Bradyarrhythmias Bradycardia: General Comments Bradycardia is defined as a heart rate below the normal range for age that may result from intrinsic injury to the sinus node, atrioventricular (AV) node, or HisPurkinje conduction system or secondary to extrinsic factors acting on native conduction The decision to intervene for bradycardia often relates to its impact on systemic perfusion, exacerbation of heart failure, or the possibility of pausedependent tachyarrhythmias Symptoms of bradycardia—such as syncope, dizziness, chest pain, shortness of breath, and fatigue—especially in younger children, are not always readily apparent The differential diagnosis of symptomatic bradycardia is broad (Box 22.1), and aside from structural congenital heart defects and cardiomyopathies, consideration should be given to endocrinopathies, infections, drug ingestion, breath-holding spells, apnea, anorexia, and central nervous system disorders In addition, various genetic abnormalities (i.e., NKX2.5, SCN5A, SCN1B and TRPM4) have been implicated in the pathogenesis of sinus node dysfunction or progressive cardiac conduction defects A simplistic yet clinically relevant approach to the patient with bradycardia from a conduction vantage point begins with abnormalities in the sinus node and progresses through block within the AV node/His-Purkinje system Box 22.1 Etiology of Bradyarrhythmias in Children and Adolescents Cardiac ▪ Congenital heart disease ▪ Heart block ▪ Endocarditis ▪ Cardiomyopathy ▪ Channelopathies ▪ Heart transplant Central/Neurologic ▪ Hypoxia ▪ Hypothermia ▪ Trauma (increased intracranial pressure) ▪ Increased vagal tone ▪ Myotonic muscular dystrophy ▪ Autonomic dysfunction ▪ Immature breathing (premature infant) ▪ Gastroesophageal reflux (infant) Metabolic ▪ Acidosis ▪ Hyperkalemia ▪ Hypokalemia ▪ Malnutrition ▪ Anorexia ▪ Hypoglycemia Rheumatologic ▪ Systemic lupus erythematosus ▪ Rheumatoid arthritis Toxins ▪ Digoxin ▪ β-blockers ▪ Calcium-channel blockers ▪ Clonidine ▪ Lithium Infections ▪ Lyme disease ▪ Chagas disease ▪ Acute rheumatic fever Sinus Node Dysfunction The sinus node is located at the cavoatrial junction between the crista terminalis and the sulcus terminalis on the epicardial surface The sinus node comprises a complex and heterogeneous arrangement of ion channels including the If (“funny hyperpolarization-activated inward current” and the L- and T-type calcium currents) responsible for a self-propagating action potential (Video 22.1) Sinus node abnormalities may be secondary to a slower than normal sinus rate (sinus bradycardia), sinus arrest, sinus pause, sinuatrial block, loss of a normal P-wave axis with default to an ectopic atrial rhythm (most often low right atrial), or complete loss of atrial activity and default to a junctional rhythm (typically narrow QRS complex) Sinus pauses or sinus arrest are almost always related to previous surgeries in the region of the sinus node or its blood supply, drugs, enhanced vagatonia, metabolic derangements, or hypoxemia Sinus node dysfunction is increasingly recognized as a late complication following congenital heart surgery either from scarring of the sinus node or injury to the sinus nodal artery or its autonomic inputs In addition, certain anatomic substrates, such as left atrial isomerism (heterotaxy), may have an absent sinus node as part of their anatomic constellations Patients with sinus node dysfunction are frequently asymptomatic; however, that does not reliably translate into the fact that those individuals would not benefit from atrial pacing Patients with sinus node dysfunction may exhibit chronotropic incompetence and have symptoms of exertional fatigue Each of these bradycardias may also occur as part of the bradycardia-tachycardia syndrome with concomitant atrial reentrant arrhythmias The primary criterion for antibradycardia pacing is the presence of symptoms