U.S. patent application number 12/136551 was filed with the patent office on 2008-11-13 for electrosurgical generator with adaptive power control.
Invention is credited to David Lee Gines.
Application Number | 20080281315 12/136551 |
Document ID | / |
Family ID | 25277394 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080281315 |
Kind Code |
A1 |
Gines; David Lee |
November 13, 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) |
Correspondence
Address: |
Tyco Healthcare Group LP
60 MIDDLETOWN AVENUE
NORTH HAVEN
CT
06473
US
|
Family ID: |
25277394 |
Appl. No.: |
12/136551 |
Filed: |
June 10, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10434019 |
May 8, 2003 |
RE40388 |
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12136551 |
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09209323 |
Dec 11, 1998 |
6228080 |
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10434019 |
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08838548 |
Apr 9, 1997 |
6033399 |
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09209323 |
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Current U.S.
Class: |
606/38 |
Current CPC
Class: |
A61B 18/1206 20130101;
A61B 2018/00875 20130101; A61B 2018/00886 20130101; A61B 2018/124
20130101; H03L 5/02 20130101; A61B 2018/00761 20130101; A61B
2018/00702 20130101 |
Class at
Publication: |
606/38 |
International
Class: |
A61B 18/14 20060101
A61B018/14 |
Claims
1. An electrosurgical generator for applying output power to
tissue, the electrosurgical generator comprising: a tissue
impedance measurement circuit configured to measure tissue
impedance; and a controller coupled to the tissue impedance
measurement circuit, the controller adapted to cycle output power
from the electrosurgical generator to cause a cycling of the tissue
impedance by applying the output power to tissue and then adjusting
the output power to at least one of a lower output value and
termination of output power, the controller further adapted to
re-apply the output power to tissue if measured tissue impedance
does not indicate tissue desiccation and to terminate output power
when the measured tissue impedance indicates tissue
desiccation.
2. The generator according to claim 1, wherein the controller
changes the output voltage to cycle the output power.
3. The generator according to claim 1, wherein the controller
changes the output current to cycle the output power.
4. The generator according to claim 1, wherein the output voltage
is cycled by lowering the output voltage once the output voltage
reaches a predetermined maximum and raising the output voltage if
the rise in measured tissue impedance does not indicate tissue
desiccation.
5. The generator according to claim 1, wherein the output power is
cycled at a frequency that is from about 1 Hz to about 20 Hz.
6. The generator according to claim 1, wherein the output voltage
does not exceed 120 volts.
7. The generator according to claim 1, further comprising a
comparator and wherein the measured tissue impedance value is
compared to a first signal representative of a desired tissue
impedance value by the comparator to produce a difference
signal.
8. The generator according to claim 7, wherein the difference
signal is input into the controller which generates a signal to
adjust the power.
9. The generator according to claim 7, wherein the first signal has
a cyclic pattern.
10. The generator according to claim 9, wherein the first signal is
a sine wave.
11. An electrosurgical generator for treating tissue by applying
energy, the electrosurgical generator comprising: a desiccation
detector configured to determine completeness of tissue
desiccation; and a controller coupled to the desiccation detector,
the controller adapted to cycle output power to cause a cycling of
tissue impedance in response to the degree of tissue desiccation,
the controller further adapted to re-apply the output power to
tissue if the desiccation detector does not indicate tissue
desiccation.
12. The generator according to claim 11, wherein the output power
is terminated by the controller upon detection of desiccated
tissue.
13. The generator according to claim 12, wherein the desiccation
detector further comprises impedance measuring circuitry adapted to
measure tissue impedance and to determine degree of tissue
desiccation based on the measured tissue impedance.
14. The generator according to claim 13, wherein the impedance
measuring circuitry adjusts the output power by adjusting the
output voltage within a predetermined voltage range.
15. The generator according to claim 11, wherein the output power
is repeatedly increased and decreased by the circuitry at a
frequency from about 1 Hz to about 20 Hz.
16. A method for applying electrosurgical energy to tissue to treat
tissue, the method comprising: a) cycling output power from an
electrosurgical generator to cause a cycling of tissue impedance by
applying output power to tissue and then adjusting output power to
at least one of a lower output value and termination of output
power; b) re-applying output power to tissue if tissue impedance
does not indicate tissue desiccation; c) allowing the tissue
impedance to fall to a predetermined minimum value and then raising
the output power to cause an increase in tissue impedance; and d)
repeating steps b and c until tissue impedance at least reaches a
predetermined value that corresponds to tissue desiccation.
17. The method according to claim 16, further comprising the step
of: lowering the output voltage once the output voltage reaches a
predetermined maximum and raising the output voltage if the rise in
measured tissue impedance does not indicate tissue desiccation.
18. The generator according to claim 1, wherein the cycling of the
output power is accomplished at a frequency from about 1 Hz to
about 20 Hz.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/434,019, filed on May 8, 2003, which is a
reissue of U.S. patent application Ser. No. 09/209,323, now U.S.
Pat. No. 6,228,080, filed on Dec. 11, 1998, which is a continuation
of U.S. patent 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.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to an electrosurgical
generator with an adaptive power control, and more particularly to
an electrosurgical 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.
[0004] 2. Background of the Disclosure
[0005] 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.
[0006] 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 RE 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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 seating of larger
diameter vessels.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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
[0017] According to one aspect of the present disclosure an
electrosurgical generator for applying output power to tissue is
disclosed. The electrosurgical generator includes a tissue
impedance measurement circuit configured to measure tissue
impedance and a controller coupled to the tissue impedance
measurement circuit. The controller is adapted to cycle output
power from the electrosurgical generator to cause a cycling of the
tissue impedance by applying the output power to tissue and then
adjusting the output power to at least one of a lower output value
and termination of output power. The controller is further adapted
to re-apply the output power to tissue if tissue impedance does not
indicate tissue desiccation and to terminate output power when the
measured tissue impedance indicates tissue desiccation.
[0018] According to another aspect of the present disclosure, an
electrosurgical generator for treating tissue by applying energy is
disclosed. The electrosurgical generator includes a desiccation
detector configured to determine completeness of tissue desiccation
and a controller coupled to the desiccation detector. The
controller is adapted to cycle output power to cause a cycling of
tissue impedance in response to the degree of tissue desiccation.
The controller is further adapted to re-apply the output power to
tissue if the desiccation detector does not indicate tissue
desiccation.
[0019] A method for applying electrosurgical energy to tissue to
treat tissue is also contemplated by the present disclosure. The
method includes the steps of: a) cycling output power from an
electrosurgical generator to cause a cycling of tissue impedance by
applying output power to tissue and then adjusting output power to
at least one of a lower output value and termination of output
power and re-applying output power to tissue if tissue impedance
does not indicate tissue desiccation. The method also includes the
steps of c) allowing the tissue impedance to fall to a
predetermined minimum value and then raising the output power to
cause an increase in tissue impedance and d) repeating steps b and
c until tissue impedance at least reaches a predetermined value
that corresponds to tissue desiccation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Particular embodiments of the present disclosure will be
described herein below with reference to the accompanying drawings.
In the following description, well-known functions or constructions
are not described in detail to avoid obscuring the present
disclosure in unnecessary detail.
[0021] FIG. 1 is a block diagram representation of an adaptive
oscillatory power curve according to the present invention.
[0022] FIG. 2(a) is a sample of experimental data for a standard
vessel sealing operation, showing output power as function of
time.
[0023] FIG. 2(b) is a sample of experimental data for a standard
vessel sealing operation, showing load impedance as a function of
time.
[0024] FIG. 2(c) is a sample of experimental data for a standard
vessel sealing operation, showing output current as a function of
time.
[0025] FIG. 2(d) is a sample of experimental data for a standard
vessel sealing operation, showing output voltage as a function of
time.
[0026] FIG. 3(a) is a sample of experimental data for an adaptive
power control generator, showing output power as fiction of
time.
[0027] FIG. 3(b) is a sample of experimental data for an adaptive
power control generator, showing load impedance as a function of
time.
[0028] FIG. 3(c) is a sample of experimental data for an adaptive
power control generator, showing output current as a function of
time.
[0029] FIG. 3(d) is a sample of experimental data for an adaptive
power control generator, showing output voltage as a function of
time.
[0030] FIG. 4(a) is a representation of a power curve for a
standard electrosurgical generator.
[0031] FIG. 4(b) is a representation of an adaptive oscillatory
power curve.
DETAILED DESCRIPTION
[0032] 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.
[0033] Charring is also reduced. 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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).
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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).
[0046] 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.
[0047] 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.
[0048] 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=5 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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 0.63 1.11 1.89 2.11 1 (SD) 1.35 1.04 0.76 0.81
1.29 0.74 1 (min) 1 12.96 1 (max) 6 17.50 2 (mean) 20 2.55 17.39
0.80 0.60 1.95 1.65 2 (SD) 1.36 0.44 1.06 0.60 1.36 0.81 2 (min) 1
15.52 2 (max) 5 17.50
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
* * * * *