In second degree AV nodal block (a.k.a. Wenckebach block or Mobitz Type I AV block), varying failure of conduction through the AV node occurs such that some P waves may not be followed by a QRS complex. Unlike 1st degree AV nodal block, a 1:1 P wave to QRS complex ratio is not maintained. Second degree type I AV block is specifically characterized by increasing delay of AV nodal conduction until a P wave fails to conduct through the AV node. This is seen as progressive PR interval prolongation with each beat until a P wave is not conducted.