To better understand the mechanisms of left ventricular (LV) mechanical dyssynchrony (LVMD), we explored the relative contributions of QRS duration (QRSd), LV ejection fraction (EF), volumes and scar to LVMD measured by gated single-photon emission tomography in a population of consecutive patients with left bundle branch block (LBBB) and right bundle branch block (RBBB) compared to controls.
Myocardial perfusion imaging studies of 275 LBBB and 83 RBBB patients from three centers were analyzed. LVMD was defined as an abnormal phase bandwidth or phase standard deviation. Hospital and gender-specific normal values were obtained from 172 controls.
The prevalence of LVMD was 85 and 40% in LBBB and RBBB, respectively. Ejection fraction, scar severity, and LBBB morphology independently explained 70% of variance seen in PhaseBW. Ejection fraction had the highest area under the curve (AUC 0.918) in the receiver operating characteristics analysis of LVMD with an optimal cut-off of 47% (sensitivity 73% and specificity 98%). Notably, QRSd was not predictive.
LV mechanical dysfunction plays a greater role than conduction abnormality in the genesis of LVMD, a finding that is intriguing in the context of contemporary literature which suggests that QRSd is the parameter that is most predictive of CRT response.