Atrial tachycardia and other ectopic atrial rhythms occur when a site outside of the sinus node, but within the atria, creates action potentials faster than the sinus node. This ectopic focus ...

The second ECG strip below shows an ectopic atrial rhythm. Note that the P wave is down in lead II and only up (not biphasic) in lead V1. If there is sinus rhythm, and the heart rate is less than ...