DESCRIPTION

• Bradycardia is diagnosed when the heart rate (ventricular rate) is slower than the lower limit of normal for age.
• There are generally only two mechanisms that cause bradycardia
  • Sinus bradycardia: a slow sinus node causing a slow atrial rate and therefore a slow ventricular rate
  • Heart block: abnormal atrioventricular (AV) node (or His-Purkinje system) with a normal atrial rate but slow ventricular rate
• Both sinus bradycardia and heart block can be transient or persistent.
• Sinus bradycardia is usually asymptomatic and rarely requires therapy; however, cardiac pacing is more likely to be required in children with congenital heart disease (CHD).
• Although mild forms of heart block can be benign, therapy is more often required than for sinus bradycardia.

EPIDEMIOLOGY

• The overall incidence and prevalence of bradycardia is very difficult to determine in children in large part due to bradycardia being a benign condition or associated with an athletic lifestyle in many children. Estimates suggest that severe bradycardia requiring pacemaker implantation only occurs in approximately 1,500 children in the United States each year.
• Persistent sinus bradycardia is common in healthy, athletic children and is often a benign finding.

ETIOLOGY

• Cardiac causes
  • Myocarditis and other cardiac sequalae of infection (including Lyme myocarditis, Chagas disease, rubella, mumps, and post–COVID-19 and multisystem inflammatory syndrome of childhood [MIS-C])
  • Injury to the conduction system during cardiac surgery/catheterization or rarely due to coronary abnormalities
  • Maternal autoimmune disease can result in neonatal heart block.
  • Inherited arrhythmias
    • Brugada syndrome
    • Long QT syndrome
    • Familial bradycardia/heart block
  • Acute rheumatic disease
  • Cardiomyopathy (including left ventricular noncompaction, certain types of genetic dilated cardiomyopathies, and muscular dystrophies)
  • CHD (such as atrial septal defects related to NKX2.5 mutations, L-transposition of the great arteries, and heterotaxy/polysplenia)
• Extracardiac causes
  • Hypoxemia (particularly apnea of prematurity)
  • Hypothermia
  • Metabolic derangements (severe hyperkalemia, hypocalcemia, and hypermagnesemia)
  • Anorexia (including anorexia nervosa)
  • High vagal tone (usually transient)
    • Nasopharyngeal stimulation
    • Breath holding spells
    • Gastroesophageal reflux
    • Coughing
    • Obstructive sleep apnea
    • Increased intracranial pressure
  • Medications
    • Digitalis
    • β-Blockers
    • Calcium channel blockers
    • Amiodarone (and most other antiarrhythmic medications)
    • Lithium
    • Clonidine
  • Hypothyroidism
  • Athletic state (usually benign)

RISK FACTORS

• Maternal anti-Ro and anti-La antibodies (associated with maternal autoimmune diseases such as systemic lupus erythematosus and Sjögren syndrome) are associated with congenital complete heart block.
• Certain forms of CHD, including congenitally corrected transposition and heterotaxy syndrome (polysplenia type), are associated with heart block.
• Heart block and sinus bradycardia can occur after CHD surgery.
  • When heart block occurs, it is usually immediately after surgery.
  • Sinus bradycardia can develop more gradually (such as in Fontan physiology in single ventricle patients).
• Athletic children are more likely to have sinus bradycardia (which is benign).

GENERAL PREVENTION

Few, if any, preventative measures are available to avoid bradycardia. However, preventative measures to avoid heart block in neonates born to mothers with autoimmune disease (such as maternal corticosteroid administration or hydroxychloroquine) have been trialed.

PATHOPHYSIOLOGY

• Heart rate is controlled by the following:
  • The cardiac conduction system, which is composed of the sinus node, AV node, and His-Purkinje system
  • Parasympathetic and sympathetic input
• Bradycardia can result from abnormalities of the sinus node (whether from intrinsic sinus node dysfunction or from abnormal parasympathetic and sympathetic input).
• Bradycardia can also result from heart block from abnormalities of the AV node (intrinsic or abnormal parasympathetic and sympathetic input) or His-Purkinje system.
• During bradycardia, different slow escape rhythms may be present (e.g., ventricular escape rhythm, junctional escape rhythm, atrial escape rhythm).
  • 1st-degree AV block (delay between atrial and ventricular conduction) does not cause bradycardia.
  • 2nd-degree AV block occurs when some but not all atrial contractions result in ventricular contractions and can cause bradycardia.
  • 3rd-degree or complete AV block is diagnosed when the atrial contractions do not affect ventricular contractions and can result in severe bradycardia.
• Bradycardia can lead to inadequate cardiac output resulting in symptoms or even sudden death.
• Premature atrial contractions that block due to AV block can result in bradycardia, but this is generally a very benign phenomenon (even when in a bigeminy pattern).

COMMONLY ASSOCIATED CONDITIONS

• Maternal autoimmune disease is associated with congenital complete heart block (due to injury from anti-Ro and anti-La antibodies).
• Infection (active myocarditis and post–COVID-19 MISC) can be associated with heart block (sometimes transient).
• Cardiac surgery can be associated with bradycardia.
  • Sinus bradycardia (atrial septal defect closure, Fontan procedure, atrial switch operation)
  • Heart block (double switch operation, tricuspid or mitral valve replacement, ventricular septal defect closure, AV canal repair)
• Athletic children frequently have sinus bradycardia or even 1st- or 2nd-degree (Mobitz type I) heart block at rest.