Causes and Characteristics of Mobitz Type II Atrioventricular Block
Pathophysiological Location and Characteristics
- Mobitz type II block occurs within or below the His bundle in the His-Purkinje system, unlike Mobitz type I which typically occurs at the AV node 1, 2
- This block is characterized by sudden failure of conduction without progressive PR prolongation before the blocked beat 1
- It is associated with a slower and more unpredictable ventricular escape mechanism 1
- Mobitz type II block does not respond to atropine but may sometimes improve with catecholamines 1
- It has greater potential to progress rapidly and unexpectedly to complete heart block 1, 2
Primary Causes
- Ischemic heart disease, particularly acute myocardial infarction, is a primary cause of Mobitz type II block 1
- Chronic ischemic cardiomyopathy is also a cause of Mobitz type II block 1
- Degenerative conduction system disease, such as Lev's and Lenegre's diseases, can cause Mobitz type II block 1
- Cardiac surgery, especially valve surgery, can lead to Mobitz type II block 1
- Transcatheter aortic valve replacement (TAVR) is a potential cause of Mobitz type II block 1
- Alcohol septal ablation can cause Mobitz type II block 1
- Myocarditis is an inflammatory condition that can cause Mobitz type II block 1
- Lymphoma is an infiltrative condition that can cause Mobitz type II block 1
- Myotonic dystrophy is a neuromuscular disease that can cause Mobitz type II block 1
- Kearns-Sayre syndrome is a neuromuscular disease that can cause Mobitz type II block 1
- Erb's dystrophy is a neuromuscular disease that can cause Mobitz type II block 1
- Antiarrhythmic drugs can cause Mobitz type II block 1
- Beta blockers can cause Mobitz type II block, although less commonly than in Mobitz type I 1
- Calcium channel blockers can cause Mobitz type II block, although less commonly than in Mobitz type I 1
- Digoxin toxicity can cause Mobitz type II block 1
- Thyroid disease, including both hypothyroidism and hyperthyroidism, can cause Mobitz type II block 1
- Adrenal disease, such as pheochromocytoma and hypoaldosteronism, can cause Mobitz type II block 1
- Catheter ablation can cause Mobitz type II block 1
Clinical Significance and Management
- Mobitz type II block is generally considered more serious than Mobitz type I block due to its location and higher risk of progression to complete heart block 2
- It is often an indication for permanent pacemaker implantation, regardless of symptoms 3
- In athletes with Mobitz type II block, comprehensive evaluation including echocardiogram and stress testing is recommended 3
- Electrophysiological studies may be required to distinguish 2:1 Wenckebach physiology from true Mobitz type II block 3
Diagnostic Considerations
- A 12-lead ECG is essential for diagnosis, with attention to QRS morphology (often wide) 1, 2
- Electrophysiological studies may be necessary in selected cases to confirm the site of block 3
- In cases with 2:1 AV block, it cannot be classified as Mobitz I or II based on surface ECG alone, and the level of block must be determined by other means 1