Diagram of Wenckebach periodicity, utilizing Lewis diagram. The PP interval is constant at 1000 msec. The PR interval gradually increases, but by decreasing increments, i.e., the second PR exceeds the first by 200 msec; the third PR exceeds the second by 100 msec; the fourth PR exceeds the third by 50 msec; and the fifth P wave fails to conduct through the AV junction. As a result of the decremental increase in PR interval, the RR intervals decrease from 1200 to 1100 to 1050 msec, prior to the pause, which measures less than the 2000 msec incorporated in two PP intervals.