Figure 5. The impact of fully preexcited QRS morphology in lead I fully on the distribution of accessory pathways (APs). Positive V1 at the STEP 1 changes the pretest AP distribution by eliminating almost all right-sided APs, only a small admixture of right-sided septal APs remains. At the STEP 2 : QRS morphology in lead I has to be provided. With increasing negativity of the QRS in lead I, the maximum probability site shift progressively to the left. A QS complex in lead I identifies left lateral (LL) AP, with 86.4% of all such APs ablated at that region, while R complex in lead I eliminates the possibility of LL AP and shifts point of maximum probability to the left posterior location (LP, 40.7%); common alternative sites are the adjacent left posteroseptal (LPS, 35.2%) and septal/epicardial (23%). With Rs or RS complex one more step is necessary to reach the final diagnosis, while with R, rS or QS complex the second step is the final step.