What does a wide QRS complex (greater than 120 ms) typically indicate?

A wide QRS complex, particularly one that measures greater than 120 milliseconds, typically indicates a ventricular origin of the rhythm. This occurs because a wide QRS complex represents a delay in the conduction of electrical impulses through the ventricles. In normal circumstances, the impulses travel through the ventricles quickly and efficiently via the His-Purkinje system, resulting in a narrow QRS complex.

When the QRS is wide, it suggests that the electrical impulses are originating from within the ventricles themselves—such as in ventricular rhythms or in the case of bundle branch blocks—leading to a slower depolarization process. Consequently, this is a hallmark sign of a problem with ventricular conduction, distinguishing it from normal conduction pathways that typically produce narrow complexes.

In contrast, a QRS complex of this duration would not be associated with atrial rhythms, which typically show a narrow QRS due to normal conduction through the AV node into the ventricles. Junctional rhythms also tend to have narrow QRS complexes unless there is an associated bundle branch block. Thus, the identification of a wide QRS complex is critical in evaluating the origin of the tachyarrhythmia and guiding subsequent management.