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AV Node Block for USMLE Step 3

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AV Node Block for the USMLE Step 3 Exam
Overview of AV Node Block
Atrioventricular (AV) node block involves delayed or failed conduction of electrical impulses from the atria to the ventricles. AV block is categorized by the degree of delay, ranging from mild (first-degree) to complete dissociation (third-degree), each with unique clinical implications and management.
Classification of AV Node Blocks
  • First-Degree AV Block:
    • Defined by a prolonged PR interval (>200 ms) with every atrial impulse conducted to the ventricles.
    • Typically asymptomatic and often discovered incidentally on ECG.
    • May result from medications (e.g., beta-blockers, calcium channel blockers), increased vagal tone, aging, or myocardial disease.
    • Generally does not require treatment unless symptomatic.
  • Second-Degree AV Block:
    • Some atrial impulses fail to conduct to the ventricles, leading to dropped QRS complexes.
    • Mobitz Type I (Wenckebach):
    • Characterized by progressive PR interval prolongation until a QRS complex is dropped.
    • Often due to reversible causes like high vagal tone or medications; typically benign and asymptomatic.
    • Rarely requires intervention unless symptomatic.
    • Mobitz Type II:
    • Sudden non-conduction of an atrial impulse without prior PR prolongation, resulting in dropped QRS complexes.
    • Often indicates structural heart disease and has a high risk of progression to complete heart block.
    • Symptomatic cases generally warrant permanent pacemaker placement.
  • Third-Degree (Complete) AV Block:
    • Complete dissociation between atrial and ventricular rhythms, with no impulses conducting through the AV node.
    • The ventricles rely on a slow escape rhythm, which can lead to severe bradycardia and symptoms such as fatigue, syncope, and even heart failure.
    • Requires immediate temporary pacing if hemodynamically unstable, followed by permanent pacemaker implantation.
AV Node Block Types
Etiology
  • Intrinsic Causes:
    • Fibrosis and Degeneration: Common in elderly patients due to age-related fibrosis (e.g., Lev’s and Lenègre’s diseases).
    • Ischemic Heart Disease: AV block may occur with inferior myocardial infarction (affecting the RCA, which supplies the AV node).
    • Inflammatory Disorders: Myocarditis, Lyme disease, sarcoidosis, and Chagas disease can infiltrate the AV node, causing varying degrees of block.
    • Congenital Heart Disease: Neonatal lupus from maternal lupus antibodies is a common cause of congenital heart block.
  • Extrinsic Causes:
    • Medications: AV nodal blocking drugs like beta-blockers, calcium channel blockers, and digoxin can exacerbate AV block.
    • Electrolyte Imbalances: Hyperkalemia and hypermagnesemia can slow AV node conduction.
    • Increased Vagal Tone: Common in athletes and during sleep; typically reversible and benign.
Clinical Presentation
  • First-Degree AV Block:
    • Typically asymptomatic, but some may report mild fatigue or dizziness.
  • Second-Degree AV Block:
    • Mobitz I: Often asymptomatic, though mild dizziness or fatigue may occur if symptoms are present.
    • Mobitz II: More likely to present with symptoms, such as fatigue, presyncope, or syncope, due to unpredictable dropped beats.
  • Third-Degree (Complete) AV Block:
    • Symptoms can include severe fatigue, dizziness, syncope, and hypotension due to loss of coordinated atrial and ventricular contraction and reliance on a slow ventricular escape rhythm.
Diagnosis
  • Electrocardiogram (ECG):
    • First-Degree AV Block: PR interval >200 ms with all P waves followed by QRS complexes.
    • Second-Degree AV Block (Mobitz I): Progressive PR interval lengthening, leading to a dropped QRS.
    • Second-Degree AV Block (Mobitz II): Fixed PR interval with random dropped QRS complexes.
    • Third-Degree AV Block: No consistent relationship between P waves and QRS complexes, indicating independent atrial and ventricular rhythms.
  • Holter Monitoring:
    • Useful for intermittent AV block or when symptoms do not correlate with initial ECG findings.
Management
  • First-Degree AV Block:
    • No treatment is typically needed unless symptomatic; in such cases, discontinuation of causative medications may be considered.
  • Second-Degree AV Block:
    • Mobitz I: Usually benign and does not require intervention; manage reversible causes if symptomatic.
    • Mobitz II: Requires a permanent pacemaker due to the risk of progression to complete heart block.
  • Complete Heart Block (Third-Degree):
    • Immediate temporary pacing may be needed in unstable patients, followed by permanent pacemaker placement for long-term management.
Key Points
  • Types of AV Block:
    • Includes first-degree, second-degree (Mobitz I and II), and third-degree (complete) AV blocks, each defined by specific ECG findings.
  • Etiology:
    • Common causes include age-related degeneration, ischemic heart disease, infiltrative diseases, medications, and electrolyte disturbances.
  • Diagnosis:
    • ECG is essential to identify the type of AV block; Holter monitoring can detect intermittent block in symptomatic patients.
  • Management:
    • First-degree and Mobitz I generally do not require intervention; Mobitz II and third-degree blocks often require permanent pacemaker placement to manage symptoms and prevent progression.

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