Left ventricular hemodynamic forces as a marker of mechanical dyssynchrony in heart failure patients with left bundle branch block

Abstract

Left bundle branch block (LBBB) causes left ventricular (LV) dyssynchrony which is often associated with heart failure. A significant proportion of heart failure patients do not demonstrate clinical improvement despite cardiac resynchronization therapy (CRT). How LBBB-related effects on LV diastolic function may contribute to those therapeutic failures has not been clarified. We hypothesized that LV hemodynamic forces calculated from 4D flow MRI could serve as a marker of diastolic mechanical dyssynchrony in LBBB hearts. MRI data were acquired in heart failure patients with LBBB or matched patients without LBBB. LV pressure gradients were calculated from the Navier-Stokes equations. Integration of the pressure gradients over the LV volume rendered the hemodynamic forces. The findings demonstrate that the LV filling forces are more orthogonal to the main LV flow direction in heart failure patients with LBBB compared to those without LBBB during early but not late diastole. The greater the conduction abnormality the greater the discordance of LV filling force with the predominant LV flow direction (r² = 0.49). Such unique flow-specific measures of mechanical dyssynchrony may serve as an additional tool for considering the risks imposed by conduction abnormalities in heart failure patients and prove to be useful in predicting response to CRT.

Figure 1

Mean ± std of the “SAx-max force/LAx-max force”-ratio during early diastolic filling (E-wave) and late diastolic filling (A-wave) respectively for the LBBB-patients (black bars) and non LBBB- patients (white bars). During E-wave the ratio was significantly larger for the LBBB patients compared to non-LBBB patients (P-value = 0.020), while there was no significant intergroup difference during A-wave (P-value = 0.185).

Figure 2

Regression analysis of the “SAx-max force/LAx-max force”-ratio versus the QRS duration for (left panel) E-wave and (right panel) A-wave. The black solid line is the best fit of a line to the data; red circles represent the non-LBBB patients and black circles the LBBB patients.

Figure 3

Schematic overview of the analysis method: (1) a binary mask is generated from the segmentation of the bSSFP short axis images and superimposed on magnitude data from the 4D flow scan, and adjusted in order to avoid possible mismatch problems between bSSFP-images and flow data, (2) the LV pressure gradients ∇p = (∂p/∂x, ∂p/∂y, ∂p/∂z) are calculated by using the velocity data and binary mask, (3) integration of pressure gradients over the LV volume at each time frame gives the LV hemodynamic force vector.

Figure 4

(Top panel) Hemodynamic force [N] where a direction of the vector is considered positive if it is mainly directed towards atrium and negative if it is mainly directed towards apex. (Bottom panel) Speed data [m/s] extracted from the 4D flow data at points near the mitral valve (MV) orifice and the aortic valve (AoV) orifice. These speed curves in combination with streamline visualizations are used to divide the cardiac cycle into systole and diastole, the latter with subphases of early (E) and late (A) filling. The x-axis shows time in seconds from R-peak.

Figure 5

Hemodynamic forces [N] in a LBBB patient (left panels) and in a non-LBBB patient (right panels) projected onto a SAx-image (top) and LAx-image (bottom). The hemodynamic force plots are colored according to diastolic phases, E-wave (blue) and A-wave (green). The “SAx-max force/LAx-max force”-ratio was defined as the ratio between the maximum force along the anteroseptal-to-inferolateral axis in the SAx-view and the maximum force along the apex-to-base axis in the LAx three-chamber view. E-wave, early diastolic filling; A-wave, late diastolic filling; LA, left atrium; LV, left ventricle.