A 45-year-old man presents with palpitations. ECG shows a regular narrow-complex tachycardia at 170 bpm with P waves visible immediately after the QRS (retrograde, with RP interval < PR interval). Carotid sinus massage terminates the arrhythmia. The mechanism is:

• A Atrial flutter with 2:1 conduction
• B AVNRT (AV nodal re-entrant tachycardia), typical slow-fast variant ✓
• C Atrial tachycardia with first-degree AV block
• D Orthodromic AVRT (accessory pathway using AV node antegrade, accessory retrograde)

Explanation

Typical AVNRT (slow-fast) is the most common regular narrow-complex SVT. The re-entrant circuit uses the slow pathway for antegrade conduction and fast pathway retrograde — producing retrograde P waves that either fall within or immediately after the QRS complex (short RP, RP <70 ms or P buried in QRS). Carotid sinus massage blocks AV node conduction, terminating the circuit. In orthodromic AVRT, the retrograde P wave appears after the QRS but with a longer RP interval (due to accessory pathway conduction distance).

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

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Written and medically reviewed by the StethoPrep medical team.