U.S. patent application number 09/747276 was filed with the patent office on 2001-09-27 for expandable-collapsible electrode structures made of electrically conductive material.
Invention is credited to Kordis, Thomas F., Panescu, Dorin, Swanson, David K., Whayne, James G..
Application Number | 20010025175 09/747276 |
Document ID | / |
Family ID | 27485980 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010025175 |
Kind Code |
A1 |
Panescu, Dorin ; et
al. |
September 27, 2001 |
Expandable-collapsible electrode structures made of electrically
conductive material
Abstract
Electrode assemblies and associated systems employ a nonporous
wall having an exterior for contacting tissue. The exterior
peripherally surrounds an interior area. The wall is essentially
free of electrically conductive material. The wall is adapted to
assume an expanded geometry having a first maximum diameter and a
collapsed geometry having a second maximum diameter less than the
first maximum diameter. The assemblies and systems include a lumen
that conveys a medium containing ions into the interior area. An
element free of physical contact with the wall couples the medium
within the interior area to a source of electrical energy to enable
ionic transport of electrical energy from the source through the
medium to the wall for capacitive coupling to tissue contacting the
exterior of the wall.
Inventors: |
Panescu, Dorin; (Sunnyvale,
CA) ; Swanson, David K.; (Mountain View, CA) ;
Whayne, James G.; (Saratoga, CA) ; Kordis, Thomas
F.; (San Jose, CA) |
Correspondence
Address: |
LYON & LYON LLP
SUITE 4700
633 WEST FIFTH STREET
LOS ANGELES
CA
90071-2066
US
|
Family ID: |
27485980 |
Appl. No.: |
09/747276 |
Filed: |
December 21, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09747276 |
Dec 21, 2000 |
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09300936 |
Apr 27, 1999 |
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6179835 |
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09300936 |
Apr 27, 1999 |
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08628928 |
Apr 8, 1996 |
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5925038 |
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60010223 |
Jan 19, 1996 |
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60010225 |
Jan 19, 1996 |
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60010354 |
Jan 19, 1996 |
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Current U.S.
Class: |
606/41 ; 600/374;
606/34; 607/105; 607/113; 607/122; 607/99 |
Current CPC
Class: |
A61B 2017/00243
20130101; A61B 2018/00113 20130101; A61B 2018/00267 20130101; A61B
2018/00839 20130101; A61N 1/06 20130101; A61N 1/056 20130101; A61B
2018/00107 20130101; A61B 2018/00214 20130101; A61B 2018/00065
20130101; A61L 31/145 20130101; A61B 2018/00083 20130101; A61B
2018/1472 20130101; A61L 29/145 20130101; A61B 2018/0022 20130101;
A61B 2017/00053 20130101; A61B 2018/1253 20130101; C08L 1/02
20130101; A61L 29/085 20130101; A61B 2017/22051 20130101; A61B
18/1492 20130101; A61B 2017/22061 20130101; A61B 2218/002 20130101;
A61L 31/10 20130101; A61L 29/085 20130101; A61B 2018/00095
20130101; A61B 2018/00148 20130101 |
Class at
Publication: |
606/41 ; 606/34;
600/374; 607/99; 607/105; 607/113; 607/122 |
International
Class: |
A61B 018/14 |
Claims
We claim:
1. An electrode assembly comprising a nonporous wall having an
exterior for contacting tissue, the exterior peripherally
surrounding an interior area, the wall being essentially free of
electrically conductive material, the wall being adapted to assume
an expanded geometry having a first maximum diameter and a
collapsed geometry having a second maximum diameter less than the
first maximum diameter, a lumen to convey a medium containing ions
into the interior area, and an element free of physical contact
with the wall that couples the medium within the interior area to a
source of electrical energy to enable ionic transport of electrical
energy from the source through the medium to the wall for
capacitive coupling to tissue contacting the exterior of the
wall.
2. An electrode assembly comprising a nonporous wall having an
exterior for contacting tissue, the exterior peripherally
surrounding an interior area, the wall being essentially free of
electrically conductive material, the wall being adapted to assume
an expanded geometry having a first maximum diameter and a
collapsed geometry having a second maximum diameter less than the
first maximum diameter, a medium containing ions filling the
interior area, and an element free of physical contact with the
wall coupling the medium to a source of electrical energy to enable
ionic transport of electrical energy from the source through the
medium to the wall for capacitive coupling to tissue contacting the
exterior of the wall.
3. An electrode assembly comprising a nonporous wall having an
exterior for contacting tissue, the exterior peripherally
surrounding an interior area, the wall being essentially free of
electrically conductive material, the wall being adapted to assume
an expanded geometry having a first maximum diameter and a
collapsed geometry having a second maximum diameter less than the
first maximum diameter, a generator of radio frequency energy, a
fluid source holding a medium containing ions, a lumen
communicating with the interior area and the fluid source to convey
into the interior area the medium containing ions, an element free
of physical contact with the wall coupled to the generator to
establish electrical contact between the medium within the interior
area and the generator to enable ionic transport of radio frequency
energy from the generator through the medium to the wall for
capacitive coupling to tissue contacting the exterior of the
wall.
4. An assembly according to claim 1 or 2 or 3 wherein the element
comprises an electrically conductive electrode in the interior
area.
5. An assembly according to claim 4 wherein the electrically
conductive electrode comprises a nobel metal.
6. An assembly according to claim 4 wherein the electrically
conductive electrode includes a material selected from the group
consisting essentially of gold, platinum, platinum/iridium, or
combinations thereof.
7. An assembly according to claim 1 or 2 or 3 wherein the medium
comprises a hypertonic solution.
8. An assembly according to claim 7 wherein the hypertonic solution
includes sodium chloride.
9. An assembly according to claim 8 wherein the sodium chloride is
present in a concentration at or near saturation.
10. An assembly according to claim 8 wherein the sodium chloride is
present in a concentration of up to about 9% weight by volume.
11. An assembly according to claim 1 or 2 or 3 wherein the
capacitive coupling of the wall is expressed in the following
relationship: {square root}{square root over
(R.sub.PATH.sup.2+X.sub.C.sup.2)}<R.sub.TISSUE where: 11 R PATH
= K S E s and K is a constant that depends upon geometry of the
wall, S.sub.E is surface area of the element, and .rho..sub.S is
resistivity of the medium containing ions, and where: 12 X C = 1 2
fC and f is frequency of the electrical energy, and 13 C = S B t
where: .epsilon. is the dielectric constant of wall, S.sub.B is the
area of the interior area, and t is thickness of the wall located
between the medium containing ions and tissue, and where
R.sub.TISSUE is resistivity of tissue contacting the wall.
12. An assembly according to claim 11 wherein R.sub.TISSUE is about
100 ohms.
13. An assembly according to claim 1 or 2 or 3 and further
including members assembled within the interior area to form a
support structure underlying the wall.
14. An assembly according to claim 13 wherein the solid support
members are made from metal material.
15. An assembly according to claim 14 wherein the metal material
includes nickel titanium.
16. An assembly according to claim 14 wherein the metal material
includes stainless steel.
17. An assembly according to claim 13 wherein the solid support
members are made from plastic material.
18. An assembly according to claim 13 wherein the solid support
members comprise elongated spline elements assembled in a
circumferentially spaced relationship.
19. An assembly according to claim 1 or 2 or 3 and further
including at least one temperature sensing element carried by the
wall.
20. A system for heating body tissue comprising a catheter tube
having a distal end, a return electrode, a fluid source of a medium
containing ions, an electrode on the distal end of the catheter
tube comprising a nonporous wall having an exterior for contacting
tissue, the exterior peripherally surrounding an interior area, the
wall being essentially free of electrically conductive material,
the wall being adapted to assume an expanded geometry having a
first maximum diameter and a collapsed geometry having a second
maximum diameter less than the first maximum diameter, a lumen to
convey the medium containing ions into the interior area, and an
electrically conductive element within the interior area free of
physical contact with the wall, and means for coupling the return
electrode and the electrically conductive element to the source of
energy to enable ionic transport of electrical energy from the
source through. the medium to the wall for capacitive coupling to
tissue to heat tissue located between the return electrode and the
electrode.
21. A system for ablating body tissue comprising a catheter tube
having a distal end, a return electrode, a fluid source of a medium
containing ions, an electrode on the distal end of the catheter
tube comprising a nonporous wall having an exterior for contacting
tissue, the exterior peripherally surrounding an interior area, the
wall being essentially free of electrically conductive material,
the wall being adapted to assume an expanded geometry having a
first maximum diameter and a collapsed geometry having a second
maximum diameter less than the first maximum diameter, a lumen to
convey the medium containing ions into the interior area, and an
electrically conductive element within the interior area free of
physical contact with the wall, and means for coupling the return
electrode and the electrically conductive element to the source of
energy to enable ionic transport of electrical energy from the
source through the medium to the wall for capacitive coupling to
tissue to ablate tissue located between the return electrode and
the electrode.
22. A system for ablating heart tissue comprising a catheter tube
having a distal end for deployment in a heart chamber, a return
electrode, a fluid source of a medium containing ions, an electrode
on the distal end of the catheter tube comprising a nonporous wall
having an exterior for contacting heart tissue, the exterior
peripherally surrounding an interior area, the wall being
essentially free of electrically conductive material, the wall
being adapted to assume an expanded geometry having a first maximum
diameter and a collapsed geometry having a second maximum diameter
less than the first maximum diameter, a lumen to convey the medium
containing ions into the interior area, and an electrically
conductive element within the interior area free of physical
contact with the wall, and means for coupling the return electrode
and the electrically conductive element to the source of energy to
enable ionic transport of electrical energy from the source through
the medium to the wall for capacitive coupling to tissue to ablate
heart tissue located between the return electrode and the
electrode.
23. A system according to claim 20 or 21 or 22 wherein the
electrically conductive element comprises an electrically
conductive electrode in the interior area.
24. A system according to claim 23 wherein the electrically
conductive electrode comprises a nobel metal.
25. A system according to claim 23 wherein the electrically
conductive electrode includes a material selected from the group
consisting essentially of gold, platinum, platinum/iridium, or
combinations thereof.
26. A system according to claim 20 or 21 or 22 wherein the medium
comprises a hypertonic solution.
27. A system according to claim 26 wherein the hypertonic solution
includes sodium chloride.
28. A system according to claim 27 wherein the sodium chloride is
present in a concentration at or near saturation.
29. A system according to claim 27 wherein the sodium chloride is
present in a concentration of up to about 9% weight by volume.
30. A system according to claim 20 or 21 or 22 wherein the
capacitive coupling of the wall is expressed in the following
relationship: {square root}{square root over
(R.sub.PATH.sup.2+X.sub.C.sup.2)}<R.sub.TISSUE where: 14 R PATH
= K S E s and K is a constant that depends upon geometry of the
wall, S.sub.E is surface area of the element, and .rho..sub.S is
resistivity of the medium containing ions, and where: 15 X C = 1 2
fC and f is frequency of the electrical energy, and 16 C = S B t
where: .epsilon. is the dielectric constant of wall, S.sub.B is the
area of the interior area, and t is thickness of the wall located
between the medium containing ions and tissue, and where
R.sub.TISSUE is resistivity of tissue contacting the wall.
31. A system according to claim 30 wherein R.sub.TISSUE is about
100 ohms.
32. A system according to claim 20 or 21 or 22 and further
including members assembled within the interior area to form a
support structure underlying the wall.
33. A system according to claim 32 wherein the solid support
members are made from metal material.
34. A system according to claim 33 wherein the metal material
includes nickel titanium.
35. A system according to claim 33 wherein the metal material
includes stainless steel.
36. A system according to claim 32 wherein the solid support
members are made from plastic material.
37. A system according to claim 32 wherein the solid support
members comprise elongated spline elements assembled in a
circumferentially spaced relationship.
38. A system according to claim 20 or 21 or 22 and further
including at least one temperature sensing element carried by the
wall.
39. A method for heating body tissue comprising the steps of
providing a catheter tube having a distal end that carries an
electrode comprising a nonporous wall having an exterior for
contacting heart tissue, the exterior peripherally surrounding an
interior area, the wall being essentially free of electrically
conductive material, the wall being adapted to assume an expanded
geometry having a first maximum diameter and a collapsed geometry
having a second maximum diameter less than the first maximum
diameter, a lumen to convey the medium containing ions into the
interior area, and an electrically conductive element within the
interior area free of physical contact with the wall, electrically
coupling a source of radio frequency energy to the electrically
conductive element and to a return electrode in contact with body
tissue, guiding the catheter tube into a body with the wall in the
collapsed geometry, causing the wall to assume the expanded
geometry at least in part by conveying a medium containing ions
into the interior area, and ohmically heating body tissue by
transmitting radio frequency energy to the electrically conductive
element for ionic transport through the medium to the wall for
capacitive coupling to tissue located between the return electrode
and the electrode.
40. A method for ablating tissue comprising the steps of providing
a catheter tube having a distal end that carries an electrode
comprising a nonporous wall having an exterior for contacting heart
tissue, the exterior peripherally surrounding an interior area, the
wall being essentially free of electrically conductive material,
the wall being adapted to assume an expanded geometry having a
first maximum diameter and a collapsed geometry having a second
maximum diameter less than the first maximum diameter, a lumen to
convey the medium containing ions into the interior area, and an
electrically conductive element within the interior area free of
physical contact with the wall, electrically coupling a source of
radio frequency energy to the electrically conductive element and
to a return electrode in contact with body tissue, guiding the
catheter tube into a body with the wall in the collapsed geometry,
causing the wall to assume the expanded geometry at least in part
by conveying a medium containing ions into the interior area, and
ohmically ablating body tissue by transmitting radio frequency
energy to the electrically conductive element for ionic transport
through the medium to the wall for capacitive coupling to tissue
located between the return electrode and the electrode.
41. A method for ablating heart tissue comprising the steps of
providing a catheter tube having a distal end that carries an
electrode comprising a nonporous wall having an exterior for
contacting heart tissue, the exterior peripherally surrounding an
interior area, the wall being essentially free of electrically
conductive material, the wall being adapted to assume an expanded
geometry having a first maximum diameter and a collapsed geometry
having a second maximum diameter less than the first maximum
diameter, a lumen to convey the medium containing ions into the
interior area, and an electrically conductive element within the
interior area free of physical contact with the wall, electrically
coupling a source of radio frequency energy to the electrically
conductive element and to a return electrode in contact with body
tissue, guiding the catheter tube into a heart chamber with the
wall in the collapsed geometry, causing the wall to assume the
expanded geometry at least in part by conveying a medium containing
ions into the interior area, and ohmically ablating heart tissue by
transmitting radio frequency energy to the electrically conductive
element for ionic transport through the medium to the wall for
capacitive coupling to tissue located between the return electrode
and the electrode.
42. A method according to claim 39 or 40 or 41 and further
including the step of controlling the transmission of radio
frequency energy, at least in part, by sensing temperature
proximate to the wall.
Description
RELATED APPLICATION
[0001] This application is a continuation-in-part of pending U.S.
application Ser. No. 08/099,994, filed Jul. 30, 1993 and entitled
"Large Surface Cardiac Ablation Catheter that Assumes a Low Profile
During Introduction into the Heart," which is itself a
continuation-in-part of pending U.S. application Ser. No.
07/951,728, filed Sep. 25, 1992, and entitled "Cardiac Mapping and
Ablation Systems."
FIELD OF THE INVENTION
[0002] The invention generally relates to electrode structures
deployed in interior regions of the body. In a more specific sense,
the invention relates to electrode structures deployable into the
heart for diagnosis and treatment of cardiac conditions.
BACKGROUND OF THE INVENTION
[0003] The treatment of cardiac arrhythmias requires electrodes
capable of creating tissue lesions having a diversity of different
geometries and characteristics, depending upon the particular
physiology of the arrhythmia sought to be treated.
[0004] For example, a conventional 8 F diameter/4 mm long cardiac
ablation electrode can transmit radio frequency energy to create
lesions in myocardial tissue with a depth of about 0.5 cm and a
width of about 10 mm, with a lesion volume of up to 0.2 cm.sup.3.
These small and shallow lesions are desired in the sinus node for
sinus node modifications, or along the AV groove for various
accessory pathway ablations, or along the slow zone of the
tricuspid isthmus for atrial flutter (AFL) or AV node slow or fast
pathway ablations.
[0005] However, the elimination of ventricular tachycardia (VT)
substrates is thought to require significantly larger and deeper
lesions, with a penetration depth greater than 1.5 cm, a width of
more than 2.0 cm, and a lesion volume of at least 1 cm.sup.3.
[0006] There also remains the need to create lesions having
relatively large surface areas with shallow depths.
[0007] One proposed solution to the creation of diverse lesion
characteristics is to use different forms of ablation energy.
However, technologies surrounding microwave, laser, ultrasound, and
chemical ablation are largely unproven for this purpose.
[0008] The use of active cooling in association with the
transmission of DC or radio frequency ablation energy is known to
force the tissue interface to lower temperature values. As a
result, the hottest tissue temperature region is shifted deeper
into the tissue, which, in turn, shifts the boundary of the tissue
rendered nonviable by ablation deeper into the tissue. An electrode
that is actively cooled can be used to transmit more ablation
energy into the tissue, compared to the same electrode that is not
actively cooled. However, control of active cooling is required to
keep maximum tissue temperatures safely below about 100.degree. C.,
at which tissue desiccation or tissue boiling is known to
occur.
[0009] Another proposed solution to the creation of larger lesions,
either in surface area and/or depth, is the use of substantially
larger electrodes than those commercially available. Yet, larger
electrodes themselves pose problems of size and maneuverability,
which weigh against a safe and easy introduction of large
electrodes through a vein or artery into the heart.
[0010] A need exists for multi-purpose cardiac ablation electrodes
that can selectively create lesions of different geometries and
characteristics. Multi-purpose electrodes would possess the
flexibility and maneuverability permitting safe and easy
introduction into the heart. Once deployed inside the heart, these
electrodes would possess the capability to emit energy sufficient
to create, in a controlled fashion, either large and deep lesions,
or small and shallow lesions, or large and shallow lesions,
depending upon the therapy required.
SUMMARY OF THE INVENTION
[0011] The invention provides electrode assemblies and associated
systems employing a nonporous wall having an exterior for
contacting tissue. The exterior peripherally surrounds an interior
area. The wall is essentially free of electrically conductive
material. The wall is adapted to assume an expanded geometry having
a first maximum diameter and a collapsed geometry having a second
maximum diameter less than the first maximum diameter. The
assemblies and systems include a lumen that conveys a medium
containing ions into the interior area. An element free of physical
contact with the wall couples the medium within the interior area
to a source of electrical energy to enable ionic transport of
electrical energy from the source through the medium to the wall
for capacitive coupling to tissue contacting the exterior of the
wall.
[0012] In a preferred embodiment, the capacitive coupling of the
wall is expressed in the following relationship:
{square root}{square root over
(R.sub.PATH.sup.2+X.sub.C.sup.2)}<R.sub.- TISSUE
[0013] where: 1 R PATH = K S E s
[0014] and
[0015] K is a constant that depends upon geometry of the wall,
[0016] S.sub.E is surface area of the element, and
[0017] .rho..sub.S is resistivity of the medium containing ions,
and
[0018] where: 2 X C = 1 2 fC
[0019] and
[0020] f is frequency of the electrical energy, and 3 C = S B t
[0021] where:
[0022] .epsilon. is the dielectric constant of wall,
[0023] S.sub.B is the area of the interior area, and
[0024] t is thickness of the wall located between the medium
containing ions and tissue, and
[0025] where R.sub.TISSUE is resistivity of tissue contacting the
wall.
[0026] The invention also provides systems and methods for heating
or ablating body tissue. The systems and methods provide a catheter
tube having a distal end that carries an electrode of the type
described above. The systems and methods electrically couple a
source of radio frequency energy to the electrically conductive
element within the electrode body and to a return electrode in
contact with body tissue.
[0027] According to this aspect of the invention, the systems and
methods guide the catheter tube into a body with the wall in the
collapsed geometry and then cause the wall to assume the expanded
geometry at least in part by conveying a medium containing ions
into the interior area of the body. The systems and methods then
ohmically heat or ablate body tissue by transmitting radio
frequency energy to the electrically conductive element for ionic
transport through the medium to the wall for capacitive coupling to
tissue located between the return electrode and the electrode.
[0028] Features and advantages of the inventions are set forth in
the following Description and Drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a plan view of a system for ablating heart tissue,
which includes an expandable electrode structure that embodies the
features of the invention;
[0030] FIG. 2 is a side elevation view of an expandable electrode
structure usable in association with the system shown in FIG. 1, in
which an inflation medium is used to expand the structure;
[0031] FIG. 3A is a side elevation view of an alternative
expandable electrode structure usable in association with the
system shown in FIG. 1, in which an inflation medium is used to
expand separate multiple chambers within the structure;
[0032] FIG. 3B is a side elevation view of an alternative
expandable electrode structure usable in association with the
system shown in FIG. 1, in which an inflation medium is used to
expand integrally formed multiple chambers within the
structure;
[0033] FIG. 3C is a top section view of the electrode structure
shown in FIG. 3B, taken generally along line 3C-3C in FIG. 3B;
[0034] FIG. 3D is a side elevation view of an alternative
expandable electrode structure usable in association with the
system shown in FIG. 1, in which an inflation medium is used to
expand a single chamber within the structure;
[0035] FIG. 3E is a top view of an alternative
expandable-collapsible electrode structure with a body having
interior coextruded webs that compartmentalize the body into
multiple interior chambers;
[0036] FIG. 4 is a side elevation view of an alternative expandable
electrode structure usable in association with the system shown in
FIG. 1, in which an open spline structure is used to expand the
structure;
[0037] FIG. 5 is the expandable electrode shown in FIG. 4, in which
a slidable sheath is used to collapse the structure;
[0038] FIG. 6 is a side elevation view of an alternative expandable
electrode structure usable in association with the system shown in
FIG. 1, in which an interwoven mesh structure is used to expand the
structure;
[0039] FIG. 7 is the expandable electrode shown in FIG. 6, in which
a slidable sheath is used to collapse the structure;
[0040] FIG. 8 is a side elevation view of an alternative expandable
interwoven mesh electrode structure usable in association with the
system shown in FIG. 1, in which an interior bladder is used to
expand the structure;
[0041] FIG. 9 is a side elevation view of an alternative expandable
foam electrode structure usable in association with the system
shown in FIG. 1;
[0042] FIG. 10 is a side elevation view of an alternative
expandable electrode structure usable in association with the
system shown in FIG. 1, in which an electrically actuated spline
structure is used to expand the structure;
[0043] FIG. 11A is a side elevation view of an alternative
expandable electrode structure usable in association with the
system shown in FIG. 1, in which the electrode structure is pleated
or creased to promote folding upon collapse;
[0044] FIG. 11B is the electrode shown in FIG. 11A in the process
of folding while collapsing;
[0045] FIG. 11C is the electrode shown in FIG. 11A as folded upon
collapse;
[0046] FIG. 12 is a side elevation view of an expandable electrode
structure usable in association with the system shown in FIG. 1, in
which a steering mechanism proximal to the structure steers the
structure at the end of a catheter tube;
[0047] FIG. 13 is a side elevation view of an expandable electrode
structure usable in association with the system shown in FIG. 1, in
which a steering mechanism within the structure steers the
structure at the end of a catheter tube;
[0048] FIG. 14 is a side elevation view of an expandable electrode
structure usable in association with the system shown in FIG. 1, in
which an axially and radially movable stilette in the structure is
used to alter the shape of the structure;
[0049] FIGS. 15A to 15E are plan views of an assembly process for
manufacturing an expandable electrode structure using an inflation
medium to expand the structure;
[0050] FIGS. 16A to 16D are plan views of an assembly process for
manufacturing an expandable electrode structure using an interior
spline structure to expand the structure;
[0051] FIG. 17 is a side elevation view of an expandable electrode
structure usable in association with the system shown in FIG. 1, in
which an electrically conductive shell is deposited on the distal
end of the structure;
[0052] FIG. 18 is a side elevation view of an expandable electrode
structure usable in association with the system shown in FIG. 1, in
which an electrically conductive foil shell is positioned for
attachment on the distal end of the structure;
[0053] FIG. 19 is an enlarged section view of the wall of an
expandable electrode structure usable in association with the
system shown in FIG. 1, in which an electrically conductive
material is coextruded within the wall;
[0054] FIG. 20 is a top view of an expandable electrode structure
having an exterior shell of electrically conductive material formed
in a segmented bull's-eye pattern;
[0055] FIGS. 21 and 22 are, respectively, side and top views of an
expandable electrode structure having an exterior shell of
electrically conductive material formed in a segmented pattern of
energy transmission zones circumferentially spaced about a
preformed, foldable body, and including multiple temperature
sensing elements;
[0056] FIGS. 23, 24A, and 24B are enlarged side views showing the
deposition of electrically conductive material to establish fold
lines on the exterior of an expandable electrode structure;
[0057] FIG. 25 is a top view of an expandable electrode structure
showing the preferred regions for attaching signal wires to an
electrically conductive shell deposited on the distal end of the
structure;
[0058] FIG. 26 is a side view of an expandable electrode structure
showing the preferred regions for attaching signal wires to an
electrically conductive shell deposited in a circumferentially
segmented pattern on the structure;
[0059] FIG. 27 is a top view of an expandable electrode structure
showing the preferred regions for attaching signal wires to an
electrically conductive shell deposited in a bull's-eye pattern on
the structure;
[0060] FIGS. 28A and 28B are, respectively side section and top
views showing the attachment of signal walls to an electrically
conductive shell deposited on the distal end of the structure, the
signal wires being led through the distal end of the structure;
[0061] FIG. 29A is an enlarged side view of the distal end of an
expandable electrode structure usable in association with the
system shown in FIG. 1, showing the attachment of an ablation
energy signal wire to the electrically conductive shell using a
mechanical fixture at the distal end of the structure;
[0062] FIG. 29B is an enlarged exploded side view, portions of
which are in section, of the mechanical fixture shown in FIG.
29A;
[0063] FIGS. 30 and 31 are side section views showing the
attachment of a signal wire to an electrically conductive shell,
the signal wire being snaked through the wall of the structure
either one (FIG. 30) or multiple times (FIG. 31);
[0064] FIG. 32 is an enlarged section view of the wall of an
expandable electrode structure usable in association with the
system shown in FIG. 1, showing the laminated structure of the wall
and the attachment of an ablation energy signal wire to the
electrically conductive shell using laser windowing techniques;
[0065] FIG. 33 is a side view, with portions broken away and in
section, of an expandable electrode structure usable in association
with the system shown in FIG. 1, showing the attachment of a
temperature sensing element to a fixture at the distal end of the
structure;
[0066] FIG. 34 is an enlarged side section view of the wall of an
expandable electrode structure usable in association with the
system shown in FIG. 1, showing ways of attaching temperature
sensing elements inside and outside the wall;
[0067] FIG. 35 is an enlarged side section view of the wall of an
expandable electrode structure usable in association with the
system shown in FIG. 1, showing a laminated structure and the
creation of temperature sensing thermocouples by laser windowing
and deposition;
[0068] FIG. 36 is a top view of an expandable electrode structure
showing the preferred regions for attaching temperature sensing
elements with respect to an electrically conductive shell deposited
on the distal end of the structure;
[0069] FIG. 37 is a side view of an expandable electrode structure
showing the preferred regions for attaching temperature sensing
elements with respect to an electrically conductive shell deposited
in a circumferentially segmented pattern on the structure;
[0070] FIG. 38 is a top view of an expandable electrode structure
showing the preferred regions for attaching temperature sensing
elements with respect to an electrically conductive shell deposited
in a bull's-eye pattern on the structure;
[0071] FIG. 39 is a side view of an expandable electrode structure
showing a pattern of holes for cooling the edge regions of an
electrically conductive shell deposited in a circumferentially
segmented pattern on the structure, the pattern of holes also
defining a fold line between the segments of the pattern;
[0072] FIGS. 40A and 40B are enlarged views of a hole formed in the
structure shown in FIG. 39, showing that the hole defines a fold
line;
[0073] FIG. 41A is a side sectional view of an expandable electrode
structure usable in association with the system shown in FIG. 1,
which is capacitively coupled to tissue;
[0074] FIG. 41B is a diagrammatic view showing the electrical path
that ablation energy follows when the electrode shown in FIG. 40A
is capacitively coupled to tissue;
[0075] FIG. 42A is an side sectional view of an alternative
expandable electrode structure usable in association with the
system shown in FIG. 1, which is capacitively coupled to
tissue;
[0076] FIG. 42B is a diagrammatic view showing the electrical path
that ablation energy follows when the electrode shown in FIG. 41A
is capacitively coupled to tissue;
[0077] FIG. 43 is a diagrammatic view of neural network usable for
predicting maximum temperature conditions when the
expandable-collapsible electrode structure carries multiple
ablation energy transmitting segments; and
[0078] FIG. 44 is a side elevation view of an expandable electrode
structure that embodies the features of the invention, used in
association with pacing and sensing electrodes.
[0079] The invention may be embodied in several forms without
departing from its spirit or essential characteristics. The scope
of the invention is defined in the appended claims, rather than in
the specific description preceding them. All embodiments that fall
within the meaning and range of equivalency of the claims are
therefore intended to be embraced by the claims.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0080] I. Overview of a System With an Expandable-Collapsible
Electrode Structure
[0081] FIG. 1 shows a tissue ablation system 10 that embodies the
features of the invention.
[0082] The system 10 includes a flexible catheter tube 12 with a
proximal end 14 and a distal end 16. The proximal end 14 carries a
handle 18. The distal end 16 carries an electrode structure 20,
which embodies features of the invention. The purpose of the
electrode structure 20 is to transmit ablation energy.
[0083] As the embodiments of FIGS. 2 through 11 show in greater
detail, the electrode structure 20 includes an
expandable-collapsible wall forming a body 22. The geometry of the
body 22 can be altered between an enlarged, or expanded, geometry
having a first maximum diameter (depicted in various forms, for
example, in FIGS. 2, 3, 4, 6, and 11A) and a collapsed geometry
having a second maximum diameter less than the first maximum
diameter (depicted in various forms, for example, in FIGS. 5, 7,
11B/C).
[0084] This characteristic allows the expandable-collapsible body
22 to assume a collapsed, low profile (ideally, less than 8 French
diameter, i.e., less than about 0.267 cm) when introduced into the
vasculature. Once located in the desired position, the
expandable-collapsible body 22 can be urged into a significantly
expanded geometry of, for example, approximately 7 to 20 mm.
[0085] The expanded geometry of the body 22, coupled with its
inherent flexibility, significantly enhance the lesion creation
characteristics of the electrode structure. Further details of the
body and the ways to alter its geometry will be provided later.
[0086] All or a portion of the wall forming the body 22 carries an
electrically conductive material that forms an electrode surface.
As the embodiments shown FIGS. 2 through 11 show in greater detail,
the electrically conductive material comprises an electrically
conductive shell 24 overlying all or a portion of the
expandable-collapsible body 22. The shell 24 serves as the
transmitter of energy that ablates body tissue. While the type of
ablation energy used can vary, in the illustrated and preferred
embodiment, the shell 24 serves to transmit radio frequency (RF)
electromagnetic energy.
[0087] The shell 24 is flexible enough to adopt to the range of
geometries, from collapsed to expanded, that the
expandable-collapsible body 22 assumes. Still, the shell 24
preferably resists stretching within this range, to thereby
minimize "thinning." Thinning of the shell 24 creates localized
changes to the shell 24, with attendant increases in resistance and
"hot spots." For this reason, the elasticity of the
expandable-collapsible body 22 and shell 24 should be selected to
fall within acceptable bounds so that the ability to fold is
retained while preserving stability during inflation. Further
details of the energy transmitting shell 24 will be provided
later.
[0088] As will be shown in greater detail later (see FIGS. 25 to
32), the shell 24 is coupled to one or more signal wires 26. The
signal wires 26 extend from the shell 24, through the catheter tube
12, to external connectors 28 on the handle 18 (see FIG. 1). The
connectors 28 electrically couple the shell 24 to a radio frequency
generator 30.
[0089] In the preferred and illustrated embodiment (see FIG. 1), a
controller 32 is associated with the generator 30, either as an
integrated unit or as a separate interface box. The controller 32
governs the delivery of radio frequency ablation energy to the
shell 24 according to preestablished criteria. Further details of
this aspect of the system 10 will be described later.
[0090] The system 10 as just described is suited for ablating
myocardial tissue within the heart. In this environment, a
physician moves the catheter tube 12 through a main vein or artery
into a heart chamber, while the expandable-collapsible body 22 of
the electrode structure 20 is in its low profile geometry. Once
inside the desired heart chamber, the expandable-collapsible body
22 is enlarged into its expanded geometry, and the shell 24 is
placed into contact with the targeted region of endocardial tissue.
Radio frequency energy is conveyed from the generator 30 to the
shell 24, as governed by the controller 32. The shell 24 transmits
radio frequency energy into tissue to a return electrode, which is
typically an external patch electrode (forming a unipolar
arrangement). Alternatively, the transmitted energy can pass
through tissue to an adjacent electrode in the heart chamber
(forming a bipolar arrangement), or between segments in the shell
24, as will be described later (also forming a bipolar
arrangement). The radio frequency energy heats the tissue forming a
lesion.
[0091] The expanded geometry of the expandable-collapsible body 22
enhances the energy transmission characteristics of the structure
20. The structure 20, when expanded, is able to form tissue lesions
that are significantly larger in terms of size and volume than the
body's initial collapsed profile during introduction would
otherwise provide.
[0092] It should also be appreciated that the
expandable-collapsible electrode structure 20 as just described is
also suited for mapping myocardial tissue within the heart. In this
use, the shell 24 senses electrical activity in the heart. The
sensed electrical activity is conveyed to an external monitor,
which processes the potentials for analysis by the physician. The
use of an expandable-collapsible electrode structure for this
purpose is generally disclosed in Edwards et al. U.S. Pat. No.
5,293,869.
[0093] It should also be appreciated that the
expandable-collapsible electrode structure 20 can be used
alternatively, or in combination with sensing electrical
activities, to convey pacing signals. In this way, the structure 20
can carry out pace mapping or entrainment mapping. The expanded
electrode structure 20 can also be used to convey pacing signals to
confirming contact with tissue before ablating. The ability to
carry out pacing to sense tissue contact is unexpected, given that
the expanded structure 20 presents a surface area significantly
greater than that presented by a conventional 4 mm/8
Felectrode.
[0094] As FIG. 44 shows, the catheter tube 20 can also carry one or
more conventional ring electrodes 21 for bipolar sensing. A
conventional pacing or unipolar sensing electrode 23 may also be
provided, appended at the distal end of the structure 20.
[0095] II. The Expandable-Collapsible Body
[0096] The expandable-collapsible body 22 is made from a material
selected to exhibit the following characteristics:
[0097] (i) the material must be capable, in use, of transition
between an expanded geometry having a first maximum diameter and a
collapsed geometry having a second maximum diameter less than the
first diameter. In this respect, the material can be formed into an
expandable-collapsible bladder or balloon body having an open
interior. The body is flexible enough to assume the expanded
geometry as a result of a normally open solid support structure
within the interior, or the opening of a normally closed support
structure within the interior, or the introduction of fluid
pressure into the interior, or a combination of such interior
forces. In this arrangement, the body is caused to assume the
collapsed geometry by an exterior compression force against the
normally open interior support structure, or the closing of the
interior support structure, or the removal of the interior fluid
pressure, or a combination of such offsetting forces.
Alternatively, the material can be a preformed body with a memory
urging it toward a normally expanded geometry. In this arrangement,
the preformed body is caused to assume the collapsed geometry by
the application of an external compression force. In this
arrangement, the preformed body can have an open interior, or can
comprise, for example, a collapsible composite foam structure.
[0098] (ii) the material must be biocompatible and able to
withstand high temperature conditions, which arise during
manufacture and use.
[0099] (iii) the material must possess sufficient strength to
withstand, without rupture or tearing, external mechanical or fluid
forces, which are applied to support and maintain its preformed
geometry during use.
[0100] (iv) the material must lend itself to attachment to the
catheter tube 12 through the use of straightforward and inexpensive
adhesive, thermal, or mechanical attachment methods.
[0101] (v) the material must be compatible with the electrically
conductive shell 24 to achieve secure adherence between the
two.
[0102] Thermoplastic or elastomeric materials that can be made to
meet these criteria include polyimide. (kapton), polyester,
silicone rubber, nylon, mylar, polyethelene, polyvinyl chloride,
and composite structures using these and other materials.
[0103] The incidence of tissue sticking to the exterior of the body
22 during use can be mediated by the inclusion of low friction
materials like PTFE. The propensity of the exterior of the body 22
to cause blood clotting and/or embolization can be reduced by
incorporating non-thrombogenic material onto or into the exterior
of the body 22.
[0104] Polyimide is particularly preferred for the
expandable-collapsible body. Polyimide is flexible, but it is not
elastic. It can withstand very high temperatures without
deformation. Because polyimide is not elastic, it does not impose
stretching forces to the shell, which could lead to electrical
conductivity decreases, as above described.
[0105] The expandable-collapsible body 22 can be formed about the
exterior of a glass mold. In this arrangement, the external
dimensions of the mold match the desired expanded internal geometry
of the expandable-collapsible body 22. The mold is dipped in a
desired sequence into a solution of the body material until the
desired wall thickness is achieved. The mold is then etched away,
leaving the formed expandable-collapsible body 22.
[0106] Various specific geometries, of course, can be selected. The
preferred geometry is essentially spherical and symmetric, with a
distal spherical contour, as FIGS. 2 to 11 show in various forms.
However, nonsymmetric geometries can be used. For example, the
expandable-collapsible body 22 may be formed with a flattened
distal contour, which gradually curves or necks inwardly for
attachment with the catheter tube 12.
[0107] The expandable-collapsible body 22 may also be blow molded
from extruded tube. In this arrangement, the body 22 is sealed at
one end using a mechanical clamp, adhesive, or thermal fusion. The
opposite open end of the body 22 is left open. The sealed
expandable-collapsible body 22 is placed inside the mold. An
inflation medium, such as high pressure gas or liquid, is
introduced through the open tube end. The mold is exposed to heat
as the tube body 22 is inflated to assume the mold geometry. The
formed expandable-collapsible body 22 is then pulled from the
mold.
[0108] A. Expansion Using Interior Fluid Pressure
[0109] In the embodiments shown in FIGS. 2 and 3A/B/C, fluid
pressure is used to inflate and maintain the expandable-collapsible
body 22 in the expanded geometry.
[0110] In this arrangement, the catheter tube 12 carries an
interior lumen 34 along its length. The distal end of the lumen 34
opens into the hollow interior of the expandable-collapsible body
22, which has been formed in the manner just described. The
proximal end of the lumen 34 communicates with a port 36 (see FIG.
1) on the handle 18.
[0111] An inflation fluid medium (arrows 38 in FIG. 2) is conveyed
under positive pressure through the port 36 and into the lumen 34.
The fluid medium 38 fills the interior of the
expandable-collapsible body 22. The fluid medium 38 exerts interior
pressure to urge the expandable-collapsible body 22 from its
collapsed geometry to the enlarged geometry desired for
ablation.
[0112] The inflating fluid medium 38 can vary. Preferably, it
comprises a liquid such as water, saline solution, or other
biocompatible fluid. Alternatively, the inflating fluid medium 38
can comprise a gaseous medium such as carbon dioxide or air.
[0113] Regardless of the type of fluid medium 38, the inflation
preferably occurs under relatively low pressures of up to 30 psi.
The pressure used depends upon the desired amount of inflation, the
strength and material used for the body 22, and the degree of
flexibility required, i.e., high pressure leads to a harder, less
flexible body 22.
[0114] More than one fluid conveying lumen 34 may be used. The
multiple lumens 34 can, for example, speed up the introduction or
removal of the inflating medium 38 from the body 22. Multiple
lumens can also serve to continuously or intermittently recycle the
inflating medium 38 within the body 22 for controlling the
temperature of the body, as will be described in greater detail
later. Multiple lumens can also be used, with at least one of the
lumens dedicated to venting air from the structure 20.
[0115] In an alternative embodiment shown in FIG. 3A, a group of
sealed bladders compartmentalize the interior of the formed body
into chambers 40. One or more lumens 42 passing through the
catheter tube 12 convey the inflating gas or liquid medium 38 into
each chamber 40, as described above. The inflated chambers 40
collectively hold the expandable-collapsible body 22 in its
expanded condition. Removal of the inflation medium 38 deflates the
chambers 40, collapsing the expandable-collapsible body 22.
[0116] The bladders defining the chambers 40 may be separately
formed by molding in generally the same fashion as the main
expandable-collapsible body 22. The bladder material need not have
the same resistance to high temperature deformation as the
expandable-collapsible body 22. If desired, the bladders may also
be deposition coated with a thermal insulating material to
thermally insulate them from the main expandable-collapsible body
22.
[0117] Alternatively, as FIGS. 3B and 3C show, the interior
chambers 40 can take the form of tubular, circumferentially spaced
ribs 41 attached to the interior of the body 22. In this
arrangement, the ribs 41 preferable constitute integrally molded
parts of the body 22.
[0118] As explained in connection with the FIG. 3A embodiment, a
single lumen may service all chambers ribs 41. However, multiple
lumens individually communicating with each rib 41 provide the
ability to more particularly control the geometry of the expanded
body 22, by selectively inflating some but not all the ribs 41 or
chambers 40.
[0119] As FIG. 3E shows, the body 22 may be extruded with interior
webs 43. When the body is in its expanded geometry, the interior
webs 43 compartmentalize the body 22 into the interior chambers 40,
as already described. As before described, multiple lumens
preferably individually communicate with each formed chamber 40 for
conveying inflation medium and for venting air.
[0120] As FIG. 3D shows, a separate, single interior chamber 124
can be used instead of the compartmentalized chambers 40 or ribs 41
shown in FIGS. 3A, 3B, and 3C to receive the inflation medium for
the exterior body 22. As will be described in greater detail later,
this arrangement creates an intermediate region 126 between the
interior of the body 22 and the exterior of the chamber 124,
through which signal wires 26 can be passed for coupling to the
shell 24.
[0121] B. Interior Support Structures
[0122] In the embodiments shown in FIGS. 4 to 7, collapsible,
interior structures 44 sustain the expandable-collapsible body 22
in the expanded geometry. The presence of the interior support
structure 44 eliminates the need to introduce air or liquid as an
inflation medium 38. Possible difficulties of fluid handling and
leakage are thereby avoided.
[0123] In the embodiment shown in FIGS. 4 and 5, the
expandable-collapsible body 22 is held in its expanded geometry by
an open interior structure 44 formed by an assemblage of flexible
spline elements 46. The spline elements 46 are made from a
resilient, inert wire, like nickel titanium (commercially available
as Nitinol material), or from a resilient injection molded inert
plastic or stainless steel. The spline elements 46 are preformed in
a desired contour and assembled to form a three dimensional support
skeleton, which fills the interior space of the
expandable-collapsible body 22.
[0124] In this arrangement, the supported expandable-collapsible
body 22 is brought to a collapsed geometry by outside compression
applied by an outer sheath 48 (see FIG. 5), which slides along the
catheter tube 12. As FIG. 5 shows, forward movement of the sheath
48 advances it over the expanded expandable-collapsible body 22.
The sliding sheath 48 encompasses the expandable-collapsible body
22, compressing the interior spline elements 46 together. The
expandable-collapsible body 22 collapses into its low profile
geometry within the sheath 48.
[0125] Rearward movement of the sheath 48 (see FIG. 4) retracts it
away from the expandable-collapsible body 22. Free from the
confines of the sheath 48, the interior support structure 44 of
spline elements 46 springs open into the three dimensional shape.
The expandable-collapsible body 22 returns to its expanded geometry
upon the spline elements 46.
[0126] In an alternative embodiment, as FIGS. 6 and 7 show, the
expandable-collapsible body 22 is supported upon a closed, three
dimensional structure 44 formed by a resilient mesh 50. The mesh
structure 50 is made from interwoven resilient, inert wire or
plastic filaments preformed to the desired expanded geometry. The
mesh structure 50 provides interior support to hold the
expandable-collapsible body 22 in its expanded geometry, in the
same way as the open structure of spline elements 46 shown in FIG.
4.
[0127] As FIG. 7 further shows, a sliding sheath 48 (as previously
described) can also be advanced along the catheter tube 12 to
compress the mesh structure 50 to collapse mesh structure 50 and,
with it, the expandable-collapsible body 22. Likewise, retraction
of the sheath 48 removes the compression force (as FIG. 6 shows),
and the freed mesh structure 50 springs open to return the
expandable-collapsible body 22 back to its expanded geometry.
[0128] By interweaving the mesh filaments close enough together,
the mesh structure 50 itself could serve as the support for the
electrically conductive shell 24, without need for the intermediate
expandable-collapsible body 22. Indeed, all or a portion of the
mesh filaments could be made electrically conductive to themselves
serve as transmitters of ablation energy. This arrangement of
interwoven, electrically conductive filaments could supplement or
take the place of the electrically conductive shell 24.
[0129] Alternatively, as FIG. 8 shows, the mesh structure 50 can be
made to normally assume the collapsed geometry. In this
arrangement, one or more interior bladders 126 can accommodate the
introduction of an inflation medium to cause the mesh structure 50
to assume the expanded geometry.
[0130] If the mesh structure 50 is tightly woven enough to be
essentially liquid impermeable , the interior bladder 126 could be
eliminated. In this arrangement, the introduction of a
biocompatible liquid, such as sterile saline, directly into the
interior of the structure 50 would cause the structure to assume
the expanded geometry.
[0131] FIG. 9 shows yet another alternative expandable-collapsible
structure. In this embodiment, a foam body 128 molded to normally
assume the shape of the expanded geometry forms the interior
support structure for the body 22. As with the interior structures
44, the presence of the foam body 128 eliminates the need to
introduce air or liquid as an inflation medium. Also like the
interior structures 44, a sliding sheath (not shown but as
previously described) can be advanced along the catheter tube 12 to
compress the foam body 128 and overlying body 22 into the collapsed
geometry. Likewise, retraction of the sheath removes the
compression force. The foam body 128, free of the sheath, springs
open to return the expandable-collapsible body 22 back to the
expanded geometry. It should be appreciated that the foam body 128
can provide interior, normally expanded support to the mesh
structure 50 in the same way.
[0132] As FIG. 10 shows, the geometry of the expandable-collapsible
body 22 can be controlled electrically. This arrangement includes
an assemblage of spline elements 132 within the body 22. The spline
elements 132 are made of a material that undergoes shape or phase
change in response to heating. Nickel titanium wire is a material
having this characteristic. Alternatively, the spline elements 132
could comprise an assembly of two metals having different
coefficients of expansion.
[0133] The body 22 overlies the spline elements 132. The spline
elements 132 are coupled to an electrical current source 134.
Current flow from the source 134 through the spline elements 132
resistively heats the elements 132. As a result, the spline
elements 132 change shape.
[0134] As FIG. 10 shows, the spline elements 132 normally present
the collapsed geometry. Current flow through the spline elements
132 causes expansion of the elements 132, thereby creating the
expanded geometry (as shown by arrows and phantom lines in FIG.
10). It should be appreciated that the spline elements 132 could
alternatively normally present the expanded geometry and be made to
contract, thereby assuming the collapsed geometry, in response to
current flow.
[0135] C. Folding
[0136] In all the representative embodiments, the
expandable-collapsible body 22 can be molded with preformed regions
52 (see FIGS. 11A/B/C) of reduced thickness, forming creases. To
create these crease regions 52, the mold has a preformed surface
geometry such that the expandable-collapsible material would be
formed slightly thinner, indented, or ribbed along the desired
regions 52. Alternatively, the use of interior coextruded webs 43,
as FIG. 3E shows, also serves to form the crease regions 52 along
the area where the webs 43 contact the interior wall of the body
22.
[0137] As FIGS. 11B/C show, the expandable-collapsible body 22
collapses about these regions 52, causing the body 22 to
circumferentially fold upon itself in a consistent, uniform
fashion. The resulting collapsed geometry can thus be made more
uniform and compact.
[0138] In the embodiments where an inflation medium 38 applies
positive pressure to expand the expandable-collapsible body 22, a
negative fluid pressure can be applied inside the
expandable-collapsible body 22 to draw the fold regions 52 further
inward. In the embodiment where the interior structure 44 of open
spline elements 46 supports the expandable-collapsible body 22, the
fold regions 52 are preferably aligned in the spaces between the
spline elements 46 to take best advantage of the prearranged
folding action.
[0139] Alternative ways of creating fold regions 52 in the body 22
will be described in greater detail later.
[0140] D. Steering
[0141] In the illustrated and preferred embodiment, a distal
steering mechanism 54 (see FIG. 1) enhances the manipulation of the
electrode structure 20, both during and after deployment.
[0142] The steering mechanism 54 can vary. In the illustrated
embodiment (see FIG. 1), the steering mechanism 54 includes a
rotating cam wheel 56 coupled to an external steering lever 58
carried by the handle 18. The cam wheel 56 holds the proximal ends
of right and left steering wires 60. The wires 60 pass with the
ablation energy signal wires 26 through the catheter tube 12 and
connect to the left and right sides of a resilient bendable wire or
leaf spring 62 adjacent the distal tube end 16 (see FIG. 12).
Further details of this and other types of steering mechanisms are
shown in Lundquist and Thompson U.S. Pat. No. 5,254,088, which is
incorporated into this Specification by reference.
[0143] In FIG. 12, the leaf spring 62 is carried within in the
distal end 16 of the catheter tube 12, to which the electrode
structure 20 is attached. As FIGS. 1 and 12 show, forward movement
of the steering lever 58 pulls on one steering wire 60 to flex or
curve the leaf spring 62, and, with it, the distal catheter end 16
and the electrode structure 20, in one direction. Rearward movement
of the steering lever 58 pulls on the other steering wire 60 to
flex or curve the leaf spring 62, and, with it, the distal catheter
end 16 and the electrode structure 20, in the opposite
direction.
[0144] In FIG. 13, the leaf spring 62 is part of a distal fixture
66 carried within the electrode structure 20 itself. In this
arrangement, the leaf spring 62 extends beyond the distal catheter
end 16 within a tube 64 inside the expandable-collapsible body 22.
The distal end of the leaf spring 62 is secured to a distal fixture
66. The distal fixture 66 is itself attached to the distal end of
the body 22. Further details of attaching the fixture 66 to the
distal end of the body 22 will be described in greater detail
later.
[0145] As FIG. 13 shows, forward movement of the steering lever 58
bends the leaf spring 62 in one direction within the
expandable-collapsible body 22, deflecting the distal fixture 66
with it. This deforms the expandable-collapsible body 22 in the
direction that the leaf spring 62 bends. Rearward movement of the
steering lever 58 bends the leaf spring 62 in the opposite
direction, having the opposite deformation effect upon the
expandable-collapsible body 22.
[0146] In either arrangement, the steering mechanism 54 is usable
whether the expandable-collapsible body is in its collapsed
geometry or in its expanded geometry.
[0147] E. Push-Pull Stiletto
[0148] In FIG. 14, a stilette 76 is attached to the distal fixture
66. The stilette extends inside the body 22, through the catheter
tube 12, to a suitable push-pull controller 70 on the handle 18
(see FIG. 1). The stilette 76 is movable along the axis of the
catheter tube 12. Moving the stilette 76 forward pushes axially
upon the distal fixture 66. Moving the stilette 76 rearward pulls
axially upon the distal fixture 66. The geometry of the body 22
elongates or expands accordingly.
[0149] The stilette 76 can be used in association with an
expandable-collapsible body 22 that is expanded by an inflation
medium 38. In this arrangement, when the expandable-collapsible
body 22 is collapsed, forward movement of the stilette 76, extends
the distal fixture 66 to further urge the expandable-collapsible
body 22 into a smaller diameter profile for introduction.
[0150] When used in association with an expandable-collapsible body
22 that is internally supported by the spline structure 46 or the
mesh structure 50, the stilette 76 can be used instead of the
slidable outer sheath 48 to expand and collapse the
expandable-collapsible body 22. Pushing forward upon the stilette
76 extends the spline structure 46 or mesh structure 50 to collapse
the expandable-collapsible body 22. Pulling rearward upon the
stilette 76, or merely releasing the pushing force, has the
opposite effect, allowing the spline structure 46 or mesh structure
50 to assume its expanded geometry.
[0151] When used with either inflated or mechanically expanded
expandable-collapsible bodies 22, pulling rearward upon the
stilette 76 also has the effect of altering the expanded geometry
by flattening the distal region of the expandable-collapsible body
22.
[0152] While the stilette 76 can be used by itself, in the
illustrated embodiment (see FIG. 14), the distal end of the
stilette 76 near the fixture 66 comprises the bendable leaf spring
62, thereby providing a radial steering function in tandem with the
axial push-pull action of the stilette 76.
[0153] There are various ways to combine the steering mechanism 54
with the stilette 76. In the illustrated embodiment, a collar 136
is retained by a heat-shrink fit within tubing 64. The collar 136
has a central aperture 138 through which a leaf spring 62 at the
end of the stilette 76 passes for movement along the axis of the
catheter tube 12. Steering wires 60 are attached to the collar 136.
Pulling on the steering wires 60 radially deflects the collar 136,
thereby bending the leaf spring 62 at the end of the stilette 76 in
the direction of the pulled steering wire 60.
[0154] F. Attachment to Catheter Tube
[0155] A sleeve 78 (see, e.g., FIG. 2) couples the near end of the
expandable-collapsible body 22 to the distal end 16 of the catheter
tube. The sleeve 78 withstands the forces exerted to expand the
expandable-collapsible body 22, resisting separation of the body 22
from the catheter tube 12. In FIG. 2, where an inflation medium 38
is used, the sleeve 78 also forms a fluid seal that resists leakage
of the medium at inflation pressures.
[0156] The sleeve 78 can be secured about the catheter tube in
various ways, including adhesive bonding, thermal bonding,
mechanical bonding, screws, winding, or a combination of any of
these.
[0157] FIGS. 15A to 15E show the details of a preferred assembly
process for an expandable-collapsible body 22 whose geometry is
altered by use of fluid pressure, such as previously shown in FIGS.
2 and 3. The body 22 is extruded as a tube 140 having an extruded
interior diameter, designated ID.sub.1 (see FIG. 15A) . The
extruded interior diameter ID.sub.1 is selected to be less than the
exterior diameter of the distal stem 142 of the catheter tube 12 to
which the body 22 will ultimately be attached.
[0158] As FIG. 15D shows, the stem 142 comprises an elongated,
stepped-down tubular appendage, which extends beyond the distal end
16 of the catheter tube 12. The distal end of the stem 142 is
sealed. The exterior diameter of the stem 142 is designated in FIG.
15D as ED.sub.S. The stem 142 includes a central lumen 152 for
carrying inflation medium. Spaced apart holes 154 on the stem 142
communicate with the lumen to convey the inflation medium into the
body 22, when attached to the stem 142.
[0159] As FIG. 15A shows, the material of the extruded tube 140 is
preferably cross linked by exposure to gamma radiation 168 or an
equivalent conventional treatment. The cross linking enhances the
capability of the material of the tube 140 to recover its shape
after mechanical deformation.
[0160] After cross linking, the extruded tube 140 is mechanically
deformed by heat molding into the body 22 having the desired
collapsed geometry, in a manner previously described. The body
geometry (see FIG. 15B) includes proximal and distal neck regions
144 and 146 and an intermediate main body region 148. The neck
regions 144 and 146 have an enlarged interior diameter (designated
ID.sub.2 in FIG. 15B) that is slightly greater than catheter stem
diameter ED.sub.S, to permit a slip fit of the body 22 over the
stem 142. The intermediate main body region 148 has an enlarged
exterior diameter selected for the collapsed geometry of the body
22. To preserve the desired wall thickness, the enlarged exterior
diameter of the tube 140 should be about twice the original
extruded outer diameter of the tube 140.
[0161] As FIG. 15C shows, the tubing ends 150 extending beyond the
neck regions 144 and 146 are cut away. As FIG. 15D shows, the body
22 is slip fitted over the stem 142. Heat is applied to shrink fit
the neck regions 144 and 146 about the stem 142 (see FIG. 15E). Due
to molding, the memory of these regions 144 and 146, when heated,
seek the original interior diameter ID.sub.1 of the tubing 140,
thereby proving a secure interference fit about the stem 142.
[0162] Preferably, after forming the interference fit between the
neck regions 144 and 146 and the stem 142, additional heat is
provided to thermally fuse the regions 144 and 146 to the stem 142.
Last, the sleeve 78 is heat-shrunk in place about the proximal neck
region 144 (see FIG. 15E). The sleeve 78 can comprise a heat-shrink
plastic material or phase changeable metal material, like nickel
titanium. Alternatively, the sleeve 78 can be heat-shrunk into
place without an intermediate thermal fusing step.
[0163] FIGS. 16A to 16D show the details of a preferred assembly
process for an expandable-collapsible body 22 whose geometry is
altered by use of an interior support structure 44 of spline
elements 46, such as previously shown in FIGS. 4 and 5. After heat
molding the body 22 in the manner shown in FIGS. 15A to 15C, the
distal neck region 146 is secured by heat shrinking about the
distal fixture 66 (see FIG. 16A). As FIG. 16A shows, the distal
fixture 66 has, preattached to it, the distal end of the spline
element structure 44, as well as any desired steering mechanism 54,
stilette 76, or combination thereof (not shown in FIGS. 16A to
16D). When initially secured to the fixture 66, the main region 148
of the body 22 is oriented in a direction opposite to the spline
element structure 44.
[0164] After securing the distal neck region 146 to the fixture 66,
as just described, the body 22 is everted about the distal fixture
66 over the spline element structure 44 (see FIG. 16B). The
proximal end of the spline element structure 44 is secured to an
anchor 156 carried by the distal catheter end 16 (see FIG. 16C),
and the everted proximal neck region 144 is then slip fitted over
the catheter stem 158. As FIG. 16C shows, the catheter stem 158 in
this arrangement does not extend beyond the neck region 144 of the
body 22.
[0165] Heat is then applied to shrink fit the neck region 144 about
the stem 158 (see FIG. 16D). Preferably, after forming this
interference fit between the neck region 144 and the stem 158,
additional heat is provided to thermally fuse the region 144 to the
stem 158. Last, the sleeve 78 is heat-shrunk in place about the
proximal neck region 144. Alternatively, the sleeve 78 can be
heat-shrunk into place without an intermediate thermal fusing
step.
[0166] III. The Electrically conducting Shell
[0167] The purpose of the electrically conducting shell 24 is to
transmit ablation energy, which in the illustrated and preferred
embodiment comprises electromagnetic radio frequency energy with a
frequency below about 1.0 GHz. This type of ablating energy heats
tissue, mostly ohmically, to form lesions without electrically
stimulating it. In this arrangement, the shell 24 should possess
the characteristics of both high electrical conductivity and high
thermal conductivity. It should also be appreciated that the shell
24 could form an antenna for the transmission of higher frequency
microwave energy.
[0168] By altering the size, location, and pattern of the shell 24,
along with adjusting the power level and time that the radio
frequency ablation energy is transmitted, the electrode structure
20 is able to create lesions of different size and geometries.
[0169] A. Shell Geometry (Thermal Convective Cooling)
[0170] In one application, the shell creates lesion patterns
greater than about 1.5 cm deep and/or about 2.0 cm wide. These
lesion patterns are significantly deeper and wider than those
created by conventional 8 F diameter/4 mm long electrodes, which
are approximately 0.5 cm deep and 10 mm wide. The deeper and wider
lesion patterns that the shell 24 can provide are able to destroy
epicardial and intramural ventricular tachycardia (VT)
substrates.
[0171] As the following Example shows, the size and location of the
shell 24 on the expandable-collapsible body 22, when expanded,
significantly affects the size and geometry of the lesions formed
by transmitting radio frequency ablation energy.
EXAMPLE 1
[0172] Finite element analysis was performed for a flexible,
expanded electrode structure 20 having a 1.4 cm diameter and a wall
thickness of approximately 200 .mu.m. The model assumed a 100 .mu.m
thick coating of gold over the distal hemisphere of the structure
20, forming the electrically conductive shell 24. The constraint
for the model was a lower limit on thickness and therefore the
thermal conductivity of the shell 24.
[0173] For the model, the percent of electrically conductive shell
24 in contact with myocardial tissue, with the balance exposed to
blood, was changed from 5%, 20%, 41%, and 100% tissue contact. Time
and power of energy transmission were also varied. Power was
changed to keep the maximum temperature of tissue under the shell
24 at 90.degree. C. Maximum lesion depth, width, and volume were
measured.
[0174] The following Table 1 presents the results:
1TABLE 1 LESION GEOMETRY AS A FUNCTION OF TISSUE vs. BLOOD CONTACT
WITH THE ELECTRICALLY CONDUCTIVE SHELL % Tis- sue Lesion Lesion
Lesion Con- Temp. Voltage Current Power Depth Width Volume tact
(.degree. C.) (Volts) (Amps) (Watts) (cm) (cm) (cm.sup.2) <5%
92.1 84 1.67 140 2.1 5.4 36 20% 89.7 81 1.55 125 2.5 4.9 41 41%
89.6 77 1.4 107 2.3 3.5 17 100% 92.3 61 0.92 56 1.4 2.6 7
[0175] The lesions created in the above Table 1 are capable of
making transmural lesions in the left ventricle and can therefore
ablate epicardial VT substrates. The Table 1 shows that lesion size
increases with an electrically conductive shell 24 presenting less
percentage contact with tissue than blood. The shell presenting
100% contact with tissue (and none with blood), compared to the
shell 24 presenting up to 41% percent of its surface to tissue had
lower lesion depths.
[0176] With less relative contact with tissue than blood, the shell
24 is more exposed to the blood pool and its convective cooling
effect. The blood cools the shell 24 it contacts. Heat is lost from
tissue under the shell 24 into the blood pool. This emulation of
active cooling of the shell 24 causes more power to be transmitted
to the tissue before maximum tissue temperatures are achieved,
thereby creating larger lesions.
[0177] Table 1 highlights the importance of relatively high thermal
conductivity for the shell 24, which can be achieved by material
selection and controlling thickness. Given the same percentage
contact with tissue versus blood, a higher thermal conductivity
results in a higher cooling effect and a corresponding increase in
lesion size.
[0178] The above Table 1 demonstrates the ability of the structure
20 carrying the shell 24 to transmit the proper amount of radio
frequency energy to create large and deep lesions.
[0179] Additional tests were performed using shells 24 with a
desirable lower percentage contact with tissue relative to blood
(less than 50%). These tests varied the time of ablation energy
transmission to gauge the effect upon lesion size.
[0180] The following Table 2 presents the results:
2TABLE 2 LESION GEOMETRY AS A FUNCTION OF TIME OF ABLATION ENERGY
TRANSMISSION, GIVEN THE SAME TISSUE vs. BLOOD CONTACT WITH THE
ELECTRICALLY CONDUCTIVE SHELL % Tissue Power Time Lesion Lesion
Contact (Watts) (Sec.) Depth (cm) Width (cm) 5% 110 25 0.5 1.6 5%
110 60 1.2 2.4 41% 67 25 0.35 1.4 41% 67 60 0.9 2.0
[0181] The above Table 2 demonstrates the ability of the structure
carrying the shell 24 to transmit the proper amount of radio
frequency energy to create wide and shallow lesions. The effect is
achieved by controlling both the delivered radio frequency power
and the time of radio frequency energy application. Wide and
shallow lesion patterns are effective in the treatment of some
endocardially located substrates and atrial fibrillation
substrates.
[0182] Tables 1 and 2 demonstrate the capability of the same
expandable-collapsible electrode structure 20 with the desirable
lower percentage contact with tissue relative to blood (less than
50%) to ablate epicardial, intramural, or endocardial substrates
with a range of lesion patterns from wide and shallow to large and
deep.
[0183] B. Surface Deposition of Shell
[0184] The electrically conductive shell 24 may be deposited upon
the exterior of the formed expandable-collapsible body 22.
[0185] In this embodiment, a mask is placed upon the surface of the
expandable-collapsible body 22 that is to be free of the shell 24.
Preferably, as generally shown in FIG. 17, the shell 24 is not
deposited on at least the proximal 1/3rd surface of the
expandable-collapsible body 22. This requires that at least the
proximal 1/3rd surface of the expandable-collapsible body 22 be
masked, so that no electrically conductive material is deposited
there.
[0186] The masking of the at least proximal 1/3rd surface of the
expandable-collapsible body 22 is desirable for several reasons.
This region is not normally in contact with tissue, so the presence
of electrically conductive material serves no purpose. Furthermore,
this region also presents the smallest diameter. If electrically
conductive, this region would possess the greatest current density,
which is not desirable. Masking the proximal region of smallest
diameter, which is usually free of tissue contact, assures that the
maximum current density will be distributed at or near the distal
region of the expandable-collapsible body 22, which will be in
tissue contact. The presence of the steering mechanism 54, already
described, also aids in placing the shell-carrying distal tip in
tissue contact.
[0187] The shell 24 comprises a material having a relatively high
electrical conductivity, as well as a relative high thermal
conductivity. Materials possessing these characteristics include
gold, platinum, platinum/iridium, among others. These materials are
preferably deposited upon the unmasked, distal region of the
expandable-collapsible body 22. Usable deposition processes include
sputtering, vapor deposition, ion beam deposition, electroplating
over a deposited seed layer, or a combination of these
processes.
[0188] Preferably (see FIG. 17), to enhance adherence between the
expandable-collapsible body 22 and the shell 24, an undercoating 80
is first deposited on the unmasked distal region before depositing
the shell 24. Materials well suited for the undercoating 80 include
titanium, iridium, and nickel, or combinations or alloys
thereof.
[0189] The total thickness of the shell 24 deposition, including
the undercoating 80, can vary. Increasing the thickness increases
the current-carrying and thermal conductive capacity of the shell
24. However, increasing the thickness also increases the potential
of shell cracking or peeling during enlargement or collapse of the
underlying expandable-collapsible body 22.
[0190] In a preferred embodiment, the deposition of the
electrically conductive shell material should normally have a
thickness of between about 5 .mu.m and about 50 .mu.m. The
deposition of the adherence undercoating 80 should normally have a
thickness of about 1 .mu.m to about 5 .mu.m.
[0191] C. Foil Shell Surface
[0192] In an alternative embodiment (see FIG. 18), the shell 24
comprises a thin sheet or foil 82 of electrically conductive metal
affixed to the wall of the expandable-collapsible body 22.
Materials suitable for the foil include platinum, platinum/iridium,
stainless steel, gold, or combinations or alloys of these
materials. The foil 82 is shaped into a predetermined geometry
matching the geometry of the expandable-collapsible body 22, when
expanded, where the foil 82 is to be affixed. The geometry of the
metal foil 82 can be accomplished using cold forming or deep
drawing techniques. The foil 82 preferably has a thickness of less
than about 0.005 cm (50 .mu.m). The foil 82 is affixed to the
expandable-collapsible body 22 using an electrically insulating
epoxy, adhesive, or the like.
[0193] The shell 24 of foil 82 offers advantages over the deposited
shell 24. For example, adherence of the shell foil 82 upon the
expandable-collapsible body 22 can be achieved without using the
deposited undercoating 80. The shell foil 82 also aids in the
direct connection of ablation energy wires 26, without the use of
additional connection pads and the like, as will be described in
greater detail later. The shell foil 82 also offers greater
resistance to stretching and cracking in response to expansion and
collapse of the underlying expandable-collapsible body 22. This
offers greater control over resistance levels along the ablation
energy transmitting surface.
[0194] D. Co-Extruded Electrically Conductive Shell
[0195] In an alternative embodiment (see FIG. 19), all or a portion
of the expandable-collapsible wall forming the body 22 is extruded
with an electrically conductive material 84. Materials 84 suitable
for coextrusion with the expandable-collapsible body 22 include
carbon black and chopped carbon fiber. In this arrangement, the
coextruded expandable-collapsible body 22 is itself electrically
conductive. An additional shell 24 of electrically conductive
material can be electrically coupled to the coextruded body 22, to
obtain the desired electrical and thermal conductive
characteristics. The extra external shell 24 can be eliminated, if
the coextruded body 22 itself possesses the desired electrical and
thermal conductive characteristics.
[0196] The integral electrically conducting material 84 coextruded
into the body 22 offers certain advantages over the external
deposited shell 24 (FIG. 17) or shell foil 82 (FIG. 18).
Coextrusion avoids the necessity of adherence between the shell 24
and the expandable-collapsible body 22. A body 22 coextruded with
electrically conducting material 84 also permits more direct
connection of ablation energy wires 34, without the use of
additional connection pads and the like. The integrated nature of
the coextruded material 84 in the body 22 protects against cracking
of the ablation energy transmitting surface during expansion and
collapse of the expandable-collapsible body 22.
[0197] The integral electrically conducting material 84 coextruded
into the body 22 also permits the creation of a family of electrode
structures 20, with the structures 20 differing in the amount of
conductive material 84 coextruded into the wall of the respective
body 22. The amount of electrically conductive material coextruded
into a given body 22 affects the electrical conductivity, and thus
the electrical resistivity of the body 22, which varies inversely
with conductivity. Addition of more electrically conductive
material increases electrical conductivity of the body 22, thereby
reducing electrical resistivity of the body 22, and vice versa. It
is thereby possible to specify among the family of structures 20
having electrically conductive bodies 22, the use of a given
structure 20 according to a function that correlates desired lesion
characteristics with the electrical resistivity values of the
associated body 22.
EXAMPLE 2
[0198] A three-dimensional finite element model was created for an
electrode structure having a body with an elongated shape, with a
total length of 28.4 mm, a diameter of 6.4 mm, and a body wall
thickness of 0.1 mm. The body of the structure was modeled as an
electric conductor. Firm contact with cardiac tissue was assumed
along the entire length of the electrode body lying in a plane
beneath the electrode. Contact with blood was assumed along the
entire length of the electrode body lying in a plane above the
electrode. The blood and tissue regions had resistivities of 150
and 500 ohm.cm, respectively.
[0199] Analyses were made based upon resistivities of 1.2 k-ohm.cm
and 12 k-ohm.cm for the electrode body.
[0200] Table 3 shows the depth of the maximum tissue temperature
when RF ablation power is applied to the electrode at various power
levels and at various levels of resistivity for the body of the
electrode.
3TABLE 3 Depth of Maximum Maximum Resistivity of Tissue Tissue the
Body Power Time Temperature Temperature (k-ohm.multidot.cm) (Watts)
(Sec) (.degree. C.) (cm) 1.2 58 120 96.9 1.1 1.2 58 240 97.9 1.4 12
40 120 94.4 0.8 12 40 240 95.0 1.0
[0201] The electrode body with higher resistivity body was observed
to generate more uniform temperature profiles, compared to a
electrode body having the lower resistivity value. Due to
additional heating generated at the tissue-electrode body interface
with increased electrode body resistivity, less power was required
to reach same maximal temperature. The consequence was that the
lesion depth decreased.
[0202] Therefore, by specifying resistivity of the body 22, the
physician can significantly influence lesion geometry. The use of a
low-resistivity body 22 results in deeper lesions, and vice versa.
The following Table 4, based upon empirical data, demonstrates the
relationship between body resistivity and lesion depths.
4TABLE 4 Resistivity Power Temperature Lesion Depth
(ohm.multidot.cm) (Watts) (.degree. C.) (cm) Time (sec) 850 94 97
1.2 120 1200 58 97 1.1 120 12,000 40 95 0.8 120
[0203] E. shell Patterns
[0204] When it is expected that ablation will occur with the distal
region of body 22 oriented in end-on contact with tissue, the shell
24 should, of course, be oriented about the distal tip of the
expandable-collapsible body 22. For this end-on orientation, the
shell 24 may comprise a continuous cap deposited upon the distal
1/3rd to 1/2 of the body 22, as FIG. 17 shows. However, when distal
contact with tissue is contemplated, the preferred embodiment (see
FIG. 20) segments the electrically conductive shell 24 into
separate energy transmission zones 122 arranged in a concentric
"bull's-eye" pattern about the distal tip of the body 22.
[0205] The concentric bull's-eye zones 122 are formed by masking
axially spaced bands on the distal region of the body 22, to
thereby segment the deposit of the electrically conductive shell 24
into the concentric zones 122. Alternatively, preformed foil shells
82 can be applied in axially spaced bands on the distal region to
form the segmented energy transmitting zones 122.
[0206] When it is expected that ablation will occur with the side
region of the body 22 oriented in contact with tissue, the shell 24
is preferably segmented into axially elongated energy transmission
zones 122, which are circumferentially spaced about the distal
1/3rd to 1/2 of the body 22 (see FIGS. 21 and 22).
[0207] The circumferentially spaced zones 122 are formed by masking
circumferentially spaced areas of the distal region of the body 22,
to thereby segment the deposit of the electrically conductive shell
24 into the zones 122. Alternatively, preformed foil shells 82 can
be applied in circumferentially spaced-apart relationship on the
distal region to form the segmented energy transmitting zones 122.
Still alternatively, the circumferentially segmented energy
transmission zones 122 may take the form of semi-rigid pads carried
by the expandable-collapsible body 22. Adjacent pads overlap each
other when the body 22 is in its collapsed geometry. As the body 22
assumes its expanded geometry, the pads spread apart in a
circumferential pattern on the body 22.
[0208] Preferably, regardless of the orientation of the zones 122
(bull's-eye or circumferential), each energy transmission zone 122
is coupled to a dedicated signal wire 26 or a dedicated set of
signal wires 26. This will be described later in greater detail. In
this arrangement, the controller 32 can direct ablation energy
differently to each zone 122 according to prescribed criteria, as
will also be described in greater detail later.
[0209] The above describes the placement of a shell 24 on the
exterior of the body 22. It should be appreciated that electrically
conductive material can be deposited or otherwise affixed to the
interior of the body 22. For example (as FIG. 44 shows), the
interior surface of the body 22 can carry electrodes 402 suitable
for unipolar or bipolar sensing or pacing. Different electrode
placements can be used for unipolar or bipolar sensing or pacing.
For example, pairs of 2-mm length and 1-mm width electrodes 402 can
be deposited on the interior surface of the body 22. Connection
wires 404 can be attached to these electrodes 100. Preferably the
interelectrode distance is about 1 mm to insure good quality
bipolar electrograms. Preferred placements of these interior
electrodes are at the distal tip and center of the body 22. Also,
when multiple zones are used, it is desired to have the electrodes
402 placed in between the ablation regions.
[0210] It is also preferred to deposit opaque markers 406 on the
interior surface of the body 22 so that the physician can guide the
device under fluoroscopy to the targeted site. Any high-atomic
weight material is suitable for this purpose. For example,
platinum, platinum-iridium. can be used to build the markers 406.
Preferred placements of these markers 106 are at the distal tip and
center of the structure 22.
[0211] P. Folding Segmented Shells
[0212] As FIGS. 21 and 22 show, segmented energy transmitting zones
122 are well suited for use in association with folding
expandable-collapsible bodies 22, as previously described in
connection with FIGS. 11A/B/C. In this arrangement, the regions
that are masked before deposition of the electrical conductive
shell comprise the folding regions 52. In this way, the regions 52
of the expandable-collapsible body 22 that are subject to folding
and collapse are those that do not carry an electrically conductive
shell 24. The electrically conductive shell 24 is thereby protected
against folding and stretching forces, which would cause creasing
and current interruptions, or increases in resistance, thereby
affecting local current densities and temperature conditions.
[0213] The selective deposition of the shell 24 in segmented
patterns can itself establish predefined fold lines 52 on the body
22, without special molding of preformed regions of the body 22 (as
FIGS. 11A/B/C contemplate). As FIGS. 23, 24A and 24B show, by
controlling the parameters by which the shell segments 122 are
deposited, predefined fold lines 52 can be created at the boarders
between the shell segments 122. These fold lines 52 are created due
to the difference in thickness between adjacent regions which are
coated with the shell 24 and those which are not.
[0214] More particularly, as FIGS. 23, 24A and 24B show, the region
between segmented shell coatings will establish a fold line 52,
when the distance between the coatings (designated x in FIGS. 24A
and B) is greater than or equal to twice the thickness of the
adjacent shell coatings 122 (designated t in FIGS. 24A and B)
divided by the tangent of one half the minimum selected fold angle
(designated .alpha..sub.MIN in FIG. 24A). This fold line
relationship is mathematically expressed as follows: 4 x 2 t Tan
MIN
[0215] The minimum selected fold angle 2.alpha..sub.MIN can vary
according to the profile of the body 22 desired when in the
collapsed geometry. Preferably, the minimum fold angle
2.alpha..sub.MIN is in the range of 1.degree. to 5.degree..
[0216] In this arrangement (see FIG. 23), the fold lines 52 created
by controlled deposition of shell segments lie uniformly along
(i.e., parallel to) the long axis of the body 22 (designated 170 in
FIG. 23).
[0217] The uncoated fold lines 52 created at the boarders between
the thicker coated shell segments 122 can also be characterized in
terms of relative electrical resistivity values. The coated
segments 122 of electrically conductive material possess higher
electrical conductivity than the uncoated fold lines 52. The
resistivity of the fold lines 52, which varies inversely with
conductivity, is thereby higher than the resistivity of the
segments 122. To achieve the desired folding effect due to
differential coating, the region in which folding occurs should
have a resistivity that is greater than about ten times the
resistivity of the segments 122 carrying electrically conductive
material.
[0218] IV. Electrical Connection to Shell
[0219] It is necessary to electrically connect the shell 24 (or
other ablation energy transmitting material 84) to the radio
frequency energy generator 30 using the one or more signal wires
26. As before described, these signal wires 26, electrically
connected to the shell 24, extend between the body 22 and the
external connectors 28 through the catheter tube 12.
[0220] The connection between the signal wires 26 and the shell 24,
whether deposited, foil layered, or coextruded, must remain intact
and free of open circuits as the expandable-collapsible body 22 and
shell 24 change geometries.
[0221] The electrical connection is preferably oriented proximate
to the geometric center of the pattern that the associated ablation
zone 122 defines. As FIGS. 25 to 27 show, the geometric center
(designated GC) varies depending upon whether the zone 122
comprises a cap pattern (as FIG. 25 shows), or a circumferential
segment pattern (as FIG. 26 shows), or a circumferential band or
bull's-eye pattern (as FIG. 27 shows). At least one electrical
connection should be present proximate to the respective geometric
center of the pattern. This ensures that maximum current density is
distributed about the geometric center of the zone and that similar
current densities are distributed at the edges of the pattern.
[0222] Regardless of the shape of the pattern, additional
electrical connections are preferably made in each ablation zone.
In the case of a cap pattern or a segment pattern, the additional
electrical connections (designated AC in, respectively, FIGS. 25
and 26) are distributed uniformly about the geometric center. In
the case of a circumferential band of a bull's-eye pattern, the
additional electrical connections (designated ACG in FIG. 27) are
distributed uniformly along the arc along which the geometric
center of the band lies.
[0223] Multiple electrical connections, at least one of which
occurs proximate to the geometric center, provide more uniform
current density distribution in the zone. These multiple
connections are especially needed when the resistivity of the shell
24 or of the corresponding patterns is high. These connections
prevent inefficient RF energy delivery due to RF voltage drops
along parts of the shell 24 or the corresponding patterns.
[0224] In a preferred embodiment of the cap or bull's-eye pattern
(see FIGS. 28A and 28B), multiple signal wires 26 are lead through
the interior of the body 22 and out through a center aperture 74 in
the distal fixture 66. Multiple signal wires 26 are preferred, as
multiple electrical connections provide a more uniform current
density distribution on the shell 24 than a single connection.
[0225] The signal wires 26 are enclosed within electrical
insulation 160 (see FIG. 28B) except for their distal ends. There,
the electrical insulation 160 is removed to exposed the electrical
conductor 162. The exposed electrical conductor 162 is also
preferably flattened by mechanical means to provide an increased
surface area. The flattened conductors 162 are affixed by an
electrically conductive adhesive proximate to the geometric center
and elsewhere at additional uniformly spaced intervals about it on
the cap pattern, as well as along the geometric center of the
concentric bands of the bull's-eye pattern, which the shell 24,
when deposited, will create.
[0226] It is preferred that the adhesive connections of the
conductors 162 to the body 22 be positioned, when possible,
relatively close to an established support area on body 22, such as
provided by the distal fixture 66. The support that the fixture 66
provides a more secure attachment area for the electrical
connections.
[0227] After the electrical connections are made, the shell 24 is
deposited in the desired pattern on the body 22, over the
adhesively attached conductors 162, in a manner previously
described. The center aperture 74 in the distal fixture 66 is
sealed closed by adhesive or equivalent material.
[0228] In an alternative embodiment (as FIGS. 29A/B show), the
distal fixture 66 can also be used to create a mechanical
connection to electrically couple a single signal wire 26 to the
geometric center of the cap of the bull's-eye pattern. In the
arrangement, the fixture 66 is made from an electrically conductive
material. As FIGS. 29A/B show, the signal wire 26 is connected by
spot welding, soldering, or electrically conductive adhesive to the
fixture 66 within the expandable-collapsible body 22. A nut 74
engaging a threaded fixture end 164 sandwiches the distal tip of
the body 22 between it and the collar 68 (see FIG. 29B). Epoxy,
which could be electrically conductive, could be used to further
strengthen the mechanical connection between the nut 74 and the
body 22 sandwiched beneath it. The shell 24 is next deposited on
the body 22 and nut 74 in a manner previously described.
[0229] Alternatively, the shell 24 can be deposited on the body 22
before attachment of the nut 74. In this arrangement, the nut 74
sandwiches the shell 24 between it and the collar 68, mechanically
establishing the desired electrical connection between the signal
wire 26 and the shell 24.
[0230] Alternatively, instead of a threaded nut connection, a heat
shrunk slip ring of nickel titanium material can be used..
Essentially, any riveting, swagging, electrically conductive
plating, or bonding technique can be used to hold the shell 24 in
contact against the collar 68.
[0231] It should be appreciated that additional solid fixtures 66
and associated electrical connection techniques can be used in
other regions of the shell 22 distant from the distal tip of the
body 22 to establish electrical contact in the circumferential
bands of the bull's-eye pattern or proximate the geometric center
and elsewhere on the circumferential segments. However, electrical
connections can be made in these regions without using fixtures 66
or equivalent structural elements.
[0232] For example, as FIG. 30 shows, insulated signal wires 26
passed into the interior of the body can be snaked through the body
22 at the desired point of electrical connection. As before
described, the electrical insulation 160 of the distal end of the
snaked-through wire 26 is removed to exposed the electrical
conductor 162, which is also preferably flattened. As also before
described, the flattened conductors 162 are affixed by an
electrically conductive adhesive 172 to body 22, over which the
shell 24 is deposited. Adhesive 172 is also preferable applied in
the region of the body 22 where the wire 26 passes to seal it. As
FIG. 31 shows, the same signal wire 26 can be snaked through the
body 22 multiple times to establish multiple electrical connections
within the same ablation zone.
[0233] In conjunction with any ablation zone pattern (see FIG. 32),
the expandable-collapsible body 22 can be formed as a laminate
structure 90. The laminate structure 90 comprises a base layer 92,
formed from an electrically insulating material which peripherally
surrounds the interior of the body- 22. The laminate structure 90
further includes one or more intermediate layers 94 formed on the
base layer 92. An ablation energy wire 26 passes through each
intermediate layer 94. Each intermediate layer 94 is itself bounded
by a layer 96 of electrically insulating material, so that the
wires 26 are electrically insulated from each other. The laminate
structure 90 also includes an outer layer 98 which is likewise
formed from an electrically insulating material.
[0234] The laminate structure 90 can be formed by successively
dipping a mold having the desired geometry in a substrate solution
of electrically insulating material. The ablation energy wires 26
are placed on substrate layers between successive dippings, held in
place by electrically conductive adhesive or the like.
[0235] After molding the laminated structure 90 into the desired
geometry, one or more windows 100 are opened through the outer
insulation layer 98 in the region which the electrically conductive
shell 24 will occupy. Each window 100 exposes an ablation energy
signal wire 26 in a chosen layer.
[0236] Various windowing techniques can be employed for this
purpose. For example, CO.sub.2 laser, Eximer laser, YAG laser, high
power YAG laser, or other heating techniques can be used to remove
insulation to the desired layer and thereby expose the desired
signal wire 26.
[0237] After windowing, the formed expandable-collapsible body 22
is masked, as before described. The shell 24 of electrically
conductive material is deposited over the unmasked area, including
the windows 100, which have been previously opened.
[0238] As FIG. 32 shows, the deposited shell 24 enters the windows
100, making electrically conductive contact with the exposed wires
26. A plating or other deposition process may be used in the window
100, before depositing the electrically conductive shell 24. The
plating f ills in the window 100 to assure good electrical contact
with the over-deposit of shell 24.
[0239] FIG. 3D shows an alternative equivalent laminated structure,
in which the chamber 124 occupies the interior of the body 22. This
creates a multiple layer structure equivalent to the laminated
structure just described. An open intermediate layer 126 exists
between the interior of the body 22 and the exterior of the chamber
124, through which signal wires 26 can be passed for electrical
connection to the shell 24. The electrical connection can be made
using either a distal fixture 66 or by snaking the wires through
the exterior body 22 (as FIG. 3D shows), both of which have already
been described.
[0240] V. Temperature sensing
[0241] A. Connection of Temperature Sensors
[0242] As before described (see FIG. 1), a controller 32 preferably
governs the conveyance of radio frequency ablation energy from the
generator 30 to the shell 24. In the preferred embodiment, the
collapsible electrode structure 20 carries one or more temperature
sensing elements 104, which are coupled to the controller 32.
Temperatures sensed by the temperature sensing elements 104 are
processed by the controller 32. Based upon temperature input, the
controller adjusts the time and power level of radio frequency
energy transmissions by the shell 24, to achieve the desired lesion
patterns and other ablation objectives.
[0243] The temperature sensing elements 104 can take the form of
thermistors, thermocouples, or the equivalent. A temperature
sensing element 104 may be located within the distal fixture 66 to
sense temperature at the distal tip, as FIG. 33 shows.
Alternatively, multiple temperature sensing elements may be
scattered at spaced apart locations on the shell 24 or
expandable-collapsible body 22, as FIG. 34 shows.
[0244] The connection of temperature sensing elements 104 to the
shell 24 or expandable-collapsible body 22 can be achieved in
various ways.
[0245] As shown in FIG. 34, when the expandable-collapsible body 22
comprises a thermal conductive material, the temperature sensing
element (designated 104A in FIG. 34) can be attached to the
interior surface of the body 22 in the region where measurement of
exterior surface temperature is desired. A thermally conductive,
but electrically insulating adhesive 106 can be used to secure the
temperature sensing element 104A to the inside of the body 22. The
temperature sensing element wires 110 extend through the catheter
tube 12 for coupling (using a suitable connector 28, shown in FIG.
1)to the controller 32.
[0246] Alternatively, the temperature sensing element (designated
104B and 104C in FIG. 34) can be attached to the exterior surface
of the body 22 in the region where measurement of temperatures is
desired. As just described, a thermally conductive, but
electrically insulating adhesive 106 can be used to secure the
temperature sensing element to the outside of the body 22.
[0247] As shown with element 104B, the electrically conductive
shell 24 can be deposited over the temperature sensing element
104B, in the manner previously described. In this way, the
temperature sensing element 104B resides under the electrically
conductive shell 24, and no discontinuities in the shell 24 are
present.
[0248] Alternatively, as shown with element 104C, the element 104C
can be masked at the time the electrically conductive shell 24 is
deposited. In this arrangement, there is no electrically conductive
material over the temperature sensing element 104C.
[0249] The signal wires 110 attached to the temperature sensing
element 104C can be attached by electrically insulating adhesive to
the outside of the expandable-collapsible body 22. Alternatively,
as shown by element 104B, the signal wires 110 can be brought from
the interior of the expandable-collapsible body 22 through the
expandable-collapsible body 22 for attachment by a thermally
conductive, but electrically insulating adhesive 106 to the outside
of the body 22. The same type of adhesive 106 can also be used to
anchor in signal wires 110 to the inside of the
expandable-collapsible body 22.
[0250] As shown in FIG. 35, temperature sensing thermocouples 112
may also be integrally formed by deposition on the
expandable-collapsible body. In this embodiment, the body 22
comprises a laminated structure 114, like that previously shown in
FIG. 31, comprising a base layer 92, an outer layer 98, and one or
more intermediate layers 94. In the laminate structure 114, the
intermediate layers 94 formed in this structure thermocouple wires
116 (t-type or other combinations). Before depositing the
electrically conductive shell 24, windowing of the laminated
expandable-collapsible body 116 in the manner previously described
exposes the thermocouple wires. A conducting material 118, which,
for a t-type thermocouple is copper or constantan, is deposited
over the exposed thermocouple wires, forming the thermocouple 112.
An electrically insulating material 120, like aluminum oxide or
silicon dioxide, is then applied over the thermocouple 112.
[0251] The electrically conducting shell 24 can be deposited over
the formed thermocouple 112. In this way, the thermocouples reside
under the electrically conductive shell 24, and no discontinuities
in the shell 24 are present. Alternatively, as thermocouple 112A
shows in FIG. 35, the thermocouple 112A can be masked at the time
the electrically conductive shell 24 is deposited. In this
arrangement, there is no electrically conductive material over the
thermocouple 112A.
[0252] B. Location of Temperature Sensing Elements
[0253] Preferably, as FIGS. 20 and 21A/B show, multiple temperature
sensing elements 104 are located on and about the shell 24 to
ascertain temperature conditions during radio frequency energy
ablation. The controller 32 uses temperature information from
temperature sensing elements to control the transmission of
ablation energy by the shell 24.
[0254] Generally speaking, at least one temperature sensing element
104 is preferably placed proximal to the geometric center of the
energy transmitting shell 24. When the shell 24 is segmented (as
FIGS. 20 and 21A/B show), at least one temperature sensing element
104 should be proximal to the geometric center of each energy
transmitting segment 122.
[0255] Preferably, as FIGS. 20 and 21A/B further show, temperature
sensing elements 104 are also placed along the edges of the shell
24, where it adjoins a masked, electrically non-conductive region
of the body 22. When the shell 24 is segmented, temperature sensing
elements 104 should be placed along the edge of each energy
transmitting segment 122. High current densities occur along these
regions where energy transmitting material adjoins non-energy
transmitting material. These edge effects lead to higher
temperatures at the edges than elsewhere on the shell 24. Placing
temperature sensing elements 104 along the edges assures that the
hottest temperature conditions are sensed.
[0256] In the case of a shell 24 segmented into adjacent energy
transmitting zones 122, it is also desirable to place at least one
temperature sensing element 104 between adjacent energy
transmitting zones, as FIGS. 20 and 21A/B show. Placing multiple
temperature sensing elements 104 in the segments 122, between the
segments 122, and along the edges of the segments 122 allows the
controller 32 to best govern power distribution to the multiple
segments 122 based upon predictions of hottest temperature
conditions. Further details of the use of multiple temperature
sensing elements, including edge temperature sensing elements, and
the use of temperature prediction methodologies, are found in
copending U.S. patent application Ser. No. 08/439,824, filed May
12, 1995, and entitled "Systems and Methods for Controlling Tissue
Ablation Using Multiple Temperature Sensing Elements."
[0257] The presence of segmented energy transmission zones 122,
each with its own prescribed placement of temperature sensing
elements 104, allows the controller 32 to govern the delivery of
power to each zone 122 separately. The controller 32 is thereby
able to take into account and react to differences in convective
cooling effects in each zone 122 due to blood flow, differences in
contact pressure and surface area between each zone 122 and the
tissue its contacts, and other nonlinear factors affecting the
power required to heat tissue adjacent each zone 122 to a
predetermined temperature to achieve the desired lesion geometry
and pattern.
[0258] Thus, whereas for any given transmission zone (like the
continuous, non-segmented shell 24 shown in FIG. 25 or each
segmented zone 122 shown in FIGS. 26 and 27), it is desirable to
allow some contact with the blood pool to allow beneficial
convective cooling effects, it is not desirable that any given zone
contact only or substantially only the blood pool. Loss of power
into the blood pool with no tissue ablation effects occurs. With
segmented zones 122, it is possible to sense, using the temperature
sensing elements 104, where insubstantial tissue contact exists. It
is thereby possible to sense and to channel available power only to
those zones 122 where substantial tissue contact exists. Further
details of tissue ablation using segmented electrode structures are
disclosed in copending U.S. patent application Ser. No. 08/139,304,
filed Oct. 19, 1993 and entitled "Systems and Methods for Creating
Lesions in Body Tissue Using Segmented Electrode Assemblies."
[0259] Further details of the use of multiple ablation energy
transmitters controlled using multiple temperature sensing elements
are disclosed in copending U.S. patent application Ser. No.
08/286,930, filed Aug. 8, 1994, and entitled "Systems and Methods
for Controlling Tissue Ablation Using Multiple Temperature Sensing
Elements".
[0260] FIG. 36 shows a preferred representative embodiment when the
shell 24 comprises a continuous cap pattern. In this arrangement,
the structure 20 carries five temperature sensing elements 104
spaced apart on the shell 24. The temperature sensing elements 104
are connected in a selected one or more of the manners previously
described.
[0261] Preferably, sensing elements Tn.sub.1 and Tn.sub.2 are
placed at diametrically opposite regions at the most proximal edge
of the shell 24. Sensing elements Tm.sub.1 and Tm.sub.2 are placed
at diametrical sides of the middle region of the shell 24, for
example, at about 50% of the radius of the structure. The sensor Tc
is placed proximal the geometric center of the shell 24. All
temperature sensors are coupled to a temperature controller, which
processes information from the sensors.
[0262] In this arrangement, the temperature controller 32 infers
the percentage of tissue contact with the shell 24 contact based
upon where significant increases in temperature conditions from an
established baseline level (for example, 37.degree. C.) are sensed
on the shell 24. These increased temperature conditions indicate
the absence of convective cooling effects, as would occur with
contact with the blood pool, thereby suggesting tissue contact. As
the preceding Tables 1 and 2 show, percentage of contact between
the shell 24 and tissue dictate effective power levels to achieve
the type of lesion desired.
[0263] The relationship between percentage shell-tissue contact and
power desired for a given lesion characteristic can be based upon
empirical or theoretical data in the manner set forth in the
preceding Example. These relationships can be set forth in look up
table format or incorporated in equivalent decision matrices, which
the controller 32 retains in memory.
[0264] For example, if large deep lesions are desired, significant
increase in temperature above the baseline at Tc, but not
elsewhere, indicates a 20% tissue contact condition, and a first
power level is commanded for the generator 30 based upon the
selected power criteria. Significant increase in temperature above
the baseline also at Tm.sub.1 and Tm.sub.2 indicates a 50% tissue
contact condition, and second power level less than the first is
commanded for the generator 30 based upon the selected power
criteria. Significant increase in temperature above the baseline
also at Tn.sub.1 and Tn.sub.2 indicates a 100% tissue contact
condition, and third power level less than the second is commanded
based upon the selected power criteria.
[0265] FIG. 37 shows a preferred representative embodiment when the
shell 24 comprises a circumferentially spaced, segmented pattern.
In this arrangement, the structure 20 carries at least four
temperature sensing elements on each shell segment.
[0266] The sensor Tc is common to all segments and is located at
the distal end of the pattern. The sensor T.sub.GC is located at
the geometric center of each segment, while the sensors T.sub.E1
and T.sub.E2 are located along opposite edges of each segment,
where the shell 24 adjoins the non-electrically conductive regions
separating the segments. An additional sensor T.sub.M is preferably
also located generally between the segments for the reasons
discussed before.
[0267] FIG. 38 shows a preferred representative embodiment when the
shell 24 comprises a bull's-eye pattern. Sensors T.sub.GC are
located at the geometric center of each segment of the pattern,
while the sensors T.sub.E1 and T.sub.E2 are located along opposite
edges of each segment, where the shell 24 adjoins the
non-electrically conductive regions separating the segments. An
additional sensor T.sub.M is preferably also located generally
between the segments for the reasons discussed before.
[0268] VI. Active Cooling
[0269] The capability of the shell 24 to form large lesions can be
enhanced by actively cooling the shell 24 while transmitting
ablation energy.
[0270] Active cooling can be accomplished by the use of multiple
lumens to cycle a cooled fluid through the expandable-collapsible
body 22 while transmitting ablation energy. Alternatively, a high
pressure gas can be transported by the lumens for expansion within
the expandable-collapsible body to achieve a comparable active
cooling effect. In yet another alternative arrangement, the cooled
medium can be conveyed outside the expandable-collapsible body 22
to achieve an active cooling effect.
[0271] With active cooling, more power can be applied, while
maintaining the same maximum tissue temperature conditions, thereby
creating larger and deeper lesions. With active cooling, the
percentage contact of the shell 24 with tissue relative to blood
can be increased above 50%, and may be as much as 100%.
[0272] Further details concerning the use of active cooling to
enhance lesion formation are found in copending U.S. patent
application Ser. No. 08/431,790, filed May 1, 1995, and entitled
"Systems and Methods for obtaining Desired Lesion Characteristics
While Ablating Body Tissue".
[0273] It should be appreciated that the entire surface of the
shell 24 need not be cooled to achieve at least some of the
benefits of active cooling. For example, only selected regions of
the shell 24 which are prone to localized edge heating effects, as
previously described, can be subjected to active cooling. The edge
effects on current densities occur at the boundary between the
shell 24 and expandable-collapsible body 22 that is free of the
shell 24 create higher temperatures. Localized cooling of these
edge regions can help minimize the effects of hot spots on lesion
formation.
[0274] In this arrangement, as FIG. 39 shows, a pattern of small
holes 174 is created in the region between segmented shell patterns
122. Liquid cooling medium is perfused from inside the body 22
through the holes 174 to provide localized cooling adjacent the
edges of the shell segments 122. It should be appreciated that hole
patterns 174 could be used elsewhere on the body 22 to provide
active cooling effects.
[0275] As FIGS. 39, 40A and 40B show, the selective establishment
of hole patterns 174 on the body 22 can also itself establish
predefined fold lines 52, eliminating the need to specially mold
preformed folding regions the body 22. The pattern of small holes
174 create fold lines 52 by the removal of material, thereby
increasing the flexibility of the body 22 along the holes 174
between adjacent regions 122. In this arrangement (see FIG. 39),
the fold lines 52 created by hole patterns 174 lie uniformly along
(i.e., parallel to) the long axis of the body 22.
[0276] VII. Obtaining Desired Lesion Characteristics
[0277] As Tables 1 and 2 in the foregoing Example demonstrate, the
same expandable-collapsible electrode structure 20 is able to
selectively form lesions that are either wide and shallow or large
and deep. Various methodologies can be used to control the
application of radio frequency energy to the shell 24 of the body
20 to achieve this result.
[0278] A. D.sub.50C Function
[0279] In one representative embodiment, the controller 32 includes
an input 300 for receiving from the physician a desired therapeutic
result in terms of (i) the extent to which the desired lesion
should extend beneath the tissue-electrode interface to a boundary
depth between viable and nonviable tissue and/or (ii) a maximum
tissue temperature developed within the lesion between the
tissue-electrode interface and the boundary depth.
[0280] The controller 32 also includes a processing element 302,
which retains a function that correlates an observed relationship
among lesion boundary depth, ablation power level, ablation time,
actual or predicted sub-surface tissue temperature, and electrode
temperature. The processing element 302 compares the desired
therapeutic result to the function and selects an operating
condition based upon the comparison to achieve the desired
therapeutic result without exceeding a prescribed actual or
predicted sub-surface tissue temperature.
[0281] The operating condition selected by the processing element
302 can control various aspects of the ablation procedure, such as
controlling the ablation power level, the rate at which the
structure 20 is actively cooled, limiting the ablation time to a
selected targeted ablation time, limiting the ablation power level
subject to a prescribed maximum ablation power level, and/or the
orientation of the shell 24, including prescribing a desired
percentage contact between the shell 24 and tissue. The processing
element 302 can rely upon temperature sensors carried by or
otherwise associated with the expandable-collapsible structure 20
that penetrate the tissue to sense actual maximum tissue
temperature. Alternatively, the processing element 302 can predict
maximum tissue temperature based upon operating conditions.
[0282] In the preferred embodiment, the electrode structure 20
carries at least one temperature sensing element 104 to sense
instantaneous localized temperatures (T1) of the thermal mass of
the shell 24. The temperature T1 at any given time is a function of
the power supplied to the shell 24 by the generator 30 and the rate
at which the shell 24 is cooled, either by convective cooling by
the blood pool, or active cooling by another cooling medium brought
into contact with the shell 24, or both.
[0283] The characteristic of a lesion can be expressed in terms of
the depth below the tissue surface of the 50.degree. C. isothermal
region, which will be called D.sub.50C. The depth D.sub.50C is a
function of the physical characteristics of the shell 24 (that is,
its electrical and thermal conductivities and size); the percentage
of contact between the tissue and the shell 24; the localized
temperature T1 of the thermal mass of the shell 24; the magnitude
of RF power (P) transmitted by the shell 24 into the tissue, and
the time (t) the tissue is exposed to the RF power.
[0284] For a desired lesion depth D50C, additional considerations
of safety constrain the selection of an optimal operating condition
among the operating conditions listed in the matrix. The principal
safety constraints are the maximum tissue temperature TMAX and
maximum power level PMAX.
[0285] The maximum temperature condition TMAX lies within a range
of temperatures which are high enough to provide deep and wide
lesions (typically between about 85.degree. C. and 95.degree. C.),
but which are safely below about 100.degree. C., at which tissue
desiccation or tissue micro-explosions are known to occur. It is
recognized that TMAX will occur a distance below the
electrode-tissue interface between the interface and D.sub.50C.
[0286] The maximum power level PMAX takes into account the physical
characteristics of the electrode and the power generation capacity
of the RF generator 30.
[0287] These relationships can be observed empirically and/or by
computer modeling under controlled real and simulated conditions,
as the foregoing examples illustrate. The D50C function for a given
shell 24 can be expressed in terms of a matrix listing all or some
of the foregoing values and their relationship derived from
empirical data and/or computer modeling.
[0288] The processing element 302 includes in memory this matrix of
operating conditions defining the D50C temperature boundary
function, as described above for t=120 seconds and TMAX=95.degree.
C. and for an array of other operating conditions.
[0289] The physician also uses the input 300 to identify the
characteristics of the structure 20, using a prescribed
identification code; set a desired maximum RF power level PMAX; a
desired time t; and a desired maximum tissue temperature TMAX.
[0290] Based upon these inputs, the processing element 302 compares
the desired therapeutic result to the function defined in the
matrix (as exemplified by the above Tables 1 and 2). The master
controller 58 selects an operating condition to achieve the desired
therapeutic result without exceeding the prescribed TMAX by
controlling the function variables.
[0291] This arrangement thereby permits the physician, in effect,
to "dial-a-lesion" by specifying a desired D.sub.50C. By taking
into account the effects of convective cooling based upon
percentage of shell contact with tissue and by using active cooling
in association with time and power control, the processing element
can achieves the desired D.sub.50C without the need to sense actual
tissue temperature conditions.
[0292] Further details of deriving the D.sub.50C function and its
use in obtaining a desired lesion pattern are found in copending
U.S. application Ser. No. 08/431,790, filed May 1, 1995, entitled
"Systems and Methods for Obtaining Desired Lesion Characteristics
While Ablating Body Tissue," which is incorporated herein by
reference.
[0293] B. Predicting Maximum Tissue Temperature
[0294] The structure 20 is cooled either by convective blood flow
(depending upon percentage contact between the shell 24 and
tissue), or by actively using another cooling medium, or both. The
level of RF power delivered to the cooled structure 20 and/or the
cooling rate can be adjusted based upon a prediction of
instantaneous maximum tissue temperature, which is designated
.PSI..sub.MAX (t).
[0295] In a preferred implementation, the prediction of
.PSI..sub.MAX is derived by a neural network, which has as inputs a
prescribed number of previous power levels, previous rates at which
heat has been removed to cool the structure 20, and previous shell
temperature.
[0296] The heat removal rate is identified by the expression .ANG.,
where
.ANG.=c.times..DELTA.T.times.RATE
[0297] where:
[0298] c is the heat capacity of the cooling medium used (in Joules
(J) per kilogram (kg) Kelvin (K), or J/kg K)
[0299] .DELTA.T is the temperature drop in the cooling medium
during passing through the structure 20 (K), and
[0300] RATE is the mass flow rate of the cooling medium through the
structure (kg/sec).
[0301] The heat generated by the structure 20 into the tissue is
the difference between the heat generated by Joule effect and the
heat removed by cooling. At a given localized shell temperature T1
and flow rate of cooling medium, the magnitude of .ANG. increases
as RF power delivered to the shell 24 increases. Together, T1 and
.ANG. represent an indirect measurement of how rapidly the
sub-surface tissue temperature is changing. Together, T1 and .ANG.
are therefore predictive of the depth and magnitude of the hottest
sub-surface tissue temperature .PSI..sub.MAX, and thus indirectly
predictive of the lesion boundary depth D.sub.50C. Large deep
lesions are predicted when T1 is maintained at a low relative
temperature (by controlling cooling rate) and the maximal predicted
tissue temperature, TMAX, is maintained at approximately 85.degree.
C. to 95.degree. C. by controlling RF power. Likewise, more shallow
lesions are predicted when T1 is maintained at a high relative
temperature and TMAX is maintained at lower values.
[0302] Further details of deriving the this prediction function and
its use in obtaining a desired lesion pattern are found in
copending U.S. application Ser. No. 08/431,790, filed May 1, 1995,
entitled "Systems and Methods for Obtaining Desired Lesion
Characteristics While Ablating Body Tissue," which is incorporated
herein by reference.
[0303] C. Segmented Shells: Duty Cycle Control
[0304] Various RF energy control schemes can also be used in
conjunction with segmented shell patterns shown in FIG. 20 (the
axially spaced, bull's-eye pattern of zones) and FIGS. 21 and 22
(the circumferentially spaced zones). For the purpose of
discussion, the zones (which will also be called electrode regions)
122 will be symbolically designated E(J), where J represents a
given zone 122 (J=1 to N).
[0305] As before described, each electrode region E(J) has at least
one temperature sensing element 104, which will be designated
S(J,K), where J represents the zone and K represents the number of
temperature sensing elements on each zone (K=1 to M).
[0306] In this mode, the generator 30 is conditioned through an
appropriated power switch interface to deliver RF power in multiple
pulses of duty cycle 1/N.
[0307] With pulsed power delivery, the amount of power (P.sub.E(J))
conveyed to each individual electrode region E(J) is expressed as
follows:
P.sub.E(J).alpha.AMP.sub.E(J).sup.2.times.DUTYCYCLE.sub.E(J)
[0308] where:
[0309] AMP.sub.E(J) is the amplitude of the RF voltage conveyed to
the electrode region E(J), and
[0310] DUTYCYCLE.sub.E(J) is the duty cycle of the pulse, expressed
as follows: 5 DUTYCYCLE E ( J ) = TON E ( J ) TON E ( J ) + TOFF E
( J )
[0311] where:
[0312] TON.sub.E(J) is the time that the electrode region E(J)
emits energy during each pulse period,
[0313] TOFF.sub.E(J) is the time that the electrode region E(J)
does not emit energy during each pulse period.
[0314] The expression TON.sub.E(J)+TOFF.sub.E(J) represents the
period of the pulse for each electrode region E(J).
[0315] In this mode, the generator 30 can collectively establish
duty cycle (DUTYCYCLE.sub.E(J)) of 1/N for each electrode region (N
being equal to the number of electrode regions).
[0316] The generator 30 may sequence successive power pulses to
adjacent electrode regions so that the end of the duty cycle for
the preceding pulse overlaps slightly with the beginning of the
duty cycle for the next pulse. This overlap in pulse duty cycles
assures that the generator 30 applies power continuously, with no
periods of interruption caused by open circuits during pulse
switching between successive electrode regions.
[0317] In this mode, the temperature controller 32 makes individual
adjustments to the amplitude of the RF voltage for each electrode
region (AMP.sub.E(J)), thereby individually changing the power
P.sub.E(J) of ablating energy conveyed during the duty cycle to
each electrode region, as controlled by the generator 30.
[0318] In this mode, the generator 30 cycles in successive data
acquisition sample periods. During each sample period, the
generator 30 selects individual sensors S(J,K), and temperature
codes TEMP(J) (highest of S(J,K)) sensed by the sensing elements
104, as outputted by the controller 32.
[0319] When there is more than one sensing element associated with
a given electrode region (for example, when edge-located sensing
elements are used, the controller 32 registers all sensed
temperatures for the given electrode region and selects among these
the highest sensed temperature, which constitutes TEMP(J).
[0320] In this mode, the generator 30 compares the temperature
TEMP(J) locally sensed at each electrode E(J) during each data
acquisition period to a set point temperature TEMP.sub.SET
established by the physician. Based upon this comparison, the
generator 30 varies the amplitude AMP.sub.E(J) of the RF voltage
delivered to the electrode region E(J), while maintaining the
DUTYCYCLE.sub.E(J) for that electrode region and all other
electrode regions, to establish and maintain TEMP(J) at the set
point temperature TEMP.sub.SET.
[0321] The set point temperature TEMP.sub.SET can vary according to
the judgment of the physician and empirical data. A representative
set point temperature for cardiac ablation is believed to lie in
the range of 40.degree. C. to 95.degree. C., with 70.degree. C.
being a representative preferred value.
[0322] The manner in which the generator 30 governs AMP.sub.E(J)
can incorporate proportional control methods, proportional integral
derivative (PID) control methods, or fuzzy logic control
methods.
[0323] For example, using proportional control methods, if the
temperature sensed by the first sensing element
TEMP(1)>TEMP.sub.SET, the control signal generated by the
generator 30 individually reduces the amplitude AMP.sub.E(1) of the
RF voltage applied to the first electrode region E(1), while
keeping the duty cycle DUTYCYCLE.sub.E(1) for the first electrode
region E(1) the same. If the temperature sensed by the second
sensing element TEMP(2)<TEMP.sub.SET, the control signal of the
generator 30 increases the amplitude AMP.sub.E(2) of the pulse
applied to the second electrode region E(2), while keeping the duty
cycle DUTYCYCLE.sub.E(.sub.2) for the second electrode region E(2)
the same as DUTYCYCLE.sub.E(1), and so on. If the temperature
sensed by a given sensing element is at the set point temperature
TEMP.sub.SET, no change in RF voltage amplitude is made for the
associated electrode region.
[0324] The generator continuously processes voltage difference
inputs during successive data acquisition periods to individually
adjust AMP.sub.E(J) at each electrode region E(J), while keeping
the collective duty cycle the same for all electrode regions E(J).
In this way, the mode maintains a desired uniformity of temperature
along the length of the ablating element.
[0325] Using a proportional integral differential (PID) control
technique, the generator takes into account not only instantaneous
changes that occur in a given sample period, but also changes that
have occurred in previous sample periods and the rate at which
these changes are varying over time. Thus, using a PID control
technique, the generator will respond differently to a given
proportionally large instantaneous difference between TEMP (J) and
TEMP.sub.SET, depending upon whether the difference is getting
larger or smaller, compared to previous instantaneous differences,
and whether the rate at which the difference is changing since
previous sample periods is increasing or decreasing.
[0326] Further details of individual amplitude/collective duty
cycle control for segmented electrode regions based upon
temperature sensing are found in copending U.S. application Ser.
No. 08/439,824, filed May 12, 1995 and entitled "Systems and
Methods for Controlling Tissue Ablation Using Multiple Temperature
Sensing Elements," which is incorporated herein by reference.
[0327] D. Segmented Shells: Differential Temperature Disabling
[0328] In this control mode, the controller 32 selects at the end
of each data acquisition phase the sensed temperature that is the
greatest for that phase (TEMP.sub.SMAX). The controller 32 also
selects for that phase the sensed temperature that is the lowest
(TEMP.sub.SMIN).
[0329] The generator compares the selected hottest sensed
temperature TEMP.sub.SMAX to a selected high set point temperature
TEMP.sub.HISET. The comparison generates a control signal that
collectively adjusts the amplitude of the RF voltage for all
electrodes using proportional, PID, or fuzzy logic control
techniques.
[0330] In a proportion control implementation scheme:
[0331] (i) If TEMP.sub.SMAX>TEMP.sub.HISET, the control signal
collectively decreases the amplitude of the RF voltage delivered to
all segments;
[0332] (ii) If TEMP.sub.SMAX<TEMP.sub.HISET, the control signal
collectively increases the amplitude of the RF voltage delivered to
all segments:
[0333] (iii) If TEMP.sub.SMAXTEMP.sub.HISET, no change in the
amplitude of the RF voltage delivered to all segments.
[0334] It should be appreciated that the generator can select for
amplitude control purposes any one of the sensed temperatures
TEMP.sub.SMAX, TEMP.sub.SMIN, or temperatures in between, and
compare this temperature condition to a preselected temperature
condition.
[0335] The generator governs the delivery of power to the segments
based upon difference between a given local temperature TEMP (J)
and TEMP.sub.SMIN. This implementation computes the difference
between local sensed temperature TEMP(J) and TEMP.sub.SMIN and
compares this difference to a selected set point temperature
difference .DELTA.TEMP.sub.SET. The comparison generates a control
signal that governs the delivery of power to the electrode
regions.
[0336] If the local sensed temperature TEMP(J) for a given
electrode region E(J) exceeds the lowest sensed temperature
TEMP.sub.SMIN by as much as or more than .DELTA.TEMP.sub.SET (that
is, if TEMP(J)-TEMP.sub.SMIN.gtoreq..DELTA.TEMP.sub.SET), the
generator turns the given segment E(J) off. The generator turns the
given segment E(J) back on when
TEMP(J)-TEMP.sub.SMIN<.DELTA.TEMP.sub.SET.
[0337] Alternatively, instead of comparing TEMP(J) and
TEMP.sub.SMIN, the generator can compare TEMP.sub.SMAX and
TEMP.sub.SMIN. When the difference between TEMP.sub.SMAX and
TEMP.sub.SMIN equals or exceeds a predetermined amount
.DELTA.TEMP.sub.SET, the generator turns all segments off, except
the segment where TEMP.sub.SMIN exists. The controller 231 turns
these segments back on when the temperature difference between
TEMP.sub.SMAX and TEMP.sub.SMIN is less than
.DELTA.TEMP.sub.SET.
[0338] Further details of the use of differential temperature
disabling are found in copending U.S. patent application Ser. No.
08/286,930, filed Aug. 8, 1994, and entitled "Systems and Methods
for Controlling Tissue Ablation Using Multiple Temperature sensing
Elements," which is incorporated herein by reference.
[0339] E. Segmented Shells (Predicted Hottest Temperature)
[0340] Because of the heat exchange between the tissue and the
electrode region 122, the temperature sensing elements 104 may not
measure exactly the maximum temperature at the region 122. This is
because the region of hottest temperature occurs beneath the
surface of the tissue at a depth of about 0.5 to 2.0 mm from where
the energy emitting electrode region 122 (and the associated
sensing element 104) contacts the tissue. If the power is applied
to heat the tissue too quickly, the actual maximum tissue
temperature in this subsurface region may exceed 100.degree. C. and
lead to tissue desiccation and/or micro-explosions.
[0341] FIG. 43 shows an implementation of a neural network
predictor 400, which receives as input the temperatures sensed by
multiple sensing elements S(J,K) at each electrode region, where J
represents a given electrode region (J=1 to N) and K represents the
number of temperature sensing elements on each electrode region
(K=1 to M). The predictor 400 outputs a predicted temperature of
the hottest tissue region T.sub.MAXPRED(t). The generator 30
derives the amplitude and duty cycle control signals based upon
T.sub.MAXPRED(t), in the same manner already described using
TEMP(J).
[0342] The predictor 400 uses a two-layer neural network, although
more hidden layers could be used. As shown in FIG. 43, the
predictor 300 includes a first and second hidden layers and four
neurons, designated N.sub.(L,X), where L identifies the layer 1 or
2 and X identifies a neuron on that layer. The first layer (L=1)
has three neurons (X=1 to 3), as follows N.sub.(1,1); N.sub.(1,2);
and N.sub.(1,3). The second layer (L=2) comprising one output
neuron (X=1), designated N.sub.(2,1).
[0343] Temperature readings from the multiple sensing elements,
only two of which--TS1(n) and TS2(n)--are shown for purposes of
illustration, are weighed and inputted to each neuron N.sub.(1,1);
N.sub.(1,2); and N.sub.(1,3) of the first layer. FIG. 43 represents
the weights as W.sub.(k,N).sup.L, where L=1; k is the input sensor
order; and N is the input neuron number 1, 2, or 3 of the first
layer.
[0344] The output neuron N.sub.(2,1) of the second layer receives
as inputs the weighted outputs of the neurons N.sub.(1,1));
N.sub.(1,2); and N.sub.(1,3). FIG. 43 represents the output weights
as W.sub.(O,X).sup.L, where L=2O is the output neuron 1, 2, or 3 of
the first layer; and X is the input neuron number of the second
layer. Based upon these weighted inputs, the output neuron
N.sub.(2,1) predicts T.sub.MAXPRED(t).
[0345] Alternatively, a sequence of past reading samples from each
sensor could be used as input. By doing this, a history term would
contribute to the prediction of the hottest tissue temperature.
[0346] The predictor 400 must be trained on a known set of data
containing the temperature of the sensing elements TS1 and TS2 and
the temperature of the hottest region, which have been previously
acquired experimentally. For example, using a back-propagation
model, the predictor 400 can be trained to predict the known
hottest temperature of the data set with the least mean square
error. Once the training phase is completed, the predictor 300 can
be used to predict T.sub.MAXPRED(t).
[0347] Other types of data processing techniques can be used to
derive T.sub.MAXPRED(t). See, e.g., copending patent application
Ser. No. 08/266,934, filed Jun. 27, 1994, and entitled "Tissue
Heating and Ablation Systems and Methods Using Predicted
Temperature for Monitoring and Control."
[0348] The illustrated and preferred embodiments use digital
processing controlled by a computer to analyze information and
generate feedback signals. It should be appreciated that other
logic control circuits using micro-switches, AND/OR gates,
invertors, analog circuits, and the like are equivalent to the
micro-processor controlled techniques shown in the preferred
embodiments.
[0349] VIII. Capacitive Coupling
[0350] In the preceding embodiments, the electrode structure 20
transmits ablation energy to tissue by exposing tissue to an
electrically conductive surface 24 carried about the exterior of
the expandable-collapsible body 22. The alternative embodiments
shown in FIGS. 41A and 42A include an electrode structure 176
comprising an expandable-collapsible body 178 having an exterior
free of an electrically conductive surface. In these embodiments,
the body 178 is capacitively coupled to tissue for the purpose of
transmitting ablation energy.
[0351] In the embodiment shown in FIG. 41A, the
expandable-collapsible body 178 is molded in same fashion as the
body 22 previously described. The body 178 includes an electrically
conductive structure 180 in contact with at least a portion of the
interior surface 182 of the body 178.
[0352] The interior conductive structure 180 can be assembled in
various ways. In the embodiment shown in FIG. 41A, the structure
180 comprises an interior shell 184 of electrically conductive
material deposited on at least a portion of the interior surface
182 of the body 178. Like the exterior shell 24 previously
described, the interior shell 184 comprises a material having a
relatively high electrical conductivity, as well as a relative high
thermal conductivity, such as gold, platinum, platinum/iridium,
among others. The shell 184 is preferably deposited upon the
exterior of the body 178 after molding using deposition process
like sputtering, vapor deposition, ion beam deposition,
electroplating over a deposited seed layer, or a combination of
these processes. The body 178 is then everted in the manner
previously described (as FIG. 16B shows) to place the deposited
shell 184 inside the everted body 178. One or more signal wires 186
are coupled to the interior shell 184 using electrically conductive
adhesive, soldering, or equivalent connection techniques.
[0353] The body 178 can be caused to assume expanded and collapsed
geometries by the introduction of an air or liquid inflation
medium, as previously described. Alternatively, the body 178 can
employ any previously described interior support structure 44 to
affect expansion and collapse. The support structures 44 could also
by electrically conductive to affect capacitive coupling, with or
without the presence of the deposited shell 184. For example, an
electrically conductive interior resilient mesh structure (like
that shown in FIG. 6), or a skeleton of flexible, electrically
conductive spline elements (like that shown in FIG. 4), or an open
cell foam structure coated with an electrically conductive material
(like that shown in FIG. 9), can be used both to provide interior
support and to provide capacitive coupling between signal wires 186
and tissue, with or without the presence of the deposited interior
shell 184. In these alternative arrangements, one or more signal
wires 184 are coupled to the electrically conductive support
structures.
[0354] FIG. 41B shows the electrical equivalent circuit 188 of the
capacitive coupling effect that the structure 176 in FIG. 41A
provides. In the electrical path 190 that the ablation energy 192
follows, the interface 194 formed among the expandable-collapsible
body 178, the conductive structure 180 contacting the inside the
body 178, and the tissue 196 contacting the outside of the body 178
functions as a capacitor (designated C), whose impedance X.sub.C is
expressed as: 6 X C = 1 2 fC
[0355] where:
[0356] f is the frequency of the radio frequency ablation energy
192, and 7 C = s t
[0357] where
[0358] .epsilon. is the dielectric constant of the material of the
expandable-collapsible body 178, which ranges from about 1.2 to
about 10.0 (multiplied by 8.85.times.10.sup.-12 Farads per meter)
for most plastic materials,
[0359] s is the surface area of the electrically conductive
structure 184, and
[0360] t is the thickness of the body 178 located between the
electrically conductive structure 180 and the contacted tissue
196.
[0361] In the electrical path 190 that the ablation energy 192
follows, the tissue 196 functions as a resistor (designated
R.sub.TISSUE) series coupled to C. Typically, R.sub.TISSUE is about
100 ohms.
[0362] To have efficient capacitive coupling to the tissue, X.sub.C
of the structure 180 must be less than R.sub.TISSUE. This
relationship assures that the desired ohmic heating effect is
concentrated in tissue.
[0363] To maximize the capacitive coupling effect, it is thereby
important to use ablation energy at higher frequencies (for
example, between 10 and 20 Mhz). It is also important to aim to
maximize C as much as possible, by controlling thickness of the
body 178 as well as by maximizing as much as possible the surface
area of contact with the electrically conductive structure 180
inside the body 178. For this reason, a continuous electrically
conductive shell 182 or equivalent mesh structure are preferred,
compared to a more open spline element structure. However, a more
dense, conductive spline element structure having many spline
elements and/or large surface area splines could be used to
maximize C, if desired.
[0364] FIG. 42A shows an alternative embodiment of an
expandable-collapsible electrode structure 198 that provides
capacitive coupling to tissue. The structure 198 comprises an
interior electrode 200 of electrically conductive material located
within interior of the body 178. The interior electrode 200
comprises a material having a relatively high electrical
conductivity, as well as a relative high thermal conductivity, such
as gold, platinum, platinum/iridium, among others. A signal wire
202 is coupled to the electrode to conduct ablation energy to
it.
[0365] In this embodiment, a hypertonic (i.e., 9%) saline solution
204 fills the interior of the body 178. The saline solution 204
serves as an electrically conductive path to convey radio frequency
energy from the electrode 200 to the body 178. The saline solution
204 also serves as the inflation medium, to cause the body 178 to
assume the expanded geometry. Removal of the saline solution 204
causes the body 178 to assume the collapsed geometry.
[0366] FIG. 42B shows the electrical equivalent circuit 206 of the
capacitive coupling effect that the structure 198 shown in FIG. 42A
provides. In the electrical path 208 that the ablation energy 210
follows, the interface 212 formed among the expandable-collapsible
body 178, the hypertonic saline solution 204 contacting the inside
the body 178, and the tissue 196 contacting the outside of the body
178 functions as a capacitor (designated C), whose impedance
X.sub.C is expressed as: 8 X C = 1 2 fC
[0367] where:
[0368] f is the frequency of the radio frequency ablation energy
210, and 9 C = S B t
[0369] where
[0370] .epsilon. is the dielectric constant of the material of the
body 178,
[0371] S.sub.B is the area of the body 178 contacting the
hypertonic saline solution 204, and
[0372] t is the thickness of the body 178 located between the
electrically conductive saline solution 204 and the tissue 196.
[0373] In the electrical path that the ablation energy follows, the
tissue 196 functions as a resistor (designated R.sub.TISSUE) series
coupled to C, which value is about 100 ohms. The path 216 through
the hypertonic saline 204 between the interior electrode 200 and
the interior surface 214 of the body 178 also functions as a
resistor (designated R.sub.PATH) series coupled to C. The value of
R.sub.PATH is expressed: 10 R PATH = K S E
[0374] where:
[0375] K is a constant that depends upon the geometry of the
structure 198,
[0376] S.sub.E is the surface area of the interior electrode 200,
and
[0377] .rho. is the resistivity of the hypertonic saline 204.
[0378] The following relationship establishes efficient capacitive
coupling between the structure 198 and tissue 196 to achieve the
desired ohmic tissue heating effect:
{square root}{square root over
(R.sub.PATH.sup.2+X.sub.C.sup.2)}<R.sub.- TISSUE
[0379] The use of capacitive coupling provides structural benefits.
It isolates possible shell adherence problems to inside the body
178 of the structure 176, where flaking and chipping of the shell
184 can be retained out of the blood pool. Capacitive coupling also
avoids potential problems that tissue sticking to exterior
conductive materials could create.
[0380] In addition to these structural benefits, the temperature
control of the ablation process (as described above in conjunction
with the structure 20) is improved using capacitive coupling. When
using a metal surface to ablate tissue, the tissue-electrode
interface is convectively cooled by surrounding blood flow. Due to
these convective cooling effects, the region of maximum tissue
temperature is located deeper in the tissue. As a result, the
temperature conditions sensed by sensing elements associated with
metal electrode elements do not directly reflect actual maximum
tissue temperature. In this situation, maximum tissue temperature
conditions must be inferred or predicted from actual sensed
temperatures, as set forth above. Using capacitive coupling in
structures 176 or 198, convective cooling of the tissue-electrode
interface by the surrounding blood flow is minimized. As a result,
the region of maximum temperature is located at the interface
between tissue and the porous electrode. As a result, the
temperature conditions sensed by sensing elements associated with
the capacitively coupled structures 176 or 198 will more closely
reflect actual maximum tissue.
[0381] IX. Conductive Polymer Surfaces
[0382] As previously mentioned in conjunction with FIG. 19, all or
a portion of the body 22 can comprise an electrically conductive
polymer. The conductivity of the polymer used preferably has a
resistivity close to the resistivity of tissue (i.e., about 500
ohm.cm). In use, the electrically conductive body 22 can be used in
association with an interior electrode 200, like that shown in FIG.
42A. In such an arrangement, a hypertonic saline solution 204 also
fills the interior of the electrically conductive body 22 (as also
shown in FIG. 42A), to serve as an electrically conductive path to
convey radio frequency energy from the electrode 200 to the body
22. In effect, in this arrangement, the electrically conductive
body 22 functions as a "leaky" capacitor in transmitting radio
frequency energy from the interior electrode 200 to tissue.
[0383] Various methodologies can be used to control the application
of radio frequency energy to capacitively coupled electrode
structures and to electrode structures having electrically
conductive bodies. The previously described D.sub.50C Function can
be used, as can the previously described Duty Cycle and Temperature
Disabling techniques. With capacitively coupled electrode
structures and electrode structures having electrically conductive
bodies, the minimal effects of convective cooling by the blood pool
enables the use of actual sensed temperature conditions as maximum
tissue temperature TMAX, instead of predicted temperatures. Because
of this, such structures also lend themselves to the use of a
proportional integral differential (PID) control technique. An
illustrative PID control techniques usable in association with
these electrode structures are disclosed in copending U.S. patent
application Ser. No. 08/266,023, filed Jun. 27, 1994, entitled
"Tissue Heating and Ablation Systems and Methods Using
Time-Variable Set Point Temperature Curves for Monitoring and
Control."
[0384] Various features of the invention are set forth in the
following claims.
* * * * *