A 55-year-old man has palpitations. ECG shows: regular narrow-complex tachycardia at 175/min, P waves not visible before QRS, short RP interval with P waves visible in the ST segment (RP < PR). Which is the most likely arrhythmia?

• A Atrial tachycardia
• B Orthodromic AVRT via accessory pathway
• C Atypical AVNRT (fast-slow)
• D Typical AVNRT (slow-fast) ✓

Explanation

Typical AVNRT (slow-fast) uses the slow pathway antegradely and fast pathway retrogradely, resulting in near-simultaneous atrial and ventricular activation. P waves are buried in or just after the QRS (short RP interval, RP <70–90 ms), often seen as pseudo-r' in V1 or pseudo-S in inferior leads. This is the most common SVT. Atypical AVNRT (fast-slow) has a long RP pattern (RP > PR). Atrial tachycardia has a long RP with P waves before QRS. Orthodromic AVRT shows retrograde P waves 70–150 ms after QRS (short-to-medium RP).

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

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Written and medically reviewed by the StethoPrep medical team.