ABIM - AV Node Block, Sick Sinus Syndrome, & Bundle Branch Block

Here are key facts for American Board of Internal Medicine (ABIM) Exam from the AV Node Block, Sick Sinus Syndrome, & Bundle Branch Block tutorial, as well as points of interest at the end of this document 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. Diagnostic criteria: Long PR interval on ECG (> 200 milliseconds). 2. Clinical significance: Usually asymptomatic and generally benign. 3. Management approach: Usually no treatment required. 4. Medication association: Can be caused by channel blockers or digoxin. 5. Risk assessment: May increase risk of atrial fibrillation. 1. Differential diagnosis:

• Mobitz Type 1 (Wenckenbach's Block): PR interval gets progressively longer until AV node completely fails.
• Mobitz Type 2: PR interval doesn't change, but ventricular depolarization is skipped.

2. Clinical manifestations:

• Type 1: Dizziness, fainting
• Type 2: Chest pain, difficulty breathing, tiring easily, hypotension

3. Management decision-making:

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

4. Prognostic implication: Type 2 can lead to complete heart block (3rd degree). 1. Diagnostic hallmark: AV dissociation with no relationship between P waves and QRS complexes. 2. Clinical presentation: Fatigue/lethargy, dizziness, fainting, slow heart beat. 3. Management standard: Pacemaker implantation required. 4. Pathophysiologic consequence: Low cardiac output deprives organs of oxygen. 1. Diagnostic criteria: QRS complex greater than 120 ms in complete bundle branch block. 2. Clinical significance:

• RBBB may occur in otherwise healthy individuals but may indicate cardiac damage in the right side.
• LBBB is indicative of left heart disease and requires investigation.

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1. Epidemiologic context: Common in highly-trained athletes due to enlarged heart muscle. 2. Etiologic considerations: Consider myocarditis, hypokalemia, hypomagnesium in differential diagnosis. 1. Pathophysiologic distinction:

• Type 1 may be physiologic in healthy athletes.
• Type 2 is pathologic and requires intervention.

2. Etiologic assessment:

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

1. Etiology classification:

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

1. Clinical manifestations: Episodes of bradycardia, sinus pauses or arrest, and junctional escape beats. 2. Pathophysiology: Any condition causing damage to the SA node, including age-related degeneration. 3. Management standard: Often requires pacemaker implantation. 1. Right Bundle Branch Block:

• ECG findings: Wide, upwardly deflected QRS in lead V1; rsR' "bunny ear" pattern in leads V1-V3; slurred S waves in I, aVL, V5 and V6.
• Clinical correlation: May indicate cardiac damage in the right side of the heart.

2. 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: Always indicative of left heart disease.

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Below is information not explicitly contained within the tutorial but important for the ABIM Exam. 1. Reversible causes: Systematic approach to identifying and treating underlying causes before committing to permanent pacemaker. 2. Risk stratification: Using electrophysiologic studies to determine need for pacemaker in borderline cases. 3. Device selection: Choosing between single-chamber, dual-chamber, and biventricular pacing based on patient characteristics. 1. Diagnostic workup: Role of ambulatory monitoring in establishing diagnosis in patients with intermittent symptoms. 2. Atrial arrhythmias: Management of associated supraventricular arrhythmias including the "tachy-brady syndrome." 3. Anticoagulation: Evidence-based approach to thromboembolism prevention in sick sinus syndrome. 1. Myocardial ischemia: Evaluating new LBBB in the context of suspected acute coronary syndrome. 2. Progressive conduction disease: Monitoring for development of higher-grade blocks in patients with bundle branch blocks. 3. Cardiac resynchronization therapy: Selection criteria for CRT in patients with LBBB and heart failure.