In second-degree atrioventricular nodal block — also known as 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 first-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 an 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. Circulation. 2009;119:e235-240.
2. Chou's Electrocardiography in Clinical Practice: Adult and Pediatric, 6e