This rhythm strip shows third-degree AV block, also called complete heart block or complete AV block. The P waves are from the sinus node, and are regular at a rate of about 120/min. (Sinus tachycardia). This is a good strip for showing your students how to "march out" the P waves to find the ones that are hidden behind QRS complexes or T waves. Knowing that the P waves are regular, it is easy to find the hidden ones.