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AV Node Block for ABIM

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AV Node Block for the American Board of Internal Medicine Exam
Overview of AV Node Block
AV node (AVN) block, or atrioventricular block, involves a delay or interruption in the conduction of electrical impulses from the atria to the ventricles. It occurs at or near the AV node and can lead to bradycardia, syncope, or heart failure. Understanding AVN block is critical for clinicians, as it impacts hemodynamics and may require urgent treatment based on severity.
Classification of AV Node Blocks
  • First-Degree AV Block:
    • Characterized by a prolonged PR interval (>200 ms) with a 1:1 atrial to ventricular conduction ratio.
    • Typically asymptomatic and often discovered incidentally on ECG.
    • Can result from increased vagal tone, aging, medications (e.g., beta-blockers, calcium channel blockers, digoxin), or underlying heart disease.
    • Usually benign and does not require treatment; however, if symptomatic, underlying causes should be addressed.
  • Second-Degree AV Block:
    • There is intermittent failure of atrial impulses to conduct to the ventricles, resulting in dropped beats.
    • Type I (Mobitz I or Wenckebach):
    • Progressive prolongation of the PR interval until a QRS complex is dropped.
    • Often due to reversible causes such as increased vagal tone, medications, or ischemia.
    • Generally benign, especially if asymptomatic, and may not require treatment; symptomatic cases may need atropine or pacing.
    • Type II (Mobitz II):
    • Sudden failure of conduction without progressive PR lengthening; consistent PR intervals with random QRS drops.
    • More likely to progress to complete heart block and is often due to structural heart disease.
    • Frequently symptomatic and associated with syncope or fatigue, necessitating evaluation and often pacemaker insertion due to the high risk of progression to third-degree AV block.
  • Third-Degree (Complete) AV Block:
    • Complete dissociation between atrial and ventricular electrical activity, with no conduction through the AV node.
    • Atrial impulses are blocked entirely, and the ventricles rely on an escape rhythm from a subsidiary pacemaker, often in the His-Purkinje system or below.
    • Associated with significant bradycardia, hemodynamic compromise, syncope, and can lead to sudden cardiac death.
    • Urgent treatment with temporary pacing, followed by permanent pacemaker placement, is usually required.
AV Node Block Types
Etiology
  • Intrinsic Causes:
    • Degenerative Disease: Most common cause in elderly patients, often due to age-related fibrosis of the conduction system, specifically Lev’s disease or Lenègre’s disease.
    • Ischemic Heart Disease: Inferior myocardial infarctions can damage the AV node due to the blood supply provided by the right coronary artery (RCA); anterior infarctions can affect the His-Purkinje system.
    • Inflammatory Disorders: Sarcoidosis, Lyme disease, Chagas disease, and myocarditis can involve the AV node.
    • Congenital Heart Disease: AVN block can be congenital, particularly in conditions like maternal lupus, resulting in neonatal lupus with congenital heart block.
  • Extrinsic Causes:
    • Medications: Beta-blockers, calcium channel blockers, digoxin, and antiarrhythmic drugs (e.g., amiodarone) can slow AV node conduction.
    • Electrolyte Abnormalities: Hyperkalemia and hypermagnesemia may affect the AV node.
    • Vagal Tone: Increased vagal tone, often due to sleep apnea, athletic training, or neurocardiogenic syncope, may lead to reversible AV block.
Clinical Presentation
  • Symptoms:
    • First-Degree and Mobitz I: Often asymptomatic; when symptomatic, may present with fatigue, lightheadedness, or mild dizziness.
    • Mobitz II and Complete Heart Block: Likely symptomatic with fatigue, presyncope, syncope, palpitations, and in some cases, signs of heart failure (e.g., dyspnea, edema).
    • Exercise Intolerance: Common in patients with significant AVN block due to decreased cardiac output.
  • Physical Exam Findings:
    • Bradycardia: Common in all types of AVN block, especially complete heart block.
    • Jugular Venous Pulsations (JVP): Cannon waves may be observed in complete AV block due to atrial contraction against a closed tricuspid valve.
    • Irregular Pulse: Often associated with dropped beats in second-degree AV block.
Diagnosis
  • Electrocardiogram (ECG):
    • First-Degree AV Block: PR interval >200 ms, with every P wave followed by a QRS.
    • Second-Degree AV Block, Mobitz I: Progressive PR interval prolongation with a subsequent dropped QRS; pattern repeats in cycles.
    • Second-Degree AV Block, Mobitz II: Fixed PR intervals with intermittent non-conducted P waves (dropped QRS).
    • Third-Degree AV Block: No relationship between P waves and QRS complexes; ventricular escape rhythm is present.
  • Holter Monitor:
    • Useful in detecting intermittent AV blocks, particularly for patients with unexplained syncope or transient symptoms.
  • Electrophysiological Studies (EPS):
    • Occasionally indicated if the location of the block is unclear or if an AV block is suspected in the context of syncope with inconclusive noninvasive studies.
Management
  • First-Degree AV Block:
    • Asymptomatic cases generally require no treatment.
    • Discontinuation of contributing medications (e.g., beta-blockers, calcium channel blockers) if symptomatic.
  • Second-Degree AV Block:
    • Mobitz I:
    • Often benign and may not require treatment if asymptomatic.
    • Symptomatic cases may respond to atropine or temporary pacing; reversible causes should be managed.
    • Mobitz II:
    • Higher risk of progression to complete AV block; therefore, pacemaker implantation is often recommended.
    • Patients with symptoms or evidence of syncope typically need a permanent pacemaker.
  • Complete Heart Block:
    • Requires immediate intervention, especially if hemodynamically unstable.
    • Temporary transcutaneous or transvenous pacing in acute settings, followed by the placement of a permanent pacemaker.
  • Pacemaker Indications:
    • Class I (Strong Indication): Complete AV block, symptomatic Mobitz II, asymptomatic Mobitz II with wide QRS, and high-grade AV block with syncope.
    • Class IIa (Moderate Indication): Asymptomatic complete AV block with low escape rates, advanced AV block in neuromuscular diseases, and atrial fibrillation with AV block.
Complications
  • Progression to Higher-Degree Block:
    • Particularly in Mobitz II, progression to third-degree AV block may occur, necessitating a pacemaker.
  • Heart Failure and Cardiogenic Shock:
    • Severe bradycardia in complete AV block may lead to reduced cardiac output and can precipitate heart failure or hypotension.
  • Sudden Cardiac Death:
    • In untreated high-grade AV block, especially complete heart block, there is a risk of sudden cardiac death due to loss of effective ventricular contraction.
Key Points
  • Classification:
    • AVN block is classified into first-degree, second-degree (Mobitz I and Mobitz II), and third-degree (complete) blocks.
  • Etiology:
    • Causes include degenerative disease, ischemia, infections, congenital factors, medications, and electrolyte abnormalities.
  • Clinical Presentation:
    • Varies from asymptomatic in mild blocks to syncope, fatigue, and heart failure symptoms in higher-grade blocks.
  • Diagnosis:
    • ECG is the primary tool; Holter monitoring and EPS are useful in detecting intermittent or uncertain blocks.
  • Management:
    • First-Degree: No treatment for asymptomatic patients; address reversible causes if symptomatic.
    • Second-Degree Mobitz I: Rarely requires treatment; monitor if asymptomatic.
    • Second-Degree Mobitz II and Complete Heart Block: High risk of progression; permanent pacemaker is generally indicated.

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