[Tachycardia with rSr' <110 ms in lead V1 -- is it always an atrio-ventricular nodal tachycardia?].

We present a case of a 15-year old girl in whom supraventricular tachycardia (SVT) with rSr' pattern in lead V1 and the QRS width of less than 110 ms, suggesting typical slow-fast AVNRT, was the presenting arrhythmia. During sinus rhythm no preexcitation was observed. Considerable variability of the tachycardia cycle length (240-370 ms), attributable to the presence of the fast and slow nodal pathways, was also observed. However, during the electrophysiological study only orthodromic atrio-ventricular reciprocating tachycardia with the left-sided superior accessory pathway as the retrograde limb was documented. Thus, rSr' pattern was due to incomplete right bundle branch block. We describe how the correct diagnosis and the site of the atrial insertion of the accessory pathway could be inferred from the careful analysis of the P' wave morphology during the tachycardia.