What indicates that a rhythm is originating in the atria rather than the SA node?

The presence of irregular P waves is indeed a significant indicator that a rhythm is originating in the atria rather than the sinoatrial (SA) node. When a rhythm is generated in the atria, various ectopic foci can take over pacing, leading to different shapes and patterns in P waves. Irregular P waves can emerge due to multiple atrial pacemaker sites or varying intervals between atrial depolarizations, which is characteristic of atrial arrhythmias.

In contrast, if the rhythm were originating from the SA node, the P waves would typically appear more regular and consistent, given that the SA node serves as the primary pacemaker of the heart under normal circumstances. Therefore, the irregularity seen in P waves strongly suggests that the rhythm is influenced by alternative atrial activity, emphasizing the importance of recognizing these patterns in assessing cardiac rhythms.