The left bundle branch is composed of 2 major divisions, the anterior superior division and the posterior inferior division. A delay or block in conduction through 1 of these in the absence of left bundle-branch block (LBBB) is called a hemiblock. A block in the anterior division is called a left anterior hemiblock (LAH), whereas a block in the posterior division is called a left posterior hemiblock (LPH). This column will review the electrocardiogram (ECG) characteristics and clinical implications of left anterior and left posterior hemiblocks followed by the combination of each with right bundle-branch block (RBBB). Trifascicular block will also be reviewed.

When an LBBB is present, we assume that both divisions have a complete block. This may not be the case. Studies have demonstrated that the left bundle branch is complex and resembles a spider web, with fibers fanning out and interconnecting.1  One would need an...

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