a typical cardiac pacemaker assembly 10 is shown in fig1 and comprises an implantable cardiac pacemaker 12 and at least one lead assembly 14 which is connected thereto . the cardiac pacemaker 12 generally includes circuitry for sensing electrical activity of the heart and for generating pacing signals as required by the heart . the lead assembly 14 , which may be either unipolar or bipolar , includes a proximal connector portion 16 that plugs into a mating connector receptacle of the pacemaker and a distal head portion 18 that may be passed along a vein to lodge within the interior of the heart . the lead assembly 14 contains electrical conducting wires which connect the proximal connector 16 to the distal head portion 18 so that electrical signals can be transmitted from the pacemaker to stimulate the heart at the tip of the distal head portion . the conductors also allow electrical signals occurring in the heart to be transmitted from the distal head portion to the pacemaker for detection by circuitry within the pacemaker . fig2 illustrates the distal head portion 18 in side elevation , partially in section . the illustrated lead 14 has a bipolar configuration and includes an outer sheath 20 , an outer electrically conducting coil 22 , an inner sheath 24 , and an inner electrically conducting coil 26 . both sheaths 20 and 24 are preferably made of any suitable biocompatible insulating material , for example polyurethane or silicone rubber . the outer and inner electrically conducting coils 22 and 26 are coaxial and preferably formed from helically wound nickel cobalt alloy wire or similar material 4 - 9 mils thick . a tubular anode electrode 28 is formed of conductive material with a central portion 30 of a first outer diameter and profiled end portions 32 and 3 of lesser outer diameters adapted to be received within outer sheath 20 . the anode electrode 28 is secured mechanically and electrically to outer coil 22 by crimps 36 . the distal end of the head 18 includes an electrically conducting tip cathode electrode 38 having a profiled head portion 40 , a shaft body portion 42 , and a first counter - bore 44 . the inner conducting coil 26 is received in the counter - bore 44 and in turn receives therein a head member 46 formed by a shaft 48 of a diameter sufficiently small to be received in the spring coil 26 and an integral enlarged head portion 50 . this head member provides support for the inner assembly of inner coil 26 and tip 38 is received in one end of insulative sheath 54 which has a plurality of resilient tines 56 , 58 which , in this case , are shown extending outwardly substantially radially from the lead assembly . tines of different shapes or other fixation means could be substituted for the illustrated tines without departing from the spirit or essential characteristics of the present invention . one end of the sheath 54 receives therein the end 34 of anode electrode 28 . the head portion 40 of the electrode 38 is shown with an axial second counter - bore 60 and three intersecting diametric grooves 62 , 64 , and 66 , as best shown in fig3 and 4 . turning now to fig5 it will be seen how the present invention is utilized . by pressing the tip of the electrode 38 into the heart tissue 68 , the tissue will shift to receive portions of the tip while penetrating the open grooves formed in the electrode . thus the electrode will have maximum surface contact with the tissue . fig3 , and 6 - 10 show the details of the head portion 40 which is generally an enlarged smooth surface of transition intersected by at least one groove 62 extending across a diameter thereof and an axial second counter - bore 60 . the preferred number of grooves is from one to three as shown in fig8 , and 3 , respectively , which together with the axial second counter - bore will form two to six separate grooves . as shown in fig4 and 7 , the grooves have a bottom or nadir which is linear along the length of the groove . in the preferred embodiment , the bottoms of the grooves are planer and are perpendicular to a longitudinal axis of the lead assembly . this configuration has the particular advantage of being able to make an intimate gripping contact with the heart tissue without requiring a screw for penetration of the heart tissue , as is the case for the above - mentioned u . s . pat . no . 4 , 679 , 572 . this configuration is also far simpler to form on the small diameter component of the present invention than is possible with the complex configurations shown by dehaan et al . referring specifically to fig6 the electrode 38 of the present invention is coated with a film or layer 70 of iridium oxide . the preferable method for coating the substrate is by chemical deposition with chloroiridic acid ( hydrogen hexochoroiridate iv hydrate ) and thermal oxidation of the coating at 300 °- 500 ° c . for 2 - 10 hours . this gives a thickness of 1 - 500 nanometers of iridium oxide . this type of coating method for thermally depositing iridium oxide is more fully explained in u . s . pat . nos . 4 , 677 , 989 and 4 , 717 , 581 , the disclosures of which are incorporated herein by reference . the surface layer preferably has a thickness on the order of 200 nanometers , although layer thickness exceeding about 100 nanometers appears to be satisfactory to obtain the desired result . preferably the electrode substrate is of a solid structure with a roughened surface to hold the irox coating thereon , although the invention is not so limited . a porous substrate would allow the iridium oxide coating will follow the lattice - work contour and a porous surface is useful in promoting cardiac tissue ingrowth and thereby reduce abrasion . the substrate may be reactively coated with iridium oxide in a conventional diode rf sputtering system . first , the substrate is positioned and maintained in good thermal contact with a water cooled platform of the sputtering system . any portion of the surface which is not to be coated is suitably masked . pre - sputtering is performed with an iridium target in pure oxygen at an ambient pressure of about 20 microns for approximately 20 minutes to one half hour . the pressure is then reduced to the range of from about 2 - 4 microns and sputtering is performed with a target power density of about 0 . 6 to 0 . 8 watts per square centimeter . the process is continued until the iridium oxide layer of the desired thickness is deposited , about three hours for a 200 nanometer thickness . the efficient transduction of the iridium oxide layer on the cathode tip results in a low threshold stimulation of excitable heart tissue in the vicinity of the stimulation site under the influence of the electric field . acute stimulation thresholds as low as approximately 0 . 2 volts have been observed in canine tests using a ring tip simulating cathode of the type described with reference to fig2 . the grooves in the tip of the present invention increase the surface contact area thereby reducing the polarizing pacing voltage . the grooves also improve stability of the tip by reducing its capability for movement . the grooves further improve pacing threshold by offering edges to discharge the pulses thereby focusing the charge . other embodiments of the present invention will be apparent to those skilled in the art to which the invention pertains from a reading of the foregoing description in conjunction with the accompanying drawings . accordingly , the invention is to be limited only as defined by the appended claims .
a lead assembly for a cardiac pacemaker is formed by an electrical conductor having one end connected to the pacemaker and a heart engaging electrode member attached to the opposite end . the electrode member has a profiled contact surface having at least one diametrically extending groove and a coating of iridium oxide thereon . the profiled electrode member offers at least a pair of edges to both grip the heart tissue to reduce relative movement of the electrode while focusing the discharge of the pulses thereby allowing reduction in the polarizing pacing voltage .