Figure 4. Illustration of the ECG analysis via WPW24.com application at the beginning of the ablation procedure. Left panel: the fully preexcited QRS is obtained with fast atrial pacing. At steps 1-4 the information regarding QRS morphology is introduced into the application. Step 1 : predominantly negative V1 nearly eliminates the left sided access accessory pathways (AP), only small admixture of left septal APs remains, the point of maximum is now at the most common right-sided region i.e. right posteroseptal (RPS). Step 2 : predominantly negative QRS in V2 now eliminates the possibility of left sided APs and makes RPS AP relatively unlikely, shifting the point of maximum probability to the parahisian region (PHIS) and adjoining right anteroseptal (RAS) region, but many other right-sided locations are also possible. Step 3 : equiphasic QRS in V3 lowers the chances of a right free wall AP and shifts the focus to septal pathways as 50% of possible APs are now at the PHIS or RAS regions. Step 4 : predominantly positive aVF, intuitively, should eliminate posterior pathways (RPS, RMS, RP i RPL), but it does more than that: it identifies a rare QRS pattern that was seen only 3 times per 509 APs and in all those three cases (from the exploratory cohort) the successful ablation was in the PHIS region. In this fourth case analysed, (from the validation cohort) the successful ablation (left panels) was also in the PHIS region.