Abstract:
An electrosurgical system comprises a generator and an instrument including a first electrode, a second electrode, and an insulating spacer separating the first and second electrodes. The generator repeatedly measures a characteristic of the radio frequency output such as the impedance between the first and second electrodes. The generator analyses the impedance measurements, and interrupts the radio frequency signal when the rate of change of the impedance is such as to indicate the onset of a “flare-out”. In this way, the power is reduced before the flare-out leads to permanent damage or failure of the instrument.

Description:
This is a Continuation of application Ser. No. 10/464,778, now U.S. Pat. No. 7,220,260, filed Jun. 19, 2003, which claims the benefit of U.S. Provisional Application No. 60/402,555, filed Aug. 12, 2002 and British Patent Application No. 0214907.8, filed Jun. 27, 2002. The disclosures of prior application Ser. No. 10/464,778, now U.S. Pat. No. 7,220,260, filed Jun. 19, 2003, which claims the benefit of U.S. Provisional Application No. 60/402,555, filed Aug. 12, 2002 are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an electrosurgical system, and in particular to one in which an electrosurgical generator provides a radio frequency cutting signal to a bipolar surgical instrument. 
     BACKGROUND OF THE INVENTION 
     A typical bipolar cutting instrument, which may also be capable of tissue coagulation, comprises first and second electrodes separated by an insulating spacer. An early example of a bipolar RF cutting device is U.S. Pat. No. 4,706,667 issued to Roos, in which the return or “neutral” electrode is set back from the active electrode. In a series of patents (including U.S. Pat. Nos. 4,674,498, 4,850,353, 4,862,890 and U.S. Pat. No. 4,958,539) Stasz proposed a variety of cutting blade designs. These were designed with relatively small gaps between two electrodes such that arcing would occur therebetween when an RF signal was applied to the blade, the arcing causing the cutting of the tissue. In an alternative arrangement, described in our co-pending patent applications GB 0130975.6 and U.S. Ser. No. 10/105,811, a device is provided in which the spacing of the electrodes is designed such that direct arcing between the electrodes does not occur, but arcing does occur between one of the electrodes and the tissue at the target site. 
     Normal use of this instrument has proved very satisfactory, but in exceptional circumstances (especially where the instrument has been used in an overly aggressive manner) a situation hereinafter referred to as a “flare-out” may develop. It is not uncommon for small particles of condensed tissue and other debris to become attached to the electrodes, and ordinarily this poses no particular problem. However in the case of a flare-out, the debris forms a conductive track between the electrodes, allowing current to flow directly therebetween. This low impedance electrical pathway from one electrode to the other, if allowed to continue for a period of several seconds, may conduct sufficient current that a failure of the device may occur. This may be by way of a failure of the insulating material forming the spacer, either by the insulating material experiencing such high temperatures that it becomes conductive, or by the temperature differentials throughout the insulator causing a physical cracking of the material. Alternatively, the extreme temperatures caused by the current flow may produce a physical melting of the electrode material itself. 
     SUMMARY OF THE INVENTION 
     The present invention provides a way in which this condition, although rarely occurring, can be prevented from causing a failure of the device. Accordingly, there is provided an electrosurgical system including a radio frequency generator, an electrosurgical instrument comprising at least first and second electrodes and an insulating spacer separating the first and second electrodes, the radio frequency generator being adapted to supply a radio frequency signal between the first and second electrodes, means for measuring a characteristic of the output of the radio frequency generator, and a controller adapted to analyse the measured characteristic and change the radio frequency signal supplied between the first and second electrodes when an aspect of the characteristic meets a predetermined criterion indicating the onset of a “flare-out”. 
     Conveniently, the characteristic of the output of the radio frequency generator which is measured is the voltage across the first and second electrodes, or alternatively the current flowing therebetween. It has been discovered that there are a number of criteria which may indicate the onset of a flare-out. These include rapid changes in the impedance experienced between the electrodes, leading to large and sudden changes in the voltage between the electrodes or the current flowing therebetween. There may be an increase in the number or amplitude of high frequency components of the current or voltage signal, or an increase in the D.C. thermionic current sensed between the electrodes. In a preferred arrangement, the predetermined criterion indicating the onset of a flare-out is the changeability of the measured characteristic, typically the rate of change of the impedance between the electrodes, or the changeability as represented by the sum of the differences between successive impedance measurements. 
     Preferably, the controller is adapted to change the radio frequency signal by reducing the power thereof when the predetermined criteria indicating the onset of a flare-out is met. Alternatively, the controller may reduce the voltage of the radio frequency signal, or even the frequency thereof. Where the radio frequency signal comprises a signal having dual components at a first and second frequency, the controller may change the signal by adjusting the relative proportions of the first and second frequency components. Preferably, however, the controller is adapted to reduce the power of the radio frequency signal, and may reduce it substantially to zero when the characteristic meets the predetermined criterion. Conveniently, the power is reduced substantially to zero for a period of at least 5 seconds, allowing time for the instrument to be withdrawn from the surgical site and the electrodes to be cleaned if necessary. Alternatively the power is reduced to zero until the operator of the instrument manually resets the instrument. 
     In one convenient arrangement, the controller is adapted to reduce the power of the radio frequency signal supplied between the first and second electrodes only when the aspect of the characteristic meets the predetermined criterion for a predetermined period of time. This serves to ensure that a false detection of a flare-out is not triggered by a transient change in the measured characteristic. The system may require a series of repeated measurements of the characteristic to all fit the predetermined criteria before action is taken. 
     Although potentially of use with other types of instrument, the present invention is primarily designed to be employed with instruments in which the first and second electrodes and the insulating spacer are such that the spacing between the electrodes is between 0.25 mm and 3.0 mm. 
     According to one preferred construction, an electrosurgical system includes a radio frequency generator, an electrosurgical instrument comprising at least first and second electrodes and an insulating spacer separating the first and second electrodes, the radio frequency generator being adapted to supply a radio frequency signal between the first and second electrodes, means for measuring the impedance between the first and second electrodes, and a controller adapted to analyse the impedance measurements and interrupt the radio frequency signal supplied between the first and second electrodes when the changeability of the impedance exceeds a predetermined threshold value. 
     The invention further resides in an electrosurgical generator for supplying radio frequency power to an electrosurgical instrument which includes at least first and second electrodes, the radio frequency generator including a radio frequency output stage having at least a pair of RF output lines for connection to the first and second electrodes respectively, a power supply coupled to the output stage for supplying power to the output stage, and a controller capable of varying the RF signal applied to the RF output lines, wherein there is provided means for measuring a characteristic of the radio frequency signal across the output lines, the controller being adapted to analyse the measured characteristic and change the radio frequency signal supplied to the output stage when an aspect of the characteristic meets a predetermined criterion indicating the onset of a “flare-out”. 
     More specifically, the present invention relates to an electrosurgical generator for supplying radio frequency power to an electrosurgical instrument which includes at least first and second electrodes, the radio frequency generator including a radio frequency output stage having at least a pair of RF output lines for connection to the first and second electrodes respectively, a power supply coupled to the output stage for supplying power to the output stage, and a controller capable of varying the RF signal applied to the RF output lines, wherein there is provided means for measuring the impedance across the output lines, the controller being adapted to analyse the impedance measurements and interrupt the radio frequency signal supplied to the output stage when the changeability of the impedance exceeds a predetermined threshold value. 
     Finally, the present invention extends to a method of cutting tissue at a target site comprising providing a bipolar cutting blade comprising first and second electrodes and an electrical insulator spacing apart the electrodes, bringing the blade into position with respect to the target site such that one electrode is in contact with tissue at the target site and the other is adjacent thereto, supplying an electrosurgical voltage to the cutting blade such that arcing does not occur in air between the first and second electrodes but that arcing does occur between one of the electrodes and the tissue at the target site, measuring the impedance between the first and second electrodes, and interrupting the electrosurgical voltage when the changeability of the impedance exceeds a predetermined threshold value. 
     The present invention will now be further described below, by way of example only, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic diagram of an electrosurgical system in accordance with the present invention; 
         FIG. 2  is a schematic side view of an electrosurgical instrument suitable for use in the system of  FIG. 1 , 
         FIG. 3  is a schematic block diagram of the generator of the system of  FIG. 1 , and 
         FIGS. 4 and 5  are schematic representations of the impedance measured across the electrodes of the system of  FIG. 1 , in normal operation and in the event of a flare-out, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a generator  10  has an output socket  10 S providing a radio frequency (RF) output for an instrument  12  via a connection cord  14 . Activation of the generator may be performed from the instrument  12  via a connection in cord  14  or by means of a footswitch unit  16 , as shown, connected to the rear of the generator by a footswitch connection cord  18 . In the illustrated embodiment footswitch unit  16  has two footswitches  16 A and  16 B for selecting a coagulation mode and a cutting mode of the generator respectively. The generator front panel has push buttons  20  and  22  for respectively setting coagulation and cutting power levels, which are indicated in a display  24 . Push buttons  26  are provided as an alternative means for selection between coagulation and cutting modes. 
     Referring to  FIG. 2 , the instrument  12  comprises a blade shown generally at  1  and including a generally flat first electrode  2 , a larger second electrode  3  and an electrical insulator  4  separating the first and second electrodes. The first electrode  2  is formed of stainless steel while the second electrode  3  is formed from copper integrally with a body portion  9 . The surface of the second electrode is plated with a biocompatible material such as stainless steel, or alternatively with a non-oxidising material such as gold, platinum or palladium. The electrical insulator  4  is formed from a ceramic material such as Al 2 O 3 . A conductive lead  5  is connected to the first electrode  2 , while lead  6  is connected to the second electrode  3 . The RF output from the generator  10  is connected to the blade  1  via the leads  5  and  6  so that a radio frequency signal having a substantially constant peak voltage (typically around 400V) appears between the first and second electrodes. When the blade  1  is brought into contact with tissue at a target site, the RF voltage will cause arcing between one of the electrodes and the tissue surface. Because the first electrode  2  is smaller in cross-sectional area, and has a lower thermal capacity and conductivity than that of the second electrode  3 , the first electrode will assume the role of the active electrode and arcing will occur from this electrode to the tissue. Electrical current will flow through the tissue to the second electrode  3 , which will assume the role of the return electrode. Cutting of the tissue will occur at the active electrode, and the blade may be moved through the tissue. 
     Referring to  FIG. 3 , the generator comprises a radio frequency (RF) power oscillator  60  having a pair of output lines  60 C for coupling via output terminals  62  to the load impedance  64  represented by the instrument  12  when in use. Power is supplied to the oscillator  60  by a switched mode power supply  66 . 
     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 lines  60 C is a voltage threshold detector  68  having a first output  68 A coupled to the switched mode power supply  16  and a second output  68 B coupled to an “on” time control circuit  70 . A micro-processor controller  72  coupled to the operator controls and display (shown in  FIG. 1 ) is connected to a control input  66 A of the power supply  66  for adjusting the generator output power by supply voltage variation and to a threshold-set input  68 C of the voltage threshold detector  68  for setting peak RF output voltage limits. Also coupled across the output lines  60 C is a current detection circuit  80  which feeds signals to the controller  72  via line  81 . 
     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 hand-piece or footswitch (see  FIG. 1 ). A constant output voltage threshold is set independently on the supply voltage via input  68 C 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 wave-form 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. 
     When the generator is first activated, the status of the control input  60 I 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 . 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 voltage 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. The operation of the generator in this way is described in detail in our European Patent Application No. 0754437, the disclosure of which is hereby incorporated by way of reference. 
     Referring back to  FIG. 2 , when the instrument  12  is in use, small particles of condensed tissue and other debris can become adhered to the edge electrode  2  and, to a lesser extent, the base electrode  3 . If the instrument is used particularly aggressively, it is possible that a conductive track of such debris can build up between the electrodes  2  and  3  across the ceramic insulator  4 . Such a conductive track is shown schematically at  11  in  FIG. 2 . If no action is taken to prevent it, this conductive track  11  will develop into a “flare-out” in which the current passing directly between the electrode  2  and the electrode  3  will cause the instrument to overheat and finally fail. The following description explains how the generator  10  detects and compensates for just such a situation. 
     At regular intervals, in this case every 10 ms the current is measured across the load  64  by the current detector  80  and the current value is sent to the controller  72 . The controller uses the current value to determine repeatedly the impedance across the load  64 . The difference between successive impedance values, irrespective of their absolute value, is calculated, and summed for 16 consecutive readings to give a first total Z 1 . The current measurements continue every 10 ms until a further 16 consecutive impedance calculations have been made, which calculations are again summed to give a second total Z 2 . If Z 1  and Z 2  are both less than the threshold criteria for the sum Q of the impedance changes, then the generator continues to supply RF signals to the instrument  12 . The process is continued with further current measurements being sent to the controller  72  every 10 ms. This normal operation is shown in  FIG. 4 , in which the voltage across the electrodes  2 ,  3  is shown by trace  31 , the current flowing by trace  32  and the impedance measured by the generator by trace  33 . 
     If a flare-out starts to develop between the electrodes  2  and  3 , the current measured across the load  64  will start to fluctuate widely, and with a high frequency of oscillation. This is shown in  FIG. 5 , with the build up to the flare-out being shown at  34  and the onset of the flare-out at  35 . In these circumstances Z 1  and Z 2  (representing difference in impedance values, not absolute impedance values) will both be above the threshold for the sum Q of the impedance changes, and this causes the controller to send a signal to the power supply  66  to cause the power to be interrupted. A typical value for Q is 1000 ohms, for a 16 measurement cycle. 
     In addition to interrupting the power supply, the controller may cause a message (such as “Clean Tip”) to be displayed by the display  24 . The controller does not allow power to be restored to the output of the generator until the surgeon has pressed a reset button to indicate that the tip has been cleaned, and will repeat the interruption process if the impedance measurements show that the flare-out conditions are still in existence when the power is recommenced. 
     It will be appreciated that criteria other than the changeability of the impedance across the output of the generator could be employed to give an indication of the onset of a flare-out. These include, non-exhaustively, the high frequency content (e.g. the number of high frequency components) of the modulation of the current or voltage signal, or the D.C. thermionic current flowing between the electrodes  2  and  3 . The latter will be measured in the manner disclosed in U.S. Pat. No. 6,547,786, the contents of which are incorporated herein by reference. 
     It will also be appreciated that, while the embodiments of the invention have been described with reference to the elimination of flare-outs, the invention could in some aspects be used to prevent overheating of electrodes without the actual existence of a flare-out. The generator, detecting criteria indicating the start of a potential overheating situation, could reduce the power or alter the radio frequency signal in other ways so as to maintain operation of the electrosurgical system operating within proper parameters. Those skilled in the art of electrosurgical generators will readily be able to establish suitable detection criteria to keep the generator operating within safe and effective limits.