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Case 005: SVT in Infant with WPW

Presentation

A 6-week-old male infant is brought to the ED for poor feeding and "fast breathing" for 1 day. Parents noticed he seems more tired than usual.

Vital Signs: - Heart rate: 280 bpm - Blood pressure: 62/40 mmHg - Respiratory rate: 60 - SpO2: 94% on room air - Temperature: 36.8°C

Exam: - Lethargic, mottled, cool extremities - Tachypneic with mild subcostal retractions - Liver palpable 4 cm below costal margin - Weak peripheral pulses

Initial Evaluation

12-Lead ECG: - Rate: 280 bpm - Regular narrow complex tachycardia - No visible P waves - R-R interval constant

Bedside Echo: - Severely depressed LV function (EF ~25%) - Dilated LV

Clinical Reasoning

Question 1: What is the rhythm diagnosis?

Answer **Supraventricular tachycardia (SVT)** - most likely AVRT (atrioventricular reentrant tachycardia) given age. Key features: - Regular narrow complex tachycardia - Rate ~280 bpm (typical infant SVT rate) - No discernible P waves (hidden in QRS/T) At this age, AVRT via accessory pathway is most common mechanism.

Question 2: Why is his LV function depressed?

Answer **Tachycardia-induced cardiomyopathy** Sustained rapid heart rate (likely >24 hours based on history) leads to: - Myocardial energy depletion - LV dysfunction - Heart failure symptoms This is REVERSIBLE with rate/rhythm control.

Question 3: Is this patient stable or unstable?

Answer **UNSTABLE** - requires immediate intervention Signs of hemodynamic compromise: - Hypotension (62/40) - Poor perfusion (mottled, cool, weak pulses) - Hepatomegaly (heart failure) - Depressed LV function - Altered mental status (lethargy)

Management

Question 4: What is the immediate treatment?

Answer **For unstable SVT with hemodynamic compromise:** 1. **Synchronized cardioversion 0.5-1 J/kg** - Sedation if possible but don't delay - Increase to 2 J/kg if unsuccessful Alternative in infant (if brief delay acceptable): - Ice to face (vagal maneuver) for 15-30 seconds - can be tried first - If no response, cardiovert **Do NOT delay cardioversion in truly unstable patient**

Question 5: After cardioversion, the rhythm converts. Post-conversion ECG shows:

ECG findings: - Rate: 140 bpm (sinus) - PR interval: 80 ms (short) - Delta wave present - QRS slightly widened

What is the diagnosis?

Answer **Wolff-Parkinson-White (WPW) syndrome** ECG triad: 1. Short PR interval (<120ms in infants) 2. Delta wave (slurred QRS upstroke) 3. Wide QRS This confirms AVRT as the SVT mechanism - orthodromic AVRT using accessory pathway retrograde.

Ongoing Management

Question 6: What is the maintenance therapy for infant WPW with SVT?

Answer **Prophylactic antiarrhythmic therapy:** First-line options for infants: - **Propranolol** (most common in infants) - **Nadolol** (alternative beta-blocker) **Medications to AVOID in WPW:** - Digoxin (can enhance accessory pathway conduction) - Verapamil (contraindicated in infants; can cause cardiovascular collapse) Duration: Often continued for 6-12 months, then may trial off medication (many outgrow by age 1)

Question 7: What is the long-term prognosis?

Answer **Good prognosis:** - ~90% of infant WPW resolves by age 1 year (accessory pathway fibrosis) - After medication trial off, monitor for recurrence - If persists to childhood, may consider ablation (typically age 5+ unless refractory) - LV function typically normalizes within weeks of rhythm control **Follow-up:** - Repeat echo in 1-2 weeks to confirm LV recovery - Cardiology follow-up - Education on recognizing SVT symptoms

Teaching Points

  1. Infant SVT rate is typically 220-300 bpm (higher than children/adults)
  2. Infants tolerate SVT poorly - quick to develop heart failure
  3. Tachycardia-induced cardiomyopathy is reversible
  4. WPW is most common accessory pathway in infants with SVT
  5. Avoid digoxin and verapamil in WPW
  6. Most infant WPW resolves by age 1