Implantable medical electrode assemblies, devices, systems, kits, and methods

A method, system and device for implanting an electrode assembly of an implantable medical device in a patient's heart. Positioning one or more radiopaque markers in a coronary sinus of the patient's heart. Positioning, by using the one or more positioned radiopaque markers as a fluoroscopic visual reference, a distal tip of a delivery catheter within a right atrium of the patient's heart so that a distal opening of a lumen of the catheter is against a septal wall of the heart at a location between the ostium of the coronary sinus and the A-V nodal area of the right atrium, and so that the tip of the catheter is generally directed toward a left ventricle of the patient's heart. Advancing the electrode assembly through the lumen of the catheter and into the septal wall.