U.S. patent number RE40,388 [Application Number 10/434,019] was granted by the patent office on 2008-06-17 for electrosurgical generator with adaptive power control.
This patent grant is currently assigned to Covidien AG. Invention is credited to David Lee Gines.
United States Patent |
RE40,388 |
Gines |
June 17, 2008 |
Electrosurgical generator with adaptive power control
Abstract
An electrosurgical generator has an output power control system
that causes the impedance of tissue to rise and fall in a cyclic
pattern until the tissue is desiccated. The advantage of the power
control system is that thermal spread and charring are reduced. In
addition, the power control system offers improved performance for
electrosurgical vessel sealing and tissue welding. The output power
is applied cyclically by a control system with tissue impedance
feedback. The impedance of the tissue follows the cyclic pattern of
the output power several times, depending on the state of the
tissue, until the tissue becomes fully desiccated. High power is
applied to cause the tissue to reach a high impedance, and then the
power is reduced to allow the impedance to fall. Thermal energy is
allowed to dissipate during the low power cycle. The control system
is adaptive to tissue in the sense that output power is modulated
in response to the impedance of the tissue.
Inventors: |
Gines; David Lee (Ft. Collins,
CO) |
Assignee: |
Covidien AG (Neuhausen am
Rheinfall, SE)
|
Family
ID: |
25277394 |
Appl.
No.: |
10/434,019 |
Filed: |
May 8, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
08838548 |
Apr 9, 1997 |
6033399 |
|
|
Reissue of: |
09209323 |
Dec 11, 1998 |
06228080 |
May 8, 2001 |
|
|
Current U.S.
Class: |
606/34;
606/42 |
Current CPC
Class: |
A61B
18/1206 (20130101); H03L 5/02 (20130101); A61B
2018/00702 (20130101); A61B 2018/00761 (20130101); A61B
2018/00875 (20130101); A61B 2018/00886 (20130101); A61B
2018/124 (20130101) |
Current International
Class: |
A61B
18/18 (20060101) |
Field of
Search: |
;606/32-34,37-42 |
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Primary Examiner: Peffley; Michael
Parent Case Text
This application is a Continuation of U.S. application Ser. No.
08/838,548, filed on Apr. 9, 1997, now U.S. Pat. No. 6,033,399, the
contents of which are incorporated herein by reference.
Claims
What is claimed:
1. An electrosurgical generator for applying output power to tissue
to desiccate tissue, the electrosurgical generator comprising: a
controller for cycling the output power to cause a cycling of the
tissue impedance; and a tissue impedance measurement circuit
coupled to said controller for measuring impedance of the tissue;
wherein the controller cycles the output power by applying a
predetermined amount of output power to said tissue, lowering the
output power upon an output voltage reaching a predetermined
maximum, re-applying the predetermined amount of output power to
said tissue of a measured tissue impedance does not indicate
desiccation of the tissue, and terminating output power when the
measured tissue impedance exceeds a predetermined value, the
predetermined value corresponding to a desiccated condition of
tissue.
2. The generator of claim 1, wherein the controller changes the
output voltage to cycle the output power.
3. The generator of claim 1, wherein the controller changes the
output current to cycle the output power.
4. The generator of claim 1, wherein the output voltage is cycled
by lowering the output voltage once it reaches a predetermined
maximum and raising the output voltage if the reduction in measured
tissue impedance does not indicate desiccation of the tissue.
5. The generator of claim 1, wherein the output power is cycled at
a frequency that is between 1 and 20 Hz.
6. The generator of claim 1, wherein the output voltage does not
exceed 120 volts.
7. The generator of claim 1, further comprising a comparator
wherein the measured tissue impedance value is compared to a first
signal representative of a desired tissue impedance value by the
comparator and a difference signal is produced.
8. The generator of claim 7, wherein the difference signal is input
to the controller which generates a signal to adjust the power.
9. The generator of claim 7, wherein the first signal has a cyclic
pattern.
10. The generator of claim 9, wherein the first signal is a sine
wave.
11. An electrosurgical generator for treating tissue by applying
energy comprising: a desiccation detector for measuring a degree of
desiccation of tissue; and a controller for minimizing the burning
of tissue comprising power control circuitry for repeatedly
increasing and decreasing output power to the tissue to be treated,
the power control circuitry coupled to the desiccation detector and
operating to adjust the output power in response to the degree of
desiccation of the tissue by applying a predetermined amount of
output power to said tissue, lowering the output power upon an
output voltage reaching a predetermined maximum, and re-applying
the predetermined amount of output power to said tissue if said
desiccation detector does not indicate desiccation of the
tissue.
12. The generator of claim 11, wherein the output power is
terminated by said controller upon detection of desiccated
tissue.
13. The generator of claim 12, wherein the desiccation detector
further comprises impedance measuring circuitry, wherein the degree
of desiccation of the tissue is determined by the impedance of the
tissue measured by the impedance measuring circuitry.
14. The generator of claim 13, wherein the circuitry adjusts the
output power by adjusting the output voltage within a predetermined
voltage range.
15. The generator of claim 11, wherein the output power is
repeatedly increased and decreased by the circuitry at a frequency
between 1 and 20 Hz.
16. A method for applying energy to tissue to treat tissue, the
method including supplying a generator having a power control
system to produce an adaptive oscillatory power curve to minimize
the heating effect on tissue, the method comprising: a) applying a
high current into a low impedance load until a maximum power is
reached; b) adjusting the output voltage to maintain constant
output power as impedance increases as tissue begins to desiccate;
c) dropping the output power in response to a rapid rise in tissue
impedance indicating the boiling of tissue; d) allowing the tissue
impedance to fall to a predetermined minimum value and then raising
the output power to cause an increase in tissue impedance; e)
repeating steps b and c until impedance reaches a maximum
value.
17. A method for applying energy to tissue to treat tissue, the
method including supplying a generator having a power control
system to produce an adaptive oscillatory power curve to minimize
the heating effect on tissue, the method comprising: a) applying a
high current into a low impedance load until a maximum power is
reached; b) adjusting the output voltage to maintain constant
output power as impedance increases as tissue begins to desiccate;
c) dropping the output power if the output voltage exceeds a
maximum value; d) raising the output power after a predetermined
period of time to cause an increase in tissue impedance; and e)
repeating steps b and c until impedance reaches a maximum
value.
18. A method for applying energy to tissue to treat the tissue,
comprising steps of: providing an electrosurgical generator;
coupling an output of the generator to tissue to be treated; and
coupling electrosurgical energy from the generator into the tissue
in an oscillatory manner until an impedance of the tissue rises to
a value that indicates that the tissue is desiccated, wherein the
electrosurgical energy is coupled into the tissue with an
oscillatory frequency that lies within a thermal bandwidth of the
tissue.
19. A method as in claim 18, wherein the frequency is within a
range of about one Hertz to about 20 Hertz.
.Iadd.20. A method of applying energy to tissue to treat the
tissue, the method including supplying a generator having a power
control system to produce an adaptive oscillatory power curve to
minimize the heating effect on tissue, the method comprising the
steps of: applying a high current into a low impedance load until a
maximum power is reached; adjusting the output voltage to maintain
constant output power as impedance increases as tissue begins to
desiccate; dropping the output power in response to a rapid rise in
tissue impedance indicating the boiling of tissue; allowing the
tissue to cool until the tissue impedance reaches a predetermined
minimum value and then raising the output power to cause an
increase in tissue impedance; and repeating the adjusting and
dropping steps until the tissue impedance reaches a maximum
value..Iaddend.
.Iadd.21. A method of applying energy to tissue to treat tissue,
the method comprising the steps of: supplying a generator having a
power control system to produce an adaptive oscillatory power curve
to minimize the heating effect on tissue; coupling electrosurgical
energy from the generator into the tissue in an oscillatory manner
until an impedance of the tissue rises to a value that indicates
that the tissue is desiccated; adjusting an output voltage to apply
output power to the tissue in a cyclical fashion such that there
are periods of increased power application and periods of reduced
power application; and allowing thermal energy to dissipate during
the periods of reduced power application..Iaddend.
.Iadd.22. The method of claim 21 wherein the electrosurgical energy
is coupled into the tissue with an oscillatory frequency that lies
within a thermal bandwidth of the tissue..Iaddend.
.Iadd.23. The method of claim 22 wherein the frequency is within a
range of about one Hertz to about 20 Hertz..Iaddend.
.Iadd.24. The method of claim 21 wherein the output voltage of the
generator is limited to 120 volts to further minimize the heating
effect on the tissue..Iaddend.
Description
BACKGROUND
1. Field of the Invention
The present invention relates to an electrosurgical generator with
an adaptive power control, and more particularly to an eleosurgical
generator that controls the output power in a manner that causes
impedance of tissue to rise and fall cyclically until the tissue is
completely desiccated.
2. Background of the Disclosure
Electrosurgical generators are used by surgeons to cut and
coagulate tissue of a patient. High frequency electrical power is
produced by the electrosurgical generator and applied to the
surgical site by an electrosurgical tool. Monopolar and bipolar
configurations are common in electrosurgical procedures.
Electrosurgical generators are typically comprised of power supply
circuits, front panel interface circuits, and RF output stage
circuits. Many electrical designs for electrosurgical generators
are known in the field. In certain electrosurgical generator
designs, the RF output stage can be adjusted to control the RMS
output power. The methods of controlling the RF output stage may
comprise changing the duty cycle, or changing the amplitude of the
driving signal to the RF output stage. The method of controlling
the RF output stage is described herein as changing an input to the
RF output stage.
Electrosurgical techniques have been used to seal small diameter
blood vessels and vascular bundles. Another application of
electrosurgical energy is tissue welding. In this application, two
layers of tissue are grasped and clamped together while
electrosurgical power is applied. The two layers are thereby welded
together. Tissue welding is similar to vessel sealing, except that
a vessel or duct is not necessarily sealed in this process. For
example, tissue welding may be used instead of staples for surgical
anastomosis. Electrosurgical power has a desiccating effect on
tissue during tissue welding or vessel sealing. As used herein, the
term "electrosurgical desiccation" is meant to encompass any tissue
desiccation procedure, including standard electrosurgical
coagulation, desiccation, vessel sealing, and tissue welding.
One of the problems associated with electrosurgical desiccation is
undesirable tissue damage due to thermal effects. The tissue at the
operative site is heated by the electrosurgical current. Healthy
tissue adjacent to the operative site can become thermally damaged
if too much heat is allowed to build up at the operative site. The
heat may conduct to the adjacent tissue and cause a large region of
tissue necrosis. This is known as thermal spread. The problem of
thermal spread becomes important when electrosurgical tools are
used in close proximity to delicate anatomical structures.
Therefore, an electrosurgical generator that reduced the
possibility of thermal spread would offer a better opportunity for
a successful surgical outcome.
Another problem that is associated with electrosurgical desiccation
is a buildup of eschar on the surgical tool. Eschar is a deposit on
an electrosurgical tool that is created from tissue that is
desiccated and then charred by heat. The surgical tools win often
lose effectiveness when they are coated with eschar. The buildup of
eschar could be reduced when less heat is developed at the
operative site.
Practitioners have known that a measurement of electrical impedance
of tissue is a good indication of the state of desiccation of the
tissue. Several commercially available electrosurgical generators
can automatically terminate output power based on a measurement of
impedance. Several methods for determining the optimal point of
desiccation are known in the field. One method sets a threshold
impedance, and terminates power once the measured impedance of the
tissue crosses the threshold. Another method terminates power based
on dynamic variations in the impedance.
A discussion of the dynamic variations of impedance of tissue can
be found in the article, Vallfors and Bergdahl "Automatically
Controlled Bipolar Electrocoagulation," Neurosurgical Review,
7:2-3, pp. 187-190, 1984. FIG. 2 in the Vallfors article shows
impedance as a function of time during heating of tissue. Valfors
reports that the impedance value of tissue proved to be close to
minimal at the moment of coagulation. Based on this observation,
Vallfors suggests a micro-computer technique for monitoring the
minimum impedance and subsequently terminating output power to
avoid charring the tissue.
A second article by Bergdahl and Vallfors, "Studies on Coagulation
and the Development of an Automatic Computerized Bipolar
Coagulator," Journal of Neurosurgery, 75:1, 148-151, July 1991,
discusses the impedance behavior of tissue and its application to
electrosurgical vessel sealing. The Bergdahl article reported that
the impedance had a minimum value at the moment of coagulation. The
Bergdahl article also reported that it was not possible to
coagulate safely arteries with a diameter larger than 2 to 2.5
millimeters. The present invention helps to overcome this
limitation by enabling electrosurgical vessel sealing of larger
diameter vessels.
U.S. Pat. No. 5,540,684 discloses a method and apparatus for
electrosurgically treating tissue in a manner similar to the
disclosures of Vallfors and Bergdahl. The '684 patent addresses the
problem associated with turning off the RF output automatically
after the tissue impedance has reached a minimum value. A storage
device records maximum and minimum impedance values, and an
algorithm computes an optimal time for terminating output
power.
U.S. Pat. No. 4,191,188 discloses a variable crest factor
electrosurgical generator. The crest factor is disclosed to be
associated with the coagulation effectiveness of the
electrosurgical waveform.
U.S. Pat. No. 5,472,443 discloses the variation of tissue impedance
with temperature. The impedance of tissue is shown to fall, and
then subsequently rise as the temperature is increased. The '443
patent shows a relatively lower temperature region (Region A in
FIG. 2) where salts, contained within the body fluids, are believed
to dissociate, thereby decreasing the electrical impedance. The
relatively next higher temperature region (Region B) is where the
water in the tissues boils away, causing the impedance to rise. The
relatively highest region (Region C) is where the tissue becomes
charred, resulting in a slight lowering of impedance.
It would be desirable to have an electrosurgical generator that
produced a clinically effective output and, in addition, reduced
the amount of heat and thermal spread at the operative site. It
would also be desirable to have an electrosurgical generator that
produced a better quality seal for vessel sealing and tissue
welding operations. It would also be desirable to have an
electrosurgical generator that desiccated tissue by applying a
minimal amount of electrosurgical energy.
SUMMARY OF THE INVENTION
The present invention relates to an electrosurgical generator
having an improved output power controller for increasing the
quality and reliability of electrosurgically sealing vessels,
sealing ducts, welding and desiccating tissue. In particular, the
output power is controlled in a manner that causes impedance of
tissue to rise and fall repeatedly until the tissue is completely
desiccated. The output power and the tissue impedance are both part
of a control system wherein the output power is cycled to thereby
cause a cycling of the tissue impedance. A basis for this invention
is an experimental observation that the electrical impedance of
tissue will usually rise when electrosurgical power is applied, and
the electrical impedance of tissue will usually fall when the
electrosurgical power is reduced or terminated. Presently available
electrosurgical generators will monitor the rising impedance of
tissue as power is applied. However, the applicant is the first to
design an electrosurgical generator with a power control system
that actively cause the impedance of the tissue to rise and fall
repeatedly until the tissue is desiccated, and thereby achieve
beneficial surgical effects.
The application of electrosurgical power is known to cause the
impedance of tissue to fall to a local minimum and then rise
monotonically thereafter. If the electrosurgical power is applied
for too long, the tissue may char and stick to the electrode.
Whereas prior designs terminated output power after the first local
minimum in the impedance measurement, the present invention
actively causes several local impedance minima to occur. Power can
be terminated in the present invention based on an impedance limit,
a time limit, or based on the responsiveness of the tissue to
changes in output power from the generator.
An advantage of the present invention is that it can coagulate
tissue with a reduced level of tissue charring. Another benefit of
the present invention is that it has improved tissue sealing
characteristics. Yet another benefit of the present invention is
that it reduces thermal spread and thereby reduces damage to
adjacent tissue. Yet another advantage of the present invention is
that it reduces the tendency for eschar buildup on the
electrosurgical tool. Yet another advantage of the present
invention is that large vessels and ducts can be electrosurgically
sealed.
It is thought that impedance of tissue can rise and fall depending
on several factors, including output power, output voltage, output
current, temperature, and pressure on the tissue exerted by
surgical graspers. The present invention addresses changes in
impedance of tissue that can be attributed to electrosurgical power
application, wherein the power can be adjusted by changing the
output voltage or the output current. The present invention causes
the tissue impedance to rise and fall repeatedly until the tissue
is completely desiccated. The present invention adjusts the output
power in a manner that is based on feedback from a tissue impedance
measurement.
According to the present invention, the impedance of the tissue
rises and falls in response to relatively low frequency cycling of
the electrosurgical power. The electrosurgical power is raised and
lowered (also referenced herein as "cycled") at a relatively low
frequency, and the impedance of the tissue is thereby caused to
rise and fall at approximately the same frequency until the tissue
becomes desiccated. The manner in which the electrosurgical power
is raised and lowered may be accomplished in several ways which
incorporate well known principles of control system design.
The frequency of power cycling in the present invention is
different from the RF modulation frequency of the electrosurgical
waveforms, which are typically in the range of one hundred
kilohertz to one megahertz. The frequency of power cycling of the
present invention is also different from the duty cycle of
generators that causes a coagulation effect on tissue, which is
typically in the frequency range above one thousand hertz. The
frequency range of power cycling in the present invention is
typically between one and twenty hertz. Both the RF modulation and
the duty cycling of present electrosurgical generators may occur
simultaneously with the power cycling of the present invention.
The frequency at which the electrosurgical power is raised and
lowered (i.e. cycled or modulated) should not be too high,
otherwise the impedance of the tissue will not be able to rise and
fall in response with an amplitude that will produce additional
benefits. Similarly, the frequency should not be too low, otherwise
the beneficial aspects of the invention will not become apparent
because the tissue will desiccate without any appreciable
modulation. The range of effective frequencies of the present
invention has been called "thermal bandwidth."
The behavior of the tissue impedance is possibly related to the
thermal time constant of the tissue. There are additional factors
that affect the tissue impedance, including the water content in
the tissue and steam. After the tissue is desiccated, which is
indicated by a high measured impedance, further application of
electrosurgical power will cause undesirable charring. Thus, it is
preferred to have impedance monitoring to determine the appropriate
time for terminating the electrosurgical power. Impedance
monitoring is also preferred so that the modulation frequency of
the electrosurgical power can be automatically adjusted and kept
within the thermal bandwidth.
It is theorized by the inventor that thermal spread during
electrosurgical desiccation is created in at least three ways. The
first is through direct thermal conduction away from the weld site.
The second is from hot steam exiting the weld site. This mechanism
is perhaps far more significant than the first, because of the
steam's high mobility. The third mechanism is the lateral spread of
current away from the weld site. It is theorized that the third
mechanism is due to steam creating a high impedance pathway between
the jaws, which forces a larger portion of the current to flow
laterally. The present invention controls the output power in a
manner that reduces thermal spread.
The present invention is relevant to all electrosurgical
generators. It has been found to be particularly relevant to
bipolar electrosurgical applications, as well as to elecosurgical
tissue welding and vessel sealing. Skilled practitioners will
recognize the value of the invention wherever tissue desiccation is
accomplished by electrosurgical methods.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram representation of an adaptive oscillatory
power curve according to the present invention.
FIG. 2(a) is a sample of experimental data for a standard vessel
sealing operation, showing output power as function of time.
FIG. 2(b) is a sample of experimental data for a standard vessel
sealing operation, showing load impedance as a function of
time.
FIG. 2(c) is a sample of experimental data for a standard vessel
sealing operation, showing output current as a function of
time.
FIG. 2(d) is a sample of experimental data for a standard vessel
sealing operation, showing output voltage as a function of
time.
FIG. 3(a) is a sample of experimental data for an adaptive power
control generator, showing output power as fiction of time.
FIG. 3(b) is a sample of experimental data for an adaptive power
control generator, showing load impedance as a function of
time.
FIG. 3(c) is a sample of experimental data for an adaptive power
control generator, showing output current as a function of
time.
FIG. 3(d) is a sample of experimental data for an adaptive power
control generator, showing output voltage as a function of
time.
FIG. 4(a) is a representation of a power curve for a standard
electrosurgical generator.
FIG. 4(b) is a representation of an adaptive oscillatory power
curve.
DETAILED DESCRIPTION OF THE INVENTION
The present invention discloses an adaptive, oscillatory power
curve which is able to reduce thermal spread in each of these areas
by applying power in a cyclical fashion, rather than continuously.
During the periods of reduced power application, thermal energy is
allowed to dissipate which reduces direct thermal conduction. Also,
the steam exits the weld site in smaller bursts, which produces
less thermal damage than one large burst. Finally, the impedance
between the jaws of the electrosurgical instrument is kept low,
which allows current to flow more directly between the jaws.
Charring is also reduced. High voltages contribute to tissue
charring, which is why it is preferable to limit the output voltage
of the electrosurgical general to 120 volts, and to periodically
reduce it to a lower value during power cycling. A relatively low
voltage is also important because it prevents electrical sparks, or
arcs, from passing through the tissue and burning small holes in
the newly sealed, or welded, tissue.
The transparency, or clarity, at the weld site has been identified
as an indicator of successful seal completion. It also gives the
surgeon visual feedback as to whether the seal is a success.
Preliminary findings indicate that this method may also increase
weld site transparency. The reason for this is unknown, but it
seems reasonable that reduced charring will allow the weld site to
remain more transparent.
Referring to FIG. 1, a block diagram of an adaptive oscillatory
power control system 10 is shown. The line designated by the letter
A represents the command input signal to the control system 10. The
command input signal A is preferably a periodic function, and in
stain embodiments the period may vary depending on the dynamics of
the tissue. The signal A is representative of the desired tissue
impedance. A measurement of tissue impedance is represented by line
B. A summing block 11 compares the command input signal A with the
measured tissue impedance B to produce a difference signal C. The
summing block 11 may be comprised of an electrical comparator
circuit as is commonly known to control systems engineers.
The difference signal C may be input to a controller 12 that
generates a control signal D. The control signal D adjusts or
terminates the output power of the electrosurgical generator by
changing the state of the RF. Output Stage 13. The controller 12
may be comprised of an algorithm in a microprocessor that
determines the conditions for power termination based on the
amplitude of the control signal. Alternatively and equivalently,
the controller 12 may be connected directly to the measured tissue
impedance B to terminate power based on the amplitude of the
measured tissue impedance B. The controller 12 may be comprised of
any combination of proportional integral, and derivative control
laws that are known to control system engineers. Other types of
control laws, such as "bang-bang " control laws, are effective
equivalents.
In one embodiment, the command input signal A has a cyclic pattern,
for example a sine wave or a square wave. The cyclic nature of the
command input signal A causes the control system 10 to regulate the
output power in a cyclic manner to achieve beneficial surgical
effects. The controller 12 monitors the difference signal C to
determine the response of the output power E. In one embodiment,
when the difference signal C is large, and the impedance
measurement B is above threshold, then the controller 12 terminates
the output power E.
The control signal D is preferably connected to an R.F. Output
Stage 13. The control signal D preferably changes a driving voltage
in the R.F. output stage to thereby change the RMS output power
from the electrosurgical generator, shown as line E in FIG. 1.
Alternatively and equivalently, the control signal D may change the
duty cycle of the R.F. Output Stage 13 thereby effectively changing
the RMS output power. Other means of changing RMS output power from
an R.F. Output Stage, such as changing current, are known to
electrical engineers.
The generator R.F. Output Stage 13 causes the electrosurgical
generator to output a power level E to the tissue 14 of the
patient. The tissue 14 becomes desiccated, thereby changing the
electrical impedance, shown by F in FIG. 1. The electrical
impedance F of the tissue is measured by an impedance measurement
circuit 15 and reported as the measured tissue impedance B. The
impedance measurement circuit 15 may be any form of electrical
circuit that measures, or estimates, electrical impedance. The
measured tissue impedance B is preferably an electrical signal that
is proportional to the actual tissue impedance F.
Electrical engineers will recognize that output power from an
electrosurgical generator can be adjusted in several ways. For
example, the amplitude of the output power can be adjusted. In
another example, the output power can be adjusted by changing the
duty cycle or the crest factor. The change or adjustment in output
power, as used herein is meant to refer any change or adjustment in
the root mean square (RMS) value of the output power of the
electrosurgical generator.
In operation, the control system 10 is designed to cycle the tissue
impedance F for preferably several cycles in order to achieve
beneficial effects. Thus, the command input signal A is a
cyclically varying signal such as a sine wave. An example of
cyclical impedance behavior of tissue is shown in FIG. 3(b). The
generator output power that caused the cyclical impedance behavior
is shown in FIG. 3(a). The cyclical behavior of the present
invention can be contrasted with a standard electrosurgical
generator wherein the output power is shown in FIG. 2(a) and the
tissue impedance is shown in 2(b).
The present invention discloses an adaptive, oscillatory power
curve which is able to reduce thermal spread in each of these areas
by applying power in a cyclical fashion, rather than continuously.
During the periods of reduced power application, thermal energy is
allowed to dissipate which reduces direct thermal conduction. Also,
the steam exits the weld site in smaller bursts, which produces
less thermal damage than one large burst. Finally, the impedance
between the jaws of the electrosurgical instrument is kept low,
which allows current to flow more directly between the jaws.
Charring is thought to be reduced by the present invention. High
voltages contribute to tissue charring, which is why it is
preferable to limit the output voltage of the electrosurgical
generator to 120 volts, and to periodically reduce it to a lower
value during power cycling. A relatively low voltage is also
important because it prevents electrical sparks, or arcs, from
passing through the tissue and burning small holes in the newly
sealed, or welded, tissue.
The transparency, or clarity, at the weld site has been identified
as an indicator of successful seal completion. It also gives the
surgeon visual feedback as to whether the seal is a success.
Preliminary findings indicate that this method may also increase
weld site transparency. The reason for this is unknown, but it
seems reasonable that reduced charring will allow the weld site to
remain more transparent.
A plot of output power vs. load impedance is called a "power
curve." A representation of a standard power curve is shown in FIG.
4(a). At low impedance, the output is typically current limited,
and this is shown as the "constant current" line segment on FIG.
4(a). At midranges of impedance, the electrosurgical generator has
a power control system that maintains the output power at a
constant level by adjusting the output voltage, as shown by the
"constant power" line segment on FIG. 4(a). Eventually, the load
impedance becomes large, and the output power cannot be maintained
without applying unacceptably high output voltages. Thus, a voltage
limit is reached, and the output power drops off because the output
current is falling and the output voltage is at a limit. The drop
in output power is shown as the "constant voltage" line segment in
FIG. 4(a).
The present invention is related to an electrosurgical generator
having an adaptive oscillatory power curve as shown in FIG. 4(b).
The adaptive oscillatory power curve is produced by a power control
system in the electrosurgical generator. The design details of the
control system can be implemented in several ways which are well
known to control system engineers.
The first part of the adaptive oscillatory power curve, shown at
the line segment I in FIG. 4(b), is similar to the standard power
curve, wherein the generator applies high current into a low
impedance load until a maximum power limit, shown as A, is reached.
In the next "leg" of the power curve, shown by line segment B,
output current begins to fall, and output voltage begins to rise as
the generator adjusts the output voltage to maintain constant
output power at the level marked by A. The generator then begins
looking for signs to indicate the onset of boiling in the tissue.
Such signs include a very rapid rise in impedance, or a high value
of voltage, such as 120 volts. The local maximum of the impedance
curve is shown by letter K in FIG. 4(b). The dotted line, marked C
and labeled V=120 V, shows the possible output power if the
generator were to maintain a voltage limit of 120 volts, which is a
preferred voltage limit. Rather than follow the V=120 V line, a
controller in the generator drops the output power. This can be
accomplished, in one embodiment, by dropping the output voltage
limit to between zero and 70 volts, and preferably 50 volts, as
shown in line segment D. In another embodiment of the control
system, the output power can be reduced by other combinations of
output current reduction and/or output voltage reduction.
As a consequence of the lower voltage limit, the output power drops
to the level indicated by H in FIG. 4(b). In certain embodiments, H
may be zero watts. At this lower output power, desiccation stops
and the tissue impedance starts to fall. A preferred lower voltage
limit of 50 volts may be used as shown by dotted line E and marked
"V=50 volts". Once the impedance has reached a local minimum, shown
by J, or after a set period of time, the power control system
raises the output power back to level A, which corresponds to an
output voltage limit of 120 volts in the preferred embodiment.
Thus, the output power rises back to level A, and the impedance
rises again, until the onset of boiling or an impedance threshold
is reached. The cyclical portion of the power curve incorporating
line segments B, D, and E is an important part of this invention
and will continue until the tissue is desiccated. When the tissue
is desiccated, the power will terminate as shown when impedance
reaches point L. In certain embodiments, point L will be
substantially the same as point K.
The behavior shown in FIG. 4(b) can be observed in FIGS. 3(a),
3(b), 3(c) and 3(d). Power oscillations between 120 watts and 20
watts in FIG. 3(a) correspond to cyclical movement between power
level A and power level H in FIG. 4(b). Impedance oscillations in
FIG. 3(b) correspond to cyclical movement between impedance level K
and impedance level J in FIG. 4(b). It will be understood by
control systems engineers that FIG. 4(b) is highly idealized, and
the cyclical behavior may not always reach exactly the same local
maxima and minima. This can be observed in FIG. 3(a), where the
local maxima of the power curve may not always reach 120 volts.
It is theorized by the inventor that the following phenomena occur.
The initial high output power initiates boiling in the tissues. The
subsequent low output power is insufficient to maintain boiling,
and hence boiling in the tissue stops. After boiling stops, if the
tissue is not completely desiccated then the impedance will fall to
a lower value. Next, the low impedance allows output power to
increase, which re-heats the tissue to the point of boiling. The
voltage is also pulled higher during the process, and remains so
until the power curve can sense the onset of boiling, and lower the
voltage, preferably back to 50 volts. The process continues until
the tissue is fully desiccated. An oscillation is one cycle of high
output power followed by low output power.
FIGS. 2(a) through 2(d) show experimental results on tissue samples
using a standard power curve. FIGS. 3(a) through 3(d) show
experimental results using an adaptive oscillatory power curve. The
general nature of the invention can be seen by comparing FIG. 2(a)
with FIG. 3(a). FIG. 2(a) shows a 100 watt electrosurgical output
that is applied continuously to tissue. As the tissue desiccates,
the impedance of the tissue rises and the output power in FIG. 2(a)
is seen to fall off below 20 watts. In contrast, FIG. 3(a) shows an
oscillating output power that varies from approximately 100 watts
to approximately 20 watts. The effects on tissue impedance can be
seen by comparing FIG. 2(b) with FIG. 3(b). The tissue impedance
resulting from the standard power curve is shown to continuously
increase in FIG. 2(b), perhaps after an initial drop. The tissue
impedance resulting from the adaptive oscillatory power curve is
shown to oscillate in FIG. 3(b) and thus has several local
minima.
Output voltage and output current show a cyclic behavior in the
adaptive oscillatory power curve. The cyclic behavior is absent in
the standard power curve. FIGS. 2(c) and 3(c) can be compared to
show the difference in output current between the standard power
curve and the adaptive oscillatory power curve. In each case the
maximum output current rises above 2 amps RMS. FIGS. 2(d) and 3(d)
can be compared to show the difference in output voltage between
the standard power curve and the adaptive oscillatory power curve.
A voltage limit, preferably in each case 120 volts, prevents arcing
that might leave pinholes in the tissue seal.
In one embodiment of the adaptive oscillatory power curve, the
generator temporarily lowers the output voltage limit to 50 volts
whenever the output voltage reaches 120 volts. This causes a
reduction in output power, and if the tissue is not completely
desiccated, a corresponding significant reduction in tissue
impedance. After the reduction in tissue impedance, the output
voltage limit is reset to 120 volts, allowing a rise in output
power. This reduction and subsequent rise in output power
constitutes a cycle.
Designers of electrosurgical generators have found that impedance
is a good indicator of the desiccation state of the tissue.
However, skilled artisans will recognize that it may not be
necessary to compute an exact value for impedance. An electrical
measurement that is proportional to the tissue impedance can be
used as a functional equivalent. In one embodiment, the control
system can properly create the adaptive oscillatory power curve
based on measurements of time, and output voltage.
Table 1 shows a comparison between two sets of tests which compare
a standard power curve with an adaptive oscillatory power curve.
Test 1 indicates use of the standard power curve, while Test 2
indicates the use of the adaptive oscillatory power curve. Size
indicates the vessel diameter in millimeters, burst pressures are
measured in p.s.i., sticking, charring, and clarity are subjective
measures ranked from 0 to 3, (where 0 represents a low value for
sticking and charring, and 0 represents a poor value for clarity),
and ts indicates thermal spread, measured in millimeters.
TABLE-US-00001 TABLE 1 Comparison of Standard Power Curve with
Adaptive Power Curve Test # samples size bp stick charring clarity
ts 1 (mean) 19 2.57 17.26 .63 1.11 1.89 2.11 1 (SD) 1.35 1.04 .76
.81 1.29 .74 1 (min) 1 12.96 1 (max) 6 17.50 2 (mean) 20 2.55 17.39
.80 .60 1.95 1.65 2 (SD) 1.36 .44 1.06 .60 1.36 .81 2 (min) 1 15.52
2 (max) 5 17.50
Table 1 illustrates that the adaptive oscillatory power curve (Test
2) has several advantages over the standard power curve (Test 1).
Most notable is the lower amount of thermal spread: a mean value of
2. 1 mm for the standard power curve, and 1.65 mm for the adaptive
oscillatory power curve. The subjective measures for sticking,
charring and clarity of the weld show that the adaptive oscillatory
power curve offer improvements over the standard power curve.
In general, the invention is an electrosurgical generator for
treating tissue, wherein the electrosurgical generator comprises a
circuit for generating a measurement of the load impedance, and an
output power controller having means for inducing multiple
oscillations of the load impedance in response to the measurement.
The load impedance refers to the impedance of the tissue being
treated by the electrosurgical generator. The circuit for
generating a measurement of the load impedance can be analog or
digital, and typically requires an output voltage sensor and an
output current sensor. The output voltage is divided by the output
current to compute a measurement of load impedance.
The means for inducing multiple oscillations of the load impedance
preferably comprises a control system which can selectively control
the output voltage to cause appropriate oscillations of the output
power. In many electrosurgical generators, an output power control
circuit has an adjustable voltage supply connected to the primary
side of an isolation transformer. The secondary winding of the
transformer is connected to an output resonant circuit. The voltage
supply has an adjuster for changing the voltage to the transformer,
and thereby changing the output voltage of the electrosurgical
generator. A digital signal may be used to control the voltage
supply.
The means for inducing multiple oscillations preferably comprise a
feedback control system, where the feedback is a measurement of the
load impedance. The control system preferably includes an algorithm
in a microprocessor. The algorithm in the microprocessor can
monitor the load impedance and determine how the load impedance is
responding to a change in the output power.
In the preferred embodiment, the control system sets an output
voltage limit of 120 volts RMS, and then controls the output power
to a user desired setting, for example 100 watts. When the
impedance is relatively low, a high current will combine with an
output voltage of less than 120 volts to yield the desired power of
100 watts. As the impedance rises, the output current will fall,
and the output voltage will be increased by the circuit to maintain
the desired output power. When the voltage limit of 120 volts is
reached, the control system will automatically lower the output
voltage to a low value, preferably 50 volts. This effectively
lowers the output power. If the tissue is not completely
desiccated, the lower output power will cause the impedance to drop
significantly. Once a local impedance minimum is detected, or after
a set period of time, the output voltage limit is reset to 120
volts by the control system, and the cycle repeats. It has been
found through experimentation that the oscillations of the load
impedance will occur in the frequency range of one to twenty hertz,
and this range has been referred to herein as the thermal
bandwidth. In one embodiment, the control system terminates the
output power after a set period of time which was three seconds.
Alternatively, the control system can terminate power when the
impedance reaches a threshold of 2000 ohms. Another alternative is
to terminate output power when the measurement of impedance
indicates that the impedance does not substantially fall in
response to a drop in the output power.
The present invention is applicable to any form of electrosurgical
coagulation. The benefits of the present invention, including
reduced thermal spread, less eschar buildup, and improved
desiccation, can be applied to both monopolar and bipolar
electrosurgical generator outputs. While a particular preferred
embodiment has been illustrated and described, the scope of
protection sought is in the claims that follow.
* * * * *