Left bundle branch block with 2:1 AV block
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Left bundle branch block (LBBB) is manifested as a wide QRS with M shaped pattern in lateral leads and wide QS complexes in anterior leads. The initial q waves are absent in lateral leads due to the loss of initial left to right activation of the septum due to left bundle branch block. There are 2 P waves for every QRS complex, with atrial rate double that of ventricular rate, denoting 2:1 AV conduction (2:1 AV block). Together with LBBB, this constitutes one form of trifascicular block. The presence of LBBB indicates that the block is infra Hisean. In fact a subsequent ECG showed complete heart block, making the indication for permanent pacing more urgent. Usually the ST segment and T waves are discordant in LBBB (opposite the predominant QRS deflection). Here the T waves in lateral leads are upright (concordant), indicating a primary T wave abnormality). Associated coronary artery disease or myocardial disease has to be thought of in this setting.