What are the causes of Mobitz type II (second-degree) atrioventricular (AV) block rhythm?

Causes of Mobitz Type II Atrioventricular Block

Mobitz type II second-degree atrioventricular block is primarily caused by structural heart disease, particularly ischemic heart disease, and is located within or below the His bundle in the His-Purkinje system. 1

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
• 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

Structural Heart Disease

• Ischemic heart disease, particularly acute myocardial infarction 1, 2
• Chronic ischemic cardiomyopathy 1
• Degenerative conduction system disease (Lev's and Lenegre's diseases) 1, 2
• Cardiac surgery, especially valve surgery 1
• Transcatheter aortic valve replacement (TAVR) 1
• Alcohol septal ablation 1

Inflammatory and Infiltrative Conditions

• Myocarditis 1, 2
• Lymphoma 1
• Infiltrative cardiomyopathy 2
• Cardiac valvular calcification 2

Neuromuscular Diseases

• Myotonic dystrophy 1
• Kearns-Sayre syndrome 1
• Erb's dystrophy 1

Medication-Related Causes

• Antiarrhythmic drugs 1
• Beta blockers (less common than in Mobitz type I) 1
• Calcium channel blockers (less common than in Mobitz type I) 1
• Digoxin toxicity 1, 2

Metabolic/Endocrine Disorders

• Hyperkalemia 2
• Thyroid disease (both hypothyroidism and hyperthyroidism) 1
• Adrenal disease (e.g., pheochromocytoma, hypoaldosteronism) 1

Iatrogenic Causes

• Catheter ablation 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 1, 2
• It is often an indication for permanent pacemaker implantation, regardless of symptoms 1, 2
• In athletes with Mobitz type II block, comprehensive evaluation including echocardiogram and stress testing is recommended 1
• Electrophysiological studies may be required to distinguish 2:1 Wenckebach physiology from true Mobitz type II block 1

Diagnostic Considerations

• A 12-lead ECG is essential for diagnosis, with attention to QRS morphology (often wide) 1
• Careful evaluation is needed to distinguish true Mobitz type II block from pseudo-AV block caused by concealed His bundle or ventricular extrasystoles 3, 4
• 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, 4
• Electrophysiological studies may be necessary in selected cases to confirm the site of block 1, 4

Important Distinctions

• Unlike Mobitz type I block, Mobitz type II is rarely associated with increased vagal tone 1, 5
• Mobitz type II block has not been reported in inferior myocardial infarction, where type I block is more common 4
• The presence of bundle branch block with Mobitz type II block suggests more extensive conduction system disease 1, 3