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.