High efficiency tissue stimulating and signal sensing electrode

A pacing lead having a porous electrode of platinum-iridium with recessed areas or grooves formed into the surface. The grooves allow for acute electrode stabilization as a result of clot formation and endocardial tissue capture during insertion and immediate immobilization upon implant. At least one layer of a porous coating of 20-200 micron diameter spherical particles are deposited on the surface of the base electrode to obtain a porous macrostructure for promoting chronic tissue ingrowth. Additionally, a microstructure surface coating is applied to increase the active surface area and enhance electrical efficiency by lowering electrochemical polarization and increasing electrical capacitance.

BACKGROUND OF THE INVENTION 
This invention relates generally to an implantable pacing lead and more 
specifically to a pacing lead having a high efficiency tissue stimulating 
and signal sensing porous electrode for use with a cardiac pacemaker and a 
method for making the porous electrode. 
For a cardiac pacemaker, implant lifetime is determined by the energy 
delivered per pulse. The pacemaker will have a longer life if the energy 
delivered per pulse is maintained at a minimum. Alternatively, the energy 
can also be used to provide for more features in the pacemaker. The design 
of an implantable pacing lead which is used with the pacemaker is 
influenced by the optimum signal for pacing stimulation. Physiologically, 
a cardiac pacemaker must be capable of generating a signal with a 
sufficient magnitude to depolarize the excitable cells of the endocardium. 
The electrode size, material, surface nature, and shape; the body tissue 
or electrolyte conductivity; and the distance separating the electrode and 
the excitable tissue, combine to determine the energy required of the 
pacemaker. Accordingly, the main factors to be considered with regard to 
the design of implantable pacing lead's electrode are: the size, surface 
nature, material and shape; the fixation of the electrode to the tissue; 
and the endocardial tissue reaction. 
In selecting a pacemaker, the current drain, and therefore the implant 
lifetime, is determined by the impedance to pacing pulses. The pacemaker 
lead's electrode must be capable of delivering a pacing pulse with a pulse 
width generally in the range of 0.01-2.0 milliseconds and 0.5 to 10.0 
volts to the tissue, and to also sense and transmit a QRS signal arising 
in the atria and ventricles of the heart to the pacemaker circuitry. 
Generally, the electrode-electrolyte system impedance is higher for 
sensing than for pacing. Pacing leads for pacing and sensing in the 
atrium, which can exhibit different stimulation and depolarization 
parameters than the ventricle, are also required. 
The electrode-endocardial tissue system impedance characteristics may be 
understood in terms of an interface component and a spreading resistance 
component. The interface component occurs within a few microns of the 
surface of the electrode. The spreading resistance component depends 
predominantly on the tissue resistivity. Generally, the former reflects 
the charge transfer characteristics of the electrode-tissue interface 
influenced mostly by the surface area and material of the electrode, and 
the latter reflects the overall size and shape of the electrode; the 
surface nature of electrode; and the resistivity of the tissue. 
The current drain of a pacemaker is determined by the impedance of the 
pacemaker circuitry, the nature of the electrode lead resistance, and the 
characteristics of the electrode tip interface with the electrolyte 
system. For a given pacemaker circuit and electrode lead design, the 
current drain is well defined. Thus, the nature of the 
electrode-endocardial tissue interface determines the overall current 
requirements of the system. 
As an additional design factor, the most significant frequency of the 
pacing pulse is on the order of 1 KHz. At this frequency, the interface 
impedance is small and most of the impedance to the pacing pulses is due 
to the bulk or spreading impedance. This is determined by the shape and 
size of the electrode tip and is generally inversely related to the radius 
of the electrode tip. 
The most significant frequency components of a signal to be sensed, i.e., 
the ventricular QRS, are in the bandwidth of 20-100 Hz. In this region, 
the interface impedance of the sensed signal becomes the most significant. 
The interface impedance is determined in large part by the microsurface 
area of the electrode tip and develops within a few microns of the 
surface. As described herein, the microsurface area of a porous electrode 
tip is the wettable surface, area which includes all of the exposed and 
interstitial porosity surfaces of the electrode tip. 
As a final design consideration, it has been determined that the pacing or 
stimulation threshold is a reflection of the electrical energy required 
for a pulse to initiate a cardiac depolarization. The stimulation 
threshold typically rises for a period of a few weeks after the implant of 
a cardiac pacemaker generally as a result of an increase in the spacing 
between the electrode and the excitable tissue. The increase occurs due to 
the development of a fibrous capsule around the electrode tip. The 
thickness of the fibrous capsule is generally dependent upon the 
mechanical characteristics of the distal end of the lead (i.e., stiff or 
flexible); the geometry of the electrode tip; and the microstructure of 
the electrode tip, such as a porous electrode surface and the electrode 
material itself. In this regard, the environment of the endocardium must 
be considered. Specifically, the constant beating of the heart can cause 
the electrode to pound and rub against the endocardium, causing irritation 
and a significant subsequent inflammatory response, which ultimately 
results in healing, and the development of a fibrotic tissue capsule about 
the electrode tip. Also, a rough surface microstructure or one with sharp 
protrusions for the electrode will tend to be abrasive or traumatic on the 
abutting heart cells, also causing irritation, which also tends to cause 
the development of a thicker fibrotic capsule. 
In view of the above characteristics of an electrode and its implantology 
issues for a cardiac pacemaker, it is clear that an electrode tip with a 
small geometric surface area (resulting in higher pacing impedance) will 
have a low current drain. However, in order to enhance sensing, the same 
electrode tip should have a large microsurface area and be of such a 
material to result in a low polarization and high capacitance which, in 
turn, results in a low sensing impedance and improved sensing. A cardiac 
pacemaker electrode tip that is constructed to be porous is therefore 
preferred in order to best satisfy these requirements. 
In a pacemaker electrode, minimal tissue reaction is desired around the 
tip, but firm intimate attachment of the electrode to the tissue is 
essential to minimize any electrode movement relative to the abutting 
tissue. A porous electrode tip with macro tissue entrapping structure 
allows rapid fibrous tissue growth into a hollow area or cavities in the 
electrode tip to facilitate and enhance attachment of the electrode to the 
heart. A reduced lead dislodgement rate is also expected as a result of 
such tissue ingrowth. A further aspect of importance is selection of 
porosity size, which must be such as to accommodate economical 
construction techniques, overall dimensional tolerances, and tissue 
response constraints. 
SUMMARY OF THE INVENTION 
The pacing tip electrode of the preferred embodiment of the present 
invention is a five square millimeter platinum-iridium (90%/10%) porous 
electrode with recessed areas or slots in the shape of a cross formed into 
the surface. The grooves allow for acute electrode stabilization tissue 
ingrowth as a result of naturally occurring clot formation during 
insertion and helps result in immediate immobilization of the electrode 
upon implant. A porous coating of 20-80 micron diameter spherical 
platinum-iridium (90%/10%) particles are deposited on the surface of the 
base electrode to obtain a porous macrostructure for chronic tissue 
ingrowth and also for extending the active surface area. Preferably, the 
particles are deposited in a two-step process where the first layer of 
particles is made up of 40 to 80 micron spheroidal particles. The second 
layer is made up of 20 to 40 micron spheroidal particles. The result is a 
clumping of the particles producing a uniformly textured surface with 
randomized particle attachment. Chronic tissue ingrowth into this clumped, 
porous macrostructure enhances the electrode stabilization. Additionally, 
a microstructure surface coating is applied on these particles to increase 
the active surface area and enhance electrical efficiency by lowering 
polarization and increasing electrical capacitance. The macrostructure is 
preferably created by sintering the platinum-iridium particles to the 
platinum-iridium electrode tip. The microstructure coating is preferably 
created by reactive sputtering of titanium nitride onto the 
platinum-iridium particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a side plane view of a pacing lead 20 according to the present 
invention. The lead 20 is provided with an elongated lead body 22 which 
includes electrical conductors (not shown) covered with an insulation 
sheath 24. The insulation sheath is preferably fabricated of silicone 
rubber, polyurethane or other suitable biocompatible, biostable polymer. 
At a proximal end 26 of the pacing lead 20 is a connector assembly 28, 
which is provided with sealing rings 30 and which carries at least one 
electrical connector pin 32, and may also. The carry an anode terminal 
ring electrical connector 33. The connector assembly 28 is constructed 
using known techniques and is preferably fabricated of silicone rubber, 
polyurethane or other suitable polymer for insulating. Connector pin (or 
pins for bipolar or multipolar leads) 32 and connector 33 are preferably 
fabricated of stainless steel or other suitable conductive material. 
At a distal end 34 of the pacing lead 20 is an electrode assembly 36 which 
is discussed in more detail below. Immediately behind the distal end of 
the electrode assembly 36 is a tine sheath 38 which includes a plurality 
of individual flexible tines 40. Tines 40 engage endocardial tissue and 
urge the electrode assembly 36 into contact with the endocardium, in a 
direction parallel to the axis of the electrode assembly 36. Tines 40 are 
more fully described in U.S. Pat. No. 3,902,501, issued to Citron et al., 
incorporated herein by reference. A fixation or lead anchoring sleeve 42, 
slidably mounted around lead body 22, serves to stabilize the pacing lead 
20 at the site of venous insertion by means of suture ties about the 
sleeve and underlying fascia. 
The electrode assembly 36 of FIG. 1 is shown in greater cross-sectional 
detail in FIG. 2. As illustrated, the electrode assembly 36 includes a 
conductive electrode 50 as well as the tine sheath 38 and the tines 40 
thereof. The conductive electrode 50 is preferably a unitary construction 
including, at its proximal end, a cylindrical portion 52 defining an axial 
bore 54. A coil-wound conductor 56 of the lead body 22 of FIG. 1 is 
inserted into the axial bore 54 and affixed in electrical contact thereto, 
for example, by mechanical crimping or welding. Proceeding toward the 
distal end of the conductive electrode 50, the conductive electrode 50 
includes a neck area 58 having a reduced diameter from the cylinder 52 
which provides a recessed area into which an interior extending ridge of 
the tine sheath 38 is inserted to provide positive engagement of the tine 
sheath 38 with the conductive electrode 50. Finally, the conductive 
electrode 50 terminates at an electrode distal tip 60. 
As illustrated in FIG. 2, the electrode distal tip 60 has a generally 
mushroom shape, such that the electrode distal tip 60 has a 
semi-hemispherical surface which is intended to provide electrical contact 
with the endocardial tissue. It should be appreciated that the electrode 
distal tip 60 may define a number of different profiles, from 
semi-hemispherical to essentially planar with rounded edges. As 
illustrated in FIG. 2 and more specifically in FIG. 3, the electrode 
distal tip 60 preferably includes at least one and preferably two or more 
recessed areas or grooves 62. The recessed areas or grooves 62 define 
generally pie-shaped segments shown in FIG. 3. These pie-shaped segments 
of the electrode distal tip 60 will be generally defined as the plateaus 
64 within the specification, although it is recognized that the plateaus 
may be semi-hemispherical in shape, or utilize other configurations. 
As discussed above in the background of the invention, the particular 
structure, i.e., the size, shape and porosity, of the electrode distal tip 
60 is of particular importance to the functioning of the pacing lead 20, 
and the cardiac pacemaker system. The grooves 62 provide a means for 
capturing blood born cells during implant of the pacing lead. 
Specifically, the recessed areas or grooves 62 in the electrode distal tip 
60 as illustrated herein, provide a capture site for blood cells and 
elements therein, including platelets, thrombin, red blood cells, and 
other elements, and the initiation of the formation of blood clotting upon 
insertion of the electrode assembly 36 into the vein of the recipient. As 
the lead body 22 of the pacing lead 20 is fed into the vein of the 
recipient, and the electrode assembly 36 proceeds to the heart, the 
platelets, thrombin, red blood cells, and other blood borne elements which 
are captured within the recessed areas or grooves 62 begin to form a 
thrombosis or blood clot. This blood clot, upon contact with the 
endocardial tissue, helps assist in affixing the electrode distal tip 60 
to the endocardial tissue, to provide immediate stabilization of the 
electrode to endocardial tissue. The grooves also help to capture some 
amount of the soft, moldable endocardial tissue to also assist in 
immediately stabilizing the electrode tip. 
It is anticipated that the grooves 62, while relatively shallow, will 
capture enough platelets, red blood cells, and other elements and 
endocardial tissue during the passage from the venous insertion point to 
the endocardium of the heart, to generally fill a majority of the recessed 
area. Accordingly, for a lead's electrode distal tip 60 having a diameter 
of between one and four millimeters, and a preferred diameter of two 
millimeters, the grooves 62 will have a depth in the range of between 0.1 
and 0.5 millimeters and a width of between about 0.2 and 1.0 millimeters; 
and preferably, about 0.4 and 0.4 millimeters, respectively. Further, it 
is preferred that the edges of the grooves 62 be radiused in order to 
minimize tissue damages. 
The electrode distal tip 60 is also treated to increase the porosity and 
active surface area, thereby enhancing the electrical efficiency by 
lowering the polarization and increasing capacitance. This texturizing 
treatment of the electrode distal tip 60 includes depositing generally 
spherical shaped small particles on the surface of the electrode distal 
tip 60, including all of the surfaces defining the recesses or grooves 62 
as well as the plateaus 64. These generally spherical particles 70 are 
illustrated in the electron microscope photograph views shown in FIG. 4 
and in FIG. 5 in greater detail. 
Preferably, the conductive electrode 50 is made of a platinum-iridium 
composition. In the preferred embodiment, the platinum-iridium alloy has a 
composition of 90% platinum, 10% iridium by weight. The generally 
spherical particles 70 are preferably platinum/iridium (90%/100%) 
particles having a generally smooth surfaced spheroidal shape. It should 
be recognized however, that the electrode and the particles may be made of 
other suitable materials, such as titanium. The diameters of the spherical 
particles 70 should be in the range of between about 10 and 200 microns 
(0.01 mm to 0.20 mm), and preferably be in the range of between 20 and 80 
microns (0.02 mm to 0.08 mm). Additionally, it is preferred that the 
spherical particles have a distribution of sizes spanning this range. 
Preferentially, two coatings of the spheroidal particles are applied to 
the base electrode. The first coating is preferred to be of particles in 
the range of 40-80 microns and the second coating 20-40 microns. 
Upon affixation to the electrode distal tip 60, the generally spheroidal 
particles 70 will create a plurality of pore sites and interstitial 
porosity for chronic ingrowth of tissue. Preferably, by affixation of the 
spheroidal particles 70 having the preferred sizes and distribution of 
size, the interstitial porosity defined by the multiple layers of 
spheroidal particles 70 will have passageway dimensions which allow the 
passage of red blood cells (typically having a six-micron (0.006 mm) 
diameter) and other blood borne elements. By allowing the migration of red 
blood cells and other blood carried substances through the interstitial 
porosity, the events resulting in chronic tissue ingrowth are initiated. 
The spheroidal particles 70 preferably have a generally smooth surface in 
order to minimize the amount of irritation of the endocardial tissue 
caused by the electrode distal tip 60 during the continuous beating of the 
heart. In addition to providing interstitial porosity for tissue ingrowth, 
the affixation of the spherical particles 70 also substantially increases 
the true surface areas of the electrode distal tip 60. Generally, by use 
of these spheroidal particles 70, the true surface area of the electrode 
distal tip 60 is increased by as much as a factor of five to twenty times. 
Following affixation of the spherical particles 70 to the electrode distal 
tips 60, the electrode distal tip 60 and the particles 70 are treated with 
a surface coating means for increasing the active electrical surface area 
and enhancing the electrical efficiency by reducing the degree of 
electrochemical polarization and increasing the electrical capacitance of 
the electrode distal tip 60 during operation of the pacemaker system. 
Preferably, a nonmetallic material such as titanium nitride is used as the 
surface coating, as depicted in the electron microscope photograph of FIG. 
6. In the electron microscope photograph of FIG. 6, a portion of the 
surface of the electrode distal tip 60 is enlarged by a factor of eight 
thousand. As may be appreciated from observing FIG. 6, the surface coating 
further increases the true surface area of the electrode distal tip 60 by 
a significant factor. 
In addition to increasing the true surface area, the surface coating 
substantially enhances the electrical characteristics of the electrode 
distal tip 60. The surface coating increases the electrode's electrical 
capacitance and lowers the polarization developed at the electrode distal 
tip 60. It should also be noted that the surface coating on the spherical 
particles 70, while appearing to create relatively sharp edges thereon, 
does not result in irritation to the endocardial tissue because the 
relative size of the crystalline structure of the surface coating is 
substantially smaller than the heart's cells, the other cardiovascular 
tissue, and the blood elements which the coating will contact (i.e., red 
blood cells having an approximate diameter of six microns). 
The surface coating is deposited in a manner such that the thicknesses of 
the surface coating attached to the spherical particles 70 is in a range 
of between one to thirty microns. While titanium nitride is the preferred 
surface coating material, other suitable nonmetallic coating materials, 
such as, for example, carbon, iridium oxide, and titanium oxide; and 
platinum oxide may also be applied as the surface coating of the electrode 
distal tip 60 following affixation of the spherical particles 70. 
FIG. 7 is a graph depicting the generalized pacing threshold performance of 
an electrode constructed according to the present invention. In FIG. 7, 
the pacing threshold energy in microjoules is depicted on the Y-axis as a 
function of time, in weeks, along the X-axis, for two exemplary lead 
designs of the prior art and the lead 20 according to the present 
invention. In the graph, the average energy threshold is based upon 
voltage thresholds at various pulse duration and assumes pacing impedance 
remains generally constant. As depicted, the increase in the average 
energy requirement within the four to six weeks following implant is 
substantial for the pacing leads of the prior art. By comparison, the lead 
20 of the present invention exhibits virtually no threshold increase, and 
remains relatively level at a lower average energy than either of the 
prior art leads. As may be appreciated, this will result in an increased 
threshold safety margin and/or a substantial increase in the useful life 
of the pacemaker system, given a fixed battery capacity since the required 
energy to stimulate the heart is low. 
FIG. 8 graphically depicts the improved cardiac signal sensing capability 
of the electrode design of the present invention. In FIG. 8 the cardiac 
signal amplitude in millivolts is depicted on the Y-axis as a function of 
time (in weeks) on the X-axis, for the lead of the present invention and a 
lead according to the prior art. As depicted, the lead of the present 
invention maintains a relatively uniform high level for the cardiac signal 
amplitude, as compared to the prior lead which has both a lower initial 
level and a reduction over the course of the first two to three weeks. The 
primary difference reduced as a result of the significantly reduced 
polarization of the lead of the present invention, as discussed in greater 
detail below. 
FIGS. 9 and 10 show a cross-sectional view and a top view of an alternative 
embodiment of an electrode distal tip electrode so for the electrode 
assembly 36 of FIG. 1. In FIGS. 9, 10, the electrode distal tip 80 
includes a plurality of generally parallel grooves 82 defining 
therebetween generally parallel strip plateau sections 84. In the 
embodiment shown in FIG. 9, three grooves 82 are illustrated; however, it 
should be understood that a limited number of additional grooves may be 
incorporated. The number of the grooves 82 is limited by the size of the 
grooves appropriate for the capture of platelets, red blood cells, and 
other blood elements and tissue as described above, and the diameter of 
the electrode distal tip 80. In the preferred configuration, the diameter 
of the electrode distal tip 80 will be in the range of between one to four 
millimeters. Preferably, the diameter of the electrode distal tip 80 is 
approximately two millimeters. 
FIGS. 11 and 12 show a cross-sectional view and a top plan view 
respectively of a second alternative embodiment of an electrode distal tip 
90 for the electrode assembly 36 of FIG. 1. The embodiment of FIGS. 11 and 
12 includes a plurality of intersecting grooves 92 which define 
therebetween generally square shaped plateaus 94. Alternatively the 
plateaus may be round in shape as shown by round plateau 95. As above, the 
grooves 92 are dimensioned so as to allow capture of platelets, red blood 
cells, and other blood elements during insertion of the lead 20 into the 
patient. In addition, as illustrated in FIG. 11, the grooves 92 are cut 
into the electrode distal tip 90 such that a base 96 of the grooves, when 
viewed in cross-section, defines a semi-hemispherical inner surface. Thus, 
in FIGS. 11 and 12, both the surfaces of the plateaus 94 and the base 96 
of the grooves 92 define semi-hemispherical surfaces. This 
semi-hemispherical base surface configuration can also be incorporated 
into the designs illustrated in FIGS. 2 and 3, as well as in the design of 
FIGS. 9 and 10. 
An additional alternative design configuration which may be incorporated 
into any of the three embodiments illustrated in FIGS. 2 and 3, 9 and 10, 
or 11 and 12, is illustrated in FIG. 13. In FIG. 13, the base of the 
grooves 102 is illustrated as either having a flat surface 104 or a 
concaved inner surface 106. Thus, in any of the electrode distal tip 
configurations of the present invention, it is contemplated that the base 
of the groove or grooves may define a concave base surface, a flat base 
surface or a semi-hemispherical base surface profile. 
Additionally, while it has been illustrated in the figures that the walls 
108 (FIG. 13) defining the grooves of the embodiments are illustrated as 
having generally flat surfaces which parallel the axis of the distal tip, 
it may be appreciated that the walls of the grooves may be generally 
angled with respect to the axis of the distal tip. In addition, the 
corners at the base and the peaks of the grooves for any of 30 the above 
described embodiments may be radiused to a radius of curvature of between 
about 0.001 mm and 0.5 mm, as opposed to having sharp edges. The grooves 
of any of the above embodiments may be formed by any of the methods 
selected from the group including stamping, milling, molding and 
electrochemical machining. The generally spherical particles 70 which are 
applied to the surfaces of the electrode distal tip 60 as discussed above 
with respect to FIG. 1 are also applied to the alternative embodiments of 
FIGS. 9 through 13, as is the titanium nitride or alternative surface 
coating treatment. The spherical particles 70 are preferably attached to 
the surfaces of the electrode distal tip by a process selected from the 
group of processes including sintering, laser fusion or welding, injection 
and molding casting. An example of one such attachment process is powder 
sintering, wherein a fractional percentage of the spherical particles are 
affixed in each of two to five successive steps. 
Finally, the surface coating of titanium nitride or alternative material, 
is applied to the spherical particles 70 of the electrode distal tips 60 
or any of the alternative embodiments of FIGS. 9 through 13 by a process 
selected from the group including sintering in an appropriate environment, 
vapor deposition, electroplating and sputtering. 
The surface coating is illustrated in FIGS. 14 and 15 which schematically 
depict top and side views respectively of the microporous surface 
structure. FIG. 14 illustrates the generally pyramid-like shape of the 
microporous coating. In FIG. 15, a side or profile view of the microporous 
surface coating illustrates the triangular peaks of the coating, and more 
importantly the areas between the peaks of the coating which provides high 
surface porosity. With the foregoing construction, incorporating the 
microporous surface coating, the surface porosity or microporosity is in 
excess of fifty percent (50%), and preferably in the range of between 
sixty-five percent (65%) and seventy percent (70%). 
A pacing lead having an electrode tip configured in accordance with the 
foregoing detailed description exhibits superior pacing performance. The 
primary reason for the superior performance is the reduction in 
polarization proximate the electrode tip. The "polarization voltage" for a 
pacing lead is herein defined to be the voltage differential developed 
between the leading edge and the trailing edge of a reference electrical 
impulse. The reference electrical impulse is a 10 mA (milliampere), one 
millisecond, square wave, constant current pulse from a pulse generator. 
The polarization voltage (Pv) for a particular pacing lead is determined 
by subtracting the leading edge voltage (V1) from the trailing edge 
voltage (V2) of the reference electrical impulse (Pv=(V2-V1). During 
measurement of the polarization voltage, the electrode is immersed in a 
0.15 molar sodium chloride (NaCl) solution, at a pH of 7, and temperature 
of 37.degree. C. 
As may be appreciated by those skilled in the art, the polarization voltage 
measured according to the above test will be influenced substantially by 
the size, shape, material, and surface nature of the electrode distal tip 
for a given pacing lead. Thus, for an electrode distal tip having a 
particular profile, and made of a particular material having a particular 
surface nature, the polarization voltages will be different for a 3 
mm.sup.2 tip and a 5 mm.sup.2 tip. Conversely, two pacing leads having 5 
mm.sup.2 electrode tips will have characterizing polarization voltages 
which will depend on their particular profile, construction, material and 
surface nature. Generally the pacing lead electrode which has a lower 
polarization voltage for any given electrode design, or more simply the 
circumferential area, (cross-sectional area for a non-round tip) will be 
the more desirable pacing lead. 
Accordingly, to further characterize pacing lead electrodes and more 
particularly the pacing lead of the present invention, a "polarization 
index" (PI) is herein defined. The polarization index is the polarization 
voltage Pv divided by the circumferential area CA (or cross-sectional 
area) of the electrode at its widest diameter. Thus, for a semi-spherical 
electrode having a diameter "d," the circumferential area CA is equal to 
.pi. (d/2).sup.2, and the polarization index is given by the following 
formula: 
EQU PI=(V2-V1)/.pi.(d/2).sup.2 or PI=Pv/CA 
The pacing leads of the present invention as disclosed in detail above have 
a polarization index PI which is less than 100 mV/mm.sup.2. More 
particularly, in the preferred embodiments, the pacing leads of the 
present invention have a polarization index PI which is less than 50 
mV/mm.sup.2. A pacing lead having an electrode tip which combines a 
surface morphology allowing tissue ingrowth and very low polarization 
index levels is highly desirable in the field of implantable cardiac 
pacing leads. 
It should be evident from the foregoing description that the present 
invention provides many advantages over pacing leads of the prior art. 
Although preferred embodiments are specifically illustrated and described 
herein, it will be appreciated that many modifications and variations of 
the present invention are possible in light of the above teaching to those 
skilled in the art. It is preferred, therefore, that the present invention 
be limited not by the specific disclosure herein, but only by the appended 
claims.