Catheter for ablating tissue inside a human body

219642/2

CATHETER FOR ABLATING TISSUE INSIDE A HUMAN BODY

BACKGROUND OF THE INVENTION a. Field of the Invention [0001] The instant invention is directed toward a brush electrode and a method forusing the brush electrode for tissue ablation. In particular, the brush electrode of thepresent invention comprises a plurality of flexible filaments or bristles for applyingablative energy (e.g., RF energy) to target tissue during the formation of spot or continuouslinear lesions. b. Background Art [0002] It is well known that benefits may be gained by forming lesions in tissue if thedepth and location of the lesions being formed can be controlled. In particular, it can bedesirable to elevate tissue temperature to around 50°C until lesions are formed viacoagulation necrosis, which changes the electrical properties of the tissue. For example,when sufficiently deep lesions are formed at specific locations in cardiac tissue viacoagulation necrosis, undesirable atrial fibrillations may be lessened or eliminated.“Sufficiently deep” lesions means transmural lesions in some cardiac applications.

[0003] Several difficulties may be encountered, however, when attempting to formadequately-deep lesions at specific locations using some existing ablation electrodes. Forexample, when forming lesions with RF energy, high temperature gradients are oftenencountered in the vicinity of the electrode. At the edges of some existing electrodes areregions of very high current density, leading to large temperature gradients and hot spots.These “edge effects” may result in the formation of undesirable coagulum and charring ofthe surface tissue. For example, undesirable coagulum may begin to form when bloodreaches around 80°C for an appreciable length of time, and undesirable tissue charring anddesiccation may be seen when tissue reaches around 100°C for an appreciable length oftime. There two types of undesirable coagulum: coagulum that adheres to and damagesthe medical device; and coagulum blood clots or curds that may enter a patient'sbloodstream, possibly resulting in other health problems for the patient. Charring of thesurface tissue may also have deleterious effects on a patient.

[0004] As the temperature of the electrode is increased, the contact time required toform an adequately-deep lesion decreases, but the likelihood of charring surface tissue and 1 219642/2 forming undesirable coagulum increases. As the temperature of the electrode is decreased,the contact time required to form an adequately-deep lesion increases, but the likelihood ofcharring surface tissue and forming undesirable coagulum decreases. It is, therefore, abalancing act trying to ensure that tissue temperatures are adequately high for long enoughto create deep lesions, while still preventing or minimizing coagulum formation and/orcharring of the surface tissue. Active temperature control may help, but the placement ofthermocouples, for example, is tricky and setting the RF generator for a certain temperaturebecomes an empirical exercise as actual tissue temperatures are generally different fromthose recorded next to the electrode due to factors such as convection and catheter design.[0005] Another difficulty encountered with existing ablation electrodes is how toensure adequate tissue contact. Current techniques for creating continuous linear lesions inendocardial applications include, for example, dragging a conventional catheter on thetissue, using an array electrode, or using pre-formed electrodes. All of these devicescomprise rigid electrodes that do not always conform to the tissue surface, especially whensharp gradients and undulations are present, such as at the ostium of the pulmonary vein inthe left atrium and the isthmus of the right atrium. Consequently, continuous linear lesionsare difficult to achieve. When forming lesions in a heart, the beating of the heart furthercomplicates matters, making it difficult to keep adequate contact between the electrode andthe tissue for a sufficient length of time to form a desired lesion. With a rigid electrode, itcan be quite difficult to maintain sufficient contact pressure until an adequate lesion hasbeen formed. This problem is exacerbated on contoured or trabeculated surfaces. If thecontact between the electrode and the tissue cannot be properly maintained, a quality lesionis unlikely to be formed.

[0006] Catheters based upon a virtual electrode may address some of the difficulties,but these catheters often require high flow rates of conductive fluid (e.g., typically around70 milliliters per minute) to maintain effective cooling for high-power RF applications.The introduction of a large amount of conductive fluid into a patient's bloodstream mayhave detrimental effects on the patient.

[0007] Thus, there remains a need for an ablation catheter that address these issueswith the existing designs and that permits the formation of uniform, transmural spot andcontinuous linear lesions on smooth or contoured surfaces. 2 219642/2

BRIEF SUMMARY OF THE INVENTION

[0008] It is desirable to be able to form adequately-deep spot or continuous linearlesions in tissue while reducing the formation of undesirable coagulum and charring of thesurface tissue, while applying a reasonable amount of RF energy, while mitigatingelectrode-tissue contact problems, and/or while reducing the amount of conductive fluid(e.g., isotonic saline) possibly entering a patient's bloodstream during the procedure. Thepresent invention is an improved ablation electrode.

[0009] In one form, the present invention comprises a wet-brush electrode thatfacilitates electrode-tissue contact in target tissue having contoured surfaces. The wet-brush electrode comprises a plurality of flexible filaments adapted to transfer ablativeenergy to target tissue, the flexible filaments having longitudinal axes and defininginterstitial spaces among the plurality of filaments, wherein the interstitial spaces areadapted to direct conductive fluid predominantly parallel to the filament longitudinal axes.This wet-brush electrode also comprises a primary conductor operatively connected to, andadapted to transfer ablative energy to, the plurality of flexible filaments; and afluid-delivery means adapted to deliver conductive fluid to the interstitial spaces.

[0010] In another form, the present invention comprises a catheter for tissue ablation.The catheter comprises an outer sheath having a distal end and a brush electrode, the brushelectrode comprising (a) a plurality of flexible filaments adapted to transfer ablative energyto target tissue during lesion formation, wherein the flexible filaments extendfrom the distal end of the outer sheath; and (b) a primary conductor in electrical contactwith the plurality of filaments. Although the brush electrode may be merely fictionallyengaged with the distal end of the outer sheath, the catheter may also comprises anattachment means for physically securing the brush electrode to the distal end of the outersheath. The filaments may be conductive filaments and/or nonconductive filaments, andthe filaments may have nonuniform cross-sectional configurations (e.g., the may betapered). Further, nonconductive tips may be present at the distal ends of at least some ofthe flexible filaments.

[0011] In yet another form, the present invention comprises a catheter for ablatingtissue inside a human body. The catheter comprises an outer sheath having a distal end; aconforming electrode adapted to apply ablative energy to target tissue, the conformingelectrode comprises an embedded portion and an exposed portion, wherein the exposed 3 219642/2 portion has a distal end, wherein a working surface is present at the distal end of theexposed portion, and wherein the exposed portion extends from the distal end of the outersheath; and a primary conductor in direct electrical contact with the conforming electrodeand adapted to carry ablative energy from an energy source to the conforming electrode.The conforming electrode may comprise a dry or wetted brush electrode having a pluralityof flexible filaments. The flexible filaments may be trimmed to give a desired tipconfiguration or a desired standoff distance between the tissue and the conductivefilaments in the brush electrode. Also, the filaments may be grouped into clusters.[0012] In still another form, the present invention comprises a catheter for tissueablation, wherein the catheter includes a shielded-tip brush electrode. In particular, thecatheter comprises an outer sheath having a distal end; a shielded-tip brush electrode at thedistal end of the outer sheath, the shielded-tip brush electrode comprising (a) a bundle offilaments adapted to transfer ablative energy to target tissue during the formation of alesion, wherein the bundle of filaments extend from the distal end of the outer sheath, andwherein the bundle of filaments has an outer surface; and (b) a primary conductor havingan uninsulated portion, wherein the uninsulated portion is in electrical contact with theplurality of filaments. Attachment means may be present to secure the shielded-tip brushelectrode to the distal end of the outer sheath.

[0013] In another form, the present invention comprises a catheter having an outersheath with a distal end; an inner sheath with a distal end; an annular channel definedbetween the outer sheath and the inner sheath, wherein the annular channel is adapted tocarry fluid; a mechanical interface supported at least in part by the distal end of the innersheath; a flexible electrode adapted to apply ablative energy to target tissue, wherein theflexible electrode is supported by the mechanical interface, wherein the flexible electrodecomprises an embedded portion and an exposed portion, and wherein the exposed portionextends from the distal end of the outer sheath and comprises a working surface; a primaryconductor adapted to carry ablative energy from an energy source to the flexible electrode,wherein the primary conductor comprises an uninsulated portion in electrical contact withthe flexible electrode; and a flexible boot at the distal end of the outer sheath, theflexible boot defining an annular fluid jacket around a booted portion of the flexibleelectrode, wherein the booted portion comprises at least a portion of the exposed portion of 4 219642/2 the flexible electrode, and wherein the annular fluid jacket is adapted to carry fluid that isin fluid communication with the annular channel.

[0014] The present invention also comprises a method of ablating tissue inside ahuman body using a flexible brush electrode affixed at a distal end of an outer sheath of acatheter. The method comprising the steps of placing an exposed portion of the brushelectrode adjacent to tissue to be treated; applying ablative energy to the exposed portion ofthe brush electrode; and forming a lesion in the tissue via coagulation necrosis.

[0015] In each of the brush electrode embodiments described above, the filamentscomprising the brush have interstitial gaps between them. The interstitial gaps are adaptedto direct fluid, when present, toward the tissue being treated.

[0016] In each of the brush electrodes described above, a secondary lead may also bepresent and may have a device (e.g., a thermocouple, a pressure sensor, and an ultrasoundsensor) operatively connected with it.

[0017] The foregoing and other aspects, features, details, utilities, and advantages ofthe present invention will be apparent from reading the following description and claims,and from reviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Fig. 1 is an isometric view of one embodiment of a catheter having a brushelectrode according to the present invention, and depicts the filaments comprising thebrush electrode extending from a distal end of an outer sheath.

[0019] Fig. 2 is an enlarged view of the circled region of Fig. 1.

[0020] Fig. 3 is similar to Fig. 2, but depicts an alternative embodiment where the brush electrode is secured at the distal end of the outer sheath by at least one suture that iscovered by a section of shrink tube.

[0021] Fig. 4 is similar to Fig. 3, but a portion of the shrink tube has been brokenaway to reveal two sutures through the outer sheath.

[0022] Fig. 5 is an isometric, cross-sectional view of the catheter depicted in Figs. 3and 4, taken along line 5-5 of Fig. 3, revealing a primary conductor making electricalcontact with the filaments comprising the brush electrode, and depicting a secondary lead(e.g., for a thermocouple) extending adjacent to the primary conductor and becomingembedded within the brush filaments. 5 219642/2 [0023] Figs. 6 and 7 depict steps that may be used to form the brush electrode depictedin, for example, Fig. 5.

[0024] Fig. 8 is similar to Fig. 5, but is a cross-sectional view of an alternativeembodiment of the brush electrode, wherein conductive filaments are interspersed amongrelatively longer nonconductive filaments.

[0025] Fig. 9 is a cross-sectional view taken along line 9-9 of Fig. 8.

[0026] Fig. 10 is an enlarged view of the circled region of Fig. 8.

[0027] Figs. 11-14 depict alternative shapes for the filaments comprising the tip of thebrush electrode.

[0028] Fig. 15 depicts an alternative embodiment of the filaments comprising thebrush electrode, wherein the individual filaments gradually taper toward their distal ends.

[0029] Fig. 16 depicts an alternative embodiment of the filaments comprising thebrush electrode, wherein the individual filaments have nonconductive tips at their distalends creating a stand-off distance.

[0030] Fig. 17 is a fragmentary, isometric view of an embodiment of the outer sheathhaving a concentric ring of sub-channels around a main or central channel through whichthe brush filaments extend.

[0031] Fig. 18 is a fragmentary, isometric view of an embodiment wherein the sheathsurrounding the filaments of the brush electrode is porous adjacent to the exposed portionof the brush electrode.

[0032] Fig. 19 is a fragmentary, isometric view of an embodiment wherein the sheathsurrounding the filaments of the brush electrode is a threaded sheath, having a spiral orhelical ridge on its outer surface, adjacent to the exposed portion of the brush electrode.[0033] Fig. 20 is a fragmentary view of a section of the threaded sheath depicted inFig. 19, surrounded by a covering shown in phantom and cross-section to create a helicalflow channel between the threaded sheath and the covering.

[0034] Fig. 21 is a fragmentary, isometric view of an embodiment wherein the sheathsurrounding the filaments of the brush electrode is a grooved sheath, having a plurality oflongitudinally-extending grooves or cuts on its outer surface, adjacent to the exposedportion of the brush electrode. 6 219642/2 [0035] Fig. 22 is a fragmentary view of a section of the grooved sheath depicted inFig. 21, surrounded by a covering (shown cross-section) to create a plurality oflongitudinally-extending flow channels between the grooved sheath and the covering.[0036] Fig. 23 is a cross-sectional view taken along line 23-23 of Fig. 21, with thecovering shown in phantom and with the longitudinally-extending flow channels clearlyvisible.

[0037] Fig. 24 is similar to Fig. 5, but depicts an isometric, cross-sectional view of acatheter wherein the primary conductor makes electrical contact with the filaments via anenergy transfer coil or spring surrounding at least the embedded portion of the brushelectrode.

[0038] Fig. 25 is similar to Figs. 5 and 24, but depicts an isometric, cross-sectionalview of a catheter wherein the primary conductor makes electrical contact with thefilaments via an energy transfer mesh or fabric surrounding at least the embedded portionof the brush electrode.

[0039] Fig. 26 is a cross-sectional view of a first embodiment of a shielded-tip brushelectrode, wherein an uninsulated portion of the primary conductor is looped around theouter surface of the brush electrode.

[0040] Fig. 27 is similar to Fig. 26, but depicts a second embodiment of a shielded-tipbrush electrode.

[0041] Figs. 28-35 depict different cross-sectional configurations for brush electrodesaccording to the present invention.

[0042] Fig. 36 is a cross-sectional view of a brush electrode wherein some ofthe filaments comprise hollow or porous members.

[0043] Fig. 37 is a cross-sectional view of a brush electrode having devices (e.g., athermocouple or other temperature sensor, a pressure sensor, or an ultrasound sensor)embedded among the conductive and nonconductive filaments.

[0044] Fig. 38 is an isometric view of a catheter having a brush electrode according tothe present invention forming a spot or point lesion on a section of tissue.

[0045] Fig. 39 is an isometric view of a catheter having a brush electrode according tothe present invention forming a linear or drag lesion on a section of tissue.

[0046] Figs. 40-42 depict a brush electrode according to the present invention formingdifferent-sized lesions based in part upon the amount of splay of the brush electrode. 7 219642/2

DETAILED DESCRIPTION OF THE INVENTION

[0047] Several embodiments of a brush electrode 10 according to the presentinvention are depicted in the figures. As described further below, the brush electrode ofthe present invention provides a number of advantages, including, for example, the abilityto form deep lesions in tissue while reducing the formation of undesirable coagulum andcharring of the surface tissue, while applying a reasonable amount of RF energy, whilemitigating electrode-tissue contact problems, and/or while reducing the amount ofconductive fluid (e.g., saline) possibly entering a patient's bloodstream during theprocedure. The present invention facilitates the formation of a deep lesion in a shorterperiod of time than required by other ablation devices, and it provides the ability to createlesions in highly perfused tissue or in fluid-rich environments. The brush electrode 10facilitates enhanced tissue contact in difficult environments (e.g., during ablation of acontoured or trabeculated surface inside a beating heart), whether creating a spot lesion 12(e.g., Fig. 38) or a continuous linear lesion 14 (e.g., Fig. 39), by readily conforming tosurface contours.

[0048] Fig. 1 is an isometric view of one embodiment of a catheter 16 having a brushelectrode 10 according to the present invention. As depicted in this figure, the cathetercomprises a catheter shaft with an outer sheath 18. In the embodiment depicted in Fig. 1,the outer sheath is formed from sections of different material (e.g., in the embodimentdepicted Fig. 1, five different sections comprise the outer sheath). These sections ofdifferent material enable the catheter 16 to have, for example, different mechanicalproperties (e.g., flexibility) at different locations along the catheter shaft. The outer sheath18 may or may not comprise these sections of different material depending upon theintended application for the catheter. Although the outer sheath 18 depicted in Fig. 1 has acircular cross section, the cross-section of the outer sheath may be other than circular.[0049] As also shown in Fig. 1, the brush electrode 10, which comprises an exposedportion 20 and an embedded portion 22 (see, e.g., Fig. 5), is present at a distal end 24 of theouter sheath 18. In particular, at the distal end of the outer sheath, the exposed portion 20of the brush electrode 10, comprising a plurality of filaments 26, may be seen (see, e.g.,Fig. 2). The exposed portion of the brush electrode may project a few millimeters from thedistal end of the outer sheath. The distance that the exposed portion of the brush electrode 8 219642/2 extends from the distal end of the outer sheath varies depending upon a number of factorsincluding the composition of the filaments comprising the brush and the particular area tobe treated with the brush electrode 10. The distal end 24 of the outer sheath 18 mayinclude a conductive or nonconductive base 28. As explained further below, the flexiblebrush electrode provides enhanced tissue contact, particularly for use on contoured ortrabeculated surfaces.

[0050] Fig. 2 is an enlarge view of the circled region of Fig. 1. As clearly shown inFig. 2, the brush electrode 10 according to this embodiment has a relatively flat workingsurface 30 at the distal end 32 of the brush electrode 10. In other words, in this depictedembodiment, all of the filaments 26 comprising the brush electrode 10 extendapproximately the same distance from the distal end 24 of the outer sheath 18. Thus, thebrush tip provides a relatively flat working surface 30 comprising the longitudinal ends ofthe filaments. The outer sheath of the catheter provides mechanical support for thefilaments and may also provide electrical shielding. As explained further below, the brushelectrode comprises a bundle of bristles or filaments that each may be constructed from avariety of different materials, including nonconductive materials, semi-conductivematerials, and conductive materials. For example, the filaments may be formed from metalfibers, metal plated fibers, carbon-compound fibers, and other materials. Very thin, carbonfibers may be used, or relatively thicker but less conductive Thunderon® acrylic fibersmay be used for the brush electrode filaments. Thunderon® is manufactured by NihonSanmo Dyeing Company Ltd. of Kyoto, Japan. Nylon fibers coated with conductivematerial may also be used. Filaments 26 constructed from metal plated fibers, like coatednylon fibers, may comprise flattened areas around their outer surfaces, resulting in thefilaments having noncircular cross-sectional shapes. The brush filaments may be insulatedfrom each other, or they may be in electrical contact with each other. As explained furtherbelow, conductive or nonconductive fluids 34 may flow within the filaments themselves(see, e.g., Fig. 36) or along the outer surface of the filaments (see, e.g., Fig 5).

[0051] Once the distance that the filaments extend from the distal end 24 of the othersheath 18 is set to a desired length, the bundle of filaments comprising the brush electrode10 may be fixed to the outer sheath 18. Figs. 3-5 depict one technique for fixing oranchoring the brush electrode 10 relative to the outer sheath using sutures. In Fig. 3, arearward suture 36 and a forward suture 38 are shown in phantom under a section of shrink 9 219642/2 tube 40 surrounding the outer surface of the outer sheath 18. The shrink tube protects thesutures and makes it easier to insert the catheter by mitigating possible snags that mayoccur due to the presence of the sutures. Fig. 4 is similar to Fig. 3, but depicts a portion ofthe shrink tube 40 broken away to reveal a portion of the two sutures 36, 38. The sutureknots 42 are clearly visible in Fig. 4.

[0052] Fig. 5 is an isometric, cross-sectional view of the catheter 16 depicted in Figs.3 and 4, taken along line 5-5 of Fig. 3. In Fig. 5, it is apparent that the rearward suture 36may be used to set the depth that the brush electrode 10 may be inserted into the distal end24 of the outer sheath 18. In this figure, the forward suture 38 pierces the filaments 26comprising the embedded portion 22 of the brush electrode 10 and thereby help preventmovement of the brush electrode relative to the outer sheath of the catheter. In theembodiment depicted in Fig. 5, conductive fluid 34 is shown flowing through a lumen 44of the outer sheath (depicted as a single, embedded channel) from a fluid source (notshown) to the brush electrode 10. When the conductive fluid 34 flows through the brushelectrode, it creates a wet-brush electrode in which impinging jets of fluid travelinginterstitially impact the tissue 46 (see, e.g., Figs. 38 and 39) at the tissue-electrodeinterface, which makes it easier to control temperature rises at the interface. Wet-brushelectrodes are discussed further below. In an alternative embodiment, the lumen 44depicted in Fig. 5 may comprise a plurality of separate lumen.

[0053] Fig. 5 also clearly depicts a primary conductor 48 having an insulated portion50 and an uninsulated portion 52. The primary conductor carries ablative energy (e.g., RFcurrent) from an energy source (not shown) to the brush electrode 10. As depicted in Fig.5, the primary conductor 48 extends within the fluid-carrying lumen 44 of the catheter,along a longitudinal axis 54 of the catheter 16. The primary conductor may comprise, forexample, insulated copper wire with an uninsulated portion in electrical contact with thebrush electrode. In this embodiment, the uninsulated portion 52 of the primary conductoris looped or noosed around the filaments comprising the brush electrode at a connectionpoint 56 (Fig. 7). At the loop or noose 58, ablative energy is transferred from the primaryconductor to the conductive filaments comprising part of the brush electrode 10. In thisembodiment, the uninsulated portion 52 of the primary conductor 48 is connected to theembedded portion 22 of the brush electrode 10 so that the connection between the primaryconductor and the brush electrode is protected within the outer sheath 18 of the catheter 16. 10 219642/2 [0054] Also clearly visible in Fig. 5 is an embedded or secondary lead 60, whichextends substantially parallel to the primary conductor 48. A distal end 62 of thesecondary lead 60 becomes embedded with the filaments 26 comprising the brushelectrode 10. As discussed further below in connection with, for example, Fig. 37, thesecondary lead 60, when present, may be operatively connected to some type of sensorembedded in the brush electrode (e.g., a thermal sensor 64, an ultrasound sensor 66, or apressure sensor 68). The brush electrode depicted in Fig. 5 acts as a surface-cooledelectrode 10.

[0055] Figs. 6 and 7 depict possible steps for forming the brush electrode 10 depictedin Figs. 1-5. In Fig. 6, a bundle 70 of conductive filaments 72 and nonconductivefilaments 74 is being formed by using the uninsulated portion 52 of the primary conductor48 to bind or tie together the filaments. In Fig. 6, the uninsulated portion has been noosedaround the bundle of filaments 70, but has not been tightened or snugged against thebundle. In Fig. 7, the uninsulated portion 52 of the primary conductor has been snugglynoosed around the connection point 56 at approximately the mid-section of the bundle offilaments that will ultimately form the brush electrode 10. The conductive filaments 72and the nonconductive filaments 74 are then bent around the connection point 56 in thedirection of the arrows 76, 78 depicted in Fig. 7. Once the filaments are folded uponthemselves about the connection point 56, they are inserted into the distal end 24 of theouter sheath 18 and positioned relative to the distal end 24 of the outer sheath 18 so that thedesired amount of the filaments extends from the distal end of the sheath and comprises theexposed portion 20 of the brush electrode 10. The ends of the filaments may then betrimmed, if desired, to create a desired shape for the working surface 30 at the distal end 32of the brush electrode 10 (see, e.g., Figs. 11-14).

[0056] Figs. 8, 9, and 10 depict an alternative embodiment of the brush electrode.This standoff brush electrode 10' includes an exposed portion 20' with a working surface30' wherein the longitudinal ends of the conductive filaments 72 are not flush with thelongitudinal ends of the nonconductive filaments 74. As shown to better advantage inFig. 10, which is an enlarged view of the circled region of Fig. 8, in this alternativeembodiment of the brush electrode, the conductive filaments 72 are interspersed amongrelatively longer nonconductive filaments 74. The relatively longer nonconductivefilaments prevent the conductive filaments from directly touching the tissue 46 (see, e.g., 11 219642/2

Fig. 40) when the working surface 30' of the brush electrode is placed normal to the tissuebeing treated. With this brush configuration and substantially perpendicular orientation ofthe brush working surface 30' relative to the tissue being treated, the brush electrode acts asa virtual electrode. If the perpendicular orientation can be maintained, there is no directcontact between the conductive filaments and the tissue, and the conductive fluid 34 (seeFig. 5) flowing through the lumen 44 of the outer sheath 18 makes the electrical contact atthe brush-tissue interface. Although Figs. 8 and 10 depict each of the conductive filaments72 as being shorter than each of the nonconductive filaments 74, the electricalcharacteristics of the brush electrode may be adjusted by having some conductive filamentsthat extend to the working surface at the tip of the brush electrode, if desired.

[0057] Fig. 9 is a cross-sectional view taken along line 9-9 of Fig. 8 and clearlydepicts the bundled filaments 70 at the connection point 56 between the filaments and theuninsulated portion 52 of the primary conductor. The secondary lead 60 is also visible inFig. 9. In this embodiment, it is possible to adjust the fluid and electrical contact at thebrush-tissue interface through appropriate selection of the conductive and nonconductivefilaments. Since this configuration of the brush electrode performs most effectively whenplaced normal or perpendicular to the tissue, a relatively short exposed portion 20' for thebrush electrode 10' may be desirable with relatively stiff filaments (e.g., Thunderon®filaments).

[0058] Figs. 11-14 depict alternative shapes for the filaments 26 comprising the tip ofthe brush electrode. The various tip configurations may provide advantages for specialapplications of brush electrodes. Fig. 11 depicts a blade-shaped distal tip 80 creating a lineof contact with the longest filaments of the brush electrode. As depicted in Fig. 11, the lineof contact at the most distal end of the brush electrode extends perpendicularly into thepage. In Fig. 12, the working surface of the electrode tip has a concave portion or channel82. The concave-tip embodiment depicted in Fig. 12 is beneficial for wrap-aroundapplications and provides advantages when ablating curved surfaces like the outer surfaceof a blood vessel. Fig. 13 depicts a convex, trough-shaped tip 84. This particularconfiguration is beneficial, for example, when reaching into troughs or depressions on acontoured surface. The distal tip could also be domed or hemispherical rather than havingthe trough-shaped contact surface shown in Fig. 13. In Fig. 14, the brush electrode has awedge-shaped tip 86. The wedge-shaped tip facilitates angular placement and increases 12 219642/2 the area of the working surface 30". The distal tip could also be conical (not shown),coming nearly to a point at the most distal end of the brush electrode, with its longestfilaments proximal to the longitudinal axis 54 of the catheter 16 (see Fig. 5). This latterconfiguration may be advantageous for point applications of ablative energy. The brushelectrodes are depicted in many of the drawings with circular cross sections, but may havedifferent cross-sectional configurations.

[0059] Fig. 15 depicts an example of a brush electrode 10" having continuouslyvarying conductivity along the longitudinal axes of the filaments. In particular, the brushelectrode comprises tapered filaments 26'. In this alternative embodiment, at least aportion of the individual filaments 26' comprising the brush electrode 10'' gradually tapertoward their distal or free ends 88. In other words, at the distal end 24 of the outer sheath18, the filaments 26' have larger cross-sectional areas than they have at their distal ends 88,adjacent to the working surface 30''' of the brush electrode 10''. The filaments 26' are thusmore conductive adjacent to the distal end of the outer sheath and less conductive at thedistal ends of the filaments. Since the filaments are more conductive adjacent to the distalend of the outer sheath, this minimizes current flow to the less conductive fluid wetting thebrush from the lumen of the outer sheath. When less of the ablative energy flows into theconductive fluid adjacent to the distal end of the outer sheath, this minimizes the energytransfer into the conductive fluid and the concomitant heating of the conductive fluidbefore it contacts the surface of the tissue. At the distal ends 88 of the filaments 26'depicted in Fig. 15, the conductivity of the filaments may be matched to the conductivity ofthe fluid to create a relatively uniform electric field at the brush-tissue interface.

[0060] The taper depicted in Fig. 15 could be an inverse taper, which may beadvantageous for certain applications. It should be noted that, in order to vary theconductivity along the length of the filaments, the filaments may also be coated or platedwith materials having different or varying electrical conductivity. For example, thefilaments, whether tapering or not, could be coated with conductive material. Theconductive material coating the filaments in the region most closely adjacent to the distalend 24 of the outer sheath 18 may be more conductive than the coating on the portion ofthe filaments most closely adjacent to the distal end of the filaments themselves. Thus, theconductivity of the filaments would be greater near the distal end of the outer sheath thannear the distal ends of the filaments, even though the cross-sectional areas of the filaments 13 219642/2 may not be changing substantially as one moves longitudinally along the filaments towardtheir distal ends. Although not specifically shown in the figures, the conductivity of all ofthe disclosed filaments may also vary radially rather than, or in addition to, varyinglongitudinally. In other words, the conductivity of the filaments may vary as one movesfrom the center of the filaments to the surface of the filaments.

[0061] Fig. 16 depicts a brush electrode 10"' in which the conductivity of thefilaments varies discontinuously. In particular, Fig. 16 depicts filaments 26'' that areconductive except at their distal ends. The distal end of each filament includes anonconductive tip 90. These nonconductive tips provide a stand-off distance when theworking surface of the brush electrode is placed substantially perpendicular to the tissuebeing treated since the conductive portions of the filaments do not actually touch the tissuein this embodiment. Similar to what occurs in the embodiment depicted in Figs. 8-10, theconductive fluid would pass through the lumen of the catheter and wet the brush. Theconductive fluid would carry the current over the stand-off distance and to the tissue,thereby acting as a virtual electrode. It should be noted that, although the embodimentdepicted in Fig. 16 shows each of the conductive filaments 26'' having a nonconductive tip90, some of the conductive filaments 26'' may extend all the way to the working surface30'''' of the brush electrode and thus would, in fact, contact the tissue during use of thebrush electrode.

[0062] Fig. 17 depicts an embodiment of the outer sheath 18' having a concentric ringof sub-channels 92 around a main or central channel 94 through which the brush filaments26 extend. The circumferential ring of sub-channels around the brush-carrying centralchannel may be used to carry conductive or nonconductive fluid, including therapeuticfluid or medicine. The embedded sub-channels depicted in this figure could define spiralor helical paths toward the distal end 24' of the outer sheath, similar to the paths orchannels 104 described below in connection with Fig. 19 and Fig. 20.

[0063] Fig. 18 depicts an embodiment wherein the sheath 18'' surrounding thefilaments of the brush electrode 10 is porous adjacent to the exposed portion 20 of thebrush electrode. An outer covering (not shown) may be placed around the outer cylindricalsurface of the porous sheath, possibly leaving an angular ring of material 96 exposed at thedistal end 24'' of the sheath 18'' adjacent to the brush electrode 10. 14 219642/2 [0064] Fig. 19 is a fragmentary, isometric view of an embodiment wherein a threadedsheath 98 surrounds the filaments of the brush electrode 10. The threaded sheath 98 has aspiral or helical ridge 100 on its outer surface. As shown to good advantage in Fig. 20,when the threaded sheath is inserted into a covering 102 (shown in phantom andcross-section), a helical flow channel 104 is created between the threaded sheath 98 and thecovering 102. Conductive fluid, nonconductive fluid, or medication may be delivered tothe tissue adjacent to the brush electrode via this flow channel.

[0065] Fig. 21 is a fragmentary, isometric view of another embodiment, wherein thesheath surrounding the filaments of the brush electrode is a grooved sheath 106. Thegrooved sheath has a plurality of longitudinally-extending grooves or cuts 108 formed onits outer surface, adjacent to the exposed portion of the brush electrode 10. As shown tobest advantage in Fig. 23, when the grooved sheath 106 is inserted into a covering 102'(shown in phantom and cross-section), a plurality of longitudinally-extending flowchannels 110 are created between the grooved sheath 106 and the covering 102'. Again,conductive fluid, nonconductive fluid, or medication may be delivered to the tissueadjacent to the brush electrode via these flow channels. Fig. 22 is a fragmentary view of asection of the grooved sheath 106 depicted in Fig. 21, surrounded by a covering 102'(shown in cross-section) to create the plurality of longitudinally-extending flow channels110 between the grooved sheath and the covering.

[0066] Figs. 24 and 25 depict alternative mechanical interfaces between the filaments26 of the brush electrode 10 and the primary conductor 48. Fig. 24 is similar to Fig. 5, butdepicts an isometric, cross-sectional view of a catheter 16' wherein the exposed portion 52of the primary conductor 48 makes electrical contact with the brush filaments 26 via anenergy transfer coil or spring 112 surrounding at least the concealed or embedded portion22 of the brush electrode 10. In this embodiment, the ablative energy is transferred to thebrush electrode 10 over a large surface area (i.e., over the entire inner surface area of thecoil 112). Thus, less damage to the filaments may occur in this embodiment than mayoccur in the embodiment depicted in Fig. 5, wherein all of the ablative energy is transferredfrom the uninsulated portion 52 of the primary conductor to the brush electrode at thesingle connection point 56. As depicted in Fig. 24, a loop of wire 114 may be present tohelp collect and stabilize the filaments 26 during assembly of the catheter 16'. This loop ofwire 114 may be anchored to, for example, the inner surface 116 of the outer sheath 18. As 15 219642/2 previously described, a secondary lead 60 may also be present in the lumen 44 of the outersheath 18.

[0067] Fig. 25 is similar to Figs. 5 and 24, but depicts an isometric, cross-sectionalview of a catheter 16" wherein the primary conductor 48 makes electrical contact with thefilaments of the brush electrode 10 via an energy transfer mesh or fabric 118 surroundingat least the concealed or embedded portion 22 of the brush electrode 10. This embodimenthas the same advantages that were just described for the embodiment depicted in Fig. 24.In another embodiment, the primary conductor 48 makes electrical contact with thefilaments of the brush electrode 10 via an energy transfer wrap (not shown), which issimilar to the mesh or fabric 118, but comprises a solid or porous sheet of conductivematerial.

[0068] Fig. 26 is a cross-sectional view of a first embodiment of a shielded-tip brushelectrode 120. In this embodiment, the uninsulated portion 52 of the primary conductor 48is looped around the outer surface of the brush electrode after passing through amechanical interface 122 supporting the filaments 26 of the brush electrode adjacent to thedistal end 124 of an inner sheath 126. Since fluid may or may not travel through the lumen128 of the inner sheath 126, the mechanical interface 122 may or may not be porous. Inthe embodiment depicted in Fig. 26, there is an outer sheath 130 surrounding the innersheath 126. The inner sheath houses the primary conductor 48 and supports themechanical interface 122 for the filaments 26 of the brush electrode 120. The primaryconductor again includes an uninsulated portion 52 that transfers ablative energy 150 (e.g.,RF energy) to the conductive filaments in the brush electrode 120. As mentioned, in thisembodiment the uninsulated portion 52 of the primary conductor forms loops or coils 132around the circumference of the brush. These loops or coils increase the surface areathrough which the ablative energy is transferred, thereby providing for more effective, andpotentially less destructive, energy transfer to the brush electrode 120.

[0069] As shown in Fig. 26, the outer sheath, which may be a typical braided sheath,is placed around the inner sheath 126, but is radially and longitudinally offset from theinner sheath. The radial offset creates an annular gap or channel 134 between the innersheath 126 and the outer sheath 130 through which conductive fluid may, for example, beintroduced to the sides of the brush electrode filaments. The conductive fluid, if present,would flow through the annular channel 134 in the direction of the arrows 136 shown at 16 219642/2 the top of Fig. 26. The longitudinal offset between the inner sheath 126 and the outersheath 130 ensures that the channel 134 for the conductive fluid extends past the distal end124 of the inner sheath 126 to the sides of the brush electrode filaments. In thisembodiment, the conductive fluid would flow through the annular channel between theinner sheath and the outer sheath, past the coils 132 of uninsulated conductive wire, into anannular fluid jacket 138 surrounding a region of the brush electrode adjacent to the distalends of the inner and outer sheaths, and then into the sides of the brush electrode itself andthrough the interstitial gaps between the filaments comprising the brush electrode. Theablative energy (e.g., the RF energy 150) is thus carried by the conductive fluid into thecore of the brush electrode and toward its working surface 140. In this embodiment, aflexible polymer nipple or boot 142, defining an outer wall of the annular fluid jacket 138,also supports the filaments in a ring 144 of direct contact extending around the perimeter ofthe filament bundle. The flexible boot or nipple may be porous. Finally, a smooth outerwall 146 to facilitate easier insertion and manipulation of the catheter in a patient maycover the outer sheath 130 and abut a corresponding edge 148 of the flexible polymernipple or boot 142. Alternatively, the outer wall material may actually form the nipple orboot in addition to forming a perimetric covering around the outer sheath. An annularlayer of porous material or mesh fabric (not shown) may be placed in the annular fluidjacket 138 to keep the brush wetted and to help prevent splaying (see Figs. 40-42) of thebrush electrode.

[0070] Fig. 27 is similar to Fig. 26, but depicts a second embodiment of a shielded tipbrush electrode 120'. The only differences between the embodiment depicted in Fig. 26and the embodiment depicted in Fig. 27 are the size of the fluid jacket 138' and theconfiguration of the flexible polymer nipple or boot 142' that supports the brush filaments.In the embodiment depicted in Fig. 27, an alternative flexible polymer nipple or boot 142'defines a smaller fluid jacket 138' and supports the filaments in a band of direct contactextending around the perimeter of the filament bundle. The band of direct contact 152supports the filaments over a larger section of the outer surface of the brush electrode thandoes the ring of direct contact 144 depicted in Fig. 26. By adjusting the configuration ofthe flexible polymer nipple or boot in this manner, the amount of conductive fluid flowinginto the brush electrode and the overall flexibility of the brush electrode can bemanipulated. 17 219642/2 [0071] It should be noted that, although the filaments depicted in Figs. 26 and 27 areshown as extending just into the distal end 124 of the inner sheath 126, the filaments mayextend further into the inner sheath and may even extend all the way to the proximal end(not shown) of the catheter.

[0072] Figs. 28-35 depict different cross-sectional configurations for brush electrodesaccording to the present invention. Interstitial spaces 156 are clearly visible in each ofthese figures. In Figs. 28-31, the brush electrode 10 has a conductive core 154. In thesefour figures, the conductive filaments 72 are shown with cross hatching, and thenonconductive filaments 74 are shown without cross hatching. Thus, the brush electrodedepicted in Fig. 28 is fully conductive and does not comprise any nonconductive filaments.In each of the embodiments depicted in Figs. 29-31, a conductive core 154 is shielded by abarrier of nonconductive filaments 74. In particular, Fig. 29 depicts a core of relativelylarge conductive filaments surrounded by two rings of nonconductive filaments ofapproximately the same size. In Fig. 30, a core 154 of relatively small conductivefilaments 72 is surrounded by two rings of relatively large nonconductive filaments 74. InFig. 31, a conductive core 154 of relatively large conductive filaments 72 is surrounded bytwo rings of relatively small nonconductive filaments 74.

[0073] Figs. 32 and 33 depict cross-sectional configurations for brush electrodes thathave conductive perimeters 158. Thus, in the embodiments depicted in Figs. 32 and 33, anonconductive core 160 of nonconductive filaments 74 is surrounded by conductivefilaments 72. Fig. 32 depicts a core of relatively small nonconductive filamentssurrounded by two rings of relatively large conductive filaments. In Fig. 33, a core ofrelatively large nonconductive filaments is surrounded by two rings of relatively smallconductive filaments.

[0074] In Fig. 34, conductive clusters 162 of relatively small filaments areinterspersed among relatively large nonconductive filaments 74. The interspersedconductive clusters may be interspersed in a specific pattern, pseudo randomly, orrandomly among the nonconductive filaments in order to achieve a desired electric fieldfrom the resulting brush electrode. In Fig. 35, nonconductive clusters 164 of relativelysmall filaments are interspersed among relatively large conductive filaments 72.

[0075] Fig. 36 is a cross-sectional view of a brush electrode wherein some of thefilaments are hollow or porous 166. Such hollow or porous filaments 166 may be used as 18 219642/2 conduits for conductive fluid, they may be used to supply therapeutic chemicals, and/orthey may provide suction ports at the brush-tissue interface to control field smearing on thetissue surface. If the filaments are porous, they may retain a small amount of fluid in poresthat are oriented at various angles to the longitudinal axis of the filaments. During anablation procedure, some of the ablative energy may dehydrate the porous filaments beforeaffecting the surrounding blood, particularly when the conductivity of the tissue lessens asthe ablation progresses. Thus, if excess ablative energy is present during an ablationprocedure, that energy may harmlessly dehydrate the porous filaments rather thannegatively affecting the tissue being ablated or the blood in the area of that tissue. In oneembodiment (not shown), some of these hollow filaments 166 do not extend to the distalend 32 (labeled on, for example, Fig. 2) of the brush electrode. For example, some of thehollow filaments 166 may only extend part way into the exposed portion 20 (labeled on,for example, Fig. 3) of the brush electrode. These shortened hollow filaments may deliverconductive fluid or therapeutic chemicals, for example, to an interior region of the bundleof brush filaments. In the embodiment depicted in Fig. 36, the other filaments 26 may beconductive or nonconductive filaments.

[0076] Fig. 37 is a cross sectional view of a brush electrode having devices 64, 66, 68embedded among the conductive and nonconductive filaments 26. The devices mayinclude, for example, pressure sensors 68 to measure contact pressure between the brushelectrode and the tissue, thermal sensors 64 (e.g., a thermocouple) at the tip of the brushelectrode to sense the brush-tissue interface temperature, or fiber optic or ultrasoundsensors 66 for in situ lesion identification and characterization. The devices may beoperatively connected to equipment (not shown) at the proximal end of the catheter bysecondary leads like the secondary lead 60 depicted in, for example, Figs. 5 and 8-16.[0077] Fig. 38 is a fragmentary, isometric view of a catheter 16 having a brushelectrode 10 according to the present invention forming a spot or point lesion 12 on asection of tissue 46. As shown in this figure, the brush electrode is placed against thetissue with its filaments in contact with or in close proximity to the tissue. The conductivefilaments are connected to, for example, an RF source (not shown) and serve as the activeelectrode. When present, conductive fluid from a fluid source (not shown) flows throughthe lumen 44 (e.g., Fig. 5) of the catheter and through the brush filaments to the workingsurface at the brush tip, thereby creating a wet-brush electrode. Rather than being localized 19 219642/2 on the tissue to create a spot or point lesion 12 as shown in Fig. 38, the brush electrode 10may be dragged along the surface of the tissue 46 to create a continuous linear lesion 14, asshown in Fig. 39. Fig. 39 is a fragmentary, isometric view of a catheter 16 having a brushelectrode according to the present invention forming a linear or drag lesion on a section oftissue.

[0078] Figs. 40-42 depict a brush electrode 10 according to the present inventionforming different size spot lesions 12 based in part upon the amount of splay of the brushelectrode. In Fig. 40, relatively light contact pressure is being used to press the brushelectrode 10 against the tissue 46 while forming a lesion 12. This application of lightpressure results in minimal splaying of the filaments comprising the brush electrode, andthus a relatively small lesion is formed. In Fig. 41, more pressure is being used to press thebrush electrode 10 into contact with the tissue 46, resulting in relatively more splaying ofthe brush electrode. As long as the efficiency of the brush electrode is not degraded toogreatly by the splaying, a relatively larger lesion 12 may thus be formed by applyingadditional pressure to press the brush electrode toward the tissue. Finally, in Fig. 42, evenmore contact pressure is being applied to the brush electrode 10 than is being applied inFigs. 40 and 41, resulting in even more splaying of the brush electrode and the formationof a relatively larger lesion 12 on the tissue 46 than is being formed in Figs. 40 and 41.

[0079] The brush electrode according to the present invention delivers ablative energyto the tissue via the conductive filaments alone, via the conductive fluid alone, or via boththe conductive filaments and the conductive fluid. In the latter two configurations, thebrush electrode is referred to as a wet-brush electrode. Since it is possible for theconductive fluid to escape from the exposed portion of the wet-brush electrode prior toreaching the working surface at the distal tip of the wet-brush electrode, there is someablative energy leakage to the surrounding blood. The leakage of ablative energy to thesurrounding blood is in part due to direct contact between the blood and the conductivefilaments and in part due to the conductive fluid escaping between the filaments to thesurrounding blood, particularly when substantial splaying of the filaments occurs (see, e.g.,Fig. 42).

[0080] The design parameters for the brush electrode include both filament and brushparameters. The filament parameters include, for example, the material and structuralproperties of the individual filaments (e.g., what material(s) each individual filament is 20 219642/2 constructed from, whether the filaments are hollow or solid, whether the filaments areporous, and how flexible or stiff the filaments are), the shape and cross-sectional areas ofthe individual filaments, and the electrical conductivity of the individual filaments. Theelectrical conductivity of the individual filaments may be constant along the length of thefilaments or may vary along the length of the filaments. Also, if the conductivity of afilament varies along its length, it may vary continuously or discontinuously. The filamentdesign parameters may be different for each filament.

[0081] The design parameters for the brush electrode include, for example, the overallshape and cross-sectional area of the brush (i.e., the overall shape and size of the filamentbundle forming the brush electrode), the tip length of the brush itself (i.e., the length of theportions of the filaments that extend the farthest from the distal end of the outer sheath),the shape of the brush tip, the length of the individual filaments relative to each other, thepacking density of the filaments comprising the brush, and the overall electrical resistanceof the brush. When both nonconductive and conductive filaments are present, theconductive filaments may be distributed evenly, randomly, or pseudo-randomly among thenonconductive filaments comprising the brush electrode.

[0082] By controlling, among other things, the cross-sectional shapes of the filaments,the cross-sectional areas of the filaments, the flexibility or stiffness of the filaments, thepacking density of the filaments, the ratio of the nonconductive filaments to the conductivefilaments, and the placement of the nonconductive and conductive filaments relative toeach other, it is possible to obtain brush electrodes having desired electrical and thermalcharacteristics, which ultimately determine the types of lesions that may be obtained whenusing the brush electrodes for ablation. As mentioned above, it is even possible to vary themechanical and electrical properties of each individual filament, if necessary, to achievedesired results.

[0083] The shapes and cross-sectional areas of the individual filaments and thepacking density of the brush electrode affect the interstitial spaces between the filaments.The interstitial spaces between the filaments determine the flow path of the conductive ornonconductive fluid when the brush electrode is being used as a wet-brush electrode. Theflow path of the conductive or nonconductive fluid determines to a great extent theelectrical and thermal characteristics of the wet-brush electrode. The use of a large numberof individual filaments defining interstitial spaces among the filaments results in efficient 21 219642/2 and effective cooling of the brush electrode and of the tissue surface. The effective coolingof the brush electrode achieved by the present invention reduces the formation of coagulumon the electrode, and the effective cooling of the tissue surface achieved by the presentinvention allows for the application of high-power ablation energy for long durations,ultimately resulting in the formation of better lesions.

[0084] During use of a brush electrode, the following operating parameters may betaken into account: the incidence angle between the brush electrode and the tissue, thestand-off distance between the brush electrode and the tissue, the power being applied, therate of fluid flow when present, and the duration of contact between the electrode and thetissue.

[0085] In one set of tests, Thunderon® filaments were used favorably in a wet-brushelectrode having a circular cross section with an overall diameter of 6-8 French, a tiplength of 2-3 millimeters, and electrical resistance of 100-150 ohms. In this embodiment,the size of the Thunderon® filaments was 40 decitex. When using this brush electrodewith zero stand-off distance, 30 watts of power, saline flowing at 12 milliliters per minute,and contact between the wet-brush electrode and the tissue occurring for 60 seconds, 5-to-6millimeter deep lesions were formed with an incidence angle of 90° between the wet-brushelectrode and the tissue. Four millimeter deep lesions were formed when the incidenceangle between the wet-brush electrode and the tissue was 0°. When a stand-off distance of1 millimeter was used during tests with similar operating parameters, a slightly less deep(on the order of 3 millimeters deep) lesion was formed.

[0086] In another set of tests, lesions 3-13 millimeters deep were created using 20-50watts of power and flow rates of 3-18 milliliters per minute with wet-brush electrodesmade from commercially available carbon fibers (e.g., carbon fibers available throughCytec Carbon Fibers LLC of South Carolina, United States of America. Isotonic salineinfusion was used in these tests. Isotonic saline is generally about twice as conductive asthe surrounding blood. In other tests, linear lesions 20-42 millimeters long and 3-8millimeters deep were created by applying 20-50 watts of power for 60 seconds in thepresence of flow rates of 3-18 milliliters per minute using wet-brush electrodes producedwith conductive filaments made from Thunderon®.

[0087] As already mentioned, when conductive fluid is used, the brush electrodebecomes a wet-brush electrode. In a wet-brush electrode, the conductive fluid serves both 22 219642/2 thermodynamic functions and electrical functions. Thermodynamically, the conductivefluid cools both the electrode and the tissue surface. As previously mentioned, effectivecooling of the electrode inhibits or prevents coagulum formation on the electrode; andeffective cooling of the tissue surface permits longer application of relatively high ablativeenergy, resulting in the formation of the deeper lesions. Electrically, the conductive fluidserves as a virtual electrode. The conductive fluid also insulates the conductive brushfilaments from the surrounding blood, which helps prevent the formation of coagulum.The conductive fluid also creates a conductivity gradient resulting from a concentrationgradient. The conductive fluid flowing through the brush interstitium has a fieldhomogenizing effect. The conductive fluid flowing through the working surface at thedistal tip of the wet-brush electrode thus helps to mitigate hot spots resulting from edgeeffects. Further, since the number of edges present in a brush electrode greatly exceeds thenumber of edges present in many existing electrodes, the energy build up at each filamentedge in a brush electrode is less than it would be for existing electrodes, assuming the samepower setting. This results in less severe edge effects when using the brush electrode ofthe present invention. The conductive fluid, when used, further smoothes or reduces theundesirable edge effects.

[0088] In the wet-brush electrode, the filaments serve both mechanical and electricalfunctions. Mechanically, the filaments create a flexible electrode that provides improvedtissue contact. The filaments also create interstitial spaces, which not only provideeffective fluid channeling, but also prevents the “virtual electrode” from being washedaway by the surrounding blood, and helps to smooth the concentration gradient of theconductive fluid. Electrically, the filaments serve as a conductive electrode.

[0089] Again, it should be noted that although the filaments are depicted in nearly allof the figures as having circular cross-sections for visual simplicity, the individualfilaments may intentionally or unintentionally have a wide variety of cross-sectionalconfigurations and areas, and need not be circular. Manufacturing irregularities may resultin various cross-sectional configurations, or filaments having a variety of different cross-sectional configurations may be intentionally selected to achieve a desired electric field atthe brush-tissue interface. The filaments also may not be perfectly aligned longitudinally.Further, the filaments may comprise a yarn of braided or twisted groups of fibers, or the 23 219642/2 filaments may comprise a roving pattern of untwisted, longitudinally-extending,substantially-parallel, conductive and nonconductive fibers.

[0090] Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from the spirit or scope of thisinvention. All directional references are only used for identification purposes to aid thereader's understanding of the present invention, and do not create limitations, particularlyas to the position, orientation, or use of the invention. It is intended that all mattercontained in the above description or shown in the accompanying drawings shall beinterpreted as illustrative only and not limiting. Changes in detail or structure may bemade without departing from the spirit of the invention as defined in the appended claims. 24