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Description  |
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TECHNICAL FIELD
This invention relates to the coagulation of vascularized tissues, and more
particularly to a miniaturized thermal cautery probe which is
endoscopically deliverable and which applies precisely controlled heat to
such tissues during a relatively short period.
BACKGROUND PRIOR ART
The use of heat for the cauterization of bleeding wounds dates back to
ancient times. Perhaps the simplest and most basic thermal cauterization
technique involves the application of a hot iron to a bleeding wound.
While this technique is somewhat effective in cauterizing large, external
wounds, the technique is not applicable to internal wounds. Nor is the
technique sufficiently precise or delimited to provide adequate
cauterization without excessive tissue damage.
In the present century, the use of radio frequency electric current
traveling through a portion of the body has been widely used to stop
bleeding. The essential ingredient in radio frequency cauterization is the
dissipation of electrical energy in resistive tissue. This dissipated
electrical energy is converted into heat, which produces a rise in
temperature of the tissue and blood. The plasma proteins in blood are
denatured in a temperature range of from 50.degree. to 100.degree. C.,
producing a sticky or congealed mass of protein. This process is familiar
in the cooking of egg white. Other processes may take place when tissue is
heated. For example, vessels may contract or shrink, thereby further
reducing the flow of blood.
Several radio frequency current generators are now commercially available
and are widely used by surgeons for both cutting and coagulating tissue.
Since the electrical current flow follows the path of least resistance,
the resulting thermal damage, or necrosis, may at times be unpredictable,
too deep and uncontrolled. The rationale for using radio frequency current
for bleeding control is that the frequency is above that which would cause
neuromuscular stimulation and yet permit sufficient power dissipation to
produce a rapid rise in temperature. Thus, used properly, electrical shock
does not occur and coagulation is accomplished.
There is currently much interest in the control of bleeding using the
modern fiberoptic endoscope, which permits visualization and therapy in
hollow organs of the body through a slender tube. Hollow channels with a
few millimeters of inside diameter permit the insertion of instruments for
the administration of therapy such as the coagulation of bleeding. Some
investigators have reported good success using radio frequency coagulation
through the endoscope in a clinical setting. But this technique has not
been widely used in practice because of its inherent risks. Several groups
have directed a laser beam through an endoscope using a special optical
wave-guide with good success in both animals and humans. However, the high
cost of laser coagulators and the as-yet unproven benefit in a controlled
clinical trial are slowing the widespread adoption of this technique.
Other problems associated with laser coagulators arise from the difficulty
in precisely directing the laser beam to a moving target, the existence of
optical hazards and the need for a gas injection system to wash away
overlying blood. Furthermore, simple laser coagulators do not
simultaneously apply heat and pressure to the wound; and the combination
of heat and pressure is considered to be more effective than heat alone.
More recently, a miniaturized thermal probe has been developed which is
endoscopically deliverable. This probe, which is described in an article
by Protell, et al., "The Heater Probe: A New Endoscopic Method for
Stopping Massive Gastro-Intestinal Bleeding" Gastroenterology, 74: 257-62
(1978), includes a heating coil mounted in a small cylindrical body with a
thermocouple. The output of the thermocouple is compared to a temperature
reference level, and the difference is used to control the power to the
probe to achieve a preset probe temperature. In use, the probe is heated
to the preset value and applied to the wound for a number of periods, each
of approximately one second in duration. Alternatively, the cold probe is
applied directly to the bleeding site, turned on and held there for a
predetermined period after reaching a target temperature. The principal
problem associated with the latter technique is the inability of the probe
to reach coagulating temperature with sufficient speed and to then cool
itself with sufficient speed to prevent excessive penetration of the heat
by diffusion. Effective coagulation requires that the bleeding site be
adequately heated. However, avoidance of thermal necrosis requires that
the heat not penetrate too deeply. The only technique providing adequate
heating of the bleeding site without producing excessive heat penetration
is heating the bleeding site at a high temperature for an extremely short
period of time. Presently existing thermal probes are not able to meet
these requirements. The problem does not stem from an inability to heat
the probe with sufficient speed as much as it does from an inability to
cool the probe with sufficient speed. Any probe can be heated rapidly by
merely utilizing a sufficiently larger heater. However, the probe can be
cooled only by the tissue with which it is in contact. Conventional probes
have been incapable of being cooled by the surrounding tissue with
sufficient speed due to their relatively high thermal mass.
Attempts have been made to design thermal cautery probes which are heated
by passing a current through the body of the probe itself instead of
through a separate heating element. An example of such probes is disclosed
in U.S. Pat. No. 3,886,944, issued to Jamshidi. The disadvantages of such
probes are twofold: first, the unavailability of a satisfactory probe
material and, second, the nonuniformity of the probe temperature.
The choice of a probe material presents a problem because the resistance of
the material must be high enough to dissipate sufficient power and the
strength of the material must be high enough to withstand forces applied
to the probe by the tissue and other objects. The Jamshidi probe utilizes
a Nichrome alloy or stainless steel as the probe material. Either material
has a relatively low resistivity, thereby making it difficult for the
probe to dissipate sufficient power without applying a great deal of
current to the probe. While probes requiring high current are acceptable
under some circumstances, they are uncceptable where the probe is to be
endoscopically deliverable since the high currents require wires which are
larger than the endoscope channels. In fact, a probe having a resistance
less than about 0.5 ohm will generally require more current than
endoscopically deliverable power leads are capable of carrying.
A probe fabricated of a low-resistivity material can dissipate adequate
power from relatively low current only by making the material extremely
thin so that the resistance of the probe is high. Yet a probe having an
extremely thin shell does not have sufficient strength to withstand
clinical use.
A probe having a relatively thick shell of a higher resistivity or
semiconductive material would be capable of dissipating adequate power at
acceptably low currents. However, a material having these properties and
which is inexpensive, easily worked, and sufficiently sturdy does not
appear to be available.
The second disadvantage mentioned above--the nonuniformity of probe
temperature--is illustrated in the Jamshidi patent. In the Jamshidi probe,
current flows outwardly from the center of the probe tip and then along
the sides of the probe. The current density--and hence the power
dissipation--varies from a maximum at the center of the probe to a minimum
at the sides of the probe. As a result, the temperature of the probe
decreases from a maximum at the center of the probe.
DISCLOSURE OF THE INVENTION
The primary object of the invention is to provide a thermal cautery probe
having a heat capacity which is sufficiently low to allow rapid heating
and cooling, thereby effectively coagulating vascularized tissue without
undue thermal necrosis.
It is another object of the invention to provide a thermal probe which is
powered for a predetermined period while measuring and displaying the
total energy delivered to the probe during that period.
It is another object of the invention to provide a thermal cautery probe
which is powered by a relatively low current.
It is another object of the invention to provide a low thermal mass cautery
probe which has a uniform temperature distribution.
It is still another object of the invention to provide a thermal probe
which receives a predetermined value of energy while measuring and
displaying the duration of the period during which the energy is
delivered.
It is yet another object of the invention to provide a low heat capacity
cautery probe which receives energy over a relatively short period in the
form of a large number of relatively short, equal energy pulses.
It is another object of the invention to provide a heating element for a
thermal cautery probe which inherently provides an indication of the
temperature of the probe's active heat transfer portion.
It is a further object of the invention to provide a washing system for a
thermal cautery probe which effectively washes blood from the wound
without interfering with cauterization, thereby facilitating
identification of the bleeding site.
It is a still further object of the invention to provide a thermal cautery
probe which does not have a tendency to adhere to coagulated tissue.
These and other objects of the invention are provided by an electrically
powered thermal probe including an active heat-transfer portion having a
low heat capacity which is in direct thermal contact with a heating
element so that heat is transferred principally by conduction. The probe
is heater active during a heating period having a duration of less than
five seconds. Yet sufficient power is applied to the heat-transfer portion
during the heating period to coagulate tissue, and the low heat capacity
of the heat-transfer portion allows rapid cooling after the heating
period. The effective impedance of the heater is greater than 0.5 ohm so
that the heater can be powered through power lines that are capable of
extending through the channels of an endoscope. The energy is delivered in
the form of a large number of relatively short pulses, each delivering the
same quantity of energy to the probe. The probe may be used in either of
two modes. In a first mode the energy to be delivered to the probe is
preset and the duration of the period during which the pulses are
delivered is displayed. Accordingly, a down-counter is preset to a number
indicative of the energy to be delivered, and each heating pulse
decrements the down-counter until a zero count is reached. During this
period, an oscillator is gated to an up-counter and the contents of the
counter is displayed to indicate the duration of the period during which
the heating pulses were delivered. In a second mode, the duration of the
period that the heating pulses are applied to the probe is preset, and the
energy delivered to the probe is displayed. Accordingly, the down-counter
is preset to a number indicative of the period during which the pulses are
to be delivered, and an oscillator is gated to the down-counter when the
pulses are being delivered until a zero count is reached. Meanwhile, the
heating pulses are applied to the up-counter, and the contents of the
counter is displayed to indicate the energy delivered to the probe during
the heating period. The heating element for the probe is preferably a
controlled breakdown diode, such as a zener or avalanche diode, which
provides good heat dissipation at low currents and has a breakdown voltage
which is temperature dependent so that it provides an electrical
indication of probe temperature. The probe temperature indication is used
to inhibit the delivery of heating pulses to the probe where the
temperature of the probe exceeds a target value. A plurality of water
jets, which are circumferentially spaced about the body of the probe,
direct water along the probe sidewalls in an axial direction, thereby
clearing blood from the bleeding site. Finally, the end of the probe is
coated with a special compound to prevent the probe from adhering to
coagulated tissues.
In operation, one of the two operating modes is selected and the
cauterization time or cauterization energy is preset, depending upon which
mode is selected. The probe is then applied to the wound while cold, and a
switch is activated to apply power to the probe. The low heat capacity of
the probe's active heat-transfer portion allows it to quickly reach a
sufficient temperature to effectively cauterize the wound and to rapidly
cool after power is removed to prevent excessive thermal penetration,
thereby minimizing necrosis.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of the fast pulse thermal cautery probe in
operation;
FIG. 2 is an exploded isometric view of the thermal cautery probe;
FIG. 3 is an isometric view of the assembled thermal cautery probe;
FIG. 4 A and B is a schematic of the circuitry for supplying power to the
thermal cautery probe;
FIG. 5 is a cross-sectional view taken along the line 5--5 of FIG. 3.; and
FIG. 6 is a graph showing the temperature at the heat transfer surface of
the probe.
BEST MODE FOR CARRYING OUT THE INVENTION
The fast pulse thermal cautery probe is illustrated in use in FIG. 1. The
probe includes a power supply and display unit 10 having a front panel 12
containing switches and indicators illustrated in greater detail in the
enlarged view. A specially constructed catheter 14 extends from the power
supply and display unit 10 to a headpiece 16 of a conventional fiberoptic
endoscope 18 extending into the mouth of a patient P to, for example, the
patient's stomach. The headpiece 16 typically includes an eyepiece through
which a surgeon S views internal cavities. However, the headpiece 16 may
alternatively interface with various optical devices of conventional
design. These devices may produce an image on a screen 20 of the position
of the probe 22 within the stomach of the patient P. The endoscope 18
typically includes one or more passages or channels extending in parallel
with the fiberoptic wave guide to allow various devices to be inserted
into internal organs of a patient. The catheter 14 extends through one of
these channels to the end of the endoscope 18 within an internal organ. A
surgeon S may then position the probe 22 against a lesion such as an ulcer
by manipulating actuator knobs generally positioned on the headpiece 16 of
conventional endoscopes 18. The endoscope channels are necessarily limited
in diameter so that the diameter of the power leads for applying power to
the probe are also limited. In practice, the diameter of the power leads
is limited to a size which is capable of delivering sufficient current for
cauterization to a probe having a resistance of at least about 0.5 ohm. A
probe having a lower resistance must receive a current which is in excess
of that which endoscopically deliverable power leads are capable of
efficiently carrying without producing excessive heating of the endoscope.
A foot switch 28 is also connected to the power supply and display unit 10
through a lead 29. As explained in greater detail hereinafter, the surgeon
S actuates the foot switch 28 to apply power to the probe 22 after the
probe has been applied to the lesion. A second switch, operating in
conjunction with the switch 28, may be actuated to supply a washing fluid
to the probe.
As best illustrated in the enlarged view of FIG. 1, the panel 12 includes
an on-off switch 30 for applying power to the unit 10 and a mode switch 32
for selecting either a "time" mode or an "energy" mode. In the time mode,
heating pulses are applied to the probe 22 for a period having a duration
determined by the number preset with conventional thumb wheel switches 34.
At the end of the period, the total energy delivered to the probe 22
during that period is shown on a conventional digital indicator 36. In the
energy mode, the energy to be delivered to the probe each time the switch
28 is activated is seleted by the thumb wheel switches 34, and the
duration of the period during which the pulses are delivered is shown on
the indicator 36. The temperature of the probe during the heating period
in either mode is selected by a temperature control knob 33.
The probe 22 is illustrated in assembled condition in FIG. 3. It is
composed of an elongated cylindrical shell 40 having a smooth, round-ended
forward portion and a cylindrical body 42 containing a number of
circumferentially spaced-apart cleaning fluid nozzles 44. The catheter 14
abuts the body 42 of the probe 22 and supplies cleaning fluid to the
nozzles 44 and heating pulses to an internal heating element in the shell
40, as explained in greater detail hereinafter.
The internal structure of the probe 22 is illustrated in greater detail in
FIGS. 2 and 5. This structure is best explained in the context of the
manufacturing procedure for the probe. Initially, a coaxial cable 50 is
prepared by trimming a portion of an insulating sheath 52 back from an
underlying coaxial metal braid 54, a coaxial dielectric insulator 56 and a
center conductor 58. In a similar manner, the braid 54 is trimmed back
from the underlying insulator 56 and conductor 58, and the insulator 56 is
trimmed back from the underlying conductor 58. As a result, each component
of the cable 50 is accessible.
After the cable 50 has been prepared as explained above, the body 42 of the
probe 22 is loosely slipped over the cable 50, and the insulator 56 and
center conductor 58 are inserted through a bore 60 of a coaxial braid
anchor 62 with the braid 54 loosely fitting inside a cylindrical portion
at the rear of the braid anchor 62. The braid 54 is then soldered to the
walls of the bore 60 by conventional means to electrically and
mechanically connect the braid anchor 62 to the braid 54 of cable 50.
A spring mount 102 is then soldered to the coax center conductor 58 and a
spring 100 is soldered onto the spring mount 102. Next, the braid anchor
62 is slipped into the body 42 with resilient fingers 109 of the braid
anchor 62 frictionally engaging the inside surface of the body 42. Next
the assembly is placed in a vertical position and a small amount of epoxy
is applied between spring mount 102 and braid anchor 62 to form a seal 70
having a shoulder 72. The seal 70 provides electrical insulation and a
seal to prevent fluids from entering the internal cavity of the probe. A
teflon seal 108 is then pressed into body 42 with the shoulder on seal 108
resting against the fingers 110 of body 42. The spring 100 now lies inside
of the cylindrical bore 106 of seal 108 with the axial tip 105 of spring
100 protruding from the bore 106 a small amount.
After the rearward components of the probe 22 are prepared and assembled as
explained above, the internal components of the shell 40 are assembled. As
best illustrated in FIG. 5, the shell 40 is generally hollow to form a
cylindrical cavity 80 surrounded by thin, cylindrical side walls 82. The
front end of the shell 40 is a solid hemispherical heat transfer portion
84 having a planar, circular rear face 86. A controlled breakdown diode,
such as a zener or avalanche diode 90, is then bonded within the cavity 80
against the rear face 86. The diode 90 includes a diode chip 92, a pair of
cylindrical conductors 94, 96 connected to opposite faces of the diode
chip 92, and an insulative coating 98 surrounding the diode chip 92 and
conductors 94, 96. Although a diode 90 having the structure shown could be
specially fabricated, the diode 90 is preferably formed by severing the
ends of commercially available diodes having a cylndrical shape. A simpler
and easier technique would be to solder a commercially available diode
chip directly to cavity 80 against the rear face 86, although other
heating devices such as a thin film resistor or conventional diode, may
also be used. However, such alternative heating devices generally require
substantially more current to dissipate the same amount of power. For
example, a 14-volt zener diode dissipates about twenty times more power
for a given current than a diode having 0.7-volt forward breakdown
voltage.
The diode 90 is mounted in the shell 40 by first tinning the rear face 86
of the heat transfer portion 84 with solder. The exposed surface of one
diode conductor 94 is then also tinned and placed in the shell 40,
preferably using an alignment jig to position the diode 90 at the center
of the shell 40. The shell 40 is then heated to fuse the solder on the
rear face 86 and the solder on the diode conductor 94. The diode 90 is
thus in direct thermal contact with the heat-transfer portion 84 so that
heat is transferred from the diode 90 to the heat-transfer portion 84
principally by conduction rather than by radiation.
After the diode 90 is soldered within the shell 40, the outer surfaces of
the shell 40 are polished and then plated with copper and gold. Finally, a
conformal coating is applied to the outer surface of the shell 82 to
prevent it from sticking to tissues after coagulation. The coating is
preferably Type R-4-3117 sold by Dow-Corning which is normally used to
seal printed circuit boards from moisture and abrasion. The coating should
be applied over an undercoat of Dow Corning 1204 Primer for best adhesion
of the conformal coating to the probe. Alternatively, the primer may be
used without the conformal coating, providing the probe with better heat
transfer characteristics to tissue but having a greater tendency to adhere
to coagulated tissues.
In a final assembly stage, the shell 40 is moved rearwardly until resilient
fingers 110 of the body 42 frictionally engage the inner walls 82 of the
shell 40. Finally, as illustrated in FIGS. 3 and 5, a catheter 112,
loosely surrounding the cable 50, is slipped onto a shoulder 114 (FIG. 5)
formed along the rear edge of the body 42, thereby completing the assembly
of the probe.
A washing fluid is pumped through the catheter 112 around the cable 50 and
enters rear openings of the nozzles 44. The washing fluid then flows along
the side of the shell 40 in an axial direction to wash blood from the
lesion, thereby facilitating the identification of bleeding sites in need
of coagulation. A variety of commercially available pumps may be used to
deliver washing fluid to the probe. However, the fluid is preferably
delivered in a pulsating fashion to allow sufficient bleeding between
washing pulses to make the site of bleeding readily visible.
It is highly advantageous to run the washing fluid along the outside of the
shell for a number of reasons. First, the washing fluid does not pass
between the heat transfer portion 84 of the probe and the tissue to be
coagulated. Consequently, it does not interfere with the transfer of heat
from the probe to the tissue. Second, the washing fluid flows along
surfaces which do not contact tissue and are thus not susceptible to
tissue clogging which would interfere with fluid flow. Finally, the fluid
stream is spread out over a sufficiently large area to prevent tissue
damage which might otherwise occur with a more concentrated fluid stream.
In the probe's assembled condition, the conductor spring 100 is somewhat
compressed so that the point 105 forcibly contacts the conductor 96. The
diode conductor 94 is connected to the shell 40 and, in turn, to the braid
54 through the body 42 and braid anchor 62. Thus, as current pulses are
applied between the center conductor 58 and the braid 54 of cable 50,
current flows through the semi-conductor junction 92, which quickly heats
the heat transfer portion 84. Because of the low heat capacity of the heat
transfer portion 84, the portion 84 not only quickly rises to a target
temperature, but it also quickly cools after heating pulses are no longer
applied to the probe 22.
The circuitry for generating the heating pulses is illustrated in FIGS. 4A
and 4B. With reference first to FIG. 4B, the foot switch 28, in its "off"
position, places a logic low at one input to a set-reset flip-flop formed
by NAND gates 200, 202. Consequently, the output of NAND gate 200 is high,
while the output of NAND gate 202 is low since its other input is biased
high through resistor 204. When the switch 28 is moved from the "off"
position, the output of NAND gate 200 is held high by the low at the
output of NAND gate 202 since its other input is biased high through
resistor 206. When the switch 28 is actuated to the "on" position, the
output of NAND gate 202 goes high, thereby causing the output of NAND gate
200 to go low. The negative-going transition at the output of NAND gate
200 is differentiated by capacitor 208 and applied to the preset (ps)
terminal of flip-flop 210, which is normally held high through resistor
212. The Q output of flip-flop 210 then goes high to produce an ENABLE
signal. It will be noted that a negative-going transition will not be
generated at the output of NAND gate 200 until the switch 28 is returned
to its "off" position and once again cycled to its "on" position.
Consequently, any contact bounce present in switch 28 as the switch
reaches its "on" position, has no effect on the operation of the circuit.
The ENABLE signal generated by flip-flop 210 has a number of functions.
First, it presets cascaded counters 220a,b,c with numbers selected by the
thumbwheel switches 34a, b,c, respectively. The data inputs to these
counters 220 are normally held low through resistors 222, but these lines
are driven high by the switches 34 so that the BCD numbers applied to each
of the counters 220a,b,c correspond to the decimal numbers appearing on
the panel 12 of the power supply and display unit 10.
The enable input also enables a synchronous oscillator 226 formed by a
one-shot, which generates a pulse train having a frequency determined by
the time constant of resistor 227 and capacitor 229 and the time constant
of resistor 231 and capacitor 233. The ENABLE signal is also inverted by
enabled NAND gate 228 to produce a negative-going transition which is
differentiated by capacitor 230 and resistor 232 to generate a
negative-going reset pulse which resets counters 234a,b,c. Thus, actuation
of the foot switch 28 resets up-counters 234a,b,c, presets down-counters
220a,b,c with a number selected by thumb wheel switches 34a,b,c and allows
oscillator 226 to begin decrementing the down-counters 220a,b,c.
With reference now to FIG. 4A, the ENABLE signal is also applied to NAND
gate 240 to initiate the delivery of heating pulses to the probe 22.
Assuming that the other input to NAND gate 240 is high, the low to high
transition of the ENABLE signal sets flip-flop 242 so that its Q output
goes low. Current then flows through resistor 244 and light-emitting diode
246. The light-emitting diode 246 is optically coupled to a
phototransistor 248 which saturates to drive transistor 250 to saturation
so that a negative power supply voltage is applied directly to the diode
90 through a fairly low impedence resistor 252. The voltage supplied to
the diode 90 is applied to a conventional integrated circuit multiplier
254 after being attenuated by resistors 256, 258 arranged in a voltage
divider configuration. The voltage on the opposite terminal of the
resistor 252 (which exceeds the voltage on diode 90 by a function of the
current-through diode 90) is similarly applied to the multiplier 254 after
being attenuated by resistors 260, 262 arranged as a voltage divider. The
multiplier 254 generates a voltage which is proportional to the product of
the voltage applied to the diode 90 and the voltage across the resistor
252. Since the voltage across resistor 252 is proportional to the
current-through diode 90, the voltage at the output of the multiplier 254
is proportional to the power being applied to the diode 90. This power
signal is applied to an operational amplifier 264 through resistor 266.
The operational amplifier has a capacitor 268 connected in its feedback
path so that it operates as an integrator. The integrated power signal at
the output of the amplifier 264 is thus a voltage proportional to the
energy which has been delivered to the diode 90 from the end of the last
COUNT pulse. The energy signal at the output of amplifier 264 is compared
by a comparator 280 to an energy reference signal generated by
potentiometer 282. When the energy delivered to the diode 90 exceeds a
value determined by the potentiometer 282, the output of the comparator
280 goes low, thereby actuating a one-shot 284 through resistor 286. The
trigger input to the one-shot 284 is normally held high through resistor
288 and the resistor 286. The one-shot 284 clears or disables the
flip-flip 242 for a predetermined period determined by the time constant
or resistor 290 and capacitor 292.
During this disabling period, the Q output of flip-flop 242 is high,
thereby causing amplifier 294 to clip at its negative supply level. This
negative voltage back-biases diode 298, thereby floating the gate of FET
transistor 296. The source to drain impedance of the FET 296 is then
greatly reduced, thereby discharging capacitor 268 and reducing the output
of the power signal integrator 264 to zero volts. At the end of this COUNT
pulse (after the disable period as determined by one-shot 284), the Q
output of flip-flop 242 once again goes low, causing the output of
amplifier 294 to float so that the gate of FET 296 is held high through
resistor 300. The source to drain impedance of the FET 296 then increases
sufficiently to allow the amplifier 264 to once again integrate the
incoming power signal. The amplifier 294 thus functions as a level
converter to interface the logic circuitry of the flip-flop 242 to the
voltage levels required by the FET 296. It switches between these two
voltage levels at a voltage determined by resistors 302, 304, which are
arranged in a voltage divider configuration.
At the end of the fifty microsecond disable period, flip-flop 242 is once
again set, thereby once again saturating transistors 248, 250 and applying
power to the diode 90. It is thus seen that as long as a logic "0" is
applied to the set terminal of flip-flop 242, measured quantities of
energy are sequentially applied to the diode 90 in a pulse train.
The foregoing explanation of the circuit operation presupposes that both
inputs to NAND gate 240 are logic "1" during the operating cycle. This
will always be the case as long as the temperature of the diode 90 is
below a preset value. However, NAND gate 240 is disabled as long as the
temperature of the diode 90 exceeds the predetermined value. Consequently,
the temperature of the probe 22 quickly rises to the predetermined value
as current pulses are repetitively applied to the diode 90; and when the
predetermined level is reached, the pulses are applied to the diode 90 at
a lower frequency to maintain the temperature constant.
In accordance with this feature, a by-pass resistor 301 is positioned
between the base and emitter of transistor 250 so that a slight amount of
current flows through diode 90 when transistors 248 and 250 are cut off.
Diode 90 is preferably an avalanche diode, and its reverse breakdown
voltage is proportional to its temperature. Consequently, the voltage
between resistors 256 and 258, which is applied to a temperature
comparator 302 is a measure of the temperature of the diode 90. This
temperature feedback voltage is compared to a reference voltage determined
by resistors 303, 306 and potentiometers 308, 310 and the temperature
controller potentiometer 33. Potentiometer 308 is varied to adjust the
bias on field effect transistor 312, thus adjusting the slope of the
temperature-to-voltage transfer curve. The potentiometer 310 is adjusted
to set the 0.degree. C. Intercept on the curve, thereby calibrating the
temperature selector 33. When the temperature, as indicated by the voltage
on the positive input to the comparator 302, exceeds the level set by
potentiometer 33, its output goes low so that a low is placed on the input
to NAND gate 240 through resistor 316. Thereafter, no additional current
pulses can be applied to the diode 90 until the temperature falls below
the preset value. The temperature comparator 302 then generates a logic
"1" to enable the NAND gate 240.
Returning now to FIG. 4B, when the switch 32a is in the "energy" position
as illustrated, the COUNT pulses produced each time a current pulse is
applied to the probe 90 are inverted by enabled NAND-gate 330 and applied
to the first counter 234a of a series of cascaded counters 234a,b,c. It
will be remembered that these counters 234 had been reset to zero by the
leading edge of the ENABLE signal when the switch 28 was initially
actuated. At the termination of each current pulse to the diode 90, the
counters 234 are incremented by a COUNT pulse. Thus, the content of the
counters 234 is an indication of the total energy which has been applied
to the probe 90 during the ENABLE pulse. The counters 234 drive the front
panel three-digit readout 36 which then continuously indicates the energy
which has been applied to the probe 90.
As long as the ENABLE signal is present, the oscillator 226 continuously
generates timing pulses regardless of the frequency at which current
pulses are applied to the diode 90. When the switch 32b is in the time
position indicated, these pulses are applied to the first counter 220a of
a series of down-counters 220a,b,c. As mentioned above, the leading edge
of the ENABLE pulse presets the counters 220 with a number corresponding
to the number selected by the thumb wheel switches 34 of the front panel
12. The counters 220 thus begin counting down from this number until a
zero count is reached. Upon the zero count from counter 220c, a high is
produced at the output of enabled NAND-gate 332 which clocks a logic "0"
on the data input (D) of flip-flop 210 to its Q output, thereby
terminating the ENABLE pulse. The ENABLE line is now logic "0", thereby
disabling the oscillator 226 so that counters 220a,b,c are no longer
decrementing. The ENABLE low also removes the low from the set input to
flip-flop 242 (FIG. 4A), thereby allowing its Q output to remain high in
order to prevent COUNT pulses from being generated which would otherwise
cause additional current pulses to be applied to the diode 90 and
increment the counters 234a,b,c. It is thus seen that when switch 32 is in
the "time" position illustrated, the switches 34 select the duration of
the period during which current pulses are applied to the probe 90, while
the front panel three-digit readout 36 indicates the total quantity of
energy delivered to the diode 90 during that period.
Repositioning the switch 32 to the "energy" mode causes the timing pulses
from oscillator 226 to be applied to the up-counters 234 while the COUNT
pulses generated for each heating pulse are applied to the down counters
220. Thus, in this alternate "energy" mode, the switches 34 select the
total energy to be delivered to the diode 90 while the front panel
three-digit readout 36 indicates the duration of the period during which
the energy was delivered to the diode 90.
During use of the probe 22, the termination of the heating pulses applied
to the probe 22 is not visually apparent. Consequently, an audible alarm
may be provided to inform the surgeon when to remove the probe from the
wound area. Accordingly, as illustrated in FIG. 4B, the Q output of
flip-flop 210 is applied to a one shot 340 and a NAND-gate 342. The output
of the NAND-gate 342 is applied to a conventional audio alarm 344,
commonly known as a "SONALERT".
Prior to initiating an ENABLE pulse, the Q output of flip-flop 210 and the
output of one-shot 340 are both high, keeping the output of NAND gate 342
low so that no current flows through the SONALERT 344. When the ENABLE
pulse occurs, NAND gate 342 becomes disabled and its output goes high,
turning on the SONALERT 344. At the termination of the ENABLE pulse, the Q
output of flip-flop 210 goes high, thereby triggering the one-shot 340,
whose output goes low, keeping NAND gate 342 disabled despite the high
input from the Q output of flip-flop 210. When the one-shot 340 output
goes high after a period determined by resistor 346 and capacitor 348, the
NAND gate 342 is again enabled, setting its output low and turning off the
SONALERT 344. Thus, the SONALERT 344 operates during the ENABLE pulse and
for a fixed time longer. This added time insures that a complete cycle
occurred with the probe in contact with the lesion, including the time
required for the probe-tissue interface temperature to fall below that for
tissue denaturization. This is generally on the order of about 0.2 second.
The foot switch 28 may also include a second switch contact 28' which, when
closed, applies power to a conventional fluid pump 360 which delivers
washing fluid to probe. As mentioned above, the pump 360 preferably
operates in a pulse-like manner to allow a sufficiently long period
between pulses to identify bleeding sites.
In operation, an endoscopist or surgeon first threads the catheter 14
through a channel in an endoscope 18 and manipulates the probe 22 at the
end of the catheter 14 until it is in a desired position against the
wound. Either thereafter or before, the surgeon S has selected either a
time or an energy mode, and he has selected either a predetermined
duration of cauterization or cauterization energy, respectively, on thumb
wheel switches | | |
