Abstract:
In a system for sensing electrical signals within a living body, and specifically for tracking location of an object in the body using impedance measurements, an isolation circuit maintains isolation between the pacing and position sensing circuits, even while the heart is being paced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims the benefit of U.S. Provisional Application No. 61/113,729 filed on Nov. 12, 2008, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to sensing of electrical signals within a living body. More particularly, this invention relates to sensing of electrical signals within a living body, while tracking an object in the body using impedance measurements. 
     2. Description of the Related Art 
     A wide range of medical procedures involve placing objects, such as sensors, tubes, catheters, dispensing devices, and implants, within the body. Position sensing systems have been developed for tracking such objects. For example, U.S. Pat. No. 5,983,126, to Wittkampf, whose disclosure is incorporated herein by reference, describes a system in which catheter position is detected using electrical impedance methods. U.S. Patent Application Publications 2006/0173251, to Govari et al., and 2007/0038078, to Osadchy, which are herein incorporated by reference, describe impedance-based methods for sensing the position of a probe by passing electrical currents through the body between an electrode on the probe and a plurality of locations on a surface of the body. 
     Systems such as those described above may be used to track the position of a catheter within a patient&#39;s heart. A physician may use the catheter, for example, for diagnostic purposes, such as electrophysiology studies, and for therapeutic purposes, such as treating cardiac arrhythmias. In the course of such procedures, the physician may wish to pace the heart, by applying a suitable electrical signal via an electrode at or near the catheter tip. For this purpose, it is common to connect a pacing generator across the same electrodes on the catheter as are used for impedance-based position sensing. 
     SUMMARY OF THE INVENTION 
     For accurate position measurement using impedance-based techniques, it is desirable that electrical currents flow between the electrodes on the catheter and the body surface electrodes without leakage to other current sinks. Pacing generators, however, typically have low input impedance, and therefore, when a pacing generator is connected across the catheter electrodes, it will tend to short-circuit the signals that are used in impedance-based position sensing. Embodiments of the present invention provide a simple, novel type of circuit that can be used to maintain isolation between the pacing and impedance-based position sensing circuits, even while the heart is being paced. 
     An embodiment of the invention provides a medical apparatus, including a probe having one or more electrodes thereon, adapted for insertion into a heart of a subject. The apparatus includes a position sensing circuit coupled to the electrodes, a pacing generator for producing electrical pacing signals to electrically activate the heart, and a coupling element, which is inserted between the pacing generator and the electrodes and the position sensing circuit and is characterized by a relatively high first impedance at a voltage that is within a predetermined range and a second impedance that is low relative to the first impedance when the voltage is without the predetermined range. 
     According to one aspect of the apparatus, the coupling element includes a pair of diodes of opposing polarities connected in parallel. 
     According to an additional aspect of the apparatus, the coupling element includes two bipolar junction transistors connected in parallel. 
     According to still another aspect of the apparatus, the pacing generator has first and second output leads and the coupling element includes first and second cross-diode pairs connected to the first and second output leads, respectively. 
     The predetermined range may be −0.7 to +0.7 volts. 
     The apparatus may include a router coupled to the pacing generator for directing an output of the pacing generator to selected ones of the electrodes, wherein the position sensing circuit and the pacing generator are simultaneously electrically connected to the selected ones of the electrodes. 
     The apparatus may include electrocardiographic circuitry coupled to the electrodes and concurrently coupled to the pacing generator via the coupling element. 
     Other embodiments of the invention provide methods for carrying out the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
         FIG. 1  is a pictorial illustration of a system for detecting areas of abnormal electrical activity and performing ablative procedures on a heart of a living subject in accordance with a disclosed embodiment of the invention; 
         FIG. 2  is a schematic diagram showing electrical connections between catheter-mounted electrodes and other components of the system shown in  FIG. 1  in accordance with a disclosed embodiment of the invention; 
         FIG. 3  schematically illustrates an impedance-based position measuring system as a component of the system shown in  FIG. 1 , in accordance with a disclosed embodiment of the invention; and 
         FIG. 4  is an electrical schematic of a circuit that is suitable for use as a coupling element in the arrangement shown in  FIG. 2 , in accordance with an alternate embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily always needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
     System Architecture 
     Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for detecting areas of abnormal electrical activity and performing ablative procedures on a heart  12  of a living subject in accordance with a disclosed embodiment of the invention. A probe or catheter  14  is a component of the system  10 , and is percutaneously inserted by an operator  16 , who is typically a physician, through the patient&#39;s vascular system into a chamber or vascular structure of the heart. The operator  16  brings the catheter&#39;s distal tip  18  into contact with the heart wall at a target site that is to be evaluated. Electrical activation maps are then prepared, according to the methods disclosed in the above-noted U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosure is herein incorporated by reference. 
     Electrical signals can be conveyed from the heart  12  through one or more electrodes  32  located at or near the distal tip  18  of the catheter  14  and through wires  34  to a console  24 . Pacing signals and other control signals areas may be conveyed from the console  24  through the wires  34  and the electrodes  32  to the heart  12 . The electrodes  32  also function as components of an impedance-based positioning system  26  for locating the catheter. Additional wire connections  28  link the console  24  with body surface electrodes  30  and other components of the positioning system  26 . Further details of the positioning system  26  are presented below. 
     Additionally, areas determined to be abnormal by evaluation of the electrical activation maps can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires  34  in the catheter to the electrodes  32 , which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically about 50° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. The principles of the invention can be applied to different heart chambers, and to mapping in sinus rhythm, and when many different cardiac arrhythmias are present. 
     The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. A positioning processor  22  is an element of an impedance-based positioning system  26  that measures location and orientation coordinates of the catheter  14 . 
     The console  24  contains a pacing generator  25 , the output of which is connected to one or more electrodes  32  on the outer surface of the catheter  14  by wires  34 . The electrodes  32  are at least dual-purpose, being employed to conduct first electrical signals from the heart  12  to the positioning processor  22  and second electrical signals from the pacing generator  25  to the heart  12 . In some embodiments, the operator  16  may cause third electrical signals, containing ablative radiofrequency energy to be conducted to the electrodes  32  from an ablation power generator  36 , which can be incorporated in the console  24 . Such techniques are disclosed in commonly assigned U.S. Pat. No. 6,814,733, which is herein incorporated by reference. 
     As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . The positioning processor  22  is preferably a computer with appropriate signal processing circuitry. The processor is coupled to drive a display monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals conveyed via the electrodes  32 . The digitized signals are received and analyzed in the console  24  to derive electro-anatomical information of medical interest. The information derived from this analysis is used to generate an electrophysiological map of at least a portion of the heart  12  or related structures such as the pulmonary venous ostia. The map may be employed for diagnostic purposes, such as locating an arrhythmogenic area in the heart, or to facilitate therapeutic ablation. 
     Other signals used by the positioning system  26  are transmitted from the console  24  through the wires  34  and the electrodes  32  in order to compute the position and orientation of the catheter  14 . 
     Typically, the system  10  includes other elements. For example, the console  24  may include an electrocardiographic device  38 , coupled to receive signals from one or more body surface electrodes, so as to provide an ECG synchronization signal to the console  24 , which may be displayed on the display monitor  29  or on a separate display (not shown). The system  10  typically also includes a reference position sensor, either on an externally-applied reference electrode attached to the exterior of the subject&#39;s body, or on another internally-placed reference catheter (not shown), which is inserted into the heart  12  and maintained in a fixed position relative to the heart  12 . By comparing the position of the catheter  14  to that of the reference catheter, the coordinates of catheter  14  are accurately determined relative to the heart  12 , irrespective of heart motion. Alternatively, any other suitable method may be used to compensate for heart motion. 
     Reference is now made to  FIG. 2 , which is a schematic diagram showing electrical connections between the electrodes  32  on the catheter  14  via a coupling adapter  40 , and other components of the system  10  ( FIG. 1 ), in accordance with a disclosed embodiment of the invention.  FIG. 2  includes ECG amplifiers  42 , which are components of the electrocardiographic device  38  ( FIG. 1 ). More particularly,  FIG. 2  illustrates how the pacing generator  25 , ECG amplifiers  42 , and impedance-based position sensing circuitry  44  may be electrically connected simultaneously to the electrodes  32 . The position sensing circuitry  44  is referred to in the figure as an accurate current location (ACL) transmitter, and operates in a manner similar to that described in the above-mentioned publication by Osadchy. Its outputs are linked to the positioning processor  22  ( FIG. 1 ). 
     Reference is now made to  FIG. 3 , which is a schematic illustration of an impedance-based position measuring system as a component of the system  10  ( FIG. 1 ), in accordance with a disclosed embodiment of the invention. The pacing generator  25  and position sensing circuitry  44  are connected to the catheter  14  as described above with reference to  FIG. 2 . A plurality of body surface electrodes  30 , which can be adhesive skin patches, are coupled to a body surface  46  (e.g., the skin) of the subject. The body surface electrodes  30  may be placed at any convenient locations on the body surface  46  in the vicinity of the site of the medical procedure. Typically, the locations of the body surface electrodes  30  are spaced apart. For example, for cardiac applications, the body surface electrodes  30  are typically placed around the subject&#39;s chest. A control unit  48 , normally disposed in the console  24  ( FIG. 1 ) drives a current between one or more of the electrodes  32  and one or more of the body surface electrodes  30 . Currents through the body surface electrodes  30  are measured by respective current measurement circuits  50 . The current measurement circuits  50  are typically configured to be affixed to a body surface patch, or, alternatively, may be situated within the console  24  ( FIG. 1 ). 
     Reverting to  FIG. 2 , the pacing generator  25  is connected to the catheter electrodes  32  via a coupling element  52  comprising cross-diodes  54 ,  56 . In this configuration, each of the cross-diodes  54 ,  56  comprises a pair of diodes coupled in parallel and having opposing polarities. The pacing generator  25  sees an open circuit for low-voltage signals (in the range between about a range of −0.7 to +0.7 V and a low impedance at voltages outside this range). Table 1 presents the impedance in Ohms of two general purpose diodes, BAS16 and BAV99, as a function of the forward voltage. Both of these diodes, which have a fast response, are suitable for the cross diodes  54 ,  56 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Diode BAS16 
                 Diode BAV99 
               
               
                 Voltage 
                 impedance 
                 impedance 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.1 
                 3333333 
                 5555556 
               
               
                 0.2 
                 833333 
                 1333333 
               
               
                 0.3 
                 176471 
                 272727 
               
               
                 0.4 
                 36364 
                 50000 
               
               
                 0.5 
                 6250 
                 7463 
               
               
                 0.6 
                 1017 
                 1000 
               
               
                 0.7 
                 171 
                 163 
               
               
                 0.8 
                 33 
                 31 
               
               
                 0.9 
                 10 
                 11 
               
               
                 1 
                 5 
                 7 
               
               
                 1.1 
                 3 
                 4 
               
               
                 1.2 
                 2 
                 4 
               
               
                 1.3 
                 2 
                 3 
               
               
                   
               
             
          
         
       
     
     Thus, the relatively high-voltage pacing signals produced by the pacing generator  25  are not significantly impeded by the cross-diodes  54 ,  56 . Low-voltage position sensing signals that are output by the position sensing circuitry  44 , however, pass directly to the catheter  14  through a router  58  and the adapter  40  without significant leakage into the pacing generator  25 . In embodiments in which the catheter  14  has a plurality of electrodes  32  ( FIG. 1 ), the router  58  directs pacing signals to selected sets of the electrodes. Irrespective of the router-directed output of the pacing generator  25 , the pacing generator  25 , ECG amplifiers  42  and position sensing circuitry  44  may be simultaneously operational via the electrodes  32  of the catheter  14 . 
     Alternate Embodiment 
     Although the embodiment shown in  FIG. 2  uses pairs of diodes to isolate the position sensing circuit from the pacing generator, other types of coupling elements with suitable non-linear V-I (voltage-current) dependence and symmetrical bidirectional conductivity may similarly be used for this purpose, for example, circuits including transistors. Such coupling elements should be characterized generally by high impedance at low voltage and a drop in impedance when the voltage reaches a certain threshold value. 
     Reference is now made to  FIG. 4 , which is an exemplary electrical schematic of a circuit  60  suitable for use as the coupling element  52  ( FIG. 2 ), in accordance with an alternate embodiment of the invention. The circuit  60  comprises a pair of bipolar junction transistors, a PNP (Positive-Negative-Positive) transistor  62  and NPN (Negative-Positive-Negative) transistor  64  and two resistors, Resistor  66  has a non-critical value in the range of a few thousand Ohms. Resistor  68  should be a few hundred ohms. General purpose transistors 2N2222 and 2N2907 are suitable for the two transistors. 
     Operation 
     Referring again to  FIG. 1 , to use the system  10 , the catheter  14  is conventionally introduced into the heart  12 , and navigated into an operating position, all the while receiving position signals from the electrodes in the position sensing circuitry  44  ( FIG. 2 ) and analyzing the position signals in the positioning processor  22 . The pacing generator  25  is activated, either continuously or intermittently, according to the requirements of the medical procedure. By suitably controlling the router  58 , the signals of the pacing generator  25  may be selectively directed to different sets of the electrodes  32 . The position sensing circuitry  44  continues to operate and receive new signals even while the pacing generator  25  is active and connected to common electrodes with the position sensing circuitry  44 . 
     Eventually, if the medical procedure is successful, or otherwise terminates, the pacing generator  25 , and optionally the position sensing circuitry  44  ( FIG. 2 ) are disabled, and the catheter  14  is withdrawn. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.