Wenckebach's phenomenon is seen in

Second-degree AV block is subclassified as Mobitz type I (Wenckebach) or Mobitz type II.In type I, the PR interval has a progressive prolongation until finally the atrial impulse is completely blocked and does not produce a QRS electrical impulse.Once the p-wave is blocked and no QRS is generated, the cycle begins again with the prolongation of the PR interval.One of the main identifying characteristics of second-degree AV block (Type 1) is that the atrial rhythm will be regular. Type II second-degree AV block is characterized by intermittent failure of conduction of the P wave without changes in the preceding PR or RR intervals.Harrison 19e pg: 1472