A 22-year-old woman presents with recurrent episodes of rapid regular palpitations (rate 180 bpm) with abrupt onset and offset, no structural heart disease. ECG during tachycardia shows narrow complex, retrograde P waves after QRS (short RP tachycardia, RP interval 80 ms). What is the most likely mechanism?

• A Atrial tachycardia with 1:1 conduction
• B Orthodromic AVRT using accessory pathway
• C AV nodal re-entrant tachycardia (AVNRT) — slow-fast type ✓
• D Junctional ectopic tachycardia

Explanation

Typical (slow-fast) AVNRT uses the slow AV nodal pathway anterogradely and the fast pathway retrogradely, resulting in near-simultaneous activation of the atria and ventricles. This produces retrograde P waves buried in or immediately after the QRS complex with a very short RP interval (RP <70 ms, or <50% of the RR interval). It is the most common paroxysmal SVT in women of reproductive age. In orthodromic AVRT, the retrograde P wave follows the QRS with a longer RP interval (typically RP >100 ms) as the accessory pathway conducts retrogradely.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.