Impact of left bundle branch block on myocardial perfusion and metabolism: A positron emission tomography study

Better understanding of pathophysiological changes, induced by left bundle branch block (LBBB), may improve patient selection for cardiac resynchronization therapy (CRT). Therefore, we assessed the effect of LBBB on regional glucose metabolism, ¹³N-NH₃-derived absolute and semiquantitative myocardial blood flow (MBF), and their relation in non-ischemic CRT candidates.

Twenty-five consecutive non-ischemic patients with LBBB underwent ¹⁸F-FDG and resting dynamic ¹³N-NH₃ PET/CT prior to CRT implantation. Regional ¹⁸F-FDG uptake, absolute MBF, and late ¹³N-NH₃ uptake were analyzed and corresponding septal-to-lateral wall ratios (SLR) were calculated. Segmental analysis was performed to evaluate "reverse mismatch," "mismatch," and "match" patterns, based on late ¹³N-NH₃/¹⁸F-FDG uptake ratios.

A significantly lower ¹⁸F-FDG uptake was observed in the septum compared to the lateral wall (SLR 0.53 ± 0.17). A similar pattern was observed for MBF (SLR 0.68 ± 0.18), whereas late ¹³N-NH₃ uptake showed a homogeneous distribution (SLR 0.96 ± 0.13). ¹³N-NH₃/¹⁸F-FDG "mismatch" and "reverse mismatch" segments were predominantly present in the lateral (52%) and septal wall (61%), respectively.

Non-ischemic CRT candidates with LBBB demonstrate lower glucose uptake and absolute MBF in the septum compared to the lateral wall. However, late static ¹³N-NH₃ uptake showed a homogenous distribution, reflecting a composite measure of altered regional MBF and metabolism, induced by LBBB.