Example of calculation of the fusion index on four paced complexes (thin lines). To identify beats with intrinsic-like morphology, the average of all intrinsic complexes (thick red dashed line) and the average of all paced complexes (thick black solid line) were constructed for each pacing intervention. An amplitude-normalized cross-correlation with the intrinsic average and with the paced average was then calculated for each paced complex. Each complex having a larger peak correlation with the intrinsic average than with the paced average was deemed to have intrinsic-like morphology and scored as a fusion beat (thin dashed lines). The fusion index represents the proportion of fusion beats to the total number of paced beats.