Abstract: Endocardial implantable cardiac leads are disclosed for applying electrical stimulation to and/or sensing electrical activity of the heart at one or more distal electrode positioned at a cardiac implantation site within a cardiac vessel adjacent to and at a desired orientation to the left ventricle or atrium of the heart. The distal electrode(s) is supported by a tubular electrode support having a diameter large enough to bear against the blood vessel wall and a support lumen that allows blood to flow through it. A retention stent extends proximally from a distal stent end fixed to the tubular electrode support to a proximal stent end. After advancement to the cardiac implantation site employing a lead delivery mechanism, the retention stent is expandable from a collapsed stent state in which the outer diameter of the retention stent is less than the inner diameter of the vessel to an expanded stent state.