What is the recommended management for a patient presenting with third-degree atrioventricular block and chronic right bundle-branch block?

Management of Complete Heart Block with Chronic Right Bundle Branch Block

Permanent pacemaker implantation is the definitive treatment for third-degree atrioventricular block, regardless of the presence of chronic right bundle branch block, and should be pursued after excluding reversible causes and stabilizing the patient. 1, 2

Immediate Assessment and Stabilization

Assess hemodynamic stability immediately by evaluating for symptomatic bradycardia including syncope, presyncope, heart failure symptoms, chest pain, hypotension, altered mental status, or dyspnea. 2, 3

• Establish continuous cardiac monitoring with pulse oximetry and frequent blood pressure checks to detect hemodynamic deterioration. 2
• Obtain intravenous access and prepare transcutaneous pacing pads during initial assessment. 2
• Acquire a 12-lead ECG to confirm third-degree AV block, determine QRS morphology (narrow versus wide escape rhythm), and evaluate for acute myocardial infarction. 2, 4

Exclude Reversible Causes First

Before proceeding to permanent pacing, systematically rule out reversible etiologies including acute MI, drug toxicity (beta-blockers, calcium-channel blockers, digoxin), electrolyte abnormalities, Lyme carditis, myocarditis, thyroid disorders, and infiltrative diseases. 1, 2, 3

• If a reversible cause is identified, provide medical therapy and supportive care, including temporary transvenous pacing if necessary, before determining need for permanent pacing. 1
• Critical pitfall: Do not implant a permanent pacemaker if the AV block completely resolves after treatment of a reversible cause—this is classified as harmful. 1, 2

Acute Medical Management

For AV-Nodal Level Block (Narrow QRS Escape)

• Administer atropine 0.5–1.0 mg IV bolus, repeating every 3–5 minutes up to a maximum cumulative dose of 3 mg for symptomatic AV-nodal level block. 1, 2, 5
• Avoid atropine doses <0.5 mg as they may paradoxically worsen the block via central vagal stimulation. 2

For Infranodal Block (Wide QRS Escape) or Persistent Symptoms

• Atropine is ineffective for infranodal (His-Purkinje) blocks—do not delay pacing in these patients. 1, 2
• Beta-adrenergic agonists (isoproterenol, dopamine, dobutamine, or epinephrine) may be considered when the likelihood of coronary ischemia is low. 1, 2, 5
• In the setting of acute inferior MI, intravenous aminophylline may be considered. 1, 2, 5

Temporary Pacing Strategy

Initiate transcutaneous pacing immediately as a bridge to transvenous pacing for hemodynamically unstable patients or those who do not respond to atropine. 2, 3

• Temporary transvenous pacing is reasonable for symptomatic or hemodynamically significant bradycardia refractory to medical therapy. 1, 3
• For anticipated prolonged temporary pacing, an externalized permanent active-fixation lead is preferred over standard passive-fixation temporary leads. 1, 2
• Do not postpone transcutaneous pacing to administer atropine in hemodynamically unstable patients. 2

Indications for Permanent Pacemaker (Class I)

Permanent pacemaker implantation is indicated for third-degree AV block at any anatomic level in the following scenarios:

• Any symptomatic bradycardia including heart failure symptoms, syncope, or ventricular arrhythmias presumed due to AV block. 1, 2
• Asymptomatic patients with high-risk features: documented asystole ≥3.0 seconds, escape rate <40 bpm, or escape rhythm below the AV node. 2
• Third-degree AV block with atrial fibrillation and bradycardia with pauses ≥5 seconds. 2
• Third-degree AV block requiring medications that cause symptomatic bradycardia. 2

Special Consideration for Chronic Right Bundle Branch Block

The presence of chronic right bundle branch block does not alter the indication for permanent pacing in third-degree AV block. 1

• The combination of complete heart block with pre-existing bundle branch block indicates infranodal disease and carries high risk for sudden death. 6
• Do not discharge asymptomatic patients with third-degree AV block and high-risk features (escape rate <40 bpm, ventricular escape rhythm, or pauses ≥3 seconds) without pacemaker placement. 2

Post-Myocardial Infarction Context

If third-degree AV block occurs in the setting of acute MI:

• Temporary pacing is indicated for medically refractory symptomatic or hemodynamically significant bradycardia. 1
• A mandatory waiting period is required before determining the need for permanent pacing, as the block may be transient. 1
• Permanent pacing is indicated after the waiting period if second-degree Mobitz type II, high-grade AV block, alternating bundle branch block, or persistent/infranodal third-degree AV block remains. 1
• Do not perform permanent pacing if transient AV block resolves during the observation period. 1

Shared Decision-Making

Engage in shared decision-making with the patient regarding pacemaker implantation, discussing procedural benefits and risks, short- and long-term complications, and alternative therapies in light of the patient's goals of care, preferences, and values. 1

• Do not implant a pacemaker in patients with significant comorbidities where pacing therapy is unlikely to provide meaningful clinical benefit, or if patient goals of care strongly preclude pacemaker therapy. 1

Critical Pitfalls to Avoid

• Never assume third-degree AV block is benign based on age alone—thorough evaluation and definitive treatment are warranted regardless of patient age. 2
• Do not rely on atropine for infranodal blocks—its effect is limited to AV-nodal level conduction. 1, 2
• Do not perform permanent pacing for asymptomatic vagally mediated AV block—this is classified as harmful. 1, 2, 3
• In the setting of acute anterior-wall MI, atropine may exacerbate ischemia and is contraindicated for infranodal block. 2