Note: Descriptions are shown in the official language in which they were submitted.
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FEVER REGULATION METHOD AND APPARATUS
This is a divisional of Canadian Patent Application Serial
No. 2,465,435 filed on November 7, 2001.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to the lowering and control of
the temperature of the human body. More particularly, the invention relates to
a
method and intravascular apparatus for cooling the whole body, especially
during
periods of fever.
Background Information
Organs in the human body, such as the brain, kidney and heart, are
maintained at a constant temperature of approximately 37 C. Hypothermia can
be clinically defined as a core body temperature of 35 C or less. Hypothermia
is
sometimes characterized further according to its severity. A body core
temperature in the range of 33 C to 35 C is described as mild hypothermia. A
body
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temperature of 28 C to 32 C is described as moderate hypothermia. A body
core
temperature in the range of 24 C to 28 C is described as severe hypothermia.
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 ischernia, 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.
Cerebral hypothermia has traditionally been accomplished through whole body
cooling to create a condition of total body hypothermia in the range of 20 C
to 30 C.
The currently-employed techniques and devices used to cause total body
hypothermia
lead to various side effects. In addition to the undesirable side effects,
present methods
of administering total body hypothermia are cumbersome.
Catheters have been developed which are inserted into the bloodstream of the
patient in order to induce total body hypothermia. For example, U.S. Patent
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 rigid metallic catheter. The 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. Thus, the Dato device
cools
along the length of a very elongated device. Use of the Dato device is highly
cumbersome due to its size and lack of flexibility.
U.S. Patent No. 5,837,003 to Ginsburg also discloses a method and apparatus
for controlling a patient's body temperature. In this technique, a flexible
catheter is
inserted into the femoral artery or vein or the jugular vein. The catheter may
be in the
form of a balloon to allow an enhanced surface area for heat transfer. A
thermally
conductive metal foil may be used as part of a heat-absorbing surface. This
device
fails to disclose or teach use of any ability to enhance heat transfer. In
addition, the
disclosed device fails to disclose temperature regulation.
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An ailment particular susceptible to treatment by cooling, either selective or
whole body, is fever or hyperthennia. There is a growing awareness of the
dangers
associated with fever. Many patients, especially after surgery and/or in the
intensive
care unit, suffer from fever. For example, it is estimated that 90% of
patients in
neurointensive care units suffering from sub-arachnoid hemorrhage have a
fever.
Further, 60% of patients in neurointensive care units suffering from intra-
cranial
hemorrhage have a fever. 80% of patients in neurointensive care units
suffering from
traumatic brain injury have a fever. These patients are typically treated with
Tylenol,
cooling blankets, or other such methods. These methods are not believed to be
very
effective; moreover, they are difficult to control.
Therefore, a practical method and apparatus that lowers and controls the
temperature of the human body satisfies a long-felt need.
SUMMARY OF THE INVENTION
In one aspect, the apparatus of the present invention can include a heat
transfer
element that can be used to apply cooling to the blood flowing in a large vein
feeding
the heart.
The heat transfer element, by way of example only, includes first and second
elongated, articulated segments, each segment having a mixing-inducing
exterior
surface. A flexible joint can connect the first and second elongated segments.
An inner
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
mixing-
inducing interior surface for inducing mixing within the pressurized working
fluid.
The mixing-inducing exterior surface may be adapted to induce mixing within a
blood
flow when placed within an artery or vein. In one embodiment, the flexible
joint
includes a bellows section that also allows for axial compression of the heat
transfer
element as well as for enhanced flexibility. In alternative embodiments, the
bellows
section may be replaced with flexible tubing such as small cylindrical polymer
connecting tubes.
In one embodiment, the mixing-inducing exterior surfaces of the heat transfer
element include one or more helical grooves and ridges. Adjacent segments of
the heat
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transfer element can be oppositely spiraled to increase mixing. For instance,
the first
elongated heat transfer segment may include one or more helical ridges having
a
counter-clockwise twist, while the second elongated heat transfer segment
includes
one or more helical ridges having a clockwise twist. Alternatively, of course,
the first
elongated heat transfer segment may include one or more clockwise helical
ridges, and
the second elongated heat transfer segment may include one or more counter-
clockwise helical ridges. The first and second elongated, articulated segments
may be
formed from highly conductive materials such as metals, thin polymers, or
doped
polymers.
The heat transfer device may also have a supply catheter with an inner
catheter
lumen coupled to the inner 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 or alternatively to the
supply
catheter. The working fluid supply may be configured to produce the
pressurized
working fluid at a temperature of about 0 C and at a pressure below about 5
atmospheres of pressure.
In yet another alternative embodiment, the heat transfer device may have three
or more elongated, articulated, heat transfer segments each having a mixing-
inducing
exterior surface, with additional flexible joints connecting the additional
elongated
heat transfer segments. In one such embodiment, by way of example only, the
first and
third elongated heat transfer segments may include clockwise helical ridges,
and the
second elongated heat transfer segment may include one or more counter-
clockwise
helical ridges. Alternatively, of course, the first and third elongated heat
transfer
segments may include counter-clockwise helical ridges, and the second
elongated heat
transfer segment may include one or more clockwise helical ridges.
The mixing-inducing exterior surface of the heat transfer element may
optionally include a surface coating or treatment to inhibit clot formation. A
surface
coating may also be used to provide a degree of lubricity to the heat transfer
element
and its associated catheter.
An embodiment of the present invention is also directed to a method of
treating fever in the body
by inserting a flexible cooling element into a vein that is in pressure
communication
with the heart, e.g., the femoral or iliac veins, the superior or inferior
vena cavae or
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both. The vena cavae may be accessed via known techniques from the jugular
vein or
from the subclavian or femoral veins, for example. The heat transfer element
in one or
both vena cavae may then cool virtually all the blood being returned to the
heart. The
cooled blood enters the right atrium at which point the same is pumped through
the
right ventricle and into the pulmonary artery to the lungs where the same is
oxygenated. Due to the heat capacity of the lungs, the blood does not
appreciably
warm during oxygenation. The cooled blood is returned to the heart and pumped
to
the entire body via the aorta. Thus, cooled blood may be delivered indirectly
to a
chosen organ such as the brain. This indirect cooling is especially effective
as high
blood flow organs such as the heart and brain are preferentially supplied
blood by the
vasculature.
A warming blanket or other warming device may be applied to portions of the
body to provide comfort to the patient and to inhibit thennoregulatory
responses such
as vasoconstriction. Thermoregulatory drugs may also be so provided for this
reason.
The method further includes circulating a working fluid through the flexible,
conductive cooling element in order to lower the temperature of the blood in
the vena
cava. The flexible, conductive heat transfer element preferably absorbs more
than
about 100 or 300 Watts of heat.
The method may also include inducing mixing within the free stream blood
flow within the vena cava. It is noted that a degree of turbulence or mixing
is
generally present within the vena cava anyway. The step of circulating may
include
inducing mixing in the flow of the working fluid through the flexible heat
transfer
element. The pressure of the working fluid may be maintained below about 5
atmospheres of pressure. "
An embodiment of the present invention also envisions a method for lowering a
fever in the body
of a patient which includes introducing a catheter, with a cooling element,
into a vena
cava supplying the heart, the catheter having a diameter of about 18 mm or
less,
inducing mixing in blood flowing over the cooling element, and lowering the
temperature of the cooling element to remove heat from the blood to cool the
blood.
In one embodiment, the cooling step removes at least about 50 Watts of heat
from the
blood. The mixing induced may result in a Nusselt number enhancement of the
flow
of between about 5 and 80.
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Possible advantages are numerous. Patients can be provided with an efficient
method of reducing fever that does not suffer from the deleterious
consequences of the prior
art. The procedure can be administered safely and easily. Numerous cardiac and
neural
settings can benefit by the hypothermic therapy. Other advantages will be
understood from
the following.
According to one aspect of the present invention there is provided a method,
comprising: providing a catheter having a cooling element attached to a distal
end thereof,
said catheter capable of insertion through the vascular system of a patient
with a fever to place
the cooling element in a vein that drains into the heart of the patient, and
further configured
such that fluid may be circulated through the cooling element thereby
transferring heat from
the blood in the vein to the cooling element; measuring a starting power
withdrawn from the
blood and a starting body temperature at the beginning of the circulating of
the fluid;
measuring a power withdrawn from the blood and the body temperature during the
circulating; measuring a power withdrawn from the blood at substantially the
time when the
body temperature equals a normothermic temperature; after fluid has circulated
through the
heat transfer element, at substantially the time when the body temperature has
been reduced
from the starting body temperature to the normothermic temperature,
calculating a
temperature which the blood would have in the absence of cooling, Tabsence of
cooling from:
(1) the normothermic temperature, (2) the starting power withdrawn from the
blood, (3) the
starting body temperature, and (4) the power withdrawn from the blood at
substantially the
time when the body temperature equals the normothermic temperature; and
comparing the
Tabsence of cooling with the normothermic temperature, to determine whether to
continue the
circulating if Tabsence of cooling is greater than the normothermic
temperature, or discontinue the
circulating if Tabsence of cooling is less than the normothermic temperature.
According to another aspect of the present invention there is provided a
computer-readable storage medium having stored thereon computer-executable
instructions
comprising code for causing a chiller console, circulating set, and a
catheter, having a heat
transfer element at a distal end thereof, which may be inserted through the
vascular system of
a patient thereby transferring heat from the blood in a vein to the heat
transfer element to:
circulate fluid through the heat transfer element, and measure a starting
power withdrawn
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from the blood and a starting body temperature of the patient at the beginning
of the
circulating of the fluid; transfer heat from the blood in the vasculature to
the heat transfer
element and thereby lower the body temperature of a patient; measure a power
withdrawn
from the blood and the body temperature during the circulating; measure a
power withdrawn
from the blood at substantially the time when the body temperature equals a
normothermic
temperature; calculate, after having circulated the fluid through the heat
transfer element, and
at substantially the time when the body temperature has been reduced to the
normothermic
temperature, a temperature which the blood would have in the absence of
cooling,
Tabsence of cooling, from: (1) the normothermic temperature, (2) the starting
power withdrawn
from the blood, (4) the starting body temperature, and (4) the power withdrawn
from the
blood at substantially the time when the body temperature equals the
normothermic
temperature; and compare the Tabsence of cooling with the normothermic
temperature, and
continuing the circulating if Tabsence of cooling is greater than the
normothermic temperature, and
discontinuing the circulating if Tabsence of cooling is less than the
normothermic temperature.
Novel features, 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 DRAWINGS
Figure 1 is an elevation view of one embodiment of a heat transfer element
according to the invention;
Figure 2 is a longitudinal section view of the heat transfer element of Figure
1;
Figure 3 is a transverse section view of the heat transfer element of Figure
1;
Figure 4 is a perspective view of the heat transfer element of Figure 1 in use
within a blood vessel;
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Figure 5 is a schematic representation of the heat transfer element being used
in an embodiment within the superior vena cava; and
Figure 6 is a flowchart showing an exemplary method of the invention.
DETAILED DESCRIPTION OF THE INVENTION
OVERVIEW
A one or two-step process and a one or two-piece device may be employed to
intravascularly lower the temperature of a body in order to treat fever. A
cooling element may
be placed in a high-flow vein such as the vena cavae to absorb heat from the
blood flowing
into the heart. This transfer of heat causes a cooling of the blood flowing
through the heart
and thus throughout the vasculature. Such a method and device may
therapeutically be used
to treat fever.
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A heat transfer element that systemically cools blood should be capable of
providing the necessary heat transfer rate to produce the desired cooling
effect
throughout the vasculature. This may be up to or greater than 300 watts, and
is at least
partially dependent on the mass of the patient and the rate of blood flow.
Surface
features may be employed on the heat transfer element to enhance the heat
transfer
rate. The surface features and other components of the heat transfer element
are
described in more detail below.
One problem with treating fever with cooling is that the cause of the
patient's
fever attempts to defeat the cooling. Thus, a high power device is often
required.
ANATOMICAL PLACEMENT
The internal jugular vein is the vein that directly drains the brain. The
external
jugular joins the internal jugular at the base of the neck. The internal
jugular veins join
the subclavian veins to form the brachiocephalic veins that in turn drain into
the
superior vena cava. The superior vena cava drains into the right atrium of the
heart
and supplies blood to the heart from the upper part of the body.
A cooling element may be placed into the superior vena cava, inferior vena
cava, or otherwise into a vein which feeds into the superior vena cava or
otherwise
into the heart to cool the body. A physician may percutaneously place the
catheter into
the subclavian or internal or external jugular veins to access the superior
vena Gaya.
The blood, cooled by the heat transfer element, may be processed by the heart
and
provided to the body in oxygenated form to be used as a conductive medium to
cool
the body. The lungs have a fairly low heat capacity, and thus the lungs do not
cause
appreciable rewarming of the flowing blood.
The vasculature by its very nature provides preferential blood flow to the
high
blood flow organs such as the brain and the heart. Thus, these organs are
preferentially
cooled by such a procedure. The core body temperature may be measured by an
esophageal probe. The brain temperature usually decreases more rapidly than
the core
body temperature. The inventors believe this effect to be due to the
preferential supply
of blood provided to the brain and heart. This effect may be even more
pronounced if
thermoregulatory effects, such as vasoconstriction, occur that tend to focus
blood
supply to the core vascular system and away from the peripheral vascular
system.
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HEAT TRANSFER
When a heat transfer element is inserted approximately coaxially into an
artery
or vein, the primary mechanism of heat transfer between the surface of the
heat
transfer element and the blood is forced convection. Convection relies upon
the
movement of fluid to transfer heat. Forced convection results when an external
force
causes motion within the fluid. In the case of arterial or venous flow, the
beating heart
causes the motion of the blood around the heat transfer element.
The magnitude of the heat transfer rate is proportional to the surface area of
the heat transfer element, the temperature differential, and the heat transfer
coefficient
of the heat transfer element.
The receiving artery or vein into which the heat transfer element is placed
has
a limited diameter and length. Thus, the surface area of the heat transfer
element must
be limited to avoid significant obstruction of the artery or vein and to allow
the heat
transfer element to easily pass through the vascular system. For placement
within the
superior vena cava via the external jugular, the cross sectional diameter of
the heat
transfer element may be limited to about 5-6 mm, and its length may be limited
to
approximately 10-15 cm. For placement within the inferior vena cava, the cross
sectional diameter of the heat transfer element may be limited to about 6-7
mm, and
its length may be limited to approximately 25-35 cm.
Decreasing the surface temperature of the heat transfer element can increase
the temperature differential. However, the minimum allowable surface
temperature is
limited by the characteristics of blood. Blood freezes at approximately 0 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 the convection heat transfer coefficient. In addition, increased viscosity
of the blood
may 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
minimum allowable surface temperature of the cooling element to approximately
5
C. This results in a maximum temperature differential between the blood stream
and
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the cooling element of approximately 32 C. For other physiological reasons,
there are
limits on the maximum allowable surface temperature of the warming element.
However, in certain situations, temperatures lower than 0 C may be used. For
example, certain patients may have blood flows such that the flow per se
prohibits or
significantly inhibits freezing. To achieve such cooling, sub-zero
temperatures may be
used. In these cases, working fluids such as perfluorocarbons may be employed.
The mechanisms by which the value of the convection heat transfer coefficient
may be increased are complex. However, it is well known that the convection
heat
transfer coefficient increases with the level of "mixing" or "turbulent"
kinetic energy
in the fluid flow. Thus it is advantageous to have blood flow with a high
degree of
mixing in contact with the heat transfer element.
The blood flow has a considerably more stable flux in the vena cava than in an
artery. However, the blood flow in the vena cava still has a high degree of
inherent
mixing or turbulence. Reynolds numbers in the superior vena cava may range,
for
example, from 2,000 to 5,000. Thus, blood cooling in the vena cava may benefit
from
enhancing the level of mixing with the heat transfer element but this benefit
may be
substantially less than that caused by the inherent mixing.
BOUNDARY LAYERS
A thin boundary layer has been shown to form during the cardiac cycle.
Boundary layers develop adjacent to the heat transfer element as well as next
to the
walls of the artery or vein. Each of these boundary layers has approximately
the same
thickness as the boundary layer that would have developed at the wall of the
artery in
the absence of the heat transfer element. The free stream flow region is
developed in
an annular ring around the heat transfer element. The heat transfer element
used in
such a vessel should reduce the formation of such viscous boundary layers.
HEAT TRANSFER ELEMENT CHARACTERISTICS AND DESCRIPTION
The intravascular heat transfer element should be flexible in order to be
placed
within the vena cavae or other veins or arteries. The flexibility of the heat
transfer
element is an important characteristic because the same is typically inserted
into a
vein such as the external jugular and accesses the vena cava by initially
passing
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though a series of one or more branches. Further, the heat transfer element is
ideally
constructed from a highly thermally conductive material such as metal in order
to
facilitate heat transfer. The use of a highly thermally conductive material
increases the
heat transfer rate for a given temperature differential between the working
fluid within
the heat transfer element and the blood. This facilitates the use of a higher
temperature
coolant, or lower temperature warming fluid, within the heat transfer element,
allowing safer working fluids, such as water or saline, to be used. Highly
thermally
conductive materials, such as metals, tend to be rigid. Therefore, the design
of the heat
transfer element should facilitate flexibility in an inherently inflexible
material.
However, balloon designs may also be employed, such as those disclosed in
co-pending U.S. Patent Application Serial No. 09/215,038, filed 12/16/98,
entitled
"Inflatable Catheter for Selective Organ Heating and Cooling and Method of
Using
the Same,", now U.S. Patent No. 6,261,312 .
It is estimated that the cooling element should absorb at least about 50 Watts
of heat when placed in the vena cava to lower the temperature of the body to
between
about 30 C and 34 C. These temperatures are thought to be appropriate to lower
most
fevers. The power removed determines how quickly the target temperature can be
reached. For example, in a fever therapy in which it is desired to lower brain
temperature, the same may be lowered about 4 C per hour in a 70 kg human upon
removal of 300 Watts.
One embodiment of the invention uses a modular design. This design creates
helical blood flow and produces a level of mixing in the blood flow by
periodically
forcing abrupt changes in the direction of the helical blood flow. The abrupt
changes
in flow direction are achieved through the use of a series of two or more heat
transfer
segments, each included of one or more helical ridges. 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
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motion within the clothes-water slurry. These surface features also tend to
increase the
surface area of the heat transfer element, further enhancing heat transfer.
Figure 1 is an elevation view of one embodiment of a cooling element 14
according to the present invention. The heat transfer element 14 includes 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 without
departing from the spirit of the invention. As seen in Figure 1, a first
elongated heat
transfer segment 20 is located at the proximal end of the heat transfer
element 14. A
mixing-inducing exterior surface of the segment 20 includes 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 without departing from the
spirit of the
present invention. 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.
The first heat transfer segment 20 is coupled to a second elongated heat
transfer segment 22 by a first bellows section 25, which provides flexibility
and
compressibility. The second heat transfer segment 22 includes 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 bellows section 27. The
third
heat transfer segment 24 includes 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.
In addition, the rounded contours of the ridges 28, 32, 36 allow the heat
transfer element 14 to maintain a relatively atraumatic profile, thereby
minimizing the
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possibility of damage to the blood vessel wall. A heat transfer element
according to
the present invention may include two, three, or more heat transfer segments.
The bellows 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 that is cycled
through
the heat transfer element 14. The structure of the bellows 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
bellows sections 25, 27 also provide for axial compression of the heat
transfer element
14, which can limit the trauma when the distal end of the heat transfer
element 14
abuts a blood vessel wall. The bellows sections 25, 27 are also able to
tolerate
cryogenic temperatures without a loss of performance. In alternative
embodiments, the
bellows may be replaced by flexible polymer tubes, which are bonded between
adjacent heat transfer segments.
The exterior surfaces of the heat transfer element 14 can be made from metal,
and may include very high thermal conductivity materials such as nickel,
thereby
facilitating 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.
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. In
particular,
one may wish to treat the bellows sections 25, 27 because stagnation of the
blood flow
may occur in the convolutions, thus allowing clots to form and cling to the
surface to
form a thrombus. 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
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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. Another
coating
that provides beneficial properties may be a lubricious coating. Lubricious
coatings,
on both the heat transfer element and its associated catheter, allow for
easier
placement in the, e.g., vena cava.
Figure 2 is a longitudinal sectional view of the heat transfer element 14 of
an
embodiment of the invention, taken along line 2-2 in Figure 1. Some interior
contours
are omitted for purposes of clarity. An inner tube 42 creates an inner lumen
40 and an
outer 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 lumen 40. At the distal end of the heat transfer
element 14, the
working fluid exits the inner 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
p olytetrafluoro ethylene or another polymer.
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 that
remains a liquid
as the working fluid. Other coolants such as Freon undergo nucleate boiling
and create
mixing through a different mechanism. Saline is a safe working fluid, 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 mixing in the working fluid is
enhanced by
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the shape of the interior surface 38 of the heat transfer element 14, the
working fluid
can be delivered to the cooling element 14 at a warmer temperature and still
achieve
the necessary cooling rate. Similarly, since mixing in the working fluid is
enhanced by
the shape of the interior surface of the heat transfer element, the working
fluid can be
delivered to the warming element 14 at a cooler temperature and still achieve
the
necessary warming rate.
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.
Figure 3 is a transverse sectional view of the heat transfer element 14 of the
invention, taken at a location denoted by the line 3-3 in Figure 1. Figure 3
illustrates a
five-lobed embodiment, whereas Figure 1 illustrates a four-lobed embodiment.
As
mentioned earlier, any number of lobes might be used. In Figure 3, the
construction of
the heat transfer element 14 is clearly shown. The inner lumen 40 is defined
by the
insulating tube 42. The outer lumen 46 is defined by the exterior surface of
the
insulating 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 Figure
3.
Although Figure 3 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.
Figure 4 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
Figure 4),
as the blood moves forward, the first helical heat transfer segment 20 induces
a
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counter-clockwise rotational inertia to the blood. As the blood reaches the
second
segment 22, the rotational direction of the inertia is reversed, causing
mixing 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 mixing throughout the
bloodstream.
During such mixing, the velocity vectors of the blood become more random and,
in
some cases, become perpendicular to the axis of the vessel. 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.
Referring back to Figure 1, 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 bellows sections 25, 27 that provide an articulating
mechanism. Bellows have a known convoluted design that provide 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 mixing both internally and externally. The modular or segmental design
allows the direction of the grooves 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 action is intended to promote mixing
to
enhance the heat transfer rate. The altemating helical design also causes
beneficial
mixing, or turbulent kinetic energy, of the working fluid flowing internally.
METHOD OF USE
The practice of the present invention is illustrated in the following non-
limiting example.
Exemplary Procedure
1. The patient is initially assessed as having a fever, resuscitated,
and stabilized.
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2. The procedure may be carried out in an angiography suite, MCU, ICU, or
surgical suite equipped with fluoroscopy.
3. An ultrasound or angiogram of the superior vena cava and external jugular
can
be used to determine the vessel diameter and the blood flow; a catheter with
an
appropriately sized heat transfer element can be selected.
4. After assessment of the veins, the patient is sterilely prepped and
infiltrated with
lidocaine at a region where the appropriate vein may be accessed.
5. The external jugular is cannulated and a guide wire may be inserted to the
superior vena cava. Placement of the guide wire is confirmed with fluoroscopy.
6. An angiographic catheter can be fed over the wire and contrast media
injected
into the vein to further to assess the anatomy if desired.
7. Alternatively, the external jugular is cannulated and a 10-12.5 french (f)
introducer sheath is placed.
8. A guide catheter is placed into the superior vena cava_ If a guide catheter
is
placed, it can be used to deliver contrast media directly to further assess
anatomy.
9. The cooling catheter is placed into the superior vena cava via the guiding
catheter or over the guidewire.
10. Placement is confirmed if desired with fluoroscopy.
11. Alternatively, the cooling catheter shaft has sufficient pushability and
torqueability to be placed in the superior vena cava without the aid of a
guide
wire or guide catheter.
12. 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 C.
13. 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 C.
14. The saline subsequently enters the cooling catheter where it is delivered
to the
heat transfer element. The saline is warmed to approximately 5-7 C as it
travels
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along the inner lumen of the catheter shaft to the end of the heat transfer
element.
15. 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 C, and in the process, heat is absorbed from the blood, cooling the
blood to 30 C to 35 C.
16. The chilled blood then goes on to chill the body. It is estimated that
less than an
hour will be required to substantially reduce a fever down to normothermia.
17. The warmed saline travels back the outer lumen of the catheter shaft and
is
returned to the chilled water bath where the same is cooled to 1 C.
18. The pressure drops along the length of the circuit are estimated to be
between 1
and 10 atmospheres.
19. 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.
20. The catheter is left in place to provide cooling for, e.g., 6-48 hours.
Of course, the use of the superior vena cava is only exemplary. It is
envisioned
that the following veins may be appropriate to percutaneously insert the heat
transfer
element: femoral, internal jugular, subelavian, and other veins of similar
size and
position. It is also envisioned that the following veins may be appropriate in
which to
dispose the heat transfer element during use: inferior vena cava, superior
vena cava,
femoral, internal jugular, and other veins of similar size and position.
Arteries may
also be employed if a fever therapy selective to a particular organ or region
of the
body is desired.
Figure 5 shows a cross-section of the heart in which the heat transfer element
14 is disposed in the superior vena cava 62. The heat transfer element 14 has
rotating
helical grooves 22 as well as counter-rotating helical grooves 24. Between the
rotating
and the counter-rotating grooves are bellows 27. It is believed that a design
of this
nature would enhance the Nusselt number for the flow in the superior vena cava
by
about 5 to 80.
In some cases, a heating blanket may be used. The heating blanket serves
several purposes. By warming the patient, vasoconstriction is avoided. The
patient is
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also made more comfortable. For example, it is commonly agreed that for every
one
degree of core body temperature reduction, the patient will continue to feel
comfortable if the same experiences a rise in surface area (skin) temperature
of five
degrees. Spasms due to total body hypothermia may be avoided. Temperature
control
of the patient may be more conveniently performed as the physician has another
variable (the amount of heating) which may be adjusted.
As an alternative, the warming element may be any of the heating methods
proposed in U.S. Patent Application Serial No. 09/292,532, filed on 4/15/99,
and
entitled "Isolated Selective 'Organ Cooling Method and Apparatus:'
Anti-shivering drugs may be used to provide the features of the heating
blanket. In this connection, meperidine is an analgesic of the phenyl
piperdine class
that is known to bind to the opiate receptor. Meperidine may be used to treat
shivering
due to post-operative anesthesia as well as hypothermia induced in a fever
suppression
treatment.
In a method according to an embodiment of the invention for treating patients
with fever, the heat transfer element as described may be placed in any of
several
veins, including the femoral, the IVC, the SVC, the subclavian, the
braichiocephalic,
the jugular, and other such veins. The heat transfer element may also be
placed in
appropriate arteries for more selective fever reduction.
The amount of cooling performed may be judged to a first approximation by
the rate of cool-down. The amount of cooling is proportional to the difference
between the temperature of the blood and the temperature of the heat transfer
element
or cooling element. Thus, if the temperature of the blood is 40 C and the
temperature
of the cooling element is 5 C, the power extracted will be greater than if the
temperature of the blood is 38 C and the temperature of the cooling element is
maintained at 5 C. Thus, the cool-down or cooling rate is generally greatest
at the
beginning of a cooling procedure. Once the patient temperature begins to
approach
the target temperature, usually normothermia or 37oC, the cooling rate may be
reduced because the temperature differential is no longer as great.
In any case, once the patient reaches the normothermie temperature, it is no
longer easy to guess whether, in the absence of the cooling therapy, the
patient would
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otherwise be feverish or whether the fever has abated. One embodiment of the
invention allows a determination of this.
First, it is noted that the power extracted can be calculated from the
temperature differential between the working fluid supply temperature and the
working fluid return temperature. In particular:
Pcathetee = M cf. AT(
Where Peatheter is the power extracted, M is the mass flow rate of the working
fluid, cf
is the heat capacity of the working fluid, and AT is the temperature
differential
between the working fluid as it enters the catheter and as it exits the
catheter.
Accordingly, P
- catheter can be readily calculated by measuring the mass flow of the
circulating fluid and the temperature difference between the working fluid as
it enters
and exits the catheter. The power removed by the catheter as determined above
may
be equated to a close approximation to the power that is lost by the patient's
body.
In general, a closed-form solution for the power P required to cool (or heat)
a body at
temperature T to temperature To is not known. One possible approximation may
be to
assume an exponential relationship:
P = a (exp13(T-To) ¨ 1)
Taking the derivative of each side with respect to temperature:
aP 13 (T ¨To)
_______ = afi e
aT
and taking the inverse of each side:
OT
op afle [1 (T ¨To)
or
ar
AT-AP
aP
where AT is the temperature differential from nominal temperature and AP is
the
measured power.
A close approximation may be obtained by assuming the relationship is linear.
Equivalently, a power series expansion may be taken, and the linear term
retained.
In any case, integrating, assuming a linear relationship, and rearranging:
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P ---- a (T-T0), where the constant of proportionality has units of
watts/degree Celsius.
One can determine the constant of proportionality a using two points during
the
therapy when both T and P are finite and known. One may be when therapy
begins,
i.e., when the patient has temperature T and the catheter is drawing power P.
Another
point may be obtained when T To and P Po.
Then, for any P, T is given by:
at T
+
absence of therapy 0
cc
An example of this may be seen in Fig. 6, which shows a flowchart of an
embodiment of a method of the invention. Referring to the figure, a patient
presents
at a hospital or clinic with a fever (step 202). Generally, such a patient
will have a
fever as a result of a malady or other illness for which hospitalization is
required. For
example, the majority of patients in ICUs present with a fever.
A catheter with a heat transfer element thereon may be inserted (step 204).
The initial power withdrawn Pstart and body temperature Tstart may be measured
(step
206), and the therapy begun (step 208). The therapy continues (step 210), and
P and T
are periodically, continuously, or otherwise measured (step 212). The measured
T is
compared to the normothennic T = To, which is usually about 37 C (step 214).
If T is
greater than TO, the therapy continues (step 210). If T is less than To, then
the power
Po is measured at T = To (step 216). By the equations above, a constant of
proportionality a may be uniquely determined (step 218) by knowledge of
Tstart, Pstart,
PO, and To. From a, Tstart, Pstart, PO, and To, Tabsence of cooling may be
determined (step
220). Tabsence of cooling is then compared to To (step 222). If Tabsence of
cooling '1'0, then
the patient is still generating enough power via their metabolism to cause a
fever if the
therapy were discontinued. Thus, therapy is continued (step 224). If Tabsence
of cooling
<= To, then the patient is no longer generating enough power via their
metabolism to
cause a fever if the therapy were discontinued. Thus, therapy is discontinued
(step
226). Variations of the above method will be apparent to those of ordinary
skill in the
art.
While the invention herein disclosed is capable of obtaining the objects
hereinbefore stated, it is to be understood that this disclosure is merely
illustrative of
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the presently preferred embodiments of the invention and that no limitations
are
intended other than as described in the appended claims.
What is claimed is:
21
