If the SA node fails to generate an impulse, a failsafe mechanism in the heart switches to the next fastest inherent rate. Usually, the atrial foci take over pacing from the SA node, and the resulting groups of ECG anomalies are known as the atrial arrhythmias. 

Impulses originating from the atria affect the morphology (appearance) of the P waves on an ECG. The atrial P wave is slower and flatter than the sinus P wave. The atrial P wave may be notched, peaked, saw-toothed, or biphasic. 

A biphasic wave may produce a positive and negative deflection in one P wave. This is an abnormal finding except in lead V1, where a biphasic P wave is normal and expected. Rhythms originating from the atria also have a narrow QRS complex.