What are the ECG characteristics of Mobitz type I (Wenckebach) and Mobitz type II second-degree AV block?

In Mobitz type I (Wenckebach) second-degree AV block, the PR interval progressively lengthens with each successive P wave until one P wave is not followed by a QRS complex, and then the cycle repeats. In Mobitz type II, the PR interval remains constant in conducted beats, but some P waves are not followed by QRS complexes, and this may occur randomly without a consistent pattern.

Both Mobitz type I and type II are forms of second-degree AV block, in which some but not all P waves are conducted to the ventricles, so on ECG some P waves lack a following QRS complex. The two types differ by their PR interval behaviour. In Mobitz type I (Wenckebach), the PR interval becomes progressively longer after each successive P wave until a P wave is dropped, and this repeating pattern is characteristic. In Mobitz type II, the PR interval is constant in the beats that are conducted, but some P waves are not followed by QRS complexes; this dropped-beat pattern may be random and irregular. The source also notes that a 2:1 AV block, where only alternate P waves conduct, may be due to either Mobitz type I or type II and can be difficult to distinguish.