How can a left bundle branch block be diagnosed on an ECG?

A left bundle branch block (LBBB) can be diagnosed on an ECG by identifying a widened QRS complex. The characteristic finding of LBBB is that the QRS duration exceeds 120 milliseconds due to the delayed conduction through the left bundle branch, leading to the activation of the left ventricle being delayed relative to the right ventricle. This results in a specific morphology on the ECG, typically presented as a broad, notched or slurred appearance in the QRS waveform in lateral leads (I, aVL, V5, V6).

The other options do not specifically indicate the presence of a left bundle branch block. A normal QT interval, for instance, does not provide any diagnostic information regarding conduction blocks; it simply reflects the time it takes for ventricular depolarization and repolarization to occur. Inverted T waves can be seen in a variety of conditions, including ischemia, but are not diagnostic for LBBB. Elevated ST segments are more indicative of conditions such as acute myocardial injury and do not relate directly to the conduction abnormalities seen with LBBB. Thus, the widened QRS complex is the definitive finding for diagnosing left bundle branch block on an ECG.