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BACKGROUND OF THE INVENTION
This invention relates to balloon catheters and similar devices useful to
apply heat within a patient's body, e.g. for angioplasty, hyperthermal
treatment of tumors, and other medical procedures.
Prior proposals for application of heat internally of the body often have
had drawbacks. The devices have been too large for certain procedures or
have otherwise been difficult to insert, remove or control. In some cases,
the devices have been too complex in construction or have been too
expensive.
We have conceived of an approach that, in a number of circumstances, can
overcome such drawbacks.
SUMMARY OF THE INVENTION
In one aspect the invention features a device and method for heating tissue
employing a chamber constructed for insertion into a patient's body, an
electrically conductive fluid preselected to produce resistive heating for
filling the chamber, a plurality of spaced electrical contacts enclosed
within the chamber and a corresponding plurality of conductors for
connecting the electrical contacts to a power supply for applying a radio
frequency electrical potential to the contacts, the contacts being exposed
to the fluid-containing space of the chamber so that radio frequency
electrical potential can cause current to flow through fluid between the
contacts, the chamber and the electrical contacts being cooperatively
constructed and arranged to cause the current to be substantially confined
to the fluid within the chamber, whereby on the basis of I.sup.2 R losses
of the radio frequency electric current flowing between the electrical
contacts, the fluid can be heated, and the fluid in turn can heat the
surrounding tissue by thermal conduction through a wall of the chamber.
Another aspect of the invention is a catheter device constructed to operate
in the above mode, which comprises a shaft, the chamber mounted on the
shaft, defined by a wall at least part of which is expandable, the chamber
being associated with an inflation/deflation lumen for flow of the
conductive fluid into the chamber after the catheter has been placed and
for emptying the chamber after the heating has been accomplished.
Preferred embodiments have the following additional features. The device is
sized for insertion into blood vessels or is sized to apply heat to the
prostate. The chamber is a balloon. The electrical contacts are mounted
directly upon the catheter shaft within the balloon. The electrical
contacts comprise radiopaque markers. The conductors that apply potential
to the contacts are enclosed within the shaft along its length, and exit
the shaft through a lumen in the shaft inside of the balloon. The device
includes a temperature sensor, and a temperature control circuit for
controlling the output of the power supply in response to information
received from the temperature sensor. The temperature sensor is located
within the chamber, or outside the chamber in contact with tissue
surrounding the chamber. The sensor and the rf power supply are operated
alternately. The sensor may be a thermistor connected to one of the
electrical contacts. The fluid is a saline solution, a conductive
radiopaque fluid, or a mixture of such fluids.
In another aspect, the invention features a method of heating tissue,
including the following steps: (1) inserting into a patient's body a
deflated balloon catheter having a shaft, two or more electrical contacts
enclosed within the balloon, and two or more conductors for connecting
each of the electrical contacts to an r.f. power supply; (2) filling the
chamber with an electrically conductive fluid preselected to produce
resistive heating, and (3) applying an electrical potential at
radiofrequency to the electrodes. The fluid is heated by radio frequency
electric current flowing through it between the electrical contacts. The
fluid in turn heats the surrounding tissue by heat transfer through the
wall of the chamber.
In another aspect, the invention features an apparatus for heating tissue,
having a shaft for insertion into a patient's body, a chamber mounted on
the shaft and filled with a fluid, and a device for inducing localized
boiling in the fluid. The boiling aids in convection of heat from the
fluid to the surrounding tissue. In preferred embodiments, the device for
inducing boiling in the fluid includes two electrical contacts enclosed
within the balloon, a power supply for applying an electrical potential to
the contacts, and a pair of conductors for connecting each of the contacts
to the power supply. The fluid is electrically conductive and the fluid is
heated by a radio frequency electric current flowing between the contacts,
the fluid in turn heating surrounding tissue by heat transfer through the
wall of the chamber. The apparatus further includes a temperature sensor,
and a temperature control circuit for controlling the output of the power
supply in response to information received from the temperature sensor.
The temperature sensor may be a pressure transducer for measuring the
pressure of the fluid, as a means of indirectly measuring the amount of
heating of the fluid on of the surrounding tissue.
The direct, bipolar heating of the fluid within the balloon according to
the invention allows for precise control of the temperature to which the
fluid is heated, or of the temperature of an adjacent body site, and
control of the duration of such heating, as is described below. The
electrical contacts provide a very low profile means of heating the fluid,
so that the deflated balloon can be easily inserted into or removed from
very narrow passages within the patient's body.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
We first briefly describe the drawings.
DRAWINGS
FIG. 1 shows a balloon catheter according to the invention.
FIG. 2 is a detailed drawing of the balloon portion of the balloon catheter
shown in FIG. 1, according to an embodiment of the invention in which a
temperature sensing device is mounted inside the balloon.
FIG. 3 is a transverse cross-section of the catheter shaft of the balloon
catheter shown in FIG. 2.
FIG. 4 is a block diagram of the RF power supply and temperature control
circuitry according to the embodiment of the invention of FIG. 2.
FIG. 5 is a block diagram of the RF power supply and temperature control
circuitry according to an embodiment of the invention in which a
temperature sensor is placed in direct contact with the tissue surrounding
the balloon.
FIG. 6 is a block diagram of the RF power supply and temperature control
circuitry according to an aspect of the invention in which a pressure
transducer is used inside of the balloon as a means of indirectly
measuring the amount of heating of surrounding tissue.
FIG. 7 is a detailed block diagram of the temperature control circuit shown
in FIGS. 4, 5 and 6.
STRUCTURE
In the embodiment of FIG. 1, balloon catheter 34 comprises a polyethylene
teraphthalate (PET) balloon 8 mounted on nylon catheter shaft -0. The
fully extended diameter of balloon 8, when inflated, ranges from 2
millimeters for coronary vascular procedures, to 20 or 35 millimeters for
hyperthermia treatment of the prostate, esophagus or colon. The volume of
the balloon ranges from 1/8 cc for the smallest balloon to 100 cc for the
largest balloon. The wall thickness of balloon 8 is about 0.001 inch.
Guidewire 46, which can extend past the distal end of the catheter, may be
used to guide the catheter through the vascular system or luminal
structure. Balloon 8 is fillable with an electrically conductive fluid 36
such as normal saline (0.9 percent NaCl in water), a conductive radiopaque
fluid, or a mixture of saline solution and a radiopaque fluid. The
exterior of the balloon is coated with a non-stick coating having a low
coefficient of friction, such as silicone or polysiloxane.
Annular electrical contacts 22 and 24 inside of balloon 8 have internal
diameters matching the portion 10a of the catheter shaft 10 which they
surround and are bonded directly to the catheter shaft. The spacing
between the contacts is approximately half the length of the balloon, and
the spacing from the respective end of the balloon is approximately one
fourth the length of the balloon, so that the balloon will heat evenly.
While the dimensions of the contacts vary according to the nature of the
medical procedure to be performed, in this embodiment it is preferable
that the contacts be in the form of annular thin-wall bands having their
axial length and diameter about equal. For the range of uses contemplated
for this embodiment, the inner diameter of the smallest contact is about
0.050 inch, and the inner diameter of the largest contact is about 0.120
inch. The contacts present a low profile, having a radial thickness of
approximately 0.002 inch. The contacts can be made of any conductive
material that is compatible with the conductive solution and the
conditions of use, but are preferably of a radiopaque metal such as
platinum or tantalum, so that they may serve as radiopaque markers during
placement of the catheter. Contacts 22 and 24 are preferably coated with
tin, so that they may be soldered by means of tin solder to 34 gauge,
multi-filament, copper wires 20 and 18, respectively. These wires, which
are TEFLON-insulated, and have outer diameters of 0.012 inch, connect
contacts 22 and 24, respectively, to opposite poles of current-controlled
(constant current) radio-frequency power supply 50. Wires 20 and 18 are
enclosed within catheter shaft 10 along its length, and exit catheter
shaft 10 through lumen 40, which is located inside of balloon 8.
RF power supply 50 preferably operates at 650 kilohertz, but can be at any
frequency within the range of about 100 kilohertz to 1 megahertz. Radio
frequency power is important to use rather than direct or low frequency
current, or microwave power, because the risk of a physiological response
or electrocution response is reduced at RF frequencies above 100 kHz
kilohertz as compared with d.c. or low frequencies, and because microwave
power would lead to radiative losses in wires 18 and 20, that can result,
e.g. in unwanted heating of catheter shaft 10. The fluid 36, while
selected to have resistive losses, has an electrical impedance low enough
that it will conduct the current supplied by RF power supply 50 at
voltages of about 100 volts or lower, so that there will be no arcing
across insulated wires 18 and 20. For example, if the current I is set at
1 amp, and the impedance R between the electrodes, through the fluid is
100 ohms, the voltage V will be 100 volts according to V=IR, and the power
P dissipated into the fluid will be 100 watts, according to P=I.sup.2 R.
In general, where two electrodes are employed, the impedance between the
electrodes will be less than 1000 ohms, preferably in the range of 50 to
500 ohms, and in the present embodiment most preferably at about 100 ohms.
In all events the shape of the balloon and the construction and spacing of
the electrical contacts are preselected so that the electrical current is
substantially confined to the interior of the balloon.
Catheter 34 plugs into RF power supply and temperature control circuitry 38
by means of a plug 39, that is keyed with respect to the particular size
of balloon catheter it is associated with, to cause the power supply to
operate at a maximum current of 1/10, 1/4, 1/2 or 1 amp. Plug 39 has seven
pins, three of which are needed to operate the catheter. During
manufacture, a jumper connection is made within plug 39 between a selected
two of the remaining four pins. The jumper connection indicates how much
current, at maximum, the RF power supply 50 should produce, depending upon
which pins the jumper connection connects. Thus, the user need only select
the appropriate catheter 34, and need not be concerned about selecting the
appropriate maximum current.
Referring to FIG. 2, in one embodiment of the invention, a bead thermistor
26, 0.014 inch in diameter and 0.020 inch long, is mounted directly upon
catheter shaft 10 between electrodes 22 and 24. Stainless steel thermistor
lead 28 connects thermistor 26 with electrode 22. A 34 gauge,
multi-filament, TEFLON coated, copper wire 30, outer diameter 0.012 inch,
which is soldered to the other stainless steel thermistor lead 32,
connects thermistor lead 32 with RF power supply and temperature control
circuitry 38 via one of the pins of the plug. Thermistor 26 fits snugly on
top of an opening 48 in the wall of catheter shaft 10 midway between
electrodes 22 and 24. Wire 30 and thermistor lead 32 are enclosed within
catheter shaft 10, and thermistor lead 32 connects with thermistor 26
through opening 48. An insulating coating of epoxy or urethane seals
thermistor 26 on top of opening 48, and secures thermistor lead 28 to
catheter shaft 10. Alternatively, thermistor lead 28 may be electrically
connected to RF power supply and temperature control circuitry 28 in the
same manner as thermistor lead 32, rather than being connected to
electrode 22.
Referring to FIG. 3, catheter shaft 10 has three lumens 12, 14, and 16.
Lumen 12 extends from the proximal end of catheter shaft 10 to the distal
end, and provides a conduit for guidewire 46. Lumen 14 extends from the
proximal end of catheter shaft 10 to an outlet in the inside of balloon 8,
and provides a conduit for fluid 36 as balloon 8 is inflated and deflated.
Lumen 16 extends from the proximal end of catheter shaft 10 to the inside
of balloon 8, and provides a conduit for wires 18 and 20, which exit lumen
16 through opening 40 in the wall of catheter shaft 10, and also provides
a conduit for wire 30 and thermistor lead 32 through opening 48 in
catheter shaft 10 that is located directly below thermistor 26, as
mentioned above.
Referring to FIG. 4, RF power supply and temperature control circuitry 38
consists of RF power supply 50, temperature control circuit 52, and solid
state switch 54. Wire 18 connects electrode 24 with RF power supply 50,
and wire 30 connects thermistor 26 with temperature control circuit 52.
Timing circuit 56 of temperature control circuit 52 toggles hold/NOT
sample line 58 so that solid state switch 54 toggles back and forth,
whereby wire 20 functions alternately as a lead connecting RF power supply
50 with electrode 22 and as a lead connecting temperature control circuit
52 with thermistor 26. (Recall that electrode 22 and thermistor 26 are
connected by wire 28.) The temperature sensing period is 1 percent of the
60 hertz cycle. When solid state switch 54 connects wire 20 with
temperature control circuit 52, temperature control circuit 52 determines
how much power, at maximum, RF power supply 50 should supply when solid
state switch 54 next connects wire 20 with RF power supply 50. By thus
multiplexing between temperature sensing and application of current to the
electrodes, the temperature control circuitry eliminates the possibility
that thermistor 26 will pick up RF noise from the electrodes 22 and 24.
Referring to FIG. 5, another embodiment of the invention is shown in which
temperature sensor 26 is placed in direct contact with tissue 44, outside
of balloon catheter 34. Wires 60 and 62 connect temperature sensor 26 with
temperature control circuit 52, and wires 20 and 18 connect electrodes 22
and 24 respectively with RF power supply 50. Temperature control circuit
52 regulates RF power supply 50 in response to the input from temperature
sensor 26.
Referring to FIG. 6, another embodiment of the invention is shown in which
the temperature sensor consists of a pressure transducer 64 in conjunction
with pressure sensing circuit 66 and pressure-to-temperature conversion
circuit 68. In this embodiment, the electrodes 22 and 24 are small enough
that the electric current density in the immediate vicinity of the
electrodes can induce localized boiling, which aids in the convection of
heat from the electrodes to the surrounding tissue 44. The balloon
material is heat-set at a temperature in excess of 100.degree. Celsuis, so
that the balloon material remains dimensionally stable when the fluid 36
within the balloon 8 boils at about 100.degree. Celsius. A flexible tube
70 provides a conduit for fluid into lumen 14 of catheter shaft 10.
Inflator 72 is used to inject fluid into flexible tube 70 until a desired
pressure is obtained, as indicated by pressure gauge 74. When RF power
supply 50 is activated, the high electric field density in the immediate
vicinity of each of the electrodes 22, 24 can induce localized boiling of
fluid 36. As the fluid 36 heats up, the boiling increases in intensity.
The boiling causes the pressure inside balloon 8 to increase. The increase
in pressure is measured by pressure transducer 64, as an indirect
indication of the amount of heating of the fluid 36, according to phase
change pressure/temperature relationships. Temperature control circuit 52
regulates RF power supply 50 in response to the input obtained from
pressure-to-temperature conversion circuit 68. Temperature display circuit
76 displays the temperature obtained from pressure-to-temperature
conversion circuit 68. Impedance stability sensor 78 detects the
initiation of boiling by sensing the instability of catheter impedance due
to the formation of vapor at the surfaces of electrodes 22 and 24.
Referring to FIG. 7, in temperature control circuit 52, linearization
network 80 linearizes the input signal from temperature sensor 26 and
delivers the linearized signal to sample and hold register 82. The signal
is deliver to amplifier buffer 84 having low-temperature reference 86.
Actual temperature display circuit 88 displays the output of amplifier
buffer 84. Control amplifier 90 compares the output of amplifier buffer 84
with a temperature set voltage 92 that is set by the user. The maximum RF
power contol circuit 94 receives the output of control amplifier 90 and
determines the level of RF power, at maximum, that the RF power supply 50
should produce. The signal from the maximum RF power control circuit 94 is
received by isolation network 96, which interfaces with RF power supply
50. The temperature set voltage 92 is received by buffer amplifier 98 and
displayed by set temperature display 100.
Timing circuit 56 toggles hold/NOT sample line 58 at 60 hertz, so that
hold/NOT sample line 58 is low during 1 percent of the cycle and high
during the other 99 percent of the cycle. Hold/NOT sample line 58 is low
when signals from temperature sensor 26 are being sampled and high when
signals from temperature sensor 26 are not being sampled. Hold/NOT sample
line 58 is received by RF output enable gate 102. The output of sample and
hold register 82 is processed by open and short sensor detector 104 to
determine whether a sensor malfunction, such as a shorted or open sensor,
has occurred. The output of open and shorted sensor detector 104 is
received by RF output enable gate 102. RF output enable gate 102 delivers
a signal to isolation network 96, which turns off RF power supply 50 when
there has been a sensor malfunction or when signals from temperature
sensor 26 are being sampled.
Divider 106 receives hold/NOT sample line 58 and delivers its output to
time elapsed display 108. Time set display 110 displays the time indicated
by time set switches 112, which are set by the user. Time compare network
114 compares the elapsed time with the time set by the user, and delivers
an output signal to output disable circuit 116. The output of output
disable circuit 116, which is active only when the elapsed time is less
than the time set by the user, is delivered to RF output enable register
118. RF output enable register 118 in turn delivers the signal to the
enable input to time elapsed display 108, and also to RF output enable
gate 102, so that RF power supply 50 may be turned off when the time set
by the user has elapsed. Switch debounce circuits 120 are provided for
time set switches 112.
The user must depress footswitch 122 in order for RF power supply 50 to
operate. While footswitch 122 is activated, and while the elapsed time is
less than the time set by the user, output disable circuit 116 delivers a
signal to RF output enable register 118, which in turn delivers the signal
to the enable input of time elapsed display 108, and also to RF output
enable gate 102 so that rf power supply 50 may be turned on. Deactivation
of footswitch 122 causes a signal to pass through elapsed time reset
register 124, in order to reset time elapsed display 108 and in order to
reset RF output enable register 118. The resetting of RF output enable
register 118 causes RF output enable gate 102 to turn off RF power suppy
50. Debounce circuit 126 is provided for footswitch 122.
Operation
Referring to FIG. 1, balloon catheter 34 may be used as a heat source
during or after angioplasty to seal the splitting of the intimal layers of
the wall of blood vessel 42 that occurs during angioplasty, and to mold
the vessel wall. The blood vessel may be a coronary artery, or a
peripheral artery such as an iliac, femoral, renal, carotid, or popliteal
artery. The user first preselects the desired therapeutic temperature
(temperature set voltage 92, FIG. 7), and sets the length of time for
which balloon 8 is to be heated (time set switches 112, FIG. 6). A
percutaneous insertion is made with a needle, and guide wire 46 is
introduced into the blood vessel 42. Balloon catheter 34 follows the wire.
If balloon 8 contains conductive radiopaque fluid, the location of balloon
8 can be monitored by means of fluoroscopy. Balloon 8 is inflated through
lumen 14 with either saline, a conductive radiopaque fluid, or a mixture
of saline and a radiopaque fluid, to a pressure of 4 to 17 atmospheres, in
order to expand the wall of blood vessel 42. The balloon remains inflated
for about 20 seconds or longer, depending on the particular blood vessel
upon which the angioplasty is being performed. Either during or after the
plastic deformation of the vessel wall, with balloon 8 inflated to at
least a low level of pressure, the user depresses footswitch 122 (FIG. 7)
to initiate the bi-polar heating between the electrodes 36. Heat is
dissipated into the fluid according to the formula P=I.sup.2 R where P is
the power that is dissipated into the fluid, I is the current that is
passed through the electrodes, and R is the resistance of the fluid. The
heat from the fluid is conducted across the balloon wall into the
surrounding tissue 44. For angioplasty procedures, RF power supply 50
supplies a maximum current of 1/4 amp, and the power dissipated into fluid
36 is about 10 to 25 watts. The fluid will heat to the temperature set by
the user, which may be in the range of 45.degree. Celsius to 80.degree.
Celsius. Heating will continue until the time set by the user has elapsed,
or until the user deactivates footswitch 122.
The balloon catheter may also be used to perform glazing or smoothing of
the vessel wall, whereby the baloon 8 is inflated to make light contact
with the wall of blood vessel 42, footswitch 122 is activated by the user
to initiate heating of the balloon, and the catheter 34 is guided through
blood vessel 42 to glaze or smooth the plaque on the vessel wall. The
balloon catheter may also be used to dehydrate, compress, and mold plaque
to improve patency.
Catheters according to the invention can be used in nonvascular
applications such as hyperthermia treatment of benign or malignant tumors,
or enlargement of the prostate gland. Hyperthermic effects begin at about
44.degree. Celsius. Heat from balloon 8 destroys the undesired cells,
which are eventually absorbed into the patient's body. When a catheter
according to the invention is used in such nonvascular applications, the
balloon 8 may be large enough that no temperature sensing device is
needed, and the fluid 36 can be left to boil of the electrodes without the
buildup of excessive pressure within the balloon. The fluid will begin to
boil locally in about 5 seconds if the balloon has a diameter of 4
millimeters.
Other embodiments are within the following claims.
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