U.S. patent application number 09/805014 was filed with the patent office on 2001-08-02 for articulation device for selective organ cooling apparatus.
Invention is credited to Dobak, John D. III, Lasheras, Juan C..
Application Number | 20010011185 09/805014 |
Document ID | / |
Family ID | 46256219 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010011185 |
Kind Code |
A1 |
Dobak, John D. III ; et
al. |
August 2, 2001 |
Articulation device for selective organ cooling apparatus
Abstract
A selective organ heat transfer device with deep irregularities
in a turbulence-inducing exterior surface. The device can have a
plurality of elongated, articulated segments, each having a
turbulence-inducing exterior surface. A flexible joint connects
adjacent elongated, articulated segments. The flexible joint may be
a rubber tube or a metal tube of a predetermined thickness. An
inner lumen is disposed within the heat transfer segments. The
inner lumen is capable of transporting a pressurized working fluid
to a distal end of the heat transfer element.
Inventors: |
Dobak, John D. III; (La
Jolla, CA) ; Lasheras, Juan C.; (La Jolla,
CA) |
Correspondence
Address: |
INNERCOOL Therapies
3931 Sorrento Valley Blvd.
San Diego
CA
92121
US
|
Family ID: |
46256219 |
Appl. No.: |
09/805014 |
Filed: |
March 12, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09805014 |
Mar 12, 2001 |
|
|
|
09215041 |
Dec 16, 1998 |
|
|
|
6254626 |
|
|
|
|
09215041 |
Dec 16, 1998 |
|
|
|
09103342 |
Jun 23, 1998 |
|
|
|
6096068 |
|
|
|
|
09215041 |
Dec 16, 1998 |
|
|
|
09052545 |
Mar 31, 1998 |
|
|
|
6231595 |
|
|
|
|
09215041 |
Dec 16, 1998 |
|
|
|
09047012 |
Mar 24, 1998 |
|
|
|
5957963 |
|
|
|
|
Current U.S.
Class: |
607/105 ;
607/113 |
Current CPC
Class: |
A61B 2017/00292
20130101; A61F 7/12 20130101; A61M 25/00 20130101; A61M 2025/006
20130101; A61B 2018/0022 20130101; A61M 2205/3606 20130101; A61F
2007/126 20130101; A61M 25/0023 20130101; A61F 2007/0056 20130101;
A61B 18/02 20130101; A61M 2025/0004 20130101; A61B 2018/0212
20130101 |
Class at
Publication: |
607/105 ;
607/113 |
International
Class: |
A61F 007/00; A61F
007/12 |
Claims
What is claimed is:
1. A selective organ heat transfer device, comprising: a flexible
catheter capable of insertion to a selected feeding artery in the
vascular system of a patient; a heat transfer element attached to a
distal end of said catheter, said heat transfer element including a
plurality of heat transfer segments, further including a flexible
joint connecting each of said heat transfer segments to adjacent
said heat transfer segments, and wherein said flexible joint
includes a flexible tube; a plurality of exterior surface
irregularities on said heat transfer element, said surface
irregularities being shaped and arranged to create turbulence in
surrounding fluid, said surface irregularities having a depth at
least equal to the boundary layer thickness of flow in the feeding
artery; and an inner coaxial tube disposed within said heat
transfer element, said inner coaxial tube being connected in fluid
flow communication with an inner coaxial tube within said
catheter.
2. The heat transfer device of claim 1, wherein said flexible tube
is made of a metal.
3. The heat transfer device of claim 2, wherein said metal is
nickel.
4. The heat transfer device of claim 2, wherein said flexible tube
has a thickness between about 0.5 and 0.8 mils.
5. The heat transfer device of claim 1, wherein said flexible tube
is made of a polymer.
6. The heat transfer device of claim 5, wherein said polymer is
selected from the group consisting of rubber, plastic, and
corrugated plastic.
7. The heat transfer device of claim 6, wherein said rubber is
latex rubber.
8. The heat transfer device of claim 1, wherein: said surface
irregularities comprise a helical ridge and a helical groove formed
on each said heat transfer segment; and said helical ridge on each
said heat transfer segment has an opposite helical twist to said
helical ridges on adjacent said heat transfer segments.
9. The heat transfer device of claim 1, wherein: said surface
irregularities comprise protrusions on said exterior surface; and
said protrusions are axially staggered along said exterior
surface.
10. The heat transfer device of claim 1, further comprising a
plurality of interior surface irregularities in said heat transfer
element, said interior surface irregularities being shaped and
arranged to create turbulence in fluid within said heat transfer
element, said interior surface irregularities having a depth at
least equal to the boundary layer thickness of flow within said
heat transfer element.
11. The heat transfer device of claim 10, wherein: said heat
transfer element comprises a plurality of heat transfer segments;
said interior surface irregularities comprise a helical ridge and a
helical groove formed within each said heat transfer segment; and
said helical ridge within each said heat transfer segment has an
opposite helical twist to said helical ridges within adjacent said
heat transfer segments.
12. The heat transfer device of claim 10, wherein: said surface
irregularities comprise protrusions on said interior surface; and
said protrusions are axially staggered along said interior
surface.
13. A selective organ heat transfer device, comprising: a flexible
coaxial catheter capable of insertion to a selected feeding artery
in the vascular system of a patient; an articulated heat transfer
element attached to a distal end of said catheter, said heat
transfer element having an exterior diameter substantially less
than the inner diameter of the selected feeding artery; a plurality
of heat transfer segments on said heat transfer element, each said
segment having a plurality of exterior surface irregularities, said
surface irregularities being shaped and arranged to create
turbulence in surrounding fluid, said surface irregularities having
a depth at least equal to the boundary layer thickness of flow in
the feeding artery; a straight flexible tube connecting each of
said heat transfer segments to adjacent said heat transfer
segments; and an inner coaxial tube disposed within said heat
transfer element, said inner coaxial tube being connected in fluid
flow communication with an inner coaxial tube within said
catheter.
14. The heat transfer device of claim 13, wherein said straight
flexible tube is made of metal.
15. The heat transfer device of claim 13, wherein: said surface
irregularities comprise a helical ridge and a helical groove formed
on each said heat transfer segment; and said helical ridge on each
said heat transfer segment has an opposite helical twist to said
helical ridges on adjacent said heat transfer segments.
16. A heat transfer device, comprising: a flexible catheter capable
of insertion to a selected vessel in the vascular system of a
patient; a heat transfer element attached to a distal end of said
catheter, said heat transfer element including a plurality of
helical mixing-inducing heat transfer segments, further including a
flexible joint connecting each of said heat transfer segments to
adjacent said heat transfer segments, wherein said flexible joint
includes a flexible tube; and an inner coaxial tube disposed within
said heat transfer element, said inner coaxial tube being connected
in fluid flow communication with an inner coaxial tube within said
catheter.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation patent application of co-pending U.S.
patent application Ser. No. 09/215,041, filed on Dec. 16, 1998,
entitled "Articulation Device for Selective Organ Cooling
Apparatus", which is a continuation-in-part patent application of
U.S. patent applications Ser. No. 09/103,342, filed on Jun. 23,
1998, and entitled "Selective Organ Cooling Catheter and Method of
Using the Same" (now U.S. Pat. No. 6,096,068) and Ser. No.
09/052,545, filed on Mar. 31, 1998, and entitled "Circulating Fluid
Hypothermia Method and Apparatus" and Ser. No. 09/047,012, filed on
Mar. 24, 1998, and entitled "Improved Selective Organ Hypothermia
Method and Apparatus" (now U.S. Pat. No. 5,957,963), the entirety
of each being incorporated by reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates generally to the modification
and control of the temperature of a selected body organ. More
particularly, the invention relates to a method and intravascular
apparatus for controlling organ temperature.
[0005] 2. Background Information
[0006] Organs in the human body, such as the brain, kidney and
heart, are maintained at a constant temperature of approximately
37.degree. C. Hypothermia can be clinically defined as a core body
temperature of 35.degree. C. or less. Hypothermia is sometimes
characterized further according to its severity. A body core
temperature in the range of 33.degree. C. to 35.degree. C. is
described as mild hypothermia. A body temperature of 28.degree. C.
to 32.degree. C. is described as moderate hypothermia. A body core
temperature in the range of 24.degree. C. to 28.degree. C. is
described as severe hypothermia.
[0007] Hypothermia is uniquely effective in reducing brain injury
caused by a variety of neurological insults and may eventually play
an important role in emergency brain resuscitation. Experimental
evidence has demonstrated that cerebral cooling improves outcome
after global ischemia, focal ischemia, or traumatic brain injury.
For this reason, hypothermia may be induced in order to reduce the
effect of certain bodily injuries to the brain as well as other
organs.
[0008] Cerebral hypothermia has traditionally been accomplished
through whole body cooling to create a condition of total body
hypothermia in the range of 20.degree. C. to 30.degree. C. However,
the use of total body hypothermia risks certain deleterious
systematic vascular effects. For example, total body hypothermia
may cause severe derangement of the cardiovascular system,
including low cardiac output, elevated systematic resistance, and
ventricular fibrillation. Other side effects include renal failure,
disseminated intravascular coagulation, and electrolyte
disturbances. In addition to the undesirable side effects, total
body hypothermia is difficult to administer.
[0009] Catheters have been developed which are inserted into the
bloodstream of the patient in order to induce total body
hypothermia. For example, U.S. Pat. No. 3,425,419 to Dato describes
a method and apparatus of lowering and raising the temperature of
the human body. Dato induces moderate hypothermia in a patient
using a metallic catheter. The metallic catheter has an inner
passageway through which a fluid, such as water, can be circulated.
The catheter is inserted through the femoral vein and then through
the inferior vena cava as far as the right atrium and the superior
vena cava. The Dato catheter has an elongated cylindrical shape and
is constructed from stainless steel. By way of example, Dato
suggests the use of a catheter approximately 70 cm in length and
approximately 6 mm in diameter. However, use of the Dato device
implicates the negative effects of total body hypothermia described
above.
[0010] Due to the problems associated with total body hypothermia,
attempts have been made to provide more selective cooling. For
example, cooling helmets or head gear have been used in an attempt
to cool only the head rather than the patient's entire body.
However, such methods rely on conductive heat transfer through the
skull and into the brain. One drawback of using conductive heat
transfer is that the process of reducing the temperature of the
brain is prolonged. Also, it is difficult to precisely control the
temperature of the brain when using conduction due to the
temperature gradient that must be established externally in order
to sufficiently lower the internal temperature. In addition, when
using conduction to cool the brain, the face of the patient is also
subjected to severe hypothermia, increasing discomfort and the
likelihood of negative side effects. It is known that profound
cooling of the face can cause similar cardiovascular side effects
as total body cooling. From a practical standpoint, such devices
are cumbersome and may make continued treatment of the patient
difficult or impossible.
[0011] Selected organ hypothermia has been accomplished using
extracorporeal perfusion, as detailed by Arthur E. Schwartz, M.D.
et al., in Isolated Cerebral Hypothermia by Single Carotid Artery
Perfusion of Extracorporeally Cooled Blood in Baboons, which
appeared in Vol. 39, No. 3, NEUROSURGERY 577 (September, 1996). In
this study, blood was continually withdrawn from baboons through
the femoral artery. The blood was cooled by a water bath and then
infused through a common carotid artery with its external branches
occluded. Using this method, normal heart rhythm, systemic arterial
blood pressure and arterial blood gas values were maintained during
the hypothermia. This study showed that the brain could be
selectively cooled to temperatures of 20.degree. C. without
reducing the temperature of the entire body. However, external
circulation of blood is not a practical approach for treating
humans because the risk of infection, need for anticoagulation, and
risk of bleeding is too great. Further, this method requires
cannulation of two vessels making it more cumbersome to perform
particularly in emergency settings. Even more, percutaneous
cannulation of the carotid artery is difficult and potentially
fatal due to the associated arterial wall trauma. Finally, this
method would be ineffective to cool other organs, such as the
kidneys, because the feeding arteries cannot be directly cannulated
percutaneously.
[0012] Selective organ hypothermia has also been attempted by
perfusion of a cold solution such as saline or perflourocarbons.
This process is commonly used to protect the heart during heart
surgery and is referred to as cardioplegia. Perfusion of a cold
solution has a number of drawbacks, including a limited time of
administration due to excessive volume accumulation, cost, and
inconvenience of maintaining the perfusate and lack of
effectiveness due to the temperature dilution from the blood.
Temperature dilution by the blood is a particular problem in high
blood flow organs such as the brain.
[0013] Therefore, a practical method and apparatus which modifies
and controls the temperature of a selected organ satisfies a
long-felt need.
BRIEF SUMMARY OF THE INVENTION
[0014] The apparatus of the present invention can, by way of
example only, include a heat transfer element which comprises first
and second elongated, articulated segments, each segment having a
turbulence-inducing exterior surface. A flexible joint can connect
the first and second elongated segments. An inner coaxial lumen may
be disposed within the first and second elongated segments and is
capable of transporting a pressurized working fluid to a distal end
of the first elongated segment. In addition, the first and second
elongated segments may have a turbulence-inducing interior surface
for inducing turbulence within the pressurized working fluid. The
turbulence-inducing exterior surface may be adapted to induce
turbulence within a free stream of blood flow when placed within an
artery. The turbulence-inducing exterior surface may be adapted to
induce a turbulence intensity greater than 0.05 within a free
stream blood flow. In one embodiment, the flexible joint comprises
a straight tube having a predetermined thickness which allows for
lateral bending of the heat transfer element.
[0015] In one embodiment, the turbulence-inducing exterior surfaces
of the heat transfer element comprise one or more helical ridges
configured to have a depth which is greater than a thickness of a
boundary layer of blood which develops within an arterial blood
flow. Adjacent segments of the heat transfer element can be
oppositely spiraled to increase turbulence. For instance, the first
elongated heat transfer segment may comprise one or more helical
ridges having a counter-clockwise twist, while the second elongated
heat transfer segment comprises one or more helical ridges having a
clockwise twist. Alternatively, of course, the first elongated heat
transfer segment may comprise one or more clockwise helical ridges,
and the second elongated heat transfer segment may comprise one or
more counter-clockwise helical ridges. The first and second
elongated, articulated segments may be formed from highly
conductive materials.
[0016] In another embodiment, the turbulence-inducing exterior
surface of the heat transfer element is adapted to induce
turbulence throughout the duration of each pulse of a pulsatile
blood flow when placed within an artery. In still another
embodiment, the turbulence-inducing exterior surface of the heat
transfer element is adapted to induce turbulence during at least
20% of the period of each cardiac cycle when placed within an
artery.
[0017] The heat transfer device may also have a coaxial supply
catheter with an inner catheter lumen coupled to the inner coaxial
lumen within the first and second elongated heat transfer segments.
A working fluid supply configured to dispense the pressurized
working fluid may be coupled to the inner catheter lumen. The
working fluid supply may be configured to produce the pressurized
working fluid at a temperature of about 0.degree. C. and at a
pressure below about 5 atmospheres of pressure.
[0018] In yet another alternative embodiment, the heat transfer
device may have three or more elongated, articulated, heat transfer
segments having a turbulence-inducing exterior surface, with
additional flexible joints connecting the additional elongated heat
transfer segments. In one such embodiment, by way of example, the
first and third elongated heat transfer segments may comprise
clockwise helical ridges, and the second elongated heat transfer
segment may comprise one or more counter-clockwise helical ridges.
Alternatively, of course, the first and third elongated heat
transfer segments may comprise counter-clockwise helical ridges,
and the second elongated heat transfer segment may comprise one or
more clockwise helical ridges.
[0019] The turbulence-inducing exterior surface of the heat
transfer element may optionally include a surface coating or
treatment to inhibit clot formation. One variation of the heat
transfer element comprises a stent coupled to a distal end of the
first elongated heat transfer segment.
[0020] In one embodiment, the catheter has a flexible metal tip and
the cooling step occurs at the tip. The tip may have
turbulence-inducing elongated heat transfer segments separated by
metal pipe sections or flexible polymer sections. The
turbulence-inducing segments may comprise helical ridges which are
configured to have a depth which is greater than a thickness of a
boundary layer of blood which develops within the blood vessel. In
another embodiment, the catheter has a tip at which the cooling
step occurs and the tip has turbulence-inducing elongated heat
transfer segments that alternately spiral bias the surrounding
blood flow in clockwise and counterclockwise directions.
[0021] The present invention also envisions a cooling catheter
comprising a catheter shaft having first and second lumens therein.
The cooling catheter also comprises a cooling tip adapted to
transfer heat to or from a working fluid circulated in through the
first lumen and out through the second lumen, and
turbulence-inducing structures on the cooling tip capable of
inducing free stream turbulence when the tip is inserted into a
blood vessel. The turbulence-inducing structures may induce a
turbulence intensity of at least about 0.05. The cooling tip may be
adapted to induce turbulence within the working fluid. The catheter
is capable of removing at least about 25 Watts of heat from an
organ when inserted into a vessel supplying that organ, while
cooling the tip with a working fluid that remains a liquid in the
catheter. Alternatively, the catheter is capable of removing at
least about 50 or 75 Watts of heat from an organ when inserted into
a vessel supplying that organ, while cooling the tip with an
aqueous working fluid. In one embodiment, in use, the tip has a
diameter of about 4 mm or less. Optionally, the turbulence-inducing
surfaces on the heat transfer segments comprise helical ridges
which have a depth sufficient to disrupt the free stream blood flow
in the blood vessel. Alternatively, the turbulence-inducing
surfaces may comprise staggered protrusions from the outer surfaces
of the heat transfer segments, which have a height sufficient to
disrupt the free stream flow of blood within the blood vessel.
[0022] In another embodiment, a cooling catheter may comprise a
catheter shaft having first and second lumens therein, a cooling
tip adapted to transfer heat to or from a working fluid circulated
in through the first lumen and out through the second lumen, and
turbulence-inducing structures on the cooling tip capable of
inducing turbulence when the tip is inserted into a blood vessel.
Alternatively, a cooling catheter may comprise a catheter shaft
having first and second lumens therein, a cooling tip adapted to
transfer heat to or from a working fluid circulated in through the
first lumen and out through the second lumen, and structures on the
cooling tip capable of inducing free stream turbulence when the tip
is inserted into a blood vessel. In another embodiment, a cooling
catheter may comprise a catheter shaft having first and second
lumens therein, a cooling tip adapted to transfer heat to or from a
working fluid circulated in through the first lumen and out through
the second lumen, and turbulence-inducing structures on the cooling
tip capable of inducing turbulence with an intensity greater than
about 0.05 when the tip is inserted into a blood vessel.
[0023] The novel features of this invention, as well as the
invention itself, will be best understood from the attached
drawings, taken along with the following description, in which
similar reference characters refer to similar parts, and in
which:
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0024] FIG. 1 is an elevation view of a turbulence inducing heat
transfer element within an artery;
[0025] FIG. 2 is an elevation view of one embodiment of a heat
transfer element which may be employed according to the
invention;
[0026] FIG. 3 is longitudinal section view of the heat transfer
element of FIG. 2;
[0027] FIG. 4 is a transverse section view of the heat transfer
element of FIG. 2;
[0028] FIG. 5 is a perspective view of the heat transfer element of
FIG. 2 in use within a blood vessel;
[0029] FIG. 6 is a cut-away perspective view of an alternative
embodiment of a heat transfer element which may be employed
according to the invention;
[0030] FIG. 7 is a transverse section view of the heat transfer
element of FIG. 6; and
[0031] FIG. 8 is a schematic representation of the invention being
used in one embodiment to cool the brain of a patient.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The temperature of a selected organ may be intravascularly
regulated by a heat transfer element placed in the organ's feeding
artery to absorb or deliver heat to or from the blood flowing into
the organ. While the method is described with respect to blood flow
into an organ, it is understood that heat transfer within a volume
of tissue is analogous. In the latter case, heat transfer is
predominantly by conduction.
[0033] The heat transfer may cause either a cooling or a heating of
the selected organ. A heat transfer element that selectively alters
the temperature of an organ should be capable of providing the
necessary heat transfer rate to produce the desired cooling or
heating effect within the organ to achieve a desired
temperature.
[0034] The heat transfer element should be small and flexible
enough to fit within the feeding artery while still allowing a
sufficient blood flow to reach the organ in order to avoid ischemic
organ damage. Feeding arteries, like the carotid artery, branch off
the aorta at various levels. Subsidiary arteries continue to branch
off these initial branches. For example, the internal carotid
artery branches off the common carotid artery near the angle of the
jaw. The heat transfer element is typically inserted into a
peripheral artery, such as the femoral artery, using a guide
catheter or guide wire, and accesses a feeding artery by initially
passing though a series of one or more of these branches. Thus, the
flexibility and size, e.g., the diameter, of the heat transfer
element are important characteristics. This flexibility is achieved
as is described in more detail below.
[0035] These points are illustrated using brain cooling as an
example. The common carotid artery supplies blood to the head and
brain. The internal carotid artery branches off the common carotid
artery to supply blood to the anterior cerebrum. The heat transfer
element may be placed into the common carotid artery or into both
the common carotid artery and the internal carotid artery.
[0036] The benefits of hypothermia described above are achieved
when the temperature of the blood flowing to the brain is reduced
to between 30.degree. C. and 32.degree. C. A typical brain has a
blood flow rate through each carotid artery (right and left) of
approximately 250-375 cubic centimeters per minute (cc/min). With
this flow rate, calculations show that the heat transfer element
should absorb approximately 75-175 watts of heat when placed in one
of the carotid arteries to induce the desired cooling effect.
Smaller organs may have less blood flow in their respective supply
arteries and may require less heat transfer, such as about 25
watts.
[0037] The method employs conductive and convective heat transfers.
Once the materials for the device and a working fluid are chosen,
the conductive heat transfers are solely dependent on the
temperature gradients. Convective heat transfers, by contrast, also
rely on the movement of fluid to transfer heat. Forced convection
results when the heat transfer surface is in contact with a fluid
whose motion is induced (or forced) by a pressure gradient, area
variation, or other such force. In the case of arterial flow, the
beating heart provides an oscillatory pressure gradient to force
the motion of the blood in contact with the heat transfer surface.
One of the aspects of the device uses turbulence to enhance this
forced convective heat transfer.
[0038] The rate of convective heat transfer Q is proportional to
the product of S, the area of the heat transfer element in direct
contact with the fluid, .DELTA.T=T.sub.b-T.sub.s, the temperature
differential between the surface temperature T.sub.s of the heat
transfer element and the free stream blood temperature T.sub.b, and
{overscore (h.sub.c)}, the average convection heat transfer
coefficient over the heat transfer area. {overscore (h.sub.c)} is
sometimes called the "surface coefficient of heat transfer" or the
"convection heat transfer coefficient".
[0039] The magnitude of the heat transfer rate Q to or from the
fluid flow can be increased through manipulation of the above three
parameters. Practical constraints limit the value of these
parameters and how much they can be manipulated. For example, the
internal diameter of the common carotid artery ranges from 6 to 8
mm. Thus, the heat transfer element residing therein may not be
much larger than 4 mm in diameter to avoid occluding the vessel.
The length of the heat transfer element should also be limited. For
placement within the internal and common carotid artery, the length
of the heat transfer element is limited to about 10 cm. This
estimate is based on the length of the common carotid artery, which
ranges from 8 to 12 cm.
[0040] Consequently, the value of the surface area S is limited by
the physical constraints imposed by the size of the artery into
which the device is placed. Surface features, such as fins, can be
used to increase the surface area of the heat transfer element,
however, these features alone cannot provide enough surface area
enhancement to meet the required heat transfer rate to effectively
cool the brain.
[0041] One may also attempt to vary the magnitude of the heat
transfer rate by varying .DELTA.T. The value of
.DELTA.T=T.sub.b-T.sub.s can be varied by varying the surface
temperature T.sub.s of the heat transfer element. The allowable
surface temperature of the heat transfer element is limited by the
characteristics of blood. The blood temperature is fixed at about
37.degree. C., and blood freezes at approximately 0.degree. C. When
the blood approaches freezing, ice emboli may form in the blood
which may lodge downstream, causing serious ischemic injury.
Furthermore, reducing the temperature of the blood also increases
its viscosity which results in a small decrease in the value of
{overscore (h.sub.c)}. Increased viscosity of the blood may further
result in an increase in the pressure drop within the artery, thus
compromising the flow of blood to the brain. Given the above
constraints, it is advantageous to limit the surface temperature of
the heat transfer element to approximately 1.degree. C.-5.degree.
C., thus resulting in a maximum temperature differential between
the blood stream and the heat transfer element of approximately
32.degree. C.-36.degree. C.
[0042] One may also attempt to vary the magnitude of the heat
transfer rate by varying {overscore (h.sub.c)}. Fewer constraints
are imposed on the value of the convection heat transfer
coefficient {overscore (h.sub.c)}. The mechanisms by which the
value of {overscore (h.sub.c)} may be increased are complex.
However, one way to increase {overscore (h.sub.c)} for a fixed mean
value of the velocity is to increase the level of turbulent kinetic
energy in the fluid flow.
[0043] The heat transfer rate Q.sub.no-flow in the absence of fluid
flow is proportional to .DELTA.T, the temperature differential
between the surface temperature T.sub.s of the heat transfer
element and the free stream blood temperature T.sub.b times k, the
diffusion constant, and is inversely proportion to .delta., the
thickness of the boundary layer.
[0044] The magnitude of the enhancement in heat transfer by fluid
flow can be estimated by taking the ratio of the heat transfer rate
with fluid flow to the heat transfer rate in the absence of fluid
flow N=Q.sub.flow/Q.sub.no-flow={overscore (h.sub.c)}/(k/.delta.).
This ratio is called the Nusselt number ("Nu"). For convective heat
transfer between blood and the surface of the heat transfer
element, Nusselt numbers of 30-80 have been found to be appropriate
for selective cooling applications of various organs in the human
body. Nusselt numbers are generally dependent on several other
numbers: the Reynolds number, the Womersley number, and the Prandtl
number.
[0045] Stirring-type mechanisms, which abruptly change the
direction of velocity vectors, may be utilized to induce turbulent
kinetic energy and increase the heat transfer rate. The level of
turbulence so created is characterized by the turbulence intensity
.theta.. Turbulence intensity .theta. is defined as the root mean
square of the fluctuating velocity divided by the mean velocity.
Such mechanisms can create high levels of turbulence intensity in
the free stream, thereby increasing the heat transfer rate. This
turbulence intensity should ideally be sustained for a significant
portion of the cardiac cycle, and should ideally be created
throughout the free stream and not just in the boundary layer.
[0046] Turbulence does occur for a short period in the cardiac
cycle anyway. In particular, the blood flow is turbulent during a
small portion of the descending systolic flow. This portion is less
than 20% of the period of the cardiac cycle. If a heat transfer
element is placed co-axially inside the artery, the heat transfer
rate will be enhanced during this short interval. For typical of
these fluctuations, the turbulence intensity is at least 0.05. In
other words, the instantaneous velocity fluctuations deviate from
the mean velocity by at least 5%. Although ideally turbulence is
created throughout the entire period of the cardiac cycle, the
benefits of turbulence are obtained if the turbulence is sustained
for 75%, 50% or even as low as 30% or 20% of the cardiac cycle.
[0047] One type of turbulence-inducing heat transfer element which
may be advantageously employed to provide heating or cooling of an
organ or volume is described in co-pending U.S. patent application
Ser. No. 09/103,342 to Dobak and Lasheras for a "Selective Organ
Cooling Catheter and Method of Using the Same," incorporated by
reference above. In that application, and as described below, the
heat transfer element is made of a high thermal conductivity
material, such as metal. The use of a highly thermally conductive
material increases the heat transfer rate for a given temperature
differential between the coolant within the heat transfer element
and the blood. This facilitates the use of a higher temperature
coolant within the heat transfer element, allowing safer coolants,
such as water, to be used. Highly thermally conductive materials,
such as metals, tend to be rigid. In that application, bellows
provided a high degree of articulation that compensated for the
intrinsic stiffness of the metal. In the present application, the
bellows are replaced with a straight metal tube having a
predetermined thickness to allow flexibility via bending of the
metal. Alternatively, the bellows may be replaced with a polymer
tube, e.g., a latex rubber tube, a plastic tube, a corrugated
plastic tube, etc.
[0048] The device size may be minimized, e.g., less than 4 mm, to
prevent blockage of the blood flowing in the artery. The design of
the heat transfer element should facilitate flexibility in an
inherently inflexible material.
[0049] To create the desired level of turbulence intensity in the
blood free stream during the whole cardiac cycle, one embodiment of
the device uses a modular design. This design creates helical blood
flow and produces a high level of turbulence in the free stream by
periodically forcing abrupt changes in the direction of the helical
blood flow. FIG. 1 is a perspective view of such a turbulence
inducing heat transfer element within an artery. Turbulent flow
would be found at point 114, in the free stream area. The abrupt
changes in flow direction are achieved through the use of a series
of two or more heat transfer segments, each comprised of one or
more helical ridges. To affect the free stream, the depth of the
helical ridge is larger than the thickness of the boundary layer
which would develop if the heat transfer element had a smooth
cylindrical surface.
[0050] The use of periodic abrupt changes in the helical direction
of the blood flow in order to induce strong free stream turbulence
may be illustrated with reference to a common clothes washing
machine. The rotor of a washing machine spins initially in one
direction causing laminar flow. When the rotor abruptly reverses
direction, significant turbulent kinetic energy is created within
the entire wash basin as the changing currents cause random
turbulent motion within the clothes-water slurry.
[0051] FIG. 2 is an elevation view of one embodiment of a heat
transfer element 14. The heat transfer element 14 is comprised of a
series of elongated, articulated segments or modules 20, 22, 24.
Three such segments are shown in this embodiment, but two or more
such segments could be used. As seen in FIG. 2, a first elongated
heat transfer segment 20 is located at the proximal end of the heat
transfer element 14. A turbulence-inducing exterior surface of the
segment 20 comprises four parallel helical ridges 28 with four
parallel helical grooves 26 therebetween. One, two, three, or more
parallel helical ridges 28 could also be used. In this embodiment,
the helical ridges 28 and the helical grooves 26 of the heat
transfer segment 20 have a left hand twist, referred to herein as a
counter-clockwise spiral or helical rotation, as they proceed
toward the distal end of the heat transfer segment 20.
[0052] The first heat transfer segment 20 is coupled to a second
elongated heat transfer segment 22 by a first tube section 25,
which provides flexibility. The second heat transfer segment 22
comprises one or more helical ridges 32 with one or more helical
grooves 30 therebetween. The ridges 32 and grooves 30 have a right
hand, or clockwise, twist as they proceed toward the distal end of
the heat transfer segment 22. The second heat transfer segment 22
is coupled to a third elongated heat transfer segment 24 by a
second tube section 27. The third heat transfer segment 24
comprises one or more helical ridges 36 with one or more helical
grooves 34 therebetween. The helical ridge 36 and the helical
groove 34 have a left hand, or counter-clockwise, twist as they
proceed toward the distal end of the heat transfer segment 24.
Thus, successive heat transfer segments 20, 22, 24 of the heat
transfer element 14 alternate between having clockwise and
counterclockwise helical twists. The actual left or right hand
twist of any particular segment is immaterial, as long as adjacent
segments have opposite helical twist.
[0053] In addition, the rounded contours of the ridges 28, 32, 36
also allow the heat transfer element 14 to maintain a relatively
atraumatic profile, thereby minimizing the possibility of damage to
the blood vessel wall. A heat transfer element may be comprised of
two, three, or more heat transfer segments.
[0054] The tube sections 25, 27 are formed from seamless and
nonporous materials, such as metal, and therefore are impermeable
to gas, which can be particularly important, depending on the type
of working fluid which is cycled through the heat transfer element
14. The structure of the tube sections 25, 27 allows them to bend,
extend and compress, which increases the flexibility of the heat
transfer element 14 so that it is more readily able to navigate
through blood vessels. The tube sections 25, 27 are also able to
tolerate cryogenic temperatures without a loss of performance. The
tube sections 25, 27 may have a predetermined thickness of their
walls, such as between about 0.5 and 0.8 mils. The predetermined
thickness is to a certain extent dependent on the diameter of the
overall tube. Thicknesses of 0.5 to 0.8 mils may be appropriate
especially for a tubal diameter of about 4 mm. For smaller
diameters, such as about 3.3 mm, larger thicknesses may be employed
for higher strength. In another embodiment, tube sections 25, 27
may be formed from a polymer material such as rubber, e.g., latex
rubber, plastic, corrugated plastic, etc.
[0055] The exterior surfaces of the heat transfer element 14 can be
made from metal except in flexible joint embodiment where the
surface may be comprised of a polymer material. The metal may be a
very high thermal conductivity material such as nickel, thereby
facilitating efficient heat transfer. Alternatively, other metals
such as stainless steel, titanium, aluminum, silver, copper and the
like, can be used, with or without an appropriate coating or
treatment to enhance biocompatibility or inhibit clot formation.
Suitable biocompatible coatings include, e.g., gold, platinum or
polymer paralyene. The heat transfer element 14 may be manufactured
by plating a thin layer of metal on a mandrel that has the
appropriate pattern. In this way, the heat transfer element 14 may
be manufactured inexpensively in large quantities, which is an
important feature in a disposable medical device.
[0056] Because the heat transfer element 14 may dwell within the
blood vessel for extended periods of time, such as 24-48 hours or
even longer, it may be desirable to treat the surfaces of the heat
transfer element 14 to avoid clot formation. One means by which to
prevent thrombus formation is to bind an antithrombogenic agent to
the surface of the heat transfer element 14. For example, heparin
is known to inhibit clot formation and is also known to be useful
as a biocoating. Alternatively, the surfaces of the heat transfer
element 14 may be bombarded with ions such as nitrogen. Bombardment
with nitrogen can harden and smooth the surface and, thus prevent
adherence of clotting factors to the surface.
[0057] FIG. 3 is a longitudinal sectional view of the heat transfer
element 14, taken along line 3-3 in FIG. 2. Some interior contours
are omitted for purposes of clarity. An inner tube 42 creates an
inner coaxial lumen 42 and an outer coaxial lumen 46 within the
heat transfer element 14. Once the heat transfer element 14 is in
place in the blood vessel, a working fluid such as saline or other
aqueous solution may be circulated through the heat transfer
element 14. Fluid flows up a supply catheter into the inner coaxial
lumen 40. At the distal end of the heat transfer element 14, the
working fluid exits the inner coaxial lumen 40 and enters the outer
lumen 46. As the working fluid flows through the outer lumen 46,
heat is transferred from the working fluid to the exterior surface
37 of the heat transfer element 14. Because the heat transfer
element 14 is constructed from a high conductivity material, the
temperature of its exterior surface 37 may reach very close to the
temperature of the working fluid. The tube 42 may be formed as an
insulating divider to thermally separate the inner lumen 40 from
the outer lumen 46. For example, insulation may be achieved by
creating longitudinal air channels in the wall of the insulating
tube 42. Alternatively, the insulating tube 42 may be constructed
of a non-thermally conductive material like polytetrafluoroethylene
or some other polymer.
[0058] It is important to note that the same mechanisms that govern
the heat transfer rate between the exterior surface 37 of the heat
transfer element 14 and the blood also govern the heat transfer
rate between the working fluid and the interior surface 38 of the
heat transfer element 14. The heat transfer characteristics of the
interior surface 38 are particularly important when using water,
saline or other fluid which remains a liquid as the coolant. Other
coolants such as freon undergo nucleate boiling and create
turbulence through a different mechanism. Saline is a safe coolant
because it is non-toxic, and leakage of saline does not result in a
gas embolism, which could occur with the use of boiling
refrigerants. Since turbulence in the coolant is enhanced by the
shape of the interior surface 38 of the heat transfer element 14,
the coolant can be delivered to the heat transfer element 14 at a
warmer temperature and still achieve the necessary heat transfer
rate.
[0059] This has a number of beneficial implications in the need for
insulation along the catheter shaft length. Due to the decreased
need for insulation, the catheter shaft diameter can be made
smaller. The enhanced heat transfer characteristics of the interior
surface of the heat transfer element 14 also allow the working
fluid to be delivered to the heat transfer element 14 at lower flow
rates and lower pressures. High pressures may make the heat
transfer element stiff and cause it to push against the wall of the
blood vessel, thereby shielding part of the exterior surface 37 of
the heat transfer element 14 from the blood. Because of the
increased heat transfer characteristics achieved by the alternating
helical ridges 28, 32, 36, the pressure of the working fluid may be
as low as 5 atmospheres, 3 atmospheres, 2 atmospheres or even less
than 1 atmosphere.
[0060] FIG. 4 is a transverse sectional view of the heat transfer
element 14, taken at a location denoted by the line 4-4 in FIG. 2.
FIG. 4 illustrates a five-lobed embodiment, whereas FIG. 2
illustrates a four-lobed embodiment. As mentioned earlier, any
number of lobes might be used. In FIG. 4, the coaxial construction
of the heat transfer element 14 is clearly shown. The inner coaxial
lumen 40 is defined by the insulating coaxial tube 42. The outer
lumen 46 is defined by the exterior surface of the insulating
coaxial tube 42 and the interior surface 38 of the heat transfer
element 14. In addition, the helical ridges 32 and helical grooves
30 may be seen in FIG. 4. As noted above, in the preferred
embodiment, the depth of the grooves, d.sub.i, is greater than the
boundary layer thickness which would have developed if a
cylindrical heat transfer element were introduced. For example, in
a heat transfer element 14 with a 4 mm outer diameter, the depth of
the invaginations, d.sub.i, may be approximately equal to 1 mm if
designed for use in the carotid artery. Although FIG. 4 shows four
ridges and four grooves, the number of ridges and grooves may vary.
Thus, heat transfer elements with 1, 2, 3, 4, 5, 6, 7, 8 or more
ridges are specifically contemplated.
[0061] FIG. 5 is a perspective view of a heat transfer element 14
in use within a blood vessel, showing only one helical lobe per
segment for purposes of clarity. Beginning from the proximal end of
the heat transfer element (not shown in FIG. 5), as the blood moves
forward during the systolic pulse, the first helical heat transfer
segment 20 induces a counter-clockwise rotational inertia to the
blood. As the blood reaches the second segment 22, the rotational
direction of the inertia is reversed, causing turbulence within the
blood. Further, as the blood reaches the third segment 24, the
rotational direction of the inertia is again reversed. The sudden
changes in flow direction actively reorient and randomize the
velocity vectors, thus ensuring turbulence throughout the
bloodstream. During turbulent flow, the velocity vectors of the
blood become more random and, in some cases, become perpendicular
to the axis of the artery. In addition, as the velocity of the
blood within the artery decreases and reverses direction during the
cardiac cycle, additional turbulence is induced and turbulent
motion is sustained throughout the duration of each pulse through
the same mechanisms described above.
[0062] Thus, a large portion of the volume of warm blood in the
vessel is actively brought in contact with the heat transfer
element 14, where it can be cooled by direct contact rather than
being cooled largely by conduction through adjacent laminar layers
of blood. As noted above, the depth of the grooves 26, 30, 34 (FIG.
2) is greater than the depth of the boundary layer which would
develop if a straight-walled heat transfer element were introduced
into the blood stream. In this way, free stream turbulence is
induced. In the preferred embodiment, in order to create the
desired level of turbulence in the entire blood stream during the
whole cardiac cycle, the heat transfer element 14 creates a
turbulence intensity greater than about 0.05. The turbulence
intensity may be greater than 0.05, 0.06, 0.07 or up to 0.10 or
0.20 or greater.
[0063] Referring back to FIG. 2, the heat transfer element 14 has
been designed to address all of the design criteria discussed
above. First, the heat transfer element 14 is flexible and is made
of a highly conductive material. The flexibility is provided by a
segmental distribution of tube sections 25, 27 which provide an
articulating mechanism. The tube sections have a predetermined
thickness which provides sufficient flexibility. Second, the
exterior surface area 37 has been increased through the use of
helical ridges 28, 32, 36 and helical grooves 26, 30, 34. The
ridges also allow the heat transfer element 14 to maintain a
relatively atraumatic profile, thereby minimizing the possibility
of damage to the vessel wall. Third, the heat transfer element 14
has been designed to promote turbulent kinetic energy both
internally and externally. The modular or segmental design allows
the direction of the invaginations to be reversed between segments.
The alternating helical rotations create an alternating flow that
results in mixing the blood in a manner analogous to the mixing
action created by the rotor of a washing machine that switches
directions back and forth. This mixing action is intended to
promote high level turbulent kinetic energy to enhance the heat
transfer rate. The alternating helical design also causes
beneficial mixing, or turbulent kinetic energy, of the working
fluid flowing internally.
[0064] FIG. 6 is a cut-away perspective view of an alternative
embodiment of a heat transfer element 50. An external surface 52 of
the heat transfer element 50 is covered with a series of axially
staggered protrusions 54. The staggered nature of the outer
protrusions 54 is readily seen with reference to FIG. 7 which is a
transverse cross-sectional view taken at a location denoted by the
line 7-7 in FIG. 6. In order to induce free stream turbulence, the
height, d.sub.p, of the staggered outer protrusions 54 is greater
than the thickness of the boundary layer which would develop if a
smooth heat transfer element had been introduced into the blood
stream. As the blood flows along the external surface 52, it
collides with one of the staggered protrusions 54 and a turbulent
wake flow is created behind the protrusion. As the blood divides
and swirls along side of the first staggered protrusion 54, its
turbulent wake encounters another staggered protrusion 54 within
its path preventing the re-lamination of the flow and creating yet
more turbulence. In this way, the velocity vectors are randomized
and turbulence is created not only in the boundary layer but
throughout the free stream. As is the case with the preferred
embodiment, this geometry also induces a turbulent effect on the
internal coolant flow.
[0065] A working fluid is circulated up through an inner coaxial
lumen 56 defined by an insulating coaxial tube 58 to a distal tip
of the heat transfer element 50. The working fluid then traverses
an outer coaxial lumen 60 in order to transfer heat to the exterior
surface 52 of the heat transfer element 50. The inside surface of
the heat transfer element 50 is similar to the exterior surface 52,
in order to induce turbulent flow of the working fluid. The inner
protrusions can be aligned with the outer protrusions 54, as shown
in FIG. 7, or they can be offset from the outer protrusions 54, as
shown in FIG. 6.
[0066] FIG. 8 is a schematic representation of the invention being
used to cool the brain of a patient. The selective organ
hypothermia apparatus shown in FIG. 8 includes a working fluid
supply 10, preferably supplying a chilled liquid such as water,
alcohol or a halogenated hydrocarbon, a supply catheter 12 and the
heat transfer element 14. The supply catheter 12 has a coaxial
construction. An inner coaxial lumen within the supply catheter 12
receives coolant from the working fluid supply 10. The coolant
travels the length of the supply catheter 12 to the heat transfer
element 14 which serves as the cooling tip of the catheter. At the
distal end of the heat transfer element 14, the coolant exits the
insulated interior lumen and traverses the length of the heat
transfer element 14 in order to decrease the temperature of the
heat transfer element 14. The coolant then traverses an outer lumen
of the supply catheter 12 so that it may be disposed of or
recirculated. The supply catheter 12 is a flexible catheter having
a diameter sufficiently small to allow its distal end to be
inserted percutaneously into an accessible artery such as the
femoral artery of a patient as shown in FIG. 8. The supply catheter
12 is sufficiently long to allow the heat transfer element 14 at
the distal end of the supply catheter 12 to be passed through the
vascular system of the patient and placed in the internal carotid
artery or other small artery. The method of inserting the catheter
into the patient and routing the heat transfer element 14 into a
selected artery is well known in the art.
[0067] Although the working fluid supply 10 is shown as an
exemplary cooling device, other devices and working fluids may be
used. For example, in order to provide cooling, freon,
perflourocarbon, water, or saline may be used, as well as other
such coolants.
[0068] The heat transfer element can absorb or provide over 75
Watts of heat to the blood stream and may absorb or provide as much
as 100 Watts, 150 Watts, 170 Watts or more. For example, a heat
transfer element with a diameter of 4 mm and a length of
approximately 10 cm using ordinary saline solution chilled so that
the surface temperature of the heat transfer element is
approximately 5.degree. C. and pressurized at 2 atmospheres can
absorb about 100 Watts of energy from the bloodstream. Smaller
geometry heat transfer elements may be developed for use with
smaller organs which provide 60 Watts, 50 Watts, 25 Watts or less
of heat transfer.
[0069] The practice of the present invention is illustrated in the
following non-limiting example.
Exemplary Procedure
[0070] 1. The patient is initially assessed, resuscitated, and
stabilized.
[0071] 2. The procedure is carried out in an angiography suite or
surgical suite equipped with flouroscopy.
[0072] 3. Because the catheter is placed into the common carotid
artery, it is important to determine the presence of stenotic
atheromatous lesions. A carotid duplex (doppler/ultrasound) scan
can quickly and non-invasively make this determination. The ideal
location for placement of the catheter is in the left carotid so
this may be scanned first. If disease is present, then the right
carotid artery can be assessed. This test can be used to detect the
presence of proximal common carotid lesions by observing the slope
of the systolic upstroke and the shape of the pulsation. Although
these lesions are rare, they could inhibit the placement of the
catheter. Examination of the peak blood flow velocities in the
internal carotid can determine the presence of internal carotid
artery lesions. Although the catheter is placed proximally to such
lesions, the catheter may exacerbate the compromised blood flow
created by these lesions. Peak systolic velocities greater that 130
cm/sec and peak diastolic velocities greater than 100 cm/sec in the
internal indicate the presence of at least 70% stenosis. Stenosis
of 70% or more may warrant the placement of a stent to open up the
internal artery diameter.
[0073] 4. The ultrasound can also be used to determine the vessel
diameter and the blood flow and the catheter with the appropriately
sized heat transfer element could be selected.
[0074] 5. After assessment of the arteries, the patients inguinal
region is sterilely prepped and infiltrated with lidocaine.
[0075] 6. The femoral artery is cannulated and a guide wire may be
inserted to the desired carotid artery. Placement of the guide wire
is confirmed with flouroscopy.
[0076] 7. An angiographic catheter can be fed over the wire and
contrast media injected into the artery to further to assess the
anatomy of the carotid.
[0077] 8. Alternatively, the femoral artery is cannulated and a
10-12.5 french (f) introducer sheath is placed.
[0078] 9. A guide catheter is placed into the desired common
carotid artery. If a guiding catheter is placed, it can be used to
deliver contrast media directly to further assess carotid
anatomy.
[0079] 10. A 10 f -12 f (3.3-4.0 mm) (approximate) cooling catheter
is subsequently filled with saline and all air bubbles are
removed.
[0080] 11. The cooling catheter is placed into the carotid artery
via the guiding catheter or over the guidewire. Placement is
confirmed with flouroscopy.
[0081] 12. Alternatively, the cooling catheter tip is shaped
(angled or curved approximately 45 degrees), and the cooling
catheter shaft has sufficient pushability and torqueability to be
placed in the carotid without the aid of a guide wire or guide
catheter.
[0082] 13. The cooling catheter is connected to a pump circuit also
filled with saline and free from air bubbles. The pump circuit has
a heat exchange section that is immersed into a water bath and
tubing that is connected to a peristaltic pump. The water bath is
chilled to approximately 0.degree. C.
[0083] 14. Cooling is initiated by starting the pump mechanism. The
saline within the cooling catheter is circulated at 5 cc/sec. The
saline travels through the heat exchanger in the chilled water bath
and is cooled to approximately 1.degree. C.
[0084] 15. It subsequently enters the cooling catheter where it is
delivered to the heat transfer element. The saline is warmed to
approximately 5-7.degree. C. as it travels along the inner lumen of
the catheter shaft to the end of the heat transfer element.
[0085] 16. The saline then flows back through the heat transfer
element in contact with the inner metallic surface. The saline is
further warmed in the heat transfer element to 12-15.degree. C.,
and in the process, heat is absorbed from the blood, cooling the
blood to 30.degree. C. to 32.degree. C.
[0086] 17. The chilled blood then goes on to chill the brain. It is
estimated that 15-30 minutes will be required to cool the brain to
30 to 32.degree. C.
[0087] 18. The warmed saline travels back down the outer lumen of
the catheter shaft and back to the chilled water bath where it is
cooled to 1.degree. C.
[0088] 19. The pressure drops along the length of the circuit are
estimated to be 2-3 atmospheres.
[0089] 20. The cooling can be adjusted by increasing or decreasing
the flow rate of the saline. Monitoring of the temperature drop of
the saline along the heat transfer element will allow the flow to
be adjusted to maintain the desired cooling effect.
[0090] 21. The catheter is left in place to provide cooling for 12
to 24 hours.
[0091] 22. If desired, warm saline can be circulated to promote
warming of the brain at the end of the procedure.
[0092] The invention has also been described with respect to
certain embodiments. It will be clear to one of skill in the art
that variations of the embodiments may be employed in the method of
the invention. For example, the tube sections may employ materials
other than metals or rubbers, so long as the materials have
characteristics similar to those described above. Accordingly, the
invention is limited only by the scope of the appended claims.
* * * * *