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. The expanded stent is lodged against the blood vessel wall and inhibits movement of the stent and distal electrode support. The expanded stent lumen is aligned with the electrode support lumen for allowing blood to flow through the aligned electrode support lumen and expanded stent lumen. The retention stent may take any of the known forms that can be introduced in the collapsed stent state within an introducer lumen or mounted to an introducer and can either self-expand upon release in the blood vessel or be mechanically expanded within the blood vessel.