Patent Publication Number: US-2021186609-A1

Title: Catheter ablation device with impedance monitoring

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation of International Patent Application No. PCT/AU2019/050844, filed Aug. 13, 2019; which claims priority from AU Patent Application No. 2018902954, filed Aug. 13, 2018. The entire contents of each of the PCT/AU2019/050844 and the 2018902954 applications are hereby incorporated by reference in their entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a catheter ablation device and a method of using such device. The device may be a microwave ablation device, with application in the field of endovascular sympathectomy or denervation such as renal artery denervation. The invention may also find application in other fields of medical ablation including the treatment of atrial and ventricular arrhythmias. 
     BACKGROUND OF THE INVENTION 
     Hypertension is a significant medical condition that leads to morbidity and mortality from end organ injury, such as strokes, heart attack and kidney failure. Many patients require multiple medications for blood pressure control and, for some patients, medications are poorly tolerated or ineffective altogether. Renal artery denervation by radiofrequency catheter ablation has emerged as a possible treatment option to control hypertension in these patients who are refractory or intolerant of medical therapy. The procedure aims to eliminate the efferent and afferent nerves that relay neural messages between the kidneys and the central nervous system, as these form essential components of neuro-hormonal reflexes that elevate blood pressure. The efferent and afferent nerves travel in the outer layer (i.e. adventitia) of the renal artery and the perinephric fat, mostly between 1 and 6 mm from the inner (luminal) surface of the renal arteries, and these nerves can potentially be destroyed by endovascular catheter ablation. 
     More recently, microwave ablation techniques have been proposed for vascular denervation, and the inventors of the present inventions have demonstrated very effective outcomes in trials of a microwave ablation device as described in WO2016/197206, the entire contents of which are included herein by reference. 
     Development of this concept has confirmed that microwave ablation using endovascular catheters has applications for renal denervation in the treatment of hypertension as well as cardiac ablation in the treatment of arrhythmias. Microwave heating is radiant and can penetrate deeply into tissue, creating large thermal lesions of more uniform temperature distribution than radiofrequency ablation. The technique does not require any catheter tip-to-tissue contact to produce heating. 
     Any discussion of documents, acts, materials, devices, articles and the like in this specification is included solely for the purpose of providing a context for the present invention. It is not suggested or represented that any of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia or elsewhere before the priority date of each claim of this application. 
     BRIEF SUMMARY OF THE INVENTION 
     In one form, the present invention provides a catheter ablation device for delivery of energy to a selected region of tissue, the device having an antenna portion including a radiating antenna electrically connectable via an electrical feedline to a source of energy, the antenna configured to generate an electromagnetic field able to ablate tissue in said selected region of tissue, wherein the device comprises an elongated catheter having an outer sheath, the device configured to allow flow of fluid along the catheter to exit through one or more orifices in or adjacent to said antenna portion, 
     wherein the device includes an impedance monitoring system comprising two electrodes, arranged respectively inside and outside said catheter sheath and configured such that in use the impedance monitoring system includes an electric circuit incorporating an ionic conductivity path through said fluid. 
     When the device is positioned within a blood vessel (or other part of the body), it can be used to measure the impedance of an electrical circuit which includes the blood pool surrounding the catheter sheath, and thus provides a system to monitor changes in calibre of the blood vessel (or other part of the body) during an ablation procedure. 
     Preferably, the device is a microwave ablation device for delivery of microwave energy, the source of energy comprising a microwave generator. Preferably, the electrodes are arranged proximal of said antenna portion. This ensures the electrodes are substantially outside the microwave field produced by the antenna. 
     Preferably, the electrodes are electrically connectable to an impedance monitoring means, configured to provide an indication to a user of the device of a measure of the impedance of said electric circuit. 
     One or both of the internal and external electrodes (being the electrodes arranged inside and outside said catheter sheath, respectively) may comprise ring or part-ring form electrodes. This assists in electrical connection with fluid with which they are in contact. 
     Said internal electrode may be supported on an outside surface of said electrical feedline. 
     Said external electrode may be supported on an outside surface of said catheter sheath. 
     Alternatively, said external electrode may be provided at a location separate from said catheter sheath. For example, it may be supported on a guiding sheath to be used in introducing the device to a patient, or may be provided as a patient return electrode independent of the catheter sheath (introduced to the body independently of the catheter sheath). 
     In a further form, the present invention provides a method of monitoring changes in vascular calibre during a vascular ablation procedure, comprising introducing a catheter ablation device to a blood vessel at a selected position with respect to a target region of tissue, the catheter ablation device including an impedance monitoring system configured to measure the impedance of an electrical circuit including a blood path in said vessel in or near said selected position, the measured impedance providing a measure of vascular calibre. 
     Preferably, the catheter ablation device comprises an elongated catheter with an outer sheath having one or more fluid flow openings, the method including: 
     providing a first electrode inside said catheter sheath and a second electrode outside said catheter sheath; 
     providing a flow of fluid within said catheter sheath, said fluid flow in electrical contact with said first electrode and passing through said one or more fluid flow openings; and 
     applying a voltage across said electrodes to enable measurement of the impedance there between, said fluid and said blood path both providing part of said electrical circuit. 
     Embodiments of the invention therefore improve the efficacy and safety of ablation procedures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the various aspects of the present invention will now be described by way of non-limiting example only, with reference to the accompanying drawings. In the drawings: 
         FIG. 1  shows a partial cross-sectional view of a microwave ablation device; 
         FIGS. 2 and 3  show, in alternative embodiments, a microwave ablation device including a thermocouple arrangement; 
         FIGS. 4A and 4B  illustrate in further detail a part of the device of  FIG. 3 , in two configurations; 
         FIGS. 5 and 6  show time-temperature graphs of trials of the device of the invention; 
         FIG. 7  shows diagrammatically a microwave ablation device including an impedance sensor arrangement; 
         FIGS. 8 and 9  illustrate alternative embodiments of the sensing electrodes of  FIG. 7 ; and 
         FIGS. 10A and 10B  show angiograms at different stages of a trial of the device of  FIG. 7 , with  FIG. 11  showing a time-impedance graph of the trial. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , microwave ablation device  10  for use in the denervation of renal arteries is shown, comprising an elongate flexible catheter suitable for passage through the vasculature. In particular, device  10  is shown in a renal artery  12 , with ablation areas indicated by reference  64 . 
     The various components of device  10  and of artery  12  and surrounding nerves are set out in further detail in WO2016/197206. Other features, including optional components, materials, dimensions, functions, procedural steps and operational parameters are also discussed in that publication. 
     Of particular note in the context of the present invention are the following components:
         Feedline  22 , formed by a coaxial cable comprising insulating outer sheath  26 , outer conductive shield  28  (e.g. braided copper wire), insulating inner sheath  30  and conductive core  32 .   Microwave radiator  24 , with radiating element (antenna)  34 , formed as a terminal part of the coaxial cable of feedline  22 , stripped of its shield and outer insulation sheath. Antenna  34  is positioned within a distal part of device  10  and encased in a tubular sheath  36  to insulate it from its environment.   Feedline  22  and radiator  24  are contained within an outer catheter sheath  46 , sized to provide sufficient internal free space around feedline  22 , to allow flow of pumped saline solution from a proximal end of the device to the distal end, to act as an irrigant and coolant. The distal ends of antenna sheath  36  and catheter sheath  46  are adhered together at a tip  42 .   To provide both a securing and centering function for the device as well as a path for irrigation fluid to exit the device, catheter sheath  46  includes one or more locating formations  48  formed by longitudinal slits  50  arranged in a ring around the sheath (e.g. six slits). The central core of the device (including feedline  22  and antenna  34 ) are relatively stiff, while sheath  46  is fabricated from a more flexible material. Pulling, at a proximal position, of feedline  22  relative to sheath  46  causes the strips of material of sheath  46  between slits  50  to deform outwardly into convex protrusions to sit against the inner walls  14  of artery  12 . Formations  48  are provided at one or more positions along the distal portion of the device, selected to ensure radiator  24  is held in the required location in accordance with the particular application. In  FIG. 1  a single distal formation  48  is shown, close to catheter tip  42 .       

       FIG. 1  also shows a structural support cover portion  40 , which terminates and seals outer conductive shield  28  and provides an outer layer to sealingly cover the transition between feedline outer sheath  26  and antenna sheath  36 , as well as providing structural support for radiator  24  in this portion. 
     Device  10  may include additional components and functionality, as understood by the skilled person, including those discussed in WO2016/197206. 
       FIG. 1  also illustrates diagrammatically the proximal end of device  10 , connecting outside a percutaneous access location PAL with a handle H, which provides connection with a patient cable  52 . Handle H is designed to allow the operator to actuate and control locating formations  48 , by relative rotation between two axially separated handle parts causing traction of feedline  22  relative to sheath  46 . As  FIG. 1  shows, the left hand part of handle H is provided with graduation markings to indicate to the operator the degree of rotation with respect to the indicium on the right hand part, the graduation markings indicating the extent of opening of locating formations  48  (e.g. 5 mm distension). Handle H also provides an interconnection between the relatively thin coaxial feedline  22  which runs to the catheter distal portion and a thicker electrical feedline  22 ′ which runs to the microwave generator, as well as a fluid interconnection between the internal lumen(s) of catheter sheath  46  and a fluid line  54 . 
     At the proximal end of patient cable  52 , fluid line  54  connects to a fluid control system  56 , which provides the saline irrigation flow though sheath  46 , while patient cable feedline  22 ′ connects to an electrical power/control unit  60 . Fluid control system  56  includes suitable pump, control and flow measurement means, allowing selective adjustment of fluid flow parameters, and may also be used to introduce other fluids such as drugs and markers into the fluid flow for delivery to the distal end of the catheter device. Electrical unit  60  includes a tunable microwave generating source for delivering power to antenna  34 . Electrical cabling  58  provides connection of other electrical components of device  10  (as discussed below) to power, monitoring and control circuitry comprised in electrical unit  60 . As will be appreciated, patient cable  52  jackets together all the cores from handle H, for convenient implementation of the device. 
     Temperature Measurement 
     Device  10  also includes a means for measuring the temperature of the distal portion of the catheter. 
     It is known to include in medical catheter devices one or more temperature sensors, such as thermocouples or thermistors. For example, for temperature monitoring using a thermocouple, a catheter is provided with a thermocouple wire pair of two different metals extending from the proximal end, through the catheter shaft and into the distal portion, where the thermocouple hot junction of the wire pair (the temperature measuring point) is located. The ends of each wire are typically stripped of their covering insulation, twisted, soldered and potted into a distal tip electrode. However, particular issues arise with regard to use of this type of device in microwave ablation devices 
     As will be understood from WO2016/197206, microwave heating is radiant and can penetrate deeply into tissue without antenna-tissue contact. The design of the catheter means the radiating antenna is both electrically insulated from the surrounding environment and separated therefrom by a zone of flowing irrigation fluid (saline). This prevents temperature rises at the catheter tip due to ohmic heating and reduces any dielectric heating along the catheter shank, thus enabling higher microwave power to be used without undesirably or uncontrollably high temperatures within the catheter. In this regard, the temperature of the catheter tip should be restricted to a maximum of around 50° C., as above this temperature there are risks of coagulum formation, tissue charring and steam pops, which can cause adverse clinical outcomes. Monitoring temperature in the distal portion of the catheter can therefore be important. Additionally, during microwave renal artery denervation, a temperature sensor in the vicinity of the catheter tip can provide a measure of renal artery blood flow velocity using a thermodilution method. This enables monitoring of arterial patency, required for safe delivery of microwave energy, as well as reduction in renal microvascular resistance, expected to occur with successful renal denervation if the patient has a high renal sympathetic tone (due to innate physiology or otherwise). 
     A natural consequence of the electrical isolation and fluid surrounding the microwave antenna is the inability to approximate the local tissue temperature by measuring the temperature of the antenna tip. 
     As illustrated in  FIGS. 2 and 3 , device  10  uses a thermocouple measurement of the temperature at the terminal part of the outer conducting shield  28  of feedline  22 . This is provided by electrically connecting a wire at that point  108  to create a thermocouple hot junction. The wire is made from a material with a Seebeck coefficient different to that of shield  28 , such that a temperature change at this junction point provides an electrical current that can be used to determine the temperature. In particular, a type-T, copper-constantan material is used, the braided wire of feedline shield  28  being plated copper. This is found to provide a measurement of temperature within an accuracy of approximately 0.5° C. in the temperature range encountered during ablation procedures. As will be understood, the region of highest temperature with the catheter will be distal of this point  108 , closer to the longitudinal center of antenna  34 , however the distal end  38  of feedline shield braid  28  is sufficiently close to provide an accurate relative measure of this maximum temperature, as discussed further below. 
     Importantly, this arrangement obviates the need for a second wire (and associated elements such as adhesive) in order to provide the thermocouple. The temperature measurement is taken of the outer shield material itself, close to or at the point where the braid ends, from which the central feedline core extends. 
     In particular, the hot junction is made by stripping the insulation from the end of the constantan wire, and soldering it to a short end portion  106  (see further detail in  FIG. 4B ) of the shield braid  28  from which the outer sheath  26  has been removed. In the variant shown in  FIG. 3 , the end of wire  100  is wrapped around the terminal portion of shield braid  28  before soldering, to create a strong, firm joint, both electrically and structurally. 
     Wire  100  runs along the length of the catheter and connects via a suitable connector in handle H to patient cable  52  and from there through electrical cabling  58  to electrical power/control unit  60 , which includes appropriate circuitry and processing means to calculate the temperature from the measured voltages. In the figures, reference  150  indicates the guiding sheath through which catheter device  10  is introduced. 
     This thermocouple system provides a means of monitoring heating adjacent to the catheter antenna, in particular to enable the user to avoid excessive temperatures during ablation, such as may result from excessively high power or failure of catheter irrigation flow. Further, monitoring temperature provides a measure of the microwave radiation at the antenna. With higher electrical power reaching the antenna, or as frequency matching between the antenna and its surrounding medium improves, the local temperature increases. Thus the temperature provides an independent measure of microwave emission, additional to measuring reflected power at the microwave generating source. 
     By way of example, in testing the device of the invention an ablation procedure under deliberately suboptimal conditions was conducted by applying 80 W of microwave power with 10 W of reflected power measured at the generator, this being a result of choosing a poorly matched frequency. With the thermocouple system a temperature at point  108  of 38° C. was measured. Repeating the test with the same forward power and selection of an optimal frequency (reducing the reflected power measured at the generator to zero), a temperature of 44° C. was measured. 
     As noted above, the temperature at the feedline braid point  108  during ablation correlates with microwave emission from the antenna. Testing of the device also demonstrated an inverse relationship between the reflected power detected by the microwave generator and the measured temperature, providing an additional independent measure of microwave energy emission. 
     Use of Thermocouple Wire as Pull Wire 
     In accordance with a further embodiment of the invention, the thermocouple wire can be used to serve the double function of temperature monitoring and catheter steering. The detail shown in  FIGS. 4A and 4B  illustrates use of thermocouple wire  100  as a pull wire, used for flexing and thus steering the distal end of the device during insertion. 
     To this end, a part of the microwave feedline  22  is provided with a flexion sheath  102 , made from a relatively non-compressible material. Flexion sheath  102  encases the feedline from a point  108  at the proximal end of antenna  34  (at the termination of conductive shield  28 ) to a point  104  where it is anchored to outer sheath  26  of the feedline, a distance of for example 30 mm, defining the longitudinal extent of the desired flexion portion of the catheter. The inner diameter of flexion sheath  102  is larger than the outer diameter of feedline sheath  26 , to provide room to accommodate thermocouple wire  100  for longitudinal movement, as discussed below. 
     Flexion sheath  102  includes along its length on one side a series of regularly spaced flexible striations  103 , which may be transverse cuts in the material, or may comprise a soft, flexible material intercalated along the length of the flexion section. In either form, these striations allow flexion sheath  102 , on that side only, to readily compress (remaining resistant to compression on the opposite side). This arrangement therefore provides a mechanism comprising a relatively incompressible ‘spine’ and a compressible arrangement of ‘ribs’, flexion enabled in the direction opposite the spine. 
     From point  104 , on the same side of the feedline  22 , a hollow cable  101  of a relatively non-compressible material (to prevent compression in the axial direction, but generally able to deflect relatively easily in the lateral direction) runs to the proximal part of the catheter, secured to the outer feedline sheath  26  by jacketing within the feedline, or alternatively secured within a lumen of the outer sheath  46 . The internal bore of cable  101  is sized to accommodate thermocouple wire  100 , and this arrangement ensures the wire is retained close to the feedline core of the catheter. 
     As  FIG. 4A  shows, the constantan wire  100  is run along the bore of cable  101  and along the inside of flexion sheath  102 , and its terminal portion is then wrapped around the distal end  106  of the conducting braid of feedline shield  28  (one or more times) and electrically joined (by secure soldering) thereto at point  108 , to produce the thermocouple hot junction. At this point the distal end of flexion sheath  102  is sealed over this electrical joint so that both of its ends are secured around feedline  22  (at points  104  and  108 ). Wire  100  is thus free to run freely from this joint point  108  to the proximal part of the catheter where it is arranged for access and manipulation by an operator. A constantan wire is selected having sufficient tensile strength to handle relatively significant tension, allowing it to reliably transfer force to the catheter tip. 
     Wire  100  thus provides a pull wire function, as known in the general field of deflecting tip catheters. When wire  100  is pulled in direction A, the wire length  110  along this flexion portion shortens, producing flexion of sheath  102  by closing or compressing of the striations  103  and resulting in the bending shown in  FIG. 4B . In the configuration  112  of maximum flexion, the striations  103  are fully closed or compressed. As will be understood, the flexion radius can be selected by choosing the particular arrangement and dimensions of the striations  103  of flexion sheath  102 , so providing a ‘tight curve’ or a ‘wide curve’ catheter, depending on the particular application. 
     When wire  100  is released, the natural elasticity of the materials of the catheter results in a return to the original, straight configuration. As will be appreciated, the wire is always retained parallel to the axial direction of the catheter along its length, so minimizing the risk of the wire fatiguing at any point. 
     In this way, the tip of the catheter can be steered by manipulation of thermocouple wire  100 , so guiding the catheter into the desired ablation position, without the need to incorporate a separate pull wire in the catheter assembly. 
     Alternative means of providing the desired directional flexibility of the catheter are of course possible, such as use of a coil-reinforced outer sheath, and/or use of a strip of stainless steel (or similar relatively incompressible material) to provide the spine of the flexion portion, the remainder of this portion of the catheter being of an elastomeric material able to compress as required, the catheter thus able to flex in a direction opposite to the location of the spine. 
     Temperature Measurement—Trials and Results 
       FIG. 5  graphically illustrates measured temperature fluctuations in use of the device of the invention during a 110 W trial ablation procedure, including arterial injection and irrigation failure. These results demonstrate that the device provides a reliable feedback measure of the conditions in the distal portion of the catheter. 
     The referenced points and phases of the procedure are: 
     A Renal angiogram 
     B Thermodilution curve produce by injection of room temperature contrast agent 
     C Catheter irrigation interrupted 
     D Resulting sharp rise in temperature 
     E Microwave ablations stopped 
     As noted above, and as  FIG. 5  illustrates (phase B), the temperature monitoring afforded by the invention can also assist in providing a measure of blood flow velocity. During microwave renal artery denervation, injection of room temperature fluid into the renal artery from the guiding sheath creates transient reductions in catheter temperature. Monitoring temperature against time provides useful information on transit times (from the guiding sheath exit to the thermocouple location) and thus renal arterial flow, and together with measures of blood pressure can be used to estimate renal microvascular resistance. 
       FIG. 6  provides an example of an in vivo microwave denervation procedure in a large animal model, and in particular illustrates changes in temperature during tuning of the microwave generating source in the range 2400-2500 MHz in order to find a frequency with maximum braid temperature rise (and thus minimal reflected power), and therefore optimize tissue coupling. 
     The referenced points and phases of the process are:
         A′ Initial baseline (blood temperature), approx. 37° C.   B′ Catheter irrigation commenced (30 ml/m)   C′ Microwave ablation commenced (110 W, 2400 MHz), from which point applied microwave frequency automatically increased to 2500 MHz over a period of 10 seconds; temperature increases rapidly   D′ Peak temperature, achieved at 2450 MHz (associated with OW reflected power measured at generator)   E′ Lowest measured temperature, indicating poor coupling (associated with 12 W reflected power measured at generator)   F′ Tuning completed on reaching 2500 MHz, at which point ablation continues at selected frequency of 2450 MHz       

     Impedance Measurement to Monitor Vascular Calibre During Ablation 
     Microwave heating is radiant and can penetrate deeply into tissue, so catheter devices of the type described in WO2016/197206 can perform deep circumferential ablation with sparing of injury to tissue adjacent to the flowing blood pool. 
     During microwave renal denervation procedures it is important to be able to monitor renal arterial calibre. Reductions in renal arterial calibre increase the risk of thermal arterial injury, as the arterial wall is brought closer to the microwave antenna and is thus exposed to more rapid heating, while the vascular contraction can result in a reduced arterial blood flow and thus a reduced rate of cooling. On the other hand, renal arterial dilatation can provide evidence of successful renal nerve ablation and provide a physiological endpoint to ensure effective therapy delivery. 
     The inventors have determined that monitoring the impedance of the blood pool around the microwave ablation catheter device  10  can provide a measure of vascular calibre. While impedance monitoring is known in cardiovascular procedures, this is generally for measuring changes in tissue impedance as the tissue heats. 
     As shown in  FIG. 7 , an embodiment of device  10  includes two electrodes  204 ,  206 , respectively positioned on the outside and the inside of catheter outer sheath  46 , at approximately the same axial position, proximal of the catheter radiator portion. In a first form, these electrodes are provided as the stripped ends of wires  200  and  202  that run the length of the catheter from the proximal end. 
     Wires  200  and  202  connect via suitable connectors in handle H to patient cable  52  and from there through electrical cabling  58  to electrical power/control unit  60 , which includes appropriate circuitry and processing means to measure, record and provide display of the impedance between electrodes  204  and  206 . 
     Once an alternating electrical potential is applied to wires  200  and  202 , with the catheter within the blood pool and the saline irrigation fluid filling the catheter distal portion, an ionic conductivity path  210  is formed from electrode  206 , along the inside of the catheter in the fluid volume surrounding feedline  22  and radiator  24 , through one or more of the six slit orifices  50 , and back along the outside of the catheter in the blood to electrode  204 . Measuring the current flow thus provides a measure of the impedance between electrodes  204  and  206 , namely the impedance of the saline volume and the blood volume through which the electrical path passes, and changes in this impedance can provide an indication of changes in the vessel calibre. As will be understood, as artery  12  expands during a denervation procedure, the electrical characteristics of the part of the electric circuit inside the catheter do not substantially change, but the lower resistive path of the part of the circuit outside the catheter has a noticeable effect on the overall impedance. 
     Hence, it is necessary that external electrode  204  is in the blood flow, and  FIGS. 8 and 9  provide detail of suitable alternative ways of realizing the electrodes. In these figures, the reference S indicates the start of the terminal portion of wires  200  and  202  where the insulation is removed. 
     In  FIG. 8 , wire  202  runs along the catheter in the space between catheter sheath  46  and feedline sheath  26 , its stripped end portion  202 ′ bent back on itself by 180° and its tip then electrically connected and secured to ring electrode  206  around feedline sheath  26 . Wire  200  similarly runs along the catheter in the space between catheter sheath  46  and feedline sheath  26 , its stripped end portion  200 ′ passing through a puncture in sheath  46 , bent back on itself and its tip then electrically connected and secured to external ring electrode  204  around catheter sheath  46 , such that both ring electrodes are longitudinally coincident at a position approximately 10-15 mm from the end of the feedline braid  28  (the proximal end of antenna  34 ). A suitable adhesive is used to seal the puncture hole. 
     In an alternative form, external electrode  204  may be provided in a manner independent of device  10 . For example, it may be disposed at or near the distal end of guiding sheath  150  (for example, adjacent to the position where a radiopaque ring is commonly located), or it may be provided as a reference patient return electrode at a suitable location. Generally, such solutions are not the preferable approach, as they necessitate use of a separate electrical connection lead to the impedance measuring circuitry of electrical power/control unit  60 . However, such an arrangement can have the advantage of reducing and simplifying the componentry of device  10 , so minimizing the calibre of the catheter sheath  46 . 
     As will be understood, it is important to terminate wires  200  and  202  before the radiator portion of the catheter, to ensure any metal components are positioned outside the microwave field and to avoid interference on both the field application and the impedance circuit that would otherwise result. Further, ring electrodes  204  and  206  are preferably not complete conducting rings, i.e. are preferably C-shaped rather than O-shaped, to avoid closing the electrical path, potentially rendering them parasitic inductors in the microwave field, which could lead to unwanted heating. 
     The alternative electrode arrangement in  FIG. 9  includes internal electrode  206  as the terminal part  202 ′ of wire  202 , bent back on itself by 180° and its tip simply secured around feedline sheath  26  by heat shrink  220 . Wire  200  passes through a puncture in sheath  46 , and external electrode  204  comprises a loop  201  of the stripped wire end portion  200 ′, passed around the outside of catheter sheath  46  and secured thereto by heat shrink or adhesive. The loop form of electrode  204 —in both of the variants illustrated in  FIGS. 8 and 9 —ensures electrical contact with the blood pool, and the loop does not electrically connect back to itself (the return point shown in  FIG. 9  is proximal of the start of the stripped insulation), to avoid closing the electrical path around the loop and the associated risk of inductive heating by the microwave field, as discussed above with reference to the embodiment of  FIG. 8 . 
     During their course along the outside of feedline sheath  26 , wires  200 ,  202  may be secured thereto by glue joints or bands of heat shrink. 
     In a further embodiment of the present invention, the inventors developed and tested an alternative version of catheter  10  in which wires  200 ,  202  were integrated within the wall material of catheter sheath  46  at fabrication, thus wholly electrically insulated from the inside or outside of the sheath. In this version, electrodes  204  and  206  were formed as incomplete ring structures (of similar form to those of the embodiment shown in  FIG. 8 ), one integrated (by melt-embedding) in the exterior surface of the catheter sheath wall, one in the interior surface. Like the wires, these electrodes were formed at fabrication of sheath  46 , to present outer and inner surfaces, respectively, flush with the corresponding surfaces of the sheath wall, so to prevent any undesirable surface discontinuities. 
     One advantage of providing both electrodes on the catheter sheath  46  is to ensure the intervening distance is functionally constant, regardless of any relative movement of the feedline within, thus avoiding any associated measurement artefact. 
     Impedance Measurement—Trials and Results 
     The concept of monitoring vascular dilatation using an impedance circuit in a denervation catheter was tested by the inventors in animal trials, the graphical output of impedance against time shown in  FIG. 11 . 
     The referenced points and phases of the procedure are:
         A″ Baseline (fluctuations of arterial size corresponding to respiration and changes in intra-abdominal pressure)   B″ Ablation start   C″ Angiogram  1  ( FIG. 10A )   D″ Balloon occlusion   E″ Angiogram ( FIG. 10B )       

     Impedance drop results from the heating effect of microwave radiation on the fluid, but impedance increases with increased rates of irrigation due to the cooling effect of room temperature saline. From the start of the microwave ablation at the end of phase A″ the impedance drops for around 30 s, due to the warming of the saline around the microwave radiator. 
     At about 72 s the injection of cold contrast media causes the steep transient in measured impedance to point C″, where the first angiogram is taken.  FIG. 10  shows the position of radiator  24 , catheter tip  42  and electrodes  204 ,  206  in the renal artery  12 . 
     At this point, balloon occlusion of the suprarenal descending aorta (balloon occlusion device  210  shown in  FIG. 10B ) results in blood pressure drop and hence mild collapse of renal artery  12 . This vascular contraction clearly translates as rising impedance during phase D″ of the procedure. 
     The second angiogram corresponds to point E″ in  FIG. 11 , which also shows contracted artery  12 . 
     In this example, an impedance change of approximately 250 ohms was observed, with a reduction of vessel calibre from approximately 6 mm to 5 mm. 
     This experiment clearly demonstrates the value of impedance monitoring as a measure of vascular calibre, and hence its value as a feedback mechanism in vascular denervation therapy. 
     In addition to providing an indication of the points and phases in the procedure discussed above, the invention can provide an indication of deployment of the locating formation(s)  48 , provided the fluid path traverses the position of a formation. Once a locating formation is deployed, then any observed change in impedance should be due solely to vascular calibre change. But during deployment the impedance is sensitive to the distension of the locating formation, and the invention can thus be used to confirm successful deployment. 
     It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention. 
     As used herein, except where the context requires otherwise, the term “comprise” and variations of the term, such as “comprising”, “comprises” and “comprised”, are not intended to exclude further additives, components, integers or steps.