What does a bundle branch block result in regarding the QRS complex?

A bundle branch block results in a wide QRS complex due to the delayed conduction through the ventricular pathways, specifically one of the bundle branches in the heart's conduction system. When one of these branches is blocked, the electrical impulse does not propagate through that pathway as quickly as it would normally. As a result, the affected ventricle depolarizes later than the unaffected ventricle.

This delay leads to a longer duration of the QRS complex on an electrocardiogram (ECG), typically measuring greater than 120 milliseconds, which is considered wide. The appearance of the QRS complex also changes, often taking on a characteristic morphology depending on which bundle branch is affected.

In contrast, a narrow QRS complex indicates that the electrical impulses are being conducted through both bundle branches efficiently and rapidly. An irregular rhythm typically signifies issues with atrial activity or conduction disturbances but does not specifically relate to the width of the QRS complex. Lastly, no variation in QRS duration would indicate normal conduction through the ventricles without any blocks, which is not the case with a bundle branch block. Hence, the presence of a wide QRS complex is the definitive sign of this condition.