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Arrhythmias - AV Node Block
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Arrhythmias - AV Node Block

AV Node Block, Sick Sinus, & Bundle Branch Block
In this tutorial, we review key aspects of AV Node Block, Sick Sinus Syndrome, & Bundle Branch Block.
First-Degree AV Block
Description:
Long PR interval on ECG (> 200 milliseconds).
Symptoms & Signs:
Asymptomatic
Treatments:
Usually, none.
Risk Factors:
Common in highly-trained athletes, due to enlarged heart muscle; Myocarditis, hypokalemia or hypomagnesium, certain medications (channel blockers or digoxin).
Clinical Concerns:
May increase risk of atrial fibrillation.
Second-Degree AV Block
Mobitz Type 1 (aka, Wenckenbach's Block) = PR interval gets progressively longer until AV node completely fails and ventricular contraction is completely skipped.
Morbitz Type 2 = PR interval doesn't change, but ventricular depolarization is skipped.
Symptoms & Signs: Type 1 = Dizziness, fainting. Type 2 = Chest pain, difficulty breathing, tiring easily, hypotension.
Treatments: Type 1 = No treatment if asymptomatic; consider medications as the source of the issue. Type 2 = Pacemaker
Risk Factors: Type 1 may be physiologic in healthy athletes. Type 2 is pathologic. Cardiac injury (fibrosis, sclerosis, scarring from heart attack), Lyme disease (Type 2), Drugs (beta blockers, calcium channel blockers, digoxin, amiodarone), vavluopathy.
Clinical Correlations: Type 2 can lead to complete heart block (3rd degree heart block).
Third-Degree AV Block
Description:
AV dissociation: No electrical communication between atria and ventricles, therefore, no relationship between P waves and QRS complexes.
Symptoms & Signs:
Fatigue/lethargy, dizziness, fainting, slow heart beat.
Treatment:
Pacemaker.
Risk Factors:
Congenital in infants from mothers with autoimmune condition or in infants born with other cardiac conditions. Acquired as result of complications in heart surgery, radiotherapy, infection (such as diphtheria or rheumatic fever), hypertension, cancer, radiofrequency ablation, medications (digoxin, calcium-channel blockers, beta blockers, tricyclic antidepressants, clonidine).
Clinical Concerns:
Low cardiac output deprives organs of oxygen.
Review of Cardiac Cycle; Cardiac Muscle Cell; Wigger's Diagram
Sick sinus syndrome
General Information
Sick sinus syndrome (aka sinus node dysfunction): Episodes of bradycardia, sinus pauses or arrest, and junctional escape beats.
Can be caused by any condition that causes damage to the SA node, including age-related degeneration; may require pacemaker implantation.
Bundle Branch Blocks
General Information
Right and Left Bundle branch blocks occur when conduction through one or both bundle branches conduction Is partially or completely interrupted. QRS complex is greater than 120 ms is a complete bundle branch block.
Right bundle branch block
Right bundle branch block can occur in otherwise healthy individuals, but may indicate cardiac damage in the right side of the heart.
On the ECG, we will see wide, upwardly deflected QRS complex in lead V1.
Additionally: rsR' bunny ear pattern in leads V1-V3; slurred S waves in I, aVL, V5 and V6.
Left bundle branch block
Left bundle branch block is indicative of left heart disease.
On the ECG, we'll see wide downwardly deflected QRS complex in V1. Additionally: Broad monophasic R wave and absent Q waves in I, V5, V6; ST and T wave displacement opposite to major deflection of QRS complex.

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