We hypothesize that a CRT-strategy targeting optimal electrical resynchronization will result in an excess increase of 4 % in LVEF compared with the routine CRT-strategy, where an 8 % increase is expected. To identify this absolute increase in LVEF and to achieve a statistical power of 80 %, the study will need a sample size of 98 patients, given a standard deviation (SD) of 7 % in both groups, and a two-sided alpha value of 0.05. Furthermore, to achieve a statistical power of > 80 % with a margen of noninferiority of 20 % for the secondary endpoint of clinical non-response to CRT (assuming a 75 % clinical response rate in the control group) we will need a sample size of 116 patients (given a two-sided alpha value of 0.05 %). Taking into consideration an expected loss of follow-up in approximately 5 % of the patients, we will include 122 patients.