A 28-year-old man has recurrent palpitations with sudden onset and termination. ECG during tachycardia shows a narrow-complex regular tachycardia at 185 bpm with retrograde P waves in the ST segment (RP interval <70 ms). Which is the MOST likely mechanism?
- A Atrial flutter with 2:1 block
- B Orthodromic AVRT via accessory pathway
- C Typical AVNRT (slow-fast pathway) ✓
- D Atrial tachycardia with 1:1 conduction
Explanation
Retrograde P waves buried in or just after the QRS complex (RP interval <70 ms, short RP tachycardia) with a regular narrow-complex tachycardia is the hallmark of typical AVNRT (atrioventricular nodal re-entrant tachycardia), the most common paroxysmal SVT. In AVNRT, anterograde conduction is via the slow pathway and retrograde via the fast pathway; retrograde atrial activation is nearly simultaneous with ventricular activation, placing P waves at the end of QRS or in early ST segment (pseudo-r' in V1, pseudo-s in inferior leads). Orthodromic AVRT typically shows RP >70 ms (P visible after QRS but with longer VA interval). Atrial tachycardia shows positive P waves before QRS.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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