What are the types of 2nd degree Atrioventricular (AV) block?

Types of Second-Degree Atrioventricular 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. 1

Mobitz Type I (Wenckebach) Block

Mobitz Type I is characterized by progressive prolongation of the PR interval on consecutive beats, culminating in a dropped QRS complex (blocked P wave). 1

Key Features:

• Progressive PR prolongation before the blocked beat creates a characteristic "group beating" pattern on ECG 1
• Usually associated with narrow QRS complexes 1
• The site of block is typically at the AV node level (supranodal), though it can occasionally occur within the His bundle 1
• Generally has a benign prognosis, especially when the QRS is narrow 2
• Can be a normal finding in well-trained athletes, particularly during sleep 1

Clinical Significance:

• When occurring with narrow QRS complexes, the block is almost always AV nodal and typically benign 2
• When occurring with bundle branch block (wide QRS), the block may be infranodal in 60-70% of cases and carries a worse prognosis 3, 2
• The clinical course is usually benign, with prognosis depending on underlying heart disease 1

Mobitz Type II Block

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. 1

Key Features:

• Constant PR intervals before and after the blocked beat—this is a sine qua non for diagnosis 4
• Usually associated with wide QRS complexes (bundle branch block) 1
• The site of block is almost always infranodal (within or below the His bundle) 1, 3
• All correctly defined Type II blocks are infranodal 3, 2

Critical Diagnostic Criteria:

• Requires a stable sinus rate—absence of sinus slowing is crucial because vagal surges can cause simultaneous sinus slowing and AV nodal block that mimics Type II 3, 2, 4
• The diagnosis cannot be established if the first post-block P wave is followed by a shortened PR interval 2, 4
• Has not been reported in inferior MI or young athletes, where Type I may be misinterpreted as Type II 3

Clinical Significance:

• Type II block is abnormal and carries a poor prognosis with high risk of progression to complete heart block 1
• Class I indication for permanent pacemaker even in asymptomatic patients 1, 5
• Frequently progresses to higher degrees of block and syncope if untreated 1

2:1 Atrioventricular Block

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. 1

Key Features:

• Shows alternating conducted and blocked P waves in a 2:1 pattern 1
• Cannot be classified as Mobitz I or II because there are no consecutive conducted beats to assess for PR interval changes 2
• Can be either nodal or infranodal in location 2
• Requires additional evaluation (stress testing or electrophysiology study) to distinguish 2:1 Wenckebach physiology from true infranodal block 1

Diagnostic Approach:

• Exercise stress testing may help differentiate—Wenckebach at the AV node typically improves with exercise, while infranodal block may worsen 1
• Electrophysiology studies may be required in select cases to determine the site of block 1

Advanced (High-Grade) AV Block

Advanced or high-grade AV block refers to situations where two or more consecutive P waves are blocked without complete loss of AV conduction. 1

• This represents a more severe form than typical second-degree block 1
• Generally considered to be intra- or infra-Hisian and treated with pacing 1
• In unusual circumstances (at night with sinus slowing), a vagal etiology may be considered, especially with narrow QRS 1

Important Clinical Pitfalls

Pseudo-AV Block:

• Concealed His bundle or ventricular extrasystoles can mimic both Type I and Type II block without true AV conduction abnormality 1, 2, 4
• This represents a diagnostic trap that can lead to unnecessary pacemaker implantation 4

Misdiagnosis of Type II:

• Atypical Wenckebach with minimal PR changes may be misinterpreted as Type II 4
• A pattern resembling narrow QRS Type II in association with obvious Type I structure in the same recording effectively rules out Type II block because coexistence is exceedingly rare 3, 2
• Vagal surges causing simultaneous sinus slowing and AV nodal block can superficially resemble Type II 3, 2, 4

QRS Width Implications:

• Narrow QRS Type I block is almost always AV nodal and benign 2
• Wide QRS Type I block is infranodal in 60-70% of cases (except in acute MI) and requires pacing 3, 2
• Type II block is invariably infranodal and always requires pacing 4