In AVNRT, the electrocardiogram typically shows which features?

• A. Regular rhythm with U waves
• B. Narrow QRS with P waves hidden
• C. Irregularly irregular rhythm with absent P waves
• D. Wide QRS with delta waves

Answer: (B)

AVNRT presents as a regular, fast tachycardia with a narrow QRS complex. The reentrant circuit is within or near the AV node, so ventricular activation happens through the normal His-Purkinje system, keeping the QRS narrow. Atrial activation occurs almost simultaneously with ventricular activation, so the P waves are often concealed within the QRS or immediately after it and thus not seen as a separate wave. This combination—regular rhythm, rapid rate, and hidden P waves with a narrow QRS—is the typical ECG description for AVNRT. U waves aren’t characteristic of AVNRT, whereas an irregular rhythm with absent P waves points to atrial fibrillation, and a wide QRS with delta waves suggests preexcitation like WPW.