USMLE/COMLEX 2 - AV Node Block, Sick Sinus Syndrome, & Bundle Branch Block

Here are key facts for USMLE Step 2 & COMLEX-USA Level 2 from the AV Node Block, Sick Sinus Syndrome, & Bundle Branch Block tutorial, as well as points of interest that are not directly addressed in this tutorial but should help you prepare for the boards. See the tutorial notes for further details and relevant links.

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1. Clinical presentation: Usually asymptomatic with long PR interval on ECG (> 200 milliseconds). 2. Management: Usually no treatment required. 3. Clinical context: Consider as a potential side effect of channel blockers or digoxin. 4. Risk assessment: May increase risk of atrial fibrillation. 1. Clinical differentiation:

• Mobitz Type 1: Presents with dizziness, fainting; PR interval gets progressively longer until AV node fails.
• Mobitz Type 2: Presents with chest pain, difficulty breathing, tiring easily, hypotension; PR interval doesn't change.

2. Management approach:

• Type 1: No treatment if asymptomatic; consider medications as source of the issue.
• Type 2: Pacemaker implantation indicated.

3. Prognostic implications: Type 2 can lead to complete heart block (3rd degree). 1. Clinical presentation: Fatigue/lethargy, dizziness, fainting, slow heart beat. 2. Diagnostic hallmark: AV dissociation with no relationship between P waves and QRS complexes. 3. Definitive management: Pacemaker implantation. 4. Clinical consequence: Low cardiac output depriving organs of oxygen. 1. Clinical constellation: Episodes of bradycardia, sinus pauses or arrest, and junctional escape beats. 2. Management: Often requires pacemaker implantation.

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1. Epidemiology: Common in highly-trained athletes due to enlarged heart muscle. 2. Differential diagnosis: Consider myocarditis, hypokalemia, hypomagnesium when evaluating first-degree AV block. 1. Clinical context:

• Type 1 may be physiologic in healthy athletes.
• Type 2 is always pathologic.

2. Etiology:

• Cardiac injury (fibrosis, sclerosis, scarring from heart attack)
• Lyme disease (particularly Type 2)
• Medication-induced: beta blockers, calcium channel blockers, digoxin, amiodarone
• Valvulopathy

1. Etiology:

• Congenital: In infants from mothers with autoimmune conditions or with other cardiac conditions.
• Acquired: Complications of heart surgery, radiotherapy, infection (diphtheria, rheumatic fever), hypertension, cancer, radiofrequency ablation.
• Medication-induced: digoxin, calcium-channel blockers, beta blockers, tricyclic antidepressants, clonidine.

1. Diagnostic criteria: QRS complex greater than 120 ms in complete bundle branch block. 2. Right Bundle Branch Block:

• Can occur in otherwise healthy individuals
• ECG findings: Wide, upwardly deflected QRS in lead V1; rsR' "bunny ear" pattern in V1-V3; slurred S waves in I, aVL, V5, V6
• Clinical significance: May indicate cardiac damage in right heart

3. Left Bundle Branch Block:

• ECG findings: Wide downwardly deflected QRS in V1; broad monophasic R wave and absent Q waves in I, V5, V6; ST and T wave displacement opposite to QRS
• Clinical significance: Indicative of left heart disease

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1. Temporary transcutaneous pacing may be necessary in emergency situations for high-grade AV blocks. 2. Isoproterenol can be used as a temporary measure to increase heart rate in symptomatic bradycardia. 3. Discontinuation of offending medications is the first step in management of drug-induced AV blocks. 1. Holter monitoring is essential for diagnosis, especially in intermittent symptoms. 2. Dual-chamber pacemakers are preferred over single-chamber to prevent pacemaker syndrome. 3. Anticoagulation therapy is often indicated due to associated atrial fibrillation. 1. Newly diagnosed LBBB requires urgent evaluation for acute coronary syndrome. 2. Rate-dependent bundle branch blocks may only appear during stress testing. 3. Alternating bundle branch blocks suggest severe conduction system disease requiring prompt intervention.