Bundle Branch Block is best described by which finding?

• A. Delay in either the left or right bundle branch
• B. PRI lengthens until QRS dropout
• C. P waves and QRS dissociate
• D. Sinus tachycardia with narrow QRS

Answer: (A)

Bundle branch block occurs when one of the main conduction pathways to the ventricles is blocked, so the impulse travels to the other ventricle first and then through the ventricular muscle to activate the rest. This makes the two ventricles activate asynchronously and widens the QRS complex. That delay or block can happen in either the left or the right bundle branch, which is why the description “delay in either the left or right bundle branch” fits best.

Because the hallmark is a widened QRS from delayed ventricular activation, this differs from AV-block findings. A gradually lengthening PR interval with a dropped QRS describes AV block (Mobitz I), not BBB. P waves and QRS dissociation point to complete AV dissociation, and sinus tachycardia with a narrow QRS indicates rapid heart rate with intact conduction.