Background: We hypothesized that signal averaged ECG during tachycardia would facilitate P wave recognition and assist in SVT diagnosis. P waves adjacent to the QRS during tachycardia would lengthen the filtered QRS and be recognized by subtracting QRS duration during sinus rhythm from that of tachycardia. Aims: to assess the feasibility of SaECG during SVT; to correlate the difference between the filtered QRS duration in SVT and sinus rhythm (ΔfQRSd) with the endocardial VA time; Methods & Results: Patients referred for an EP study and ablation of any SVT were included. A SAECG was acquired during SVT and compared with another during SR. 40 patients were included, 20 had AVNRT and 20 AVRT. For AVNRT, the P wave was detected as a pseudo-late potential in 16 patients. In 4 patients, P wave was invisible and presumed within the confines of the QRS. The mean ΔfQRSd was 2017 ms and the VA time was 1415 ms. For AVRT a distinct P wave separated from the QRS was detected in all patients. The ΔfQRSd was 10742ms and the VA time was 9631 ms. ΔfQRSd was longer during AVRT than AVNRT (p<0.0001). Over all, the ΔfQRSd correlated with the longest VA time (R=0.796). Motion artifact and sensing of T waves during tachycardia were 2 confounders. Conclusion: SaECG provides a rapid adjunct to the 12 lead ECG and is capable of identifying P waves and facilitating diagnosis of SVT mechanism.