Benefit of CRT:
It is not completely understood how cardiac resynchronization therapy improves mechanical LV function in patients with heart failure and LBBB. Electrical resynchronization can reduce the LBBB-induced mechanical interventricular dyssynchrony between the right and the left ventricle and the intraventricular dyssynchrony within the left ventricle. Minimizing dyssynchrony has been shown to improve global LV function, increase LV filling time, decrease septal discordinated activity and reduce mitral regurgitation, thus improving hemodynamic17,29. To assess the impact of CRT on LV remodeling, three CRT studies have been conducted. In these studies, serial transthoracic Doppler echocardiography were used to assess reverse LV remodeling in advanced systolic heart failure. Patients were randomized to receive CRT with optimal medical therapy or medical therapy alone1,30. CRT resulted in a significant decrease in LV size, assessed as LV end‐diastolic and end‐systolic diameters or as LV volumes as early as 1 month8,31,32, compared with control patients. There is further progressive reduction in LV diameter and LV volume at 6 months, which are sustained at 1 year in 65–75% of patients33–35. The progressive reduction in LV volume with CRT is associated with restoration of mitral valve annular diameter and mitral subvalvular geometry towards normal. The changes in LV cavity shape and geometry of the mitral valve apparatus are associated with reduction in the severity of mitral regurgitation. In the MIRACLE study, the severity of mitral regurgitation had decreased significantly with CRT at 3 months, and this improvement was maintained at 6 and 12 months36. There is some evidence that the decrease in the severity of mitral regurgitation precedes the reduction in LV volume and the associated changes in LV and mitral valve apparatus architecture37. Reverse LV remodeling requires continuous CRT. This was demonstrated clearly when CRT was discontinued after 3 months in one small open‐label study37. After 3 months of CRT‐induced reverse LV remodeling, cessation of CRT resulted in rapid abolition of the LV volume reduction with concomitant recurrent LV dilatation, progressive deterioration in LVEF and recurrent mitral regurgitation.