Second-Degree Heart Block Type 2 (Mobitz II): Causes and Differential Diagnosis
Overview and Key Distinction
Mobitz Type II second-degree AV block is an infranodal conduction abnormality characterized by sudden failure of AV conduction with constant PR intervals before and after the blocked beat, typically requiring permanent pacemaker implantation due to high risk of progression to complete heart block. 1, 2
Anatomic Location
- Mobitz Type II block occurs within or below the His bundle in the His-Purkinje system (infranodal location), distinguishing it from Mobitz Type I which occurs at the AV node 2, 3
- The block is almost always infranodal when correctly diagnosed 3, 4
- Type II block is typically associated with a wide QRS complex due to the infranodal location 1
Primary Causes
Degenerative/Structural Disease
- Degenerative conduction system disease (Lev's and Lenegre's diseases) represents a primary cause 2
- Sclerodegenerative conduction system disease is a common manifestation 5
- Extensive myocardial necrosis, particularly with anterior wall involvement 1
Ischemic Heart Disease
- Anterior wall myocardial infarction is a common acute cause 2
- Anterior MI-associated AV block is usually infra-Hisian with high mortality due to extensive necrosis 1
- Type II block has not been reported in inferior MI (where Type I predominates) 4
Congenital Heart Disease
- Repaired tetralogy of Fallot commonly presents with second-degree AV block 2
- Congenitally corrected transposition of the great arteries can manifest with AV block 2
Other Structural Causes
- Cardiac valvular calcification affecting the conduction system 5
- Infiltrative cardiomyopathy (sarcoidosis, amyloidosis) 5
- Myocarditis (including viral causes) 2, 5
Metabolic/Toxic
- Hyperkalemia can precipitate Type II block 5
- Drug toxicity (though medications more commonly cause Type I block) 5
Critical Differential Diagnosis Considerations
Pseudo-AV Block
- Concealed His bundle or ventricular extrasystoles confined to the specialized conduction system can mimic Type II block without actual myocardial depolarization 3, 4
- This represents pseudo-AV block and must be excluded 1, 3
Misdiagnosis of Type I as Type II
- Vagal surge causing simultaneous sinus slowing and AV nodal block can superficially resemble Type II block but is benign 3, 4
- Absence of sinus slowing is an important criterion for true Type II block 3, 4
- Type II block requires a stable sinus rate for accurate diagnosis 4
- Narrow QRS Type I block coexisting with apparent Type II pattern effectively rules out true Type II block, as coexistence is exceedingly rare 3, 4
2:1 AV Block
- 2:1 AV block cannot be classified as Type I or Type II based on ECG alone, but can be determined as nodal or infranodal 3, 4
- Requires additional testing (exercise stress test or electrophysiologic study) to determine the anatomic level 1
Nonconducted PACs
- Nonconducted premature atrial contractions may mimic second-degree AV block 5
- Atrial tachycardia with block is another common mimic 5
Diagnostic Pitfalls to Avoid
- Cannot diagnose Type II block if the first post-block P wave is followed by a shortened PR interval or the P wave is not discernible 3, 4
- Type II block has not been reported in young athletes where Type I may be misinterpreted 4
- Bundle branch block presence does not define Type II block—Type I with bundle branch block is infranodal in 60-70% of cases (except in acute MI) 3, 4
Clinical Significance and Prognosis
- Type II block is more serious than Type I due to higher risk of progression to complete heart block 2
- Untreated chronic second-degree block below the His bundle has poor prognosis with frequent progression to higher degrees of block and syncope 1
- Permanent pacemaker implantation is indicated even in asymptomatic patients with Type II block, particularly when associated with fascicular block 1
- Five-year survival is significantly better with pacing (78%) compared to unpaced patients (41%) 6