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Description  |
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BACKGROUND Of THE INVENTION
1. Field of the Invention
The present invention relates generally to the selective modification and
control of a patient's body temperature. More particularly, the present
invention provides methods and apparatus for treating hypothermia or
hyperthermia by inserting a catheter into a blood vessel of the patient
and selectively controlling the temperature of a portion of the catheter
within the blood vessel. Heat is transferred to or from blood flowing
through the vessel and the patient's body temperature may thereby be
increased or decreased as desired.
2. Description of the Background Art
Under ordinary circumstances the thermoregulatory system of the human body
maintains a near constant temperature of about 37.degree. C. (98.6.degree.
F.). Heat lost to the environment is precisely balanced by heat produced
within the body.
Hypothermia is a condition of abnormally low body temperature. 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 from 32.degree. C. to
35.degree. C. is described as "mild" hypothermia, 30.degree. C. to
32.degree. C. is called "moderate," 24.degree. C. to 30.degree. C. is
described as "severe," and a body temperature less than 24.degree. C.
constitutes "profound" hypothermia. Although the above ranges provide a
useful basis for discussion, they are not absolutes and definitions vary
widely in the medical literature.
Accidental hypothermia results when heat loss to the environment exceeds
the body's ability to produce heat internally. In many cases,
thermoregulation and heat production are normal but the patient becomes
hypothermic due to overwhelming environmental cold stress. This is a
relatively common condition, often resulting from exposure to the
elements. Hypothermia may also occur in patients exposed to mild cold
stress whose thermoregulatory ability has been lessened due to injury or
illness. For example, this type of hypothermia sometimes occurs in
patients suffering from trauma or as a complication in patients undergoing
surgery.
Hypothermia of either type is a dangerous condition which can have serious
medical consequences. In particular, hypothermia interferes with the
ability of the heart to pump blood. Hypothermia may be fatal for this
reason alone. Additionally, low body temperature seriously interferes with
the enzymatic reactions necessary for blood clotting. This sometimes
results in bleeding that is very difficult to control, even when normal
clotting factor levels are present. These effects and other adverse
consequences of hypothermia lead to drastically increased mortality rates
both among victims of trauma and in patients undergoing surgery.
Simple methods for treating hypothermia have been known since very early
times. Such methods include wrapping the patient in blankets,
administering warm fluids by mouth, and immersing the patient in a warm
water bath. Even these simple methods may be effective if the hypothermia
is not too severe. These simple methods are limited in their effectiveness
however. Wrapping the patient in blankets ultimately depends on the
patient's own production of heat to rewarm his body. In even moderate
cases of hypothermia, or in the case of an ill or injured patient, the
patient may simply be too weak or exhausted to produce sufficient heat.
Oral administration of a warm fluid requires that the patient be conscious
and capable of swallowing the fluid. Since loss of consciousness occurs
early in hypothermia, this method is also limited to moderate cases.
Finally, immersion of the patient in a warm water bath is often simply
impractical. For example, immersion of a patient undergoing surgery would
obviously be undesirable. Furthermore, the immersion technique is time
consuming and may be ineffective in that it requires the transmission of
warmth from the patient's skin surface into the body core before the
benefit of the warmth can be realized.
For this reason, methods have been devised to allow for the direct warming
of a patient's blood. These methods involve removing blood from the
patient, warming the blood in external warming equipment, and delivering
the blood back into the patient. While such methods are much more
effective than any of the simple methods previously described, they are
disadvantageous for other reasons. First, the apparatus involved is quite
cumbersome. Second, some danger is involved in even the temporary removal
of significant quantities of blood from an already weakened patient. In
fact, a further drop in body temperature is often experienced when blood
is first removed for warming in the external apparatus. It would be
desirable for these reasons to provide a method and apparatus for directly
warming blood in situ, i.e., within the patient's body.
Hyperthermia, a condition of abnormally high body temperature, may result
from exposure to a hot environment, overexertion, or fever. Body core
temperatures can range from 38.degree. C.-41.degree. C. due to fever and
may be substantially higher in cases of exposure and overexertion. Like
hypothermia, hyperthermia is a serious condition and can be fatal. Also
like hypothermia, simple methods for treating hyperthermia, for example,
immersion of the patient in a cool water bath or administration of cool
fluids, have long been known. Generally, these simple methods for treating
hyperthermia suffer from the same drawbacks and limited effectiveness as
the simple hypothermia treatments noted above.
It would therefore be desirable to develop more effective methods for
lowering the body temperature of hyperthermic patients. Furthermore, it is
sometimes beneficial to induce an artificial low-temperature condition
(induced hypothermia) within a patient by artificial cooling. This may be
desirable, for example, to reduce a patient's requirement for oxygen
during surgery or during a condition of cardiovascular collapse.
To achieve these goals, methods have been used in which a patient's blood
is removed from his body, cooled in external cooling apparatus, and
returned to his body. This external cooling suffers from the same
disadvantages as the external warming previously described. External
cooling requires cumbersome apparatus and the temporary removal of blood
entails some degree of risk to the patient. It would therefore be
desirable to devise a method and apparatus for cooling blood within the
patient's body.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for modifying and
controlling a patient's body temperature. According to the present
invention, a catheter is inserted percutaneously into a blood vessel of
the patient. By controlling the temperature of a portion of the catheter
lying within the blood vessel, heat may be selectively transferred to or
from blood flowing through the vessel. The patient's body temperature may
thereby be increased or decreased as desired. Some embodiments of
apparatus suitable for practicing the present invention will provide means
for treating hypothermia by warming a patient's blood. Other embodiments
will provide means for treating hyperthermia or inducing a desired
condition of hypothermia by cooling the patient's blood.
Because blood circulates rapidly through the vascular system, the
beneficial effect of warming or cooling blood within the vessel will be
quickly felt throughout the patient's body. In situ modification of blood
temperature is further advantageous in that blood is not removed from the
patient. Additionally, no external pump is needed to circulate the blood.
Injury to blood components from the pump is thereby eliminated.
Furthermore, the required apparatus is much simpler, less cumbersome, and
easier to use than the external blood warming or cooling apparatus
previously known.
A catheter suitable for practicing the present invention will include means
for warming or cooling at least a portion of the catheter inserted into
the blood vessel. It is desirable that such a catheter have a relatively
small cross-section so as not to unnecessarily impede blood flow through
the vessel. On the other hand, a large heat transfer surface area will
facilitate rapid heat transfer between the catheter and the blood.
Structural features may therefore be included to increase the surface area
of the temperature controlled region.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a catheter according to the present invention inserted
percutaneously into a blood vessel of a patient;
FIG. 2 depicts a catheter suitable for increasing the temperature of a
patient's blood by electrical resistance heating;
FIG. 3 depicts the distal end of a catheter having a resistance heating
element and a temperature sensor;
FIG. 4 depicts the distal end of a catheter having an optical wave guide
and an optical diffusing tip for converting laser energy into heat;
FIG. 5 depicts a catheter in which heat is transferred down a thermally
conductive shaft between the distal end of the catheter and heating or
cooling apparatus at the proximal end of the shaft;
FIG. 6 depicts a catheter in which a heated or cooled fluid flows through a
balloon, which provides for an increased surface area at the distal end;
FIG. 7 depicts a catheter having a resistance heating element at its distal
end and a balloon having longitudinal ribs to further increase the heat
transfer surface area;
FIG. 8A depicts a catheter having longitudinal fins at the distal end of
the catheter body;
FIG. 8B depicts a catheter having radial ribs at the distal end of the
catheter body; and
FIG. 8C depicts a catheter having a spiral fin to increase the heat
transfer area at the distal end of the catheter.
FIG. 9 depicts a catheter having a balloon which is heated by current
flowing into two electrodes.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The present invention provides methods and apparatus for selectively
modifying and controlling a patient's body temperature by warming or
cooling the patient's blood in situ. According to the present invention, a
catheter is inserted through a puncture or incision into a blood vessel in
the patient's body. By warming or cooling a portion of the catheter, heat
may be transferred to or from blood flowing within the vessel and the
patient's body temperature may thereby be increased or decreased as
desired. During the procedure, the patient's body core temperature may be
independently monitored and treatment may continue until the patient's
core temperature approaches the desired level, usually the normal body
temperature of about 37.degree. C. Such methods will find use in treating
undesirable conditions of hypothermia and hyperthermia and may also be
used to induce an artificial condition of hypothermia when desired, e.g.,
to temporarily reduce a patient's need for oxygen. In such a case, the
patient's temperature may be reduced several degrees Celsius below the
normal body temperature.
FIG. 1 depicts a distal end 15 of a catheter 10 according to the present
invention. The catheter has been inserted through the patient's skin into
a blood vessel BV. Blood flow through the vessel is indicated by a set of
flow arrows F. Preferably, the catheter will be inserted into a relatively
large blood vessel, e.g., the femoral artery or vein or the jugular vein.
Use of these vessels is advantageous in that they are readily accessible,
provide safe and convenient insertion sites, and have relatively large
volumes of blood flowing through them. In general, large blood flow rates
facilitate quicker heat transfer into or out of the patient.
For example, the jugular vein may have a diameter of about 22 French, or a
bit more than 7 millimeters (1 French=0.013 inches=0.33 mm). A catheter
suitable for insertion into a vessel of this size can be made quite large
relative to catheters intended for insertion into other regions of the
vascular system. Atherectomy or balloon angioplasty catheters are
sometimes used to clear blockages from the coronary artery and similar
vessels. These catheters commonly have external diameters in the range
between 2 and 8 French.
In contrast, it is anticipated that a catheter according to the present
invention will typically have an external diameter of about 10 French or
more, although this dimension may obviously be varied a great deal without
departing from the basic principles of the claimed invention. It is
desirable that the catheter be small enough so that the puncture site can
be entered using the percutaneous Seldinger technique, a technique well
known to medical practitioners. To avoid vessel trauma, the catheter will
usually be less than 12 French in diameter upon insertion. Once in the
vessel however, the distal or working end of the catheter can be expanded
to any size so long as blood flow is not unduly impeded.
Additionally, the femoral artery and vein and the jugular vein are all
relatively long and straight blood vessels. This will allow for the
convenient insertion of a catheter having a temperature controlled region
of considerable length. This is of course advantageous in that more heat
may be transferred at a given temperature for a catheter of a given
diameter if the length of the heat transfer region is increased.
Techniques for inserting catheters into the above mentioned blood vessels
are well known among medical personnel. Although the method of the present
invention will probably be most commonly employed in a hospital, the
procedure need not be performed in an operating room. The apparatus and
procedure are so simple that the catheter may be inserted and treatment
may begin in some cases even in an ambulance or in the field.
The distal end 15 of the catheter may be heated or cooled as desired and
held at a temperature either somewhat above or somewhat below the
patient's body temperature. Blood flowing through the vessel will thereby
be warmed or cooled. That blood will be circulated rapidly throughout the
patient's circulatory system. The beneficial effect of warming or cooling
the patient's blood in the vicinity of the catheter will thereby be spread
very quickly throughout the entire body of the patient.
FIGS. 2 and 3 depict a catheter suitable for treating hypothermia by
increasing the temperature of a patient's blood. As depicted in FIG. 2,
the catheter has a preferably flexible catheter body 20. Disposed within
the catheter body are a pair of electrical conduction leads 22 and 23 and
a temperature measurement lead 25.
Electrical conduction leads 22 and 23 are connected to a resistance heating
element 28, as depicted in FIG. 3. Electrical current provided by a power
source (not shown) is converted to heat within the heating coil. That heat
warms distal end 15 of the catheter and is thereby transferred to blood
flowing through the vessel.
Temperature measurement lead 25 is connected to a temperature sensor 30.
The temperature sensor facilitates the control of current flow through the
heating coil. It is important to closely monitor the temperature of the
distal end of the catheter and thus the flow of heat into the patient's
blood. Care must be taken not to overheat the blood while still providing
an adequate rate of heat transfer into the patient. The provision of a
sensitive temperature sensor at the distal end of the catheter will help
to achieve this goal.
FIG. 4 depicts an alternate embodiment of a catheter having means for
transferring energy from an external power source to distal end 15 of
catheter body 20. In this embodiment, laser energy from a laser light
source (not shown) is transmitted along optical wave guide 35. The wave
guide directs the laser energy into optical diffusing tip 37, which
converts the laser energy to heat. From diffusing tip 37, the heat
radiates outward into distal end 15 of the catheter and from there into
the patient's blood stream.
FIG. 5 depicts another catheter suitable for practicing the present
invention. This embodiment has a thermally conductive shaft 40 running the
length of catheter body 20. Shaft 40 is made of a metal or other material
having a high thermal conductivity. By heating or cooling the proximal end
42 of shaft 40 with an external heating or cooling apparatus 45, heat will
be caused to flow either into or out of the distal end 47 of the shaft. In
the embodiment depicted, the distal end of the shaft is fitted with heat
transfer vanes 50, which add to the surface area of the shaft and thereby
promote more effective heat transfer between the catheter and the
patient's blood stream.
FIG. 6 depicts still another means for transferring heat to or from the
distal end of a catheter. In this embodiment, catheter body 20 has two
lumens running through it. Fluid flows from the proximal end of the
catheter through in-flow lumen 60, through a heat transfer region 62, and
back out through out-flow lumen 64. By supplying either warmed or cooled
fluid through inflow lumen 60, heat may be transferred either to or from
the patient's blood stream.
In the embodiment depicted, heat transfer region 62 is in the form of a
balloon 70. Use of a balloon will be advantageous in some embodiments to
provide an increased surface area through which heat transfer may take
place. Balloon inflation is maintained by a pressure difference in the
fluid as it flows through in-flow lumen 60 and out-flow lumen 64. The
balloon should be inflated to a diameter somewhat less than that of the
inside diameter of the blood vessel so as not to unduly impede the flow of
blood through the vessel.
FIG. 7 depicts a catheter having an internal resistance heating element 28
and a balloon 70, which is shown inflated. In this embodiment, the
increased surface area provided by the inflated balloon is further
augmented by the presence of a set of longitudinal fins 75 on the surface
of the balloon. Alternatively, longitudinal fins 75, radial ribs 77, or
one or more spiral fins 79 may be disposed directly on the body 20 of a
catheter as shown in FIGS. 8A, 8B and 8C. Ordinarily, longitudinal ribs
will be most advantageous because they restrict blood flow through the
vessel less than other configurations. In fact, these ribs insure that the
balloon will not block the flow of blood through the vessel because a flow
path will always be maintained (between the ribs) regardless of how much
the balloon is inflated.
Inclusion of a balloon on a catheter employing resistance heating allows
for designs in which current is conducted through the fluid which fills
the balloon. The catheter depicted in FIG. 9 has a catheter body 20 about
which is disposed an inflatable balloon 70. The balloon is inflated by
injecting a suitable fluid into the balloon through central balloon
inflation lumen 80. In this embodiment, current flows from an external
source of electrical power (not shown) through conduction wires 82 and 84
to electrodes 86 and 88.
A suitable fluid will allow current to flow between electrodes 86 and 88.
Common saline solution, for example, contains dissolved ions which can
serve as charge conductors. Electrical resistance within the fluid will
cause the fluid to be heated, thus providing the desired warming of the
catheter. The amount of warming will be dependant upon the voltage between
the electrodes, the distance between them, and the resistivity of the
fluid. The relation between these quantities is fairly simple; one skilled
in the art will have no difficulty selecting appropriate values.
Resistance heating catheters like those depicted in FIGS. 3, 7 and 9 may
use DC or low frequency AC power supplies. However, it may be desirable to
use a higher frequency power supply. For example, it is known that the
risk of adverse physiological response or electrocution response may be
lessened at frequencies within the range of about 100 kilohertz to 1
megahertz. Power supplies that operate at these frequencies are commonly
referred to as radio-frequency, or RF, power supplies.
A catheter according to the present invention should be designed to
optimize the rate of heat transfer between the catheter and blood flowing
through the vessel. While a large surface area is desirable in order to
maximize heat transfer, care must be taken so that the catheter does not
unduly restrict blood flow through the vessel. Furthermore, the
temperature of the catheter should be carefully controlled to prevent
undesirable chemical changes within the blood. This is especially
important when applying heat to the blood as blood is readily denatured by
even moderately high temperatures. The exterior temperature of a catheter
for warming blood should generally not exceed about 42.degree.
C.-43.degree. C.
It is estimated that a catheter whose surface temperature is controlled
between 37.degree. C. and 42.degree. C. will provide a body core warming
rate of approximately one to two degrees Celsius per hour in a patient
starting out with severe hypothermia. This estimate is highly dependant on
a number of factors including the rate of blood flow through the vessel,
the initial body temperature of the patient, the external surface area of
the catheter through which heat is conducted, etc. The actual rate
achieved may vary substantially from the above estimate.
The above estimate provides a starting point for a rough estimate as to the
level of power transferred from the catheter to the patient's body and
therefore of the size of the power supply required by the system.
Regardless of the exact means of power transmission chosen, resistance
heating coil, laser and diffusing tip, direct conduction or fluid
circulation, an appropriate power supply will be required to provide heat
to the system.
The sum of heat entering and leaving a patient's body can be written as:
.DELTA.H=H.sub.c +H.sub.i -H.sub.e
where .DELTA.H is the sum of all heat transferred, H.sub.c is the heat
transferred from the catheter to the patient, H.sub.i the heat produced by
the patient internally, and H.sub.e the heat lost from the patient to the
environment. If one assumes, as will ordinarily be the case in a healthy
patient, that the body's internal thermoregulatory system will produce
just enough heat to offset heat lost to the environment, then the equation
is made simple:
.DELTA.H=H.sub.c.
The above equation can be written in terms of the change in the patient's
internal body temperature over time as follows:
mc(.DELTA.T/.DELTA.t)=(.DELTA.H.sub.c /.DELTA.t)
where m is the body mass of the patient, c is the specific heat of the
patient's body, (.DELTA.T/.DELTA.t) is the time rate of change of the
patient's internal body temperature, (.DELTA.H.sub.c /.DELTA.t) is the
time rate of heat delivery from the catheter to the patient.
If one assumes a patient having a body mass of 75 kilograms and a specific
heat of 4186 joules/.degree.C.-kg (assumes the specific heat of the human
body to be the same as that of water, the actual value will be somewhat
different), then a warming rate of 1.degree. C. per hour (3600 seconds)
will require the catheter to transfer heat to the patient at a rate of
about 87 watts (1 watt=1 joule/sec).
However, as an estimate of the desirable size of a power supply to be used
with a catheter of the present invention, this estimate is almost
certainly too low. This is true for a number of reasons. First, it was
assumed for the sake of convenience that the patient's internal system
would produce an amount of heat equal to that lost to the environment. In
a hypothermic patient this will obviously not be the case. Almost by
definition, hypothermia occurs when a person's ability to produce heat
internally is overwhelmed by heat lost to the environment. The catheter
will have to make up the difference so the power level required will need
to be greater for that reason alone.
Additionally, the above estimate does not allow for power losses between
the power supply and whatever warming means is utilized. Such losses could
include resistance losses in electrical transmission lines between the
power supply and a resistance heating element, inherent inefficiencies and
other losses in a system having a laser and a diffusing tip, heat losses
along a thermally conductive shaft or fluid circulation lumen, and the
like. Any such losses which do occur will need to be compensated for by
additional power supply capacity.
Furthermore, it would be undesirable to limit the performance of a catheter
according to the present invention by limiting the size of the power
supply used. It would be preferable instead to use a power supply capable
of providing power considerably in excess of that actually needed and then
controlling the delivery of that power according to the measured
temperature of the catheter itself. As mentioned previously, this can be
readily accomplished by including a sensitive temperature sensor within
the body of the catheter. Nevertheless, the above calculation can be used
as a useful estimate of the likely lower bound for sizing a power supply
for use in a catheter according to the present invention.
An alternative estimate can be made by comparing the likely performance of
the various embodiments described herein with the power requirements for
the external blood warming apparatus presently known. Such external
warming apparatus generally requires a supply of power on the order of
1000-1500 watts and sometimes more. A device according to the present
invention will most likely require considerably less power than that.
First, the present invention requires no external pump to circulate the
blood; this function is provided by the patient's own heart. Accordingly,
no power is needed to drive such a pump. Secondly, the present invention
is considerably less complicated than external blood warming systems.
Known systems circulate the blood over a relatively lengthy path from the
patient, through the warming element, and back into the patient. It is
expected that more heat is lost over this lengthy path than will be lost
in any device according to the present invention.
Thus, the power required by external blood circulation and warming systems
of the type previously known can be used as a rough estimate of the likely
upper limit for power required by a system according to the present
invention. It is most likely that such a system will best be equipped with
a power supply having a capacity somewhere between the two rough estimates
described above. It is therefore contemplated that a suitable power supply
will be capable of providing peak power somewhere in the range between 100
and 1500 watts, probably being in the range between 300 and 1000 watts.
The ranges specified are an estimate of suitable peak power capability.
The power supply will most commonly be thermostatically controlled in
response to a temperature sensor in the body of the catheter. The actual
effective power transmitted to the patient will therefore typically be
much less than the peak power capacity of the system power supply.
With respect to a catheter for cooling, the temperature and power
constraints are not as limiting as is the case in a catheter for warming
blood. Care should merely be taken to avoid freezing the blood or inducing
shock to the patient from too rapid cooling.
Blood is essentially water containing a number of suspended and dissolved
substances. As such, its freezi | | |
