Patent Publication Number: US-2021186365-A1

Title: Methods for determining a position of a first medical device with respect to a second medical device, and related systems and medical devices

Description:
FIELD 
     This document relates to medical devices. More specifically, this document relates to methods for determining a position of a first medical device with respect to a second medical device, and related systems and medical devices. 
     SUMMARY 
     The following summary is intended to introduce the reader to various aspects of the detailed description, but not to define or delimit any invention. 
     Systems of medical devices are disclosed. According to some aspects, a system of medical devices includes first medical device having an elongate member. The elongate member has a proximal portion defining a proximal end, a distal portion defining a distal end, and a lumen extending longitudinally therethrough from the proximal end to the distal end. The system further includes a second medical device including a needle advanceable through the lumen from the proximal end towards the distal end. The system further includes a capacitor including a first electrical conductor supported by the elongate member, and a second electrical conductor supported by one of the elongate member and the second medical device. The first electrical conductor and second electrical conductor are spaced apart and electrically insulated from each other. As the needle is advanced through the lumen, a capacitance of the capacitor is a measure of a longitudinal position of the needle with respect to the elongate member. The system further includes a capacitance sensor electrically connectable to the first electrical conductor and the second electrical conductor and configured to sense the capacitance of the capacitor as the needle is advanced through the lumen, and to generate a sensor signal based on the sensed capacitance. The system further includes a processor in communication with the capacitance sensor and configured to receive and process the sensor signal from the capacitance sensor, and generate a processor signal based on the sensor signal. The system further includes an output device in communication with the processor and configured to receive the processor signal and generate an output based on the processor signal. The output is an indicator of the longitudinal position of the needle with respect to the elongate member. 
     In some examples, the first electrical conductor and second electrical conductor are supported by the elongate member. The elongate member can include a sidewall extending longitudinally between the proximal end and the distal end, and radially between an inner surface and an outer surface, and the first electrical conductor can include a first plate embedded in the sidewall on a first side of the lumen. The second electrical conductor can include a second plate embedded in the sidewall on a second side of the lumen opposite the first side. At least one of the first electrical conductor and the second electrical conductor can extend continuously from the proximal portion to the distal portion. Alternatively, at least one of the first electrical conductor and the second electrical conductor can be positioned within the distal portion, for example proximate the distal end. 
     In some examples, the system includes a second capacitor. The second capacitor can include a third electrical conductor including a third plate, and a fourth electrical conductor including a fourth plate. The third plate and fourth plate can be spaced apart and electrically insulated from each other. As the needle is advanced through the lumen, a capacitance of the second capacitor can be an additional measure of a longitudinal position of the needle with respect to the elongate member. The third plate can be embedded in the sidewall on the first side of the lumen, and spaced proximally from the first plate. The fourth plate can be embedded in the sidewall on the second side of the lumen, spaced proximally from the second plate. 
     In some examples, the first electrical conductor is supported by the elongate member, and the second electrical conductor is supported by the second medical device. In some examples, the needle is the second electrical conductor. In some such examples, the elongate member can include a sidewall extending longitudinally between the proximal end and the distal end, and radially between an inner surface and an outer surface. The first electrical conductor can be embedded in the sidewall and can extend continuously from the proximal portion to the distal portion. 
     In some examples, the system further includes a first shield electrode supported by the elongate member and spaced radially outwardly and electrically insulated from the first electrical conductor. The first shield electrode can be electrically connectable to the capacitance sensor and configured to shield the first electrical conductor from parasitic capacitance. In some examples the system further includes a second shield electrode supported by the elongate member and spaced radially outwardly and electrically insulated from the second electrical conductor. The second shield electrode can be electrically connectable to the capacitance sensor and configured to shield the second electrical conductor from parasitic capacitance. 
     In some examples, the output device includes a light. The output can include illumination of the light when the needle is at a predetermined longitudinal position with respect to the elongate member. In some examples, the output device includes a screen. The output can include a GUI showing an image of the longitudinal position of the needle with respect to the elongate member. 
     Medical devices are also disclosed. According to some aspects, a medical device includes an elongate member having a proximal portion defining a proximal end, a distal portion defining a distal end, a sidewall extending longitudinally between the proximal end and the distal end and radially between an inner surface that and an outer surface, and a lumen defined by the inner surface and extending longitudinally through the elongate member from the proximal end to the distal end. The medical device further includes a capacitor including a first electrical conductor supported by the sidewall and a second electrical conductor spaced apart and electrically insulated from the first electrical conductor and supported by the sidewall. A first electrical connector is electrically connected to the first electrical conductor and electrically connectable to a capacitance sensor. A second electrical connector is electrically connected to the second electrical conductor and electrically connectable to the capacitance sensor. 
     In some examples, the first electrical conductor includes a first plate embedded in the sidewall on a first side of the lumen, and the second electrical conductor includes a second plate embedded in the sidewall on a second side of the lumen opposite the first side. The first plate and the second plate can extend continuously from the proximal portion to the distal portion. 
     In some examples, the first electrical conductor is longitudinally spaced from the second electrical conductor. 
     In some examples, the first electrical conductor includes a first plate embedded in the sidewall on a first side of the lumen, and the second electrical conductor includes a second plate embedded in the sidewall on a second side of the lumen opposite the first side. The first plate and the second plate can be positioned within the distal portion, for example proximate the distal end. 
     In some examples, the medical device includes a second capacitor. The second capacitor can include a third electrical conductor including a third plate, and a fourth electrical conductor including a fourth plate. The third plate and fourth plate can be spaced apart and electrically insulated from each other. The third plate can be embedded in the sidewall on the first side of the lumen, spaced proximally from the first plate, and the fourth plate can be embedded in the sidewall on the second side of the lumen, spaced proximally from the second plate. 
     In some examples, the capacitor further includes a first shield electrode associated with the elongate member and spaced radially outwardly and electrically insulated from the first electrical conductor. The first shield electrode can be configured to shield the first electrical conductor from parasitic capacitance. 
     In some examples, the capacitor further includes a second shield electrode associated with the elongate member and spaced radially outwardly and electrically insulated from the second electrical conductor. The second shield electrode can be configured to shield the second electrical conductor from parasitic capacitance. 
     Methods for determining a position of a first medical device with respect to a second medical device are also disclosed. According to some aspects, a method includes: a. advancing a second medical device into a lumen of a first medical device, from a proximal end of the first medial device towards a distal end of the first medical device; b. during step a., sensing a capacitance of a capacitor associated with the first medical device and the second medical device, whereby the capacitance is a measure of a longitudinal position of the second medical device with respect to the first medical device; and c. generating an output based on the capacitance, wherein the output is an indicator of the longitudinal position of the second medical device with respect to the first medical device. 
     In some examples, step a. includes passing the second medical device through a gap between a first electrical conductor and second electrical conductor of the capacitor. 
     In some examples, step a. includes, while advancing the second medical device, advancing a second electrical conductor of the capacitor with respect to a first electrical conductor of the capacitor. 
     In some examples, step a. includes passing the second medical device past the capacitor. 
     In some examples, step c. includes generating the output when the capacitance reaches a predetermined value. 
     In some examples, step c. includes generating an image of the longitudinal position of the needle with respect to the elongate member and updating the image as the capacitance changes. 
     In some examples, the method further includes adjusting a position of the second medical device with respect to the first medical device based on the output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are for illustrating examples of articles, methods, and apparatuses of the present disclosure and are not intended to be limiting. In the drawings: 
         FIG. 1  is a perspective view of a first example system of medical devices, showing a first medical device, second medical device, and control unit, spaced apart and disconnected from each other; 
         FIG. 2  is a perspective view of the system of  FIG. 1 , showing the second medical device inserted into the first medical device, and the first medical device connected to the control unit; 
         FIG. 3  is a longitudinal cross section taken along line  3 - 3  in  FIG. 2 , schematically showing the control unit and related electrical connections; 
         FIG. 4  is a transverse cross section taken along line  4 - 4  in  FIG. 2 ; 
         FIG. 5  is a simplified graph showing the change in sensor signal as the second medical device of  FIGS. 1 to 4  is advanced through the first medical device of  FIGS. 1 to 4 ; 
         FIG. 6  is a cross-section similar to that of  FIG. 3 , showing a first medical device, second medical device, control unit, and electrical connections of a second example system; 
         FIG. 7  is a simplified graph showing the change in sensor signal as the second medical device of  FIG. 6  is advanced through the first medical device of  FIG. 6 ; 
         FIG. 8  is a cross-section similar to that of  FIG. 3 , showing a first medical device, second medical device, control unit, and electrical connections of a third example system; 
         FIG. 9  is a cross-section similar to that of  FIG. 4 , taken through the first electrical conductor of the first medical device and the second medical device of the third example system; 
         FIG. 10  is a simplified graph showing the change in sensor signal as the second medical device of  FIGS. 8 and 9  is advanced through the first medical device of  FIGS. 8 and 9 ; 
         FIG. 11  is a perspective view of a fourth example system of medical devices, showing a first medical device, second medical device, and control unit, spaced apart and disconnected from each other; 
         FIG. 12  is a longitudinal cross section similar to that of  FIG. 3 , schematically showing the control unit and related electrical connections; 
         FIG. 13  is a transverse cross section similar to that of  FIG. 4 ; and 
         FIG. 14  is a cross-section similar to that of  FIG. 3 , showing a first medical device, second medical device, control unit, and electrical connections of a fifth example system. 
     
    
    
     DETAILED DESCRIPTION 
     Various apparatuses or processes or compositions will be described below to provide an example of an embodiment of the claimed subject matter. No example described below limits any claim and any claim may cover processes or apparatuses or compositions that differ from those described below. The claims are not limited to apparatuses or processes or compositions having all of the features of any one apparatus or process or composition described below or to features common to multiple or all of the apparatuses or processes or compositions described below. It is possible that an apparatus or process or composition described below is not an embodiment of any exclusive right granted by issuance of this patent application. Any subject matter described below and for which an exclusive right is not granted by issuance of this patent application may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicants, inventors or owners do not intend to abandon, disclaim or dedicate to the public any such subject matter by its disclosure in this document. 
     Generally disclosed herein are systems of medical devices that include one or more capacitors, and one or more capacitance sensors. For example, a system of medical devices can include a first medical device (e.g. a catheter in the form of a sheath or dilator) that in use is advanced into a patient&#39;s body to a target location (e.g. advanced via a femoral vein to a patient&#39;s heart) and a second medical device (e.g. a perforation device including a needle) that in use is passed through the first medical device towards the target location. A capacitor can be associated with the first medical device and the second medical device, and a capacitance sensor can be connected to the capacitor, to sense the capacitance of the capacitor as the second medical device is advanced through the first medical device. The capacitor can be configured so that the capacitance of the capacitor changes as the second medical device is advanced through the first medical device. The capacitance can thus be used as a measure of a position of the second medical device with respect to the first medical device. 
     For example, the capacitor can include a pair of electrical conductors that are supported by the first medical device, so that as the second medical device is advanced through the first medical device, the second medical device passes through a gap between the pair of electrical conductors. For further example, the capacitor can include a pair of electrical conductors that are supported by the first medical device, so that as the second medical device is advanced through the first medical device, the second medical device passes beside (but not between) the pair of electrical conductors. For further example, the capacitor can include a pair of electrical conductors, where one of the electrical conductors is supported by the first medical device, and one of the electrical conductors is supported by the second medical device, so that as the second medical device is advanced through the first medical device, the relative position of the electrical conductors changes. 
     In the above examples, as the second medical device is advanced through the first medical device, the capacitance of the capacitor changes, and by sensing the capacitance of the capacitor, the relative position of the first medical device and second medical device can be determined. For example, by sensing the capacitance, it can be determined whether a perforating tip of a needle is approaching a distal end of the catheter, or has passed beyond a distal end of the catheter. This can facilitate ease of use of the medical devices, and enhance patient safety. 
     Referring now to  FIGS. 1 and 2 , a first example system  100  of medical devices is shown. In the example shown, the system  100  includes a first medical device  102  in the form of a catheter, and a second medical device  104  in the form of a perforation device. The catheter can be, for example, a sheath, a dilator, or an alternative device that is intended for use by passing another medical device therethrough (e.g. coaxially therethrough). The perforation device can be, for example, a mechanical perforation device, or a radiofrequency (RF) perforation device. In alternative examples, the second medical device can be alternative type of medical device that is intended for use by being passed through another medical device. 
     Referring also to  FIGS. 3 and 4 , in the example shown, the first medical device  102  includes a hub  106  and an elongate member  108  extending from the hub  106 . The elongate member  108  has a proximal portion  110  defining a proximal end  112  of the elongate member  108 , a distal portion  114  defining a distal end  116  of the elongate member  108 , and a lumen  118  (shown in  FIGS. 3 and 4 ) extending longitudinally through the elongate member  108  from the proximal end  112  to the distal end  116 . The elongate member  108  includes a sidewall  120 , which extends longitudinally between the proximal end  112  and the distal end  116 , and radially between an inner surface  122  (shown in  FIGS. 3 and 4 ) that defines the lumen  118 , and an outer surface  124 . 
     Referring still to  FIGS. 1 to 4 , in the example shown, the second medical device  104  includes a hub  126  and a needle  128  extending from the hub  126 . The needle  128  has a proximal portion  130  defining a proximal end  132  of the needle  128 , and a distal portion  134  defining a distal end  136  of the needle  128 . The distal end  136  of the needle  128  includes a perforating tip  138 . As shown in  FIG. 2 , the needle  128  is advanceable through the hub  106  and through the lumen  118 , from the proximal end  112  of the elongate member  108  towards the distal portion  114  of the elongate member  108 , to position the perforating tip  138  of the needle  128  proud of the distal end  116  of the elongate member  108 . 
     Referring to  FIGS. 3 and 4 , the system further includes a capacitor  140 , which is associated with the first medical device  102  and the second medical device  104 . As used herein, the term “associated with” indicates that the first medical device  102 , second medical device  104 , and capacitor  140  are configured so that the capacitance of the capacitor  140  changes as a function of the position of the second medical device  104  with respect to the first medical device  102 , as will be described below. 
     Referring still to  FIGS. 3 and 4 , the capacitor  140  includes a first electrical conductor  142 , and a second electrical conductor  144 . In the example shown, the first electrical conductor  142  and the second electrical conductor  144  are both supported by the elongate member  108 . In alternative examples (as will be described below), the first electrical conductor can be supported by the elongate member, and the second electrical conductor can be supported by the second medical device. That is, in general, the first electrical conductor can be supported by the elongate member, and the second electrical conductor can be supported by the elongate member or the second medical device (i.e. supported by one of the elongate member and the second medical device). As used herein, the term “supported by” indicates that the referenced electrical conductor is integral with, embedded in, connected to, mounted to, adhered to, affixed to, or otherwise secured to the referenced part, so that the referenced electrical conductor moves with the referenced part. 
     Referring still to  FIGS. 3 and 4 , the first electrical conductor  142  and second electrical conductor  144  are spaced apart and are electrically insulated from each other, to form the capacitor  140 . More specifically, the first electrical conductor  142  is in the form of a first plate  146 , which is embedded in the sidewall  120  on a first side of the lumen  118 . The second electrical conductor  144  is in the form of a second plate  148 , which is embedded in the sidewall  120  on a second side of the lumen  118  opposite the first side. The first electrical conductor  142  and second electrical conductor  144  are spaced apart by a portion of the sidewall  120 , and by the lumen  118 . 
     The first plate  146  and second plate  148  can be, for example, a metal such as copper. The first plate  146  and second plate  148  can optionally be in the form of a tape. The first plate  146  and the second plate  148  can be, for example, up to about 3 mm in width, and about 0.035 mm or more in thickness. In alternative examples, the first plate and second plate can be of another size (e.g. only nanometers thick), provided that they are not in electrical contact. The first plate  146  and the second plate  148  can optionally be curved, as shown in  FIG. 4 , to match the curve of the sidewall  120 . 
     In the example shown, the first plate  146  and second plate  148  are each embedded in the sidewall  120 , and each extend continuously along the entire length of the elongate member  108 , from the proximal portion  110  to the distal portion  114 . In alternative examples, the first plate and second plate can extend along less than the entire length of the elongate member, such as along a majority of the length of the elongate member, or along only a small section of the elongate member. For example, the first plate and the second plate can be relatively short in length (e.g. about 5 mm), and can be positioned within the distal portion, proximate the distal end (e.g. right at the distal end, or slightly proximal of the distal end). 
     Referring still to  FIGS. 3 and 4 , in the example shown, the system  100  further includes first  150  and second  152  shield electrodes, which are supported by the elongate member  108 . The first  150  and second  152  shield electrodes are spaced radially outwardly from the first  142  and second  144  electrical conductors, respectively, and are electrically insulated from the first  142  and second  144  electrical conductors, respectively, by electrical insulation  154 . In the example shown, the electrical insulation  154  is a strip of insulative material; however, in alternative examples, electrical insulation could be provided by the material of the sidewall. As will be described in further detail below, the first  150  and second  152  shield electrodes can shield the first  142  and second  144  electrical conductors, respectively, from parasitic capacitance. 
     Referring still to  FIGS. 3 and 4 , the system further includes a control unit  156 . The control unit  156  houses a capacitance sensor  158 , a processor  160 , and an output device  162 , described in further detail below. 
     Referring still to  FIGS. 3 and 4 , the capacitance sensor  158  can be, for example, a commercially available capacitance sensor such as the one sold by Texas Instruments Incorporated under model number FDC1004. In the example shown, the capacitance sensor  158  is electrically connectable to the first electrical conductor  142  and the second electrical conductor  144 , as well as to the first shield electrode  150  and the second shield electrode  152 . Specifically, in the example shown, the system includes a first electrical connector  164  (e.g. a wire) that is electrically connected to the first electrical conductor  142 , and extends from the first electrical conductor  142  for electrical connection with the capacitance sensor  158  (e.g. for connection with a capacitive sense port of the capacitance sensor  158 ), a second electrical connector  166  (e.g. a wire) electrically connected to the second electrical conductor  144  and extending from the second electrical conductor  144  for electrical connection with the capacitance sensor  158  (e.g. for connection with a ground port of the capacitance sensor), a third electrical connector  168  (e.g. a wire) electrically connected to the first shield electrode  150  and extending from the first shield electrode  150  for electrical connection with the capacitance sensor  158  (e.g. for connection with a first shield port of the capacitance sensor), and a fourth electrical connector  170  (e.g. a wire) electrically connected to the second shield electrode  152  and extending from the second shield electrode  152  for electrical connection with the capacitance sensor  158  (e.g. to a second shield port of the capacitance sensor). 
     The first through fourth electrical connectors  164 - 170  can optionally be removably electrically connectable to the capacitance sensor  158 , for example via a common male connector (shown in  FIG. 1 ) that connects to a female connector of the control unit  156 . 
     The capacitance sensor  158  can sense the capacitance of the capacitor  140  as the needle  128  is advanced through the lumen  118 . More specifically, in the example shown, in use, the capacitance sensor  158  can continuously sense the capacitance of the capacitor  140  as the needle  128  is advanced through the lumen  118 . Furthermore, the capacitance sensor  158  can activate the shield electrodes  150 ,  152  to shield the capacitor  140  from the effects of parasitic capacitance (e.g. due to other instruments in the vicinity of the elongate member  108 ). 
     As shown in  FIG. 5 , as the needle  128  is advanced into the lumen  118 , the capacitance of the capacitor  140  will change, due to the effect of the needle  128  on the dielectric permittivity between the first electrical conductor  142  and the second electrical conductor  144 . The capacitance can thus be used as a measure of the longitudinal position of the needle  128  with respect to the elongate member  108 . For example, the capacitance (e.g. the value of the capacitance, or a trend in the capacitance, or a change in the capacitance) can be an indicator of when the perforating tip  138  is well shrouded within the elongate member  108  (and thus when a patient&#39;s anatomy is protected from being perforated by the needle  128 ), or when the perforating tip  138  is at the distal end  116  of the elongate member  108  (i.e. when the needle  128  is ‘primed’ for use), or when the perforating tip  138  has passed beyond the distal end  116  of the elongate member  108  and is exposed (and thus when a patient&#39;s anatomy is not protected from being perforated by the needle  128 ). 
     Referring back to  FIG. 3 , the capacitance sensor  158  is in communication with the processor  160 , and the processor  160  is in communication with an output device  162 . The capacitance sensor  158  can sense the capacitance of the capacitor  140  as the needle  128  is advanced through the lumen  118 , and can generate a signal (referred to herein as a “sensor signal”) based on the sensed capacitance. The processor  160  is configured to receive and process the signal from the capacitance sensor  158 , and to generate a signal (referred to herein as a “processor signal”) based on the sensor signal. The output device  162  can receive the processor signal, and can generate an output based on the processor signal. 
     In some examples, the output device  162  can include a light, and the processor signal can cause illumination of the light, or change in color of the light. For example, when the sensor signal indicates that the capacitance has reached a predetermined value that corresponds to the perforating tip  138  being at a predetermined longitudinal position with respect to the elongate member  108  (e.g. the perforating tip being at the distal end  116  of the elongate member  108 ), the processor can signal the output device  162  to change the color of the light from green to red. 
     In some examples, the output device  162  can include a screen that shows a graphical user interface (GUI). The processor signal can cause the output device  162  to generate an image showing the longitudinal position of the needle  128  within the elongate member  108 . For example, as the needle  128  is advanced through lumen  118  and the capacitance changes, the image can change based on the capacitance. 
     Referring now to  FIG. 6 , an alternative example system is shown. In the example of  FIG. 6 , features that are like those of  FIGS. 1 to 5  will be referred to with like reference numerals, incremented by  500 . 
     In the example of  FIG. 6 , the first medical  602  device includes several discrete capacitors (i.e. a first capacitor  640   a,  a second capacitor  640   b,  a third capacitor  640   c,  and a fourth capacitor  640   d ), spaced along the length of the elongate member  608 . Each capacitor includes a pair of electrical conductors in the form of plates, which are embedded in the sidewall  620  on opposed sides of the lumen  618 , and are spaced apart and electrically insulated from each other. That is, the first capacitor  640   a  includes a first plate  646   a  and a second plate  648   a,  the second capacitor  640   b  includes a third plate  646   b  and a fourth plate  648   b,  which are spaced proximally from the first plate  646   a  and second plate  648   a,  the third capacitor  640   c  includes a fifth plate  646   c  and a sixth plate  648   c,  which are spaced proximally from the third plate  646   b  and fourth plate  648   b,  and the fourth  640   d  capacitor includes a seventh plate  646   d  and an eighth plate  648   d,  which are spaced proximally from the fifth plate  646   c  and sixth plate  648   c.  Each plate is electrically connectable to the capacitance sensor  658 . For example, the first  646   a,  third  646   b,  fifth  646   c,  and seventh  646   d  plates can be connected to a common ground port of the capacitance sensor  658 , and the second  648   a,  fourth  648   b,  sixth  648   c,  and eighth  648   d  plates can be connected to a respective capacitance sense port of the capacitance sensor  658 . 
     Referring still to  FIG. 6 , the system further includes a set of shield electrodes  650   a - d  and  652   a - d,  each of which is associated with one of the plates  646   a - d  and  648   a - d,  respectively, and each of which is connected to the capacitance sensor  658  (e.g. via shield ports). 
     Similarly to the example of  FIGS. 1 to 5 , the system  600  includes a control unit  656 , which houses the capacitance sensor  658 , a processor  660  that is in communication with the capacitance sensor  658  and configured to receive and process sensor signals from the capacitance sensor  658  and generate processor signals based on the sensor signals, and an output device  662  in communication with the processor  660  and configured to receive the processor signals and generate an output based on the processor signals. 
     Referring also to  FIG. 7 , as the needle  628  is advanced into the lumen  618 , the capacitance of each capacitor  640   a - d  will change as the needle  628  passes by that capacitor. The capacitance can thus be used as a measure of the longitudinal position of the needle  628  with respect to the elongate member  608 . As noted above, the capacitance can be an indicator of when the perforating tip  638  is well shrouded within the elongate member  608  (and thus when a patient&#39;s anatomy is protected from being perforated by the needle  628 ), or when it is at the distal end  616  of the elongate member  608  (i.e. when it is ‘primed’ for use), or when it has passed beyond the distal end  616  of the elongate member  608  and is exposed (and thus when a patient&#39;s anatomy is not protected from being perforated by the needle  628 ). 
     Referring now to  FIGS. 8 to 10 , an alternative example is shown. In the example of  FIGS. 8 to 10 , features that are like those of  FIGS. 1 to 5  will be referred to with like reference numerals, incremented by  700 . 
     In the example of  FIGS. 8 to 10 , the first  842  and second  844  electrical conductors of the capacitor  840  are in the form of a first ring  846  and a second ring  848 , respectively. The first ring  846  and second ring  848  are embedded in the sidewall  820  in the distal portion  814 , and are longitudinally spaced apart. The first ring  846  and second ring  848  are connected to the capacitance sensor  858  (e.g. to a capacitance sense port and a ground port of the capacitance sensor, respectively). 
     Similarly to the example of  FIGS. 1  the  5 , the system further includes shield electrodes  850 ,  852 , each of which is associated with one of the electrical conductors  842 ,  844 , respectively, and each of which is connected to the capacitance sensor  858 . 
     Furthermore, similarly to the example of  FIGS. 1 to 5 , the system includes a control unit  856 , which houses the capacitance sensor  858 , a processor  860  that is in communication with the capacitance sensor  858  and configured to receive and process sensor signals from the capacitance sensor  858  and generate processor signals based on the sensor signals, and an output device  862  in communication with the processor  860  and configured to receive the processor signals and generate an output based on the processor signals. 
     In the example of  FIGS. 8 to 10 , as the needle  828  is advanced through the lumen  818 , the needle  828  passes beside, but not between, the electrical conductors  842 ,  844  of the capacitor  840 . As shown in  FIG. 10 , as the needle  828  passes beside the electrical conductors  842 ,  844 , a spike is observed in the sensor signal. The spike indicates that the perforating tip  838  of the needle  828  is at the longitudinal position of the capacitor  840 . 
     In alternative examples, rather than rings, the electrical conductors can be in the form of longitudinally spaced apart plates. 
     Referring now to  FIGS. 11 to 13  a further alternative example is shown. In the example of  FIGS. 11 to 13 , features that are like those of  FIGS. 1 to 5  will be referred to with like reference numerals, incremented by  1000 . 
     In the example of  FIGS. 11 to 13 , the second electrical conductor  1144  of the capacitor  1140  is supported by the second medical device  1104 . Particularly, in the example shown, the needle  1128  itself serves as the second electrical conductor  1144 , and is electrically connected to the capacitance sensor  1158 . The first electrical conductor  1142  is embedded in the sidewall  1120  of the elongate member  1108 , is generally tubular, and extends from the proximal portion  1110  of the elongate member  1108  to the distal portion  1114  of the elongate member  1108 . 
     In the example shown, the system  1100  further includes a shield electrode  1150 , which is embedded in the sidewall  1120  of the elongate member  1108  radially outwardly of the first electrical conductor  1142 , is generally tubular, and extends from the proximal portion  1110  of the elongate member  1108  to the distal portion  1114  of the elongate member  1108 . Insulation  1154  is positioned between the first electrical conductor  1142  and the shield electrode  1150 . 
     In the example shown, the needle  1128 , the first electrical conductor  1142 , and the shield electrode  1150  are electrically connectable to the capacitance sensor  1158 . Similarly to the example of  FIGS. 1 to 5 , the system  1110  includes a control unit  1156 , which houses the capacitance sensor  1158 , a processor  1160  that is in communication with the capacitance sensor  1158  and configured to receive and process sensor signals from the capacitance sensor  1158  and generate processor signals based on the sensor signals, and an output device  1162  in communication with the processor  1160  and configured to receive the processor signals and generate an output based on the processor signals. 
     Similarly to the examples of  FIGS. 1 to 5 , as the needle  1128  is advanced through the lumen  1118 , the capacitance of the capacitor  1140  will change as a function of the longitudinal position of the needle  1128 , and can be used as a measure of the longitudinal position of the needle  1128  with respect to the elongate member  1108 . 
     Referring now to  FIG. 14 , a further alternative example is shown. In the example of  FIG. 14 , features that are like those of  FIGS. 1 to 5  will be referred to with like reference numerals, incremented by  1300 . 
     The example of  FIG. 14 , the system  1400  includes a set of capacitors, i.e. a first capacitor  1440   a,  a second capacitor  1440   b,  a third capacitor  1440   c,  and a fourth capacitor  1440   d.  Each capacitor  1440   a - d  includes a respective first electrical conductor  1442   a - d  in the form of a ring (not shown in transverse section, but similar to the transverse section shown in  FIG. 13 ). The rings are longitudinally spaced apart and embedded in the sidewall  1420 . The needle  1428  itself serves as the second electrical conductor  1444  of each capacitor  1440   a - d.    
     In the example shown, the system  1400  further includes a respective shield electrode  1450   a - d  associated with each ring. The shield electrodes  1450   a - d  are embedded in the sidewall  1420  of the elongate member  1408 , and spaced radially outwardly of each first electrical conductors  1442   a - d,  respectively. Insulation  1454  is positioned between each first electrical conductor  1442   a - d  and each shield electrode  1450   a - d,  respectively. 
     The needle  1428 , the first electrical conductor  1442   a - d  of each respective capacitor  1440   a - d,  and the shield electrodes  1450   a - d  are electrically connectable to the capacitance sensor  1458 . Similarly to the example of  FIGS. 1 to 5 , the system  1400  includes a control unit  1456 , which houses the capacitance sensor  1458 , a processor  1460  that is in communication with the capacitance sensor  1458  and configured to receive and process sensor signals from the capacitance sensor  1458  and generate processor signals based on the sensor signals, and an output device  1462  in communication with the processor  1460  and configured to receive the process or signals and generate an output based on the processor signals. 
     As the needle  1428  is advanced through the lumen  1418 , the capacitance of each capacitor  1440   a - d  will change as a function of the longitudinal position of the needle  1428 , in a similar fashion to that shown in  FIG. 7 , and can be used as a measure of the longitudinal position of the needle  1428  with respect to the elongate member  1408 . 
     In any of the above examples, in order to reduce parasitic capacitance, “out-of-phase” sensing can be used. That is, the electrical conductor that is connected to the ground port of the capacitive sensor can be connected to a shield output of the electrical conductor that is connected to the capacitive sense port, to cancel out some sources of parasitic capacitance. 
     In any of the above examples, in order to mitigate parasitic capacitance, reference electrodes can be mounted in the elongate member, to normalize the first electrical conductor and/or second electrical conductor with reference capacitive signals. 
     The devices and systems described above can be used in various medical procedures, but may be particularly useful in transseptal perforation procedures, in which a dilator (i.e. a first medical device) is advanced via the femoral vein towards the heart and positioned adjacent the fossa ovalis of the atrial septum, and then a transseptal perforation device (i.e. a second medical device) is advanced into and through the lumen of the dilator, from the proximal end of the dilator towards the distal end of the dilator. In such procedures, as the transseptal perforation device is advanced through the lumen of the dilator, capacitance can be sensed, to provide a measure of the longitudinal position of the transseptal perforation device with respect to the dilator. That is, as described above, capacitance can be sensed as the transseptal perforation device is passed through a gap between a first electrical conductor and second electrical conductor of the capacitor (as shown in  FIGS. 1 to 5 and 6 to 7 ), and/or capacitance can be sensed as a second electrical conductor of the capacitor is advanced with respect to a first electrical conductor of the capacitor (as shown in  FIGS. 11 to 13, and 14 ), and/or capacitance can be sensed as the transseptal device passes past the capacitor (as shown in  FIGS. 8 to 10 ). 
     As described above, an output can be generated based on the capacitance, to provide an indication of the longitudinal position of the transseptal perforation device with respect to the dilator. For example, an output can be generated when the capacitance reaches a predetermined value. The output can be, for example, in the form of an image, or a light. This can help an operator to ensure that the perorating tip of the perforation device is shrouded within the dilator until it is ready for use by the operator. 
     Optionally, based on the output, the position of the transseptal perforation device can be adjusted with respect to the dilator. For example, if a red light illuminates before the user is ready to perforate the fossa ovalis, the user can withdraw the transseptal perforation device proximally, until a green light illuminates. 
     While the above description provides examples of one or more processes or apparatuses or compositions, it will be appreciated that other processes or apparatuses or compositions may be within the scope of the accompanying claims. 
     To the extent any amendments, characterizations, or other assertions previously made (in this or in any related patent applications or patents, including any parent, sibling, or child) with respect to any art, prior or otherwise, could be construed as a disclaimer of any subject matter supported by the present disclosure of this application, Applicant hereby rescinds and retracts such disclaimer. Applicant also respectfully submits that any prior art previously considered in any related patent applications or patents, including any parent, sibling, or child, may need to be re-visited.