AV Node Block for USMLE Step 2
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AV Node Block for the USMLE Step 2 Exam
Overview of AV Node Block
Atrioventricular (AV) node block describes delayed or failed conduction of electrical impulses from the atria to the ventricles. It can range from mild (first-degree) to complete (third-degree) block and is associated with bradycardia and potential hemodynamic compromise.
Classification of AV Node Blocks
- First-Degree AV Block:
- Characterized by a PR interval >200 ms on ECG, with each atrial impulse conducting to the ventricles.
- Typically asymptomatic and found incidentally; no dropped QRS complexes.
- Commonly associated with increased vagal tone, medications (e.g., beta-blockers, calcium channel blockers), and aging.
- Does not require treatment unless symptomatic.
- Second-Degree AV Block:
- Occurs when some atrial impulses do not reach the ventricles, leading to intermittently dropped QRS complexes.
- Mobitz Type I (Wenckebach):
- PR interval gradually lengthens until a QRS complex is dropped.
- Often benign and related to reversible causes (e.g., increased vagal tone, medications).
- Generally does not require treatment unless symptomatic.
- Mobitz Type II:
- Constant PR interval with unexpected non-conducted P waves and dropped QRS complexes.
- More serious, often due to structural heart disease, with higher risk of progressing to complete heart block.
- Symptomatic patients generally require pacemaker placement.
- Third-Degree (Complete) AV Block:
- Complete failure of AV conduction, with no relationship between atrial and ventricular rhythms.
- The ventricles rely on a slow escape rhythm, often resulting in significant bradycardia.
- Symptoms often include dizziness, fatigue, syncope, and signs of low cardiac output.
- Requires immediate intervention with temporary pacing and usually a permanent pacemaker.
Etiology
- Intrinsic Causes:
- Fibrosis and Degeneration: Most common cause in elderly patients; due to age-related changes.
- Ischemic Heart Disease: Inferior myocardial infarctions can affect the AV node, especially if the right coronary artery (RCA) is involved.
- Inflammatory Disorders: Sarcoidosis, Lyme disease, Chagas disease, and myocarditis can impact AV conduction.
- Congenital Heart Disease: Neonatal lupus from maternal lupus antibodies may lead to congenital heart block.
- Extrinsic Causes:
- Medications: AV nodal blocking agents, such as beta-blockers, calcium channel blockers, and digoxin, may cause AV block.
- Electrolyte Abnormalities: Hyperkalemia and hypermagnesemia may exacerbate AV block.
- Increased Vagal Tone: Seen in athletes and during sleep, reversible AV block related to high vagal tone often resolves without intervention.
Clinical Presentation
- First-Degree AV Block:
- Usually asymptomatic; may occasionally cause mild fatigue or dizziness if symptomatic.
- Second-Degree AV Block:
- Mobitz I: Often asymptomatic; if symptomatic, may present with mild fatigue or lightheadedness.
- Mobitz II: More likely to cause symptoms such as presyncope, fatigue, and syncope, especially with multiple dropped beats.
- Third-Degree (Complete) AV Block:
- Frequently symptomatic, presenting with syncope, severe fatigue, dizziness, or signs of heart failure due to slow ventricular escape rhythm.
Diagnosis
- Electrocardiogram (ECG):
- First-Degree AV Block: PR interval >200 ms without dropped QRS complexes.
- Second-Degree AV Block (Mobitz I): Gradually lengthening PR interval before a dropped QRS.
- Second-Degree AV Block (Mobitz II): Fixed PR interval with random dropped QRS complexes.
- Third-Degree AV Block: No association between P waves and QRS complexes, with independent atrial and ventricular rhythms.
- Holter Monitor:
- Useful for detecting intermittent AV block and assessing correlation with symptoms like syncope or dizziness.
Management
- First-Degree AV Block:
- Usually does not require intervention unless symptoms are present, in which case causative medications may need to be stopped.
- Second-Degree AV Block:
- Mobitz I: Typically benign; address reversible causes if symptomatic, but often does not require treatment.
- Mobitz II: Due to high risk of progression, a permanent pacemaker is often indicated, especially in symptomatic patients.
- Complete Heart Block (Third-Degree):
- Requires urgent intervention with temporary pacing in unstable patients, followed by permanent pacemaker placement.
Key Points
- Types of AV Block:
- Includes first-degree, second-degree (Mobitz I and II), and third-degree (complete) AV blocks.
- Etiology:
- Common causes include fibrosis, ischemia, medications, and high vagal tone.
- Diagnosis:
- ECG is essential for determining AV block type; Holter monitoring is useful for intermittent blocks.
- Management:
- First-degree and Mobitz I generally do not need intervention; Mobitz II and complete heart blocks often require pacemakers due to high risk of progression and symptoms.