Patent Publication Number: US-2021183268-A1

Title: Fistula Cannulation Simulator

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 15/296,680, having a filing date of Oct. 18, 2016, which claims filing benefit of U.S. Provisional Patent Application Ser. No. 62/330,404, having a filing date of May 2, 2016, both of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     To cannulate an arteriovenous (AV) fistula for hemodialysis, it is necessary to insert two large gauge (usually 15 g to 17 g) needles through a patient&#39;s skin and into a vessel having high blood flow rate. AV fistula cannulation is an almost entirely tactile skill that requires locating the AV fistula in the subcutaneous tissue and proper assessment of the fistula orientation and depth followed by accurate placement of the needle end within the lumen of the vessel. Fistulas are identified in practice by the tactile pressure difference of the vessel longitude and circumference and by local area vibration caused by the mixed venous and arterial blood flow. The procedure is performed in clinical settings by dialysis nurses and technicians and requires great skill in correctly identifying the proper location and orientation of the fistula, as well as obtaining proper needle insertion into the vessel lumen. Patients who opt for home dialysis treatment must learn how to perform this skill themselves, generally without any prior experience in vascular access. 
     To improve the dialysis capabilities of medical personnel, nurses and technicians are trained on simulators. Unfortunately, no such simulators exist that have been designed for patients to train for self-cannulation. Moreover, even existing simulators offer only limited training effectiveness. Typical existing cannulation simulators are formed in shapes that mimic human or animal body parts (e.g., arms) and include a fixed artificial fistula covered with an artificial flesh material. While the anatomical form of the simulators can add to the perceived realism of the devices, the fixed placement and materials of the components decreases realism. Repeated piercing of the surface material to access the artificial fistula fixed beneath the surface material can reveal previous students&#39; attempt locations and decrease effectiveness of the training. Movement or replacement of the surface material can be carried out at added expense, but the gel-like artificial flesh can be difficult to replace and the replacement procedure can be messy if the simulator includes synthetic blood. 
     Due to such issues, the skills gained by use of existing simulators provide fairly limited proficiency in real-world application. Difficulties can arise following training not only due to the fixed, static nature of existing simulators but also because it is difficult to capture in existing training simulators the large degree of variation in fistula depth, orientation, skin thickness and size between patients that will be faced by clinicians. 
     Patients wishing to self-cannulate away from the clinical setting may have the opportunity to use existing simulators, but the insufficiencies described for clinical settings still apply. Currently, estimates show that only 0.8% of hemodialysis patients choose to engage in home treatment. However, it is commonly held that outcomes are better when patients choose home treatment and nephrologists prefer patients be on home dialysis. The major barrier to home dialysis is patient fear of needle sticks, but proper training could alleviate such fears. As technology improves and the pool of hemodialysis patients expands, this market is expected to grow, as will the need for better training options. 
     What is needed in the art are cannulation simulators that more closely mimic an AV fistula cannulation experience. For instance, a cannulation simulator that addresses the real-world variables that may be encountered by health-care workers during actual dialysis practice such as little or no visual cues as to the location of the fistula could be of great benefit. A cannulation simulator that provides for patients to safely train for self-cannulation that can closely represent an actual AV fistula cannulation experience would also be of great benefit. 
     SUMMARY 
     A cannulation simulation device is disclosed. The cannulation simulation device includes an artificial fistula that can include an upper barrier, a lower barrier, and an opening there between that simulates the lumen of a fistula blood vessel. The upper barrier can define a radial curvature and an axial length that can simulate the radial curvature and longitudinal length of a blood vessel. The lower barrier can include a conductive material, e.g., a metal. During use, the conductive material can be utilized to register the passage of the conductive tip of a needle tip and provide information with regard to the exit of the needle tip out of the fistula opening and through the lower barrier. This feature can be used to register to a trainee that the needle has passed out of the fistula opening. 
     An artificial fistula can be associated with a vibration motor and a controller. The controller can be utilized to vibrate an artificial fistula via the associated vibration motor. An artificial fistula can optionally include a sensor that can be in communication with the open lumen area of the artificial fistula and function to register and communicate the presence of a needle tip within the opening of the artificial fistula. 
     In one embodiment, a device can include a support surface that defines a plurality of cut-outs in the surface, and each cut-out can retain an artificial fistula therein. 
     A device can also include a pad that can be removably located over the upper barrier of an artificial fistula. For instance, in one embodiment, a pad can be removably located on a support surface that defines a plurality of cut-outs in the surface. In this embodiment, the pad can cover all of the cut-outs and the artificial fistulas retained therein and can function as an artificial flesh over the fistulas. Optionally, a device can include several different pads of different thickness, compression characteristics, etc. so as to provide a different “feel” to the simulator surface and provide for cannulation simulations of a variety of difficulties. 
     In one embodiment, a support surface and a pad can both have a same surface area shape and size, such as a circle. As such, the pad can be removed, rotated and/or reversed, and replaced on the support surface to again fully cover the support surface and artificial fistulas. This can minimize the presence of visual and tactile cues on the pad that could designate the locations of the underlying fistulas and can extend the useful life of the pad. 
     In one embodiment, the support surface can rotate about an axis, which can be utilized to vary the location of the artificial fistulas underneath the pad and prevent memorization by trainees of fistula locations on a simulator. 
     According to another embodiment, a personal cannulation simulation device is disclosed. A personal cannulation simulation device can include a sleeve that is configured for temporary attachment to a subject; for instance, by having a shape that generally conforms to a human arm. In one embodiment, the sleeve can be of a closed and substantially cylindrical construction and can be configured to be slipped over the hand and onto the arm. In another embodiment, the sleeve can include an opening along a length of the sleeve and can include a closure. In this embodiment, a sleeve can be configured to wrap around an arm and optionally be held in place by use of the closure. A sleeve can be impenetrable to a needle (e.g., include a metal or a hard plastic) and can protect a wearer from accidental needle puncture during training. 
     A personal cannulation simulation device can also include an artificial fistula and a pad as described above. The artificial fistula can be between the sleeve and the pad, can be separable from the pad and/or sleeve, or of unitary construction with the pad and/or sleeve. 
     In one embodiment, a personal cannulation simulation device can include multiple removably attachable components so as to be assembled to mimic an individual&#39;s fistula depth, orientation, size, etc. 
     A cannulation simulation device can also include a flesh simulant material located on either side of the artificial fistula. The flesh simulant material can be a unitary component of a pad (e.g., the pad can include one or more channels of the flesh simulant material for location of the artificial fistula) or may be removably attachable to the pad, a sleeve, and/or the artificial fistula. 
     Also disclosed are methods for utilizing the cannulation simulation devices. For instance, following assembly of a device, a vibration motor can be turned on causing an artificial fistula to vibrate. A trainee (either a clinician or a patient) can then use tactile interaction to explore the upper surface of the pad and locate this vibrating artificial fistula. The trainee can then insert a dialysis needle into the fistula opening following determination of the direction and side edges of the fistula. Successful entrance of the needle tip into the fistula opening can be ascertained by use of sensors within the fistula openings. Should the trainee pass the needle tip through the fistula opening and out of a lower barrier of the artificial fistula, contact between the needle tip and a conductive surface on the lower barrier can be registered to signify that the needle tip has gone too far. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figure, in which: 
         FIG. 1  is a top view of a support surface. 
         FIG. 2  is a perspective view of a support surface. 
         FIG. 3  is a side view of components of a cannulation simulator. 
         FIG. 4  is a top view of an opening of a support surface and an artificial fistula retained therein. 
         FIG. 5  is an end view of an artificial fistula. 
         FIG. 6  illustrates several components of an artificial fistula. 
         FIG. 7  illustrates a fistula needle and hub. 
         FIG. 8  illustrates a method of utilizing a cannulation simulator. 
         FIG. 9  illustrates a personal cannulation simulator in cross section. 
         FIG. 10  illustrates a personal cannulation simulator located on an arm. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present disclosure. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     A cannulation simulation device and methods for using the device are disclosed herein. The device can be used as an instructive instrument designed to teach dialysis technicians/nurses/patients to cannulate arteriovenous (AV) fistulas for hemodialysis. The simulator facilitates cannulation training by use of a more realistic experience as compared to existing cannulation simulators and can include real-time feedback to the trainee to indicate successful, as well as unsuccessful, cannulation of the lumen of the artificial fistula. Moreover, the simulator has been designed to provide for long life without development of visual or eidetic indicators as to the location of the artificial fistulas, so as to improve the capabilities of individuals trained by use of the device. While the present disclosure is primarily directed to description of arteriovenous cannulation, it will be readily understood by the person skilled in the art that the invention is not so limited but extends to techniques such as cannulation of arteriovenous grafts for hemodialysis. 
     In one embodiment, the simulator comprises a support, multiple artificial fistulas held in various locations in conjunction with the support, and a removable pad of synthetic flesh overlaying the upper surface of the support and the artificial fistulas. By way of example,  FIG. 1  illustrates a top view of a support  10  and  FIG. 2  illustrates a perspective view of a support  10 . The support  10  includes a series of cut-outs  12  within the support  10 . The cut-outs  12  can generally have a cross-sectional dimension  14  of about 0.5 inches; for instance, from about 0.25 inches to about 1 inch. In addition, the cut-outs  12  can have a length  15  of several inches; for instance, from about 5 inches to about 12 inches or from about 7 inches to about 10 inches in some embodiments. As shown, the cut-outs  12  can define curvature(s) or can be generally straight along the length  15 . 
     As shown in  FIG. 2 , the support  10  can be supported by legs  16 . The legs  16  can be of any suitable length. For instance, the legs  16  can be designed such that the device can be held on a table or bench top, in which case the legs  16  can be relatively short, e.g., about 8 inches to about 14 inches, or about 12 inches in one embodiment. Alternatively, the device can be self-standing, in which case the legs  16  can be long enough (e.g., about 24 inches to about 40 inches) to locate the upper surface  18  of support  10  at a convenient height for use. In any case, the legs  16  can be of a height such that device components, e.g., wiring, controllers, motors, etc. can be retained beneath the support  10 . 
     In general, the device can be portable. As such, the support  10  can generally be about 36 inches or less in width as measured from one side to an opposite side of the upper surface  18  of the support  10 , e.g., from about 15 inches to about 24 inches in some embodiments. In addition, the support  10  and legs  16  can be formed of any suitable material, e.g., wood, plastic, or combinations of materials. 
     In the embodiment of  FIG. 1  and  FIG. 2 , the support  10  has a generally round upper surface  18 . The round surface  18  can be of benefit in some embodiments as by placing the artificial fistulas in conjunction with a round support  10 , there can be few or no spatial reference points for students, ensuring that students will rely on tactile sensing to determine the location and the size of the target fistula held in conjunction with the support  10 . A round upper surface  18  can also enable instructors to rotate and flip a flesh-simulation pad (described further herein) that is held on the upper surface  18  of the support  10  and has a size and shape that corresponds to the upper surface  18  of the support  10  while maintaining alignment between the upper surface  18  of the support  10  and the pad held thereon. This can prevent the formation of visual cues on the flesh-simulator pad that could indicate underlying fistula locations and can also extend the life of the flesh-simulator pad. 
       FIG. 3  presents a side-view of components of a device including a support  10 , an artificial fistula  20  held in conjunction with the support  10 , and a flesh-simulation pad  22  that can be removably located so as to align with the upper surface  18  of support  10  and can cover the artificial fistula  20 . 
     The pad  22  can be a flesh-simulation material as is generally known in the art. For instance, the pad  22  can include an opaque gel-like or foam filler to simulate subcutaneous tissue and an outer cover to simulate skin. The pad  22  can generally be from about 0.5 inches to about 1 inch in total thickness. In one embodiment, the device can include multiple pads of varying thicknesses or various degrees of hardness (e.g., Shore A hardness) that can provide varying degrees of difficulty to a trainee. For instance, a relatively thin pad of about 0.5 inches can be used in an easier or beginning training session and a higher degree of difficulty can be obtained by substituting a thicker pad, e.g., about 1 inch, on the support  10 . Alternatively, multiple thin pads can be combined to provide a more difficult simulation experience. 
     As mentioned previously, the temporary fixation of the pad  22  on the upper surface  18  of the support  10  can also be used to extend the life of the pad  22  and prevent the formation of visual cues on the pad  22  with regard to the location of the underlying fistula  20 . In particular, the pad  22  can be lifted, rotated, and/or flipped and replaced on the upper surface  18  of the support  10 . Beneficially, such changes can be carried out without creating a mess as the device need not include an artificial blood in the simulated flesh, as is the case for previously known cannulation simulators (though inclusion of an artificial blood in the device is certainly not prohibited). By changing the orientation of the gel pad the previous attempts at cannulation are obfuscated, preventing students from following needle marks to find a fistula. Further, the difficulty of the cannulation can be increased by increasing the thickness, density, and/or hardness of the pad  22 , simulating difference in the depth and tenacity of the flesh. 
     An artificial fistula  20  can be held in conjunction with each of the openings  12  in the support  10 .  FIG. 4  presents a top view of an artificial fistula  20  held in a cut-out  12 . Each fistula  20  is suspended within a cut-out  12 ; for instance, by use of rubber bushings  24  and the like. The suspension mechanism can at least partially isolate the fistula  20  from the support  10  that surrounds the cut-out  12 . As such, vibration of the fistula  20  can be better confined to the fistula and minimal vibration can transfer to the support  10 . Partial isolation of the vibrations of the fistula  20  can better simulate the vibrations of an actual fistula in real-world practice. 
       FIG. 5  presents an end-view of an artificial fistula  20 . The fistula  20  includes an upper barrier  26  and a lower barrier  28  that define a cannulation opening  27  there between. In the illustrated embodiment, the upper barrier  26  and the lower barrier  28  can be formed of different materials, but this is not a requirement of the devices. For instance, the upper barrier  26  can be a portion of a rubber or silicone tube that has a cross-sectional width w similar to that of a typical fistula. For instance, the side-to-side width (e.g., diameter) of the upper barrier  26  can be from about 6 millimeters to about 10 millimeters, or about 8 millimeters in one embodiment. The semi-circular cross-sectional shape of upper barrier  26  defining a radial curvature can also be beneficial as it can more closely resemble the shape of a natural fistula for the trainee. A fistula  20  can be held in a cut-out  12  such that a portion of the upper barrier  26  extends above the upper surface  18  of the support  10 . For instance, the upper barrier  26  of a fistula  20  can protrude above the upper surface  18  of the support  10  by a distance of from about 2 millimeters to about 4 millimeters. 
     The lower barrier  28  can be of any suitable shape and material. In one embodiment, the lower barrier  28  can be formed of an electrically conductive material, such as a metal mesh. In this embodiment, penetration of the lower barrier  28  of the fistula  20  can be noted by a closed electrical circuit created between the conductive material of the lower barrier  28  and a conductive material of the needle tip. A signal (e.g., visual and/or auditory) can then be generated when the needle passes through the fistula opening  27  and makes physical contact with the charged mesh on the other side. Of course, alternatively, the electric signal generator and detector may be reversed, with the needle providing an electric potential and the mesh acting as a detector. As illustrated in  FIG. 8 , if the tip of the needle  42  passes through the opening  27  and through the lower barrier  28 , a signal (e.g., an optical and/or auditory sound) can be emitted; signifying cannulation failure because the lower barrier  28  of the fistula  20  has been penetrated. For instance, the signal can be generated when the metal tip of the needle  42  contacts the charged wire mesh forming the lower barrier  28  of the fistula  20 . 
     Referring again to  FIG. 5 , the fistula  20  can include a cradle  25  that can be utilized to suspend the fistula  20  in a cut-out  12  of a support  10  and to support the other components of the fistula  20 , including the upper barrier  26  and the lower barrier  28 , as shown. 
     The cradle  25  can also support a component of a sensor  30  that can be utilized to detect successful penetration of the tip of a needle  42  into the opening  27 . By way of example, the cradle  25  can support a strip  34  of IR-light emitting diodes (LEDs)  30  spaced along the length  15  of the fistula  20  that radiate emission within the opening  27 . Proper positioning of a cannulation needle  42  with the tip within the opening  27  of the simulated fistula  20  can be determined by an infrared (IR) detector  40  ( FIG. 7 ) attached to the needle  42  that can detect emission from the IR-LEDs  30  placed beneath the fistula opening  27  in the cradle  25 . The emission from the IR-LEDs  30  cannot permeate beyond the fistula opening  27  as the upper barrier  26  and the overlying pad  22  can be opaque. When the tip of the needle  42  pierces the upper barrier  26 , the detector  40  can create a signal upon detection of the IR emission from the IR-LEDs  30 . For example, the detector  40  in the end of the cannulation needle  42  can create a signal  44  (e.g., optical and/or auditory signal) when the student successfully inserts the needle tip within the cannulation opening  27  of the fistula  20 . 
     Of course, the detection method is not limited to IR and other optical or other detection systems may be utilized. Moreover, the location of the various sensor components may be reversed. For instance, an IR-LED can be attached to the tip of a fistula needle, and an IR detector can be located under or within the fistula opening. Missed cannulations of the fistula can thus be noted by the absence of a signal generation by the IR detector. 
     In the real-world cannulation of a fistula, the proper alignment of a needle in a vessel is evidenced by the “flashback” of blood within the IV tubing of the cannulation set that includes a large gauge needle, IV tubing with connectors, and flow arrestor. Previously known simulation systems include tubes filled with simulated blood that are connected to external reservoirs, allowing the simulator to “bleed.” Cannulated synthetic veins in such systems can continue to leak between simulations and can lead to wet, messy simulators as the artificial blood will not coagulate a puncture as would living tissue. The presently disclosed systems can avoid such problems as proper cannulation of a fistula can be determined by signal generation without the need for synthetic blood. 
     Signals generated or received by the cannulation needle can be used to generate an indication for immediate user feedback, an indication only revealed to the instructor, and/or a recording of the trainee&#39;s attempts. These indications can be of any type, e.g., visual, tactile, and/or auditory, and recordings can be catalogued by computer program. 
     In addition to other components, a device can include vibration motors  32  in communication with each of the artificial fistulas  20 . For example, one or more vibration motors  32  can be in mechanical communication with a cradle  25  or some other component of each fistula  20  so as to vibrate the upper barrier  26  of the fistula  20  and provide a tactile cue through the pad  22  to a trainee of the location, width, and direction of the underlying fistula  20 . 
     Any suitable vibration motor  32  is encompassed, such as those commonly found in cell phones that can be used to mimic the vibrations of fistula. The vibration motor(s)  32  can be in communication with a control system (not shown) as is known in the art. Each motor  32  can be separately controlled or, alternatively, multiple motors  32  can be in communication with a single controller. For instance, multiple vibration motors  32  of a single fistula  20  can be controlled together. As shown in  FIG. 8 , in one embodiment, a series of vibration motors  32  can be located beneath the fistula opening  27 ; for instance, in mechanical communication with the cradle  25  of a fistula  20 . During use, the vibration motor(s)  32  of only one fistula  20  of the device can be activated at a time to denote a single fistula that the trainee will then attempt to locate and cannulate. This is not a requirement, however, and in other embodiments, multiple fistulas can be activated at one time. 
     According to one embodiment, a simulator can be designed as a personal simulation device for use in training a patient for self-cannulation. According to this embodiment, a simulator can include a support in the form of a removable sleeve that is held in conjunction with a single artificial fistula. Similar to the multiple-fistula device described above, a personal simulation device can also include a pad of synthetic flesh overlaying the upper surface of the sleeve and the artificial fistula. 
     One embodiment of a personal simulation device  101  is illustrated in  FIG. 9 . As shown, a simulation device  101  can include a sleeve  128  as a support for the artificial fistula. In one embodiment, a portion of the sleeve  128  can function as a lower barrier for the artificial fistula  120 . In other embodiments, the lower barrier can be applied to the sleeve  128  during assembly. In the cross-sectional view of  FIG. 9 , the device is illustrated as covering a portion of a patient&#39;s limb  130 , e.g., a forearm, leg, etc. 
     The sleeve  128  can be of a material that can prevent puncture by the trainee (generally a person wearing the device on their limb  130 ) with the training needle. For instance, the sleeve  128  can include a metal or a relatively hard plastic (e.g., a polyvinyl chloride, a polypropylene, etc.). The sleeve can be formed so as to be pulled over or wrapped around a patient&#39;s limb. For instance, one embodiment of a sleeve can be in the form of a generally cylindrical tube that can slide over a wearer&#39;s hand and into place on the limb. In another embodiment, the sleeve is not in the form of a closed tube, but rather, is open along the length of the cylinder. In this embodiment, the sleeve can be wrapped around the arm. Optionally, and depending upon the flexibility of the sleeve material, etc., a sleeve can include a closure such as a buckle, snap, button, or other commonly used securing techniques. As such, following location around a patient&#39;s limb, the sleeve can be secured in place. Currently available products on the market may also be used as a sleeve of a simulation device such as, without limitation, the Needle Resistant Arm Sleeves AG8TW by HexArmor®. Optionally, the sleeve  128  can include a conductive material, such as a metal mesh as described above, for the lower barrier so as to aid in training when a portion of the sleeve serves as the lower barrier of the artificial fistula. 
     A personal fistula cannulation simulation device  101  can include an artificial fistula  120  that includes an upper barrier  126  as described previously and a portion of the sleeve  128  as a lower barrier that define a cannulation opening  127  there between. Alternatively, the lower barrier can be formed of a different material that is applied to the sleeve. For instance, in one embodiment, the upper and lower barriers can be formed from a unitary tube that is assembled on the sleeve  128 . The artificial fistula  120  can generally be of a size and shape as described previously. 
     A personal simulation device  101  can optionally include penetration sensors and vibrational components as described above for a larger, clinical-type device. For instance, a device  101  can be electrically connectable to a motor as described above that can vibrate all or a portion of the upper barrier  126  or a portion of the sleeve  128  so as to provide a vibration in the area of the artificial fistula  120  similar to what will be encountered by the patient during self-cannulation. 
     In one embodiment, a personal simulation device can include separable components that can provide for the device to be personalized for a user. For instance, the artificial fistula  120  can be separable from the sleeve  128  so as to be securable thereto in a variety of orientations. During formation, a technician that is familiar with a patient&#39;s fistula can locate the artificial fistula  120  on the sleeve  128  so as to mimic the orientation of the patient&#39;s actual fistula. Placement can be carried out with temporary attachment, for instance by use of a temporary adhesive or hook and pile attachments (e.g., Velcro®) or alternatively by use of a more permanent adhesive. 
     The personal device  101  can also include a pad  122  that can cover the artificial fistula and function as a flesh simulation. For instance, the pad can have a thickness of from about 0.25 inches to about 1 inch and be formed of materials as discussed previously. 
     In order to create a generally uniform upper surface on the device  101  and thereby offer minimal tactile cues as to the location of the fistula below the flesh simulant pad, a device can provide for the artificial fistula  120  to be generally surrounded on the top and sides with an artificial flesh such as the pad  122 . For instance, the pad  122  can be formed with a channel or cavity within which the artificial fistula  122  can be located. Alternatively, multiple pieces of pad materials can be located on the sleeve  128  so as to surround the artificial fistula  120  and provide a uniform upper surface with little or no visual cues as to the location of the underlying fistula  120 . 
     The channel or cavity formed in a pad  122  that can be adjacent to the top and sides of the fistula  120  may be formed by the manufacturer or may be custom designed by a nurse/technician removing material from a pad  122  to mimic a patient&#39;s specific fistula orientation. The pad  122  with imbedded artificial fistula  120  can then be located on the sleeve  128 . 
       FIG. 10  illustrates a perspective view of a personal simulation device  101  following location on a patient&#39;s limb  130 . As can be seen, in this embodiment, the sleeve  128  can be formed so as to be wrapped around the limb  128 , following which it can be secured with buckles, snap closures, buttons, hook and loop closure, or other commonly-used temporary securing techniques. A sleeve  128  that is open along the side and includes a closure that can be preferred in some embodiments, as this can provide for a device with a variable cross-sectional size. 
     As shown, during use, a trainee can target a needle  142  to the artificial fistula  120 . In the illustrated embodiment, the pad  122  is not shown. In one embodiment, during use, a trainee can locate the device on their own limb and over their own fistula. The trainee can then be taught to locate the artificial fistula  120  that lies beneath the pad by tactile exploration of the pad following which the trainee can be taught to penetrate the pad with a needle  142  and successfully locate the tip of the needle within the lumen  127  of the artificial fistula  120 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.