FCPS Paediatrics TOACS · Supraventricular Tachycardia (SVT)

✅ Candidate's answer:
• ECG findings in SVT:
  - Narrow QRS complex (<0.08 seconds in infants, <0.09 seconds in older children).
  - Regular tachycardia with rate >220 bpm in infants (>180 bpm in children).
  - No visible P waves – P waves are often buried in the QRS complex (AVNRT) or retrograde (AVRT).
  - Fixed RR intervals – no variation.
• Distinguishing SVT from sinus tachycardia:
  - Sinus tachycardia: Gradual onset and offset, rate varies with activity/emotion/fever, normal P wave axis (positive in I, II, aVF), PR interval normal.
  - SVT: Abrupt onset and offset, rate is fixed and does not vary with activity, P waves are abnormal (absent, retrograde, or buried), PR interval may be prolonged or absent.
  - Vagal maneuvers: Adenosine terminates SVT but only slows sinus tachycardia.
  - ECG in sinus rhythm: Look for delta wave (WPW), short PR, or other clues to mechanism.

✅ Candidate's answer:
• Mechanisms of SVT in children (Nelson Ch 484.3):
  1️⃣ AVRT (Atrioventricular reentry tachycardia) – most common in infants (<1 year).
    - Orthodromic AVRT: Antegrade conduction down the AV node, retrograde up the accessory pathway (narrow QRS). Associated with WPW (delta wave in sinus rhythm).
    - Antidromic AVRT: Antegrade down accessory pathway, retrograde up AV node (wide QRS – rare).
  2️⃣ AVNRT (Atrioventricular nodal reentry tachycardia) – common in adolescents (dual AV node pathways).
  3️⃣ Atrial flutter (intra-atrial reentry) – common after congenital heart disease repair (Fontan, Mustard).
  4️⃣ Atrial ectopic tachycardia (automatic) – incessant, may cause cardiomyopathy.
  5️⃣ Junctional ectopic tachycardia (JET) – post-operative (tetralogy of Fallot).
• Most common mechanism in infants: AVRT (accessory pathway) – accounts for ~90% of SVT in infants <1 year. Often associated with WPW (delta wave in sinus rhythm).

✅ Candidate's answer:
• Step 1: Vagal maneuvers (while preparing IV access and monitoring):
  - Ice to face: Cover the infant's face (eyes, nose, mouth) with a small bag of ice water for 15-30 seconds – effective in infants (triggers diving reflex).
  - Valsalva: Not effective in infants.
  - Carotid sinus massage and ocular pressure are contraindicated (dangerous in infants).
• Step 2: Adenosine (if vagal maneuvers fail):
  - First dose: 0.1 mg/kg rapid IV push (max 6 mg) – followed by a saline flush (2-3 mL).
  - Second dose: 0.2 mg/kg rapid IV push (max 12 mg) if no effect.
  - Adenosine terminates AVRT by blocking the AV node (anterograde limb).
  - Side effects: transient asystole, flushing, chest pain, bronchospasm (rare).
• Step 3: Alternative medications (if adenosine fails):
  - Esmolol (IV beta-blocker): 500 mcg/kg over 1-2 min (contraindicated in asthma).
  - Procainamide (IV class Ia): 15 mg/kg over 30-60 min – useful for refractory SVT or atrial flutter.
  - Amiodarone (IV class III): 5 mg/kg – reserve for unstable/refractory cases.
  - Avoid verapamil in infants <1 year – risk of cardiovascular collapse.
• If unstable (hypotension, poor perfusion, altered consciousness):
  - Synchronized cardioversion – 0.5-1 J/kg, increase to 2 J/kg if needed.

✅ Candidate's answer:
• Mechanism: Adenosine is a purine nucleoside that binds to A1 receptors in the AV node, causing transient AV block (slows conduction and increases refractoriness). It terminates reentry circuits that use the AV node as the anterograde limb (AVRT, AVNRT).
• Dosing:
  - First dose: 0.1 mg/kg rapid IV push (max 6 mg).
  - Second dose: 0.2 mg/kg rapid IV push (max 12 mg).
  - Administer via a central or large peripheral vein with a rapid flush (2-3 mL of saline).
• Side effects (transient):
  - Flushing (most common).
  - Bronchospasm (contraindicated in asthma).
  - Chest pain, dyspnea.
  - Transient asystole (few seconds – usually resolves spontaneously).
  - Headache, nausea.
• Contraindications:
  - Asthma (relative) – risk of bronchospasm.
  - Second- or third-degree heart block (unless pacemaker).
  - Sick sinus syndrome.
  - Do not use in wide complex tachycardia (may cause VF).
• Half-life: <10 seconds – very short action.

✅ Candidate's answer:
• Unstable SVT (hypotension, shock, altered consciousness):
  - Immediate synchronized cardioversion – 0.5-1 J/kg, increase to 2 J/kg if unsuccessful.
  - Sedation if time permits (midazolam, ketamine).
  - If IV access is available: Adenosine can be administered while preparing for cardioversion.
  - Do not delay cardioversion for IV access.
• Post-cardioversion: Obtain ECG in sinus rhythm, monitor for recurrence, and start oral propranolol or digoxin.
• If the patient is pulseless: Follow PALS algorithm – CPR, defibrillation (2 J/kg → 4 J/kg), epinephrine, amiodarone.

✅ Candidate's answer:
• Long-term management of infant SVT:
  1️⃣ If no WPW (no delta wave in sinus rhythm):
    - Propranolol (1-2 mg/kg/day, divided TID) or digoxin (10-15 mcg/kg loading, then 5-10 mcg/kg/day) – both effective.
  2️⃣ If WPW (delta wave present):
    - Propranolol or flecainide – first-line.
    - Digoxin and verapamil are contraindicated in WPW (risk of preexcited AF → VF).
  3️⃣ Echocardiogram: To exclude structural heart disease (especially if WPW).
  4️⃣ Follow-up: Monitor for recurrence. Many infants will outgrow SVT by 12-18 months.
  5️⃣ Medication weaning: After 6-12 months of being event-free, medication can be weaned (usually after 1 year of age).
  6️⃣ Catheter ablation: Reserved for refractory cases, or older children (>15 kg) with symptomatic WPW.
• Parent education: Teach parents to recognize signs of SVT (poor feeding, pallor, irritability) and when to seek emergency care.

✅ Candidate's answer:
• WPW syndrome: A form of preexcitation caused by an accessory pathway (Kent bundle) connecting the atria to the ventricles. Characterized by:
  - Short PR interval (for age).
  - Delta wave (slurred upstroke of the QRS).
  - Wide QRS complex (due to fusion of AV node and accessory pathway conduction).
• Risk: SVT (orthodromic AVRT) and atrial fibrillation with rapid conduction over the accessory pathway → VF → sudden death.
• Why digoxin is contraindicated in WPW:
  - Digoxin shortens the refractory period of the accessory pathway and slows conduction through the AV node → in atrial fibrillation, the accessory pathway can conduct rapidly (without AV node delay) → rapid ventricular rate → ventricular fibrillation and sudden death.
• Other contraindicated drugs: Verapamil (calcium channel blockers) – same mechanism.
• Safe medications: Beta-blockers (propranolol), flecainide, amiodarone, sotalol.
• Definitive treatment: Catheter ablation (curative, success >95%).

✅ Candidate's answer:
• Diagnosis: Atrial flutter (intra-atrial reentry) – common after atrial surgery (Fontan, Mustard, ASD closure, TOF repair).
• ECG features: Sawtooth flutter waves (II, III, aVF), atrial rate 250-400 bpm, variable AV block (2:1, 3:1, 4:1).
• Acute management (stable):
  - Anticoagulation: If duration >48 hours, transesophageal echo (TEE) to rule out thrombus, or anticoagulate for 3-4 weeks before cardioversion.
  - Synchronized cardioversion: 0.5-1 J/kg (first-line for acute conversion).
  - If unstable: Immediate synchronized cardioversion.
  - Pharmacologic: Amiodarone, ibutilide, or procainamide.
• Long-term management:
  - Catheter ablation: Of the flutter circuit (cavotricuspid isthmus-dependent or intra-atrial) – curative.
  - Anticoagulation: Lifelong if high-risk (Fontan, CHD with residual lesions).
  - Rate control: Beta-blockers or digoxin (if no WPW).

✅ Candidate's answer:
• Atrial ectopic tachycardia (AET): An automatic (non-reentrant) tachycardia originating from an ectopic atrial focus. It is often incessant and can cause tachycardia-induced cardiomyopathy if left untreated.
• ECG: Abnormal P wave morphology (axis), rate varies (warm-up/cool-down), no abrupt onset/offset.
• Treatment:
  - Class Ic antiarrhythmics: Flecainide (1-3 mg/kg/day) – first-line.
  - Class III: Amiodarone or sotalol (second-line).
  - Beta-blockers: May be used as adjunctive therapy.
  - Catheter ablation: Difficult in infants, but possible if refractory and weight >15 kg.
  - If cardiomyopathy develops: Heart failure management (diuretics, ACE inhibitors). The cardiomyopathy often reverses after successful tachycardia control.

✅ Candidate's answer:
• Diagnosis: Junctional Ectopic Tachycardia (JET) – common post-operative arrhythmia after congenital heart surgery (tetralogy of Fallot, VSD).
• ECG: Narrow QRS tachycardia (rate 180-250 bpm) with AV dissociation (P waves independent of QRS).
• Management:
  - Cooling the patient – decrease core temperature to 34-35°C (reduces automaticity).
  - Amiodarone (IV 5 mg/kg, then infusion).
  - Dexmedetomidine – sedative that reduces sympathetic tone.
  - Procainamide – alternative.
  - Avoid catecholamines (may worsen JET).
  - If refractory: Atrial overdrive pacing or ECMO.
• Prognosis: Usually resolves within 24-48 hours with treatment.

✅ Candidate's answer:
• Inheritance: Most WPW cases are sporadic, but familial WPW exists and is inherited in an autosomal dominant pattern with variable penetrance. Mutations in PRKAG2 and MYH6 have been associated with familial WPW.
• Family screening:
  - First-degree relatives of a patient with WPW should undergo ECG screening (especially if the patient has a family history of WPW or sudden death).
  - If an asymptomatic relative has WPW on ECG, further evaluation (Holter, EP study) is recommended if they participate in competitive sports or have symptoms.
  - Asymptomatic family members with no ECG findings do not need further screening unless they develop symptoms.
• Genetic testing: Not routinely performed unless there is a strong family history of WPW or PRKAG2 syndrome (WPW + HCM + conduction disease).

✅ Candidate's answer:
• AHA/ACC guidelines for WPW and sports:
  - Symptomatic WPW patients should undergo catheter ablation before participating in competitive sports. After successful ablation, patients can return to sports after 3-6 months (if no arrhythmia recurrence).
  - Asymptomatic WPW patients: Can participate in sports if they undergo an invasive EP study that demonstrates a low-risk accessory pathway (SPERRI >250 ms, AP ERP >250 ms, no inducible AF).
  - High-risk activities: Competitive sports (basketball, soccer, football, swimming) are high-risk and should be avoided until risk stratification or ablation is completed.
• Recommendation for this patient: Since he has symptoms (palpitations, near-syncope), he should not participate in competitive sports until he undergoes EP study and ablation. After successful ablation, he can return to sports.

✅ Candidate's answer:
• Ebstein anomaly is a congenital heart defect characterized by downward displacement of the tricuspid valve into the right ventricle.
• Association with WPW: Up to 30% of patients with Ebstein anomaly have WPW – due to the presence of multiple accessory pathways (often right-sided).
• Clinical implications:
  - High risk of recurrent SVT and preexcited atrial fibrillation.
  - Risk of sudden cardiac death is higher than in isolated WPW.
• Management:
  - Catheter ablation is the treatment of choice – but may be more complex due to multiple pathways.
  - Medical therapy: Beta-blockers, flecainide.
  - Avoid digoxin and verapamil.
  - ICD placement may be needed for high-risk patients.
  - Transition: Lifelong follow-up with a cardiologist experienced in adult congenital heart disease.

✅ Candidate's structured answer:
• “Your baby has a condition called Supraventricular Tachycardia – a fast heart rhythm that starts in the upper chambers of the heart. This causes the heart to beat too fast, which can make your baby feed poorly and feel tired.”
• “The good news is that this is a very treatable condition. We have stopped the fast rhythm today using a medication called adenosine. For the future, we will start a daily medication (propranolol) to prevent it from happening again.”
• “Most babies outgrow this by 12-18 months of age as their hearts mature. In most cases, this does not cause any long-term heart problems.”
• “You will need to give the medication regularly and watch for signs of a fast heartbeat (pale, irritable, poor feeding). If you think it's happening again, bring your baby to the nearest emergency department.”
• “We will see your baby regularly to monitor growth and adjust the medication. Your baby will grow up to be a healthy child.”