Abstract:
The disclosure relates to systems and methods for electronic intubation of bodily passages such as the nasolacrimal duct. The disclosed technology can be embodied in an apparatus that includes an electrically conductive probe configured to be extendible through a body passage, and a conductive tool for locating the probe in the body passage. The apparatus also includes a signal generator configured to generate a feedback signal, and a circuit coupled to the probe and the conductive tool. The circuit includes a comparison module that is configured to compare a voltage developed at the probe with a reference voltage and cause a generation of the feedback signal if at least a portion of the conductive tool touches the probe. The circuit is further configured to prevent the generation of the feedback signal when no portion of the conductive tool touches the probe.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/481,126, filed Apr. 29, 2011, the entire content of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to the field of medicine and, in particular, to new and useful methods and apparatus for enhanced intubation of the lacrimal ducts and the like. 
       BACKGROUND 
       [0003]    Lacrimal fluids or “tears” are normally supplied continuously to the eye socket from the lacrimal gland. Normally, excess fluid is drained through canaliculi or small passageways commencing adjacent the inner corner of the eye. The fluid is collected in the lacrimal sac and then drained via the nasolacrimal duct, which leads the fluid to the inferior nasal meatus. 
         [0004]    If the lacrimal ducts become blocked, the fluid can no longer flow to the nasal meatus. Such closure can result from congenital anomalies, accidents, inflammation and advanced aging, as well as other physiological conditions. The result is that the eye is continually brimming over with tear fluid causing discomfort to the patient. 
         [0005]    Nasolacrimal intubation has been a standard of care for severe epiphora, failed pediatric probings and irrigations, as well as an adjunct to dacriocystorhinostomy. The challenges of retrieving the metal portion of the silicone tubes as they descend in the posterior nasal cavity have resulted in various methods and devices. Because the procedures can cause bleeding of mucosa and turbinates, visibility and thus tube retrieval may be difficult. Many areas of the world still use a clamp to probe blindly in the nose to find the other end of the tube. 
       SUMMARY 
       [0006]    In one aspect, an apparatus includes an electrically conductive probe configured to be extendible through a body passage, and a conductive tool for locating the probe in the body passage. The apparatus also includes a signal generator configured to generate a feedback signal, and a circuit coupled to the probe and the conductive tool. The circuit includes a comparison module that is configured to compare a voltage developed at the probe with a reference voltage and cause a generation of the feedback signal if at least a portion of the conductive tool touches the probe. The circuit is further configured to prevent the generation of the feedback signal when no portion of the conductive tool touches the probe. The apparatus further includes a power source for powering the signaling mechanism to generate the signal, wherein the circuit is configured such that a current through the probe is below a first predetermined threshold. 
         [0007]    In another aspect, an apparatus includes an electrically conductive probe configured to be extendible through a body passage a signal generator configured to generate a feedback signal, and a circuit coupled to the probe. The circuit includes a comparison module that is configured to compare a voltage developed at the probe with a reference voltage and cause a generation of the feedback signal if at least a portion of the probe touches a conductive tool used to locate the probe in the body passage. The circuit is further configured to prevent the generation of the feedback signal when no portion of the conductive tool touches the probe. The circuit is configured such that a current through the probe is below a first predetermined threshold. 
         [0008]    In another aspect, an apparatus includes a signal generator configured to generate a feedback signal, and a circuit configured to be coupled to an electrically conductive probe that is extendible through a body passage and a conductive tool for locating the probe within the body passage. The circuit includes a comparison module that is configured to compare a voltage developed at the probe with a reference voltage and to cause a generation of the feedback signal if at least a portion of the probe touches the conductive tool. The circuit is further configured to prevent generation of the feedback signal when no portion of the conductive tool touches the probe. The circuit is further configured such that a current through the probe is below a first predetermined threshold. 
         [0009]    In another aspect a method includes providing a circuit. The circuit is configured to be coupled to an electrically conductive probe that is extendible through a body passage, and a conductive tool for locating the probe within the body passage. The circuit includes a comparison module that is configured to compare a voltage developed at the probe with a reference voltage and cause a generation of a feedback signal. The circuit is further configured to prevent the generation of the feedback signal when no portion of the conductive tool touches the probe, and to limit the current through the probe to a value below a first predetermined threshold. The method also includes locating, based on the feedback signal, the probe when at least a portion of the probe touches the conductive tool. 
         [0010]    Implementations of the apparatuses and method described above can include any combination of the following. 
         [0011]    The signal generator can be selected from a group consisting of a visual signal generator, an audible signal generator, and a tactile signal generator. At least a portion of the circuit can be disposed on the conductive tool. At least a portion of the circuit can be disposed on a housing coupled to the probe. A response time of the comparison module can be greater than a second predetermined threshold. The second predetermined threshold can be 7 μs or between 2 μs and 7 μs. A shunt resistor can be connected in parallel to the probe and the conductive tool. The shunt resistor can be configured to limit current through the probe below the first predetermined threshold. The comparison module can include a comparator that is configured to withstand electrostatic discharge up to a third predetermined threshold value. Connectors can be provided to electrically couple the circuit with the probe and the conductive tool. The body passage can be a nasolacrimal duct. Any of the apparatuses above can be provided as a kit in a sterile package. 
         [0012]    The apparatus and methods described herein can help, for example, where a nasal endoscope is not readily available, as an option to nasolacrimal surgeons. The combination of a feedback signal and proprioceptive sense of anatomy can facilitate efficient nasolacrimal intubation. The apparatus can include a circuit that limits the current (and resulting power dissipation) through the probe. Such limiting of the current in the probe allows for implementation of a continuity tester that can be safely used in human subjects without causing pain, discomfort or other adverse conditions that can result from high current flow through sensitive tissues. The methods and systems described herein allows for fast intubation through sensitive human body parts such as the lacrimal ducts. 
         [0013]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
         [0014]    Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram of an example of a system for facilitating intubation through a body passage 
           [0016]    FIGS.  2  and  3 A- 3 B each show a probe set and tool used on a patient. 
           [0017]      FIGS. 4-6  are schematic diagrams of example circuits used in intubation systems. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In nasolacrimal intubation procedures, retrieving the metal portion (or conducting probe portion) of a silicone tube as the probe descends in the posterior nasal cavity is challenging. This is due, at least in part, to the lack of visibility in the area where the probe is located and pulled out from using a tool such as a hook. One approach to overcome the challenges of locating the probe within a human or animal body is to use a conductive retrieval tool in conjunction with an electronic circuit that provides a feedback signal to the user when the tool and the probe are electrically coupled within a body passage. However, the use of such a circuit is challenging for various reasons. For example, because tissues within a body passage may conduct electricity, an electrical coupling between the probe and the conductive tool may be established if the probe and tool are sufficiently proximate to one another, but not actually touching. This can result in undesirable false positives and potential injuries to the tissues within the body passage if the user attempts to retrieve the probe when the tool is not actually touching the probe. Furthermore, if the current through the probe is above a certain level (for example, 200 μA), the current can burn the tissue as well as increase the chances of the false positives. On the other hand, implementing a system that is adequately sensitive and generates suitable feedback signals under such low current conditions is also challenging. 
         [0019]    The systems and methods described herein can allow safe and efficient intubation through a body passage (e.g., nasolacrimal intubation) by reducing false positives and keeping a current through the probe below a predetermined threshold value that is safe yet still highly effective for use in human and animal subjects. This is made possible by new circuit designs that realize the above goals without compromising effectiveness or sensitivity of the overall systems. 
         [0020]    The systems and methods described herein can be implemented in many ways, and some useful implementations are described below. 
         [0021]      FIG. 1  is a block diagram of an example of a system  5  for facilitating intubation through a body passage. System  5  includes a probe  12  that can be inserted into a body passage such as a nasolacrimal duct. System  5  also includes a conductive tool  18  that is used to locate one end of probe  12  in the body passage, and locate and remove an end portion of the probe from the body passage. System  5  further includes a circuit  14  for detecting a physical contact between probe  12  and tool  18  within the body passage. Circuit  14  can be configured such the circuit prevents a current from passing through the probe  12  and the tool  18  unless they are in physical contact with one another. This reduces the possibility of false positives resulting from current passing from probe  12  to tool  18  through intervening body tissues when probe  12  and tool  18  are proximate to one another but not in actual physical contact. This can be done, for example, by connecting a shunt resistor in parallel across probe  12  and tool  18 . When probe  12  and tool  18  are in contact with one another, circuit  14  can be configured to activate a signal generator  10  to generate a feedback signal. 
         [0022]    Signal generator  10  can be an audio signal generator such as a buzzer. For example, a Sonalert® 1.5V, 3 mA SNP2 buzzer can be used. In some implementations, signal generator  10  can be a visual signal generator such as a light emitting diode (LED) or a display device such as a liquid crystal display (LCD) or LED screen. In some implementations, signal generator  10  can be a tactile signal generator such as a vibrator. Circuit  14  can be powered by, for example, a battery, e.g., a disposable or rechargeable battery. In some implementations, circuit  14  can include a powering module that includes a housing for a 1.5V battery. 
         [0023]    Although  FIG. 1  shows various modules, in some implementations, two or more of these modules can be combined. For example, circuit  14  and/or signal generator  10  can be implemented in a miniaturized form (such as on an integrated circuit chip) and disposed on a portion of the probe or the tool.  FIG. 3B  illustrates an example of such a system where circuit  14  is disposed in or on a handle-shaped housing  37  attached to probe  12 . In some implementations, housing  37  can include a switch  39  for switching circuit  14  on or off. In such cases, system  5  can be packaged, e.g., as a sterile package, as one self-contained system. The package can be sealed and sterilized using standard techniques such as radiation or chemicals, e.g., ethylene oxide gas, to form a kit. In some implementations, a portion of system  5  (e.g., probe  12  and circuit  14 ) can be packaged as a self-contained apparatus (e.g., using a housing  37 ) that can be used in conjunction with a commercially available tool  18 . In some implementations, the self-contained apparatus can be provided as a sterile (and possibly disposable) unit that a clinician can use for an intubation procedure by simply opening the sealed package, removing the device using sterile procedures, and then performing the medical intubation without the need to connect any electrical leads to probes or other medical tools. 
         [0024]    In some implementations, circuit  14  and signal generator  10  can be provided together in as a unit configured to accept an electrical connection from a probe  12  and tool  18  through appropriate leads. The leads can be made long enough to allow the unit to be sufficiently away from the sterile field of a surgical environment. The leads can be configured to be attached to probe  12  and tool  18  using electrical connectors such as an alligator clips or banana connectors. 
         [0025]      FIG. 2  illustrates parts of an example of a nasolacrimal intubation set (also known as a canaliculus intubation set). Although the complete set may comprise a pair of thin metal probes, only probe  12  is shown. In a complete set, probe  12  and a similar second probe can be fitted to respective ends of a tube  16  of silicone rubber which is limp and flexible. 
         [0026]    Probe  12  can include an enlarged distal end portion or “olive”  18  and a tapered enlargement  20  adjacent to the proximal end of the probe  12 . The end portion  18  is rounded and the enlargement  20  has an external size comparable to that of the tube  16 . In some implementations, the probes are formed from a relatively fine tempered steel wire which is electrically conductive. 
         [0027]    Probe  12  can move through the patient&#39;s lacrimal duct when pushed in a downward direction. The end portion  18  prevents the fine wire from puncturing the patient&#39;s tissue and is made, for example, from solder attached to the end of probe  12 . Enlargement  20  can be formed in similar fashion. End portion  18  is generally spherical so that there is a limited possibility of damage caused by the end of the probe being forced to penetrate or scratch soft tissue. 
         [0028]    Probe  12  can be used together with a tool  21  having a handle portion  22  with an aligned extension  24  terminating in a hook portion  26 . The extension and hook portions  24 ,  26  are formed of stiff, electrically conductive wire and the hook includes a recess which extends forwardly and inwardly. The recess has a width corresponding generally to the diameter of the probe  12 . 
         [0029]    In use, probe  12  is pushed downwardly through the lacrimal duct and into the inferior meatus of the patient or subject. At this point, end portion  18  of the probe is in engagement with the lower wall of the meatus and the probe must be deflected to allow the end portion to be withdrawn. The probe can be deflected without undue force, which would otherwise result in damage to surrounding tissue, using the tool  21  to withdraw the end of probe  12  from the nasal passage, but the practitioner typically must first locate and “hook” the probe. 
         [0030]    As indicated in  FIG. 2 , tool  21  is pushed into the nasal passage with extension  24  projecting inwardly. Because it is difficult or even impossible to look into the nasal passage, the usual practice is to feel around for the probe  12 , and, once found to gently withdraw the tool to hook the end portion or “olive”  18  in the recess of hook portion  26 . The tool is then withdrawn with end portion  18  trapped in the recess so that the tool can be used to draw the probe through the nasal passage and outwardly as illustrated in  FIG. 3A . 
         [0031]    This draws tube  16  into the lacrimal duct. The process can be repeated using the other probe (not shown) in the opposite canaliculus outflow system and after intubation of both systems the tube is cut leaving a portion in the canaliculus systems. The tube is then left in place for a period determined by the type of damage being corrected or repaired and then subsequently removed. 
         [0032]      FIG. 2  shows a circuit that includes a pair of wires  28  and  30 , connected to opposite contacts of a signal generator  10 . Each wire has a free end connected to an alligator clip or other means  34  and  36  for attaching the wire to a conductive part of the probe  12  and tool  21 . A battery or other power source or supply  32  sufficient to power signal generator  10 , is provided at any appropriate location in circuit  14 . 
         [0033]    Signal generator  10  may, for example, be a low voltage buzzer, tone generator, LED (light-emitting-diode), vibrator, or other device that can generate an audible, visual, tactile, or other signal that can be perceived by a practitioner, when circuit  14  is closed to connect the power supply or source  32  to signal generator  10 . Signal generator  10  may even generate a combination of two or more different types of signals, e.g., a tone and a rapidly blinking light, or a tone and a tactile signal, to advise the practitioner that he or she has made contact between the tool and the probe, even when the practitioner is not directly looking at the signal generator  10 , or when there is other noise in the operating location. 
         [0034]    Power source  32  may be a battery holder for receiving a battery, within the meaning of this disclosure since the battery will normally be a consumable and replaceable part used with the apparatus. The compact size and simple and cheap construction of circuit  14  makes it readily available to practitioners in any country. 
         [0035]    Although attachable means in the form of clips  34  and  36  are illustrated, the circuit can be incorporated into tool  21 , which may contain signal generator  10  and battery  32  along with a single wire or lead  30  and clip  34  for engaging a conductive part of probe  12 , which is still outside the patient. 
         [0036]    In addition, attachment means other than clips and of any known type can be used for connecting the circuit  14  to conductive portions of the tool and probe. Contact between tool extension  24  and probe  12  amounts to closing a simple switch for circuit  14 , but this simple action gives the practitioner a quick and easy way of finding the end of probe  12 . 
         [0037]    Disclosed herein are improved circuits that replace the circuits found in the prior art and allow for low power and low current implementation. Such a low power device can be used for fast positioning of a tube into the lacrimal duct. The new circuits can be configured to limit the current (and resulting power dissipation) through the probe below a predetermined threshold. Such limiting of the current in the probe allows for implementation of the new devices that can be safely used in human subjects without causing pain, discomfort, or other adverse conditions that can result from high current flow through sensitive tissues. 
         [0038]      FIG. 4  is a circuit diagram illustrating of an example of a circuit  14 . These circuits are current limited, voltage limited, and/or resistant to a reasonable electrostatic discharge levels. In some implementations, circuit  14  uses a voltage divider to detect continuity between two connected probes or a probe and a tool. For safety, the current and/or voltage on the probes can be limited. In some implementations, safety and stability can be further improved by battery isolation. In some implementations, a voltage divider configuration can be used in circuit  14  for limiting the current through the probe. 
         [0039]    In some implementations, circuit  14  can include two voltage divider configurations. The first voltage divider can include three resistors  45 ,  47 , and  39  with values R1, R2, and R_Probe, respectively. To limit the worst case current through resistor  49 , the value R2 can be selected assuming the value R_Probe to be zero. Such an assumption increases a likelihood of preventing a current flow through resistor  49  in the presence of any tissue was between the probe and the tool. The circuit shown in  FIG. 4  can be designed, for example, using the following design parameters:
       a) Maximum current in probes R_Probe_Imax=200 μA   b) Typical current in the probes: about half the maximum value, i.e. R_Probe_Ityp=R_Probe_Imax/2   c) Soft moist tissue conductivity: R_Probe=2KΩ   d) System voltage: V1=Vcc=3V       
 
         [0044]    The value resistor  45  is therefore given as: 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                      
                     
                         
                     
                      
                     2 
                   
                   = 
                     
                    
                   
                     V 
                      
                     
                         
                     
                      
                     
                       1 
                       / 
                       R_Probe 
                     
                      
                     _Ityp 
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     30 
                      
                     
                         
                     
                      
                     K 
                      
                     
                         
                     
                      
                     Ω 
                   
                 
               
             
           
         
       
     
         [0045]    In some implementations, a standard resistor value, such as 33KΩ can be chosen for resistor  45 . 
         [0046]    In some implementations, resistor  47  functions as a shunt resistor such that a majority of the current passes through resistor  47  and not the probe and the tool. Under such conditions, a low resistance value, such as 330Ω or 470Ω can be chosen as a value R1 for resistor  47 . When a current path through the probe is closed, resistor  49  (or the resistance value R_probe) represents the current path. The current through resistor  45  is given by: 
         [0000]        R 2 —   I dry= V 1/( R 1 +R 2) 
         [0000]    The voltage across the probe and the tool (when dry) is given by: 
         [0000]        R _Probe —   V dry= V 1 —   R 2 —   I dry* R 2 
         [0000]    The current through the probe when the probe is wet is given as: 
         [0000]        R _Probe —   I wet= V 1/( R _Probe —   thR wet+ R 2) 
         [0000]      wherein 
         [0000]        R _Probe —   thR wet=( R 1* R _Probe)/( R 1+ R _Probe) 
         [0047]    The voltage across the probe and the tool, when the probe is wet, is given by: 
         [0000]        R _Probe —   V wet= R _Probe —   I wet* R _Probe —   thR wet 
         [0000]    When the probes are touching, the current through the probe is given by: 
         [0000]        R _Probe —   I touch= V 1/ R 2 
         [0048]    In some implementations, the circuit includes a comparison module  50 , which can include, for example, a comparator such as the U1 MCP6546 DIP8 comparator shown in  FIG. 4 . In some cases, the comparator can be chosen such that it has a sufficiently wide hysteresis loop that prevents the comparator from self oscillating. In some implementations, it may also be of interest to choose a relatively slow comparator to prevent the self oscillations. Threshold conditions for hysteresis and speed can be used in choosing the comparator. In some implementations, a comparison module  50  with a response time of 7 or more can be selected. In some implementations, the reference voltage of the comparison module is such that the reference voltage is less than the voltage when the probes are wet and more than the voltage when they are touching. 
         [0049]    The voltage across the probe and the tool can be selected, for example, to be a multiple of an input offset voltage of comparator  50 . The ratio between the value R2 and the parallel combination of the values R1 and R_Probe can be used to determine the voltage across the probe and the tool. In some implementations, the difference between the voltage across resistor  49  and the reference voltage is determined to be about half way between the sense voltage (i.e., the voltage at the probe) and zero such that comparison module  50  can detect a difference that is larger than a threshold amount. This can ensure that the comparison module  50  does not detect small differences that could potentially render the comparison module unstable. For example, the reference voltage V_Ref can be given by: 
         [0000]        V _Ref= R _Probe —   V wet/2 
         [0050]    In some implementations, a second voltage divider configuration can be used to provide the reference instead of a voltage regulated source. Therefore, the reference voltage levels can be made to change substantially proportionally with the voltage across resistor  49  when the electrodes are not touching. By allowing the relative voltages to be maintained while the rail voltage changes circuit  14  can be configured to be powered using unregulated battery power. In some implementations, this can reduce component count in circuit  14  and/or increase battery life. The second voltage divider configuration can be implemented, for example using resistors  52  and  54  with values R3 and R4, respectively. Resistors  52  and  54  can be chosen such that even when two metal tools or probes touch each other the current shunted will be lower than a predetermined threshold. In some implementations, the value R4 can be chosen as: 
         [0000]        R 4 =R 2*2 
         [0051]    In some cases, resistor  54  can be chosen to have values such as 66 KΩ or 61.9 KΩ. The current through resistor  54  is given by: 
         [0000]        R 4 —   I= ( V 1− V _Ref)/ R 4
 
         [0052]    The value R3 for resistor  52  can be chosen as, for example: 
         [0000]        R 3 =V _Ref/ R 4 —   I    
         [0053]    In some implementations, the response time of comparison module  50  is selected to be more than a predetermined threshold, e.g., 7 μs. The slow response time can help improve the stability of comparison module  50  by reducing the possibility of high frequency oscillation. This in turn can also improve precision of the comparison module. In some implementations, the comparison module can include a comparator with internal electrostatic discharge protection. This reduces the need for a separate electrostatic discharge protection circuit, thereby reducing the component count of the circuit. For example, internal electrostatic discharge protection for comparison module  50  can be provided by using a comparator that has an open collector output with a sufficiently large current rating, e.g. 30 mA. In some implementations, using a buzzer with an internal oscillator further reduces the component count for circuit  14 . 
         [0054]    In some implementations where the exact calculated values for the resistors are not available, approximate or substantially close available values can be used. In some implementations, resistor  56  can be used to set a gain for signal generator  58  (e.g., a volume of a buzzer or beeper). The value R5 of resistor  56  can be set experimentally. In some implementations, other alarms or alerting modules can be used in place of, or in conjunction with, the buzzer as the signal generator. For example, the circuit can include a display that visually renders establishment of continuity. The circuit can also include a mechanism for providing haptic or tactile feedback in place of or in conjunction with the buzzer. In some implementations, a light emitting diode (LED) can be used as the signal generator. In such cases, a diode such as a Zener diode can be used to adjust the light emission from the LED. 
         [0055]    The source of power for the voltage rails (Vcc) can be, for example, a battery  62  that includes, for example, two 1.5V AA cells for a total of 3.0 volts. In some implementations, electricity from a building electric system can also be used as the source of power. However, the use of a battery can provide isolation from utility power lines as an additional safety factor. In some implementations, circuit  14  can be housed in a box made from non-conductive material. In some implementations, the circuit includes another diode, such as the Schottky diode  60  to prevent the circuit from malfunctioning if the batteries are installed backwards. 
         [0056]    The circuits described herein can be implemented in various ways. For example, the circuits can be implemented on a printed circuit board. In some implementations, the circuits can include one or more integrated circuits. The circuits described herein can be implemented using more or fewer circuit elements than what is depicted in  FIG. 4 . 
         [0057]    Circuit  14  is designed such that the current or power through the probes do not exceed a threshold condition. The threshold condition can be chosen based on a level of current or power that can be tolerated by the corresponding human body part. For example, the circuit can be designed such that the current does not exceed 200 μA. The threshold conditions can be determined in various ways, for example, by experimentally or theoretically determining a resistance of a specific portion of the human body. 
         [0058]      FIG. 5  illustrates an example of another circuit, circuit  15 , which can be used as an alternative to circuit  14 . Circuit  15  includes a double pole double throw (DPDT) switch  65  between the probes (or the probe and the tool)  66  and  68 . Switch  65  can be electronically controllable and can be automatically switched at an even duty by an oscillator with a predetermined periodicity.  FIG. 6  illustrates a device  17  that can be used to implement switch  65 . In some implementations, device  17  can be a solid state device with internal electrostatic discharge protection. The frequency of oscillation can be selected to be non-interfering with frequency used for utility power transmission (e.g., 60 Hz, 50 Hz, or 400 Hz). Switch  65  can be implemented by configuring the device  17  to reverse the connections to the probes  66  and  68  every time the clock  70  cycles. In some implementations, the switching speed of DPDT switch  65  can be selected to be faster than the response time of comparison module  50 . This can mean that the opening of switch  65  between states does not change the output of comparison module  50 . In some implementations and as shown in  FIG. 5 , a capacitor  70  can be added to introduce additional hysteresis that also further reduces the response time of comparison module  50 . 
       Other Embodiments 
       [0059]    The methods and systems described herein can also be adapted to other medical procedures utilizing probes extending through body passages where the need exists for locating one end of the probe. While some specific embodiments have been described in detail, other embodiments, aspects, advantages, and modifications are within the scope of the following claims.