How should atrioventricular (AV) block be managed, including first-degree, second-degree Mobitz I (Wenckebach), second-degree Mobitz II, and third-degree blocks?

Management of Atrioventricular Block

The management of AV block is determined primarily by the degree of block, presence of symptoms, and anatomic location of the conduction disturbance—with permanent pacing indicated for all Mobitz II, high-grade, and third-degree AV block regardless of symptoms, while first-degree and Mobitz I blocks require pacing only when symptomatic. 1

First-Degree AV Block

Asymptomatic Patients

• No treatment is required for asymptomatic first-degree AV block with PR interval <300 ms. 1, 2
• Permanent pacemaker implantation is not indicated and should be avoided in asymptomatic patients—this is a Class III (potentially harmful) recommendation. 1, 2
• Athletes with asymptomatic first-degree AV block can participate in all competitive sports unless excluded by underlying structural heart disease. 2

Symptomatic Patients (PR ≥300 ms)

• Permanent pacemaker implantation is reasonable (Class IIa) when symptoms are clearly attributable to profound first-degree AV block (typically PR >300 ms), causing hemodynamic compromise or "pseudo-pacemaker syndrome" symptoms such as fatigue, exercise intolerance, dizziness, or dyspnea. 1, 2
• These symptoms result from loss of AV synchrony, leading to decreased cardiac output and increased pulmonary capillary wedge pressure. 2

Diagnostic Evaluation

• For symptomatic patients or those with PR ≥300 ms: Perform ambulatory ECG monitoring (Class IIa) to correlate symptoms with rhythm and detect intermittent higher-grade block. 1, 2
• Exercise stress testing is reasonable (Class IIa) for patients with exertional symptoms to assess whether the PR interval shortens appropriately (normal response) or worsens (suggests infranodal disease). 1, 2

High-Risk Features Requiring Closer Monitoring

• Coexisting bundle branch block or bifascicular block significantly increases risk of progression to complete heart block. 2
• Neuromuscular diseases (myotonic dystrophy, Kearns-Sayre syndrome, Emery-Dreifuss muscular dystrophy) warrant close monitoring due to unpredictable progression—permanent pacing with defibrillator capability may be considered even if asymptomatic (Class IIb). 1, 2

Second-Degree AV Block: Mobitz Type I (Wenckebach)

General Characteristics

• Mobitz I typically represents AV nodal-level block with progressive PR prolongation before a dropped QRS, usually associated with narrow QRS complexes. 1
• The traditional teaching that Mobitz I is benign requires reconsideration based on recent evidence. 3

Management Algorithm

Asymptomatic Patients:

• In patients <45 years of age or those with physiologic causes (high vagal tone, athletes, sleep), observation without pacing is appropriate. 1, 3
• In patients ≥45 years of age, even asymptomatic Mobitz I may not be benign—five-year survival is significantly reduced compared to the general population (53.5% vs 68.6%, p<0.001), and pacemaker implantation should be considered. 3
• Critical caveat: Mobitz I with wide QRS or bundle branch block is infranodal in 60-70% of cases and should be treated as high-risk, similar to Mobitz II. 4

Symptomatic Patients:

• Permanent pacing is indicated (Class I) for symptomatic bradycardia with documented Mobitz I causing syncope, presyncope, dizziness, fatigue, or heart failure symptoms. 1

Reversible Causes:

• Permanent pacing should not be performed for vagally mediated AV block (Class III). 1
• Exclude reversible causes: medications (beta-blockers, calcium channel blockers, digoxin), hyperkalemia, acute myocardial infarction (especially inferior MI), Lyme disease, or acute rheumatic fever. 2, 5, 6

Second-Degree AV Block: Mobitz Type II

Critical Distinction

• Mobitz II represents infranodal block (His-Purkinje system), typically with wide QRS complexes and constant PR intervals before dropped beats. 1, 7, 4
• This is a precursor to complete heart block and carries high risk of sudden progression to third-degree block and Stokes-Adams syncope. 1, 7

Management

• Permanent pacemaker implantation is recommended (Class I) for all Mobitz II block, regardless of symptoms. 1
• This is particularly important when associated with fascicular block (Class IIa). 1
• Do not wait for symptoms to develop—the risk of sudden complete heart block and death is too high. 7, 4

Diagnostic Pitfalls

• 2:1 AV block cannot be classified as Mobitz I or II from surface ECG alone—the anatomic site must be determined by QRS width, clinical context, and sometimes electrophysiology study. 7, 4
• Mobitz II has never been reported in inferior MI or young athletes—apparent "Mobitz II" in these settings is likely misdiagnosed Mobitz I. 4
• Concealed His bundle or ventricular extrasystoles can mimic both Mobitz I and II (pseudo-AV block). 4

High-Grade and Third-Degree (Complete) AV Block

Management

• Permanent pacemaker implantation is recommended (Class I) for all acquired high-grade or third-degree AV block not attributable to reversible causes, regardless of symptoms. 1
• Symptomatic third-degree AV block is a Class I indication, with observational studies strongly suggesting permanent pacing improves survival, especially if syncope has occurred. 1
• Asymptomatic third-degree AV block is a Class IIa indication for permanent pacing. 1

Special Populations

• Infiltrative cardiomyopathies (cardiac sarcoidosis, amyloidosis) with second-degree Mobitz II, high-grade, or third-degree block: permanent pacing with defibrillator capability is reasonable (Class IIa) if meaningful survival >1 year is expected. 1
• Neuromuscular diseases (muscular dystrophy, Kearns-Sayre syndrome) with any second- or third-degree block or HV interval ≥70 ms: permanent pacing with defibrillator capability is recommended (Class I). 1
• Lamin A/C gene mutations with PR >240 ms and LBBB: permanent pacing with defibrillator capability is reasonable (Class IIa). 1

Acute Myocardial Infarction Context

• Inferior MI: AV block is usually transient, vagally mediated, and AV nodal—often resolves without intervention. 2, 4
• Anterior MI: New bundle branch block or bifascicular block with first-degree AV block may warrant prophylactic temporary pacing due to high risk of progression. 2
• Revascularization should be considered in patients with AV block who have not received reperfusion therapy. 2

Medication-Induced AV Block

• If AV block is caused by guideline-directed medical therapy (beta-blockers, calcium channel blockers, digoxin, amiodarone) for which there is no alternative and continued treatment is clinically necessary, permanent pacing is recommended (Class I). 1
• If the medication is non-essential and can be discontinued, permanent pacing is not indicated (Class III). 2

Atrial Fibrillation with AV Block

• Permanent pacing is recommended (Class I) for patients with permanent atrial fibrillation and symptomatic bradycardia. 1

Common Pitfalls to Avoid

• Do not implant pacemakers for isolated, asymptomatic first-degree AV block—this is Class III (potentially harmful). 1, 2
• Do not assume narrow QRS Mobitz I is always benign—in patients ≥45 years, consider pacing even if asymptomatic. 3
• Do not confuse Mobitz I with wide QRS as benign—this is infranodal in 60-70% of cases and requires pacing. 4
• Do not pace vagally mediated AV block—this is Class III (not indicated). 1
• Do not delay pacing in Mobitz II waiting for symptoms—sudden progression to complete block can be fatal. 1, 7, 4
• Exclude reversible causes before committing to permanent pacing: medications, electrolyte abnormalities (especially hyperkalemia), acute MI, Lyme disease, acute rheumatic fever. 2, 5, 6