Method and apparatus for thermally destroying a layer of an organ

Method and apparatus are disclosed for thermally destroying a layer of an organ such as the mucosal layer of the gallbladder. The apparatus includes a catheter having an elongated member having a plurality of lumens therein. At the distal end of the elongated member is an electrode for emitting radiofrequency current to the mucosal layer. Also at the distal end is a capacitive balloon electrode surrounding the current-emitting electrode for containing an electrolyte solution and for distributing the radiofrequency current to the mucosal layer. The balloon electrode is expanded with the electrolyte solution to conform and make contact with the mucosal layer. The electrolyte solution has a resistivity significantly less than the resistivity of the gallbladder wall, as well as the gallbladder bile, to cause a concentrated power deposition in the mucosal layer. The distal end of the catheter is endoscopically inserted into the body of the gallbladder by a retrograde route through the duodenum, common bile duct and cystic duct. While the balloon electrode is being expanded, the bile present in the gallbladder is drained through one of the lumens in the elongated member. The apparatus also includes a radiofrequency generator for supplying radiofrequency current to the current-emitting electrode. The current-emitting electrode is energized for a period of time to cause the mucosal layer to be heated for a predetermined period of time to thermally coagulate the mucosal layer of the gallbladder and cystic duct. A dispersive electrode is positioned on the skin of the patient's body to facilitate a complete circuit back to the generator without causing trauma to the patient.

TECHNICAL FIELD 
This invention relates to catheters and, in particular, method and 
apparatus including a catheter for heating the layer of an organ. 
BACKGROUND OF THE INVENTION 
Gallstones are a common problem in the United States and the most frequent 
cause of gallbladder inflammation. About 500,000 cholecystectomies are 
performed each year with an overall medical cost nearing approximately two 
billion dollars. Patients of all ages with cholecystitis have a mortality 
rate of 1.3% to 5%. For those over 65 years of age, the rate increases to 
10%. When empyema of the gallbladder is present, the mortality rate is 
close to 29%. As the population ages, there will be more and more 
poor-risk patients with troublesome gallstones. 
Removal of only gallstones, without a cholecystectomy, offers promise of 
reducing risk, but only solves the problem temporarily. Nearly 50% of the 
patients having a surgical cholecystostomy with the gallbladder left 
intact will have a recurrence of gallstones within three years or less, 
and 80% will develop stones within 15 years. As yet, there are no 
long-term follow-up studies for nonsurgical removal of gallstones by 
extracorporeal shockwave lithotripsy or by methyl-tertiary-butyl ether 
treatment. However, one study evaluating the recurrence of gallstones 
following another nonsurgical method of removal, bile-acid treatment, 
found the recurrence rate to be 24% at one year, rising to 58% at two 
years, 63% at three years, and 100% at five years. Other bile-acid 
treatment studies have indicated somewhat lower rates of recurrence, but 
50% eventual recurrence seems to be generally accepted. Therefore, the 
need for preventing the recurrence of gallstones is significant. This is 
further heightened by eliminating the problem with a nonsurgical solution 
that will reduce the costs and the mortality rate associated with surgical 
methods. 
SUMMARY OF THE INVENTION 
The foregoing problems are solved and a technical advance is achieved by a 
method and apparatus for thermally destroying the layer of an organ such 
as by thermally coagulating the mucosal layer of the gallbladder to 
reduce, if not eliminate, the recurrence of gallstones. Illustratively, 
the apparatus includes a catheter comprising an elongated member having a 
current-emitting electrode positioned about the distal end thereof for 
emitting radiofrequency current. The catheter also includes a capacitive 
balloon electrode that is positioned about the distal end of the elongated 
member and surrounds the current-emitting electrode for performing a 
number of advantageous functions. First, the balloon electrode distributes 
the radiofrequency current from the current-emitting electrode to the 
layer. This facilitates a more uniform distribution of the emitted current 
to the mucosal layer. Next, the balloon electrode is expanded with an 
electrolyte solution for making contact with the mucosal layer and for 
containing the electrolyte solution. The electrolyte solution has a 
resistivity less than the resistivity of the mucosal layer for selectively 
heating the mucosal layer. 
The change in resistivity from a lesser to a higher level at the interface 
of the electrolyte solution and the mucosal layer causes a concentrated 
deposition of power within the mucosal layer of the gallbladder. The 
degree of resistivity change at the interface controls the concentration 
of power deposited in the mucosal layer and consequently the selective 
heating of the mucosal layer. The greater is the resistivity change; the 
greater is the concentration of power in the mucosal layer. Furthermore, a 
power deposition peak occurs at the interface of the electrolyte solution 
and the mucosal layer. 
The capacitive balloon electrode is also expanded for conforming the shape 
of the gallbladder and the balloon electrode together. This brings the 
capacitive balloon electrode in physical and electrical contact with the 
mucosal layer for distributing the radiofrequency current to the mucosal 
layer in a more uniform manner. The balloon electrode also contains the 
electrolyte solution and brings the solution within close proximity of the 
mucosal layer. As a result, electrolyte solutions that have resistivity 
levels much lower than that of the mucosal layer, as well as the 
gallbladder bile, may be used to expand the balloon electrode. This 
containing feature is particularly advantageous since some of the lower 
resistivity level solutions are caustic to the surrounding tissue. These 
lower resistivity level solutions can also be made radio-opaque, thereby 
allowing fluoroscopic visualization of the balloon electrode in the 
gallbladder prior to applying the radiofrequency current. A filling lumen 
extending longitudinally through the elongated member and a plurality of 
sideports about the distal end thereof are utilized to fill and expand the 
balloon electrode with the electrolyte solution. 
A second lumen is also provided in the elongated member for draining the 
bile from the gallbladder as the balloon electrode expands. Draining the 
bile from the gallbladder permits the lower resistivity level electrolyte 
solution to be positioned in close proximity to the mucosal layer. 
The wall of the balloon electrode comprises a relatively thin flexible 
material such as latex which has a relatively large predetermined 
capacitance. Consequently, the impedance of the capacitive balloon 
electrode to the radiofrequency current is relatively low. The 
radiofrequency current is thus negligibly impeded by the capacitive 
balloon electrode. 
The catheter also includes a sensor positioned about the distal end of the 
member for sensing a temperature of an environment about the distal end. 
Illustratively, this sensor includes a thermistor for sensing the 
temperature of the current-emitting electrode and the surrounding 
electrolyte solution which is indicative of the temperature of the mucosal 
layer. A pair of electrical conductors connected to the thermistor extends 
through a third lumen in the elongated member to a control circuit. In 
response to the sensed temperature, the control circuit regulates the 
amount of radiofrequency current supplied by a generator to the 
current-emitting electrode. 
The radiofrequency current is supplied to the current-emitting electrode 
via a conductor extending through a fourth lumen in the elongated member 
to the current-emitting electrode. 
The apparatus also includes the radiofrequency current generator and the 
control circuit that cooperate to supply current to the current-emitting 
electrode. The electrode is used to heat the mucosal layer of the 
gallbladder to a predetermined temperature for a predetermined period of 
time to thermally coagulate the mucosal layer. Consequently, the mucosal 
layer is thermally destroyed along with the mucosal layer of the cystic 
duct for advantageously preventing the recurrence of gallstones within the 
gallbladder. The selective deposition of power in the mucosal layer also 
prevents thermal destruction of the outside wall of the gallbladder and 
the surrounding tissue. 
To significantly reduce the risks associated with surgery, the method of 
the invention includes inserting the distal end of the catheter 
endoscopically into the gallbladder by a retrograde route through the 
duodenum, common bile duct and cystic duct.

DETAILED DESCRIPTION 
Depicted in FIG. 1 is illustrative apparatus 123 including a catheter 100 
having a distal end 105 that is inserted into an organ such as body 114 of 
gallbladder 102 for thermally coagulating and destroying the inner mucosal 
layer 101 of the gallbladder. The distal end of the catheter is 
endoscopically introduced into the body of the gallbladder by a retrograde 
route through the duodenum 117, common bile duct 118, and cystic duct 119. 
Catheter 100 includes an elongated member 103 with a current-emitting 
electrode 104 positioned about the distal end thereof for emitting 
radiofrequency current to the mucosal layer of the gallbladder. Also 
positioned about the distal end of the elongated member and surrounding 
the current-emitting electrode is capacitive balloon electrode 106, which 
is expandable for making physical and electrical contact with the mucosal 
layer of the gallbladder. 
The balloon electrode is expanded with an electrolyte solution 107 that has 
a resistivity level less than the resistivity level of the mucosal layer 
for concentrating the deposition of radiofrequency power in the mucosal 
layer. The mucosal layer 101 of gallbladder 102 has a relatively high 
level of electrical resistivity approximating, for example, 500 ohm-cm. 
The electrical resistivity of the electrolyte solution is at a much lower 
level and approximates, for example, 10 ohm-cm. The solution resistivity 
is also selected to be less than the resistivity of the gallbladder bile, 
which typically approximates 70 ohm-cm. The change in resistivity from 10 
ohm-cm to 500 ohm-cm represents a significant gradient in the order of 
one and a half orders of magnitude. The radiofrequency heating method of 
this invention exploits this low-to-high level change in electrical 
resistivity from the electrolyte solution to the mucosal layer at the 
interface thereof. A large change in resistivity causes a concentrated 
power deposition in the inner mucosal layer of the gallbladder, because 
there is greater power dissipation in media of higher electrical 
resistivity. It is this power deposition as a function of distance from 
the current-emitting electrode that produces selective heating. 
A spherical model of a gallbladder is depicted in FIG. 3 to illustrate the 
selective heating feature of the invention. A current-emitting point 
electrode 300 is positioned at the center of a spherical gallbladder 301 
of 1.0 cm inner radius filled with bile 302 having a resistivity of 70 
ohm-cm. Beyond 1 cm is the inner mucosal layer 303 and outer gallbladder 
wall 304 of resistivity approximating 500 ohm-cm. Beyond the outer 
gallbladder wall is the liver (not shown) with approximately the same 
resistivity. The deposited power density as a function of radial distance 
from the electrode is illustrated by curve 305. The deposited power 
density at any point on curve 305 depicted in FIG. 5 is J.sup.2 p, where J 
is the current density (current I divided by area) and p is the electrical 
resistivity. At any radius r, the current density J=I/(4.pi.r.sup.2). 
Current density J decreases as the inverse square of the distance from the 
electrode in the center of the sphere. From the central electrode, the 
radiofrequency current first encounters the low-resistivity bile and then 
the high-resistivity bladder wall and extrabladder tissues. It is the 
deposited power that produces heating. At a distance beyond the bile of 
the gallbladder (r&gt;1.0 cm), the current density continues to decrease, but 
the resistivity rises to a high level such as 500 ohm-cm. Consequently, 
there is a peak in power deposition in the inner mucosal layer 303 of the 
gallbladder where the resistivity gradient at the interface is the largest 
changing from 70 ohm-cm to 500 ohm-cm. As a result, there is a 
concentration of deposited power in and selective heating of the mucosal 
layer at the bile-gallbladder interface. 
Returning the reader's attention to FIG. 1, balloon electrode 106 is 
expanded with an electrolyte solution 107 having a resistivity lower than 
that of the gallbladder bile. This further enhances the resistivity 
gradient at the interface and more selectively concentrates the power 
deposition and heating in the mucosal layer. By way of example, the 
electrolyte solution used to expand balloon electrode 107 comprises a 
solution of 5% saline with potassium iodide added to provide 
radio-opacity. The addition of 5 grams of potassium iodide to 100 ml of 5% 
saline reduces the resistivity of the latter by 25%. The resistivity of 
the resulting mixture is 10 ohm-cm at 37.degree. centigrade. The 
resistivity gradient at the electrolyte solution-mucosal layer interface 
is now 10/500, rather than 70/500 when, as previously described, bile 
carried the radiofrequency current. The steeper resistivity gradient 
enhances selective power deposition and heating in the mucosal layer. 
Depicted in FIG. 2 is a cross-sectional view of the elongated member 103 of 
the catheter. The elongated member includes a plurality of lumens 201-204 
that extend the entire longitudinal length of the member. Filling lumen 
201 is for transporting the electrolyte solution from the proximal end 115 
of the catheter to the distal end 105. As shown in FIG. 1, a plurality of 
side ports 108 about the distal end connected to filling lumen 201 
facilitates the expansion of capacitive balloon electrode 106. The balloon 
electrode is attached in a well-known manner to the distal end of the 
catheter and expands with the electrolyte solution to make electrical and 
physical contact with mucosal layer 101 of the gallbladder. The balloon 
electrode also contains the electrolyte solution and prevents caustic 
solutions from causing possible undesired injury to surrounding tissue. 
The shape of the gallbladder conforms to the expanded electrode to provide 
a more even distribution of radiofrequency current to the mucosal layer. 
Positioning of the balloon electrode and contact with the mucosal layer is 
visualized and verified by any one of a number well-known techniques such 
as fluoroscopy. Such technique is enhanced with the radio-opaque 
electrolyte solution. 
The balloon electrode comprises a thin wall or layer of material, such as 
well-known latex, having a thickness ranging from two to ten thousandths 
of an inch. Since the balloon wall is very thin, the capacitance thereof 
is thus quite large approximating, for example, 10,000 pF. The reactance 
of this capacitive electrode to, for example, a two megahertz 
radiofrequency current signal is thus relatively low such as 7 ohms. As a 
result, the radiofrequency signal is negligibly impeded by the balloon 
wall. 
A second lumen 202 included in elongated member 103 transports gallbladder 
bile from distal end 105 of the catheter to proximal end 115 for drainage 
therefrom. Depicted in FIG. 4 is an enlarged view of the distal end of 
elongated member 103. This drainage occurs as a result of expanding the 
balloon electrode and consequently forcing the gallbladder bile into the 
drainage lumen. 
Positioned about the distal end 105 of the catheter is sensor 109 such as a 
well-known thermistor for sensing the temperature of the surrounding 
environment. A volume of this environment includes the current-emitting 
electrode, the electrolyte solution and the mucosal layer of the 
gallbladder. Since a direct reading of the mucosal layer temperature is 
not practically feasible, the temperature of the electrolyte solution and 
current-emitting electrode are utilized to closely approximate the 
temperature of mucosal layer 101. This approximation is derived from a 
number of experimental measurements along with knowing the power 
deposition characteristics of the radiofrequency current from the 
current-emitting electrode as previously described with respect to the 
spherical model. A third lumen 203 in member 103 of the catheter provides 
a channel for extending a pair of electrical conductors 110 that are 
connected to temperature sensor 109. 
The electrical conductors from the thermistor are connected to a well-known 
control circuit 113 included in radiofrequency generator 112 for 
controlling the amount of current supplied to current-emitting electrode 
104 as a function of temperature. The sensed temperature is used to 
control the amount of current supplied to the current-emitting electrode 
so as to maintain the temperature of mucosal layer 101 at a temperature 
such as 50.degree. centigrade to thermally coagulate and destroy the 
layer. Since the power deposition gradient is the steepest at the mucosal 
layer, the outer layer 115 of the gallbladder is not thermally destroyed 
since the temperature therein is less than the 42.degree. centigrade 
temperature necessary to thermally destroy living tissue. 
A fourth lumen 204 is also included in elongated member 103 for housing an 
electrical conductor 111 for interconnecting current-emitting electrode 
104 and radiofrequency current generator 112. A large dispersive electrode 
120 is placed on the skin 121 of the patient's body (not shown) to receive 
the radiofrequency current emitted from the balloon electrode and 
conducted through the body of the patient and to reduce trauma to the 
body. Another electrical conductor 116 interconnects the dispersive 
electrode and the radiofrequency current generator. 
The method for thermally destroying, or more specifically, thermally 
coagulating the mucosal layer of the gallbladder includes inserting the 
distal end of the catheter into body 114 of gallbladder 102 endoscopically 
by a retrograde route through the duodenum, common bile duct and cystic 
duct. The capacitive balloon electrode is then expanded with the 
electrolyte solution for making electrical and physical contact with the 
gallbladder. The balloon catheter also conforms the gallbladder thereto to 
provide a more uniform distribution of the radiofrequency current from the 
current-emitting electrode. The use of the electrolyte solution with a 
resistivity lower than gallbladder bile and of the thin wall balloon 
electrode permits the concentrated deposition of power within the mucosal 
layer of the gallbladder. This concentrated deposition of power causes the 
selective heating of the mucosal layer to a thermally destructive 
temperature without destroying or killing outer layer 122 of the 
gallbladder. The mucosal layer of the cystic duct is also thermally 
destroyed to prevent the recurrence of gallstones therein. The temperature 
of the mucosal layer is maintained at a predetermined temperature such as 
50.degree. centigrade for a predetermined period of time to ensure the 
thermal coagulation and thermal destruction of the entire mucosal layer. 
Heating times for thermally coagulating and destroying the mucosal layer 
of the gallbladder in dogs has been derived from experiments in which 
heating times varied from 6- 15 minutes at approximately 50.degree. C. for 
dogs ranging in weight from 8-23 kg. The percentage of mucosal layer 
destroyed varied from 90-100%. 
It is to be understood that the above-described method and apparatus for 
thermally destroying the layer of an organ is merely an illustrative 
embodiment of the principles of this invention and that other apparatus 
and methods may be devised by those skilled in the art without departing 
from the spirit and scope of this invention. In particular, different 
electrolyte solutions with varying resistivities may be employed along 
with different degrees of radio-opacity. In another embodiment, the 
capacitive balloon electrode may be eliminated and the distal end of the 
elongated member slit along the longitudinal axis thereof to form several 
strips therein. The current-emitting electrode is internally affixed to 
the distal tip of the member and operated to expand the strips radially 
and hold the electrode in the center of the gallbladder. The expanded 
strips engage and make contact with the inner mucosal layer of the 
gallbladder. Experiments with this conductive electrode catheter indicated 
that an electrode temperature of 72.degree. was needed to maintain the 
mucosal layer at 50.degree. C. while the outer wall temperature was 
approximately 41.degree. C. The radiofrequency generator utilized in these 
experiments had a tentative frequency of two megahertz with signal 
amplitude ranging between 0-2 amps rms of radiofrequency current.