Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND or TI~E INVENTION
The present invention relates generally to an improved
implantable electrode for delivering electrical stimulation pulses
to an organ, and more particularly to an improved implantable elec-
trode means for delivery of electrical s-timulation pulses to the
heart.
Heart pacer therapy and technology has expanded intensively,
and with improvements available in batteries and circuitry, a need
has now arise for improvement of the functional characteristics of
the elec~rode. Recent improvements in batteries and a corresponding
reduction of current consumption of the circuitry within the pulse
generator have made it both desirable and necessary to improve
electrode performance, particularly improvements relating to the
current requirements of the electrode. Furthermore, most of the
complications resulting from heart pacer therapy may be traced to the
electrodes, and thus physical properties appear to also require
improvement.
Studies have been conducted, and certain improvements in
electrodes and leads have been made in the past. Specifically, metal
fatigue has presented problems which may result in lead fractures.
Furthermore, improved lead sealing and electrode geometries have
been developed, with one typical lead struc-ture being disclosed in
United States Patent No. 4,033,355, July 5, 1977,Amundson, and
entitled "Electrode Lead Assembly for Implantable Devices', assigned
to applican-t herein.
Porous electrode structures have been known in the past,
with the pores being formed in a solid structure and having a size
of less than approximately lOO microns. Such structures have been
of assistance, particularly in the compatibility of the porous
structure to accommodate ingrowth into the channels of active heart
tissue. Further improvements are available from the electrode
structure of the present invention, with -these improvements including
both electrical performance and accommodation of desirable heart
tissue ingrowth. In electrical performance, improvements are found
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,articul.irly in -the reduction of polarization. Stimulation thresholds
have been significantly reduced, with those of the present structure
having been found to be as low as one third to one half of the
requirements of conventional electrodes. The electrode of the
present inven-tion consists of a filamentary member, wherein
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the individual filaments may be compressed together to form a substantially
solid body, and wherein the filaments preferably have a diameter of less than
about 100 microns and form less than approximately 20% of the entire volume
of the filamentary or fibrous electrode. In one embodiment, the individual
fi laments may be compressed of compacted or compacted within a grid en~losure,
with the individual elements forming the grid being slightly larger than the
filaments forming the confined filamentary electrode, with the grid enclosure
being adapted to confine, restrict, or otherwise retain the filamentary electrode
i ntact.
Organ stimulation, particularly heart stimulation, may be described
in terms of electrical field theory. Excitation is initiated by virtue of an
electrical stimulus which exceeds a certain threshold level. The electrical
field, therefore, may be characterized as a force being applied to the system.
Relating the field strength to the electrode structure, the maximum field
strength is believed to be present at the surface of the electrode, with the
magnitude of the field being generally inversely proportionally to the electrode
size. In ordinary electrodes, the excitable tissue normally becomes spaced
from the surface of the electrode by the growth of a barrier or layer of fibrotic
material. Accordingly, the field strength must be related to that field which is
available or effective at the interface which develops between the excitable
tissue and the fibrotic tissue. Hence, any reduction in the formation or growth
of fibrotic tissue should reasonably be calculated or increase the magnitude of
the effective field at the excitable tissue boundary and thus increase the ultimate
effectiveness of the electrode.
Furthermore, in normal heart pacers, particularly those of the
demand/inhibit type or those which respond in synchrony to a natural heart
signal, the electrode performs a sensing function as well as a stimulation
function. The sensing impedance of the amplifiers typically used range in the
order of 20,000 ohms, and any source impedance will, of course, be in series
with the input impedance. The electrode of the present invention permits the
source impedance to remain low, thereby enhancing the overall sensing capability
of the electrode. As has been indicated, the polarization is exceptionally low,
this being believed to be due to the large real surface area available for polari-
zation considerations. However, it has also been found that the effective surface
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area of the structure is small for stimulation considerations. This appears
to provide significant advantages for both operating parameters.
SUMMARY OF THE INVENTION
Turning now to the aspects of the present invention, the implantable
electrode consists of a compressed or compacted bundle of metallic filaments
or fibers, with the bundle being designed so as to substantially completely
envelop the surface of the conductive lead which couples the pulse generator
to the electrode. The individual fibers or filaments forming the fibrous member
have a diameter of between about 10 microns and 100 microns, with the pre-
ferred diameter being about 20 microns. The preferred filamentary diameter
of 20 microns is due to the general match of the cell size with the filament size,
with cell size being generally in the area of about 20 microns.
The solid filamentary material forms between about 3% to 30% of
the volume of the member, with the lower ranges, such as approximately 5%
being preferred. The balance of the volume of the fibrous member is, of
course, open. When less than about 3% of the volume is formed of the filamen-
tary material, the structure appears to become overly porous, and when more
than about 20% of the volume of the member is formed of the filamentary material,
the finished structure appears to become generally quasi solid and may in some
cases become somewhat less pliant in those de~es wherein this is a desired
f eatu re.
As a result of this structural design, it has been found that substan-
tially less energy is required for stimulation of the heart. A red~ction in polari-
zation occurs, thereby further enhancing the performance of the electrode and
reducing the energy required for constant and consistent organ stimulation over
extendeci periods of time. Ingrowth occurs between the surrounding tissue and
the electrode, thereby improving electrical response, and furthermore enhances
the performance of the device for reducing the tendency toward dislodgement
whiie also reducing abrasir,n.
Therefore, it is a primary object of the present invention to provide
an improved implantable electrode structure which is particularly adapted for
use in the electrical stimulation of an organ, such as the heart, with the
el ectrode comprising a metal I ic fi I amentary bundle.
It is a further object of the present invention to provide an improved
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implantable electrode means in the form of a fibrous tip member consisting of a
filamentary probe which is arranged to be electrically coupled to a remotely
disposed pulse generator, and being designed to substantially completely
envelop the electrically conductive member which extends from the pulse genera-
tor to the electrode.
It is yet a further object of the present invention to provide an
improved implantable electrode means for use with heart pacemakers, wherein
the electrode is formed as a bundle or compress of metallic fibers, and wherein
the metal lic fibers have a diameter of less than about 100 microns and form
between 3% and 30%.
A principal object is to provide an implantable electrode means for
electrical stimulation of an organ from an electrical signal generator and
including an electrically conductive lead member extending from said electrical
signal generator to said implantable electrode means and having a surface in
electrical contact with said implantable electrode means, said implantable
electrode means comprising a plurality of electrically conductive metallic
filaments retained together as a bundle to form a substantially solid body, with
the filaments forming the bundle having a diameter of less than about 100
microns and forming between 3% and 30% of the volume of said fibrous member.
Other and further objects of the present invention will become
apparent to those skilled in the art upon a study of the following specification,
appended claims and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a flow diagram of a typical procedure which may be
employed in the preparation of the implantable electrode means of the present
invention;
Figure 2 is a side elevational view illustrating one typical embodi-
ment of an implantabie electrode means of the present invention in combination
with a typical pulse generator and conductive lead assembly;
Figure 3 is a sectional view on an enlarged scale taken through the
diameter of one embodiment of the lead and electrode, and illustrating one
m~n~ner in which the electrode may be secured to the conductive lead;
Figure 4 is a perspective view, partially broken away, and further
illustrating the manner in which the implantable electrode of Figure 3 is coupled
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to the conductive lead;
Figure 5 is a sectional view taken along the line and in the direction
of the arrows 5-S of Figure 3;
Figure 6 is a sectional view on an enlarged scale from Figure 2,
and illustrating an alternate preferred embodiment of the lead and electrode,
and illustrating the details of the alternate preferred embodiment;
Figure 7 is a perspective view, partially broken away, and further
illustrating certain details of the embodiment of Figure 6, and the manner in
which the electrode is coupled to the conductive lead; and
Figure 8 is a sectional view taken along the line and in the direction
of the arrows 8-8 of Figure 7.
DE~RIPTION OF THE_PREFERRED EMBODIMENTS
In accordance with one of the alternate preferred embodiments of
the present invention, and with particular attention being directed to Figures
2-5 of the drawings, it will be seen that the cardiac pacer assembly generally
designated 10 includes a pulse generator with lead assembly 14 extending from
the coupling zone or StatiOn 12 to the conductive electrode 15. Conductive
electrode 15 is, of course, exposed and is generally in physical contact with
the tissue to be stimulated, such as the heart muscle in the case of a cardiac
pacer device. It will be understood that the configuration of the electrode as
illustrated at 15 is merely one of many such designs, with the arrangement
being appropriately adapted for either myocardial or epicardial type electrodes.
The arrangement of the device illustrated in Figure 2 is that of a
unipolar cardiac pacer, and the assembly may be in the form of that assembly
disclosed and claimed in United States Patent No. 3, 882,707. The circuitry
for the pulse generator 1~ may be in the form of that circuitry disclosed in
United States Patent No. 4,041,953.
It will be appreciated, of course, that bipolar leads may be prepared
utilizing the features of the present invention. For purposes of comprehending
the concept, however, a unipolar device is disclosed for simplicity.
Attention is now directed to Figures 3 and 4 of the drawings which
illustrate the details of the lead assembly and the manner in which the implant-
able electrode is secured thereto. Specifizally, as illustrated in Figure ~, the
conductor system generally designated 20 includes a pair of parallel spans 21
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~nd 22, each of which is arranged in a helical pattern about a
common axis. The coiled conductors have a uniform outer diameter,
as is apparent in the structure shown in Figures 3 and 4. A pair of
coaxially disposed tubular insulating sheaths 24 and 25 are illustra-
ted to enclose the coiled conductors 21 and 22, it being appreciated,
of course, that a single insulating sheath such as illustrated in the
embodiment of Figures 6-8 inclusive may be employed as well. If
desired, a double insulating sheath, such as is illustrated in
Figure 3 may be employed, with such an arrangement being disclosed
in United States Patent No. 4,0033,355.
In the embodiment illustrated in Figures 3-5 inclusive,
electrode 15 is shown coupled electrically and mechanically to the
outer circumference of the coil formed by helically wound conductors
21 and 22. In order to seal the lead assembly, a conductive cap
member is provided, as shown at 26, with the filamentary metallic
compress 27 being secured both electrically and mechanically to the
outer surface thereof. It will be appreciated that other sealing
techniques may be employed as desired.
Attention is now directed to Figures 6-8 of the drawings
wherein the other of the alternate preferred embodiments is illustra-
ted. Specifically, the conductor system, again generally designated
20, (this being designated with the same reference numeral as has
been utilized in the corresponding component of Figure 3) includes a
pair of parallel spans 21 and 22, each of which is arranged in a
helical pattern ~out a common axis, as is true in the alternate
embodiment of Figures 3 and ~. A tubular insulating sheath 30
encloses the coiled conductors 21 and 22.
In the embodiment illustrated in Figures 6-8 inclusive,
electrode 31 is shown coupled electrically and mechanically to the
outer circumference of the coil formed by helically wound conductors
21 and 22. ~lectrode 31 includes a bundle of filamentary fibers 32,
enclosed within a metallic grid 33 of a composition similar to that
of the fialements forming the enclosed bundle.
For materials of construction, it will be appreciated
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.hat pl.atinum i.s preferred, with other substances being suitable
such as, for example, Elgiloy, titanium, or platinum-iridium
alloys. Elgiloy is the trade mark of Elgin Watch Company and is
unique to a specific composition which is 40% cobalt, 20%
chromium, 15% nickel, 22.9% iron, 2% manganese and 0.1% carbon.
~oth Elgiloy and 90:10 platinum-iridium alloys are widely used as
materials of construction for implantable electrodes.
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,
The material of construction for conductors 21 and 22 is preferably
MP35N, nickel-chromium-cobalt alloy, this material being inert to body fluids,
and possessing mechanical properties which include the requisite flexural
characteristics, tensile strength, and resistance to fatigue.
PREPARATION OF_IMPLANTABLE ELECT_ODES
In order to prepare the implantable electrode, such as fibrous
member 27, a bundle of metallic filaments of platinum are compressed together
to form a substantially solid body, and prepared for sintering. The compress
is placed within~a suitable cavity or chamber, and heated to a temperature of
1500 C. for a period of about 2 hours. This raises the temperature of the
metal sufficiently so as to provide substantial VapOr pressure for the platinum,
and thus form the desired cohesive compress. The diameter of the individual
filaments is, in this example, 18 microns, although filament diameters ranging
from between about 10 microns and 100 microns may be suitably employed. As
a further consideration, the volumetric parameters provide for the structure
to b~preferably 95 percent open, with the metallic filaments forming the
balance of 5% of the volume. It has been found, however, that up to approxi-
mately 20% of the volume may be metallic filaments.
In order to prepare the implantable electrode of the embodiment
illustrated in Figures 6-8, a bundle of metallic filaments of platinum are com-
pressed together to form a substantially solid bundle, and the bundle is then
placed w jthin the confines of the grid enclosure. Specifically, the grid
enclosure is formed of the same material as the metallic filaments, and normally
has a reticulated pattern of l50 x 150 filaments per inch, with each filament
having a diameter of 0. 001 inches. In certain instances, a grid having some-
what larger diameter lines may be employed, such as up to approximately 0.002
inches, with one such grid having filament 0.0017 inches in diameter being
commercially available. ln this embodiment, the bundle is approximately 90%
open, with the metallic filaments forming the balance of 10% of the volume.
Stated another way, the material is 10% dense. The individual filaments form-
ing the bundle have a diameter of from between about 10 microns and 100 microns
although in the specific example, the filaments have a diameter of 20 microns.
In order to form a solid compress or bundle, the filaments within
the grid enclosure are heated to a temperature of 1500C. for a period of about
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2 hours to permit sintering to occur.
As has been indicated, the metallic filament structure of the
present invention contributes to a reduction of fibrosis. The reduction is
believed due to the utilization of the small diameter metallic filament, together
with the provision of permitting approximately 80% or more of the structure to
be open. The organ being stimulated provides certain ingrowth into the
electrode element, with the ingrowth contributing to a red~ction in abrasion,
and a more uniform electrical response and function. Therefore, the active
heart tissue is disposed more closely adjacent to the surface portions of the
electrode which generate the field which performs the actual stimulation.
Furthermore, polarization during stimulation is substantially reduced, with
measurements indicating that polarization is almost eliminated for most normal
cardiac pacer functions. In addition, the sensing function is formed effectively
by the structure of the present invention.
While the utilization of a non-woven filamentary mass is contem-
plated and discussed herein, it will also be appreciated that a woven or knitted
pattern may also be employed to form the overall structure. In such an
arrangement, the parameters of filament size and volume considerations remain
as set forth above.
While various techniques may be employed to bond the electrode
structure to the conductive leads, it has been found that diffusion bonding is
generally preferred. This is particularly desirable or applicable to the
structure illustrated in Figures 6-8.
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