What causes a 2nd degree (second degree) heart block?

Causes of Second Degree Heart Block

Second-degree heart block is primarily caused by conduction system disease, ischemic heart disease, medications, and structural heart abnormalities, with the specific etiology varying by block type (Mobitz I vs II). 1, 2

Mobitz Type I (Wenckebach) Second-Degree AV Block

Pathophysiology and Location

• Typically occurs at the level of the AV node 3, 4
• Characterized by progressive PR interval prolongation before a dropped QRS complex 3, 4
• Usually associated with a narrow QRS complex unless pre-existing bundle branch block is present 4

Common Causes

• Increased vagal tone (most common cause in healthy individuals) 3, 4
• Medications:
  • Beta-blockers 5, 1
  • Non-dihydropyridine calcium channel blockers 5, 1
  • Digoxin 5, 1
• Acute inferior wall myocardial infarction 2, 4
• Electrolyte abnormalities 5, 2
• Inflammatory conditions affecting the heart 2
• Highly trained athletes (can be a normal variant) 3, 6

Mobitz Type II Second-Degree AV Block

Pathophysiology and Location

• Occurs within or below the His bundle in the His-Purkinje system 1, 7
• Characterized by sudden failure of conduction without PR prolongation before the blocked beat 1, 7
• Usually associated with wide QRS complexes 1, 4
• Higher risk of progression to complete heart block 1, 2

Common Causes

• Ischemic heart disease, particularly anterior wall 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
• Myocarditis 1, 2
• Infiltrative conditions:
  • Sarcoidosis 2
  • Amyloidosis 2
  • Lymphoma 1
• Neuromuscular diseases:
  • Myotonic dystrophy 1
  • Kearns-Sayre syndrome 1
  • Erb's dystrophy 1
• Medications (less commonly than in Mobitz type I):
  • Antiarrhythmic drugs 1
  • Beta blockers 1
  • Calcium channel blockers 1
  • Digoxin toxicity 1

2:1 AV Block

• Cannot be classified as Mobitz I or II based on surface ECG alone 8, 4
• May be nodal or infranodal in origin 8, 4
• Causes similar to both Mobitz I and II, depending on the site of block 8, 4
• In children, presence of 2:1 block on initial ECG is associated with higher risk of progression to complete heart block 6

Other Considerations

Congenital Heart Disease

• Repaired tetralogy of Fallot commonly associated with second-degree AV block 3
• Less commonly seen after repair of ventricular septal defects 3
• Congenitally corrected transposition of the great arteries can present with AV block 5

Infectious Causes

• Lyme disease can affect the cardiac conduction system 5, 2
• Viral myocarditis 1, 2

Diagnostic Pitfalls

• Nonconducted premature atrial contractions may mimic second-degree AV block 2
• Atrial tachycardia with block can resemble second-degree AV block 2
• Concealed His bundle or ventricular extrasystoles can produce patterns that mimic both types of second-degree block (pseudo-AV block) 8, 4
• Vagal surges can cause simultaneous sinus slowing and AV nodal block, which may be misinterpreted as Mobitz type II block 7

Clinical Significance

• Mobitz type II block is generally considered more serious than Mobitz type I due to higher risk of progression to complete heart block 1, 2
• Second-degree block occurring during exercise suggests disease in the His-Purkinje system and is associated with poor prognosis 3
• In children with concerning second-degree block, 30% progress to complete heart block or require a pacemaker during long-term follow-up 6