Assessment of electrical and mechanical function in the paced LBBB failing heart using tagged MRI and epicardial mapping

The method by which pacing improves the mechanical function of the failing left bundle branch block (LBBB) heart is poorly understood. In this study, LBBB was induced in 7 dogs, after which the hearts were paced to failure over approximately 3 weeks. In the MR scanner, tagged cine images and nearly simultaneous electrograms and LV pressure recordings were acquired for right atrial (RAP), left ventricular (LVP), and bi-ventricular (BVP) pacing. Electrograms were correlated spatially and temporally to the MR images, and 3D strain was calculated through time at points corresponding to electrode locations. Distinctive patterns of electrical and mechanical activation are evident in movies from these data. Mechanical function, characterized by dP/dt, indicated significant improvement for both LVP and BVP relative to RAP. The improvement is inversely correlated to spatial variation in the magnitude of circumferential strain (Ecc) across the left ventricle, but uncorrelated to the spread of electrical activation times across the ventricular epicardium. Contractile function is not predicted by epicardial mapping alone but may be predicted by the pattern of myocardial strain evaluated using MRI.