Final answer:
A bundle branch block occurs when the electrical signals in the heart are delayed or blocked in the bundle branches, resulting in a prolonged QRS complex on an ECG.
Step-by-step explanation:
A bundle branch block occurs when the electrical signals in the heart are delayed or blocked in the bundle branches, which are responsible for transmitting the signals to the ventricles. This can be seen on an ECG as a prolonged QRS complex, where the QRS complex represents the depolarization and contraction of the ventricles. The specific QRS duration criteria for diagnosing bundle branch block may vary slightly depending on which branch is affected. However, generally, a QRS duration greater than or equal to 120 milliseconds is considered indicative of bundle branch block.