USMLE/COMLEX 1 - AV Node Block, Sick Sinus Syndrome, & Bundle Branch Block

Here are key facts for USMLE Step 1 & COMLEX-USA Level 1 from the AV Node Block, Sick Sinus Syndrome, & Bundle Branch Block tutorial, as well as points of interest at the end 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. Long PR interval on ECG (> 200 milliseconds). 2. Usually asymptomatic. 3. Common in highly-trained athletes, due to enlarged heart muscle. 4. Risk factors include myocarditis, hypokalemia, hypomagnesium, and certain medications (channel blockers or digoxin). 1. Mobitz Type 1 (Wenckenbach's Block): PR interval gets progressively longer until AV node completely fails and ventricular contraction is skipped. 2. Mobitz Type 2: PR interval doesn't change, but ventricular depolarization is skipped. 3. Treatment: Type 1 - no treatment if asymptomatic; Type 2 - pacemaker. 4. Type 1 may be physiologic in healthy athletes, while Type 2 is pathologic. 1. AV dissociation: No electrical communication between atria and ventricles, therefore, no relationship between P waves and QRS complexes. 2. Symptoms include fatigue, dizziness, fainting, slow heart beat. 3. Treatment: Pacemaker. 4. Can be congenital (in infants from mothers with autoimmune conditions) or acquired (from complications in heart surgery, infection, medications). 1. QRS complex greater than 120 ms in a complete bundle branch block. 2. Right Bundle Branch Block: wide, upwardly deflected QRS complex in lead V1. 3. Left Bundle Branch Block: wide downwardly deflected QRS complex in V1, indicative of left heart disease.

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1. May increase risk of atrial fibrillation. 2. Caused by medications including channel blockers or digoxin. 1. Type 1 symptoms: Dizziness, fainting. 2. Type 2 symptoms: Chest pain, difficulty breathing, tiring easily, hypotension. 3. Risk factors for Type 2 include cardiac injury (fibrosis, sclerosis, scarring from heart attack), Lyme disease, and drugs (beta blockers, calcium channel blockers, digoxin, amiodarone). 4. Type 2 can lead to complete heart block (3rd degree). 1. Low cardiac output deprives organs of oxygen. 2. Acquired causes include radiotherapy, infection (such as diphtheria or rheumatic fever), hypertension, cancer, and medications (digoxin, calcium-channel blockers, beta blockers, tricyclic antidepressants, clonidine). 1. Characterized by episodes of bradycardia, sinus pauses or arrest, and junctional escape beats. 2. Can be caused by any condition that causes damage to the SA node, including age-related degeneration. 3. May require pacemaker implantation. 1. Right Bundle Branch Block ECG findings: rsR' "bunny ear" pattern in leads V1-V3; slurred S waves in I, aVL, V5 and V6. 2. Left Bundle Branch Block ECG findings: Broad monophasic R wave and absent Q waves in I, V5, V6; ST and T wave displacement opposite to major deflection of QRS complex. 3. RBBB can occur in otherwise healthy individuals but may indicate cardiac damage in the right side of the heart. 4. LBBB is indicative of left heart disease.

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Below is information not explicitly contained within the tutorial but important for USMLE & COMLEX 1. 1. Atropine may temporarily improve symptoms in Mobitz Type I but is generally ineffective in Type II or third-degree block. 2. Lenegre's disease and Lev's disease are degenerative diseases of the conduction system that can lead to AV blocks. 3. Exercise testing in first-degree AV block may reveal progression to higher-grade block. 1. Tachycardia-bradycardia syndrome is a variant with alternating episodes of fast and slow rhythms. 2. Risk of thromboembolism is high, and anticoagulation is often required. 3. Chronotropic incompetence (inability to increase heart rate appropriately with exercise) is a common feature. 1. New-onset LBBB should raise suspicion for acute myocardial infarction. 2. Bifascicular block (RBBB plus left anterior or posterior fascicular block) increases risk of developing complete heart block. 3. Rate-dependent bundle branch block occurs only at certain heart rates.