A 55-year-old man has a narrow complex regular tachycardia at 170 bpm. Adenosine 6 mg IV is given and causes brief AV block, revealing no change in the tachycardia rate — the rhythm continues unchanged through the AV nodal block. The most likely diagnosis is:
- A Atrial tachycardia ✓
- B AV nodal re-entrant tachycardia (AVNRT)
- C Atrial flutter with 2:1 block
- D AV re-entrant tachycardia (AVRT) using accessory pathway
Correct answer: A. Atrial tachycardia
Explanation
When adenosine causes transient AV block but the tachycardia continues unchanged (P waves march through the AV block), the tachycardia is AV-node independent — confirming atrial tachycardia. AVNRT and AVRT both terminate with adenosine because the AV node is an integral part of the re-entry circuit. Atrial flutter at 2:1 block would show a rate of ~150 bpm and reveal 300 bpm flutter waves during AV block. Atrial tachycardia typically shows distinct P-wave morphology differing from sinus.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.