Patent Publication Number: US-9415188-B2

Title: Bioimpedance-assisted placement of a medical device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/283,395, filed Oct. 27, 2011, now U.S. Pat. No. 8,801,693, which claims the benefit of U.S. Provisional Application No. 61/408,181, filed Oct. 29, 2010, and titled “Bioimpedance-Assisted Catheter Placement,” each of which is incorporated herein by reference in its entirety into this application. 
    
    
     BRIEF SUMMARY 
     Briefly summarized, embodiments of the present invention are directed to a system and method for guiding a catheter or other medical device to a desired target destination within the vasculature of a patient via bioimpedance measurements. The target destination in one embodiment includes placement of the catheter such that a distal tip thereof is disposed proximate the heart, e.g., the junction of the right atrium and superior vena cava. 
     In one embodiment the method for guiding the catheter comprises introducing the catheter into a vessel of the patient, the catheter defining a lumen through which fluids can be infused into the vasculature of the patient. The catheter is advanced toward a target destination within the vasculature. A first impedance value based on intravascular detection of at least one electrical property related to a first tissue surface of the vessel, such as electrical current and voltage, is calculated to enable determination of the proximity of a distal end of the catheter to the target destination. 
     These and other features of embodiments of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of embodiments of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more particular description of the present disclosure will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. Example embodiments of the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a block diagram of an example system with which embodiments of the present invention can be practiced; 
         FIG. 2  is a simplified view of a patient and a catheter being inserted therein using the system of  FIG. 1 ; 
         FIG. 3  is a partial cutaway view of the catheter of  FIG. 2  disposed in a vessel of a vasculature of the patient according to one embodiment; 
         FIG. 4  is a simplified schematic of portions of the system of  FIG. 1 , according to one embodiment; 
         FIG. 5  is a simplified view of a heart and surrounding vasculature of a patient together with the catheter of  FIG. 2 ; 
         FIG. 6  is a perspective view of a distal portion of a catheter including a guiding stylet disposed therein according to one embodiment; 
         FIG. 7  is a side view of a distal portion of a catheter including a directional flap according to one embodiment; 
         FIG. 8  is a side view of a distal portion of a catheter including two directional flaps according to one embodiment; 
         FIGS. 9A and 9B  are partial cross sectional side views showing a distal portion of a catheter disposed in a vessel and including a deployable wing according to one embodiment; 
         FIG. 10  is a partial cross sectional side view showing a catheter disposed in a vessel and including an electrode pair according to one embodiment; 
         FIG. 11  is a cross sectional bottom view of a catheter disposed in a vein according to one embodiment; 
         FIG. 12  is a cross sectional bottom view of a catheter disposed in an artery according to one embodiment; and 
         FIG. 13  is a partial cutaway view of the catheter of  FIG. 2  disposed in a vessel of a vasculature of the patient according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF SELECTED EMBODIMENTS 
     Reference will now be made to figures wherein like structures will be provided with like reference designations. It is understood that the drawings are diagrammatic and schematic representations of exemplary embodiments of the present invention, and are neither limiting nor necessarily drawn to scale. 
     For clarity it is to be understood that the word “proximal” refers to a direction relatively closer to a clinician using the device to be described herein, while the word “distal” refers to a direction relatively further from the clinician. For example, the end of a catheter placed within the body of a patient is considered a distal end of the catheter, while the catheter end remaining outside the body is a proximal end of the catheter. Also, the words “including,” “has,” and “having,” as used herein, including the claims, shall have the same meaning as the word “comprising.” 
     Embodiments of the present invention are generally directed to a system and method for guiding to a desirable anatomic location a medical device, such as a peripherally inserted central catheter (“PICC”) or other catheter. In particular, certain embodiments to be discussed describe assisting placement of a catheter or other medical device within the vasculature of the body of a patient such that a distal tip thereof is disposed proximate the heart, e.g., the junction of the right atrium (“RA”) and superior vena cava (“SVC”). In one embodiment, guidance of a catheter tip to such a location is achieved by using bio-impedance measurements, which can enhance clinical efficacy and improve patient safety. Thus, a mapping between body impedance and intravascular anatomic location can be achieved in one embodiment. Note that the catheters to be described for placement within the patient by way of the systems and methods discussed herein include those defining one or more lumens for the infusion and aspiration of fluids from the vasculature. It should be remembered, however, that other types of catheters and medical devices can be placed using the principles described herein. As such, the discussion to follow should not be construed as limiting in any way. 
     In brief, tissue impedance is a location-specific phenomenon within the patient vasculature. For example, in the thoracic cavity there is measurable tissue impedance difference between the different heart chambers as well as between atrial tissue and adjacent vessels, including the inferior vena cava (“IVC”) and the SVC. Indeed, atrial tissue mainly includes myocardial tissue that exhibits a relatively high electrical conductivity, and thus a relatively low impedance. In contrast, vascular tissue, e.g., regions of the vena cava (the IVC and the SVC), includes mainly smooth muscle cells that are much thinner than the atrial tissue and therefore possess a relatively low electrical conductivity, and thus a relatively high impedance. At the junction of the RA and SVC, the atrial tissue and vascular tissue meet one another and thus define an impedance “border zone” where relatively low impedance tissue meets relative high impedance tissue. This region is but one example where relative differences in impedance are found within the patient vasculature. 
     In accordance with one embodiment, a system is disclosed for enabling such impedance variations to be monitored during advancement of a catheter or other medical device within the vasculature of the patient so as to enable positioning of a distal tip of the catheter at a desired target destination. The system in one embodiment includes, among other components, a purpose-specific electrical circuit, processor, and display for monitoring intravascular bioimpedance via electrodes disposed on a distal portion of the catheter. The system and methods described herein provide a clinician with guidance to assist in directing the distal tip of the catheter to the desired target destination via feedback of impedance detected by the electrodes during catheter advancement through the vasculature. Further, the system can be employed to confirm the catheter distal tip position after catheter advancement is complete. Again, note that the catheter positioned by the system and methods discussed herein is merely representative of one of many different types of catheters or other suitable indwelling medical devices. 
     Reference is first made to  FIGS. 1 and 2  which depict various components of a catheter placement system (“system”), generally designated at  10 , configured in accordance with one example embodiment of the present invention. As shown, the system  10  generally includes a console  20 , display  30 , ultrasound probe  40 , sensor  50 , and impedance components  60 , each of which is described in further detail below. 
     Note that the particular components to be employed in guiding a catheter via impedance measurements are shown here in the system  10 , which system also includes additional catheter insertion and guidance functionality, including a pre-insertion ultrasound-based vessel visualization modality and a magnetic-based catheter tip guidance modality, as will be discussed below. This notwithstanding, it is understood that the impedance-based catheter guidance modality, also discussed below, can be employed independent and apart from the other catheter insertion and advancement assistance features of the system  10 . Indeed, the system  10  may only include an impedance-based catheter guidance modality, in one embodiment. As such, the present discussion presents merely one example of an environment in which embodiments of the present invention can be practiced. 
       FIG. 2  shows the general relation of the above-referenced components to a patient  70  during a procedure to place a catheter  72  into the patient vasculature through a skin insertion site  73 .  FIG. 2  shows that the catheter  72  generally includes a proximal portion  74  that remains exterior to the patient and a distal potion  76  that resides within the patient vasculature after placement is complete. The system  10  is employed to ultimately position a distal tip  76 A of the catheter  72  in a desired position within the patient vasculature. In one embodiment, the desired position for the catheter distal tip  76 A is proximate the patient&#39;s heart, such as in the lower one-third (⅓ rd ) portion of the SVC. Of course, the system  10  can be employed to place the catheter distal tip in other locations. The catheter proximal portion  74  further includes a hub  74 A that provides fluid communication between the one or more lumens of the catheter  72  and one or more extension legs  74 B extending proximally from the hub. 
     The console  20  of  FIGS. 1 and 2  can take one of a variety of forms and optionally houses various system components. A processor  22 , including non-volatile memory such as EEPROM for instance, is included in the console  20  for controlling system function and intravascular impedance calculations during operation of the system  10 , thus acting as a control processor. A digital controller/analog interface  24  is also included with the console  20  and is in communication with both the processor  22  and other system components to govern interfacing between the ultrasound probe  40 , sensor  50 , the impedance components  60 , and other system components. 
     In greater detail, the impedance components  60  of the console  20  include means for measuring electrical current delivered to electrodes disposed on the catheter  72 , as will be described. In the present embodiment, the means for measuring current includes an ammeter  64  implemented as a sampling circuit or other suitable form. Means for measuring voltage across the electrodes is also included. In the present embodiment, the means for measuring voltage includes a voltmeter  66  implemented as a sampling circuit or other suitable form. Of course, other devices can be employed to achieve the functionality of the aforementioned means. A radiofrequency (“RF”) or current source  62  is also included for providing an electrical current to the catheter electrodes, as will be described. In addition to these components, other components for enabling impedance intravascular detection can also be added to the system  10 , catheter  72 , or both. As shown in  FIG. 1 , the ammeter  64 , the voltmeter  66 , and the RF source  62  are operably connected to the processor  22  and ports  52  to enable interoperability therewith. Note that the aforementioned components can be disposed in locations other than the console  20 . 
     As mentioned, the system  10  further includes ports  52  for connection of console components with the sensor  50  and optional components  54  including a printer, storage media, keyboard, audio speaker, etc. The ports  52  in one embodiment are USB ports, though other port types or a combination of port types can be used for this and the other interfaces connections described herein. A power connection  56  is included with the console  20  to enable operable connection to an external power supply  58 . A battery or other suitable internal power supply  57  can also be employed, either with or exclusive of the external power supply  58 . Power management circuitry  59  is included with the digital controller/analog interface  24  of the console to regulate power use and distribution. 
     The display  30  in the present embodiment is integrated into the console  20  and is used to display impedance and other information to the clinician during the catheter placement procedure. In another embodiment, the display may be separate from the console. As will be seen, the content depicted by the display  30  changes according to which mode the catheter placement system is in: ultrasound vessel visualization, magnetic-based catheter guidance, impedance-based catheter guidance, etc. In one embodiment, a console button interface  32  and buttons included on the ultrasound probe  40  can be used to immediately call up a desired mode to the display  30  by the clinician to assist in the placement procedure. In one embodiment, information from multiple modes, such as magnetic and impedance-based catheter guidance, may be displayed simultaneously. Thus, the single display  30  of the system console  20  can be employed for ultrasound guidance in accessing a patient&#39;s vasculature, magnetic-based guidance during catheter advancement through the vasculature, and impedance-based guidance and/or confirmation of catheter distal tip placement with respect to a desired target destination within the vasculature, for instance. In one embodiment, the display  30  is an LCD device. 
     The ultrasound probe  40  is employed in connection with the first modality mentioned above, i.e., ultrasound (“US”)-based visualization of a vessel, such as a vein, in preparation for insertion of the catheter  72  into the vasculature. Such visualization gives real time ultrasound guidance for introducing the catheter into the vasculature of the patient and assists in reducing complications typically associated with such introduction, including inadvertent arterial puncture, hematoma, pneumothorax, etc. 
     The handheld probe  40  includes a head that houses a piezoelectric array for producing ultrasonic pulses and for receiving echoes thereof after reflection by the patient&#39;s body when the head is placed against the patient&#39;s skin proximate the prospective insertion site  73  ( FIG. 2 ). The probe  40  further includes a plurality of control buttons, which can be included on a button pad. In one embodiment, the modality of the system  10  can be controlled by the control buttons, thus eliminating the need for the clinician to reach out of the sterile field, which is established about the patient insertion site prior to catheter placement, to change modes via use of the console button interface  32  ( FIG. 1 ). 
     As such, in one embodiment a clinician employs the first (US) modality to determine a suitable insertion site and establish vascular access, such as with a needle or introducer, then with the catheter. The clinician can then seamlessly switch, via button pushes on the probe button pad, to another modality, such as magnetic-based or impedance-based catheter guidance, without having to reach out of the sterile field. These latter modes can then be used to assist in advancement of the catheter  72  through the vasculature toward an intended target destination. 
       FIG. 1  shows that the probe  40  further includes button and memory controller  42  for governing button and probe operation. The button and memory controller  42  can include non-volatile memory, such as EEPROM, in one embodiment. The button and memory controller  42  is in operable communication with a probe interface  44  of the console  20 , which includes a piezo input/output component  44 A for interfacing with the probe piezoelectric array and a button and memory input/output component  44 B for interfacing with the button and memory controller  42 . Note that the console button interface and probe interface can, in one embodiment, include a touch screen, voice command, or other suitable functionality to enable ease of system control for the clinician. 
     The sensor  50  is employed by the system  10  during operation in the magnetic sensing mode to detect a magnetic field produced by magnetic elements included in a stylet that is removably received in the lumen of the catheter  72 . As seen in  FIG. 2 , the sensor  50  is placed on the chest of the patient during catheter insertion. The sensor  50  is placed on the chest of the patient in a predetermined location, such as through the use of external body landmarks, to enable the magnetic field of the stylet magnetic elements, disposed in the catheter  72  as described above, to be detected during catheter transit through the patient vasculature. The magnetic elements of the stylet magnetic assembly are co-terminal with the distal end  76 A of the catheter  72  ( FIG. 2 ) such that detection by the sensor  50  of the magnetic field of the magnetic elements provides information to the clinician as to the position and orientation of the catheter distal end during its transit within the vasculature. 
     In greater detail, the sensor  50  is operably connected to the console  20  of the system  10  via a cable and one or more of the ports  52 , as shown in  FIG. 1 . Note that other connection schemes between the sensor and the system console can also be used without limitation. As just described, the magnetic elements are employed in the stylet  100  to enable the position of the catheter distal end  76 A ( FIG. 2 ) to be observable relative to the sensor  50  placed on the patient&#39;s chest. Detection by the sensor  50  of the stylet magnetic elements is graphically displayed on the display  30  of the console  20  during magnetic guidance mode. 
     In this way, a clinician placing the catheter is able to generally determine the location and/or orientation (e.g., which way the distal tip  76 A of the catheter  72  is pointing) of the catheter distal end  76 A within the patient vasculature relative to the sensor  50  and detect when catheter malposition, such as advancement of the catheter along an undesired vein, is occurring. In one embodiment, the magnetic assembly can be tracked using the teachings of one or more of the following U.S. Pat. Nos. 5,775,322; 5,879,297; 6,129,668; 6,216,028; and 6,263,230. The contents of the afore-mentioned U.S. patents are incorporated herein by reference in their entireties. Note again that buttons included on either the console  20  or the ultrasound probe  40  can be used to control system functionality during ultrasound mode, magnetic-based catheter guidance mode, or impedance-based catheter guidance mode. 
     Note that the system described herein in one embodiment can include additional functionality wherein determination of the proximity of the catheter distal tip relative to a sino-atrial (“SA”) or other electrical impulse-emitting node of the heart of the patient can be determined, thus providing enhanced ability to accurately place the catheter distal tip in a desired location proximate the node. Also referred to herein as “ECG” or “ECG-based tip confirmation,” this additional modality of the system enables detection of ECG signals originating from the SA node in order to place the catheter distal tip in a desired location within the patient vasculature. Note that the ECG modality can be seamlessly combined with the other modalities of the system as described herein, namely ultrasound, magnetic-based catheter tracking, and impedance-based tracking to be described further below. Further details regarding this ECG modality and the other modalities described above can be found in U.S. Patent Application Publication No. 2011/0015533, filed Sep. 29, 2010, and entitled “Stylets for use with Apparatus for Intravascular Placement of a Catheter,” which is incorporated herein by reference in its entirety. 
       FIG. 2  shows that the catheter  72  is operably connected to the sensor  50  atop the patient&#39;s chest via a tether  78 , with the sensor in turn operably connected to the console  20  and its included components via a cable. In this way, the electrode is operably connected to the RF source  62 , the ammeter  64 , the voltmeter  66 , the processor  22 , the display  30 , and the other system components employed during operation thereof. 
       FIG. 3  shows a distal portion  76  of the catheter  72  disposed in a vessel  80  of the patient  70 , as inferred in  FIG. 2 . As shown, the distal portion  76  includes the electrode array  90 , including first and second electrodes  90 A and  90 B that are operably connected to conductive wires or the like longitudinally extending proximally in the catheter wall, for instance, and operably connecting to the tether  78  ( FIG. 2 ) so as to operably connect the electrodes  90 A,  90 B of the electrode array  90  with the RF source  62  ( FIG. 1 ). It is appreciated that the electrode array and constituent electrodes can be configured in a variety of ways and that the shape, number, position, and type of electrodes can vary from what is depicted and described herein. For example, the electrodes can be included proximate a distal end of a stylet that is removably received within a lumen of the catheter. These and other possible configurations are therefore contemplated. 
       FIG. 4  depicts a simplified schematic of the various components directly involved in measuring an impedance of the tissue surface  92  of a vessel and the operating relationship to one another, according to one embodiment. The components include the RF source  62 , which provides an RF current to the electrode array  90 , including the electrodes  90 A and  90 B that bound either side of the tissue surface  92  under evaluation. The magnitude of the current provided by the RF source  62  can be measured by the ammeter  64  and forwarded to the processor  22  of the console  20 . The magnitude of the voltage difference between the two electrodes  90 A and  90 B across the tissue under evaluation can be measured by the voltmeter  66  and forwarded to the processor  22 . 
     With the system  10  and catheter  72  configured as shown in  FIGS. 3 and 4 , the catheter  72  can be accurately positioned within the patient vasculature by first disposing the catheter within a vessel of the vasculature such that the electrode array  90  proximate the distal tip  76 A thereof is adjacent to a tissue surface, such as the tissue surface  92  of  FIG. 3 , thus providing electrical communication between the electrode array  90  and the tissue surface  92  of the vessel  80 . In one embodiment, such electrical communication is achieved by positioning the catheter  72  within the vessel  80  such that the electrodes  90 A,  90 B physically touch the tissue surface  92  of the vessel  80 , as in  FIG. 3 . An electrical RF current produced by the RF source  62  can then be provided to the electrode array  90 . In one embodiment, the current includes a predetermined frequency and is of relatively low power. Thus, with the distal portion  76  of the catheter disposed against a tissue surface  92 , i.e., the inner wall of the vessel  80  in the present embodiment and as shown in  FIG. 3 , the RF current is provided to the electrode array  90  and measured by the ammeter  64 . The resultant voltage difference between the electrodes  90 A and  90 B across the tissue surface  92  is measured by the voltmeter. The magnitudes of the RF current and voltage are forwarded to the processor  22 . 
     Upon receipt of the current and voltage data from the ammeter  64  and voltmeter  66  respectively, the processor can calculate the impedance in the region of the tissue surface under evaluation, also referred to herein as bioimpedance, according to the equation:
 
Impedance( Z )=Volts( V )/Current( I ).  (1)
 
     As such, in the present embodiment, the processor includes suitable control algorithms with embedded software to sample the current and voltage data (and any other biophysical parameters), in order to automatically calculate the bioimpedance. The resulting impedance data as calculated by the processor  22  or other suitable system component can be depicted on the display  30  for observation by the clinician. In addition, audio tones or other suitable signals can be output by the speaker  54  or other suitable output device so as to provide additional feedback to the clinician. For instance, upon reaching the junction of the RA and the SVC, an area where a significant change in tissue impedance is encountered, the display can indicate the detected position of RA/SVC junction, and the audio speaker  54  can emit a predetermined audio tone to indicate the desired anatomic target location. 
     The above process can be iterated in real time as the catheter distal tip  76 A is advanced in the vessel so as to provide real-time updating as to the calculated impedance value according to the present position of the distal tip of the catheter  72 . For instance, a first impedance calculation is calculated and displayed for a first location within the vessel of the catheter distal tip, then a second impedance calculation is calculated and displayed for a second distal tip location. Such a process can be iteratively performed and the resultant impedance values compared so as to enable a clinician to discern when the catheter distal tip is disposed at a desired target location, such as the RA/SVC junction, for instance. 
       FIG. 5  shows a heart  96  of a patient, including areas in and proximate thereto of varying impedance, including the SVC  98 , the RA/SVC junction  100 , and the RA  102 . The SVC  98 , for instance includes a relatively high impedance, such as about 130-140 ohms, in one example, while the RA/SVC junction  100  is at a relatively lower impedance of about 118 ohms for instance. The RA  102  is of even lower impedance, such as about 84 ohms in one example. Such location-based variations in tissue impedance values can be employed by the system  10  to determine the location of the electrode array  90  of the catheter  72  ( FIGS. 1, 3 ). Further, knowledge of the distance from the electrode array  90  to the distal tip  76 A of the catheter enables the position of the catheter tip to be determined within the vasculature, thus enabling its precise placement at a desired target location. Note that the above impedance values are for purposes of illustration and should not be considered limiting. 
     In light of the above, therefore, comparison of subsequent impedance calculations for successive catheter distal tip locations in the vessel can indicate proximity to a desired target location. For instance, a relatively small decrease in impedance values between first and second tissue surfaces can indicate that the electrodes have passed from the SVC  98  to the RA/SVC junction  100  ( FIG. 5 ), while a relatively larger impedance decrease between the first and second interior surfaces can indicate that the electrodes have passed from the SVC to the RA  102 . This or other suitable processes can be expanded to use multiple electrode arrays, multiples impedance readings, etc. In one embodiment, only a single impedance reading may be necessary to determine the location of the catheter distal tip with respect to the RA/SVC or RA, for instance. 
     As indicated above, measurement of impedance values at a given catheter location, followed by movement of the catheter and subsequent measurement at the new location, can be iteratively performed so as to determine when the catheter has been desirably placed, such as proximate the RA/SVC junction, for instance. It is appreciated that in one embodiment, the system  10  includes suitable algorithms to calculate, track, store, and display the impedances at the various discrete catheter locations and the impedance change as the catheter is advanced within the vasculature. Further the system  10  can include various functionality to depict and display the tracked data in a user-friendly visual format for depiction on the display  30 , including electronic circuits for displaying the impedance data and/or other biophysical parameters in digital and/or analog format. Note that example insertion sites for the catheter into the patient&#39;s vasculature include the arm (cephalic vein), neck (jugular vein) and the groin (femoral artery). Other insertion sites can, of course, be used. 
     In one embodiment, communication ports and software can be included with the system  10  to enable biophysical parameters sensed and/or employed by the system, e.g., impedance, current, and voltage, to be exported for use by other medical equipment, such as clinical vital sign equipment, hemodynamic systems, anesthesia systems, electrophysiology lab systems, computers, storage systems, data analysis systems, etc. 
     In other embodiments, the electrode array of the system can vary from what is described herein for use in identifying and confirming the specific anatomic location within the vasculature and proximate the heart, including bipolar and/or monopolar electrodes that are included with a catheter, included stylet, or other indwelling medical device. Further, in one embodiment, the impedance values detected by the system described herein can be used to map the vasculature about the heart, which data can be correlated with radiographically acquired landmarks of the patient&#39;s anatomy. 
     As mentioned above, the electrodes  90 A,  90 B are operably connected to the console  20  by the tether  78  via the sensor  50 , in one embodiment. In this case, the tether  78  and/or associated connectors are configured to penetrate through a sterile barrier surrounding a sterile field established about the patient&#39;s catheter insertion site without compromising the sterile field so as to enable the electrodes  90 A,  90 B to operably connect with the console  20 . Examples of and further details regarding such sterile field breaching can be found in U.S. Patent Application Publication No. 2011/0015533, which is incorporated by reference above. 
     In a further embodiment, it is appreciated that multiple electrodes or electrode arrays can be included or associated with the catheter such that multiple impedance measurements can be made simultaneously at differing locations along the length of the distal portion of the catheter. In yet another embodiment and as mentioned above, it is appreciated that ECG-based catheter tip location can be used in concert with the impedance-based location techniques described herein. In such a configuration, the ECG-based location method can be used to direct the catheter distal tip to a generally preferred area, after which impedance-based location can be employed to precisely place the catheter distal tip at a desired location within the vasculature. Further details regarding such ECG-based location can be found in U.S. Patent Application Publication No. 2011/0015533, incorporated by reference above. 
     As mentioned, in one embodiment it is necessary to position the catheter  72  within the vessel  80  such that the electrodes  90 A,  90 B of the electrode array  90  are in physical contact with the interior tissue surface  92  of the vessel, as seen in  FIG. 3 . In one embodiment, this can be achieved by deviating the distal portion of the catheter within the blood stream of the vessel in which it is disposed.  FIG. 6  shows an example of an apparatus for such a deviation, including a stylet  110  disposed within the lumen  72 A of the catheter  72  such that a distal portion of the stylet extends distal to the distal tip  76 A of the catheter. The stylet  110  includes a diversion flap  112  pivotably mounted at a distal end of the stylet so as to be able to be selectively moved between an aligned position and the deviated positions shown in phantom in  FIG. 6 . An actuating wire  114  is attached to the flap  112  and extends through the length of the stylet  110  so as to enable a clinician external to the patient to selectively deviate the flap. Deviation of the flap from its aligned position of  FIG. 6  causes the flap to interfere with the blood flow through the vessel, which in turn causes the stylet  110  and the distal portion  76  of the catheter  72  to be pushed to one side of the vessel, thus enabling the electrodes  90 A,  90 B to physically contact the interior surface of the vessel. Once physical contact of the electrodes  90 A,  90 B is no longer needed, the actuating wire  114  can be moved to bring the flap  112  into alignment, thus stopping interfering engagement of the flap with the vessel blood flow. Note that the particular configuration and shape of the flap and stylet can vary from what is shown and described herein. Also, the stylet can be extended from the catheter distal tip  76 A either less or further than what is shown in  FIG. 6 . 
       FIG. 7  shows a catheter diversion feature according to another embodiment, wherein a diversion flap  122  is included at the distal tip  76 A of the catheter  72  and has operably connected thereto an actuating wire  124  for enabling a clinician to selectively fold the flap from the aligned position to the phantom deviated position shown in  FIG. 7 . The flap  122  operates in similar fashion to the flap  112  of  FIG. 6  in causing deviation of the catheter  72  in the blood stream so that at least a portion of the electrodes  90 A,  90 B can contact the vessel interior surface. 
       FIG. 8  shows a catheter diversion feature according to another embodiment, wherein two diversion flaps  122  are included at the catheter distal end  76 A, each being operably connected to a separate actuating wire  124  so that deviation of the catheter distal portion is selectively achieved by actuating one or both of the flaps in order to deviate the distal portion in a particular direction. In one embodiment, the flaps can be used in concert to maintain the catheter in a central portion of the vessel so as to enable the catheter to be guided past difficult or tortuous vascular anatomy and to reduce vessel wall damage. Note that the number, shape, size, and particular design of the flaps disclosed herein can vary from what is shown and described. 
       FIGS. 9A and 9B  show a catheter diversion feature according to another embodiment, wherein a distal portion of the catheter  72  includes a deployable flap, or wing  132 , for interacting with the vessel blood flow to cause deviation of the distal portion  76  of the catheter toward the wall of the vessel  80  in which the catheter is disposed. In turn, this enables contact to be made between the interior wall of the vessel  80  and the catheter electrodes of the electrode array  90 . As shown in  FIG. 9B , in the present embodiment the wing  114  is triangular or semi-pyramidal in shape, though in other embodiments other wing shapes are possible. The wing can be selectively extensible/collapsible in one embodiment. 
     In one embodiment, it is appreciated that impedance-based guidance and measurement within a vasculature can be employed to detect regions of abnormality within vessels. For instance, an impedance measuring catheter or other intravascular device employing the methods as described herein can be used to detect plaque locations within coronary arteries, such as early-stage atherosclerotic lesions including foam cells and fatty deposits within intima. Such plaque deposits are unstable and are prone to rupture, which can expose the subendothelial plaque to blood flow. This in turn can lead to platelet clot formation and unstable angina or acute myocardial infarction. Detection of such regions via impedance difference measurement with respect to surrounding vessel tissue can enable prophylactic treatment (e.g., angioplasty, stents) to be commenced to alleviate any danger therefrom. 
     Impedance-based guidance and measurement can also be employed in one embodiment to detect pre-stenotic lesions in veins and/or arteries. It is noted that stenosis of atherosclerotic coronary and peripheral arteries, as well as central and peripheral veins (including veins included in an AV access circuit for hemodialysis) is a common problem often treated with angioplasty. Detection of such regions via impedance difference measurement (“mapping”) as described herein with respect to surrounding vessel tissue can enable prophylactic treatment to be commenced to prevent problems in risk areas such as those prone to restenosis and/or de novo stenosis while still in early-stage development in the vessel wall and prior to significant vessel constriction. In one embodiment, a solid body catheter or catheter including a lumen is employed for carrying the impedance electrodes for detecting stenotic and/or pre-stenotic lesions. An example configuration is shown in  FIG. 10 , wherein the catheter  72  including the electrodes  90 A,  90 B is disposed within the vessel  80  such that the electrodes are positioned adjacent a pre-stenotic lesion  140 . Differing impedance measurements on and around the lesion  140  can indicate its presence to a clinician, who may then treat the area as needed. In another embodiment, the impedance electrodes are included on an angioplasty balloon assembly, a stent assembly, or a drug-eluting balloon assembly so that the appropriate treatment (e.g., angioplasty, stenting, drug delivery) can be administered immediately after detection of the pre-stenosis region. Note that the detection of stenotic and/or pre-stenotic lesions may require, in one embodiment, the use of an RF source frequency distinct from that for impedance-based catheter tip placement. 
     In yet another embodiment, it is appreciated that impedance measurement within a vessel can be employed to ensure that access to an intended one of an artery or vein has been achieved. It is appreciated that during endovascular procedures inadvertent cannulation of a vein instead of an intended artery (or vice versa) can produce adverse effects during procedures including cardiac catheterization, central venous catheter placement, etc. Measurement of impedance values for portions of an interior wall of a vessel after access thereto is achieved can indicate whether the vessel is an artery or vein, thus enabling a clinician to confirm that the proper vessel type has been accessed. It is noted that impedance values for arteries generally fall between those of veins and myocardial tissue. This relationship enables discrimination between veins and arteries to be achieved. Thus, arteries, such as those typically cannulated during endovascular procedures (femoral, subclavian, brachial, etc.) can be identified by their impedance. Veins can be similarly identified, thus reducing the potential for adverse events related to incorrect vessel puncture. 
     The above impedance relationship is depicted in  FIGS. 11 and 12 , wherein in  FIG. 11  a relatively thick-walled vein vessel  80  having a thickness t 1  is shown. The catheter  72  including electrodes such as the electrode  90 B is disposed within the vein vessel  80 . Impedance measurements taken by the electrodes can enable the clinician to determine whether the catheter  72  is disposed within a vein or artery, as described above.  FIG. 12  shows a corresponding situation for the catheter  72  disposed in an artery vessel  80  having a thickness t 2  that is thinner relative to the thickness t 1  of the vein. As mentioned, the measured impedance of the artery will be generally lower than that for the vein but higher than that for heart-related tissue. 
       FIG. 13  shows that, in one embodiment, extended electrode wires  150  can be added to one or both of the electrodes  90 A,  90 B so as to enable the electrode array  90  to be in operable contact with the tissue surface  92  when the catheter  72  itself is not disposed adjacent the surface of the vessel. The extended electrode wires can be configured in a compliant manner so as to enable the wires to deform as necessary during advancement of the catheter  72  through the vasculature yet maintain contact with the tissue surface  92  so that impedance measurements can be taken when desired. In the present embodiment, four curved electrode wires  150  are attached to each electrode  90 A,  90 B. Note, however, that the number, size, shape, extension, and other configurations of the extended electrode wires can vary from what is shown and described herein. 
     Embodiments of the invention may be embodied in other specific forms without departing from the spirit of the present disclosure. The described embodiments are to be considered in all respects only as illustrative, not restrictive. The scope of the embodiments is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.