Abstract:
The present invention relates to the field of electrosurgery, and more particularly to systems and methods for ablating, cauterizing and/or coagulating body tissue using radio frequency energy. More in particular, the systems utilize voltage threshold means for controlling the voltage applied to tissue in a cycle-to-cycle manner.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This is a continuation-in-part of U.S. patent application Ser. No. 10/995,600, filed on Nov. 22, 2004, which was a continuation of U.S. patent application Ser. No. 10/135,135, filed on Apr. 30, 2002, now U.S. Pat. No. 6,821,275, which was a continuation of U.S. patent application Ser. No. 09/631,040, filed on Aug. 1, 2000, now U.S. Pat. No. 6,413,256, and also claims the benefit of U.S. Provisional Patent Application No. 60/555,777 filed Mar. 24, 2004, the full disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of electrosurgery, and more particularly to systems and methods for ablating, cauterizing and/or coagulating body tissue using radio frequency energy. More in particular, the systems utilize voltage threshold means for controlling the voltage applied to tissue in a cycle-to-cycle manner. 
   Radio frequency ablation is a method by which body tissue is destroyed by passing radio frequency current into the tissue. Some RF ablation procedures rely on application of high currents and low voltages to the body tissue, resulting in resistive heating of the tissue which ultimately destroys the tissue. These techniques suffer from the drawback that the heat generated at the tissue can penetrate deeply, making the depth of ablation difficult to predict and control. This procedure is thus disadvantageous in applications in which only a fine layer of tissue is to be ablated, or in areas of the body such as the heart or near the spinal cord where resistive heating can result in undesirable collateral damage to critical tissues and/or organs. 
   It is thus desirable to ablate such sensitive areas using high voltages and low currents, thus minimizing the amount of current applied to body tissue. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a method and apparatus for treating tissue using an electrosurgical system. The system includes an electrosurgical system having an RF generator, a treatment electrode electrically coupled to the RF generator and positioned in contact with target tissue to be treated, and a spark gap switch positioned between the RF generator and the target tissue. The spark gap includes a threshold voltage and is configured to prevent conduction of current from the RF generator to the tissue until the voltage across the spark gap reaches the threshold voltage. 
   A method according to the present invention includes the steps of using the RF generator to apply a voltage across the spark gap switch, the spark gap switch causing conduction of current from the RF generator to the target tissue once the voltage across the spark gap reaches the threshold voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional side elevation view of a first embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 2  is an end view showing the distal end of the device of  FIG. 1 . 
       FIG. 3  is a graphical representation of voltage output from an RF generator over time. 
       FIG. 4A  is a graphical representation of voltage potential across a body tissue load, from an ablation device utilizing voltage threshold ablation techniques as described herein. 
       FIG. 4B  is a graphical representation of voltage potential across a body tissue load, from an ablation device utilizing voltage threshold ablation techniques as described herein and further utilizing techniques described herein for decreasing the slope of the trailing edge of the waveform. 
       FIGS. 5A through 5D  are a series of cross-sectional side elevation views of the ablation device of  FIG. 1 , schematically illustrating use of the device to ablate tissue. 
       FIG. 6A  is a cross-sectional side view of a second embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 6B  is an end view showing the distal end of the device of  FIG. 6A . 
       FIGS. 7A and 7B  are cross-sectional side elevation view of a third embodiment of an ablation device utilizing principles of the present invention. In  FIG. 7A , the device is shown in a contracted position and in  FIG. 7B  the device is shown in an expanded position. 
       FIG. 8A  is a perspective view of a fourth embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 8B  is a cross-sectional side elevation view of the ablation device of  FIG. 8A . 
       FIG. 9A  is a perspective view of a fifth embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 9B  is a cross-sectional side elevation view of the ablation device of  FIG. 9A . 
       FIG. 10  is a cross-sectional side elevation view of a sixth ablation device utilizing principles of the present invention. 
       FIG. 11A  is a perspective view of a seventh embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 11B  is a cross-sectional side elevation view of the ablation device of  FIG. 11A . 
       FIG. 11C  is a cross-sectional end view of the ablation device of  FIG. 11A . 
       FIG. 12A  is a perspective view of an eighth embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 12B  is a cross-sectional side elevation view of the ablation device of  FIG. 12A . 
       FIG. 13A  is a cross-sectional side elevation view of a ninth embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 13B  is a cross-sectional end view of the ablation device of  FIG. 13A , taken along the plane designated  13 B- 13 B in  FIG. 13A . 
       FIG. 14A  is a cross-sectional side elevation view of a tenth embodiment of an ablation device utilizing principles of the present invention. 
       FIG. 14B  is a front end view of the grid utilized in the embodiment of  FIG. 14A . 
       FIG. 15A  is a cross-sectional side elevation view of an eleventh embodiment. 
       FIG. 15B  is a cross-sectional end view of the eleventh embodiment taken along the plane designated  15 B- 15 B in  FIG. 15A . 
       FIG. 15C  is a schematic illustration of a variation of the eleventh embodiment, in which the mixture of gases used in the reservoir may be adjusted so as to change the threshold voltage. 
       FIGS. 16A-16D  are a series of drawings illustrating use of the eleventh embodiment. 
       FIG. 17  is a series of plots graphically illustrating the impact of argon flow on the ablation device output at the body tissue/fluid load. 
       FIG. 18  is a series of plots graphically illustrating the impact of electrode spacing on the ablation device output at the body tissue/fluid load. 
       FIG. 19  is a schematic illustration of a twelfth embodiment of a system utilizing principles of the present invention, in which a spark gap spacing may be selected so as to pre-select a threshold voltage. 
       FIG. 20  is a perspective view of a hand-held probe corresponding to the invention with a voltage threshold mechanism at the interior of a microporous ceramic working surface. 
       FIG. 21  is a sectional view of the working end of the probe of  FIG. 20 . 
       FIG. 22  is a greatly enlarged cut-away schematic view of the voltage threshold mechanism and microporous ceramic working surface of  FIG. 21 . 
       FIG. 23  is a cut-away schematic view of an alternative voltage threshold mechanism with multiple spark gaps dimensions. 
       FIG. 24  is a cut-away schematic view of an alternative voltage threshold mechanism with a microporous electrode. 
       FIG. 25  is a sectional view of an alternative needle-like probe with a voltage threshold mechanism at it interior. 
       FIG. 26  is a sectional view of an alternative probe with a voltage threshold mechanism at it interior together with an exterior electrode to allow functioning in a bi-polar manner. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Several embodiments of ablation systems useful for practicing a voltage threshold ablation method utilizing principles of the present invention are shown in the drawings. Generally speaking, each of these systems utilizes a switching means that prevents current flow into the body until the voltage across the switching means reaches a predetermined threshold potential. By preventing current flow to tissue until a high threshold voltage is reached, the invention minimizes collateral tissue damage that can occur when a large amount of current is applied to the tissue. The switching means may take a variety of forms, including but not limited to an encapsulated or circulated volume of argon or other fluid/gas that will only conduct ablation energy from an intermediate electrode to an ablation electrode once it has been transformed to a plasma by being raised to a threshold voltage. 
   The embodiments described herein utilize a spark gap switch for preventing conduction of energy to the tissue until the voltage potential applied by the RF generator reaches a threshold voltage. In a preferred form of the apparatus, the spark gap switch includes a volume of fluid/gas to conduct ablation energy across the spark gap, typically from an intermediate electrode to an ablation electrode. The fluid/gas used for this purpose is one that will not conduct until it has been transformed to conductive plasma by having been raised to a threshold voltage. The threshold voltage of the fluid/gas will vary with variations in a number of conditions, including fluid/gas pressure, distance across the spark gap (e.g. between an electrode on one side of the spark gap and an electrode on the other side of the spark gap), and with the rate at which the fluid/gas flows within the spark gap—if flowing fluid/gas is used. As will be seen in some of the embodiments, the threshold voltage may be adjusted in some embodiments by changing any or all of these conditions. 
   A first embodiment of an ablation device  10  utilizing principles of the present invention is shown in  FIGS. 1-2 . Device  10  includes a housing  12  formed of an insulating material such as glass, ceramic, siliciumoxid, PTFE or other material having a high melting temperature. At the distal end  13  of the housing  12  is a sealed reservoir  20 . An internal electrode  22  is disposed within the sealed reservoir  20 . Electrode  22  is electrically coupled to a conductor  24  that extends through the housing body. Conductor  24  is coupled to an RF generator  28  which may be a conventional RF generator used for medical ablation, such as the Model Force 2 RF Generator manufactured by Valley Lab. A return electrode  30  is disposed on the exterior surface of the housing  12  and is also electrically coupled to RF generator  28 . 
   A plurality of ablation electrodes  32   a - 32   c  are located on the distal end of the housing  12 . Ablation electrodes  32   a - 32   c  may be formed of tungsten or any conductive material which performs well when exposed to high temperatures. In an alternative embodiment, there may be only one ablation electrode  32 , or a different electrode configuration. A portion of each ablation electrode  32   a - 32   c  is exposed to the interior of reservoir  20 . Electrodes  22  and  32   a - 32   c , and corresponding electrodes in alternate embodiments, may also be referred to herein as spark gap electrodes. 
     FIGS. 5A through 5D  illustrate the method of using the embodiment of  FIG. 1 . Referring to  FIG. 5A , prior to use the reservoir  20  is filled with a fluid or gas. Preferably, an inert gas such as argon gas or a similar gas such as Neon, Xenon, or Helium is utilized to prevent corrosion of the electrodes, although other fluids/gases could be utilized so long as the electrodes and other components were appropriately protected from corrosion. For convenience only, the embodiments utilizing such a fluid/gas will be described as being used with the preferred gas, which is argon. 
   It should be noted that while the method of  FIGS. 5A-5D  is most preferably practiced with a sealed volume of gas within the reservoir  20 , a circulating-flow of gas using a system of lumens in the housing body may alternatively be used. A system utilizing a circulating gas flow is described in connection with  FIGS. 15A-15B . 
   The distal end of the device  10  is placed against body tissue to be ablated, such that some of the electrodes  32   a ,  32   b  contact the tissue T. In most instances, others of the electrodes  32   c  are disposed within body fluids F. The RF generator  28  ( FIG. 1 ) is powered on and gradually builds-up the voltage potential between electrode  22  and electrodes  32   a - 32   c.    
   Despite the voltage potential between the internal electrode  22  and ablation electrodes  32   a - 32   c , there initially is no conduction of current between them. This is because the argon gas will not conduct current when it is in a gas phase. In order to conduct, high voltages must be applied through the argon gas to create a spark to ionize the argon and bring it into the conductive plasma phase. Later in this description these voltages may also be referred to as “initiating voltages” since they are the voltages at which conduction is initiated. 
   The threshold voltage at which the argon will begin to immediately conduct is dependent on the pressure of the argon gas and the distance between electrode  22  and surface electrodes  32   a - 32   c.    
   Assume P 1  is the initial pressure of the argon gas within reservoir  20 . If, at pressure P 1 , a voltage of V 1  is required to ignite plasma within the argon gas, then a voltage of V&gt;V 1  must be applied to electrode  22  to ignite the plasma and to thus begin conduction of current from electrode  22  to ablation electrodes  32   a - 32   c.    
   Thus, no conduction to electrodes  32   a - 32   c  (and thus into the tissue) will occur until the voltage potential between electrode  22  and ablation electrodes  32   a - 32   c  reaches voltage V. Since no current flows into the tissue during the time when the RF generator is increasing its output voltage towards the voltage threshold, there is minimal resistive heating of the electrodes  32   a - 32   c  and body tissue. Thus, this method relies on the threshold voltage of the argon (i.e. the voltage at which a plasma is ignited) to prevent overheating of the ablation electrodes  32   a ,  32   b  and to thus prevent tissue from sticking to the electrodes. 
   The voltage applied by the RF generator to electrode  22  cycles between +V and −V throughout the ablation procedure. However, as the process continues, the temperature of the tip of the device begins to increase, causing the temperature within the reservoir and thus the pressure of the argon to increase. As the gas pressure increases, the voltage needed to ignite the plasma also increases. Eventually, increases in temperature and thus pressure will cause the voltage threshold needed to ignite the plasma to increase above V. When this occurs, flow of current to the ablation electrodes will stop ( FIG. 5D ) until the argon temperature and pressure decrease to a point where the voltage required for plasma ignition is at or below V. Initial gas pressure P 1  and the voltage V are thus selected such that current flow will terminate in this manner when the electrode temperature is reaching a point at which tissue will stick to the electrodes and/or char the tissue. This allows the tip temperature of the device to be controlled by selecting the initial gas pressure and the maximum treatment voltage. 
   The effect of utilizing a minimum voltage limit on the potential applied to the tissue is illustrated graphically in  FIGS. 3 and 4A .  FIG. 3  shows RF generator voltage output V RF  over time, and  FIG. 4A  shows the ablation potential V A  between internal electrode  22  and body tissue. As can be seen, V A  remains at 0 V until the RF generator output V RF  reaches the device&#39;s voltage threshold V T , at which time V A  rises immediately to the threshold voltage level. Ablation voltage V A  remains approximately equivalent to the RF generator output until the RF generator output reaches 0 V. V A  remains at 0 V until the negative half-cycle of the RF generator output falls below (−V T ), at which time the potential between electrode  22  and the tissue drops immediately to (−V T ), and so on. Because there is no conduction to the tissue during the time that the RF generator output is approaching the voltage threshold, there is little conduction to the tissue during low voltage (and high current) phases of the RF generator output. This minimizes collateral tissue damages that would otherwise be caused by resistive heating. 
   It is further desirable to eliminate the sinusoidal trailing end of the waveform as an additional means of preventing application of low voltage/high current to the tissue and thus eliminating collateral tissue damage. Additional features are described below with respect  FIGS. 14A-18 . These additional features allow this trailing edge to be clipped and thus produce a waveform measured at the electrode/tissue interface approximating that shown in  FIG. 4B . 
   Another phenomenon occurs between the electrodes  32   a - 32   c  and the tissue, which further helps to keep the electrodes sufficiently cool as to avoid sticking. This phenomenon is best described with reference to  FIGS. 5A through 5D . As mentioned, in most cases some of the electrodes such as electrode  32   c  will be in contact with body fluid while others (e.g.  32   a - 32   b ) are in contact with tissue. Since the impedance of body fluid F is low relative to the impedance of tissue T, current will initially flow through the plasma to electrode  32   c  and into the body fluid to return electrode  30 , rather than flowing to the electrodes  32 ,  32   b  that contact tissue T. This plasma conduction is represented by an arrow in  FIG. 5A . 
   Resistive heating of electrode  32   c  causes the temperature of body fluid F to increase. Eventually, the body fluid F reaches a boiling phase and a resistive gas/steam bubble G will form at electrode  32   c . Steam bubble G increases the distance between electrode  22  and body fluid F from distance D 1  to distance D 2  as shown in  FIG. 5B . The voltage at which the argon will sustain conductive plasma is dependent in part on the distance between electrode  22  and the body fluid F. If the potential between electrode  22  and body fluid F is sufficient to maintain a plasma in the argon even after the bubble G has expanded, energy will continue to conduct through the argon to electrode  32   c , and sparking will occur through bubble G between electrode  32   c  and the body fluid F. 
   Continued heating of body fluid F causes gas/steam bubble G to further expand. Eventually the size of bubble G is large enough to increase the distance between electrode  22  and fluid F to be great enough that the potential between them is insufficient to sustain the plasma and to continue the sparking across the bubble G. Thus, the plasma between electrodes  22  and  32   c  dies, causing sparking to discontinue and causing the current to divert to electrodes  32   a ,  32   b  into body tissue T, causing ablation to occur. See  FIG. 5C . A gas/steam insulating layer L will eventually form in the region surrounding the electrodes  32   a ,  32   b . By this time, gas/steam bubble G around electrode  32   c  may have dissipated, and the high resistance of the layer L will cause the current to divert once again into body fluid F via electrode  32   c  rather than through electrodes  32   a ,  32   b . This process may repeat many times during the ablation procedure. 
   A second embodiment of an ablation device  110  is shown in  FIGS. 6A and 6B . The second embodiment operates in a manner similar to the first embodiment, but it includes structural features that allow the threshold voltage of the argon to be pre-selected. Certain body tissues require higher voltages in order for ablation to be achieved. This embodiment allows the user to select the desired ablation voltage and to have the system prevent current conduction until the pre-selected voltages are reached. Thus, there is no passage of current to the tissue until the desired ablation voltage is reached, and so there is no unnecessary resistive tissue heating during the rise-time of the voltage. 
   As discussed previously, the voltage threshold of the argon varies with the argon pressure in reservoir  120  and with the distance d across the spark gap, which in this embodiment is the distance extending between electrode  122  and ablation electrodes  132   a - 132   c . The second embodiment allows the argon pressure and/or the distance d to be varied so as to allow the voltage threshold of the argon to be pre-selected to be equivalent to the desired ablation voltage for the target tissue. In other words, if a treatment voltage of 200V is desired, the user can configure the second embodiment such that that voltage will be the threshold voltage for the argon. Treatment voltages in the range of 50V to 10,000V, and most preferably 200V-500V, may be utilized. 
   Referring to  FIG. 6A , device  110  includes a housing  112  formed of an insulating material such as glass, ceramic, siliciumoxid, PTFE or other high melting temperature material. A reservoir  120  housing a volume of argon gas is located in the housing&#39;s distal tip. A plunger  121  is disposed within the housing  112  and includes a wall  123 . The plunger is moveable to move the wall proximally and distally between positions  121 A and  121 B to change the volume of reservoir  120 . Plunger wall  123  is sealable against the interior wall of housing  112  so as to prevent leakage of the argon gas. 
   An elongate rod  126  extends through an opening (not shown) in plunger wall  123  and is fixed to the wall  123  such that the rod and wall can move as a single component. Rod  126  extends to the proximal end of the device  110  and thus may serve as the handle used to move the plunger  121  during use. 
   Internal electrode  122  is positioned within the reservoir  120  and is mounted to the distal end of rod  126  such that movement of the plunger  121  results in corresponding movement of the electrode  122 . Electrode  122  is electrically coupled to a conductor  124  that extends through rod  126  and that is electrically coupled to RF generator  128 . Rod  126  preferably serves as the insulator for conductor  124  and as such should be formed of an insulating material. 
   A return electrode  130  is disposed on the exterior surface of the housing  112  and is also electrically coupled to RF generator  128 . A plurality of ablation electrodes  132   a ,  132   b  etc. are positioned on the distal end of the housing  112 . 
   Operation of the embodiment of  FIGS. 6A-6B  is similar to that described with respect to  FIGS. 5A-5B , and so most of that description will not be repeated. Operation differs in that use of the second embodiment includes the preliminary step of moving rod  126  proximally or distally to place plunger wall  123  and electrode  122  into positions that will yield a desired voltage threshold for the argon gas. Moving the plunger in a distal direction (towards the electrodes  132   a - 132   c ) will decrease the volume of the reservoir and accordingly will increase the pressure of the argon within the reservoir and vice versa. Increases in argon pressure result in increased voltage threshold, while decreases in argon pressure result in decreased voltage threshold. 
   Moving the plunger  126  will also increase or decrease the distance d between electrode  122  and electrodes  132   a - 132   c . Increases in the distance d increase the voltage threshold and vice versa. 
   The rod  126  preferably is marked with calibrations showing the voltage threshold that would be established using each position of the plunger. This will allow the user to move the rod  126  inwardly (to increase argon pressure but decrease distance d) or outwardly (to decrease argon pressure but increase distance d) to a position that will give a threshold voltage corresponding to the voltage desired to be applied to the tissue to be ablated. Because the argon will not ignite into a plasma until the threshold voltage is reached, current will not flow to the electrodes  132   a ,  132   b  etc. until the pre-selected threshold voltage is reached. Thus, there is no unnecessary resistive tissue heating during the rise-time of the voltage. 
   Alternatively, the  FIG. 6A  embodiment may be configured such that plunger  121  and rod  126  may be moved independently of one another, so that argon pressure and the distance d may be adjusted independently of one another. Thus, if an increase in voltage threshold is desired, plunger wall  123  may be moved distally to increase argon pressure, or rod  126  may be moved proximally to increase the separation distance between electrode  122  and  132   a - 132   c . Likewise, a decrease in voltage threshold may be achieved by moving plunger wall  123  proximally to decrease argon pressure, or by moving rod  126  distally to decrease the separation distance d. If such a modification to the  FIG. 6A  was employed, a separate actuator would be attached to plunger  121  to allow the user to move the wall  123 , and the plunger  126  would be slidable relative to the opening in the wall  123  through which it extends. 
   During use of the embodiment of  FIGS. 6A and 6B , it may be desirable to maintain a constant argon pressure despite increases in temperature. As discussed in connection with the method of  FIGS. 5A-5D , eventual increases in temperature and pressure cause the voltage needed to ignite the argon to increase above the voltage being applied by the RF generator, resulting in termination of conduction of the electrodes. In the  FIG. 6A  embodiment, the pressure of the argon can be maintained despite increases in temperature by withdrawing plunger  121  gradually as the argon temperature increases. By maintaining the argon pressure, the threshold voltage of the argon is also maintained, and so argon plasma will continue to conduct current to the electrodes  132   a ,  132   b  etc. This may be performed with or without moving the electrode  122 . Alternatively, the position of electrode  122  may be changed during use so as to maintain a constant voltage threshold despite argon temperature increases. 
     FIGS. 7A and 7B  show an alternative embodiment of an ablation device  210  that is similar to the device of  FIGS. 6A and 6B . In this embodiment, argon is sealed within reservoir  220  by a wall  217 . Rather than utilizing a plunger (such as plunger  121  in  FIG. 6A ) to change the volume of reservoir  220 , the  FIGS. 7A-7B  embodiment utilizes bellows  221  formed into the sidewalls of housing  212 . A pullwire  226  (which may double as the insulation for conductor  224 ) extends through internal electrode  222  and is anchored to the distal end of the housing  212 . The bellows may be moved to the contracted position shown in  FIG. 7A , the expanded position shown in  FIG. 7B , or any intermediate position between them. 
   Pulling the pullwire  226  collapses the bellows into a contracted position as shown in  FIG. 7A  and increases the pressure of the argon within the reservoir  220 . Advancing the pullwire  226  expands the bellows as shown in  FIG. 7B , thereby decreasing the pressure of the argon. The pullwire and bellows may be used to pre-select the threshold voltage, since (for a given temperature) increasing the argon pressure increases the threshold voltage of the argon and vice versa. Once the threshold voltage has been pre-set, operation is similar to that of the previous embodiments. It should be noted that in the third embodiment, the distance between electrode  222  and ablation electrodes  232   a - c  remains fixed, although the device may be modified to allow the user to adjust this distance and to provide an additional mechanism for adjusting the voltage threshold of the device. 
   An added advantage of the embodiment of  FIG. 7A  is that the device may be configured to permit the bellows  221  to expand in response to increased argon pressure within the reservoir. This will maintain the argon pressure, and thus the threshold voltage of the argon, at a fairly constant level despite temperature increases within reservoir  220 . Thus, argon plasma will continue to conduct current to the electrodes  132   a    132   b  etc and ablation may be continued, as it will be a longer period of time until the threshold voltage of the argon exceeds the voltage applied by the RF generator. 
     FIGS. 8A through 13B  are a series of embodiments that also utilize argon, but that maintain a fixed reservoir volume for the argon. In each of these embodiments, current is conducted from an internal electrode within the argon reservoir to external ablation electrodes once the voltage of the internal electrode reaches the threshold voltage of the argon gas. 
   Referring to  FIGS. 8A and 8B , the fourth embodiment of an ablation device utilizes a housing  312  formed of insulating material, overlaying a conductive member  314 . Housing  312  includes exposed regions  332  in which the insulating material is removed to expose the underlying conductive member  314 . An enclosed reservoir  320  within the housing  212  contains argon gas, and an RF electrode member  322  is positioned within the reservoir. A return electrode (not shown) is attached to the patient. The fourth embodiment operates in the manner described with respect to  FIGS. 5A-5D , except that the current returns to the RF generator via the return electrode on the patient&#39;s body rather than via one on the device itself. 
   The fifth embodiment shown in  FIGS. 9A and 9B  is similar in structure and operation to the fourth embodiment. A conductive member  414  is positioned beneath insulated housing  412 , and openings in the housing expose electrode regions  432  of the conductive member  414 . The fifth embodiment differs from the fourth embodiment in that it is a bipolar device having a return electrode  430  formed over the insulated housing  412 . Return electrode  430  is coupled to the RF generator and is cutaway in the same regions in which housing  412  is cutaway; so as to expose the underlying conductor. 
   Internal electrode  422  is disposed within argon gas reservoir  420 . During use, electrode regions  432  are placed into contact with body tissue to be ablated. The RF generator is switched on and begins to build the voltage of electrode  422  relative to ablation electrode regions  432 . As with the previous embodiments, conduction of ablation energy from electrode  422  to electrode regions  432  will only begin once electrode  422  reaches the voltage threshold at which the argon in reservoir  420  ignites to form a plasma. Current passes through the tissue undergoing ablation and to the return electrode  430  on the device exterior. 
   The sixth embodiment shown in  FIG. 10  is similar in structure and operation to the fifth embodiment, and thus includes a conductive member  514 , an insulated housing  512  over the conductive member  512  and having openings to expose regions  532  of the conductive member. A return electrode  530  is formed over the housing  512 , and an internal electrode  522  is positioned within a reservoir  520  containing a fixed volume of argon. The sixth embodiment differs from the fifth embodiment in that the exposed regions  532  of the conductive member  514  protrude through the housing  512  as shown. This is beneficial in that it improves contact between the exposed regions  532  and the target body tissue. 
   A seventh embodiment is shown in  FIGS. 11A through 11C . As with the sixth embodiment, this embodiment includes an insulated housing  612  (such as a heat resistant glass or ceramic) formed over a conductive member  614 , and openings in the insulated housing  612  to expose elevated electrode regions  632  of the conductive member  614 . A return electrode  630  is formed over the housing  612 . An internal electrode  622  is positioned within a reservoir  620  containing a fixed volume of argon. 
   The seventh embodiment differs from the sixth embodiment in that there is an annular gap  633  between the insulated housing  612  and the elevated regions  632  of the conductive member  614 . Annular gap  633  is fluidly coupled to a source of suction and/or to an irrigation supply. During use, suction may be applied via gap  633  to remove ablation byproducts (e.g. tissue and other debris) and/or to improve electrode contact by drawing tissue into the annular regions between electrode regions  632  and ground electrode  630 . An irrigation gas or fluid may also be introduced via gap  633  during use so as to flush ablation byproducts from the device and to cool the ablation tip and the body tissue. Conductive or non-conductive fluid may be utilized periodically during the ablation procedure to flush the system. 
   Annular gap  633  may also be used to deliver argon gas into contact with the electrodes  632 . When the voltage of the electrode regions  632  reaches the threshold of argon delivered through the gap  633 , the resulting argon plasma will conduct from electrode regions  632  to the ground electrode  630 , causing lateral sparking between the electrodes  632 ,  630 . The resulting sparks create an “electrical file” which cuts the surrounding body tissue. 
   An eighth embodiment of an ablation device is shown in  FIGS. 12A and 12B . This device  710  is similar to the device of the fifth embodiment,  FIGS. 9A and 9B , in a number of ways. In particular, device  710  includes a conductive member  714  positioned beneath insulated housing  712 , and openings in the housing which expose electrode regions  732  of the conductive member  714 . A return electrode  730  is formed over the insulated housing  712 . Internal electrode  722  is disposed within an argon gas reservoir  720  having a fixed volume. 
   The eighth embodiment additionally includes a pair of telescoping tubular jackets  740 ,  742 . Inner jacket  740  has a lower insulating surface  744  and an upper conductive surface  746  that serves as a second return electrode. Inner jacket  740  is longitudinally slidable between proximal position  740 A and distal position  740 B. 
   Outer jacket  742  is formed of insulating material and is slidable longitudinally between position  742 A and distal position  742 B. 
   A first annular gap  748  is formed beneath inner jacket  740  and a second annular gap  750  is formed between the inner and outer jackets  740 ,  742 . These gaps may be used to deliver suction or irrigation to the ablation site to remove ablation byproducts. 
   The eighth embodiment may be used in a variety of ways. As a first example, jackets  740 ,  742  may be moved distally to expose less than all of tip electrode assembly (i.e. the region at which the conductive regions  732  are located). This allows the user to expose only enough of the conductive regions  732  as is needed to cover the area to be ablated within the body. 
   Secondly, in the event bleeding occurs at the ablation site, return electrode surface  730  may be used as a large surface area coagulation electrode, with return electrode surface  746  serving as the return electrode, so as to coagulate the tissue and to thus stop the bleeding. Outer jacket  742  may be moved proximally or distally to increase or decrease the surface area of electrode  746 . Moving it proximally has the effect of reducing the energy density at the return electrode  746 , allowing power to be increased to carry out the coagulation without increasing thermal treatment effects at return electrode  746 . 
   Alternatively, in the event coagulation and/or is needed, electrode  730  may be used for surface coagulation in combination with a return patch placed into contact with the patient. 
     FIGS. 13A-13B  show a ninth embodiment of an ablation device utilizing principles of the present invention. The ninth embodiment includes an insulated housing  812  having an argon gas reservoir  820  of fixed volume. A plurality of ablation electrodes  832  are embedded in the walls of the housing  812  such that they are exposed to the argon in reservoir  832  and exposed on the exterior of the device for contact with body tissue. A return electrode  830  is formed over the housing  812 , but includes openings through which the electrodes  832  extend. An annular gap  833  lies between return electrode  830  and housing  812 . As with previous embodiments, suction and/or irrigation may be provided through the gap  833 . Additionally, argon gas may be introduced through the annular gap  833  and into contact with the electrodes  832  and body tissue so as to allow argon gas ablation to be performed. 
   An internal electrode  822  is positioned within reservoir  820 . Electrode  822  is asymmetrical in shape, having a curved surface  822   a  forming an arc of a circle and a pair of straight surfaces  822   b  forming radii of the circle. As a result of its shape, the curved surface of the electrode  820  is always closer to the electrodes  832  than the straight surfaces. Naturally, other shapes that achieve this effect may alternatively be utilized. 
   Electrode  822  is rotatable about a longitudinal axis and can also be moved longitudinally as indicated by arrows in  FIGS. 13A and 13B . Rotation and longitudinal movement can be carried out simultaneously or separately. This allows the user to selectively position the surface  822   a  in proximity to a select group of the electrodes  832 . For example, referring to  FIGS. 13A and 13B , when electrode  822  is positioned as shown, curved surface  822   a  is near electrodes  832   a , whereas no part of the electrode  822  is close to the other groups of electrodes  832   b - 832   d.    
   As discussed earlier, the voltage threshold required to cause conduction between internal electrode  822  and ablation electrodes  832  will decrease with a decrease in distance between the electrodes. Thus, there will be a lower threshold voltage between electrode  822  and the ablation electrodes (e.g. electrode  832   a ) adjacent to surface  822   a  than there is between the electrode  822  and ablation electrodes that are farther away (e.g. electrodes  832   b - d . The dimensions of the electrode  822  and the voltage applied to electrode  822  are such that a plasma can only be established between the surface  822   a  and the electrodes it is close to. Thus, for example, when surface  822   a  is adjacent to electrodes  832   a  as shown in the drawings, the voltage threshold between the electrodes  822   a  and  832   a  is low enough that the voltage applied to electrode  822  will cause plasma conduction to electrodes  832   a . However, the threshold between electrode  822  and the other electrodes  832   b - d  will remain above the voltage applied to electrode  822 , and so there will be no conduction to those electrodes. 
   This embodiment thus allows the user to selectively ablate regions of tissue by positioning the electrode surface  822   a  close to electrodes in contact with the regions at which ablation is desired. 
     FIG. 14A  shows a tenth embodiment of an ablation device utilizing voltage threshold principles. The tenth embodiment includes a housing  912  having a sealed distal end containing argon. Ablation electrodes  932   a - c  are positioned on the exterior of the housing  912 . An internal electrode  22  is disposed in the sealed distal end. Positioned between the internal electrode  922  and the electrodes  932   a - c  is a conductive grid  933 . 
   When electrode  922  is energized, there will be no conduction from electrode  922  to electrodes  932   a - c  until the potential between electrode  922  and the body tissue/fluid in contact with electrodes  932   a - c  reaches an initiating threshold voltage at which the argon gas will form a conductive plasma. The exact initiating threshold voltage is dependent on the argon pressure, its flowrate (if it is circulating within the device), and the distance between electrode  922  and the tissue/body fluid in contact with the ablation electrodes  932   a - c.    
   Because the RF generator voltage output varies sinusoidally with time, there are phases along the RF generator output cycle at which the RF generator voltage will drop below the voltage threshold. However, once the plasma has been ignited, the presence of energized plasma ions in the argon will maintain conduction even after the potential between electrode  922  and the body fluid/tissue has been fallen below the initiating threshold voltage. In other words, there is a threshold sustaining voltage that is below the initiating threshold voltage, but that will sustain plasma conduction. 
   In the embodiment of  FIG. 14A , the grid  933  is spaced from the electrodes  932   a - c  by a distance at which the corresponding plasma ignition threshold is a suitable ablation voltage for the application to which the ablation device is to be used. Moreover, the electrode  922  is positioned such that once the plasma is ignited, grid  933  may be deactivated and electrode  922  will continue to maintain a potential equal to or above the sustaining voltage for the plasma. Thus, during use, both grid  933  and electrode  922  are initially activated for plasma formation. Once the potential between grid  933  and body tissue/fluid reaches the threshold voltage and the plasma ignites, grid  933  will be deactivated. Because ions are present in the plasma at this point, conduction will continue at the sustaining threshold voltage provided by electrode  922 . 
   The ability of ionized gas molecules in the argon to sustain conduction even after the potential applied to the internal electrode has fallen below the initiating threshold voltage can be undesirable. As discussed, an important aspect of voltage threshold ablation is that it allows for high voltage/low current ablation. Using the embodiments described herein, a voltage considered desirable for the application is selected as the threshold voltage. Because the ablation electrodes are prevented from conducting when the voltage delivered by the RF generator is below the threshold voltage, there is no conduction to the ablation electrode during the rise time from 0V to the voltage threshold. Thus, there is no resistive heating of the tissue during the period in which the RF generator voltage is rising towards the threshold voltage. 
   Under ideal circumstances, conduction would discontinue during the periods in which the RF generator voltage is below the threshold. However, since ionized gas remains in the argon reservoir, conduction can continue at voltages below the threshold voltage. Referring to  FIG. 4A , this results in the sloping trailing edge of the ablation voltage waveform, which approximates the trailing portion of the sinusoidal waveform produced by the RF generator ( FIG. 3 ). This low-voltage conduction to the tissue causes resistive heating of the tissue when only high voltage ablation is desired. 
   The grid embodiment of  FIG. 14A  may be used to counter the effect of continued conduction so as to minimize collateral damage resulting from tissue heating. During use of the grid embodiment, the trailing edge of the ablation voltage waveform is straightened by reversing the polarity of grid electrode  933  after the RF generator has reached its peak voltage. This results in formation of a reverse field within the argon, which prevents the plasma flow of ions within the argon gas and that thus greatly reduces conduction. This steepens the slop of the trailing edge of the ablation potential waveform, causing a more rapid drop towards 0V, such that it approximates the waveform shown in  FIG. 4B . 
     FIGS. 15A and 15B  show an eleventh embodiment utilizing principles of the present invention. As with the tenth embodiment, the eleventh embodiment is advantageous in that it utilizes a mechanism for steepening the trailing edge of the ablation waveform, thus minimizing conduction during periods when the voltage is below the threshold voltage. In the eleventh embodiment, this is accomplished by circulating the argon gas through the device so as to continuously flush a portion of the ionized gas molecules away from the ablation electrodes. 
   The eleventh embodiment includes a housing  1012  having an ablation electrodes  1032 . An internal electrode  1022  is positioned within the housing  1012  and is preferably formed of conductive hypotube having insulation  1033  formed over all but the distal-most region. A fluid lumen  1035  is formed in the hypotube and provides the conduit through which argon flows into the distal region of housing  1012 . Flowing argon exits the housing through the lumen in the housing  1012 , as indicated by arrows in  FIG. 15A . A pump  1031  drives the argon flow through the housing. 
   It should be noted that different gases will have different threshold voltages when used under identical conditions. Thus, during use of the present invention the user may select a gas for the spark gap switch that will have a desired threshold voltage. A single type of gas (e.g. argon) may be circulated through the system, or a plurality of gases from sources  1033   a - c  may be mixed by a mixer pump  1031   a  as shown in  FIG. 15C , for circulation through the system and through the spark gap switch  1035 . Mixing of gases is desirable in that it allows a gas mixture to be created that has a threshold voltage corresponding to the desired treatment voltage. In all of the systems using circulated gas, gas leaving the system may be recycled through, and/or exhausted from, the system after it makes a pass through the spark gap switch. 
     FIGS. 16A through 16D  schematically illustrate the effect of circulating the argon gas through the device of  FIG. 15A . Circulation preferably is carried out at a rate of approximately 0.1 liters/minute to 0.8 liters/minute. 
   Referring to  FIG. 16A , during initial activation of the RF generator, the potential between internal electrode  1022  and ablation electrode  1032  is insufficient to create an argon plasma. Argon molecules are thus non-ionized, and the voltage measured at the load L is 0V. There is no conduction from electrode  1022  to electrode  1032  at this time. 
     FIG. 16B  shows the load voltage measured from internal electrode  1022  across the body fluid/tissue to return electrode  1030 . Once the RF generator voltage output reaches voltage threshold V T  of the argon, argon molecules are ionized to create a plasma. A stream of the ionized molecules flows from electrode  1022  to electrode  1032  and current is conducted from electrode  1032  to the tissue. Because the argon is flowing, some of the ionized molecules are carried away. Nevertheless, because of the high voltage, the population of ionized molecules is increasing at this point, and more than compensates for those that flow away, causing an expanding plasma within the device. 
   After the RF generator voltage falls below V T , ion generation stops. Ionized molecules within the argon pool flow away as the argon is circulated, and others of the ions die off. Thus, the plasma begins collapsing and conduction to the ablation electrodes decreases and eventually stops. See  FIGS. 16C and 16D . The process then repeats as the RF generator voltage approaches (−V T ) during the negative phase of its sinusoidal cycle. 
   Circulating the argon minimizes the number of ionized molecules that remain in the space between electrode  1022  and electrode  1032 . If a high population of ionized molecules remained in this region of the device, their presence would result in conduction throughout the cycle, and the voltage at the tissue/fluid load L would eventually resemble the sinusoidal output of the RF generator. This continuous conduction at low voltages would result in collateral heating of the tissue. 
   Naturally, the speed with which ionized molecules are carried away increases with increased argon flow rate. For this reason, there will be more straightening of the trailing edge of the ablation waveform with higher argon flow rates than with lower argon flow rates. This is illustrated graphically in  FIG. 17 . The upper waveform shows the RF generator output voltage. The center waveform is the voltage output measured across the load (i.e. from the external electrode  1032  across the body tissue/fluid to the return electrode  1030 ) for a device in which the argon gas is slowly circulated. The lower waveform is the voltage output measured across the load for a device in which the argon gas is rapidly circulated. It is evident from the  FIG. 17  graphs that the sloped trailing edge of the ablation waveform remains when the argon is circulated at a relatively low flow rate, whereas the trailing edge falls off more steeply when a relatively high flow rate is utilized. This steep trailing edge corresponds to minimized current conduction during low voltage phases. Flow rates that achieve the maximum benefit of straightening the trailing edge of the waveform are preferable. It should be noted that flow rates that are too high can interfere with conduction by flushing too many ionized molecules away during phases of the cycle when the output is at the threshold voltage. Optimal flow rates will depend on other physical characteristics of the device, such as the spark gap distance and electrode arrangement. 
   It should also be noted that the distance between internal electrode  1022  and external electrode  1032  also has an effect on the trailing edge of the ablation potential waveform. In the graphs of  FIG. 18 , the RF generator output is shown in the upper graph. V PRFG  represents the peak voltage output of the RF generator, V T1  represents the voltage threshold of a device having a large separation distance (e.g. approximately 1 mm) between electrodes  1022  and  1032 , and V T2  represents the voltage threshold of a device in which electrodes  1022 ,  1032  are closely spaced—e.g. by a distance of approximately 0.1 mm. As previously explained, there is a higher voltage threshold in a device with a larger separation distance between the electrodes. This is because there is a large population of argon molecules between the electrodes  1022 ,  1032  that must be stripped of electrons before plasma conduction will occur. Conversely, when the separation distance between electrodes  1022  and  1032  is small, there is a smaller population of argon molecules between them, and so less energy is needed to ionize the molecules to create plasma conduction. 
   When the RF generator output falls below the threshold voltage, the molecules begin to deionize. When there are fewer ionized molecules to begin with, as is the case in configurations having a small electrode separation distance, the load voltage is more sensitive to the deionization of molecules, and so the trailing edge of the output waveform falls steeply during this phase of the cycle. 
   For applications in which a low voltage threshold is desirable, the device may be configured to have a small electrode spacing (e.g. in the range of 0.001-5 mm, most preferably 0.05-0.5 mm) and non-circulating argon. As discussed, doing so can produce a load output waveform having a steep rising edge and a steep falling edge, both of which are desirable characteristics. If a higher voltage threshold is needed, circulating the argon in a device with close inter-electrode spacing will increase the voltage threshold by increasing the pressure of the argon. This will yield a highly dense population of charged ions during the phase of the cycle when the RF generator voltage is above the threshold voltage, but the high flow rate will quickly wash many ions away, causing a steep decline in the output waveform during the phases of the cycle when the RF generator voltage is below the threshold. 
   A twelfth embodiment of a system utilizing principles of the present invention is shown schematically in  FIG. 19 . The twelfth embodiment allows the threshold voltage to be adjusted by permitting the spark gap spacing (i.e. the effective spacing between the internal electrode and the ablation electrode) to be selected. It utilizes a gas-filled spark gap switch  1135  having a plurality of internal spark gap electrodes  1122   a ,  1122   b ,  1122   c . Each internal electrode is spaced from ablation electrode  1132  by a different distance, D 1 , D 2 , D 3 , respectively. An adjustment switch  1125  allows the user to select which of the internal electrodes  1122   a ,  1122   b ,  1122   c  to utilize during a procedure. Since the threshold voltage of a spark gap switch will vary with the distance between the internal electrode and the contact electrode, the user will select an internal electrode, which will set the spark gap switch to have the desired threshold voltage. If a higher threshold voltage is used, electrode  1122   a  will be utilized, so that the larger spark gap spacing D 1  will give a higher threshold voltage. Conversely, the user will selected electrode  1122   c , with the smaller spark gap spacing, if a lower threshold voltage is needed. 
   It is useful to mention that while the spark gap switch has been primarily described as being positioned within the ablation device, it should be noted that spark gap switches may be positioned elsewhere within the system without departing with the scope of the present invention. For example, referring to  FIG. 19 , the spark gap switch  1135  may be configured such that the ablation electrode  1132  disposed within the spark gap is the remote proximal end of a conductive wire that is electrically coupled to the actual patient contact portion of the ablation electrode positioned into contact with body tissue. A spark gap switch of this type may be located in the RF generator, in the handle of the ablation device, or in the conductors extending between the RF generator and the ablation device. 
     FIGS. 20-26  illustrate additional embodiments of a surgical probe that utilizes voltage threshold means for controlling ablative energy delivery to tissue at a targeted site. In general,  FIG. 20  depicts an exemplary probe  1200  with handle portion  1202  coupled to extension member  1204  that supports working end  1205 . The working end  1205  can have any suitable geometry and orientation relative to axis  1208  and is shown as an axially-extending end for convenience. A hand-held probe  1200  as in  FIG. 20  can be used to move or paint across tissue to ablate the tissue surface, whether in an endoscopic treatment within a fluid as in arthroscopy, or in a surface tissue treatment in air. In this embodiment, the exterior sheath  1206  is an insulator material ( FIG. 21 ) and the probe is adapted to function in a mono-polar manner by cooperating with a ground pad  1208  coupled to the targeted tissue TT (see  FIGS. 20 and 21 ). The system also can operate in a bi-polar manner by which is meant the working end itself carries a return electrode, as will be illustrated in  FIG. 26  below. 
   Referring to  FIGS. 20 and 21 , the working end  1205  comprises a microporous ceramic body  1210  that cooperates with an interior voltage threshold mechanism or spark gap switch as described above. In one embodiment in  FIG. 21 , the ceramic body  1210  has interior chamber  1215  that receives a flowable, ionizable gas that flows from a pressurized gas source  1220  and is extracted by a negative pressure source  1225 . In this embodiment, it can be seen that gas flows through interior lumen  1228  in conductive sleeve  1230 . The gas is then extracted through concentric lumen  1235  that communicates with negative pressure source  1225  as indicated by the gas flow arrows F in  FIG. 21 . Any suitable spacer elements  1236  (phantom view) can support the conductive sleeve  1230  within the probe body to maintain the arrangement of components to provide the gas inflow and outflow pathways. As can be seen in  FIG. 21 , the conductive sleeve  1230  is coupled by electrical lead  1238  to electrical source  1240  to allow its function and as electrode component with the distal termination  1241  of sleeve  1230  on one side of a spark gap indicated at SG. 
   The interior surface  1242  of ceramic body  1210  carries an interior electrode  1244 A at the interior of the microporous ceramic. As can be seen in enlarged cut-away view of  FIG. 22 , the ceramic has a microporous working surface  1245  wherein a micropore network  1248  extends through the thickness TH of the ceramic body surface overlying the interior electrode  1244 A. The sectional view of  FIG. 21  illustrates the pore network  1248  extending from working surface  1245  to the interior electrode  1244 A. The function of the pore network  1248  is to provide a generally defined volume or dimension of a gas within a plurality of pores or pathways between interior electrode  1244 A and the targeted tissue site TT. Of particular importance, the cross-sectional dimensions of the pores is selected to insure that the pores remain free of fluid ingress in normal operating pressures of an underwater surgery (e.g., arthroscopy) or even moisture ingress in other surgeries in a normal air environment. It has been found that the mean pore cross-section of less than about 10 microns provides a suitable working surface  1245  for tissue ablation; and more preferably a mean pore cross-section of less than about 5 microns. Still more preferably, the mean pore cross-section is less than about 1 micron. In any event, the microporous ceramic allows for electrical energy coupling across and through the pore network  1248  between the interior electrode  1244 A and the targeted tissue site TT, but at the same time the microporous ceramic is impervious to liquid migration therein under pressures of a normal operating environment. This liquid-impervious property insures that electrical energy will ablatively arc through the pore network  1248  rather than coupling with water or moisture within the pore network during operation. 
   In  FIG. 21 , it also can be seen that working surface  1245  is defined as a limited surface region of the ceramic that is microporous. The working end  1205  has a ceramic glaze  1250  that covers the exterior of the ceramic body except for the active working surface  1245 . Referring now to  FIG. 22 , the thickness TH of the microporous ceramic body also is important for controlling the ablative energy-tissue interaction. The thickness TH of the ceramic working surface can range from as little as about 5 microns to as much as about 1000 microns. More preferably, the thickness TH is from about 50 microns to 500 microns. 
   The microporous ceramic body  1210  of  FIGS. 20-22  can be fabricated of any suitable ceramic in which the fabrication process can produce a hard ceramic with structural integrity that has substantially uniform dimension, interconnected pores extending about a network of the body—with the mean pore dimensions described above. Many types of microporous ceramics have been developed for gas filtering industry and the fabrication processed can be the same for the ceramic body of the invention. It has been found that a ceramic of about 90%-98% alumina that is fired for an appropriate time and temperature can produce the pore network  1248  and working surface thickness TH required for the ceramic body to practice the method the invention. Ceramic micromolding techniques can be used to fabricate the net shape ceramic body as depicted in  FIG. 21 . 
   In  FIGS. 21 and 22 , it can be understood how the spark gap SG (not-to-scale) between conductor sleeve  1230  and the interior electrode  1244 A can function to provide cycle-to-cycle control of voltage applied to the electrode  1244 A and thus to the targeted treatment site to ablate tissue. As can be understood in  FIG. 22 , a gas flow F of a gas (e.g., argon) flows through the interior of the ceramic body to flush ionized gases therefrom to insure that voltage threshold mechanism functions optimally, as described above. 
     FIG. 23  illustrates another embodiment of working end that included multiple conductor sleeves portions  1230  and  1230 ′ that are spaced apart by insulator  1252  and define different gap dimensions from distal surface  1241  and  1241 ′ to interior electrode  1244 A. It can be understood that the multiple conductor sleeves portions  1230  and  1230 ′, that can range from 2 to 5 or more, can be selected by controller  1255  to allow a change in the selected dimension of the spark gap indicated at SG and SG′. The dimension of the spark gap will change the voltage threshold to thereby change the parameter of ablative energy applied to the targeted tissue, which can be understood from the above detailed description. 
     FIG. 24  illustrates a greatly enlarged cut-away view of an alternative microporous ceramic body  1210  wherein the interior electrode  1244 B also is microporous to cooperate with the microporous ceramic body  1210  in optimizing electrical energy application across and through the pore network  1248 . In this embodiment, the spark gap again is indicated at SG and defines the dimension between distal termination  1241  of conductor sleeve  1230  and the electrode  1244 B. The porous electrode  1244 B can be any thin film with ordered or random porosities fabricated therein and then bonded or adhered to ceramic body  1210 . The porous electrode also can be a porous metal that is known in the art. Alternatively, the porous electrode  1224 B can be vapor deposited on the porous surface of the ceramic body. Still another alternative that falls within the scope of the invention is a ceramic-metal composite material that can be formed to cooperate with the microporous ceramic body  1210 . 
     FIG. 24  again illustrates that a gas flow indicated by arrows F will flush ionized gases from the interior of the ceramic body  1210 . At the same time, however, the pores  1258  in electrode  1244 B allow a gas flow indicated at F′ to propagate through pore network  1248  in the ceramic body to exit the working surface  1245 . This gas flow F′ thus can continuously flush the ionized gases from the pore network  1248  to insure that arc-like electrical energy will be applied to tissue from interior electrode  1244 B through the pore network  1248 —rather than having electrical energy coupled to tissue through ionized gases captured and still resident in the pore network from a previous cycle of energy application. It can be understood that the percentage of total gas flow F that cycles through interior chamber  1215  and the percentage of gas flow GF′ that exits through the pore network  1248  can be optimized by adjusting (i) the dimensions of pores  1258  in electrode  1244 B; (ii) the mean pore dimension in the ceramic body  1210 , the thickness of the ceramic working surface and mean pore length, (iv) inflow gas pressure; and (v) extraction pressure of the negative pressure source. A particular probe for a particular application thus will be designed, in part by modeling and experimentation, to determine the optimal pressures and geometries to deliver the desired ablative energy parameters through the working surface  1245 . This optimization process is directed to provide flushing of ionized gas from the spark gap at the interior chamber  1215  of the probe, as well as to provide flushing of the micropore network  1248 . In this embodiment, the micropore network  1248  can be considered to function as a secondary spark gap to apply energy from electrode  1224 B to the targeted tissue site TT. 
   In another embodiment depicted in  FIG. 25 , it should be appreciated that the spark gap interior chamber  1215 ′ also can be further interior of the microporous ceramic working surface  1245 . For example,  FIG. 25  illustrates a microprobe working end  1260  wherein it may be impractical to circulate gas to a needle-dimension probe tip  1262 . In this case, the interior chamber  1215 ′ can be located more proximally in a larger cross-section portion of the probe. The working end of  FIG. 25  is similar to that of  FIG. 21  in that gas flows F are not used to flush ionized gases from the pore network  1248 . 
     FIG. 26  illustrates another embodiment of probe  1270  that has the same components as in  FIGS. 22 and 24  for causing electrical energy delivery through an open pore network  1248  in a substantially thin microporous ceramic body  1210 . In addition, the probe  1270  carries a return electrode  1275  at an exterior of the working end for providing a probe that functions in a manner generally described as a bi-polar energy delivery. In other words, the interior electrode  1244 A or  1244 B comprises a first polarity electrode (indicated at (+)) and the return electrode  1275  (indicated at (−)) about the exterior of the working end comprises a second polarity electrode. This differs from the embodiment of  FIG. 21 , for example, wherein the second polarity electrode is a ground pad indicated at  1208 . The bi-polar probe  1270  that utilizes voltage threshold energy delivery through a microporous ceramic is useful for surgeries in a liquid environment, as in arthroscopy. It should be appreciated that the return electrode  1275  can be located in any location, or a plurality of locations, about the exterior of the working end and fall within the scope of the invention. 
   The probe  1270  of  FIG. 26  further illustrates another feature that provided enhanced safety for surgical probe that utilizes voltage threshold energy delivery. The probe has a secondary or safety spark gap  1277  in a more proximal location spaced apart a selected dimension SD from the interior spark gap indicated at SG. The secondary spark gap  1277  also defines a selected dimension between the first and second polarity electrodes  1230  and  1275 . As can be seen in  FIG. 26 , the secondary spark gap  1277  consists of an aperture in the ceramic body  1210  or other insulator that is disposed between the opposing polarity electrodes. In the event that the primary spark gap SG in the interior chamber  1215  is not functioning optimally during use, any extraordinary current flows can jump the secondary spark gap  1277  to complete the circuit. The dimension across the secondary spark gap  1277  is selected to insure that during normal operations, the secondary spark gap  1277  maintains a passive role without energy jumping through the gap. 
   Several embodiments of voltage threshold ablation systems, and methods of using them, have been described herein. It should be understood that these embodiments are described only by way of example and are not intended to limit the scope of the present invention. Modifications to these embodiments may be made without departing from the scope of the present invention, and features and steps described in connection with some of the embodiments may be combined with features described in others of the embodiments. Moreover, while the embodiments discuss the use of the devices and methods for tissue ablation, it should be appreciated that other electrosurgical procedures such as cutting and coagulation may be performed using the disclosed devices and methods. It is intended that the scope of the invention is to be construed by the language of the appended claims, rather than by the details of the disclosed embodiments.