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BACKGROUND OF THE INVENTION
It is well known that cardiac arrhythmias, such as atrial or ventricular
fibrillation, can be overcome by applying electrical energy to the
fibrillating myocardium. This procedure, defibrillation, can be
accomplished by applying the electrical energy either to the chest of the
patient by means of conductive-metal paddles held in place by medical
personnel or, during the course of cardiac surgery, by holding
conductive-metal paddles in direct contact with the surface of the heart.
Such procedures are well known and have been found to be generally
effective.
More recently, implantable defibrillators have been proposed for
automatically detecting the onset of the cardiac arrhythmia and for
automatically correcting such arrhythmia. These automatic defibrillators
may employ conformal electrodes, which are maintained in contact with the
surface of the heart or, electrodes on an intravascular catheter, or some
combination of these. In any case, the electrodes act to impart the
desired electrical energy to the heart muscle to achieve defibrillation.
With the intravascular catheter electrode approach, it has been found that
although less electrical energy need be imparted to the heart than in the
exterior chest paddles approach, more energy is needed than in the system
wherein the electrodes are placed directly in contact with the heart
surface. In other words, it has been found that physically placing the
electrodes in contact with the exterior of the heart will provide a more
efficient use of the electrical energy, thereby reducing the amount of
energy required. Obviously, energy consumption is of the utmost importance
in any implanted medical-electronic device.
In the automatic defibrillators, previously under consideration, the
defibrillation electrodes have been designed for application to the heart
by entering the chest cavity and by sewing the electrodes to the heart or
positioning the electrodes on the surface of the heart. At times, such
electrode implantation may be accomplished during the course of cardiac
surgery, such as during a bypass operation. However, even when such heart
surgery is not independently required, the previous surface electrodes
required that the chest cavity be opened in order to implant the
defibrillating electrodes. This surgical procedure requires intubation of
the lungs and exposes the surfaces of the lungs to possible infection.
Additionally, in order for the surgeon to have sufficient working space to
effectively position and apply the electrodes, it may be necessary to
perform an additional surgical procedure involving spreading two adjacent
ribs or splitting the sternum. Accordingly, at the present time, in order
to apply any type of cardiac electrodes to the surface of the heart, it is
necessary to perform major surgery. Nevertheless, it is desirable to be
able to implant the electrodes without the necessity of entering the
pleural space, thereby maintaining the integrity of the pleural cavity.
Moreover, known surface electrodes suffer from the disadvantage that less
than uniform energy density results from a discharge. Higher energy
densities appear at the electrode edges, and at higher discharge levels,
damaged tissue could result at the heart surface. It is, of course,
desirable that the discharge be uniform over the entire electrode surface
and that no regions of high energy density be present.
SUMMARY OF THE INVENTION
The present invention is generally related to the field of electrical
defibrillation and, more specifically, to particular cardioverting
electrode configurations for use in implantable defibrillators, as well as
to methods for the implantation of such electrodes.
"Cardioverting" or "cardioversion" as used herein is intended to encompass
the correction of a number of arrhythmic heart conditions, both lethal and
nonlethal. Those arrhythmic heart conditions include atrial tachycardia,
atrial flutter, atrial fibrillation, junctional rhythms, ventricular
tachycardia, ventricular flutter, ventricular fibrillation, and any other
non-pacemaking related arrhythmic condition which may be corrected by
applying electrical shocks, which are of a magnitude substantially greater
than pacing shocks, to the heart. Obviously then "defibrillation" is
included in the term cardioversion as a method of applying electrical
shocks to the heart to defibrillate fibrillating atria or fibrillating
ventricles.
In one embodiment, the cardioverting electrode is of rectangular shape
which is designed for insertion through the soft tissues outside the
pleural cavity and then to be arranged in contact with the heart. The
electrode has a specific configuration which enhances energy efficiency,
while providing optimum transfer of electrical energy to the heart, and
preventing high-current densities from existing at the edges of the
electrode.
The electrode is formed of a metallic mesh or screen which is sandwiched
between two layers of a chemically inert, electrical insulation material.
In this manner, the portions of the electrode facing away from, i.e., not
in contact with the heart surface, are electrically insulated from the
body. An alternative, yet a somewhat less efficient, embodiment is to use
only a single layer of insulation on the back and to stitch such layer to
the screen. The defibrillation electrode may be provided with an
additional electrode, which is utilized to provide a cardiac pacing
function. Means are also provided to permit disconnection of the
electrical lead to the pacing tip, after the surgical implantation has
taken place, at such time when it may be reasonably assumed that cardiac
pacing is not required.
Additionally, the inventive electrode may also be used as a pick-up
electrode in an electrocardiogram (ECG) system to detect the electrical
activity of the heart. Since both functions need not occur simultaneously,
the same electrical lead can be used both in the defibrillator function
and in the ECG function.
Implantation of the electrode by the inventive method consists of the steps
of first making a skin incision on the interior thoracic or abdominal
wall, and then positioning the electrode on the surface of the heart by
using a hand-held instrument to separate the tissue planes and to create a
tunnel inside the thorax, but outside the pleural cavity, through the soft
tissues surrounding the heart. Upon creating the tunnel, one or more
electrodes may be placed into the tunnel and arranged proximate the
surface of the heart. In one embodiment, two electrodes are placed on
opposing sides of the heart and means are provided whereby the proximal
ends of the electrodes may be sutured to the adjacent tissue, in order to
provide positive electrode fixation. In another manner of practicing the
inventive method, a first tunneled space is located between the interior
surface of the sternum and the exterior surface of the heart's
pericardium, and a second tunnel is created on the heart's inferior
surface between the pericardium and the diaphragm.
In order to perform the inventive method described above, a specialized
inventive implantation tool is provided which cooperates with the
electrode being implanted to permit relatively easy placement of the
electrode in relation to the heart and subsequent withdrawal of the
implantation tool.
In one embodiment of the electrode, the edge surface of the electrode is
constructed so as to have a higher impedance to current flow than the
central portion, thereby providing efficient, and relatively uniform,
energy transfer by eliminating the so-called "edge effect". Such impedance
is controlled by the use of a mechanically embodied electrical filter
having spaced holes located over the edges of the mesh electrode.
It is, therefore, one object of the present invention to provide
defibrillating electrodes, which are highly efficient in transferring
electrical energy to the heart.
Another object of the present invention is to provide defibrillator
electrodes, which minimize any electrical damage that may be done to the
heart muscle during defibrillation.
It is a further object of the present invention to provide a defibrillation
electrode, which may be implanted and arranged next to the heart in a
manner requiring only a minimum amount of surgery.
Still another object of the present invention is to provide a
defibrillation electrode which also has a pacing tip.
It is yet another object of the present invention to provide an electrode
which may be used both as a defibrillation electrode and as a pick-up
electrode for ECG signals.
Another object of the present invention is to provide a method for
implanting a defibrillator electrode adjacent to the heart without the
need for major surgery.
A further object of the present invention is to provide a method for
inserting electrodes into the body by making only a skin incision and
utilizing a specialized tool to create a tunnel inside the thorax but
outside the pleural cavity.
It is still a further object of the present invention to provide a
specialized tool for use in inserting the inventive electrode by means of
the inventive method.
Yet another object of the present invention is to provide a defibrillating
electrode having a heart-pacing tip wherein the lead connecting the pacing
tip to the pacing apparatus, located externally of the patient, is
provided with a separable connector so that after surgery a temporary
pacing lead may be disconnected and removed.
It is another object of the present invention to provide an implantable
defibrillation electrode having a specialized receiving pocket at its
forward-most end to receive the leading edge of an implantation tool,
which will permit insertion of the electrode through the soft tissues of
the thorax, while permitting the insertion tool to be withdrawn after
placement of the electrode.
The manner in which these and other objects are accomplished by the present
invention, as well as the many attendant advantages thereof, will become
more readily apparent when reference is made to the following description,
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective of the inventive electrode;
FIG. 2 is a cross section through the electrode of FIG. 1 taken along sight
line 2--2;
FIG. 3 is a perspective of the bottom surface of the inventive electrode of
FIG. 1;
FIG. 4 is a perspective of a tool suitable for inserting the inventive
electrode;
FIG. 5 is a cross section of the insertion tool of FIG. 4 taken along plane
5--5;
FIG. 6 is a perspective view of another embodiment of a tool suitable for
inserting the inventive electrode;
FIG. 7 is a schematic representation showing the insertion of the inventive
electrode into the body;
FIG. 8 is a schematic representation showing another manner of insertion of
the inventive electrode into the body;
FIG. 9 is a perspective of the inventive electrode arranged on the
insertion tool prior to implantation of the electrode; and
FIG. 10 is a cross-sectional view of the separable coupler utilized in the
embodiment of FIG. 1; and
FIG. 11 is a perspective of another embodiment of the inventive electrode.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring first to FIG. 1, the inventive electrode 10 is formed in a
substantially rectangular configuration, with side dimensions ranging from
approximately 1.5 to 4 cm and 3 to 6 cms, respectively. Preferably, the
electrode 10 is approximately 4.times.6 cm. In special cases the electrode
may also be formed as a square. The actual metallic electrode element is a
mesh or screen 12, which may be formed of titanium or platinum.
Alternately, the electrode may be formed of expanded platinum. The mesh is
a 150 mesh, having 150 elements or individual wires per inch. The wire
diameter is chosen to be between 1 and 3 mils. The electrode mesh 12 is
first prepared by spot welding together the wires located around the
periphery of the mesh. After spot welding, the excess lengths of wires are
then ground or machined flush, so as to produce a smooth edge and to form
a continuous border.
The body of the electrode may be formed of two layers of Silastic with the
metallic electrode element sandwiched therebetween. The electrode 10 is
formed by providing a first bottom layer 14 then applying the titanium
mesh 12 and, finally, having a top layer 16 and applied thereupon. The
thickness of the assembly should be approximately 1 to 3 cm. The top layer
16 has a rectangular aperture cut into it so that the titanium mesh 12 may
make electrical contact with the surface tissue of the heart. In order to
provide structural strength, a reinforcing mesh of Dacron may be embedded
in both of the Silastic layers and, at the very least, should be used in
the bottom layer 14.
The inventive electrode 10 may also be provided with a pacing button 18,
which is centrally arranged in the metal mesh 12, and which is
electrically insulated from the mesh 12 by means of a suitable insulator
20. The pacing button 18 may be formed of platinum or some other suitably
inert conductor. The pacing button 18 should extend approximately 1 to 3
millimeters above the surface of the titanium mesh 12. The electrical lead
for the pacing button 18 is seen at 22 and is provided with a specialized
detachable coupler 24, located at the end of the conductor 22. The
detachable coupler 24 will be further described and shown in more detail
hereinbelow. Issuing from this connector 24 is another conductor 26 which
is connected to a suitable plug 28 for connection to the appropriate
electronic cardiac pacing apparatus. Upon implanting the electrode 10, the
conductor 26 is arranged externally to the patient with the coupler 24
located just inside the skin of the patient. In this way, if necessary
following surgery, the lead 26 is available to make a connection for the
heart pacing function. Subsequently, after the patient's condition has
been stabilized, and when it appears to the physician that the pacing
function will not be required, the lead 26 may be pulled from the coupler
24 with no additional surgical procedures required.
The mesh 12 of the electrode 10 is connected to the proper source of
electrical current through an insulated cable 30 having a suitable
electrical connector 32 located at one end. The other end of the cable 30
is electrically connected to the mesh 12 at a low resistance joint located
inside an insulated boot 33.
As stated above, it is an object of the present invention to provide an
energy efficient electrode which does not require creating large incisions
or openings in the thorax, in order to effect direct surgical placement of
the electrodes. It is also another object of the present invention to
construct such electrodes so as to minimize the possibility of any damage
to the heart caused when using the electrode for defibrillation. In this
regard, it has been found that when employing defibrillation electrodes
having opposed conductors, as contemplated by the present invention, there
is present in the electric field between these two conductors a phenomenon
commonly associated with parallel plate capacitors, to wit, the edge
effect. Briefly stated the edge effect involves the electric field between
two plates of a capacitor wherein the electric field is normal to the
plates except near the plate edges, at which place the electric-field
lines tend to bulge outwardly. These bulging field lines which are
concentrated near the edges of the capacitor plates (in the present case
they are located near the edges of the titanium mesh) produce a higher
current density than that which is present over the central portion of the
metal electrode surface.
In order to eliminate the adverse effects of higher current densities due
to the edge effect, as well as to simultaneously maximize the surface area
available to reduce the current densities over the metallic electrode
surface, the present invention provides a plurality of holes, which are
cut through the top layer 16 of Silastic down to the surface of the mesh
12, in the vicinity of the edges of the mesh. These holes are shown
typically at 34 in FIG. 1 and, as may be seen, the surface of the margin
of the titanium mesh screen 12 is exposed therethrough. It has been found
that by means of these holes 34, the edge effect which creates high
current densities can be substantially eliminated and also that the mesh
12, which shows through the holes 34, increases the surface area of the
electrode which is available to contact the heart surface, thereby also
tending to lower the current densities on the metallic electrode elements.
The two layers and the metallic mesh may be securely fastened together by
placing stitches 36 around the periphery of the electrode 10 with a
conventional sewing machine using Dacron thread or the like.
The leads 22, 30 are preferably formed of a special, extremely flexible,
electric cable, which is particularly suitable for use with implanted
cardiac electrodes. Such flexibility is extremely important so as to
permit cardiac activity without trauma. This cable is commonly known as
tinsel and is formed having a central strand of a polyester yarn and
around such central strand are wound six or more conductive strands of
silver. Each conductive strand comprises, in turn, its own polyester yarn
core and concentrically wound conductive strands. This tinsel wire has an
exceedingly long life in the face of mechanical stress, such as flexure.
Additionally, since it is formed of silver, the tinsel wire has an
extremely low electrical resistance. The tinsel wire cable 30 must be
electrically connected to the titanium mesh 12 and this operation is
accomplished on the bottom surface of the mesh by crimping, welding, or
some other similar electrical connecting operation. Alternately the cable
30 may be attached to the mesh 12 by means of a specialized clip which is
described in copending application, Ser. No. 34731, entitled Device and
Method for Making Electrical Contact.
Turning now to FIG. 2, the arrangement of the various elements making up
the electrode 10 of FIG. 1 is shown in cross section, taken along sight
line 2--2 of FIG. 1. In such cross section, the top Silastic layer 16 is
bonded to the bottom Silastic layer 14 and the titanium mesh electrode 12
is sandwiched therebetween. It is seen that the holes 34 expose an
additional surface area of the mesh 12 to the heart surface. The pacing
tip 18 is connected to its flexible cable 22 and the tip 18 is
electrically insulated from the mesh 12 by means of an insulator 20.
Also seen in the cross section of FIG. 2 is a layer of Dacron mesh which
may be utilized as a strengthening element in either or both Silastic
layers of the sandwich. In this embodiment the mesh is placed in the lower
layer 14 and is seen in cross section at 42 and at the leading edge of the
assembly where stresses occur during implanting. Similar Dacron mesh could
also be used to strengthen the top layer 16. The specialized pouch or
pocket arrangement 44 which interacts with the specialized insertion or
implantation tool, for placement of the electrode 10 with minimum surgical
involvement, is shown in cross section also. The pocket or pouch 44 is
formed by continuing the top layer 16 of Silastic down over the leading
edge of the electrode so as to form a lip 48 which extends parallel to the
bottom layer 14 of the electrode, thereby forming a pocket 50 across the
entire width of the electrode to receive the insertion tool. Additionally,
a portion of Dacron mesh 52 may be embedded in the pocket or pouch 44 in
order to provide added strength to the pocket to prevent tearing by the
insertion tool.
FIG. 3 is a perspective from the bottom of the inventive electrode 10 and
in this view the pocket arrangement 44 is seen in more detail. Also seen
in FIG. 3 is the Dacron mesh 42, which is embedded inside the bottom
Silastic layer 14. The specialized pocket 44, which is intended for use
during insertion of the inventive electrodes, is raised above the bottom
layer 14 so as to form the desired pocket 50. The opening of the pocket 50
must be directed backwards and opening towards the rear of the electrode,
for reasons which will become clear below. The additional Dacron mesh
strengthening layer is seen at 52. The location of the joint between the
electrical conductor 30 and the wire mesh 12 is in the area shown
generally at 54 and, after such suitable electrical connection, the boot
33 is placed over the joint area to provide both electrical insulation and
mechanical strain relief.
The inventive method for implanting the present defibrillation electrode
and pacer tip will be set forth below, however, preliminarily thereto it
is necessary to show and describe a preferred embodiment of a suitable
insertion tool for use in practicing such inventive method. FIG. 4 shows a
perspective view of a preferred embodiment of the insertion tool which is
formed essentially as an elongated, flat, mandrel-type probe 60. The probe
60 has an elongated flat handle portion 62 and a blunt but rigid leading
edge 64 which is utilized to form a tunnel through the soft-tissue plane
during the electrode insertion process. The plane of the handle 62 changes
somewhat to form the upraised leading edge 64.
FIG. 5 is a cross section of a portion of the insertion tool 60 of FIG. 4
and shows the location of the upraised leading edge portion 64, in
relation to the plane of handle 62. The leading edge portion 64 is
provided to interact with the pocket 50 formed in the bottom surface of
the electrode 10 and, as may be seen by comparing FIGS. 2 and 5, in such
interaction portion 64 will slide easily into the pocket 50 in one
direction and the electrode will then lie parallel the plane of the handle
62. Accordingly, the relative motion which is possible between the
insertion tool 60 and the electrode 10 is unidirectional. After insertion
of the electrode, the tool 60 is withdrawn, thereby permitting the leading
edge portion 64 to slip out of the pocket 50, while the electrode is
retained in place next to the heart. Additionally, upon insertion of the
leading edge portion 64 into the pocket 50 of the electrode 10, the
electrode itself becomes part of the means for insertion into the human
body.
FIG. 6 shows another embodiment of an insertion tool 70, which has an
elongated handle portion 72 and a leading edge portion 74. The leading
edge portion 74 is inserted into the pocket 50 in the electrode. This
insertion tool 70 is formed as a planar instrument.
As described above, it is an object of the present invention to provide a
method of implanting a defibrillation electrode system by making only a
skin incision, i.e., not involving major chest surgery, on either the
interior thoracic or abdominal wall. Hence, by means of a specially
provided hand-held instrument, the tissue planes are separated and a
tunnel is created inside the thorax but outside the pleural cavity through
the soft tissues which surround the heart. After forming a tunnel, one or
more electrodes may be inserted into the tunnel and arranged proximate the
surface of the heart. The present invention contemplates the forming of
one tunnel between the interior surface of the thorax and the exterior
surface of the heart's pericardium and, attendantly, the insertion of one
or more electrodes through such tunnel for ultimate placement proximate
the heart. Additionally, the present invention teaches another tunnel
created on the posterior surface of the heart's pericardium, between the
pericardium and the diaphragm, and also placement of cardiac electrodes
through this second tunnel.
Turning to FIG. 7, the silhouette of the thoracic region of a patient 100
is shown and the incision locations are shown at 102 and 104. In the
silhouette of FIG. 7 the location and general outline of the patient's
heart is seen at 106. In regard to the abdominal incision 102, whereat the
incision tool 62 may be seen partially protruding through this incision,
the tunnel is being created which is located on the inferior surface of
the heart's pericardium between the pericardium and the diaphragm. The
inventive electrode 10 is shown in position and retained on the insertion
tool 60. The two leads 22 and 30 are also shown protruding from the
incision 102.
Referring for a moment to FIG. 8, the inventive electrode 10 is shown
installed on the specialized insertion tool 60 and the leading edge 64 of
the insertion tool 60 having been inserted into the pocket 50 formed in
the back surface of the inventive electrode 10. As may be seen in phantom,
the leading hard edge 64 of the insertion tool 60 is at the front of the
electrode 10. During insertion through the soft tissue planes, the
electrode 10 and the hard leading edge 64 of the insertion tool cooperate
to form the tunnel.
Referring back to FIG. 7, it may be seen that the tunnels are being formed
as indicated above. In relation to the upper incision 104, which is made
in the interior thoracic region, a tunnel 110, similar to 108, is being
created between the interior surface of the thorax and the anterior
surface of the heart's pericardium and the handle 62 of the insertion tool
60 is shown protruding through the incision 104, as are the leads 22 and
30.
In FIG. 9, it may be seen that by completely inserting the insertion tool
60 and tunnels 108 and 110 are fully formed, and upon withdrawing the
insertion tool 60, the leading edge portion 64 of the tool slips out of
the pocket 50 of the electrode 10 and the electrode remains at its
original location. FIG. 9 shows the electrodes in place after having been
inserted through the tunnels formed in the soft tissue planes and after
having the insertion tool withdrawn from the tunnels. In FIG. 9, the leads
are shown extending through the incisions; however, these leads will be
ultimately disposed of in accordance with the desired outcome, i.e.,
whether or not pacing is required and whether or not the defibrillator is
of a completely implanted type.
There are also other procedures which may be followed in applying the
inventive electrodes to the heart and the apparent order of combination
preferences might be as follows. A superior vena cava electrode in
combination with a tunneled diaphragmatic electrode. Another combination
might be a substernal patch in combination with a diaphragmatic patch,
wherein both electrodes were introduced via a subziphoid route. Of course,
the locations as shown in FIG. 9 are another combination.
FIG. 10 is a cross sectional view of the inventive releasable lead
coupling, shown at 24 in FIG. 1. As mentioned above, this type of coupling
is intended for use with the pacer tip so that after implantation of the
present electrode an electrical connection is available for quick hook-up
to a heart pacing device so that if necessary, cardiac pacing may be
easily and quickly achieved. The inventive coupling is provided so that
when it becomes clear that cardiac pacing will not be required, the lead
may be pulled free, with the coupling remaining in the patient, and no
further surgery will be required.
In FIG. 10, an outer casing 130 of the coupling 24 surrounds a female-type
metallic connector 132 which is electrically connected to the pacing lead
22. Forming an electrical joint with the female connector 132 is a male
plug 134 which is electrically connected by either soldering or crimping
to the external pacing lead 26. The casing 130 of the coupling is
constructed such that there is a tight mechanical bond formed at 136
between the pacing lead insulation 22 and the casing 130, whereas in the
case of the external pacing lead 26, the housing 130 is formed having an
excessively large passage, shown typically at 138, which is of greater
diameter than the outside diameter of the pacing lead 26. Such greater
diameter is provided so that when it is desired to disconnect the external
pacing lead, a tensional force may be applied to the external lead 26 so
that parts 132 and 134 will be separated, thereby permitting the external
lead 26 to be withdrawn from the patient's body.
FIG. 11 shows an alternate embodiment of the inventive electrode. In this
embodiment, the electrode 150 is constructed as was electrode 10 of FIG.
1, except that top layer 16 is provided with a plurality of apertures 152
in place of the single large opening. These apertures 152 are formed by
crossing members 154 formed in the top layer 14. This multiple aperture
embodiment still permits use of the pacing tip 18, which is then relocated
in one of the several available apertures.
These cross members 154 and additional stitches 156 have been found
advantageous in maintaining the original form and function of the
electrode. After implantation of an electrode the tissue adjacent to the
metallic mesh has been found to adhere and grow on and through the
metallic mesh. When this happens, the tissue gets behind the mesh and
tends to force it away from the back layer. This ultimately distorts the
electrode shape and degrades its performance by adversely affecting the
contact surface. The additional stitches 156 keep the mesh firmly affixed
to the back portion.
It is, of course, understood that the above detailed description is
intended by way of example only and is not intended to limit the present
invention in any way, except as set forth in the following claims.
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