Look at the comparison between the V1 and V5 QRS complexes of the tachycardia ECG with those same leads of the sinus rhythm ECG. Note what almost appears to be an incomplete right bundle branch block pattern (rSr' in V1) during the tachycardia. It isn't present in the slower ECG. It is possible that this could represent a rate-related minor conduction delay but it is more likely caused by a retrogradely generated P-wave that ends just after the QRS, creating this pseudo-incomplete RBBB. In fact, this pattern is often seen in SVT due to so-called AV node reentry. It is also visible in V5. SVT of this type is the most common of the recurrent reentrant SVTs, and it is due to the presence of dual AV nodal pathways. Continue reading the answer