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right anterior RV (site 3) and apical RV region (site 4). At this time (mid-QRS) the RV breakthrough

minimum (site 1) has enlarged and spread in all directions. The breakthrough at site 2 is of a wave
front initiated by the left-anterior fascicular branch of the left bundle branch of the specialized
conduction system. At the end of QRS (panel D), the RV minima from the various breakthroughs
coalesce and cover the entire RV epicardium with negative potentials (left panel). Thus, normal RV

epicardial activation occurs through coalescence of multiple breakthrough events. LV epicardial
potentials during mid-QRS (Figure 5.4C, right) display an extensive positive maximum over the
lateral-apical region, reflecting endocardium-to-epicardium propagation of a wave front in the LV
free wall. AS the QRS progresses, the maximum migrates superiorly from lateral LV apex toward

the posterolateral LV base (Figure 5.4D, right). The potential patterns in Figure 5.4 and their
evolution in time are representative of all normal subjects in the study, with some inter-individual
differences. Figure 5.4E is a summary of all epicardial breakthrough sites mapped by ECGI in
seven subjects. These sites are consistent with those obtained by direct intraoperative mapping 288 .

The last area to activate was the LV base in four subjects, the right ventricular outflow tract (RVOT)
in two subjects, and both the LV base and RVOT in one subject.

Normal Ventricular Activation Isochrones

Figure 5.5, top panels show the activation sequences (isochrones) mapped with ECGI in
three subjects. The general global sequence is similar in all normal subjects and is consistent with
the evolution of the epicardial potential maps. However, some inter-individual differences exist in

the details. In subject #1, epicardial activation starts at two adjacent sites on a line perpendicular
to the left anterior descending coronary artery (LAD) (sites 1 and 2 in the right panel). Earliest
activation in subject #3 (middle panel) is on an elongated region parallel to the LAD (site 1) and in
the left anterior paraseptal region (site 2). Subject #4 (left panel) shows multiple areas of early

activation: the RV breakthrough region (site 1), left paraseptal (site 2), left apical (site 3) and pos-
terior LV (site 4). In the three subjects shown, the LV base was last to activate (blue). The bottom
panels of Figure 5.5 show directly-mapped isochrones from isolated undiseased human hearts 287 .
Note the correspondence between each ECGI map (top) and its directly-mapped counterpart

(below).