A new technique has rendered the study technique obsolete, making it inappropriate to continue the study due to lack of therapeutic interest.

The purpose of the study is to assess the feasibility of cardiac resynchronization under electroanatomic guidance in the event of primary implantation failure due to catheterization failure or instability in the coronary sinus ostium

As part of this study, to avoid surgery, the doctor will use a 3-dimensional (3D) guidance technique to position the heart probes in an optimal position during a new endovascular intervention.

Inclusion Criteria:

• Patient with indication for implantation of a classic multisite pacemaker;
• New York Heart Association II/IV patient despite optimal medical treatment;
• Patient with left ventricular ejection fraction ≤35%;
• Patient with a widened QRS > 120ms;
• Patient with at least one lateral coronary sinus vein visualized on angiography during the 1st procedure (if performed);
• Patient with failure of resynchronization via the endovenous route due to difficulty in accessing the coronary sinus or instability in the coronary sinus;
• Patient having been informed and having signed an informed consent form;
• Patient able to understand the information related to the study (absence of linguistic, psychological, cognitive problem, etc.), and to complete questionnaires.

Exclusion Criteria:

• Patient with resynchronization failure due to occlusion of the superior vena cava;
• Patient with no usable lateral coronary sinus vein;
• Patient with a life expectancy < 3 months;
• Patient wearing a vena cava filter;
• Patient with active sepsis;
• Pregnant or likely to be pregnant (of childbearing age, without effective contraception) or breastfeeding;
• Patient participating in another clinical trial, or in a period of exclusion from another clinical trial;
• Patient not benefiting from a social security scheme.