Second-Degree Atrioventricular Block Classification and Management

Introduction to Second-Degree AV Block

Second-degree AV block is classified into two main types: Mobitz Type I (Wenckebach) and Mobitz Type II, with an additional category of 2:1 AV block that cannot be classified as either type, as stated by the American College of Cardiology 1, 2

Mobitz Type I is characterized by progressive prolongation of the PR interval on consecutive beats, culminating in a dropped QRS complex (blocked P wave), according to the American Academy of Family Physicians 3, 4
The site of block is typically at the AV node level (supranodal), though it can occasionally occur within the His bundle, as noted by the American College of Cardiology 1, 2
Progressive PR prolongation before the blocked beat creates a characteristic "group beating" pattern on ECG, as described in Circulation 5
Can be a normal finding in well-trained athletes, particularly during sleep, as reported in Circulation 6

Mobitz Type II is characterized by constant PR intervals before and after a blocked P wave, representing an all-or-none conduction pattern without visible changes in AV conduction time, as defined by the American College of Cardiology 1, 2
The site of block is almost always infranodal (within or below the His bundle), according to the American College of Cardiology 1, 2
Type II block is abnormal and carries a poor prognosis with high risk of progression to complete heart block, as stated in Circulation and the American Academy of Family Physicians 4, 6
Represents a Class I indication for permanent pacemaker even in asymptomatic patients, as recommended by the American Academy of Family Physicians and other guideline societies 4

2:1 AV block represents a special category where every other P wave is blocked, and it cannot be classified as either Type I or Type II, as described by the American College of Cardiology 1, 2
Shows alternating conducted and blocked P waves in a 2:1 pattern, as noted by the American College of Cardiology 1, 2
Requires additional evaluation (stress testing or electrophysiology study) to distinguish 2:1 Wenckebach physiology from true infranodal block, as recommended in Circulation 6

Advanced or high-grade AV block refers to situations where two or more consecutive P waves are blocked without complete loss of AV conduction, as defined by the American College of Cardiology 1, 2
Represents a more severe form than typical second-degree block, as described in Circulation 5
Generally considered to be intra- or infra-Hisian and treated with pacing, as noted in Circulation 5