What are the causes and mechanisms of Supraventricular Tachycardia (SVT) in pediatric patients?

Causes and Mechanisms of SVT in Children

Accessory pathway-mediated tachycardia (AVRT) is the dominant mechanism in pediatric SVT, accounting for over 70% of cases in infants and approximately 55% in adolescents, while AVNRT becomes progressively more common with age, increasing from 9-13% in infants to 30-50% in teenagers. 1

Age-Related Distribution and Presentation

Temporal Patterns

• Approximately half of all pediatric SVT presents within the first 4 months of life, with subsequent age-related peaks occurring at 5-8 years and after 13 years of age 1
• The mechanism shifts dramatically with age: accessory pathway-mediated tachycardia predominates in infancy but decreases progressively through adolescence 1

Clinical Presentation by Age

• Congestive heart failure occurs in up to 20% of infants with SVT, particularly those with incessant tachycardia, and may rarely necessitate mechanical cardiopulmonary support during initial therapy 1
• In older children and adolescents, symptoms typically include palpitations, light-headedness, chest discomfort, anxiety, dyspnea, or fatigue 2

Primary Mechanisms in Pediatric Populations

Accessory Pathway-Mediated Tachycardia (AVRT)

• Reentry through manifest or concealed accessory pathways represents the most common mechanism overall in children 3, 4
• Manifest Wolff-Parkinson-White (WPW) syndrome shows ventricular pre-excitation on baseline ECG with delta waves 5
• Concealed accessory pathways (unidirectional retrograde accessory pathways) conduct only in the retrograde direction and account for a substantial proportion of pediatric SVT 3
• Cardiac arrest may be the initial manifestation of pre-excitation, even in previously asymptomatic patients 1

Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

• AVNRT frequency increases progressively with age, from 9-13% in infants to 30-50% in teenagers 1
• The mechanism involves reentry within or near the AV node, utilizing dual AV nodal pathways (fast and slow) 3, 4
• More commonly diagnosed in adolescents than younger children 6

Focal Atrial Tachycardia

• Accounts for 10-15% of SVT across all pediatric age groups 6
• Results from enhanced automaticity or triggered activity from an ectopic atrial focus 3, 4
• Can be either automatic or reentrant in mechanism 4

Junctional Ectopic Tachycardia

• Caused by enhanced (abnormal) automaticity from an ectopic focus in the AV junction (including the His bundle) 1
• Typically seen in infants postoperatively after cardiac surgery for congenital heart disease 1
• Uncommon in adults but represents an important mechanism in the pediatric population 1

Sinoatrial Node Reentry

• Involves reentry within or around the sinoatrial node 3, 4
• Less common than other mechanisms but represents a distinct entity in children 4

Associated Conditions

Congenital Heart Disease

• Atrial flutter in children is predominantly observed after repair of congenital heart disease 1
• Macroreentrant atrial tachycardias occur with circuits defined by atrial scars from prior heart surgery, with location determining ablation approach and risks 1
• Multiple reentry circuits can be present in patients with surgical history 1

Atrial Fibrillation in Children

• Uncommon in childhood, accounting for less than 3% of supraventricular arrhythmias 1
• May occur as a consequence of AVRT or AVNRT in adolescents 1
• Can be associated with repaired congenital heart disease 1

Electrophysiologic Mechanisms

Reentry Mechanisms

• Reentry phenomena represent the most common underlying mechanism, requiring a circuit with areas of differential conduction and refractoriness 2, 4
• Can occur at multiple anatomic levels: sinoatrial node, atrial muscle, AV node, or through accessory pathways 3, 4

Automaticity

• Enhanced automaticity causes focal atrial tachycardia and junctional ectopic tachycardia 1, 4
• Results from abnormal spontaneous depolarization of cardiac tissue 4

Risk Stratification Considerations

Sudden Cardiac Death Risk

• The absence of prior symptoms does not preclude risk in patients with pre-excitation, as cardiac arrest may be the initial presentation 1
• Risk stratification with 24-hour ambulatory monitoring or treadmill exercise testing is often considered for children with pre-excitation to assess persistence 1
• There is a small but definite risk of sudden death in patients with WPW syndrome, whether symptomatic or not 7

Hemodynamic Consequences

• SVT can cause more hemodynamic instability in children than adults, particularly in infants 6
• Ventricular tachycardias (though less common) cause more hemodynamic instability than SVT and require closer monitoring 6

Key Distinguishing Features from Adult SVT

Mechanism Distribution

• Ectopic mechanisms are more common in children than adults, with focal atrial and junctional tachycardias representing a larger proportion 4
• The relative frequency of accessory pathway-mediated tachycardia decreases with age as patients transition to adulthood 1

Natural History

• SVT in young patients varies significantly from adults in terms of mechanism, risk of heart failure or cardiac arrest, risks of interventional therapy, natural history, and psychosocial impact 1
• Many children experience spontaneous resolution of accessory pathway conduction over time 1