AV Node Block for USMLE Step 1
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AV Node Block for the USMLE Step 1 Exam
Overview of AV Node Block
Atrioventricular (AV) node block occurs when the conduction of electrical impulses from the atria to the ventricles is delayed or blocked at or near the AV node. This can lead to symptoms of bradycardia, syncope, or even heart failure in severe cases.
Classification of AV Node Blocks
- First-Degree AV Block:
- Defined by a prolonged PR interval >200 ms on ECG.
- All atrial impulses are conducted to the ventricles, maintaining a 1:1 ratio.
- Generally asymptomatic and often found incidentally.
- Causes include high vagal tone, certain medications (e.g., beta-blockers, calcium channel blockers), and aging.
- Usually does not require treatment unless symptomatic.
- Second-Degree AV Block:
- Occurs when some atrial impulses fail to conduct to the ventricles, resulting in dropped beats.
- Mobitz Type I (Wenckebach):
- Progressive lengthening of the PR interval until a QRS complex is dropped.
- Often due to increased vagal tone or medications; usually benign and may not need treatment.
- Mobitz Type II:
- Sudden non-conduction of atrial impulses without prior PR prolongation, leading to a dropped QRS complex.
- More likely to progress to a complete AV block and is often associated with structural heart disease.
- May require pacemaker placement, especially if symptomatic.
- Third-Degree (Complete) AV Block:
- Complete dissociation between atrial and ventricular activity, with no impulse conduction through the AV node.
- The ventricles rely on an escape rhythm from a lower pacemaker, resulting in significant bradycardia.
- Often presents with symptoms of fatigue, dizziness, or syncope.
- Urgent treatment with pacing is typically required to prevent hemodynamic instability.
Etiology
- Intrinsic Causes:
- Age-Related Degeneration: Fibrosis of the conduction system, commonly seen in elderly patients.
- Ischemic Heart Disease: Inferior wall myocardial infarctions can impact the AV node.
- Inflammatory or Infectious Conditions: Myocarditis, Lyme disease, and sarcoidosis can affect the AV node and surrounding tissue.
- Congenital Heart Disease: Congenital AV block, often seen in infants of mothers with lupus, results from antibodies crossing the placenta.
- Extrinsic Causes:
- Medications: Beta-blockers, digoxin, calcium channel blockers, and antiarrhythmics can slow AV node conduction.
- Electrolyte Abnormalities: Hyperkalemia and hypermagnesemia can exacerbate AV block.
- Increased Vagal Tone: Common in athletes or during sleep, causing benign AV block that typically resolves when vagal tone decreases.
Clinical Presentation
- First-Degree AV Block:
- Often asymptomatic; may present with mild fatigue or dizziness if symptomatic.
- Second-Degree AV Block:
- Mobitz I: Usually asymptomatic; some patients may experience mild dizziness or fatigue.
- Mobitz II: More likely to cause symptoms such as fatigue, lightheadedness, or syncope, especially with multiple dropped beats.
- Third-Degree (Complete) AV Block:
- Symptoms are more pronounced due to the slow escape rhythm.
- Commonly presents with syncope, severe fatigue, and signs of reduced cardiac output (e.g., hypotension, weakness).
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 followed by a dropped QRS.
- Second-Degree AV Block (Mobitz II): Consistent PR intervals with sudden dropped QRS complexes.
- Third-Degree AV Block: P waves and QRS complexes occur independently, with no consistent relationship.
- Holter Monitoring:
- Useful for detecting intermittent AV block in symptomatic patients, particularly with syncope or transient symptoms.
Management
- First-Degree AV Block:
- Usually does not require treatment unless symptomatic.
- Consider stopping causative medications if symptomatic.
- Second-Degree AV Block:
- Mobitz I: Typically requires no treatment; address any reversible causes.
- Mobitz II: Often necessitates pacemaker placement due to the risk of progression to complete heart block.
- Complete Heart Block (Third-Degree):
- Requires immediate intervention with temporary pacing if unstable, followed by permanent pacemaker placement.
Key Points
- Types of AV Block:
- Classified into first-degree, second-degree (Mobitz I and II), and third-degree (complete) blocks based on ECG findings and conduction delay.
- Etiology:
- Common causes include age-related degeneration, ischemia, medications, and increased vagal tone.
- Diagnosis:
- ECG is the primary tool for identifying the degree of AV block; Holter monitoring is useful for detecting intermittent block.
- Management:
- First-degree block typically does not need intervention; second-degree (Mobitz II) and third-degree blocks often require pacemaker placement to prevent complications.