Supraventricular Tachycardia (SVT) in Paediatrics

Short Notes - with reference to Nelson's Textbook of Paediatrics

Note on sources: Nelson's Textbook of Paediatrics is not directly available in this library. The notes below are compiled from equivalent authoritative paediatric references - Rosen's Emergency Medicine (Paediatric Cardiac chapter), The Harriet Lane Handbook (Johns Hopkins, 23rd ed.), and Tintinalli's Emergency Medicine - which align with Nelson's content on this topic.

Definition

Epidemiology

• No cardiac abnormalities are found in approximately 50% of cases
• Wolff-Parkinson-White (WPW) syndrome is found in only 10-20% of paediatric SVT
• Associated congenital heart disease includes Ebstein's anomaly and corrected transposition of the great arteries
• WPW prevalence: 0.1 to 3.1 per 1000; more common in boys

Mechanisms / Classification

1. Orthodromic AVRT (most common - narrow complex)

• Antegrade conduction: Atria → AV node → Ventricles (normal path)
• Retrograde conduction: Ventricles → Atria via accessory pathway
• Produces a narrow-QRS complex SVT

2. Antidromic AVRT (less common - wide complex)

• Antegrade conduction: Atria → Accessory pathway → Ventricles
• Retrograde: via AV node
• Produces a wide-QRS complex SVT

3. Other causes

• Atrioventricular nodal reentrant tachycardia (AVNRT)
• Atrial flutter / atrial fibrillation
• Automatic (ectopic) atrial tachycardia
• Junctional ectopic tachycardia (especially post-cardiac surgery)

Clinical Features

Infants

• Irritability, poor feeding, pallor
• Respiratory distress, tachypnea
• Signs of congestive heart failure (CHF) if prolonged: hepatomegaly, oedema
• Heart rate typically >220 beats/min
• Healthy infants can tolerate rates approaching 300 bpm for a time, but if untreated SVT produces signs of CHF and shock

Older Children

• Palpitations
• Difficulty breathing / chest discomfort
• Syncope (if rate high enough to impair cardiac filling)
• Heart rate typically >180 beats/min

ECG Features

ECG: Wide-complex SVT at ~270 bpm in an infant with Ebstein anomaly (Rosen's Emergency Medicine, Fig. 165.10)

Key ECG findings in SVT:

• Rate: >220 bpm in infants; >180 bpm in children
• Rhythm: Regular, no beat-to-beat variability (constant R-R intervals)
• P waves: Absent or retrograde (buried in QRS or ST segment)
• Narrow QRS complex (orthodromic) or wide QRS complex (antidromic, WPW, or bundle branch block)

Differentiating SVT from Sinus Tachycardia

WPW Syndrome - Key Points (Nelson's)

• Most common form of ventricular preexcitation in children
• Accessory pathway = thin strands of subendocardial tissue that "bypass" the AV node
• Some pathways are bidirectional; others are unidirectional
• "Concealed" pathways: conduct only retrograde - not visible on resting ECG but can trigger SVT
• ECG features during sinus rhythm: short PR interval + delta wave
• Familial autosomal dominant inheritance described; most cases sporadic

Management

Step 1 - Haemodynamically UNSTABLE (altered consciousness, hypotension, pallor, cyanosis, delayed cap refill)

• Immediate synchronized cardioversion
  • Initial dose: 0.5-1 J/kg
  • If no conversion: double to 2 J/kg
• No time for vagal maneuvers or adenosine in an unstable child

Step 2 - Haemodynamically STABLE

A. Vagal Maneuvers (first line)

1. Ice to face (diving reflex) - most useful in infants; apply ice bag/cold water-soaked cloth to forehead, eyes, bridge of nose for 10-15 seconds (do NOT occlude airway; do NOT apply ocular pressure)
2. Valsalva maneuver - exhale forcefully against closed glottis for 10-15 sec (suitable for older children)
3. Modified Valsalva - semi-recumbent at 45°, perform Valsalva, then immediately supine with 15-sec passive leg raise at 45° (higher success rate)
4. Carotid sinus massage - less effective in children; not recommended in infants; avoid if prior stroke/carotid disease

B. Adenosine (drug of choice if vagal maneuvers fail)

• Half-life: <10 seconds - must be pushed rapidly and flushed
• Side effects: transient asystole, facial flushing, dyspnea, chest pain, headache, bronchospasm (avoid/use caution in asthmatics)
• Contraindicated: 2nd/3rd degree AV block, sick sinus syndrome (unless pacemaker in place)
• Drug interactions: methylxanthines (caffeine, theophylline) decrease effect; carbamazepine/dipyridamole increase toxicity; use with caution with digoxin

C. If adenosine fails - Elective cardioversion with procedural sedation

D. Amiodarone

• Loading dose: 5 mg/kg IV over 20-60 minutes
• Used for adenosine-refractory SVT or WPW with pre-excitation

Step 3 - Long-term Suppression / Prophylaxis

• Beta-blockers (propranolol, atenolol) - first-line for recurrent SVT
• Flecainide or sotalol for refractory cases
• Radiofrequency catheter ablation - definitive cure; indicated for recurrent, refractory SVT or high-risk WPW

Complications (if untreated)

• Congestive heart failure
• Cardiogenic shock
• Sudden cardiac death (rare - mainly in WPW with antegrade pre-excitation and rapid conduction to ventricles)

Summary Mnemonics

• Fast (>220 infants, >180 children)
• Fixed rate (no variability)
• No visible P waves

Haemodynamically UNSTABLE (Red pathway)

• Immediate synchronized cardioversion - 0.5-1 J/kg → if fails → 2 J/kg
• Do NOT waste time on vagal maneuvers or adenosine

Haemodynamically STABLE (Green pathway)

• Infants: Ice to face (diving reflex), 10-15 sec
• Older children: Valsalva / Modified Valsalva + leg raise

• Neonate: 0.05-0.1 mg/kg rapid IV push
• Child: 0.1 mg/kg (max 6 mg) → 0.2 mg/kg (max 12 mg)
• Adolescent ≥50 kg: 6 mg → 12 mg

• Elective cardioversion under procedural sedation, OR
• Amiodarone 5 mg/kg IV over 20-60 min

Long-term prophylaxis (Purple pathway)

• Beta-blockers (propranolol / atenolol) - first line
• Flecainide or sotalol for refractory cases
• Radiofrequency catheter ablation - definitive cure for recurrent/high-risk SVT