What characteristic is indicative of a junctional rhythm on an ECG?

A junctional rhythm on an ECG is primarily characterized by an absence or inversion of the P wave. In this rhythm, the electrical impulse originates from the junction between the atria and ventricles, rather than from the sinoatrial (SA) node. As a result, the P wave can either be missing entirely or may appear inverted, depending on its relation to the QRS complex.

This absence reflects the atrial depolarization occurring either after or simultaneously with ventricular depolarization, leading to the distinctive appearance of the junctional rhythm on the ECG. The overall heart rate may be normal or slightly decreased, but the defining feature is indeed the alteration in the P wave appearance or its absence. Understanding this characteristic helps in differentiating junctional rhythms from other types of arrhythmias.