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Left bundle branch block is characterised by a wide QRS, which is 120 msec or more. The left precordial leads V5 and V6 will show a tall RR’ or M shaped QRS complex. A QS complex with a notch or W shaped QRS may be seen in right precordial leads V1 and V2. A small initial r wave may be seen in these leads. Secondary ST segment and T wave changes are seen associated, in the form of a discordant ST – T. The ST segment and T wave are in opposite in direction to the dominant QRS. Hence the ST segment is depressed and T waves inverted in lateral leads. ST segment may be elevated with upright T waves in right precordial leads.
Unlike right bundle branch block, left bundle branch block is usually associated with structural heart disease. Coronary artery disease is an important cause of left bundle branch block. In fact a new onset left bundle branch block in the setting of acute myocardial infarction is considered equivalent to ST elevation myocardial infarction and is an indication for thrombolytic therapy.