What are the clinical treatment guidelines for 2nd degree atrioventricular (AV) block?

Clinical Treatment Guidelines for Second-Degree AV Block

Permanent pacemaker implantation is recommended for patients with acquired second-degree Mobitz type II AV block, regardless of symptoms, due to high risk of progression to complete heart block and associated mortality. 1

Types of Second-Degree AV Block and Their Significance

Second-degree AV block is characterized by intermittent failure of atrial impulses to conduct to the ventricles and is classified into two main types:

1. Mobitz Type I (Wenckebach):

   • Progressive PR interval prolongation before blocked P wave
   • Block typically occurs at AV node level (when QRS is narrow)
   • Generally benign prognosis when isolated
   • Usually does not require pacing unless symptomatic or occurs below the AV node

2. Mobitz Type II:

   • Constant PR intervals before and after blocked P wave
   • Block typically occurs in His-Purkinje system (infranodal)
   • Higher risk of progression to complete heart block
   • Requires permanent pacing regardless of symptoms

Management Algorithm Based on Type and Symptoms

For Mobitz Type I (Wenckebach):

• If asymptomatic with narrow QRS: Observation is appropriate 1
• If symptomatic:
  • Ambulatory ECG monitoring to establish symptom-rhythm correlation 1
  • Exercise testing if exertional symptoms present 1
  • Consider permanent pacing if symptoms correlate with bradycardia 1
• If occurring below AV node (infranodal): Permanent pacing recommended regardless of symptoms 2

For Mobitz Type II:

• Permanent pacing is recommended regardless of symptoms 1
• This is a Class I recommendation (Level of Evidence: B-NR) per ACC/AHA/HRS guidelines 1
• High risk of progression to complete heart block with potential for sudden cardiac death 1, 2

For 2:1 AV Block:

• Cannot be classified as Type I or II based on surface ECG alone 2, 3
• Management depends on:
  • QRS width (narrow suggests AV nodal block, wide suggests infranodal)
  • Symptoms
  • Clinical context
• Consider electrophysiological study to determine level of block 1
• Permanent pacing recommended if infranodal or symptomatic 1

Special Considerations

Reversible Causes:

• For AV block due to reversible causes (medications, Lyme disease, thyroid dysfunction), treat underlying cause before considering permanent pacing 1
• If AV block persists despite treatment of reversible cause, permanent pacing is recommended 1

Specific Clinical Scenarios:

• Neuromuscular diseases (myotonic dystrophy, Kearns-Sayre): Permanent pacing recommended for second-degree AV block regardless of symptoms 1
• Infiltrative cardiomyopathies (sarcoidosis, amyloidosis): Permanent pacing with possible defibrillator capability recommended 1
• Drug-induced AV block: If on chronic stable doses of necessary medications, reasonable to proceed with permanent pacing 1

Acute Management:

• For symptomatic bradycardia with hemodynamic compromise:
  • Atropine is reasonable for AV nodal block 1
  • Consider temporary transvenous pacing if refractory to medical therapy 1
  • Temporary transcutaneous pacing may be considered as bridge to transvenous pacing 1

Pacemaker Selection

• For patients with preserved LV function: Dual-chamber pacing preferred to maintain AV synchrony 4
• For patients with LV dysfunction (EF ≤50%): Consider biventricular pacing to prevent heart failure progression 5

Common Pitfalls and Caveats

1. Misdiagnosis of Type II block: Ensure PR intervals are truly constant before and after blocked P wave; vagal surges can mimic Type II block 6
2. Pseudo-AV block: Concealed His bundle or ventricular extrasystoles can mimic second-degree AV block 3
3. 2:1 AV block classification: Cannot be definitively classified as Type I or II without electrophysiological study 2
4. Overlooking infranodal block: Type I block with bundle branch block is often infranodal (60-70% of cases) and requires pacing 2
5. Failure to recognize vagally-mediated AV block: Permanent pacing should not be performed in asymptomatic vagally-mediated AV block 1

The management of second-degree AV block requires careful assessment of the block type, location, symptoms, and underlying conditions to determine appropriate treatment strategy, with permanent pacing being essential for Mobitz type II block due to its poor prognosis.