U.S. patent application number 09/924485 was filed with the patent office on 2002-03-07 for electrosurgical generator and system.
This patent application is currently assigned to Gyrus Medical Limited. Invention is credited to Goble, Colin Charles Owen, Goble, Nigel Mark.
Application Number | 20020029036 09/924485 |
Document ID | / |
Family ID | 27451302 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020029036 |
Kind Code |
A1 |
Goble, Nigel Mark ; et
al. |
March 7, 2002 |
Electrosurgical generator and system
Abstract
An electrosurgical system including an electrode assembly having
two electrodes for use immersed in an electrically conductive fluid
has a generator with control circuitry for rapidly reducing the
delivered radio frequency output power by at least 50% within at
most a few cycles of the peak radio frequency output voltage
reaching a predetermined threshold limit. In this way, tissue
coagulation can be performed in, for example, saline without
significant steam generation. The same peak voltage limitation
technique is used in a tissue vaporization or cutting mode to limit
the size of the steam pocket at the electrodes and to avoid
electrode burning. In a blended mode, the output voltage is
alternately limited to a value appropriate for coagulation and a
value appropriate for cutting or vaporization.
Inventors: |
Goble, Nigel Mark; (Nr.
Cardiff, GB) ; Goble, Colin Charles Owen; (South
Glamorgan, GB) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
8th Floor
1100 North Glebe Road
Arlington
VA
22201-4714
US
|
Assignee: |
Gyrus Medical Limited
|
Family ID: |
27451302 |
Appl. No.: |
09/924485 |
Filed: |
August 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09924485 |
Aug 9, 2001 |
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09048717 |
Mar 26, 1998 |
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09048717 |
Mar 26, 1998 |
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08642121 |
May 2, 1996 |
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6293942 |
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Current U.S.
Class: |
606/38 ; 606/34;
606/39; 606/40 |
Current CPC
Class: |
A61B 2018/162 20130101;
A61B 2018/1861 20130101; A61B 18/148 20130101; A61B 2018/00875
20130101; A61B 18/1482 20130101; A61B 18/082 20130101; A61B
2018/00755 20130101; A61B 2018/126 20130101; A61B 2018/00892
20130101; A61B 2018/00505 20130101; A61B 18/14 20130101; A61B
2018/143 20130101; A61M 2205/6027 20130101; A61B 2017/00482
20130101; A61B 2018/1472 20130101; A61B 18/1402 20130101; A61B
2018/00178 20130101; A61B 2018/00666 20130101; A61B 18/149
20130101; A61B 2018/00625 20130101; A61B 18/1492 20130101; A61B
2018/1432 20130101; A61B 2018/1213 20130101; A61B 2018/00589
20130101; A61B 18/1485 20130101; A61B 2018/00988 20130101; A61B
2018/1435 20130101; A61M 2205/6018 20130101; A61B 18/1206
20130101 |
Class at
Publication: |
606/38 ; 606/39;
606/40; 606/34 |
International
Class: |
A61B 018/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 1995 |
GB |
9512889.8 |
Jun 23, 1995 |
GB |
9512888.0 |
Dec 29, 1995 |
GB |
9526627.6 |
Mar 6, 1996 |
GB |
9604770.9 |
Claims
What is claimed is:
1. An electrosurgical generator for supplying power to an
electrosurgical instrument, the generator comprising a radio
frequency output stage having at least a pair of electrosurgical
output connections for the delivery of radio frequency power to the
instrurnent, and control circuitry operable to limit the radio
frequency peak output voltage developed across the output
connections to at least first and second predetermined threshold
values and, in a blend mode of the generator, to alternate
constantly between said first and second threshold values.
2. A generator according to claim 1, wherein the first threshold
value is in the range of from 150V to 200V and the second threshold
value is in the range of from 250V to 600V, the voltages being peak
voltages.
3. A generator according to claim 2, wherein the control circuity
is operable, in said blend mode, to limit said output voltage
alternately to said first and second threshold values at an
alternation rate in the range of from 5 Hz to 2kHz.
4. A generator according to claim 3, wherein the control circuit is
operable, in said blend mode, to limit said output voltage
alternately to said first and second threshold values at an
alternation rate in the range of from 20 Hz to 1 kHz.
5. A generator according to claim 1, wherein the output stage
includes a radio frequency switching device for delivering power
signal to the output connections, wherein the control circuitry
includes sensing means for deriving a sensing signal representative
of the radio frequency peak output voltage developed across said
output connections, and a reference signal generator for generating
reference signals representative of said first and second threshold
values, wherein the sensing means further includes a comparator
arranged to compare said sensing signal with said reference signals
to produce a control signal for actuating the switching device such
as to reduce said delivered output power when the control signal
produced by the comparator is indicative of the respective said
threshold value having been reached, and wherein the reference
signal generator and the comparator are operable, in said blend
mode, to compare said sensing signal alternately with the reference
signal representative of the first threshold value and the
reference signal representative of the second threshold value.
6. A generator according to claim 5, wherein the reference signal
generator includes a switching circuit operable to apply said
reference signals alternately to a reference input of said
comparator.
7. A generator according to claim 6, wherein said switching circuit
is software-controlled.
8. An electrosurgical system comprising an electrosurgical
generator for generating radio frequency power and an
electrosurgical instrument coupled to the generator, the instrument
having an electrode structure for operation immersed in an
electrically conductive liquid, wherein the system has a first mode
of operation in which tissue is treated by the application of heat
in the region of the electrode structure without forming an
electrode-enveloping vapour pocket, and a second mode of operation
in which the tissue is locally vaporised by energy transmitted from
the electrode structure via an electrode-enveloping vapour pocket,
said first and second modes being defined by different respective
electrical control parameters selected in the generator, and
wherein the system has a third, blended mode of operation produced
by constantly alternating between the first and second modes.
9. A system according to claim 8, wherein the said different
control parameters are first and second radio frequency output
voltage values, and wherein the generator includes control
circuitry operable, when the blended mode is required, to limit the
radio frequency output voltage of the generator alternately to said
first and second values.
Description
RELATED APPLICATION
[0001] The present application is a continuation-in-part of
application Ser. No. 08/642,121, filed May. 2, 1996.
FIELD OF THE INVENTION
[0002] This invention relates to an electrosurgical generator for
delivering an electrosurgical current particularly but not
exclusively in intracavitary endoscopic electrosurgery. The
invention also relates to an electrosurgical system comprising the
combination of a generator and an electrode assembly. The term
"intracavitary" is used in this specification to denote
electrosurgery in which living tissue is treated by least invasive
surgical access to a body cavity. This may involve "underwater
electrosurgery", a term denoting that the surgery is performed
using an electrosurgical instrument with a treatment electrode or
electrodes immersed in liquid at the operation site. The invention
has particular application in the fields of urology, hysteroscopy
and arthroscopy.
BACKGROUND OF THE INVENTION
[0003] Intracavitary endoscopie electrosurgery is useful for
treating tissue in anatomical or surgically created cavities of the
body which can be accessed by methods involving minimal trauma to
the patient, be this through a natural body passage or one created
artificially. The cavity is distended to provide space for gaining
access to the operation site to improve visualisation and to allow
for manipulation of instruments. In low volume body cavities,
particularly where it is desirable to distend the cavity under
higher pressure, liquid rather than gas is more commonly used due
to better optical characteristics and because it washes blood away
from the operative site. Conventionally. a non-electrolyte solution
such as glycine is used as the fluid distension medium when
electrosurgery is being used, glycine being electrically
non-conductive.
[0004] The limited surgical access encountered during intracavitary
endoscopic procedures makes the removal of tissue pieces derived
from a typical electrosurgical loop cutting electrode both
difficult and time consuming. Vaporisation of tissue whereby the
tissue is reduced to smoke and water vapour is a preferable
technique in these situations, rather than the piecemeal removal of
tissue. The products of vaporisation may be removed following
dissolution within a liquid irrigating medium.
[0005] With regard to underwater endoscopic electrosurgery, the
applicants have found that it is possible to use a conductive
liquid medium such as normal saline in place of glycine. Normal
saline is the preferred distension medium in underwater endoscopic
surgery when electrosurgery is not contemplated or a non-electrical
tissue effect such as laser treatment is being used. Although
normal saline (0.9% w/v; 150 mmol/l) has an electrical conductivity
somewhat greater than that of most body tissue, it has the
advantage that displacement by absorption or extravasation from the
operative site produces little physiological effect and the
so-called water intoxication effects of glycine are avoided.
[0006] Effective electrosurgical treatment of tissue which is
totally immersed in liquid at the application site is difficult to
achieve because the heat generated by the flow of electrical
currents in both the tissue being treated and surrounding
conductive liquid tends to cause boiling of the liquid. The
operating electrode is intermittently surrounded by water vapour
rather than liquid, with consequent large variations in the
electrical impedance of the load presented to the generator
supplying the electrosurgical power to the electrode. Whilst this
variation is mitigated by use of a non-conductive liquid, it cannot
be eliminated entirely due to the release of body fluids at the
operative site which elevates the electrical conductance of the
liquid. Changes in tissue type also alter the load impedance. These
effects result in difficulty in controlling the electrosurgical
output to produce consistent effects on the tissue being treated.
As a result, high powers are commonly employed to overcome this
performance variation.
SUMMARY OF THE INVENTION
[0007] According to a first aspect of this invention, an
electrosurgical generator for supplying radio frequency power to an
electrical instrument, comprises a radio frequency output stage
having at least a pair of electrosurgical output connections for
the delivery of radio frequency power to the instrument, and
control circuitry operable to limit the radio frequency peak output
voltage developed across the output connections to at least first
and second predetermined threshold values and, in a blend mode of
the generator, to alternate constantly between said first and
second threshold values. The output stage preferably comprises a
resonant output circuit coupled to the output connections and a
switching device coupled to the resonant output circuit, and
wherein the control circuitry is operable to actuate the switching
device to reduce the delivered radio frequency power. The switching
device is preferably connected between the resonant output circuit
and one of a pair of supply rails of the power supply means, and
connected so as to switch current repeatedly through the resonant
output circuit at its resonant frequency. In order to cause a
control overshoot, in terms of the degree to which the delivered
power is reduced when the output voltage reaches the predetermined
threshold, the control circuitry is so arranged and coupled to the
switching device that it is capable of reducing the "on" time of
the switching device during individual radio frequency switching
cycles sufficiently rapidly to cause a 50% reduction in delivered
output power within 100 .mu.s of the predetermined threshold having
been reached. This allows surgery to be performed in a conductive
fluid field, in particular in a saline solution. Large and rapid
changes in load impedance can occur substantially without causing
unwanted electrosurgical effects. For example, when it is desired
to produce electrosurgical desiccation, any increase in impedance
due to vaporisation of surrounding saline in the region of an
electrode of the instrument which might otherwise lead to unwanted
arcing at the required power level for effective desiccation can be
largely prevented. When electrosurgical tissue cutting or tissue
vaporisation is required, output voltage limitation can be used to
prevent electrode burning and/or excessive tissue vaporisation. In
the blended mode, the above two states are used alternately,
wherein a pocket of vapour continually forms and collapses in rapid
succession.
[0008] The control circuitry may include a control line feeding a
first power reduction control signal to the radio frequency output
stage. The output stage, which may be a radio frequency power
oscillator, typically has as the oscillating element a radio
frequency power device. and in the preferred embodiment, the
control circuitry is arranged such that at least a 50% reduction in
output power is brought about in a period of less than 20 .mu.s
after the output voltage reaches the predetermined threshold by
reducing the period of conduction of the device during individual
cycles of the radio frequency output signal. Such alteration in the
period of conduction is advantageously achieved independently of
any variation in supply voltage to the radio frequency power
device. In practice, the reduction in output power is brought about
using a single control variable, i.e. the peak output voltage or
peak-to-peak output voltage, independently of supply voltage and
independently of the delivered output power which varies according
to the load impedance and the supply voltage. Thus, triggering of a
power reduction occurs at the same preset output voltage threshold
but at different output power and load impedance values, according
to circumstances.
[0009] As an adjunct to direct control of the radio frequency
output stage, the means for causing a reduction in output power may
include a further control line which is coupled to the power supply
means, the control circuitry being arranged such that a second
power reduction signal is fed to the power supply means to effect a
reduction in the average power supply voltage supplied to the
output stage. Typically, the rate of reduction of power due to
lowering of the power supply voltage is comparatively slow, but the
combination of two means of control can produce a larger range of
available output power levels.
[0010] In the preferred generator the control circuitry has a first
output coupled to a radio frequency power device in the output
stage to reduce the radio frequency duty cycle thereof and a second
output coupled to the power supply to effect a reduction in the
average power supply voltage supplied to the output stage, the said
reductions occurring in response to the sensing signal reaching a
respective predetermined threshold value, depending on the mode of
treatment required.
[0011] In the case of the power supply being a switched mode power
supply having output smoothing components, the supply circuit may
be arranged such that the second power reduction control signal has
the effect of disabling the supply circuit, e.g. by gating the
pulsed output. Accordingly, a high-speed control response is
obtained with the supply voltage falling relatively slowly after
the initial step power reduction to enable the radio frequency duty
cycle of the power device to be increased again, thereby allowing
further high-speed power reductions if necessary.
[0012] The technique of directly controlling the radio frequency
output stage can be performed by repeatedly producing, firstly, a
rapid reduction in the cycle-by-cycle conduction period of the
power device from a peak level to a trough level when the
respective output threshold is reached, followed by, secondly, a
progressive increase in the conduction period until the conduction
period again reaches its peak level, the radio frequency output
voltage being monitored during the progressive increase. This rapid
reduction and progressive increase sequence may be repeated until
the peak conduction period level can be reached without the output
voltage exceeding the respective output threshold due to the supply
voltage from the switched mode power supply having fallen
sufficiently since it was disabled. Re-enabling of the supply
circuit typically occurs after a delay, and conveniently at the end
of the first switched mode switching cycle in which the output
voltage has not reached the threshold for the whole of the
switching cycle. It will be appreciated that, during the blended
mode of operation, the repeated reduction and restoration of the
cycle-by-cycle conduction period of the power device typically
occurs many times during each period of tissue coagulation or
vaporisation. In other words, it occurs at a much faster rate than
the rate of alternation between states.
[0013] The output stage preferably includes an output resonant
circuit having a Q which is sufficiently high to remove switching
noise from the switching device or devices of the stage without
unduly slowing the response to the output voltage reaching the
predetermined threshold. Typically, the Q is sufficient to achieve
a crest factor below 1.5, the crest factor being the ratio of the
peak and r.m.s. values of the output voltage waveform.
[0014] The generator may have an output impedance in the range of
from 100 ohms to 250 ohms, and preferably between 130 and 190 ohms.
Such a generator has its radio frequency output stage operable to
produce a CW (continuous wave) output, i.e. with a 100% duty cycle
or without on/off pulse width modulation at a frequency lower than
the r.f. oscillation frequency. In effect, the output stage may
operate as an open loop stage with a power/load impedance
characteristic having a peak (preferably a single peak) at about
150 to 160 ohms and with the curve decreasing continuously with
decreasing impedance below the peak and increasing impedance above
the peak.
[0015] Another view of the preferred generator is that of apparatus
for supplying radio frequency power to an electrosurgical
instrument for operation in an electrically conductive fluid
medium, the generator comprising a radio frequency output stage
having a radio frequency power device and at least a pair of
electrosurgical output connections for the delivery of radio
frequency power to electrodes, power supply means coupled to the
output stage, and control circuitry including sensing means for
deriving a sensing signal representative of the radio frequency
output voltage developed across the output connections, and means
responsive to the sensing signal for causing a reduction in
delivered output power when the sensing signal is indicative of a
predetermined output voltage threshold having been reached, wherein
the control circuitry is arranged such that the reduction in output
power is effected by reducing the period of conduction of the
device during individual cycles of radio frequency oscillation,
preferably independently of the supply voltage to the device.
[0016] The generator has at least a pair of electrosurgical output
connections for the delivery of radio frequency power to the
instrument, means coupled to the output stage for supplying power
to the output stage, and control circuitry including sensing means
for deriving a sensing signal representative of the radio frequency
output voltage developed across the output connections and means
responsive to the sensing signal for causing at least a 50%
reduction in delivered output power when the sensing signal is
indicative of a predetermined output voltage threshold having been
reached, the said reduction being effected within a period of 20
.mu.s or less.
[0017] The invention also includes an electrosurgical system
comprising an electrosurgical generator for generating radio
frequency power and an electrosurgical instrument coupled to the
generator, the instrument having an electrode structure for
operation immersed in an electrically conductive liquid, wherein
the system has a first mode of operation in which tissue is treated
by the application of heat in the region of the electrode structure
without forming an electrode-enveloping vapour pocket, and a second
mode of operation in which the tissue is locally vaporised by
energy transmitted from the electrode structure via an
electrode-enveloping vapour pocket, said first and second modes
being defined by different respective electrical control parameters
selected in the generator, and wherein the system has a third,
blended mode of operation produced by constantly alternating
between the first and second modes. The electrode structure may
include a distal treatment electrode and a liquid contact electrode
spaced proximally from the distal electrode, both electrodes being
for use surrounded by the conductive liquid and each being
connected to a respective one of the pair of output connections the
control stage being operable to reduce the reduction time of the
power device when the conductive liquid at the distal electrode is
vaporised. The electrosurgical instrument may provide an electrode
structure having juxtaposed first and second electrodes for
immersion in the conductive liquid, the first and second electrodes
respectively forming a tissue treatment electrode at an extreme
distal end of the instrument and a return electrode proximally
spaced from the tissue contact electrode.
[0018] The preferred system is operable in a tissue desiccation
mode, a tissue cutting or vaponsation mode and a blended mode, and
comprises a generator for generating radio frequency power and an
electrosurgical instrument coupled to the generator, the instrument
having an electrode structure for operation immersed in a
conductive liquid, wherein the generator includes a mode selection
control and has power control circuitry for automatically adjusting
the radio frequency power suppled to the electrode structure to
limit the peak generator output voltage to a first value when the
desiccation mode is selected and to at least one second value when
the cutting or vaporisation mode is selected, the second value or
values being higher than the first value, and to continually
alternated first and second values when the blended mode is
selected. The first and second values are advantageously in the
ranges of from 150V to 200V, and from 250V to 600V respectively,
these voltages being peak voltages.
[0019] From a method aspect, the invention provides an
electrosurgical system comprising an electrosurgical radio
frequency generator coupled to an electrode assembly having a
treatment electrode; introducing the electrode assembly into a
selected operation site with the treatment electrode contacting the
tissue to be treated and with the tissue and the treatment
electrode immersed in a conductive liquid; actuating the generator;
and controlling the radio frequency power applied to the treatment
electrode by the generator so as constantly to alternate between
(a) a first treatment state in which the tissue to be treated is
heated with the liquid adjacent the electrode maintained
substantially at its boiling point without creating a vapour layer
surrounding the electrode, and (b) a second treatment state in
which said tissue is vaporised via a layer of vapour from the
conductive liquid which is maintained around the electrode without
overheating of the electrode, thereby to treat the tissue in a
blended mode of operation in which the tissue is vaporised and
neighbouring tissue is coagulated. The radio frequency power supply
to the electrode may be automatically adjusted by alternately
limiting the output voltage to predetermined first and second
voltage values, the first voltage value being used for desiccation
and the second voltage value, which is higher than the first
voltage value, being used for cutting or vaporisation to yield the
required blended effect.
[0020] Alternatively, the output voltage may be modulated to
produce a pulsed waveform in which bursts of radio frequency power
are separated by periods of zero voltage output. Constant
alternation between tissue desiccation and tissue vaporisation
occurs, desiccation being obtained at the beginning of each burst
prior to formation of a vapour pocket around the treatment
electrode, and afterwards due to residual heat in the treatment
electrode. Each time the vapour pocket forms, tissue vaporisation
occurs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The invention is illustrated by way of example in the
drawings in which:
[0022] FIG. 1 is a diagram showing an electrosurgical system in
accordance with the invention;
[0023] FIG. 2 is a fragmentary view of a first electrode assembly
for tissue desiccation, shown in use and immersed in a conductive
liquid;
[0024] FIG. 3 is a load characteristic graph illustrating the
variation in load impedance produced by an electrode assembly such
as that shown in FIG. 2 when used in a conductive liquid, according
to the delivered output power;
[0025] FIG. 4 is a fragmentary view of a second electrode assembly
for tissue vaporisation, shown in use immersed in a liquid;
[0026] FIG. 5 is a fragmentary view of a third electrode assembly
for tissue cutting, or for combined tissue cutting and desiccation
in a blended mode of operation;
[0027] FIG. 6 is a block diagram of a generator in accordance with
the invention;
[0028] FIG. 7 is a block diagram of part of the control circuity of
the generator of FIG. 6;
[0029] FIG. 8 is a waveform diagram showing a typical RF output
voltage variation pattern obtained with the generator of FIGS. 6 to
8, the voltage being shown varying with time according to
variations in load impedance and generator output stage supply
voltage;
[0030] FIG. 9 is a circuit diagram of part of the generator of
FIGS. 6 and 7;
[0031] FIG. 10 is a graph showing the variation of output power
produced by the generator as a function of the load impedance
presented to it by the electrode assembly, the output power
variation being shown in two operation modes of the generator;
and
[0032] FIG. 11 is a graph showing the variation of output power for
generator as a function of load impedance after modification of the
generator characteristics in response to output voltage
sensing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0033] Historically, underwater electrosurgery has been the most
demanding electrosurgical application in terms of instrument
engineering. The reason for this is that the electrosurgical power
requirement is very high, specifically because it is necessary to
create arcs for cutting and tissue disruption in circumstances in
which power is dissipated quickly by the surrounding liquid.
Consequently, high currents are used to ensure vaporisation of
liquid surrounding the electrode. Power levels up to 300 watts are
commonly used. Conventionally, underwater electrosurgery is
performed using a non-conductive fluid or irrigant to eliminate
electrical conduction losses. Glycine, which is commonly used, has
the disadvantage that in the course of an operation, veins may
become severed and irrigant may be infused into the circulation.
This absorption causes among other things a dilution of serum
sodium which can lead to a condition known as water
intoxication.
[0034] Accordingly, the applicants propose use of a conductive
liquid medium such as normal saline, electrosurgery being performed
with using a system comprising a generator and an instrurnent, the
instrument having a dual-electrode structure with the saline acting
as a conductor between the tissue being treated and one of the
electrodes, hereinafter called the "return electrode". The other
electrode is located adjacent to or applied directly to the tissue.
This other electrode is hereinafter called the "active
electrode".
[0035] Such a system is shown in FIG. 1. The generator 10 has an
output socket 10S providing a radio frequency (RF) output for an
instrument in the form of a handpiece 12 via a connection cord 14.
Activation of the generator may be performed from the handpiece 12
via a control connection in cord 14 or by means of a footswitch
unit 16, as shown, connected separately to the rear of the
generator 10 by a footswitch connection cord 18. In the illustrated
embodiment, footswitch unit 16 has two footswitches 16A and 16B for
selecting a desiccation mode and a vaporisation mode of the
generator respectively. The generator front panel has push buttons
20 and 22 for respectively setting desiccation and vaporisation
power levels, which are indicated in a display 24. Push buttons 26
are provided as an alternative means for selection between
desiccation and vaporisation modes, and as a means of setting a
blended mode.
[0036] Handpiece 12 mounts a detachable electrode assembly 28
having a dual electrode structure, as shown in the fragmentary view
of FIG. 2.
[0037] FIG. 2 is an enlarged view of the distal end of electrode
assembly 28. At its extreme distal end the assembly has an active
electrode 30 which, in this embodiment, is formed as a series of
metal filaments connected to a central conductor 32. The filaments
may be made of stainless steel. Proximally of the active electrode
30 and spaced from the latter by a longitudinally and radially
extending insulator 34 is a return electrode 36. The return
electrode 36 is arranged coaxially around the inner conductor 32 as
a sleeve 38 which extends as a tubular shaft 40 to the proximal end
of the assembly 28 where it is connected in the handpiece 12 to
conductors in the connection cord 14. Similarly, the inner
conductor 32 extends to the handpiece and is connected to a
conductor in cord 14. The electrode assembly 28 has an insulating
sheath 42 which covers shaft 40 and terminates proximally of the
insulator 34 to leave the distal end of shaft 40 exposed as the
return electrode 36.
[0038] In operation as a desiccation instrument, the electrode
assembly 28 is applied as shown in FIG. 2 to the tissue 44 to be
treated, the operation site being immersed in a normal saline (0.9%
w/v) solution, here shown as a drop 46 of liquid surrounding the
distal end portion of the electrode assembly 28. The liquid
immerses both the active electrode 30 and the return electrode
36.
[0039] Still referring again to FIG. 2, the metallic filaments
forming the active electrode 30 are all electrically connected
together and to the inner conductor 32 of the electrode assembly to
form a unitary active electrode. Insulator 34 is an insulating
sleeve, the distal end portion of which is exposed proximally of
the exposed part of the active electrode 30. Typically, this sleeve
is made of a ceramic material to resist damage from arcing. The
return electrode terminates at a point short of the end of the
insulator 36 so that it is both radially and axially spaced from
the active, or tissue contacts electrode 30. The surface area of
the return electrode is considerably greater than that of the
active electrode 30. At the distal end of the electrode assembly,
the diameter of the return electrode is typically in the region of
from 1 mm to 3 mm, with the longitudinal extent of the exposed part
of the return electrode being typically between 1 mm and 5 mm with
the longitudinal spacing from the active electrode being between 1
mm and 5 mm.
[0040] In effect, the electrode assembly is bipolar, with only one
of the electrodes (30) actually extending to the distal end of the
unit. This means that the return electrode, in normal
circumstances, remains spaced from the tissue being treated and a
current path exists between the two electrodes via the tissue and
the conductive liquid which is in contact with the return electrode
36.
[0041] The conductive liquid 46 may be regarded, as far as the
delivery of bipolar electrosurgical energy is concerned, as a low
impedance extension of the tissue. Radio frequency currents
produced by the generator 10 flow between the active electrode 30
and the return electrode 36 via the tissue 44 and the immersing
conductive liquid 46. The particular electrode arrangement shown in
FIG. 2 is most suitable for tissue desiccation.
[0042] The axial as well as radial separation between the
electrodes avoids the small spacing of the conventional bipolar
arrangement in which both electrodes are tissue-contacting. As a
result, there is less danger of unwanted arcing across the
insulation surface, which allows comparatively high power
dissipation for desiccation treatment, and, in the case of tissue
cutting or vaporisation, prevents excessive arcing which can lead
to inter-electrode insulation damage.
[0043] The immersing saline solution may be provided from a conduit
(not shown) forming part of the instrument 12. Thus, the invention
may take the form of an electrosurgical system for the treatment of
tissue immersed in a conductive fluid medium, comprising an
electrosurgical instrument having a handpiece and an instrument
shaft, and, on the end of the shaft, an electrode assembly, the
assembly comprising a tissue treatment electrode which is exposed
at the extreme distal end of the instrument, and a return electrode
which is electrically insulated from the tissue treatment electrode
and has a fluid contact surface spaced proximally from the exposed
part of the tissue treatment electrode, the system further
comprising a radio frequency generator coupled to the electrode
assembly of the instrument, a reservoir of electrically conductive
fluid, such as the normal saline solution, and a conduit, typically
and integral part of an endoscope, for delivering the liquid from
the reservoir to the region of the electrode assembly. Pressure for
delivering the liquid may be provided by a pump forming part of the
apparatus.
[0044] Since in this embodiment of electrode assembly 28, the
active electrode 30 is made of stainless steel filaments in the
form of a brush, the electrode is flexible, providing a
reproducible tissue effect which is comparatively independent of
the application angle of the electrode to the tissue surface. The
flexibility of the electrode 30 also results in a differential
contact area of the active electrode dependent on the applied
pressure, allowing variations in the breadth of desiccation over
the surface of the tissue, reducing procedure time.
[0045] Desiccation occurs by virtue of radio frequency currents
passing between the active electrode 30 and the conductive liquid
46 via the outer layer of the tissue 44 immediately beneath and in
an area surrounding the active electrode 30. The output impedance
of the generator is set at a level commensurate with the load
impedance of the electrode assembly when used as shown in FIG. 2
with both electrodes in contact with the conductive liquid 46. In
order to sustain this matched state for tissue desiccation, the
output power of the generator is automatically controlled in a
manner which will be described below so that vapour bubbles of
significant size are substantially prevented from appearing at the
active electrode 30, thereby avoiding a consequent increase in load
impedance. In this way, the active electrode can be continually
wetted by the conductive liquid so that, whilst the tissue water is
removed by thermal desiccation, the impedance reaches an upper
limit corresponding to the point at which the conductive liquid
starts to boil. As a result, the system is able to deliver high
power levels for desiccation without unwanted conductive liquid
vaporisation leading to unwanted tissue effects.
[0046] The electrical behaviour of the electrode assembly when the
electrodes 30 and 36 are immersed in the conductive liquid 46 is
now considered with reference to the graph of FIG. 3.
[0047] When power is first applied, there is presented to the
generator an initial load impedance r which is governed by the
geometry of the electrode and the electrical conductivity of the
conductive liquid. The value of r changes when the active electrode
touches the tissue. The higher the value of r, the greater is the
propensity of the conductive liquid to vaporise. As power is
dissipated in the tissue and the conductive liquid, the conductive
liquid increaes in temperature. In the case of normal saline, the
temperature coefficient of conductivity is positive and the
corresponding impedance coefficient is therefore negative so that
the impedance initially falls. Thus, the curve in FIG. 3 indicates
a fall in load impedance as the delivered power is increased, the
impedance falling through point A to a minimum at point B, at which
point saline in immediate contact with the electrode reaches
boiling point. Small vapour bubbles now form on the surface of the
active electrode and the impedance starts to rise as shown by the
curve rising from point B to point C. Thus, once the boiling point
has been reached, the arrangement displays a dominant positive
power coefficient of impedance so that small increases in power now
bring about large increases in impedance.
[0048] As the vapour bubbles form, there is an increase in the
power density at the remaining active electrode to saline interface
(the exposed area of the active electrode not covered by vapour
bubbles) which further stresses the interface, producing more
vapour bubbles and thus even higher power density. This is a
runaway condition, with an equilibrium point only occurring once
the electrode is completely enveloped in vapour. The only means of
preventing the runaway condition is to limit applied voltage,
thereby preventing power dissipation into higher impedance loads.
Thus, for a given set of variables, there is a power threshold
corresponding to point C at which this new equilibrium is
reached.
[0049] In the light of the foregoing, it will be appreciated that
the region between points B and C in FIG. 3 represents the upper
limit of desiccation power which can be achieved. The transition
from point "C" in the vaporise equilibrium state will follow the
power impedance curve for the RF stage of the generator (shown as a
dotted line in FIG. 3).
[0050] Upon formation of an electrode-enveloping vapour pocket, the
impedance elevates to about 1 k.OMEGA., as shown by point D in FIG.
3, the actual impedance value depending on a number of system
variables. The vapour is then sustained by discharges across the
pocket between the active electrode and the vapour/saline
interface.
[0051] This state of affairs is illustrated by the diagram of FIG.
4 which shows an alternative electrode assembly 28A having a
hemispherical or ball electrode 30A in place of the brush electrode
30 of the embodiment of FIG. 2. As before, the return electrode 36A
is proximally spaced from the active electrode 30A by an
intervening insulator 34A. The ball electrode is preferred for
tissue vaporisation.
[0052] Once in the vaporisation equilibrium state, the vapour
pocket, shown by the reference 50 in FIG. 4, is sustained by
discharges 52 across the vapour pocket between the active electrode
30A and the vapour to saline interface. The majority of power
dissipation occurs within this pocket with consequent heating of
the active electrode. The amount of energy dissipation in this
conduction is a function of the delivered power. It will be noted
from FIG. 3 that the vaporisation mode, indicated by the dotted
boundary lines, can be sustained at much lower power levels than
are required to bring about formation of the vapour pocket. The
impedance/power characteristic consequently displays hysteresis.
Once the vaporisation mode has been established, it can be
maintained over a comparatively wide range of power levels, as
shown by the inclined part of the characteristic extending on both
sides of point D. However, increasing the delivered output power
beyond that represented by point D causes a rapid rise in electrode
temperature, potentially damaging the electrode. It should be noted
that, if power were delivered at the same level as point "C", the
resulting voltages would cause electrode destruction. The normal
operating point for an electrode used for vaporisation is
illustrated by the point "D". This point is defined uniquely by
combination of the impedance power characteristic for the electrode
in conjunction with the vaporise voltage limit. To collapse the
vapour pocket and to return to desiccation mode requires a
significant power reduction back to point A, direct contact between
the active electrode and the saline being reestablished and the
impedance falling dramatically. The power density at the active
electrode also falls so that the temperature of the saline now
falls below boiling point and the electrode is then once again in a
stable desiccation equilibrium.
[0053] In a third, blended mode of operation, the vapour pocket
continually and successively forms, collapses and reforms as the
generator is controlled so as to alternate between the electrical
conditions required for coagulation or desiccation and those
required for tissue cutting or vaporisation. The blended mode is
particularly useful for procedures involving vascular tissue
removal, the combined tissue cutting and desiccation effect largely
preventing blood loss.
[0054] Referring to FIG. 5, an electrode assembly which may
typically be used in the blended mode has an exposed treatment
portion 30B in the form of a rigid needle, bent to form a hook. As
in the other electrode assemblies described above, the fluid
contact portion of the return electrode 36B is set back from the
active electrode in the direction of a treatment axis 52. One
dimensional characteristic of this electrode is that the ratio of
(i) the length of the shortest conductive path between the return
electrode 36B and the furthermost point of the active electrode
treatment portion 30B (shown as "b" in FIG. 5) and (ii) the length
of the shortest conductive path between the return electrode 36B
and the treatment portion 30B (shown as "a" plus "c" in FIG. 5), is
in the range of from 1.25:1 and 2:1. The applicants have found that
optimum values for this ratio tend to be 1.5:1 or 1.6:1
upwards.
[0055] In use, the hooked end of the treatment portion 30B is
applied to the surface of tissue to be severed and dragged along to
produce a cut line. As this movement is executed, tissue is
vaporised during the period when a vapour pocket is present over
the active electrode, and surrounding tissue is then desiccated
when the pocket collapses, thereby reducing bleeding.
[0056] The generator to be described hereinafter has the ability to
sustain both the desiccation mode and the vaporisation mode, and to
oscillate between the two in the blended mode. Whilst in general
the electrode assemblies illustrated in FIGS. 2 and 4 can be used
in either mode, the brush electrode of FIG. 2 is preferred for
desiccation due to its wide potential area of coverage, and the
ball electrode of FIG. 4 is preferred for vaporisation due to its
small active electrode/return electrode surface area ratio.
Although the blended mode can be used with either of these
electrode assemblies, that of FIG. 5 has particular advantages as
will be clear from the preceding description. As can be seen from
FIG. 4, during the cut or vaporisation mode, tissue vaporisation
occurs when the vapour pocket 50 intersects the tissue surface,
with the electrode assembly preferably being held spaced above the
tissue surface by a small distance (typically 1 mm to 5 mm).
[0057] The runaway condition which occurs when the delivered power
reaches the level shown by point C in FIG. 3 is exacerbated if the
generator has a significant output impedance, because the output
voltage can then suddenly rise. With increased power dissipation
and without the presence of the cooling liquid around the active
electrode 30, the electrode temperature rises rapidly with
consequent damage to the electrode. This also produces
uncontrollable tissue disruption in place of the required
desiccation. For this reason, the preferred generator has an output
source impedance which, approximately at least matches the load
impedance of the electrode structure when wetted.
[0058] The preferred generator now to be described allows both
desiccation electrosurgery substantially without unwanted cell
disruption, and electrosurgical cutting or vaporisation
substantially without electrode burning. Although intended
primarily for operation in a conductive liquid distension medium,
it has application in other electrosurgical procedures, e.g. in the
presence of a gaseous distension medium, or wherever rapid load
impedance changes can occur.
[0059] Referring to FIG. 6, the generator comprises a radio
frequency (RF) power oscillator 60 having a pair of output
connections 60C for coupling via output terminals 62 to the load
impedance 64 represented by the electrode assembly when in use.
Power is supplied to the oscillator 60 by a switched mode power
supply 66.
[0060] In the preferred embodiment, the RF oscillator 60 operates
at about 400 kHz, with any frequency from 300 kHz upwards into the
HF range being feasible. The switched mode power supply typically
operates at a frequency in the range of from 25 to 50 kHz. Coupled
across the output connections 60C is a voltage threshold detector
68 having a first output 68A coupled to the switched mode power
supply 16 and a second output 68B coupled to an "on" time control
circuit 70. A microprocessor controller 72 coupled to the operator
controls and display (shown in FIG. 1), is connected to a control
input 66A of the power supply 66 for adjusting the generator output
power by supply voltage variation and to a threshold-set input 68C
of the voltage threshold detector 68 for setting peak RF output
voltage limits.
[0061] In operation, the microprocessor controller 72 causes power
to be applied to the switched mode power supply 66 when
electrosurgical power is demanded by the surgeon operating an
activation switch arrangement which may be provided on a handpiece
or footswitch (see FIG. 1). A constant or alternating output
voltage threshold is set via input 68C according to control
settings on the front panel of the generator (see FIG. 1).
Typically, for desiccation or coagulation the threshold is set at a
desiccation threshold value between 150 volts and 200 volts. When a
cutting or vaporisation output is required, the threshold is set to
a value in the range of from 250 or 300 volts to 600 volts. These
voltage values are peak values. Their being peak values means that
for desiccation at least it is preferable to have an output RF
waveform of low crest factor to give maximum power before the
voltage is clamped at the values given. Typically a crest factor of
1.5 or less is achieved.
[0062] When a blended output is required, the voltage threshold set
via input 68C is constantly alternated between the value for
desiccation or coagulation and the value for cutting or
vaporisation.
[0063] When the generator is first activated, the status of the
control input 601 of the RF oscillator 60 (which is connected to
the "on" time control circuit 70) is "on", such that the power
switching device which forms the oscillating element of the
oscillator 60 is switched on for a maximum conduction period during
each oscillation cycle. The power delivered to the load 64 depends
partly on the supply voltage applied to the RF oscillator 60 from
the switched mode power supply 66 and partly on the load impedance
64. If the supply voltage is sufficiently high, the temperature of
the liquid medium surrounding the electrodes of the electrosurgical
instrument (or within a gaseous medium, the temperature of liquids
contained within the tissue) may rise to such an extent that the
liquid medium vaporises, leading to a rapid increase in load
impedance and a consequent rapid increase in the applied output
voltage across terminals 12. This is an undesirable state of
affairs if a desiccation output is required. For this reason, the
voltage threshold for a desiccation output is set to cause trigger
signals to be sent to the "on" time control circuit 70 and to the
switched mode power supply 66 when the threshold is reached. The
"on" time control circuit 70 has the effect of virtually
instantaneously reducing the "on" time of the RF oscillator
switching device. Simultaneously, the switched mode power supply is
disabled so that the voltage supplied to oscillator 60 begins to
fall.
[0064] Subsequent control of the "on" time of individual cycles of
the oscillator 60 will be understood by considering the internal
configuration of the "on" time control circuit 20 which is shown in
FIG. 7. The circuit comprises an RF sawtooth generator 74
(synchronised at the RF oscillation frequency by a synchronisation
signal derived from the oscillator and applied to a synchronisation
input 74I), and a ramp generator 76 which is reset by a reset pulse
from the output 68B of the voltage threshold detector 68 (see FIG.
6) produced when the set threshold voltage is reached. This reset
pulse is the trigger signal referred to above. The "on" time
control circuit 70 further comprises a comparator 78 for comparing
the sawtooth and ramp voltages produced by the sawtooth and ramp
generators 74 and 76 to yield a square wave control signal for
application to the input 601 of the RF oscillator 60. As shown by
the waveform diagrams in FIG. 7, the nature of the sawtooth and
ramp waveforms is such that the mark-to-space ratio of the square
wave signal applied to the oscillator 60 progressively increases
after each reset pulse. As a result, after a virtually
instantaneous reduction in "on" time on detection of the output
voltage reaching the set voltage threshold, the "on" time of the RF
oscillator is progressively increased back to the original maximum
value. This cycle is repeated until the supply voltage for the
oscillator from power supply 66 (FIG. 6) has reduced to a level at
which the oscillator can operate with the maximum conduction period
without the output voltage breaching the set voltage threshold as
sensed by the detector 68.
[0065] The output voltage of the generator is important to the mode
of operation. In fact, the output modes are defined purely by
output voltage, specifically the peak output voltage. The absolute
measure of output voltage is only necessary for multiple term
control. However, a simple term control (i.e. using one control
variable) can be used in this generator in order to confine the
output voltage to predetermined limit voltages. Thus, the voltage
threshold detector 68 shown in FIG. 6 compares the RF peak output
voltage with a preset DC threshold level, and has a sufficiently
fast response time to produce a reset pulse for the "on" time
control circuit 70 within one RF half cycle.
[0066] Before considering the operation of the generator further,
it is appropriate to refer back to the impedance/power
characteristic of FIG. 3. It will be appreciated that the most
critical control threshold is that applicable during desiccation.
Since vapour bubbles forming at the active electrode are
non-conducting, the saline remaining in contact with the electrode
has a higher power density and consequently an even greater
propensity to form vapour. This degree of instability brings about
a transition to a vaporisation mode with the same power level due
to the runaway increase in power density at the active electrode.
As a result, the impedance local to the active electrode rises.
Maximum absorbed power coincides with the electrode condition
existing immediately before formation of vapour bubbles, since this
coincides with maximum power distribution and the greatest wetted
electrode area. It is therefore desirable that the electrode
remains in its wetted state for the maximum desiccation power. Use
of voltage limit detection brings about a power reduction which
allows the vapour bubbles to collapse which in turn increases the
ability of the active electrode to absorb power. For this reason,
the generator described in this specification includes a control
loop having a large overshoot, in that the feedback stimulus of the
peak voltage reaching the predefined threshold causes a large
instantaneous reduction in power. This control overshoot ensures a
return to the required wetted state.
[0067] In the generator described above with reference to FIGS. 6
and 7, power reduction in response to voltage threshold detection
takes place in two ways:
[0068] (a) an instantaneous reduction in RF energy supplied to the
resonant output circuit of the oscillator, and
[0069] (b) a shut down of DC power to the oscillator for one or
more complete cycles of the switched mode power supply (i.e.
typically for a minimum period of 20 to 40 .mu.s).
[0070] In the preferred embodiment, the instantaneous power
reduction is by at least three quarters of available power (or at
least half voltage) from the DC power supply, but continuous
voltage threshold feedback continually causes a reduction in
delivered power from the DC power supply. Thus, a high speed
response is obtained in the RF stage itself, with the DC supply
voltage tracking the reduction to enable the RF stage to return to
a full duty cycle or mark-to-space ratio, thereby enabling further
rapid power reductions when the voltage threshold is again
breached.
[0071] The effect of this process on the RF output voltage is shown
in the waveform diagram of FIG. 8, containing traces representative
of the output voltage, the oscillator supply voltage, and the load
impedance during a typical desiccation episode over a Ims
period.
[0072] Starting on the lefthand side of the diagram with the supply
voltage approximately constant, the output voltage increases with
increasing load impedance to a point at which the output voltage
threshold is reached, whereupon the above-described instantaneous
reduction in oscillator "on" time occurs. This produces a rapid
decrease in the RF output voltage, as shown, followed by a
progressive increase, again as described above. When the output
voltage reaches the threshold voltage, the voltage threshold
detector 68 (shown in FIG. 6) also disables the power supply,
leading to a gradual decrease in the supply voltage. As a result.
when the "on" time of the oscillator device has once again reached
its maximum value, illustrated by point a in FIG. 8, the threshold
voltage has not been reached. However, the load impedance begins
rising again, causing a further, albeit slower, increase in the
output voltage until, once again, the threshold voltage is reached
(point b). Once more, the "on" time of the oscillator is instantly
reduced and then progressively increased, so that the output
voltage waveform repeats its previous pattern. Yet again, the
threshold voltage is reached, again the output voltage is instantly
reduced (at point c), and again the "on" time is allowed to
increase. On this occasion, however, due to the supply voltage
having further reduced (the power supply still being disabled), the
output voltage does not reach the threshold level (at point d)
until a considerably longer time period has elapsed. Indeed, the
length of the period is such that the output voltage has failed to
reach the threshold voltage over a complete switching cycle of the
power supply, so that it has in the meantime been enabled (at point
e).
[0073] During this period the power supplied to the electrode has
been sufficient to further increase the load impedance. The erratic
impedance behaviour is typical of the commencement of vapour
formation. Consequently, when the threshold voltage is next reached
(at point e), several successive cycles of "on" time reduction and
increase occurring one after the other are required (see f)
combined with a further disabling (see g) of the power supply in
order to maintain the voltage below the threshold.
[0074] It will be seen, then, that the control circuitry 70, 72
(FIG. 6) operates dynamically to control the output voltage both
sufficiently rapidly and to a sufficient degree to maintain the
voltage at a level consistent with, in this case, the level
required for desiccation without tissue disruption due to arcing.
The same technique can be used with a different threshold voltage
to limit the output voltage to prevent electrode burning and/or
excessive tissue vaporisation. In the latter case, the voltage
limit may be set to a level between 250 volts (preferably 300
volts) and 600 volts.
[0075] Due to the high power density at the active electrode during
the vaporisation mode, the great majority of delivered power is
dissipated in the proximity of the electrode. In the vaporisation
mode, it is desirable that a minimum of saline heating occurs, but
that any tissue which encroaches the vapour boundary of the active
electrode is vaporised. In the vaporisation mode, the vapour is
sustained by arcs within the vapour pocket as described above with
reference to FIG. 4. Increasing the output voltage during
vaporisation results in increased volume of tissue removal due to
the increased size of the vapour pocket. Collapse of the vapour
pocket during tissue vaporisation has greater consequence, due to
the increased necrosis as a result of the greater power dissipation
in the surrounding saline. Vapour pocket collapse can be prevented
by, firstly, arranging for the electrode impedance in vaporisation
mode to be such that the instrument is in an unmatched condition as
regards impedance, with result that the resonant output circuit Q
is high and the output voltage does not change so rapidly as with
lower load impedances and, secondly, the active electrode has a
significant heat capacity that sustains the vapour pocket for a
significant period.
[0076] An unwanted increased in the size of the vapour pocket can
be prevented by limiting the peak output voltage during the
vaporisation mode, which may be conveniently carried out by
substituting a different threshold value for the voltage threshold
detector 68 (see FIG. 6) when in the vaporisation mode.
[0077] The circuitry of the RF oscillator 60, voltage threshold
detector 68. and "on" time control circuit 70 (shown in FIG. 6) in
the preferred generator in accordance with the invention is shown
in FIG. 8.
[0078] Referring now to FIG. 9, the RF oscillator comprises a IGBT
(insulated gate bipolar transistor) 80 acting as an RF switching
device which pumps energy into a parallel resonant circuit
comprising the primary winding 82P of transformer 82 and a
parallel-connected resonating capacitor 84. RF power is supplied
from the transformer secondary winding 82S via isolating capacitors
86, 88 to RF output terminals 62. Power for the oscillator
transistor 80 is supplied on a high voltage supply line 90 which is
connected to the output of the switched mode power supply 66 (shown
in FIG. 6). Supply line 90 is decoupled by capacitor 92.
[0079] The oscillator feedback loop runs from the resonant primary
winding 82P (on the opposite side of the winding from the supply
line 90) via a phase shift network comprising capacitor 94,
resistor 96, and clamping diodes 98, 100, and via a field effect
transistor (FET) 104, the voltage controlled monostable represented
by comparator 78 and associated components, and the driver 108,
which is connected to the gate of transistor 80.
[0080] The voltage on that side of the primary winding 82P which is
coupled to transistor 80 is substantially sinusoidal and alternates
at a frequency defined by the parallel resonant combination of the
winding inductance and capacitor 84. Typically the voltage swing is
greater than twice the supply voltage on supply line 90, falling
below ground voltage in negative half-cycles.
[0081] The phase-shift network 94, 96, 98, 100 provides a
positive-going square wave which is 90.degree. phase-advanced with
respect to the primary voltage. Thus, FET 104 is turned on
approximately when the voltage on primary winding 82P has just
reached its minimum value, and off when it has just reached its
maximum value. When FET 104 is turned on a timing capacitor is
rapidly discharged and the output of comparator 78 is turned off.
The driver 108 is non-inverting and consequently transistor 80 is
also turned off at this point. It follows that the transistor "off"
point is repeatable and has a constant phase relationship with
respect to the primary voltage by virtue of the feedback path
described above. The logic of the feedback path is also such that
the feedback signal fed to the gate connection of transistor 80 has
a logic level of "1" when the primary voltage is decreasing (and
the potential difference across the primary winding 82P is
increasing). The "off" point occurs substantially at a primary
voltage peak, i.e. when the primary voltage is at its minimum value
in the present case.
[0082] Unlike the "off" point, the "on" point of transistor 80 is
variable as will now be described. The instant at which the logic
level at the output of comparator 78 and on the base of device 80
changes to "1" depends on the reference voltage applied to the
inverting input 78I of comparator 78. As a result, the delay
between device 80 switching off and switching on is determined by
this comparison of voltage applied to input 78I of comparator 78.
In other words, an "on" signal to device 80 is delayed with respect
to switching off by a period which is in accordance with the
reference voltage on the inverting input. This reference voltage is
dependent on the voltage appearing across resistor 112 which is
part of a potential divider consisting also of resistor 114 and
potentiometer 116. Potentiometer 116 sets the minimum switching on
delay, corresponding to the maximum duty cycle of transistor 80.
The voltage appearing across resistor 112 is variable and
represents the control range of "on" time adjustment between 25% of
the maximum duty cycle and 100%. Timing capacitor 110 is charged by
variable resistor 118 (preset for an appropriate time constant)
from a low voltage supply line 120.
[0083] Comparing FIG. 9 with FIG. 7, it will be appreciated that
the voltage on the non-inverting input 78N of comparator 78 has a
sawtooth waveform as shown in FIG. 7, the waveform being produced
by the repeated triggering of FET 104 and discharging of capacitor
110, each discharging being followed by charging of a capacitor
through resistor 118.
[0084] The voltage across resistor 112 is normally at a minimum
value, and is increased when the RF output voltage from the
generator reaches a predetermined peak threshold value. The
circuitry which brings about this effect will now be described.
[0085] Output voltage detection is provided by the capacitive
divider chain 122, 124 connected across the RF output, the tap
between the capacitors feeding the primary winding of an isolating
transformer 126. Resistors 128 and 130 connected across the primary
and secondary windings of transformer 126 respectively provide
damping to avoid unwanted resonances and to filter high frequency
components which may occur during arcing at the active electrode.
The resulting sensing voltage appearing at the secondary winding of
transformer 126 is then fed to two comparators 132 and 134. At this
point, it should be appreciated that only the positive-going half
cycles of the sense voltage are used for peak output voltage
threshold detection.
[0086] Each comparator 132, 134 has two inputs, one connected to
the transformer 126 to receive the sense voltage, and one connected
to a respective reference voltage input 136, 138 (labelled CLAMP
and BOOST in FIG. 9). Reference voltages applied to these inputs
136, 138 are computer generated set voltage thresholds for the
desiccate and vaporisation modes respectively. Selection of the
operating mode is brought about by a control signal (MODE) applied
to control inputs 140, and the logic chain comprising decoder 141,
and gates 142, 144, 146, and 148. Mode selection signals applied at
inputs 140 cause operation of the decoder 141 such that in the
desiccation mode a logic level "1" is set on the input to gate 142.
In vaporisation mode, logic level "0" is set on this input,
effectively disabling the output of comparator 132 via NOR gate
144, the output threshold detection then being fed through NOR gate
146. It will therefore be appreciated that the CLAMP voltage
applied to input 136 is the reference voltage setting the threshold
value for the peak output voltage during desiccation, while the
BOOST voltage applied to input 138 sets the threshold value of the
peak output voltage in the vaporisation mode. When a blended output
is signalled, the decoder 141 produces a square wave output,
causing togging between logic levels "1" and "0" at a rate which is
appropriate for the connected electrode assembly, which may be
between 5 Hz and 2 kHz, and more typically between 25 Hz and 1 kHz.
The actual rate of alternation is determined by the rates at which
the vapour pocket forms and collapses for any given electrode. A
larger electrode assembly tends to require an alternation rate
towards the lower end of the 25 Hz to 1 kHz range, while a small
electrode assembly may be able to operate effectively in a blended
mode at 1 kHz. When the electrode assembly is operating properly in
blended mode the repeated formation and collapse of the vapour
pocket is audible as a buzz or whine, depending on the frequency of
alternation.
[0087] When the output voltage reaches the set threshold value
(i.e. a "limit" voltage), transistor 150 is switched on. This
transistor is capable of charging capacitor 152 from 1.5V to 4V in
a period of 50 ns. The base charge of transistor 150 is sufficient
to enlarge very narrow pulses from the voltage detection circuitry
and therefore ensures that capacitor 152 attains maximum voltage
for only marginly detected limit voltages at the RF output. The
function of capacitor 152 is to provide progressively lower
reference voltages for comparator 78 after a limit voltage
detection. Thus, the voltage on the emitter of transistor 150 has a
waveform as shown at the output of the ramp generator 76 in FIG. 7.
In this way, the turn-on instant of device 80 is instantly retarded
when the RF output voltage reaches the preset threshold value, and
is subsequently progressively advanced as the voltage across
resistor 112 slowly decreases. The discharge rate of capacitor 152
is determined by the parallel combination of resistor 112 in
parallel with resistor 114 plus resistor 116.
[0088] Switching energy provided by transistor 80 is converted by a
series inductor 154P into a current drive into the resonant primary
winding 82P. The action of series inductor 154P smoothes energy
injection into the resonant output circuit represented by winding
82P and capacitor 84 and prevents excessive initial current through
transistor 80, and excessive swinging of the voltage input to
winding 82P above the voltage on supply line 90.
[0089] Under full power conditions, the initial switch-on of
transistor 80 occurs at an initial resonant voltage maximum across
the resonant circuit. This creates a switch-on current zero as the
inductor 154P is completely depleted of energy after each cycle.
Current in this inductor rapidly builds up until a point is reached
at which the voltage on winding 82P becomes negative. The inductor
154P then releases its energy into this reverse bias. The current
zero at switch-off is then guaranteed by a blocking diode 156 which
prevents the return of energy from the resonant circuit to the
inductor 154P.
[0090] When the switch-on time of transistor 80 is reduced due to
the output voltage reaching the predetermined set threshold, the
primary voltage amplitude across winding 82P decreases to the
extent that the primary peak amplitude is less than the supply
voltage. In particular, the voltage minimum at the end of primary
winding 82P coupled to transistor 80 no longer swings beyond the
ground voltage. Energy can now no longer be released from inductor
154P back into the resonant circuit. A secondary path for stored
energy in inductor 154P is provided by the fact that this inductor
is the primary winding of a transformer 154 which has a second
winding 154S coupled via a diode 158 to the supply line 90.
Residual energy stored in inductor 154P at switch-off causes
forward biasing of diode 158 through which the energy is recovered
back into the supply. This recovery mechanism permits partial
resonant primary amplitude levels without damaging switching
transistor 80 by uncoupled energy creating excessive voltage.
[0091] The relationship between "on" time of transistor 80 and
switching energy depends on a number of variables such as the
initial energy storage of the resonant circuit 82P, 84, the loading
on the output terminals 62 (which affects the Q of the resonant
circuit), and the loading as it affects oscillation frequency,
which all affect the non-linear energy storing rate of inductor
154P.
[0092] As has been described above, detection of the output voltage
reaching a predetermined threshold value not only causes the duty
cycle of the switching transistor 80 to be instantly reduced, but
it also disables the switched mode power supply 66 (shown in FIG.
6). This disabling effect is produced by feeding a signal from the
output of the logic chain 142 to 148 via a filter 160 to remove RF
transients to a DISABLE output 68A, which is connected to the
switched mode power supply 66.
[0093] The generator output impedance is set to about 160 ohms. The
effect of this choice will be evident from the following
description with reference to FIGS. 10 and 11 which are graphs
showing the variation of output power which can be produced by the
generator into different load impedances.
[0094] Referring to FIG. 10, the power delivered to the load is
here shown as a function of load impedance for two different
oscillator supply voltage settings. In both cases, it will be seen
that, to the left of the power/impedance peak, an increase in load
impedance leads to an increase in output power and, hence, an
increase in output voltage. At higher impedances, to the right of
the peaks, the voltage continues to increase, albeit less
aggressively, as impedance increases.
[0095] One of the features of the preferred generator in accordance
with the invention is that the output stage operates as an open
loop oscillator with an output impedance (corresponding to the
peaks in FIG. 10) of about 160 ohms. This is considerably lower
than the output impedance of conventional generators used for
underwater electrosurgery, and contributes to the ability to
prevent runaway arcing behaviour and consequent excessive tissue
damage and electrode burn-out.
[0096] It should be understood that for desiccation, steam envelope
generation at the electrode and arcing should be prevented.
Conversely, for cutting or vaporisation, steam envelope generation
and arcing are required, but to a level consistent with achieving
the required tissue effect and the avoidance of electrode burn-out.
Operating points for low and high power desiccation and cutting or
vaporisation are shown in FIG. 10.
[0097] A feature of the combination of the generator in accordance
with the invention and an electrode assembly having two adjacent
electrodes is that the output is virtually bistable. When operating
in desiccation mode, the entire electrode surface is in contact
with an electrically conductive medium and therefore the load
impedance is comparatively low, consequently inhibiting the rise in
output voltage to a level sufficient for arcing. Conversely, when
in cutting or tissue vaporisation mode, the entire active electrode
surface is covered with a layer of vapour which is of much higher
impedance, and the vapour pocket is sustained by arcing within it
so that nearly all of the power dissipation occurs within the
vapour envelope. In order to traverse from a desiccation mode to
the cutting mode, a high power burst is required, hence the
positioning of the power/load curve peak between the desiccation
and cutting operation points on the curve. By allowing the output
power to increase with impedance in this way, a high power burst of
sufficient energy to create arcing is achieved despite the low
impedance presented by the electrodes. As the supply voltage to the
oscillator is increased, it has a greater propensity to flip into
the cut mode, whilst at lower supply voltage levels, the bistable
nature of the output, although more pronounced, tends towards the
desiccation state.
[0098] The bistable properties arise not only from the electrode
impedance behaviour, but also from the shape of the power/load
impedance curve. The flatter the load curve, the more constant the
output power across a band of impedances and the less pronounced
the effect.
[0099] Referring to FIG. 10, it will be appreciated that in the cut
or tissue vaporisation mode, a power equilibrium point is achieved
by virtue of the decreasing output power as impedance increases. In
the desiccation mode, the equilibrium is less straightforward,
because there are two impedance change mechanisms. The first
mechanism is the heating of the conductive medium and/or tissue
which, due its positive coefficient of conductivity, results in a
falling impedance initially, so that when power is first applied,
the operating point moves towards the lefthand side of the diagram
in FIG. 10. Consequently, there is a well-defined equilibrium point
defined by the reduction in impedance with increasing power supply
voltage, and the consequent reduction in delivered output power.
However, when the saline medium or tissue fluids in contact with
the active electrode start to boil, small water vapour bubbles
begin to form which increase the impedance. When the generator is
about to flip into the cutting mode, impedance rise due to steam
formation is dominant. The impedance change therefore becomes
positive with increasing supply voltage, and the operating point
moves towards the righthand side of the diagram, which allows
greater input power as a result of the shape of the load curve,
causing a rapid change to cutting or vaporisation mode. As steam
formation continues to increase, the increasing impedance causes a
fall-off in delivered output power.
[0100] The applicants have found that the inherent equilibria
described above may be insufficient to maintain a stable
coagulation state or a stable cutting state. It is for this reason,
that the RF output voltage from the RF oscillator 60 (FIG. 6) is
limited, the limiting occurring extremely rapidly, typically with a
response time of 20 .mu.s or less. Excessive radio frequency
interference is avoided by linear variation of the oscillator
switching device "on" time in response to a feedback signal from
the voltage threshold detector. This technique is used in
conjunction with the RF oscillator having a comparatively low
output Q when matched to the load, this Q being sufficient to
suppress switching noise without inordinately damping the response
to output voltage threshold detection.
[0101] By way of example, the effect of voltage threshold control
for a particular electrode configuration is shown in FIG. 11. The
heavy lines 200, 202 indicate the modified power/load impedance
characteristics. For desiccation, shown by line 200, the switched
mode power supply is set to produce a peak (matched) open loop
output power of between 75 watts and 110 watts, with the actual
peak power in this case being about 90 watts. For cutting and
vaporisation (shown by line 202), the peak power can be between 120
watts and 175 watts. In this case it is 150 watts. As examples, the
voltage thresholds are set at 180 volts peak for desiccation and
300 volts peak for cutting, as illustrated by the hyperbolic
constant voltage lines 204 and 206 respectively. The
power/impedance curves follow the respective constant voltage
threshold lines to the right of their intersection with the
unmodified open loop curves 208 and 210. Thus, it will be
understood that the desiccation threshold line represents the
maximum voltage that can be achieved in the desiccation mode before
arcing is produced, whilst the cut threshold line limits the
cutting or tissue vaporisation performance to achieve the desired
tissue effect and. in the extreme, to avoid electrode burn-out. The
desiccation threshold line also represents a voltage insufficient
to achieve arcing for cutting or vaporising tissue.
[0102] A significant feature of the generator characteristic for
electrosurgical cutting or tissue vaporisation is that at peak
power (matched impedance) the load impedance lies between the
impedances corresponding to the threshold voltages at that power
level. In contrast, in the desiccation mode, the power/load
impedance characteristic has a power peak at an impedance lying
below the desiccation threshold line at that power level.
[0103] In practice, the output power in the desiccation mode will
be higher than in the cutting or tissue vaporisation mode. The
reason for this statement (despite the apparent contradiction with
the load curves in FIG. 11) is that the equilibrium points
described above lie at different points on the respective curves.
To ensure cutting, the high peak power of the higher curve is
required to reach the cut threshold line (corresponding to 300
volts peak). The cutting mode then follows the cutting or
vaporisation threshold line. The cutting operating point is defined
by the load impedance created when a suitable level of arcing is
occurring. Typically, the load impedance in these circumstances is
greater than 1000 ohms. Thus, although a full 150 watt peak power
is available to ensure that vapour pockets are formed to promote
arcing for cutting, the actual power drawn during cutting or tissue
vaporisation for this particular electrode example may be between
30 watts and 40 watts. This situation is more easily understood if
reference is also made to FIG. 3.
[0104] In the desiccation mode, the operating point is determined
by the positive power coefficient of impedance arising from steam
generation. Consequently, the equilibrium naturally occurs in the
region of the peak of the desiccation mode power/load impedance
curve.
[0105] The invention is useful for dissection, resection,
vaporisation, desiccation and coagulation of tissue and such
combinations of these functions with particular application in
hysteroscopic, laparoscopic, colposcopic (including vaginal
speculum) and open surgical procedures on the female genital tract
and adnexal related diseases. Hysteroscopic operative procedures
may include: removal of submucosal fibroids, polyps and malignant
neoplasms; resection of congenital uterine anomalies such as a
septum or subseptum; division of synechiae (adhesiolysis); ablation
of diseased or hypertrophic endometrial tissue; and haemostasis.
Laparoscopic operative procedures may include: removal of
subserosal and pedunculated fibroids, ablation of ectopic
endometrium, ovarian cystectomy and ovarian drilling procedures;
oophorectomy, salpingo-oophorectomy, subtotal hysterectomy and
laparoscopically assisted vaginal hysterectomy (LAVH) as may be
performed for benign or malignant diseases laparoscopic uterosacral
nerve ablation (LUNA); fallopian tube surgery as correction of
ectopic pregnancy or complications arising from acquired
obstructions; division of abdominal adhesions; and haemostasis.
[0106] The invention is also useful in the lower female genital
tract, including treatment of the cervix, vagina and external
genitalia whether accessed directly or using instrumentation
comprising generally speculae and colposcopes. Such applications
include: vaginal hysterectomy and other pelvic procedures utilising
vaginal access; LLETZ/LEEP procedure (large loop excision of the
transformation zone) or excision of the transformation zone of the
endocervix; removal of cystic or septic lesions; ablation of
genital or venereal warts; excision of benign and malignant
lesions; cosmetic and surgical repairs including vaginal prolapse;
excision of diseased tissue; and haemostasis.
[0107] The invention is also useful for dissection, resection,
vaporisation, desiccation and coagulation of tissue and such
combinations of these functions with particular application in
arthroscopic surgery as it pertains to endoscopic and percutaneous
procedures performed on joints of the body including but not
limited to such techniques as they apply to the spine and other
non-synovial joints. Arthroscopic operative procedures may include:
partial or complete meniscectomy of the knee joint including
meniscal cystectomy; lateral retinacular release of the knee joint;
removal of anterior and posterior cruciate ligaments or remnants
thereof; labral tear resection, acromioplasty, bursectomy and
subacromial decompression of the shoulder joint; anterior release
of the temperomandibular joint; synovectomy, cartilage debridement,
chondroplasty, division of intra-articular adhesions, fracture and
tendon debridgement as applies to any of the synovial joints of the
body; including thermal shrinkage of joint capsules as a treatment
for recurrent dislocation, subluxation or repetitive stress injury
to any articulated joint of the body; discectomy either in the
treatment of disc prolapse or as part of a spinal fusion via a
posterior or anterior approach to the cervical, thoracic and lumbar
spine or any other fibrous joint for similar purposes; excision of
diseased tissue; and haemostasis.
[0108] The invention is also useful for dissection, resection,
vaporisation, desiccation and coagulation of tissue and such
combinations of these functions with particular application in
urological endoscopic (urethroscopy, cystoscopy, ureteroscopy and
nephroscopy) and percutaneous surgery. Urological procedures may
include: electro-vaporisation of the prostate gland (EVAP) and
other variants of the procedure commonly referred to as
transurethral resection of the prostate (TURP) including but not
limited to interstitial ablation of the prostate gland by a
percutaneous or perurethral route whether performed for benign or
malignant disease; transurethral or percutaneous resection of
urinary tract tumours as they may arise as primary or secondary
neoplasms and further as they may arise anywhere in the urological
tract from the calyces of the kidney to the external urethral
meatus; division of strictures as they may arise as the
pelviureteric junction (PUJ), ureter, ureteral orifice, bladder
neck or urethra; correction of ureterocoele; shrinkage of bladder
diverticular; cystoplasty procedures as they pertain to corrections
of voiding dysfunction; thermally induced shrinkage of pelvic floor
as a corrective treatment for bladder neck descent; excision of
diseased tissue; and haemostasis.
[0109] The invention is also useful for dissection, resection,
vaporisation, desiccation and coagulation of tissue and such
combinations of these functions with particular application in
surgery on the ear, nose and throat (ENT) and more particularly
procedures performed on the oropharynx, nasopharynx and sinuses.
These procedures may be performed through the mouth or nose using
speculae or gags or using endoscopic techniques such as functional
endoscopic sinus surgery (FESS). Functional endoscopic sinus
procedures may include: removal of chronically diseased inflamed
and hypertrophic mucus linings, polyps and neoplasms from the
various anatomical sinuses of the skull; excision of diseased
tissue; and haemostasis. Procedures on the nasopharynx may include:
removal of chronically diseased inflamed and hypertrophic mucus
linings, polyps and neoplasms from the turbinates and nasal
passages; submucus resection of the nasal septum; excision of
diseased tissue; and haemostasis. Procedures on the oropharynx may
include: removal of chronically diseased inflamed and hypertrophic
tissue, polyps and neoplasms particularly as they occur related to
the tonsil, adenoid, epi- and supraglottic region, and salivary
glands; as an alternative method to the procedure commonly known as
laser assisted uvulopalatoplasty (LAUP); excision of diseased
tissue; and haemostasis.
[0110] It is evident from the scope of applications of the
invention that it has further additional applications for
dissection, resection. vaporisation, desiccation and coagulation of
tissue and such combinations of these functions in general
laparoscopic, thoracoscopic and neurosurgical procedures being
particularly useful in the removal of diseased tissue and
neoplastic disease whether benign or malignant.
[0111] Surgical procedures using a system incorporating the
generator of the present invention include introducing the
electrode assembly to the surgical site whether through an
artificial (cannula) or natural conduit, which may be in an
anatomical body cavity or space such as the human uterus or one
created surgically either using the invention or another technique.
The cavity or space may be distended during the procedure using a
fluid or may be naturally held open by anatomical structures. The
surgical site may be bathed in a continuous flow of conductive
fluid such as saline solution either to fill and distend the cavity
or to create a locally irrigated environment around the tip of the
electrode assembly in a gas filled cavity or on an external body
surface or other such tissue surfaces exposed during part of a
surgical procedure. The irrigating fluid may be aspirated from the
surgical site to remove products created by application of the RF
energy, tissue debris or blood. The procedures may include
simultaneous viewing of the site via an endoscope or using indirect
visualisation means.
* * * * *