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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to cardiac diagnostic and chronic therapeutic leads,
and more particularly to fixation leads in which an electrode includes an
anchoring element.
The utility of cardiac pacing leads is well recognized, both for carrying
pulse stimulation signals to the heart from a pacemaker, and for
monitoring heart electrical activity from outside the body. Many such
leads are sufficiently flexible and small in diameter for intravenous
introduction to a cardiac cavity, whereupon an electrode at the distal end
of the lead is implanted into the endocardium to secure the lead. For this
purpose, helical coils, barbs and other anchoring elements are provided,
typically as part of the electrode.
The anchoring element must be sufficiently sharp to penetrate the
endocardium and secure the electrode against becoming detached, for
example due to contractions of the myocardium. During a critical period
immediately after implant and prior to full fibrotic growth, usually three
to twelve weeks, the anchoring element must provide substantially the
entire force maintaining the electrode in its selected location. Given
these requirements, it is not surprising that an effective anchoring
element can become entangled in the vein, heart valve or other tissue
encountered during its intravenous insertion.
The problem has given rise to numerous proposed solutions. For example,
U.S. Pat. No. 3,974,834 to Kane granted Aug. 17, 1976 shows a sleeve which
shrouds the sharp tip of a fixation helix, but collapses in accordion-like
fashion as the helix is turned into the endocardium. In U.S. Pat. No.
4,282,885 to Bisping granted Aug. 11, 1981, a protective core is
surrounded by the helix, and is movable axially relative to the helix. A
wire, attached to the core, extends through the lead and can be pulled
after lead insertion to withdraw the core, exposing the helix. U.S. Pat.
No. 4,146,036 to Dutcher et al granted Mar. 27, 1975 discloses an
extensible and retractable core surrounded by the helix.
Other solutions involve making the fixation element movable. For example,
in U.S. Pat. No. 4,180,080 to Murphy granted Dec. 25, 1979, a spiral coil,
normally recessed within a guide tube, can be rotated whereby it emerges
beyond the tube. U.S. Pat. No. 3,844,292 to Bolduc granted Oct. 29, 1974
discloses a plunger outside of the body which, after release of two
locking mechanisms, is movable to push outward a barb-like tip. A somewhat
similar arrangement, involving a platinum piston movable to push a
harpoon-shaped anchor beyond the end of a tubular electrode, is shown in
U.S. Pat. No. 4,258,724 to Balat et al granted Mar. 31, 1981.
Such devices, while satisfactory in certain respects, are undesirable in
that leads employing them must have a larger diameter. They often require
additional tools, for example a stylet-type screw driver for rotating the
helix. Further, such devices are often overly complex, diminishing their
reliability and raising the possibility of a current leakage path between
conductors of bipolar leads.
Therefore, it is an object of the present invention to provide a smooth,
rounded covering for the anchoring element of a cardiac endocardial
electrode to facilitate intravenous insertion of the electrode.
Another object of the invention is to provide such a covering which is
soluble in body fluids, thereby to expose the anchoring element at a
specified time after its initial insertion into the body.
Yet another object is to provide a simple, non-mechanical means for
covering fixation mechanisms during intravenous insertion of a pacing
electrode having an anchoring element, without requiring any longitudinal
relative movement between the electrode and anchoring element.
SUMMARY OF THE INVENTION
To achieve these and other objects, there is provided an intravascular lead
implantable inside a patient's body. The lead includes an electrode having
a fixation element for effecting penetration into endocardial tissue at a
selected location to secure the electrode at the selected location. The
lead includes one or more flexible electrical conductors, and one or more
flexible, biocompatible dielectric sheaths surrounding the conductors
along substantially their entire length. A coupling means electrically and
mechanically joins the electrode to a distal end of the conductor, whereby
the conductor and electrode transmit electrical signals from the selected
location to the lead proximal end. A biocompatible covering surrounds the
fixation element and facilitates intravascular movement of the electrode.
The covering is soluble in bodily fluids and has a thickness selected to
allow at least a predetermined minimum time for the intravascular
insertion of the lead and electrode, and for the positioning of the
electrode at least proximate to the selected location, before the covering
dissolves sufficiently to expose the fixation element and permit the
penetration.
Mannitol, and other sugar derivatives, have been found suitable for forming
the covering, which can be produced by dipping the fixation element into a
beaker containing the mannitol or other covering constituent at a
temperature slightly above its melting point. The fixation element is
removed, cooling the element, along with a portion of material adhering to
it. Alternatively, the covering can be preformed as a capsule, with a bore
formed in the capsule for accommodating the fixation element. An adhesive
is then used to join the covering to the electrode, with the fixation
element inside the bore.
Another aspect of the present invention is an apparatus for facilitating
intravascular insertion of a cardiac pacing electrode. The apparatus
includes a biocompatible, non-pyrogenic covering substantially surrounding
a fixation element of an electrode. The covering is soluble in bodily
fluids and has a thickness selected to allow at least a predetermined time
for intravascular insertion of the electrode at least proximate a selected
location inside the body of the patient, before the covering dissolves
sufficiently to expose the fixation element to permit penetration of the
fixation element into body tissue at the selected location.
As another aspect of the invention, there is disclosed a process for
coating a fixation element of a body implantable electrode, including the
steps of:
(a) selecting a biocompatible, non-pyrogenic material soluble in bodily
fluids and having a melting point substantially above normal body
temperature, and heating the material to a temperature slightly above its
melting point;
(b) dipping a fixation element of a body implantable electrode into a
solution of the material maintained at said temperature;
(c) removing the fixation element, along with an initial portion of the
material adhering to the fixation element, from the solution and
permitting them to cool a sufficient time for said initial portion to at
least partially solidify;
(d) dipping said fixation element and initial portion into the solution for
a time sufficient to permit a subsequent portion of the material to adhere
to the initial portion; and
(e) removing the fixation element, initial portion and subsequent portion
from the solution, and permitting them to cool to an ambient temperature.
Steps (d) and (e) may be repeated until the thickness of the covering is
sufficient for the desired dissolving time.
A covering in accordance with the present invention, whether preformed or
applied through dip coating, forms a smooth, blunt distal tip for its
associated lead, allowing an expeditious, intravenous insertion of the
lead, without concern that the fixation element will snag upon, tear or
otherwise damage the vein or any other tissue as it travels toward the
heart. A short time after the electrode reaches the selected cardiac
chamber, there is a sufficient dissolving of the covering such that the
fixation element is exposed and ready to penetrate the endocardium.
Due to the many materials suitable for the covering, which include various
salts and polyvinylpyrrolidone as well as the aforementioned sugar
derivatives, and further due to controlling the covering thickness, a wide
range of dissolving times is available, so that a particular covering can
be tailored to the expected time for a particular procedure. Further
refinement is provided by the preformed capsule, due to enhanced control
over size, thickness and surface area of the covering.
IN THE DRAWINGS
For a better appreciation of these and other features and advantages,
reference is made to the following detailed description and the drawings,
in which:
FIG. 1 is a side sectional view of the distal end region of an implantable,
positive fixation, cardiac lead;
FIG. 2 is an enlarged side view showing the lead of FIG. 1 provided with a
soluble covering in accordance with the present invention;
FIG. 3 is a view similar to that of FIG. 2, illustrating the covering at an
intermediate stage of its formation;
FIG. 4 is a side view of a lead provided with a second embodiment covering;
FIG. 5 is a side elevation of a lead provided with a third embodiment
covering comprising a molded tip;
FIGS. 6 and 7 are side views of leads provided with coverings comprising
pre-molded tips; and
FIGS. 8 and 9 are top and side views, respectively, of a tool used in
forming soluble coverings pursuant to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to the drawings, there is shown in FIG. 1 the distal end region
of an implantable, positive fixation, cardiac lead 16. Devices such as
lead 16 typically are inserted intravenously, for example into the
subclavian vein or the cephalic vein, and progressively moved toward the
heart until the distal end reaches a selected cardiac chamber. With the
distal tip positioned at a selected location, the lead proximal end, still
outside of the body, is maneuvered to implant the distal tip into the
endocardium. The implanted lead transmits electrical signals between the
selected location in the heart and the lead proximal end, for one or both
of two purposes: to monitor heart electrical activity at the selected
location, and to carry stimulating signals to the selected location from a
pulse generator (not shown) connected to the lead proximal end.
To transmit the electrical signals there is provided an electrical
conductor, shown in FIG. 1 as a double-wound coil 18 formed of a nickel
alloy. The coil provides maximum flexibility for conforming to the vein,
with minimal stress on the conductor. At the distal end of the lead is an
electrode 20, electrically and mechanically coupled to coil 18 by a
platinum alloy crimp tube 22. A flexible, dielectric sheath 24 surrounds
the coil and crimp tube. A suitable material for the sheath is silicone
rubber.
Electrode 20 is porous, having a screen 26 formed of a platinum alloy.
Screen 26 aids in chronic fixation, whereby fibrous connective tissue
intertwines with the screen to firmly secure electrode 20. Fibrous
encapsulation, however, can take weeks, and it is essential to provide a
means for positively securing the lead distal end during the time
immediately following implantation. To this end, there is provided a
fixation helix 28 of platinum alloy. Helix 28 has a sharp point at its
distal end, which readily penetrates the endocardium. Upon initial
penetration, the helix is manipulated from the proximal end of lead 16,
whereby it rotates clockwise, to further penetrate the tissue, to the
point of firmly securing electrode 20 at the designated endocardial
location.
A problem associated with helix 28 is that its sharp tip is capable of
snagging and becoming entangled with the blood vessel wall, venous valves,
or heart valve. Consequently, the physician using lead 16 typically is
advised to rotate the helix counterclockwise, which tends to draw the
sharp point of the helix away from the tissue it encounters, to minimize
the potential for entanglement. Alternatively, protective devices, such as
those discussed above, have been employed.
FIG. 2 illustrates lead 16 with a covering or tip 30 mounted at its distal
end in accordance with the present invention. Tip 30 is solid, and adheres
to helix 28, screen 26 and the distal end of sheath 24. The outer surface
of tip 30 is generally spheroid. However, the precise surface
configuration is not so important as the fact that tip 30 is smooth,
rounded and blunt, and that it completely surrounds fixation helix 28 to
protect intravascular and other tissue from the fixation helix,
particularly its sharp point 32.
Tip 30 is composed of a non-toxic, biocompatible and non-pyrogenic
material. Also, the material must be soluble in body fluids (particularly
blood), within a temperature range encompassing normal body temperature
(37.degree. C.). Further, the material of tip 30 must maintain its
structural integrity in an environment of body fluids at or about normal
body temperature, in that it should not undergo plastic or elastic
deformation as it dissolves. Usually the structural integrity requirement
is satisfied if the melting point of the tip is substantially above normal
body temperatures, in fact preferably 60.degree. C. or higher.
In one example, tip 30 has been formed of mannitol, chemical formula
C.sub.6 H.sub.14 O.sub.6. Mannitol has a melting point of about
167.degree. C., and one gram dissolves in about 5.5 milliliters of water,
with solubility being greater in hot water. Mannitol in a glass beaker was
first heated to a temperature between 177.degree. and 182.degree. C.,
slightly above its melting point, and maintained at that temperature. The
distal end of lead 16 was immersed in the mannitol solution for a brief
time and withdrawn. A portion of the mannitol adhered to helix 28, forming
a core portion 34 as illustrated in FIG. 3. Away from the beaker, helix 28
and core portion 34 were allowed to cool a sufficient time for the core
portion to solidify. This cooling required about five seconds.
Following cooling, the lead distal end was dipped into the mannitol melt
once again, then withdrawn after about one second. A second portion of the
mannitol melt, adhering to core portion 34, helix 28 and screen 26, was
sufficient in combination with the core portion to form tip 30 as
illustrated in FIG. 2. The spheroid tip configuration results from the
natural surface tension of the mannitol melt as it solidifies. A tip
formed in this manner dissolves in water heated to about 38.degree. C. in
about three and one-half minutes.
Repeated trials of this example have yielded consistently satisfactory lead
tips. The results indicate that the precise temperature of the mannitol
melt is not critical, so long as it is maintained in liquid form, slightly
above the melting point. Likewise, the duration of each dip coating is not
critical, although it must be sufficient for adherence of subsequent
mannitol layers while not so long as to melt mannitol previously
solidified onto the lead. Finally, as the number of dip coatings required
is largely a function of desired tip size, certain tip designs may require
substantially more than the two dip coatings described.
The dissolving time for tip 30 in body fluid is controlled principally by
the tip material, surface area and thickness so that increased dissolving
times can be provided, if desired, by increasing the tip thickness. The
main concern is that tip 30 be of sufficient size to ensure that lead 16
can be directed intravenously to the selected cardiac chamber prior to
exposure of helix 28, particularly at point 32. Also of concern is that
within a reasonably short time after insertion, tip 30 is completely
dissolved to expose helix 28 for the implanting of electrode 20.
While the above example involves mannitol, other sugar derivatives stable
at temperatures below 60.degree. C. are suitable substitutes, for example
dextrose, sorbose, sucrose, and glucosamine. Also usable are certain
salts, for example sodium chloride, potassium chloride and sodium
carbonate. A further suitable constituent is polyvinylpyrrolidone (PVP).
These materials are suitable, as well, with non-helical fixation elements.
The dip process described above, when used to form tip 30, tends to trap
air which expands due to heating, and can cause undesirable formation of
bubbles in the tip. In such cases, it is advantageous to control the
degree to which the lead distal end is submerged into the constituent
melt. For example, FIG. 4 illustrates a lead 36 in which a tip 40 entirely
surrounds a fixation helix 42. Tip 40 does not abut a sheath 44, but
leaves a proximal portion 46 of a screen 48 exposed. As a result of such
controlled submersion, air can escape through proximal portion 46, and
later the proximal portion facilitates ethylene oxide sterilization of the
lead distal end.
For improved control over the size and shape of the soluble tip, a tip 50
for a lead 52 is formed by a casting or injection molding of a constituent
melt. The tip surrounds a helix 54 and screen 56, to abut a sheath 58. As
indicated at 60, the profile of the tip along its side is linear, while a
rounded blunt distal end 62 is retained. This allows a reduction of tip
diameter to the nominal lead diameter, to further facilitate intravenous
insertion.
Another method of controlling the size and shape of the tip is to preform
the tip by casting or injection molding. FIG. 6 illustrates a lead 64 with
a screen 65 and fixation helix 66 affixed to a conductor which is
surrounded by a sheath 68. Surrounding the helix and screen is a preformed
tip or capsule 70, fixed to sheath 68 using an adhesive at 72. Such
adhesive can be the molten material itself in the case of mannitol, a
heated syrup of fructose and sucrose which solidifies upon cooling, or
syrups of other sugars (mannitol, sorbitol, etc.). A large diameter
opening 74 is formed in capsule 70 to accommodate screen 65, while a
cylindrical bore 76 of greater depth accommodates helix 66. As indicated
by broken lines at 77 and 78, bore 76 can be shaped to provide a
substantially uniform thickness in capsule 70 if desired. One or more
openings, as indicated at 79, can be formed if desired to facilitate
sterilization and increase the capsule surface area exposed to bodily
fluids during lead insertion.
Although it requires an adhesive not needed in the dip coating or direct
mold approach, a preformed tip has several significant advantages. First,
it affords maximum control over the tip size and shape, so that a
comparatively precise dissolving time can be achieved by appropriately
selecting tip constituents and tip thickness. The resulting consistency
among many tips renders the preformed tip the preferred choice for mass
production. Also, the preformed tip requires less of the tip constituent.
In particular, no constituent is provided where none is needed--namely, in
the cylinder defined by the fixation helix. This factor contributes to
more predictable dissolving times as well as material savings. Preformed
tips are more amenable to being installed or replaced on site. Finally,
constituents that cannot be melted because of their thermal instability,
but which have the desired dissolving properties, can be formed into a tip
such as tip 70 by compression or molding. Examples of such constituents
are lactose, lecithin in combination with other materials, and
glucosamine.
In FIG. 7, an alternative preformed tip or capsule 80 is mounted to a lead
82 over its fixation helix 84 and screen 86. A helical bore 88 is formed
in tip 80, of a size and shape to accommodate helix 84, so that tip 80 is
secured to lead 82 by turning it clockwise upon the helix. So mounted, tip
80 depends less upon an adhesive, and may not require any adhesive to
connect it to a sheath 90.
Shown in FIGS. 8 and 9 is a forming tool 92, which is an aluminum block
including two opposed sections 94 and 96 held together by socket head
screws 98 and 100. The opposing walls of sections 94 and 96 are cut away
to form cavities 102, 104 and 106 when sections 94 and 96 are mounted with
respect to each other as shown in the figures. Spacers, indicated at 108,
maintain a slight gap between sections 94 and 96, preferably of about
0.005 inches.
The cavities are substantially identical in shape, although they can be
formed in different sizes corresponding to tips of different selected
sizes. Cavity 104, for example, includes a tip forming segment 110 and an
upper chamfered segment 112. Tip forming segment 110 has the desired
cylindrical sides and rounded base to form the desired blunt tip, while
chamfered segment 112 facilitates insertion of tips prior to their
shaping, and also serves as a temporary catch basin for excess melted tip
material.
Forming tool 92 is used to control the size and shape of a soluble tip
formed by the dip process described in connection with FIGS. 2-4. More
particularly, the lead distal end is dipped in the mannitol solution,
permitted to cool, then dipped again, this process being repeated a
sufficient number of times to form a tip larger than the desired size.
Then, after heating forming tool 92 to a desired temperature, preferably
slightly over the mannitol melting point, the tip formed by dip coating is
momentarily inserted into the desired one of cavities 102-106, then
quickly withdrawn, the desired time within the chosen cavity being a
fraction of a second. During this brief insertion, the heated cavity wall
melts the excess mannitol, whereupon the melt is removed by draining
through the gap formed by spaces 108. Part of the excess mannitol melt is
collected briefly in chamfered segment 112 before drainage. While not
necessary, the tip can be rotated slightly about a vertical axis while
inserted, to further ensure the desired cylindrical sides and blunt end.
Whether formed by dip coating, direct molding or molded separately for
later attachment, a soluble tip in accordance with the present invention
renders cardiac pacing lead implantation safer and less traumatic to the
patient. As it is moved toward the heart through a selected vein, the tip
dissolves, but at a sufficiently slow rate to prevent exposure of the
fixation helix or other fixation element, until the electrode is at least
near to its selected location along the endocardium. The smooth, rounded
and blunt tip in fact expedites intravascular lead insertion. The tip
thickness and constitu | | |
