A 13-year-old with palpitations during exercise has ECG showing a short PR interval of 0.08 seconds and a delta wave. He loses consciousness during a school football match. Cardiac monitoring shows a wide-complex tachycardia at 200 bpm. What is the mechanism of this arrhythmia?

• A Antidromic AVRT using the accessory pathway for antegrade conduction ✓
• B Enhanced AV node automaticity with retrograde conduction
• C Orthodromic AVRT using the accessory pathway for retrograde conduction
• D AV nodal reentrant tachycardia (AVNRT) with aberrant conduction

Explanation

This adolescent has Wolff-Parkinson-White (WPW) syndrome with pre-excitation (delta wave, short PR). The wide-complex tachycardia during exercise represents antidromic AVRT, where antegrade conduction occurs via the accessory pathway (producing wide QRS) and retrograde conduction returns through the AV node. In antidromic AVRT, the ventricle is activated entirely through the bypass tract, producing a maximally pre-excited (broad) QRS. Orthodromic AVRT (more common) uses the accessory pathway retrogradely, producing a narrow QRS. This distinction is critical as IV adenosine is potentially dangerous in WPW with atrial fibrillation.

Reference: Ghai Essential Pediatrics, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.