricular depolarization are not affected.