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, as recommended by the American Academy of Family Physicians 1, 2

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, according to the American Academy of Family Physicians 2
Type II block is typically associated with a wide QRS complex due to the infranodal location, as noted by the American Academy of Family Physicians 1, 3

Degenerative conduction system disease (Lev's and Lenegre's diseases) represents a primary cause, as reported by the Praxis Medical Insights 2
Extensive myocardial necrosis, particularly with anterior wall involvement, is a common cause, as reported by the European Heart Journal 4
Anterior wall myocardial infarction is a common acute cause, according to the Praxis Medical Insights 2
Anterior MI-associated AV block is usually infra-Hisian with high mortality due to extensive necrosis, as reported by the European Heart Journal 4

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, and requires additional testing (exercise stress test or electrophysiologic study) to determine the anatomic level, as recommended by the American Heart Association 6

Type II block is more serious than Type I due to higher risk of progression to complete heart block, as reported by the Praxis Medical Insights 2
Untreated chronic second-degree block below the His bundle has poor prognosis with frequent progression to higher degrees of block and syncope, as reported by the Journal of the American College of Cardiology 5
Permanent pacemaker implantation is indicated even in asymptomatic patients with Type II block, particularly when associated with fascicular block, as recommended by the American Academy of Family Physicians 1, 3