This is a good example of atrial fibrillation with a rapid ventricular response. The rate is around 150 per minute, and the rhythm is almost regular. Show your students how to "march out" the QRS complexes so they can see the irregularity. When the rate is around 150, also check for atrial flutter with 2:1 conduction. In this case, none of the 12 leads shows strong evidence of flutter waves, but it is a possibility when the rhythm is almost regular. Fortunately, the initial treatment for both AFib and AFlutter with rapid response are the same.