Diseases of the conduction system and bradyarrhythmias

Editor-in-Chief: Alena Goldman, MD

Bradyarrhythmias can be broadly divided into two categories: those at the atrial level that are caused by sinus node dysfunction, and those at the subatrial level resulting in AV conduction disturbances.

1. Sinus Node Dysfunction
SSS is defined by the electrocardiographic criteria,as clinical symptoms are often vague.
 * Sick Sinus Syndrome (SSS): refers to an abnormal impulse formation in the sinus node, and/or abnormal impulse conduction. It is frequently associated with AV nodal conduction disturbances, where alternating tachycardia (more commonly atrial fibrillation (AF) or atrial flutter, although can be seen with other supraventricular tachycardias) and bradycardia can be seen in up to 50% of patients.
 * Here is the list of findings that can be seen with SSS:


 * unexplained persistent bradycardia without warning
 * sinus arrest or sinus exit block
 * paroxysmal atrial fibrillation (PAF) followed by sinus arrest
 * SA block/arrest or severe sinus bradycardia following a cardioversion of atrial fibrillation
 * Alternating bradycardia and atrial tachyarrhythmias
 * Slow ventricular response in atrial fibrillation in patients off any nodal agents reflects AV nodal disease that often co-insides with a significant SA nodal dysfunction and an unstable sinus rhythm upon conversion
 * Inadequate sinus acceleration with exercise

Etiology
SA node dysfunction may be a result of intrinsic or extrinsic factors.
 * Intrinsic Factors:
 * Idiopathic degenerative disease (fibrosis) associated with aging: most common cause overall
 * SA Nodal Artery Involvement
 * Atherosclerosis
 * Vasculitis
 * Embolus
 * Infiltrative diseases
 * Amyloidosis
 * Hemochromatosis
 * Malignancies
 * Inflammatory conditions
 * Myocarditis
 * Pericarditis
 * Myotonic Dystrophy
 * Freidrech’s Ataxia
 * Collagen Vascular Diseases
 * SLE
 * Scleroderma
 * After cardiac surgery:
 * Corrective Cardiac Surgery for Congenital Heart Disease yabekref1 hayesref2
 * ASD repair
 * Transposition of the great arteries
 * Transplant
 * Congenital/genetic SA node dysfunction
 * Familial SSS is rare, but some familial cases have been reported:
 * SCN5a mutations
 * HCN4 mutations
 * Extrinsic Factors:
 * Drugs
 * Antiarrhythmics: classes Ia, Ic, III.
 * Antihypertensives:
 * Beta-blockers
 * Calcium Channel Blockers
 * Digitalis
 * Lithium: SA nodal abnormalities seen in up to 50% of patients
 * Dilantin
 * Cimetidine
 * Electrolyte Disturbances
 * Endocrine abnormalities
 * Inferior Myocardial Infarction (Bezold-Jarish phenomenon)
 * Autonomic Nervous System Influence
 * CSP
 * Vasovagal response
 * Miscellaneous
 * High Intracranial Pressure
 * Obstructive Sleep Apnea

SA Block
SA Block can be classified as the following:
 * 1) First degree SA Block: due to a prolonged Conduction
 * 2) Second degree SA Block: due to intermittent Conduction
 * 3) Type 1: Progressive prolongation
 * 4) Type 2: Sudden failure of conduction
 * 5) Third Degree SA Block: No conduction