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. There is an irregular R-R interval. Sometimes when the block is consistent, the QRS complexes are said to demonstrate "group beating".
A second degree type I AV block occurs when conduction within the AV node itself is delayed in this progressive manner. It does not necessarily indicate intrinsic conduction disease and rarely requires a pacemaker to be implanted. A second degree type I AV block can be caused by AV blocking medications or increased vagal tone. AV nodal ischemia during an inferior MI can cause AV nodal blocks as well.
Note that if every second P wave is not conducted, there will not be enough time to see PR prolongation. This is called 2:1 AV block as depicted below:
When 2:1 AV block is present, the rhythm may be second degree type I or second degree type II AV block. Exercising the patient will increase AV nodal conduction and help distinguish these two. If second degree type I AV block is present, then the heart rate will increase and the progressive prolongation of PR intervals prior to a non-conducted P wave will be apparent. If second degree type II AV block is present, there may be no change with exercise.
1. Surawicz B et al. ACC/AHA recommendations for the standardization and interpretation of the electrocardiogram. Circulation. 2009;119:e235-240.
2. Chou's Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
By Steven Lome