Note: Descriptions are shown in the official language in which they were submitted.
ll(~SS6Z
The invention relates to a device for use in moni-
toring a biological signal from a patient while an electro-
surgical appliance which applies a high r.f. voltage is being
simultaneously used.
An ECG device is known from U.S. Patent No. 3,915,154.
The screen or shield of the cable connected to the ECG elec-
trodes is linked to a screen surrounding the ECG amplifier.
This device is suitable for suppressing noise voltages ema-
nating from the mains. However, ECG signals are not detected
in this device in the case where an r.f. signal having a
magnitude of several hundred volts is fed to the tissue.
Even if an r.f. filter is interposed before the
ECG amplifier, it will nevertheless be impossible to detect
the ECG signals reliably enough. In fact radiation outside
and inside the screen will affect the operation of the device.
According to the present invention, there is provided
a device which monitors a biological function of a pati~nt
during use of an electro-surgical appliance which delivers
high r.f. voltages to the tissue under consideration, com-
prising at least one sensor which, when linked to the patient,provides a signal representative of the biological function,
which signal is supplied via a screened r.f. filter to an
amplifier, the filter and amplifier being disposed within
a Faraday cage which is connected to a reference potential
and insulated from a surrounding earthed or grounded container.
Thus, in a device according to the invention, the radiation
emana ing from the filter inductors is screened, and so is
radiation emanating from the Faraday cage.
-1-
ll(~SS6;~
It is advantageous for the inductors of the filter
at the input of the amplifier to be separately screened.
In accordance with an important aspect, the invention
provides a device for monitoring a biological function of
a patient during use of an electro-surgical applicance which
delivers high r.f. voltages to the tissue under consideration,
the device comprising a reference electrode and two active
electrodes adapted to contact the body of the patient, thereby
providing a signal representative of the biological function,
a signal amplifier having two input terminals, a shielded
conductor and a filter for connecting the electrodes with
the amplifier, the reference electrode, and the conductor
shield being interconnected, and each of the active elec-
trodes, being connected to an amplifier input terminal, an
ungrounded shield surrounding said filter and said amplifier,
and a grounded container surrounding and insulated from the
ungrounded shield.
Preferably the device is so arranged that the capacity
between the ~araday cage and the external screening container,
which is earthed when the device is in use, is in the range
3~ to 60 pF. As a result the r.f. signal is to some extent
short-circuited, but the inflowing r.f. currents are not
so large that the r.f. vcltage drop in the tissue of the
patient becomes excessive.
--2--
~,
ll~SS62
Where the transducer means requires a voltage feed,
as opposed to ECG electrodes which require no applied voltage,
this voltage feed may be applied via a further filter or
filters whose inductors are fitted and screened in the screened
Faraday cage, in which case the power supply used may be
the power supply which is used for energizing the amplifier.
A specific embodiment of a device according to
the present invention will now be described by way of example
with reference to the accompanying drawing wherein:-
Figure 1 is a circuit diagram of circuitry, includ-
ing an ECG amplifier, fitted in a screened Faraday cage;
One form of electro-surgical appliance, with which
a device according to the present invention can be used,
comprises a radio-frequency source of which one electrode
having a relatively large surface area is arranged to be
coupled to the back of a patient, and of which the other
electrode consists for example of the knife used by the sur-
geon. During an operation, bleeding is stopped by sending
r.f. currents amplitude-modulated at 50 Hz through the tissue
under the control of the surgeon by use of a pedal-opera~ed
control device. In the first place, coagulation occurs,
in the second place there is a certain amount of carbonisation,
and in ~he third place the tissue dries out. All these things
contribute to stopping the bleeding. The applied voltage
is preferably in the range of 300-400. The frequency is
preferably in the range 750 kHz-1.6 MHz; such a frequency
does not upset the nerve-cells of the tissue.
~I~'S56Z
The device illustrated in this embodiment comprises
electrodes or transducer means which, for ECG monitoring,
may be one reference electrode and two ECG electrodes. The
reference electrode is connected to a metal container 10
which acts as a Faraday cage. The two active electrodes
are connected to the input terminals 2,4 of a symmetrically
constructed filter 6, which provides an attenuation of about
70 dB at 1 MHz. The filter has inductors Ll and L2 which
are individually screened as indicated in Figure 1. The
output of the filter is connected to an ECG amplifier 8 (the
attenuation by 70 dB prevents the amplifier from being over-
loaded by the modulation signal of the r.f. signal), which
is fitted in a part 11 of the Faraday cage 10, the whole
of which is enclosed in and insulated from a screening con-
tainer 17. The capacity between the cage 10 and the screen-
ing container 17 is in the range 30-60 pF. This arrangement
results in the r.f. signal being to some extent short-cir-
cuited, thus limiting the change in potential of the cage
10. The ECG amplifier 8 is of the fully symmetrical type,
with a gain of about 40 times over a frequency range extend-
ing to below one Hz. The amplifier comprises matched re-
sistances, thus avoiding any resistance adjustment. Follow-
ing the ECG amplifier 8 is an attenuator including two diodes
parallel Dl, D2 coupled in opposition, which has the effect,
should overloading occur, of clamping the signal level.
The output signal of the ECG amplifier 8 (of about 41 mV)
is fed via a capacitor C6 to a self-oscillating multivibrator
12.
ll(~SS6Z
When the device is energized, the multivibrator
acts in the following manner:
Feedback to the positive terminal - see figure
1 - sets up an initial voltage of about 66mV (10 V 150) at
the positive input. The capacitor C7, which has zero volts
across it at the moment when the device is switched on, is
charged up via the feedback resistor R5. After a certain
time, the capacitor C7 reaches 66 mV, and then the output
changes abruptly from its positive level to its negative
level. The potential of the positive input is then at -66mV,
with the result that the capacitor C7 then charges towards
-lOV until the voltage of -66mV is reached and the cycle
then repeats continuously. The frequency of oscillation
is about 1 kHz.
As mentioned, the output signal of about 41 mV
of the ECG amplifier is fed to the input of the multivibrator
12. This is to modulate the charging of the capacitor C7
and thereby modulate the pulse-width of the multivibrator
12. This modulation depends unambiguously on the ECG signals.
A coupling capacitor C8, having a value of 2.2nF, differentiates
the output signal of the multivibrator (rectangular-wave
signal) and passes only the flanks to a light-emitting diode
D3. As a result, only a brief glimmer of light occurs at
each change of state of the multivibrator 12. This manner
of modulation saves energy.
' ~
S56Z
A d.c. voltage supply arrangement 14 for the ECG
amplifier comprises a rectifier bridge fitted in the cage
10. To the rectifier bridge is connected a toroidal winding
which is connected via a single turn 15 to a toroidal winding
of an inverter, shown in Figure 3, disposed outside the cage
10. The capacity between the toroidal windings is about 0.7
pF and the capacity of each toroidal winding is about 1.4
pF.
The light-emitting diode D3 transmits light to
a phototransistor of the demodulator via a light pipe LP,
and the capacitance between diode D3 and the phototransistor
can be as small as desired by appropriately selecting the
distance between them.
An additional advantage is that the transmission
path of the power supply is different from the transmission
path of the output signal. This prevents any cross-modula-
tion which might otherwise occur. It is also an advantage
that the capacity is at its lowest in the transmission path
of the output signal.
The amplitude of the r.~. signal may be optionally
reduced by appropriately positioning the ECG electrode.
Since the patient is always receiving and transmitting a
signal at 50 Hz, the positioning of the electrodes may be
checked with this signal. ~y measuring the strength of the
- ~5 50 Hz signal received at the electrodes, it can be determined
whether or not the skin under the electrodes must be further
abraded (the d.c. or low frequency resistance must be below
i -~
11(~S562
5 k-nL), and whether they must be fitted such that they are
on a line at right-angles to the r.f. current. When the
electrodes are properly fitted, a filter which filters off
the remainder of the 50 Hz signal is coupled into the output
stage (the demodulator).
~,
: Whereas the above described device has been described
in connection witb monitoring of ECG signals, it will be
;~ appreciated that it can be readily modified as appropriate
for monitoring other biological signals, for example those ~::
;
~ 10 representing blood pressure and temperature, EEG and EMG.
.~!' In the case of monitoring blood pressure and temperature
signals, the voltage on the secondary side of the transformer
in the Faraday cage is reduced, and the voltage is fed out
to the transducers via a filter (which is likewise fitted
in the cage and has individually screened inductors).
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