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.