A 6-year-old presents with recurrent syncopal episodes during exercise. ECG shows a short PR interval, delta waves, and a wide QRS complex. Holter monitor demonstrates paroxysmal supraventricular tachycardia. What is the underlying mechanism of tachycardia in this condition?

• A Accessory pathway-mediated re-entry: antegrade via AV node and retrograde via accessory pathway (orthodromic AVRT) ✓
• B AV nodal re-entry using slow and fast pathways within the AV node
• C Triggered automaticity from delayed after-depolarizations in the accessory pathway
• D Abnormal automaticity in the sinus node causing inappropriate sinus tachycardia

Explanation

The ECG findings (short PR, delta wave, wide QRS) describe Wolff-Parkinson-White (WPW) syndrome, caused by an accessory pathway (Bundle of Kent) bypassing the AV node. The typical paroxysmal SVT in WPW is orthodromic AVRT: antegrade conduction travels through the AV node (normal, slow) and retrograde through the accessory pathway (fast), creating a re-entrant circuit — the QRS is narrow during this tachycardia. AVNRT uses dual AV node pathways without an accessory pathway and does not cause pre-excitation pattern on resting ECG. Exercise-related syncope in WPW raises concern for pre-excited atrial fibrillation with rapid ventricular response.

Reference: Ghai Essential Pediatrics, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.