In this open, prospective, randomized, parallel group, German-American, two-center, Phase I study the standard-approach CRT-implantation will be compared to a guided CRT-implantation after determining the site of latest contraction and placement of the electrode as close as possible to this site, using a 5-step-scheme of imaging methods determining the site of latest contraction prior to implantation.

Patients will be allocated to two groups with either

1. Guided implantation of the left ventricular lead
2. Empirical implantation of the left ventricular lead (current clinical standard)

For Treatment Group a) the target vessel, which is the vessel with smallest distance between RLC and CVS is determined in advance and provided to the implanting physician. In case, this vessel is not amenable for lead positioning, a second vessel which is the second closest to the RLC is defined.

For Treatment Group b), the physician has to choose the vessel where to place the electrode without knowing the exact spatial orientation between that particular side-branch and the RLC.

In all patients, standard biventricular devices and leads are to be used. Aspects that are evaluated are:

1. Technical success in placing the electrode in the target vessel.
2. Failure due to dislocation during the implantation procedure or phrenic nerve stimulation.

Also, procedure duration, radiation exposure, and complications occurring during the procedure as well as post- operatively will be documented.

Inclusion Criteria:

• Congestive heart failure,
• Ejection fraction <35%,
• Complete left bundle branch block >120ms,
• NYHA II,III or IV

Exclusion Criteria:

• Patients not fulfilling the inclusion criteria or not capable/mental able to understand and follow the instructions;
• Impaired kidney function (GFR <30ml/min;
• Pregnant or breastfeeding women