Patent ID: 7945337

Claim:
An implantable medical electrical lead, comprising: an elongated lead body having a proximal end and terminating at a distal end wall, a conductor extending from the proximal end of the elongated body toward the distal end wall, an electrode connected to the distal end wall of the elongated body, the electrode being adapted for stimulating myocardial tissue via intimate contact with the tissue the electrode being electrically coupled to the conductor, the electrode comprising a conductive structure having a first, interior surface defining a closed cavity within the electrode and having a second, exterior surface, wherein the first, interior surface of the conductive structure defines a first electrode surface; an insulative housing wrapping around the second, exterior surface of the conductive structure and having a port that circumscribes an area of the second, exterior surface of the conductive structure to define a second electrode surface; an ionically conductive fluid medium filling the cavity and being in intimate contact with the first electrode surface; and an insulated helical fixation member coupled to the insulative housing and extending distally therefrom; wherein, when a current is delivered, via the conductor, to the electrode, a first current density is generated at the first electrode surface and a second current density is generated at the second electrode surface, the first current density being smaller than the second current density; and when the helical fixation member is engaged in tissue, the second electrode surface forms a high impedance and low polarization tissue-stimulating electrode.