System for cutting biological tissue

A system for controlling the operation of a high frequency biological tissue cutting device for cutting biological tissue with high-frequency current having a HF generator component designed so that the amplitude of the voltage (UG) applied to the tissue can be changed. At least one adjustment device is provided to adjust one of the characteristic values (K) of the high frequency generator component to a 1st desired value (b), related to the output of an indicator device that indicates by an electrical signal (d) the size and intensity of the electric arc occurring between the surgical probe and the tissue during cutting. A desired-value transmitter provides a second desired value (c) representing the desired size and intensity of the electric arc. An evaluation unit receives the output signal (d) of the indicator device and the 2nd desired value representing the intensity of the electric arc (c) and provides therefrom a desired value (b) output for the adjustment device such that the maximum changing speed of the desired value (b) is at least one magnitude smaller than the speed with which the adjustment device adjusts the characteristic value (K) of the high-frequency generator component.

TECHNICAL FIELD 
The present invention relates to a system for cutting biological tissue 
with high-frequency current. 
STATE OF THE ART 
High-frequency currents are employed in surgery to cut biological tissue or 
to coagulate, i.e. stop the bleeding. During cutting, an almost continuous 
high-frequency power is supplied. A problem in high-frequency surgery is 
the right dosage of the power during cutting. If the power is set too low, 
the tissue undergoes mechanical stress, it cannot be cut quickly or the 
cutting procedure comes to a complete halt. On the other hand, setting the 
high-frequency power too high results in a strong electric arc between the 
surgical probe and the tissue. This electric arc causes major tissue 
necrosis which impairs the healing process. Too strong an electric arc has 
also other drawbacks. The essential disadvantage is a partial 
rectification of the high-frequency current due to the electric arc, which 
leads to the danger of nerve and muscle stimulation in the patient. Such 
muscle and nerve stimulation may result in sudden, unexpected movements by 
the patient even if the patient is under full narcosis. In this event, the 
surgeon no longer has control, and there is high risk that the patient 
will be injured by the surgical probe. Moreover, too strong an electric 
arc decomposes the tissue and in the case of underwater cutting, as e.g., 
in urology, the rinsing fluid may even be thermally dissociated. Both 
processes generate explosive gas mixtures which can lead to dangerous 
explosions in the patient's body when operating in body cavities. 
The power required for cutting and the size and intensity of the resulting 
electric arc are also influenced by many exterior parameters. The main 
influencial factors are, e.g., the specific electric conductivity of the 
tissue being cut, dependent, on the one hand, on the type of tissue and, 
on the other hand, on the rate at which the tissue is desiccating, 
the momentary cutting speed, 
the momentary cutting depth, 
the shape of the surgical probe, 
the dimensions of the surgical probe, 
the specific electric conductivity of a rinsing fluid that might be 
present: this conductivity can change even during the cutting due to blood 
flow and electrolytes, 
the configuration of the operation site, respectively the distribution of 
high-ohmic and low-ohmic tissue elements there, 
the momentary current density distribution in the patient's body: this 
current density distribution can change extremely rapidly and drastically, 
in particular, if an electric arc is ignited between the surgical probe 
and the tissue to be cut. 
An adjustment of one of the characteristic values of the high frequency 
surgical generator, which are, e.g. 
the high-frequency current delivered to the patient, 
the high-frequency voltage applied to the patient, 
the high-frequency power input to the patient, and 
the no-load voltage set at the generator, can compensate for only some of 
the influences due to the external parameters. Thus, e.g. an adjustment 
proposed in DP-A-0285 962 of the output voltage to a constant value 
largely compensates for the influences: cutting depth and cutting speed. 
An altered specific electric conductivity of the tissue, e.g. due to 
desiccation of the tissue requires changing the output voltage, can 
therefore not be influenced, in particular, by such an adjustment. 
The optimum setting of the high-frequency generator is when there is a 
small electric arc between the surgical probe and the tissue. On the one 
hand, the electric arc ensures a cutting-friendly dot-shaped transmission 
of the high-frequency current from the surgical probe to the tissue, but 
on the other hand does not lead to the described drawbacks of a strong 
electric arc. 
German patent P 25 04 280, describes an apparatus for cutting and/or 
coagulating human tissue with high-frequency current having a indicator 
device which shows the size and intensity of the electric arc occurring 
between the probe and the tissue by means of an electric signal and which 
contains an adjustment device which regulates the strength of the current 
of the high-frequency current delivered to the patient and thereby also 
the high-frequency power input to the patient in such a manner that the 
size and intensity of the electric arc corresponds to a pre-set value. 
Measurements during operations conducted with surgical generators whose 
output setting is carried out according to this adjustment principle show 
distinct advantages over operations with generators lacking such controls. 
Even if the paramenters also influencing the necessary generator setting 
change drastically, such as electric conductivity of the tissue, the 
degree at which the tissue is desiccating, cutting speed, cutting depth, 
shape and dimensions of the surgical probe, etc., one and the same setting 
of the desired value for the size and intensity of the electric arc can be 
operated with. As there is hardly any scab formation, the power input to 
the patient could be decreased in some cases to a third compared to 
similar operations with a generator without an electric arc adjustment. 
Nonetheless, the adjustment has some drawbacks. They can be described if 
the physical effects connected with cutting at the operation site with a 
burning electric arc are examined more closely. The electric arc is not 
dependent solely on the power dosage. A number of other physical effects 
influence the size and intensity of the electric arc. 
First of all, the electric voltage between the surgical probe and the 
tissue must be sufficiently high that an electric arc can even ignite. 
This requires, on the one hand, a suitably high no-load voltage of the 
generator, but also the presence, on the other hand, of a high-ohmic or 
insulating layer between the surgical probe and the tissue. If the 
surgical probe is covered with crust, this layer may, under circumstances, 
be formed by a coat of dried, coagulated blood and adhering remains of 
tissue. If there is a small gap between the surgical probe and the tissue, 
air or a only minimally conductive rinsing fluid forms the high-ohmic or 
insulating layer. If the surgical probe comes into contact with the tissue 
and it has a clean surface, this high-ohmic or insulating layer is formed 
by a vapor layer created when the cell fluid vaporizes. The thickness of 
the resulting vapor layer depends on the electric input power. 
The thickness of the high-ohmic or insulating layer then itself influences 
the electric arc and its effects. The thicker the high-ohmic or insulating 
layer, the greater the sparking distance of the electric arc and the 
greater the amount of power is converted into energy at the arcing point 
of the electric arc. This causes some of the described drawbacks when a 
strong electric arc occurs. As the sparking distance of the electric arc 
increases, the interrelationship between the high-frequency current in the 
electric arc and the high-frequency voltage at the electric arc becomes 
more and more non-linear. This increases the non-linear signals, primarily 
harmonious with the momentary generator frequency, caused by the 
high-frequency current and high-frequency voltage in the electric arc. 
These are, on the one hand, the harmonic 2nd, 3rd, 4th, and higher order, 
whose frequencies are the twofold, threefold, fourfold, . . . of the 
momentary frequency of the output signal and it is the harmonic 0 (zero) 
order which describes the rectifier effect of the electric arc. This 
rectifier component created in the electric arc is responsible for the 
nerve and muscle stimulation. 
The thickness of the vapor layer as a thermal effect does not immediately 
follow the momentary power input. Thus the adjustment system has a dead 
time. This is especially noticeable when starting to cut. Between the 
point of switching on the generator and the point when the electric arc 
first ignites, there is an not to be neglected interval; it sometimes 
takes several seconds until cutting actually commences. It is a known fact 
in adjustment technology that adjustment systems which contain dead times 
are very difficult to stabilize. 
Moreover, the electric arc does not burn evenly the whole time on the 
surface of the surgical probe. The electric arc is, if the voltage is 
sufficiently high, ignited there where the vapor layer is the thinnest. 
The high concentration of energy generated by the electric arc at the 
arcing point of the high-frequency current vaporizes the cell fluid there, 
the arcing point then quickly becomes the point with the thickest 
insulating layer. The electric arc then ignites at another point. In this 
way, the electric arc scans the entire surface of the surgical probe and 
ultimately vaporizes the cell fluid along its entire surface. The site and 
the sparking distance of the electric arc is so random that the burning of 
the electric arc must be considered a stochastic process. This effects the 
spectrum of high-frequency current and high-frequency voltage. Thus, e.g. 
the spectral ranges created by the electric arc are not of constant 
amplitude, the speed of change reaching the upper limit which is 
predetermined by the working frequency. As a result there is, in addition, 
a broadband noise in the frequency spectrum, used in EP-A0O 219 568 to 
detect the electric arc. 
If such stochastic fluctuations influence the measured values employed for 
adjustment, the incidental fluctuations have to be compensated by 
averaging. The measurement of the stochastic processes therefore requires 
a finite measuring period. This, on the other hand, means that the 
adjustment cannot occur at any desired rate. Due to this finite period, 
which passes until there is an unequivocal control value, the electric 
arcs cannot be adjusted to a constant momentary value. An additional 
problem in adjusting electric arcs lies in the known physical fact that 
the non-linear interrelationship between the high-frequency voltage and 
the high-frequency current in the electric arc has partially negative 
rises, i.e. it can happen that when raising the momentary voltage, the 
momentary current decreases and when lowering the momentary voltage the 
momentary current rises. It is known that such processes can excite 
oscillations and destabilize adjustments. 
DESCRIPTION OF THE INVENTION 
The object of the present invention is thus to design the system for 
cutting biological tissue with high-frequency currents in such a manner 
that a stable adjustment is obtained despite the afore-described dead 
times, the required averaging and the threat of destabilization of the 
adjustment by the physical effects of the electric arc. 
Accordingly, in order to indicate the size and intensity of the electric 
arc, the system is combined with an adjustment of at least one of the 
characteristic values of the generator. At least one of the characteristic 
values of the generator is adjusted to a 1st desired value. In this way, 
the effect of one component of the external parameters on the cutting 
behavior is eliminated. Preferably, the characteristic value is adjusted 
to a desired value which influences the external parameter that has the 
most influence on the cutting process in the type of surgery just being 
conducted. If the type of tissue and the desiccation state only change 
slowly, but the cutting depth or cutting speed have to be varied 
continually, it is advantageous to adjust the output voltage. 
In setting the high-frequency current or the high-frequency power to a 1st 
desired value, the influence of the specific electric conductivity on the 
cutting behavior is largely eliminated; in this event the influence of the 
cutting depth and cutting speed on the cutting behavior remain. 
These uninfluenceable effects of external parameters by the respective 
setting of the characteristic value of the generator are compensated for 
in that the 1st desired value is not constant but is gained by a 
comparison of the electric signal of a indicator device for the size and 
intensity of the electric arc with a 2nd desired value. The gaining of the 
1st desired value occurs in an evaluation unit to which, on the one hand, 
the electric output signal of the indicator device for the size and 
intensity of the electric arc is transmitted and, on the other hand, to 
which the 2nd desired value is transmitted. For stable adjustment, it is 
necessary that the 1st desired value generated in the evaluation unit for 
the adjustment device changes slower by at least one order of magnitude 
than the adjustment device which needs time to adjust the characteristic 
value to the desired value. 
Short-term changes of the external parameters are thus regulated by quick 
operating adjustment of the characteristic value in its effect on the 
cutting behavior. Averaged over a longer period, the size and intensity of 
the electric arc are constant and determined by the 2nd desired value. 
The 2nd value for the size and intensity of the electric arc are supplied 
by the desired-value transmitter. In the simplest case, the desired-value 
transmitter supplies a fixed desired value. Usually the surgeon can 
influence the 2nd desired value supplied by the desired value transmitter 
and adapt it to the goal of the operation. Very small 2nd desired values 
for the size and intensity of the electric arc lead to incisions with 
minimal necrosis and minimal muscle and nerve stimulation. This setting is 
selected if, e.g., cutting is in the vicinity of nerve centers and there 
is a danger that the patient will twitch because these nerves have been 
stimulated. Such sudden movements by the patient make surgery more 
difficult and present the risk that the surgeon may cut too deeply and 
thereby seriously injure the patient. 
In surgery in which much tissue is to be removed, e.g., in the case of 
prostatectomy up to 100 g of prostate tissue, a higher setting of the 
desired value for the size and intensity of the electric arc permits quick 
cutting. As at the beginning of this type of surgery, the tissue is 
removed in several layers, a greater degree of necrosis in the top layers 
is no problem, because these necrotic sections of tissue will be removed 
in the course of the operation. 
The invented combination of an adjustment device for a characteristic value 
of the generator component and the indicator device for the size and 
intensity of the electric arc yields further advantages for the design of 
the system for cutting biological tissue. 
Thus the adjustment of the characteristic value does not have to occur 
precisely; minor deviations in the adjustment are compensated for over a 
long period by the readjustment of the 1st desired value. 
Moreover, it is not necessary that one of the output signals of the 
generator such as output voltage, output current, output power, no-load 
voltage, etc. is immediately utilized as the adjusted characteristic 
value. But rather, characteristic values occurring within the generator 
component can be adjusted if they simply have an unequivocal relationship 
to the output values of the generator. Thus, it suffices, e.g., if during 
a final high power stage of the generator component, which is set up so 
that it has little interior resistance and its output voltage, therefore, 
is almost proportional to the voltage of its direct current supply except 
for a not very great decline in voltage at this interior resistance, to 
adjust this direct current voltage. In such a case, the complexity of the 
circuit for the adjustment device may be substantially reduced. 
The combination of the adjustment device for one of the characteristic 
values with the indicator device for the size and intensity of the 
electric arc hereto also permits utilizing physical processes for the 
indicator devices, which are too slow for a direct adjustment for the 
constant size and intensity of the electric arc. For instance, the printed 
patent of the German patent DE 25 04 280 describes as a particular 
advantage of the adjustment of the 3rd and higher harmonic that only this 
harmonic permits a rapid adjustment. If, due to the adjustment to a 
constant desired value for the size and intensity of the electric arc, 
only adjustment deviations and slowly changing processes have to be 
compensated, the especially easy-to-be-measured 0 (zero) harmonic can be 
utilized to indicate the size and intensity of the electric arc with 
adequate success. If, by way of illustration, the indication of the size 
and intensity of the electric arc are combined via the 0 harmonic with an 
adjustment device for the output voltage of the generator component, the 
following working mechanism for the entire system is yielded: for varying 
cutting speed and cutting depth, the constant adjustment of the output 
voltage achieves almost constant cutting conditions and constant size and 
intensity of the electric arc. A readjustment of the output voltage is 
only necessary if the conductivity of the tissue at the site of the 
operation changes. This occurs either if the incision leads into a tissue 
area of a different type of tissue, e.g. from muscle tissue into fatty 
tissue, or if the operation area desiccates slowly due to constant 
heating. In this event, if the output voltage is at first constant, the 
average size and intensity of the electric arc is reduced. As a result, 
the low-frequency parts and, in particular, also the rectified current 
decrease. The output signal of the electric arc adjustment becomes 
smaller, the evaluation unit raises the 1st desired value for the 
adjustment device until the signal of the indicator device in turn equals 
the 2nd desired value for the size and intensity of the electric arc. 
If a physical effect, which permits quick dectection of the size and 
intensity of the electric arc, is utilized to indicate the electric arc, 
as for example the evaluation of the harmonics of a higher order contained 
in the generator current, the evaluation unit has to restrict the changing 
speed of its output signal, thus the 1st desired value supplied to the 
adjustment device, by suitable means in such a manner that the rising 
speed is at least one order of magnitude smaller than the adjustment speed 
of the adjustment device. 
In an especially favorable design of the present invention, the restriction 
of the changing speed occurs in that first the momentary deviation of the 
output signal of the indicator circuit for the size and intensity of the 
electric arc is compared to the 2nd desired value. A possible circuit 
realization of this comparison can occur by establishing the difference of 
the two signals, perferably with a differential amplifier. The thereby 
formed difference signal will initially still change quickly. If then the 
temporal average value of the signal is established, this output signal is 
suited to be transmitted as the 1st desired value to the adjustment 
device. Circuits for averaging are public knowledge. The simplest 
realization is an RC low-pass filter with a defined cutting-off frequency 
of f.sub.g1. 
In another advantageous embodiment of the evaluation unit, a difference 
signal is also formed from the output signal of the indicator device for 
the size and intensity of the electric arc and the 2nd desired value for 
the size and intensity of the electric arc. This output signal is 
transmitted to a circuit with a temporally integrating function, as e.g. 
can be realized in a known manner with the aid of a capacitive feedback 
differential amplifier. In this case, the size and intensity of the 
electric arc is changed until the output signal of the indicator device 
for the size and intensity of the electric arc equals the 2nd desired 
value for the size of the electric arc without any permanent adjustment 
deviation. A possibly necessary direct current voltage offset for the 1st 
desired value, transmitted to the adjustment device, sets in automatically 
due to this design. 
Due to averaging or integration, the 1st desired value changes only slowly. 
Because of the previously described dead times until the ignition of an 
electric arc, it is usually useful to only raise the output signal of the 
generator very slowly. If the electric arc occurs then, the 1st desired 
value is only a little larger than would be optimum in the transient state 
of the adjustment despite the dead times. The size and intensity of the 
electric arc is then also almost optimum. The adjustment speed of the 1st 
desired value for a higher characteristic value of the generator component 
is in this case at least one order of magnitude smaller than the 
adjustment speed of the adjustment device. If too high an electric arc 
occurs in this case, either because the external parameters have changed 
or because the dead time lasted too long that the 1st desired value 
transmitted to the adjustment device rose far above the optimum value, the 
too strongly burning electric arc would then cause the previously 
described drawbacks. In this event, the evaluation unit is designed in 
such a manner that the changing speed of the 1st desired value in the 
direction which means a downward adjustment of the adjusted characteristic 
value of the generator component is at least one order of magnitude larger 
than in the case of an upward adjustment. 
A special problem in an adjustment which maintains a constant size and 
intensity of the electric arc is the time passing between activating the 
generator and the 1st ignition of the arc. At this point, the tissue to be 
cut is at body temperature. Now the tissue must first be heated to the 
boiling point of the cell fluid and sufficient cell fluid must vaporize 
until the surgical probe and the tissue are completely isolated from one 
another, not until then can the electric arc ignite. Measurements show 
that in this case several seconds can pass between activating the 
generator and igniting the electric arc, especially if the surgical probe 
is pressed strongly against the tissue when the generator is activated. In 
this event, the 1st desired value for the adjustment device for adjusting 
one of the characteristic values of the generator rises steadily during 
the entire time that the electric arc has not ignited. Without any special 
measure, a much too large an electric arc burns once the electric arc has 
ignited. This disadvantage can be avoided if, due to a suited circuit, the 
1st desired value cannot rise above a preset limit. Thus, measurements 
show that, e.g., in dentistry no voltages with an effective value higher 
than 250 V are needed if the current required for the surgery is returned 
low-ohmically to the generator. In this case, it is advantageous to adjust 
the output voltage of the generator as the characteristic value to a 
constant value but not to permit higher desired values for the adjustment 
device than are suited for an output voltage of 250 V. 
Frequently a bottom limit suited for the application can also be given as 
the characteristic value of the generator which is adjusted by the 
adjustment unit to the 1st desired value. Thus values of at least 150 V 
are required for the amplitude of the voltage applied to the tissue in 
order to be able to ignite an electric arc at all. In this case, it is 
advantageous if the desired value for the adjustment device is restricted 
in such a manner that the limit of the characteristic value suited for the 
application is not undercut and, of course, the suited top value is not 
exceeded. 
In this event, the required adjustment range for the adjustment of the 
characteristic value of the generator is restricted permitting achieving 
higher adjustment speeds and greater adjustment stability. 
In another advantageous embodiment of the present invention, the problem 
of, at times, long delays between activating the generator and igniting 
the electric arc is avoided by extending the evaluation unit by a circuit 
component which recognizes whether or not an electric arc has ignited. In 
its simplest form, this circuit component consists of a comparator which 
indicates whether the output signal of the indicator device for the size 
and intensity of the electric arc differs from zero. If the output signal 
of the indicator device is zero, there is no electric arc. In this case, a 
fixed desired value is passed on as the 1st desired value for the 
adjustment device. The 1st desired value is not formed from the comparison 
of the 2nd desired value with the output signal of the indicator device 
for the size and intensity of the electric arc until the electric arc has 
ignited. Naturally, in practical realization, the circuit will be designed 
in such a manner that averaging or integration circuits do not run into a 
limit during the time when there is no electric arc. The transient effect 
of the adjustment would otherwise be needlessly slowed down. 
During some surgery, the surgeon already activates the generator some time 
prior to touching the tissue with the surgical probe. This is especially 
the case if he only has to remove a small amount of tissue yet has to be 
particularly careful. In this event, the time between the activation of 
the generator and the occurance of the electric arc is extended further. 
The state that the surgical probe has not yet touched the tissue can be 
determined by monitoring the impedance Z occurring between the surgical 
probe and the tissue. In another advantageous embodiment, the system for 
cutting biological tissue is, therefore, supplemented with a circuit for 
determining the momentary impedance and its output signal is also 
transmitted to the evaluation device. In connection with the output signal 
of the indicator device for the size and intensity of the electric arc, 
the 1st desired value can then be adapted better to the events of the 
operation. 
In an especially advantageous embodiment of the present invention, the 1st 
desired value for the adjustment device is set to a preset low value as 
long as a high-ohmic impedance Z indicates that the surgical probe has not 
touched the tissue. The characteristic value of the generator is not set 
to the greater value prescribed by the evaluation unit until the impedance 
Z undercuts a preset limit Zu. The limit Zu depends on the application of 
the system. If the described system is part of a high-frequency surgical 
generator for dentistry, the measurements of the inventors show that if 
the generator frequency is 350 kHz and there is no tissue contact, the 
load impedance of the high-frequency generator in the case of a surgical 
system with an applied by-pass electrode is more than 20 k.OMEGA. and 
distinctly less if the tissue is contacted. In this case, it is useful to 
select an impedance value of Zu=20 k.OMEGA. as a threshold value. 
The indicator device for the load impedance does not necessarily have to 
pass on an analogue value to the evaluation unit. The decision whether the 
surgical probe has touched the tissue or not can already be made in the 
indicator device for determining the load impedance. In this event, 
forming the ratio of the generator voltage U divided by the generator 
current I(Z=U/I) otherwise needed for detemining the impedance is 
obviated. It suffices to give the comparator a value v.sub.1 *I which is 
proportional to the generator current I and a value v.sub.2 *U which is 
proportional to the generator voltage. The output signal of the comparator 
will change its switch state exactly when v.sub.1 *I equals v.sub.2 *U 
(v.sub.1 *I=v.sub.2 *U). Thus the limit for switching Zu is given by 
Zu=v.sub.2 /v.sub.1. The factors v.sub.1 and v.sub.2 can easily be set 
with state-of-the-art processes, e.g. by means of voltage-divider 
circuits. In that event, the output signal of the comparator has only the 
two switching states which show whether the momentary load impedance Z of 
the system for cutting biological tissue is greater or smaller than Zu. 
This signal can then be transmitted to the evaluation unit as the output 
signal of the indicator device for the load impedance. There it can serve 
without any or only little further processing as a switching signal from a 
given low value to the variable value determined from the indicator device 
for the size of the electric arc as the 1st desired value for the 
adjustment device. 
All state-of-the-art analogue operating circuits having the properties 
defined in the claims hereto may be utilized for the practical 
construction of the evaluation unit. As the 1st desired value which is 
transmitted to the adjustment device for a characteristic value of the 
high-frequency generator component from the evaluation unit only needs to 
change slowly, the evaluation unit can also be realized by digital circuit 
elements in an especially advantageous embodiment. Some new high-frequency 
surgical generators on the market already contain microprocessors. In this 
case, the overall system can be particulary easily adapted to the 
different surgical purposes. It suffices to change the program of the 
microprocessor in order to obtain different characteristic values or 
different limits.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 shows a basic circuit diagram of the system for cutting biological 
tissue (1) in conjunction with the most frequently used mode of 
tissue-to-system contacting. One clamp of the system is conductively 
connected to the surgical probe (7). The surgical probe is often also 
referred to as active electrode or cutting electrode. The second clamp is 
usually conductively connected to a second, large-surface electrode (10), 
which is usually disposed away from the operation site. This second 
electrode is often referred to as by-pass electrode, neutral electrode or 
passive electrode. Between the surgical probe (7) and the second electrode 
is the biological tissue to be cut (8). During the incision, a high-ohmic 
or insulating layer (11), which is penetrated by an electric arc (9), 
forms between the surgical probe (7) and the tissue (8). The system for 
cutting biological tissue consists of a generator component (2) which 
generates the high-frequency power needed for cutting. Required for the 
system for cutting biological tissue is a generator component (2) whose 
output power can be changed by an electronic signal (a). It is not of 
significance for the present invention which output values of the 
generator component such as output voltage, output current, output power, 
no-load voltage is primarily influenced by signal a. All these values are 
interconnected via the characteristic values of the generator and the 
impedance fixed by the external circuit. 
As further component the system for cutting biological tissue has a device 
for adjusting at least one of the characteristic values of the generator. 
In this system, signal (b) represents the 1st desired value to which the 
characteristic value (K) is adjusted. In the drawing, the device for 
adjusting the characteristic value is drawn in such a manner that a value 
(K) which can be detected at the output of the generator component is 
adjusted. Only a value occurring in the generator component can also be 
adjusted instead as the characteristic value (K) if it has an equivocal 
relationship to the output value of the generator component. The 1st 
desired value (b) occurring in the system for cutting biological tissue is 
not a set, fixed value, but rather is set by the following described 
components of the overall system. First there is an indicator device (4) 
which indicates the size and intensity of an electric arc (9) burning 
between the surgical probe (7) and the tissue (8) by an electric signal 
(d) All hitherto state-of-the-art circuits for detecting an electric arc 
with an electric signal can be used as indicator units, thus in particular 
all the methods described in the German patent 2504280. The output signal 
(d) of the indicator device (4) for the size and intensity the electric 
arc is transmitted to an evaluation unit (6) which forms the 1st desired 
value (b) for the device for adjusting at least one of the characteristic 
values of the generator component from the output signal (d) and a desired 
value c given by a desired-value transmitter (5). It is important for the 
functioning of the overall system that the 1st desired value (b) changes 
by at least one order of magnitude slower than the adjustment device for 
adjusting the required characteristic value requires. Possible embodiments 
of the evaluation unit (6) are described in the respective description and 
are made more apparent in the following figures. 
FIG. 2 shows a diagrammatic view of an advantageous embodiment of the 
evaluation unit (6). The difference signal e is formed from the input 
signals (d), (c) of the evaluation unit (6) by a difference establishing 
circuit (12). There are many possibilities familiar to those versed in the 
art for the realization of the circuit, one of them is the depicted 
circuit having an operational amplifier. Subsequently, the average value 
of the difference signal (e) is formed by the circuit (13). The simplest 
form of linear averaging can occur as described by an RC-low pass filter. 
Other circuits with low-pass functions are just as suited to solving this 
object, in particular also active low passes. In their case, the generally 
also needed addition of an offset value to the output signal is especially 
easy to solve. Averaging is, however, not restricted to the linear average 
value as low passes form. Particularly advantageous can be forming the 
root mean square of the difference signal. In this case, however, the 
complexity of the circuit increases considerably. 
FIG. 3 shows a diagrammatic view of another advantageous embodiment of the 
evaluation unit (6). Like in FIG. 2, a difference signal e is formed from 
the input signals (d), (c) of the evaluation unit (6) by a subtraction 
circuit (12). This is followed by a circuit with an integrating effect 
(14) like those, e.g., that can be realized with the aid of a capacitive 
regenerative operational amplifier. The special advantages of this circuit 
arrangement is that the size and intensity of the electric arc can be 
adjusted to the 2nd desired value (c) without any permanent adjustment 
deviation even if the 1st desired value (b) has to be provided with an 
offset value which changes with the time or with the surgical temperature. 
FIG. 4(a) shows a diagrammatic view of another advantageous embodiment of 
the evaluation unit (6). Like in FIGS. 2 and 3, a difference signal e is 
formed from the input signals (d), (c) of the evaluation unit (6) by a 
difference establishing circuit (12). This is followed by a circuit having 
a low-pass behavior (15) and/or with an integrating effect. Contrary to 
the circuits described in FIGS. 2 and 3, this circuit has different time 
constants depending on whether the signal (d) indicating the size and 
intensity of the electric arc is larger or smaller than the 2nd desired 
value (c). The simplest possibility of realizing such different time 
constants is to couple the storing element of the circuit for averaging or 
the integration circuit to the previous circuit via different resistances. 
Switching can occur, as shown diagrammatically in FIG. 4 by a diode (D1). 
The same function could, however, also be obtained via controlled 
switches. 
The intention of the time diagram in FIGS. 4(b) and 4(c) is to make the 
function of the circuit more apparent. First the input signals (c) and (d) 
of the evaluation unit (6) are entered in the diagram. The desired 2nd 
value (c) indicating the desired size and intensity of the electric arc 
should be constant the entire time (C.sub.0) (interrupted line). For 
signal 2 describing the actual size and intensity of the electric arc at 
the time t.sub.1 the size and intensity of the electric arc has suddenly 
become larger, by the value D. In response to this, the output signal b of 
the evaluation unit representing the 1st desired value for the following 
adjustment device (3) lowers. Due to the low-pass behavior of the 
averaging or integrating circuit (15), it takes a finite time span of 
T.sub.1 until the output signal b reaches a determined deviation B from 
the previoulsy assumed value B.sub.0. The next thing drawn in the diagram 
at the time t.sub.2 is a deviation D of the signal d from the value 
c=C.sub.0 in the direction indicating the decreased size of the electric 
arc. Now it takes a considerably longer time span T.sub.2 until the signal 
b reaches the same deviation B of the value B.sub.0. An inventive element 
is that the circuit (15) is designed in such a manner that time span 
T.sub.2 is substantially larger than time span T.sub.1, thus it can be 
said T.sub.2 &gt;&gt;T.sub.1. The drawn Time course of signals c, d and b are 
only intended to illustrate the basic behavior of the circuit. They cannot 
be measured in the drawn manner in a closed adjustment loop in the 
actually realized circuit. Any change in the signal b immediately results 
in a change in the adjusted characteristic value of the generator 
component and therefore also in a change in the size and intensity of the 
electric arc and a change in signal d. A temporal constant deviation in 
signal d by the value D is thus possible in a closed adjustment loop. The 
signals can, however, be measured in a similar manner if the adjustment 
loop, as is common in basic tests, be opened at any point. 
FIG. 5 shows another advantageous embodiment of the evaluation unit. The 
circuit block 6a is an embodiment of the evaluation unit 6 previously 
completely described in 
FIGS. 1-4. Before the output signal is transmitted to the adjustment device 
as the 1st desired value (b), there is now a limit circuit 16 which 
prevents signal b from rising above the limit B.sub.max. 
FIG. 6 shows another advantageous embodiment of the evaluation device. The 
circuit block 6a is an embodiment of the evaluation device 6 previously 
completely described in 
FIGS. 1-4. Before the output signal is transmitted to the adjustment device 
as the 1st desired value (b), there is now a limit circuit 17 which 
prevents signal b from rising above the top limit B.sub.max and dropping 
below the bottom limit B.sub.min. 
FIG. 7 shows another advantageous embodiment of the evaluation device. The 
circuit block 6b is an embodiment of the evaluation device 6 previously 
completely described in FIGS. 1-6. The output signal of the circuit 
component (6b) is only transmitted as the 1st desired value (b) to the 
adjustment device if there is an electric arc. In the times in which there 
is no electric arc, the preset value B1 is applied for the signal b by the 
change-over switch (19). The decision concerning the change-over occurs in 
the circuit component (18) which, in the simplest embodiment, consists of 
a comparator which determines whether the size and intensity of the 
electric arc indicated by signal d differs from zero. 
FIG. 8 describes another advantageous embodiment of the present invention. 
FIG. 8 shows in another diagrammatic view the entire system for cutting 
biological tissue. In addition to the function units shown in FIG. 1, 
there is the indicator device (20) for the impedance Z with its output 
signal g which is also transmitted to the evaluation unit (6). The 
impedance Z is the load impedance which occurs at the output clamps (A1, 
A1') of the overall system through connection to the biological tissue. By 
including the impedance Z in the forming of the 1st desired value (b) for 
the adjustment device, more attention can be paid to the momentary 
surgical conditions at the site of the surgery. 
FIG. 9 shows a simple but especially advantageous embodiment of the 
evaluation device (6) which, in addition, includes the value of the 
momentary impedance Z in its assessment. Block 6c is one of the 
embodiments of the evaluation device 6 described in the FIGS. 1 to 7. In 
addition, there is now a comparison circuit provided which determines 
whether the momentary impedance is larger or smaller than the preset value 
of Zu. If Z&gt;Zu, the surgical probe is not in contact with the tissue. In 
this case, a diminished value B2 is transmitted as the 1st desired value b 
to the adjustment device of the characteristic value of the generator 
component by the change-over switch (22).