AVRT (Accessory Pathway-Mediated): Most common SVT mechanism in infants. Utilizes accessory bypass tract (e.g., Wolff-Parkinson-White syndrome). Orthodromic AVRT (antegrade AV node, retrograde pathway) produces narrow QRS. Antidromic AVRT (antegrade pathway, retrograde AV node) produces wide QRS.
AVNRT: Rare in infancy; increases in childhood and adolescence. Involves slow and fast functional pathways within AV node,. Associated with exercise and syncope.
Atrial Flutter: Macroreentrant circuit,. Neonates frequently have normal hearts; older children often have congenital heart disease (e.g., stretched atria from tricuspid atresia, Ebstein anomaly, Fontan procedure).
Ectopic Atrial Tachycardia (EAT): Automatic focus in atrial myocardium remote from sinus node. Often incessant, leading to tachycardia-induced cardiomyopathy.
Junctional Ectopic Tachycardia (JET): Automatic focus near His bundle. Most commonly occurs postoperatively after congenital heart surgery,.
Clinical Presentation
Infants/Neonates: May present with heart failure if unrecognized for 6–24 hours. Symptoms include poor feeding, ashen color, restlessness, irritability, tachypnea, and hepatomegaly. Heart rates range from 240–300 beats/min.
Older Children/Adolescents: Abrupt onset of palpitations, precordial discomfort, fatigue, dizziness, or dyspnea,,. Heart rates typically >180 beats/min. Exacerbated by caffeine or decongestants.
Fetal Presentation: Hydrops fetalis due to in utero heart failure. Mortality approaches 50% without treatment.
Diagnostic Evaluation
Electrocardiography (ECG) Features vs. Sinus Tachycardia
Heart Rate: SVT exhibits fixed, rapid rates (>230 beats/min in infants) lacking normal beat-to-beat variability seen in sinus tachycardia.
P Waves: Visible in only 50–60% of neonates on standard ECG. Abnormal P-wave axis (normal is positive in I and aVF) strongly argues against sinus tachycardia,.
Transesophageal Lead: Detects P waves effectively when surface ECG is obscure.
Differential Diagnosis of Narrow QRS Tachycardia
Arrhythmia
P Waves
P-QRS Relation
Response to Adenosine
Sinus Tachycardia
Normal morphology & axis
1:1
Transient slowing; AV block.
Ectopic Atrial Tachycardia
Abnormal, different from baseline
Usually 1:1
No effect or transient AV block,.
Atrial Flutter
Saw-tooth appearance (>240/min)
2:1 or 1:1
Transient AV block unmasks flutter waves; rarely terminates,.
AV Nodal Reentry (AVNRT)
Usually not visible (masked by QRS)
1:1
Sudden termination.
AV Reentry (AVRT)
Inverted (retrograde VA conduction)
1:1
Sudden termination.
Junctional Ectopic (JET)
Normal (AV dissociation) or inverted
Complete AV dissociation diagnostic
No effect on rate; transient retrograde VA block.
Management
Acute Termination
Vagal Maneuvers: Ice bag over entire face for 15–30 seconds in infants,. Valsalva maneuver, straining, or head-standing in older children. Eyeball pressure contraindicated..
Adenosine: Drug of choice for stable patients. Rapid IV push 0.1 mg/kg (up to 6 mg). May increase to 0.2 mg/kg (up to 12 mg) if ineffective. Contraindicated without direct current (DC) cardioversion standby due to risk of initiating atrial fibrillation.
DC Cardioversion: Indicated immediately for hemodynamically unstable patients or urgent heart failure. Synchronized shock at 0.5–2 J/kg,.
Chronic Pharmacotherapy
Medication
Class
Pediatric Indications & Nuances
Beta-Blockers (Propranolol, Atenolol, Nadolol)
Class II
Mainstay therapy for patients without antegrade accessory pathways (Non-WPW). Atenolol/Nadolol dose: 0.5–2 mg/kg/day,.
Digoxin
Class IV/Misc
Effective in infants. Contraindicated in WPW syndrome; increases antegrade conduction via bypass tract, risking ventricular fibrillation,.
Verapamil
Class IV
Utilized in older children. Contraindicated in infants <1 year due to reduced cardiac output, hypotension, and cardiac arrest risk. Contraindicated in WPW.
Amiodarone / Sotalol / Flecainide
Class III / IC
Indicated for resistant tachycardias. Flecainide limited to patients with normal ventricular function.
Catheter Ablation
Definitive, potentially curative therapy utilizing radiofrequency or cryoablation,.
Success Rates: 90–98% depending on pathway location.
Indications (Class I): Ventricular dysfunction, refractory to medications, avoiding long-term medication, hemodynamic compromise during SVT, resuscitated cardiac arrest (WPW), or WPW with syncope.
Complications: AV block, perforation, thrombosis. Higher complication incidence in children weighing <15 kg.
Fetal SVT Management
Maternal Transplacental Therapy: First-line treatment. Digoxin, flecainide, and sotalol are most common.
Hydropic Fetus: Reduced transplacental bioavailability necessitates combination therapy. Direct fetal intramuscular (IM) digoxin injection into buttock/thigh rapidly decreases time to conversion, bypassing placental transfer issues.
