P waves resulting from impulses that originate outside the SA node are called?

P waves that originate from impulses outside the sinoatrial (SA) node are indeed referred to as P prime waves. The SA node is known as the primary pacemaker of the heart, but sometimes ectopic foci can generate impulses, leading to P waves that differ in morphology from those traditionally initiated by the SA node. These waves are designated as P prime to signify that they arise from an alternative pacemaker site rather than the normal conduction pathway initiated by the SA node.

This distinction is important in the context of electrocardiography, as recognizing these variations can help in diagnosing various arrhythmias or cardiac conditions. The presence of P prime waves can indicate that the heart is responding to increased activity from other pacemaker cells in the atria, which may suggest underlying issues such as atrial enlargement, ectopic atrial rhythms, or other forms of atrial activity that diverge from the norm established by the SA node.