What is a Mobitz 2 (second-degree atrioventricular) heart block?

What is Mobitz Type II Second-Degree Atrioventricular Block?

Mobitz Type II second-degree AV block is a pathological cardiac conduction disorder characterized by periodic failure of atrial impulses to conduct to the ventricles with constant PR intervals before and after the blocked P waves, occurring within or below the His bundle in the His-Purkinje system. 1

Electrocardiographic Definition

• P waves occur at a constant rate (<100 bpm) with periodic single nonconducted P waves, while all other P waves before and after the blocked beat have constant PR intervals 1
• The PR interval remains unchanged after the blocked beat, which is the sine qua non of Mobitz type II block 2
• The QRS complex is typically wide (≥120 ms), reflecting the infranodal location of the block 3, 4
• An unchanged PR interval distinguishes this from Mobitz type I (Wenckebach), where progressive PR prolongation occurs before the blocked beat 5

Anatomic Location and Pathophysiology

• The block occurs within or below the His bundle in the His-Purkinje system, not at the AV node 3, 4
• This infranodal location results in sudden failure of conduction without progressive PR prolongation 3
• The ventricular escape mechanism is slower and more unpredictable compared to AV nodal blocks 3
• His bundle electrogram studies consistently demonstrate prolongation of the H-Q interval with block localized within the His-Purkinje system 6

Clinical Significance and Prognosis

• Mobitz type II block has a high risk of progression to complete heart block and sudden cardiac death 5, 3, 6
• 75% of patients experience syncopal attacks due to the unpredictable nature of progression 6
• The block does not respond to atropine (unlike AV nodal blocks) but may sometimes improve with catecholamines 3
• This is considered more serious than Mobitz type I block due to its location and higher risk of progression 3

Common Causes

Structural and degenerative causes:

• Ischemic heart disease, particularly acute myocardial infarction 3, 4
• Degenerative conduction system disease (Lev's and Lenegre's diseases) 3
• Chronic ischemic cardiomyopathy 3

Iatrogenic causes:

• Cardiac surgery, especially valve surgery 3
• Transcatheter aortic valve replacement (TAVR) 3
• Alcohol septal ablation 3
• Catheter ablation 3

Infiltrative and inflammatory causes:

• Myocarditis 3
• Lymphoma 3

Medication-related causes:

• Antiarrhythmic drugs 3
• Beta blockers (less commonly than in Mobitz type I) 3
• Calcium channel blockers (less commonly than in Mobitz type I) 3
• Digoxin toxicity 3
• Immune checkpoint inhibitors (e.g., pembrolizumab) 7

Neuromuscular diseases:

• Myotonic dystrophy 3
• Kearns-Sayre syndrome 3
• Erb's dystrophy 3

Important Diagnostic Pitfalls

• A 2:1 AV block cannot be classified as Mobitz type I or type II based on surface ECG alone 3, 2
• A stable sinus rate is required for diagnosis because a vagal surge can cause simultaneous sinus slowing and AV nodal block that mimics Mobitz type II 2
• Atypical forms of Wenckebach AV block may be misinterpreted as Mobitz type II when PR intervals appear constant before the block 2
• Concealed His bundle or ventricular extrasystoles may mimic Mobitz type II block (pseudo-AV block) 2
• Exercise testing or electrophysiological studies may be required to distinguish 2:1 Wenckebach physiology from true Mobitz type II block 1, 3

Management Implications

• Mobitz type II second-degree AV block is a Class I indication for permanent pacemaker implantation, even in asymptomatic patients 5
• Transcutaneous pacing pads should be placed immediately due to high risk of progression to complete heart block 5
• Prophylactic pacemaker implantation is indicated in all patients with correctly identified Mobitz type II block to prevent Adams-Stokes syndrome and sudden death 2, 6
• In rare cases of tachycardia-dependent Mobitz type II block, beta-blocker therapy may be considered to prevent high sinus rates 8