What is the management approach for a patient presenting with a second-degree atrioventricular (AV) block?

Management of Second-Degree Atrioventricular Block

The management of second-degree AV block depends critically on distinguishing Mobitz type II from Mobitz type I: Mobitz type II requires permanent pacing regardless of symptoms, while Mobitz type I typically requires pacing only if symptomatic or associated with hemodynamic compromise. 1

Initial Assessment and Risk Stratification

Determine the Type of Second-Degree Block

Mobitz Type II characteristics:

• Constant PR intervals in conducted beats with sudden dropped beats 2, 3
• Almost always infranodal (below the AV node) 2, 4
• Often associated with wide QRS complex (bundle branch block) 3, 4
• High risk of progression to complete heart block 2, 5
• Never occurs with sinus slowing - simultaneous sinus slowing suggests vagal-mediated type I block, not type II 3, 4

Mobitz Type I (Wenckebach) characteristics:

• Progressive PR interval prolongation before the dropped beat 3, 4
• Usually AV nodal (supranodal) location 2, 4
• Typically narrow QRS complex 3, 4
• More benign prognosis, especially without organic heart disease 5

2:1 AV block cannot be classified as type I or type II but requires assessment of QRS width, clinical context, and response to atropine or exercise to determine the site of block 3, 4

Assess Clinical Context

In acute myocardial infarction:

• Inferior MI: Type I block is common, usually AV nodal, often transient and vagally mediated 1
• Anterior MI: Type II or high-grade block suggests extensive necrosis with infranodal disease and carries high mortality 1
• Mobitz type II in acute MI is a Class I indication for temporary pacing 1

Assess for hemodynamic compromise:

• Hypotension, altered mental status, chest pain, dyspnea, or signs of shock 1
• These findings mandate immediate intervention regardless of block type 1

Management Algorithm

For Mobitz Type II Block

Permanent pacing is recommended (Class I) regardless of symptoms because all correctly defined type II blocks are infranodal and carry high risk of progression to complete heart block 1, 3, 4

In acute settings (e.g., MI):

• Apply transcutaneous pacing patches immediately 1
• Administer IV atropine (0.5 mg every 3-5 minutes, maximum 3 mg) for symptomatic bradycardia, though response is often poor with infranodal block 1
• Insert temporary transvenous pacemaker if hemodynamically unstable or high-risk features present 1
• Consider urgent revascularization if patient has not received reperfusion therapy 1

For Mobitz Type I Block

Management depends on symptoms and hemodynamics:

Asymptomatic with normal hemodynamics (Class III - no pacing indicated):

• No specific treatment required 1
• Monitor for progression, especially if associated with bundle branch block 1
• Avoid or use caution with AV nodal blocking agents (beta-blockers, calcium channel blockers, digoxin, amiodarone) 1

Symptomatic or hemodynamically compromised:

• IV atropine (0.5 mg every 3-5 minutes, maximum 3 mg) as first-line therapy 1
• Temporary pacing if atropine fails (Class I indication) 1
• Consider permanent pacing if symptoms persist after treating reversible causes 1

In acute MI with inferior wall involvement:

• Often requires no treatment unless severe hypotension present 1
• Usually resolves spontaneously or after reperfusion 1
• Atropine is first-line if treatment needed 1

For 2:1 AV Block

Determine the likely site of block:

• Narrow QRS + young patient or inferior MI = likely AV nodal (better prognosis) 2, 4
• Wide QRS + anterior MI or elderly patient = likely infranodal (requires pacing) 2, 4
• Response to atropine: improvement suggests nodal block; no response suggests infranodal 4
• Exercise testing: PR shortening suggests nodal; worsening block suggests infranodal 1

Management:

• If infranodal features present, treat as Mobitz type II with permanent pacing 4
• If nodal features present, manage as Mobitz type I based on symptoms 4

Special High-Risk Scenarios Requiring Immediate Pacing Consideration

New bifascicular block with second-degree AV block in acute MI:

• Class I indication for temporary pacing 1
• High risk of progression to complete heart block 1

Neuromuscular diseases (myotonic dystrophy, Kearns-Sayre syndrome):

• Permanent pacing recommended even for second-degree block due to unpredictable progression (Class I) 1
• Consider ICD capability if meaningful survival >1 year expected 1

Infiltrative cardiomyopathy (sarcoidosis, amyloidosis):

• Permanent pacing with ICD capability is reasonable (Class IIa) for second-degree Mobitz type II or high-grade block 1

Reversible Causes to Address

Before permanent pacing, evaluate and treat:

• Medications: beta-blockers, calcium channel blockers, digoxin, amiodarone 1
• Electrolyte abnormalities: hyperkalemia, hypomagnesemia 1
• Acute ischemia requiring revascularization 1
• Lyme disease or other infectious/inflammatory causes 1

If AV block completely resolves with treatment of reversible cause, permanent pacing should NOT be performed (Class III: Harm) 1

However, if AV block persists despite treating the reversible cause, permanent pacing IS recommended (Class I) 1

Critical Pitfalls to Avoid

Do not misdiagnose type I as type II:

• Coexistence of obvious type I pattern elsewhere in the same recording (e.g., Holter) effectively rules out type II block 3, 4
• Sinus slowing concurrent with AV block suggests vagal-mediated type I, not type II 3, 4
• Type II block has never been reported in inferior MI or young athletes 3

Do not delay pacing for asymptomatic Mobitz type II:

• Even asymptomatic patients require permanent pacing due to high risk of sudden progression to complete heart block 1, 6
• Case reports document progression from asymptomatic 2:1 block to complete heart block during anesthesia induction 6

Do not assume narrow QRS excludes infranodal disease:

• Type I block with bundle branch block is infranodal in 60-70% of cases (excluding acute MI) and requires pacing 3, 4

Recognize pseudo-AV block:

• Concealed His bundle or ventricular extrasystoles can mimic both type I and type II block 3, 4
• Nonconducted premature atrial contractions can mimic second-degree block 2

Prognosis

Mobitz type I without organic heart disease:

• Relatively benign course with low risk of progression 5
• In one study of 19 patients followed mean 1395 days, only one required pacing for symptoms 5

Mobitz type I with organic heart disease:

• Prognosis related to severity of underlying cardiac disease, not the conduction abnormality itself 5
• In one study, 16 of 37 patients died during follow-up, primarily from heart failure or MI, not sudden death 5

Mobitz type II:

• High risk of progression to complete heart block and sudden death 2
• Requires permanent pacing regardless of symptoms or underlying disease 1