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The invention relates to an arrangement for the high frequency coagulation
of protein for surgical purposes, comprising a means for measuring the
protein impedance and a coagulation instrument.
With the coagulation devices known hitherto, it is not possible to
automatically achieve a determinate coagulate in different body tissue and
with different instruments. It is known that a thermoelement be
incorporated into the coagulation instrument. However, conditioned by the
dimensions of the sensor and by the wiring, limits are set to such
incorporation of thermoelements in instruments. Even with the most
complicated of sensors, it is always only the sensor temperature and not
the critical tissue temperature which can be determined.
Medical science demands a controllable and close localization of the
coagulation focus, which is to remain limited to the desired area in order
that no damage is sustained by important structures in the immediate
vicinity. It is important that the coagulation be limited only to
denaturation of the tissue and that no charring with scabbing take place
at the tips of the instrument.
The reprint in "Zentralblatt fur Chirurgie", Vol. 85, 1960 No. 19, p. 1052
to 1063, has disclosed working without a thermoelement. In the known
arrangement, use is also made of recording of the resistance to monitor
the coagulation. If the resistance is made directly measurable and is
recorded while the coagulation is in course, without any delay in time, it
is possible to immediately detect the beginning of the rise in resistance
and to reduce the current and the temperature by hand or by corresponding
automatic feedback. Thus, scabbing and blistering with ensuing rupture of
the coagulate is avoided. The said publication does not pay regard to the
fact that especial importance is to be attached to the high frequency
power set at the beginning of the coagulation. If the power is too low, no
coagulation takes place. If the power is preselected to be too high, then
this gives rise to tissue burns. Furthermore the initial power to be set
is dependent on the volume of the desired coagulation focus and the
differing conductivity of the tissue. This inhomogeneity of the tissue to
be coagulated and the varying electrode surfaces, particularly when the
electrodes and their spacing is small, make it difficult for the operating
surgeon to decide on what power range is to be preselected before the
start of coagulation. Even if the power is initially set to a low value,
during its subsequent increase it is not possible to set the optimum power
by hand due to thermal time constants and dynamic events taking place.
Furthermore, the tests described in the reprint show that the resistance
curve takes an extremely flat course, so that it is extremely difficult to
determine when coagulation has taken place to the desired extent. As
mentioned above, an optimal result is not achieved by subsequently
regulating the current down, as is proposed in the paper.
An object underlying the invention is to develop an arrangement for the
high frequency coagulation of protein in such a way that the power output
is automatically controlled in an optimal manner from the beginning of the
coagulation operation and, after the desired coagulation volume has been
formed, is switched off in a dependable manner without any extra check on
the part of the operating surgeon.
This object is accomplished according to the invention in that the protein
impedance is continuously measured and the differential quotient of said
impedance is formed and is used to control the high frequency power
employed.
In the invention it was discovered that the differential quotient of the
protein impedance curve is an important indication not only for the final
formation of the coagulate but also for the initial setting. It is
therefore precisely in the initial state that uncontrolled occurences in
the coagulation are avoided. It has furthermore proved that continuous
coagulation shortly before the zero value of the differential quotient is
reached leads to optimal, homogenous coagulation zones without burns. With
the arrangement according to the invention, the desired size of the
coagulate can be achieved in a simple way by presetting the differential
quotient to a desired value which determines time and power value.
The invention is based on the finding that, regardless of the respective
tissue structure, with bipolar coagulation instruments and with the use of
different power values over different periods of time, there is given rise
to protein impedance curves which in each case take the same
characteristic course. The differential quotient of the protein impedance
is of especial significance for the initial regulation of the high
frequency power and for the attainment of optimal coagulation.
Embodiments of the invention are described below with the help of drawings
in which
FIG. 1 is a block diagram of an analog embodiment of the arrangement
according to the invention,
FIG. 2 is a diagram illustrating the course of typical protein or tissue
impedance curves for different power and time, and
FIG. 3 is a block diagram of a digital embodiment of the arrangement
according to the invention.
The block diagram illustrated in FIG. 1 indicates a tissue G into which a
coagulation instrument is introduced. The values of the tissue impedance Z
are continuously established by means of an impedance measuring means 10
and are differentiated in a differentiator 11. The output signal of the
differentiator 11 is hence a measure for the differential quotient of the
tissue impedance curve. This signal is supplied to an I-controller 12, the
output of which is connected to an analog storage 14 through an analog
switch 13. The I-controller 12 is furthermore supplied with a voltage
which can be set through a potentiometer P1 and corresponds to a certain
preset value of the differential quotient. This voltage is also supplied
to a first comparator V1 which compares it with the output signals of the
differentiator 11, i.e. with the differential quotient of the tissue
impedance curve. This arrangement serves for the high frequency power to
be optimally set at the beginning of the coagulation operation, depending
on the desired volume of coagulation, whereby the power and the duration
of its use are correlated with one another, as may be seen from the
diagram in FIG. 2. By means of the potentiometer P1, the rate of
coagulation, i.e. the volume per unit of time, can be preselected.
A monitor 17 is also connected to the differentiator 11 and provides
acoustic and/or optical indication on the course of the differential
quotient. In particular, upon variation of the differential quotient, the
pitch of tone can likewise vary in the same direction. The monitor 17 can
be adapted to be preset from the impedance measuring means 10 for various
orders of magnitude of the impedance.
A second comparator V2 receives a voltage set through a potentiometer P2
and likewise compares this voltage with the output values of the
differentiator 11. The potentiometer P2 is set to a value of the
differential quotient which is near zero. In particular, shortly before
the minimum of the impedance time curve (differential quotient =zero), the
value of the differential quotient of the differentiator 11 sinks to said
value near zero. Then the comparator V2 determines conformity and switches
off a high frequency final stage 20, for instance through a relay with
contact r.
The way in which the arrangement according to FIG. 1 operates is as
follows. When the switch S closes, the high frequency final stage 20 is
switched on and power is gradually built up through the control circuit of
elements 10,11,12,13 and 14. The impedance value is delivered as a
continuous signal by the impedance measuring means 10 to the
differentiator 11 which in turn applies a continuous signal to the
I-controller 12. After the switching on, the I-controller 12 delivers an
output voltage rising linearly from zero as long as the voltage difference
between the voltage from the potentiometer P1 and the voltage which is at
the output of the I-controller 12 and corresponds to the present
differential quotient is not equal to zero. The output voltage of the
I-controller 12 is applied through the analog switch 13 to the analog
storage 14 which, by way of example, may take the form of sample and hold
circuit. The output of the high frequency final stage 20 is then increased
according to the value delivered by the analog storage. If the value of
the differential quotient reaches the value set by the potentiometer P1,
then the comparator V1 opens the analog switch 13, so that the high
frequency final stage 20 is operated with constant power in accordance
with the value stored in the analog storage 14.
The heating of the tissue now progresses, the impedance gradually reducing
as shown by the curves in FIG. 2. The value of the differential quotient
runs from a negative initial value towards zero in accordance with the
progression of coagulation. When the value set through the potentiometer
P2 is reached, the comparator V2 switches off the high frequency final
stage 20, because then the desired coagulation is concluded. Otherwise,
depending on the position of the working point on the power-load
resistance characteristic curve of the coagulate, the power might rise
steeply directly after the zero value of the differential quotient, which
could lead to tissue burns and scabbing.
The diagram in FIG. 2 illustrates curves with different power values
between 3 and 6 watts, the coagulate sizes being the same in each case. It
is of interest to note the different negative gradient in the initial part
of the curves with different HF power. This value can be used for
automatically setting the coagulation power at the start of the
coagulation. If the power is set in such a way through the potentiometer
P1, the desired coagulate is always obtained in the same time, even under
different conditions.
It is hence apparent that at the beginning of the entire operation, a
controlled and regulated setting of the power can be achieved by the
potentiometer P1.
With the help of FIG. 1, an analog embodiment of the arrangement according
to the invention was described. However, the arrangement according to the
invention can also be designed for a digital mode of operation. In this
case, the measured impedance values are digitalized with a high, sampling
frequency. The differential quotient can then be determined by a computer
which passes the corresponding values on to the elements as are indicated
in FIG. 1, these being of analog or digital design.
FIG. 3 illustrates the use of a computer in the arrangement of digital
design according to the invention. A computer 30 is controlled by way of a
program 31 which, depending on the desired parameters, in accordance with
the settings of potentiometers P1 and P2, initiates the various program
steps in the computer. The continuously measured impedance values are
digitalized and supplied to the computer. The computer has a desired curve
storage 32 storing various impedance curves and the values of the
differential quotient of these impedance curves according to time. As
indicated at 33, curve comparison then takes place with comparison of the
differential quotients of the measured impedance curve and the selected
desired curve. The value derived therefrom is supplied to the computer 30
which then applies a signal corresponding to the output signal of the
analog storage 13 to the high frequency final stage 20. As in the analog
embodiment according to FIG. 1, the control is such that the high
frequency power is kept as constant as possible.
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