Normal sinus rhythm with second-degree (Mobitz) atrioventricular block. Each cycle commences with a P wave, but occasionally the P wave is not followed by a QRS and another P wave appears. This is called a “dropped beat” and is the fundamental defect in a second-degree or Mobitz block. First look at tracing A. (Don't be disturbed by the fact that the QRS complexes go down instead of up. Waves are waves! Their direction depends on the particular lead used to record the tracing.) Notice that each successive PR interval lengthens until finally 1 P wave stands alone and a beat is dropped. Also notice that after the beat is dropped, the PR intervals commence again to progressively lengthen until another beat is dropped. This strange pattern of PR intervals was first described by a cardiologist named Wenckebach. Therefore, this type of second-degree block is called a Mobitz 1 or Wenckebach block. In tracing B, notice that all PR intervals are identical. They may be normal in length or delayed, but they are all the same; even after a beat is dropped, they resume their duration. This is called a Mobitz 2 block. In this particular example, the ratio of P waves to QRS complexes is 2 : 1. Therefore, the R-R intervals are regular. With any other ratio, eg, 3 : 1 or 4 : 1, the R-R interval would appear irregular.