(A) Time-to-peak radial strain was similar among the 6 standard segments at baseline. (B) Distribution of the sites of earliest and latest mechanical activation after permanent right ventricular pacing. The septal and anteroseptal segments were most frequently the sites of earliest mechanical activation, whereas the lateral and posterior segments were most frequently the sites of latest mechanical activation. A = anterior; AS = anteroseptal; I = inferior; L= lateral; P = posterior; S = septal.

The color-coded curves represent the 6 segments (light blue = septal; yellow = anteroseptal; red = anterior; green = lateral; purple = posterior; dark blue = inferior). (A) Before His bundle ablation and pacemaker implantation, there was no significant delay in time-to-peak radial strain among the 6 segments. (B) After 4 years of permanent right ventricular pacing, a marked heterogeneity in time-to-peak strain for the 6 segments was observed. A time difference >130 ms was present between the time-to-peak radial strain of the septal (arrow) and posterolateral (open arrow) segments. (C) In this patient, upgrade of the conventional pacemaker to a biventricular pacemaker was performed. More than 1 year after "upgrade," the radial strain curves revealed restoration of synchronous left ventricular activation, with no significant delay between the time-to-peak radial strain of the different segments.

Left ventricular (LV) ejection fraction at different time points for 11 patients who were upgraded to biventricular (BiV) pacing after a mean of 4.1 ± 2.0 years of permanent right ventricular (RV) pacing. After permanent RV pacing, LV ejection fraction had significantly decreased. This was partly reversed after upgrade to BiV pacing. *As assessed with repeated measures analysis of variance. Bonferroni post-hoc testing revealed significant differences between baseline and RV pacing (p < 0.001) and between RV pacing and BiV pacing (p < 0.01).