The ECG and ladder diagram reproduced here are from the original 1924 publication of Wilhelm Mobitz, a Dutch Physician. They show repetitive sequences in which there is progressive prolongation of the PR interval culminating in a P wave that is blocked in the AV junction and does not propagate to the ventricles. This is referred to as Mobitz type I 2nd degree AV block with Wenckebach periodicity, although it is often shortened to: “Mobitz I block”, “type I block” or “Wenckebach block”. It results in varying ratios of the P wave to the QRS complex, i.e. 2:1, 3:2, 4:3---9:8 etc and causes group beating. The electrocardiographic expression of group beating is the repetitive pattern of 2 or more QRS complexes followed by a pause.