08. Pediatric Supraventricular Tachyarrhythmias
Supraventricular Tachycardia (SVT) is the most frequent pathological tachyarrhythmia in pediatric practice, requiring rapid bedside differentiation from physiological sinus tachycardia.
Figure 8.1: Clinical 12-lead ECG of Supraventricular Tachycardia displaying rapid, regular, narrow-complex tachycardia (>250 bpm) with absent preceding P waves. Rendered with solid white background.
1. Clinical Presentation & Hemodynamic Assessment
- Infants: Inability to articulate symptoms leads to presentation in congestive heart failure (irritability, diaphoresis during feeds, tachypnea, hepatomegaly, gray/mottled skin). Heart rate is typically fixed between $220-300\text{ bpm}$.
- Older Children: Abrupt sensation of rapid palpitations, chest discomfort, lightheadedness, and neck pulsations (frog sign). Heart rate typically $180-240\text{ bpm}$.
2. Diagnostic Differentiation: SVT vs Sinus Tachycardia
| Clinical / ECG Feature | Supraventricular Tachycardia (SVT) | Sinus Tachycardia |
|---|---|---|
| Heart Rate | Infants $>220\text{ bpm}$; Children $>180\text{ bpm}$. | Infants $<220\text{ bpm}$; Children $<180\text{ bpm}$. |
| Beat-to-Beat Variability | Strictly fixed, constant R-R intervals with zero respiratory variation. | Phasic variability with fever, activity, and respiration. |
| Onset & Termination | Abrupt ("on/off" switch mechanism). | Gradual acceleration and deceleration. |
| P-Wave Morphology | Hidden in QRS, or abnormal retrograde (negative in II, III, aVF). | Normal upright P waves in I, II, aVF preceding every QRS. |
| Response to Adenosine / Vagal | Abrupt termination to sinus rhythm, or no response. | Gradual, transient slowing followed by gradual re-acceleration. |
3. Mechanisms of Pediatric SVT
- Atrioventricular Reentrant Tachycardia (AVRT): Involves an accessory pathway (WPW or concealed accessory pathway); accounts for $>70\%$ of SVT in neonates and infants.
- AV Nodal Reentrant Tachycardia (AVNRT): Involves dual AV nodal pathways (slow and fast); becomes the dominant mechanism in adolescents ($>50\%$).
- Atrial Ectopic Tachycardia (AET) & Permanent Junctional Reciprocating Tachycardia (PJRT): Incessant automatic or reentrant tachycardias carrying a high risk of Tachycardia-Induced Cardiomyopathy (TIC).
- Atrial Flutter: Rapid atrial rate ($300-450\text{ bpm}$) with variable AV conduction; common in newborns with structurally normal hearts (frequently permanently curable after single cardioversion).
4. Evidence-Based Emergency Management Algorithm
PEDIATRIC SVT │ ┌───────────────────┴───────────────────┐ Hemodynamically UNSTABLE Hemodynamically STABLE │ │ Synchronized DC Cardioversion Vagal Maneuvers (0.5 - 1.0 J/kg) (Ice bag to infant face 15s) │ │ If fails: Increase to 2 J/kg If unsuccessful: IV Rapid Adenosine (0.1 mg/kg -> max 6 mg; 0.2 mg/kg -> max 12 mg)
- Maintenance Medical Pharmacotherapy:
- Infants $<1$ year: Propranolol ($2-4\text{ mg/kg/day}$ divided tid) or Flecainide ($50-100\text{ mg/m}^2\text{/day}$).
- Electrophysiology Study & Catheter Ablation:
- Definitive curative therapy indicated in children $>15\text{ kg}$ or with refractory/incessant SVT.
5. Clinical Pearls & Diagnostic Traps
Clinical Pearl: Never apply ocular pressure in a child to induce vagal tone (risk of retinal detachment). Use an ice bag applied over the forehead and midface (diving reflex) without occluding the airway.