Data processing: structural design – modeling – simulation – and em – Simulating electronic device or electrical system – Power system
Reexamination Certificate
1999-12-22
2001-08-14
Teska, Kevin J. (Department: 2123)
Data processing: structural design, modeling, simulation, and em
Simulating electronic device or electrical system
Power system
C703S020000
Reexamination Certificate
active
06275786
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a device for monitoring the application of a two-art neutral electrode during unipolar high-frequency surgery.
BACKGROUND OF THE INVENTION
During high-frequency surgery, if the actual area of contact between the neutral electrode and the patients body is too small there is a risk of receiving burns from the neutral electrode due to the large current densities which then occur.
Devices for monitoring the application of neutral electrodes in high-frequency surgery are known from DE 32 39 640 C2 or EP 0 390 937 A1. The DE 32 39 640 C2 reference discloses such a device having an impedance sensor with a resonant circuit comprising a secondary coil of a transformer and HF input capacitors of preferably two partial electrode surfaces. The impedance sensor detects the transition impedances of two partial electrode surfaces connected in series with the patient's tissues by application of a patient auxiliary current, and is moderated by the transition impedances. If this resonance network is excited at its exact resonant frequency, the patient transition resistance R alone becomes visible. This operating point can be attained in principle by positive voltage feedback. However, it has been found that such arrangements are vulnerable to interference signals produced by HF generators. As such, a positive voltage feedback would have to be tapped behind a filter, the phase shift of which depends on the tolerances of the components used to construct the filter. This phase shift has a large impact on the resulting closed loop oscillation frequency of the system. The parallel resonant circuit is then no longer excited at its exact resonant frequency, which results in measurement errors. Furthermore, such a system is typically not very accurate due to the tolerances of the relatively large number of components involved.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a device for unipolar HF surgery which excites the resonant circuit in such a manner that a more precise measurement of the transition impedance of the patient's tissues is made possible.
This and other objects of the invention are achieved by provision of a resonant circuit which is excited by alternating voltages of variable frequencies in the range of a resonant frequency, and a peak value detector which detects an alternating voltage peak value at the resonant frequency.
Contrary to conventional measuring devices, which are affected by phase errors caused by component tolerances, the device according to the invention dispenses with any use of feedback signals, which could be altered by large interference signals. Instead, the peak value detector determines the alternating voltage peak value at the resonant frequency. While the frequency of the exciting voltage continuously varies, generally being repeatedly passed over in a frequency sweep, the peak value detector, which has a time constant substantially greater than the duration of a single frequency variation or of a single sweep, stores the detected peak value. This corresponds to the patient transition resistance at the resonant frequency, which itself is not determined at all in accordance with the invention, but which is the voltage which would be measured when the resonance circuit is excited at the exact resonance frequency. Measuring errors of conventional devices which attempt to match the exact resonant frequency are avoided by the selection of the peak value detected during the frequency variation as the patient transition resistance.
To permit self-testing of the impedance sensor's function, i.e., without a HF generator, a further development of the invention provides for a reference resonant circuit that can be connected instead of the normal resonant circuit via a switch. It is advantageous for the reference resistance at the resonant frequency to be greater than the highest patient transition resistance at which activation of the HF generator power is still permitted. This ensures that no HF use is possible if the switch fails on being switched back to the resonance network.
The systems or devices for monitoring the application of neutral electrodes known from the cited prior art produce a signal if the adherence of the neutral electrode has become so poor that the HF generator may no longer be activated. With the HF generator switched on, the HF output is suddenly shut off due to the excessively high transition resistance of the neutral electrode. Conventional devices are therefore unsuitable for calling the surgeon's attention to a deterioration of the adherence of the neutral electrode or for supplying information regarding the quality of the application while the neutral electrode is being applied. It is therefore desirable to provide continuous monitoring of the application of the neutral electrode and determination of the quality of the application even while the neutral electrode is being applied.
In this regard, the invention is preferably capable of determining how well the neutral electrode is functioning even during its application. With this being the case, during the surgical operation, it is possible to determine if the current density is rising and will soon reach a critical value at which the generator can no longer be activated. Moreover, this also makes it possible to determine whether the neutral electrode is suitable for the planned surgery on the basis of its physical properties, its expansion and its placement, or if the neutral electrode is inadequate for the planned surgery.
In a system for unipolar HF surgery with a simple, divided neutral electrode, the difference of the currents flowing through the two parts of the neutral electrode, aside from the transition resistance, can typically be analyzed in a manner that is known per se as a parameter for describing the quality of the application. Thus, in addition to the impedance sensor, which detects the series-connected transition resistances of the two partial electrode surfaces by application of a patient auxiliary current, it may be desirable to provide a current asymmetry sensor as well.
The current asymmetry sensor detects the difference between the HF currents flowing through the two partial electrode surfaces when the HF output signal is activated, and the rectified value of the patient auxiliary current between the two partial electrode surfaces when the HF output signal is deactivated. The output signals of the impedance sensor and of the current asymmetry sensor are supplied to an electronic control and analysis means which derives a signal for monitoring the sensor function from the signals of the impedance sensor and of the current asymmetry sensor. A control and analysis unit can also use the momentary value of the current asymmetry in relation to a HF current, as well as the first derivation of this relative current asymmetry in time, as criteria for this in addition to the momentary transition resistance.
Furthermore, for practical purposes the electronic control and analysis means sets the output signals of the impedance sensor and of the current asymmetry sensor in relation to the selected generator setting. In such case, a fuzzy-logic function linking the individual parameters together can be used for the analysis and linking processes in addition to fixed criteria for the transition impedance and the asymmetry.
Finally, the invention can have an optical and/or acoustic indicator such as a bar indicator, which displays the quality of the application of the neutral electrode and warns the surgeon of an impending failure of the neutral electrode.
REFERENCES:
patent: 4651280 (1987-03-01), Chang et al.
patent: 4741334 (1988-05-01), Irnich
patent: 4754757 (1988-07-01), Feucht
patent: 5406503 (1995-04-01), Williams, Jr. et al.
patent: 5720744 (1998-02-01), Eggleston et al.
patent: 32 39 640 C2 (1987-01-01), None
patent: 0 390 937 A1 (1990-10-01), None
Jankauskas et al.; ECG sampling unit for electrosurgical environments; IEEE 1988 Bioengineering Conf.; pp. 79-81.
Jones Hugh
St. Onge Steward Johnston & Reens LLC
Storz Endoskop GmbH
Teska Kevin J.
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