What characteristic identifies a left bundle branch block (LBBB) on an ECG?

A left bundle branch block (LBBB) on an ECG is primarily characterized by a wide QRS complex that typically appears as a "notching" or "slurring" in the terminal portion of the QRS waveform, specifically in leads V5 and V6. This is indicative of a delay in left ventricular depolarization due to the block in the left bundle branch of the conduction system.

The correct characteristic for identifying LBBB involves the alteration in the morphology of the QRS complex, leading to a wide QRS that can sometimes also appear notched. The criteria used for diagnosing LBBB commonly includes a QRS duration of greater than 120 milliseconds as well as the appearance of the aforementioned notching or characteristic shape changes predominantly in the lateral leads.

Other options present characteristics of an ECG that are not typical for LBBB, such as a shorter PR interval or normal QRS duration, which would not align with the expected findings in a patient with LBBB. Additionally, while inverted QRS complexes can be seen in other conditions, they are not indicative of LBBB specifically.