Abstract:
An implantable medical device having at least one elongated electrical function conductor that transmits therapeutic signals or diagnostic signals or both, and an electrode pole connected to the function conductor to deliver electrical current or field or sense electrical potentials in surrounding tissue during use, or both. Includes a sensing device which is connected to a field-generating electrode pole and potential-sensing electrode pole, and a reference pole, and which is designed to detect generated electrical potentials via the potential-sensing electrode pole in relation to the reference pole, and to generate an output signal which represents a detected electrical potential. Also has a control device connected to the sensing device to evaluate an output signal generated by the sensing device, and to control the medical device as a function of the potential detected by the sensing device.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application 61/424,685 filed on 20 Dec. 2010, the specification of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    At least one embodiment of the invention relates to a permanently or temporarily implantable device having an elongated electrical conductor. 
         [0004]    2. Description of the Related Art 
         [0005]    Such devices, for example electrode lines for electrostimulation, have the disadvantage that their electrical conductor may heat up during magnetic resonance imaging due to the fact that the alternating magnetic fields that are present induce considerable electrical currents in the electrical conductor. In addition, such induced currents may be delivered to surrounding tissue via electrode poles of the electrode line, resulting in undesired heating of the tissue, for example. For this reason, there is presently little or no possibility for cardiac pacemaker patients to be tested using magnetic resonance imaging. 
         [0006]    Implantable cardiac pacemakers or defibrillators (also jointly referred to below as cardiac stimulators or implantable pulse generators (IPG)) are typically connected to at least one stimulation electrode line, which at its proximal end which is provided for connection to the cardiac pacemaker or defibrillator has a standardized electrical terminal, and at its distal end which is provided for placement in the heart has one or more electrode poles. Such an electrode pole is used to deliver electrical pulses to the (myocardial) tissue of the heart or for sensing electrical fields in order to sense an activity of a heart. For these purposes, electrode poles typically form electrically conductive surface sections of an electrode line. Electrode poles are typically provided as an annular electrode in the form of a ring around the electrode line, or in the form of a point electrode or tip electrode at the distal end of the electrode line. At their proximal end the electrode poles are connected in an electrically conductive manner via one or more electrical conductors to contacts of the electrical terminal of the electrode line. Thus, the electrode lines at their proximal end extend between the contacts of the electrical terminal, and at the distal end one or more electrical conductors which electrically connect the one or more electrode poles to the one or more contacts extend between the electrode poles. These electrical conductors may be used on the one hand for transmitting stimulation pulses to the electrode poles, and on the other hand for transmitting electrical signals received via the electrode poles to the proximal end of the electrode line, and in the description below are also referred to in each case as a function line. Such function lines are electrical conductors which are necessary for the functions of the particular electrode line, and are thus subject to the risk of electrical currents being induced therein as the result of external alternating magnetic fields which, for example, may lead to undesired heating of the function lines or the electrode poles connected thereto, or may result in the discharge of corresponding currents via the electrode poles to surrounding tissue, and thus heating of the surrounding tissue. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The object of at least one embodiment of the invention is to provide a device which eliminates the above-described problem. 
         [0008]    According to at least one embodiment of the invention, this object is achieved by a permanently or temporarily implantable medical device having at least one elongated electrical function conductor for transmitting therapeutic signals or diagnostic signals or both, and an electrode pole connected to the function conductor by means of which electrical current may be delivered to surrounding bodily tissue during use, or by means of which electrical potentials may be sensed in surrounding tissue during use, or both, wherein
       a sensing device is provided which is connected to a field-generating electrode pole and to a potential-sensing electrode pole, and which in the event of a current output via the field-generating electrode pole is designed to detect generated electrical potentials via the potential-sensing electrode pole in relation to a reference potential, and to generate an output signal which represents a detected electrical potential,   and a control device which is connected to the sensing device and is designed to evaluate an output signal generated by the sensing device, and to control the medical device as a function of the potential detected by the sensing device.       
 
         [0011]    At least one embodiment of the invention includes an implanted system having a connected electrode line having at least two electrode poles, or an elongated electronic implant having electrode poles whose electrical field of the one electrode pole, as a field-generating electrode pole which is able to heat the surrounding tissue as the result of induced currents, for example by interaction in magnetic resonance imaging (MRI), is sensed by a further electrode pole system as sensing electrode poles, and generates one or more signals which is/are evaluated by a device connected to the electrode poles, and which is/are used for activating the implant with regard to a safety measure. 
         [0012]    Since the potential which is to be detected is a potential resulting from high-frequency, external interference fields, the potential itself is a signal having a high-frequency signal curve. 
         [0013]    The field-generating electrode pole may at the same time be one of the sensing electrode poles, which includes at least one reference pole as a further electrode pole which supplies the reference potential. 
         [0014]    Electrode poles which are mounted anyway on the electrode as therapeutic/diagnostic poles are preferably selected as nonfield-generating, sensing electrode poles, so that no additional electrode poles are necessary. 
         [0015]    The implant-side input which is provided for connection to the field-generating electrode pole is preferably used as the reference potential-supplying reference pole for the signal of the sensing electrode pole. 
         [0016]    Alternatively, an implant-side input which is provided for connection to a third electrode pole may be used as the reference potential-supplying reference pole for the signal of the sensing electrode pole. 
         [0017]    Alternatively, an electrode pole formed by an implant housing may be provided as the reference potential-supplying reference pole for the signal of the sensing electrode pole. 
         [0018]    In principle, the medical device may also be designed to process a combination of various signals which originate, for example, from various sensing electrode poles with respect to different reference potentials. For this purpose, the sensing device may include a mixer for combining signals. Alternatively, an evaluation unit or evaluation device of the sensing unit may be designed to process the various signals in parallel, i.e., unmixed. A higher degree of interference immunity results in both cases. 
         [0019]    The evaluation device may also be designed to evaluate only signals in specific frequency ranges, for example signals &gt;1 MHz, or bands with bandwidths of &lt;5 MHz around typical MRI HF frequencies such as 42, 64, 128 MHz, for example. The sensing device may have an appropriate filter for this purpose. 
         [0020]    Furthermore, the evaluation device may be designed to evaluate only signals above a specific threshold, for example in the frequency range around 64 MHz &gt;1V. 
         [0021]    An electrode pole having low heating potential, i.e., an electrode pole for which a low degree of interference is expected, is preferably provided as the sensing electrode pole. The evaluation device may be designed to select the sensing electrode pole among the available electrode poles. For example, a tip electrode of a coaxially configured standard electrode may be selected as a field-generating electrode pole. An annular electrode is correspondingly selected as a sensing electrode pole. 
         [0022]    According to one preferred embodiment variant, the selection of the electrode poles (field-generating, sensing) is externally programmable. 
         [0023]    The evaluation device may be designed to determine the cumulative effect of the signal which represents the detected potential, for example by integration, averaging, or determining the effective value. 
         [0024]    Alternatively, the evaluation device may be designed to determine the maximum effect of the signal, generally pulsed, which represents the detected potential, and for this purpose may be designed as a maximum value detector, for example. 
         [0025]    The control device is preferably designed to switch the medical device and/or the implant-side electrode circuitry to a safe mode for the particular operating environment, in particular in the event of an extreme electromagnetic field effect, in response to a corresponding output signal of the evaluation device when the output signal of the evaluation device indicates such an operating environment. The control device is preferably designed to perform this switching in a time-limited manner, for example for a specific, programmable period of time, or for a latency period following the last recorded interference. 
         [0026]    According to another preferred embodiment variant, the medical device may have a telemetry device which is at least indirectly connected to the evaluation device. In this case the evaluation device is preferably designed to externally communicate the occurrence of electrode heating above a threshold, or an electrical signal amplitude indicating such, via the telemetry device. This may be carried out in real time, or after analysis for providing information to the medical practitioner via a home monitoring system, for example. 
         [0027]    The medical device, in particular when it has the form of an electrode line, preferably has a converter which is distally located in the immediate proximity of the sensing electrode poles, and which converts the signal energy detected at the sensing electrode poles for more efficient (undisturbed) transmission to the evaluation unit. 
         [0028]    The converter may be a transformer which transforms the signal representing the detected potentials to a higher value in order to conduct the signal via high-impedance lines, and therefore with immunity to RF interference, to the evaluation unit. 
         [0029]    Alternatively, the converter may be a piezoelectric converter or in general an electroacoustic converter which converts detected potentials to mechanical signals, so that they may be conducted acoustically, i.e., mechanically and therefore with immunity to RF interference, to the evaluation unit. 
         [0030]    According to another alternative, the converter is a rectifier or demodulator which converts the detected potentials to a useful signal in the form of a direct current signal. Any interferences (RF) may then be easily filtered out upstream from the evaluation unit. 
         [0031]    Another alternative provides a converter in the form of an electrooptical converter, for example a light-emitting diode (LED), which converts detected potentials to a useful signal in the form of a light signal which may then be conducted without interference to the evaluation device. When the useful signal is a mechanical, in particular acoustic, or an optical useful signal, a second converter may be provided in the vicinity of the evaluation unit, for example at the proximal end of an electrode line, which converts the particular useful signal back to an electrical signal. The sensitivity to interference is correspondingly reduced due to the fact that this advantageously occurs just upstream from the evaluation unit. 
         [0032]    In addition to the embodiments described herein other alternative embodiments may include some or all of the disclosed features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    At least one embodiment of the invention is explained in greater detail with reference to the figures, which show the following: 
           [0034]      FIG. 1  shows as an implantable medical device an implantable cardiac stimulator  10  and an implantable electrode line  20  connected thereto. 
           [0035]      FIG. 2  shows a typical temperature curve at the electrode tip. 
           [0036]      FIG. 3  shows an example of how a field-generating electrode pole (tip electrode) of a conductor having intense interference injection cooperates with sensing electrode poles in the form of annular electrodes. E*dl results in a voltage which is a measure of the field strength E, and thus of the heating of the tip. 
           [0037]      FIGS. 4A and 4B  show fields at the electrode tip; the field-generating electrode pole (in this case, the tip electrode) is at the same time one of the sensing poles in this instance. 
           [0038]      FIGS. 5A and 5B  show two variants of an implantable pulse generator, each having a device for evaluating an electrical signal of the electrode, which is correlated with the heating of an electrode pole. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    The implantable cardiac stimulator  10  may be a cardiac pacemaker or a cardioverter/defibrillator (ICD). In the illustrated exemplary embodiment, the cardiac stimulator  10  is a ventricular cardiac pacemaker and defibrillator. Other known cardiac stimulators are dual-chamber cardiac pacemakers for stimulating the right atrium and the right ventricle, or biventricular cardiac pacemakers, which in addition to the right ventricle are also able to stimulate the left ventricle. 
         [0040]    Such stimulators typically have a housing  12 , which is generally made of metal and is therefore electrically conductive, and which may be used as a large-surface electrode pole. A connector housing  14 , also referred to as a header, is typically affixed to the exterior of the housing  12 . Such a header typically has contact sockets for accommodating plug contacts. The contact sockets have electrical contacts  16  which are connected via appropriate conductors to an electronics system situated in the housing  12  of the cardiac stimulator  10 . 
         [0041]    The electrode line  20  likewise represents an implantable medical device within the meaning of at least one embodiment of the invention. Electrode poles in the form of a point electrode or tip electrode  22  and an annular electrode  24  present in the vicinity thereof are situated in a manner known per se at the distal end of the electrode line  20 . The electrode poles  22  and  24  are designed in such a way that, depending on the function of a cardiac stimulator to which the electrode line  20  is connected, they are provided to sense electrical potentials of the cardiac tissue (myocardium) or to supply electrical signals, for example for delivering stimulation pulses to the cardiac tissue which surrounds them.  FIG. 1  shows the manner in which the electrode poles, i.e., the tip electrode  22  and the annular electrode  24 , and for the present application, the electrode line  20 , are located in the apex of a right ventricle of a heart. 
         [0042]    The tip electrode  22  and the annular electrode  24  are in each case electrically connected via at least one electrical conductor  26  to a plug contact  28  at the proximal end of the electrode line  20 . The plug contact  28  has electrical contacts which correspond to the electrical contacts  16  of the contact socket in the connector housing  14  of the implantable cardiac stimulator. The electrical conductors  26  in the electrode line  20  may be designed as somewhat elongated cable conductors or as helically coiled conductors. Such conductors, which connect functional electrode poles to electrical contacts of the plug contact at the proximal end of the electrode line  20  in an electrically conductive manner, are referred to as function conductors within the scope of this description, since, for example, they transmit electrical signals used for the treatment from the plug contact to the particular electrode pole, or conduct sensed signals which represent electrical potentials from the particular electrode pole to the plug contact, and are thus used for the fundamental function of the medical device. 
         [0043]    The electrical conductors  26  which connect the electrode poles  22  and  24  to the electrical contacts of the plug  28  of the electrode line  20  are enclosed over most of their length by an insulating sheath, resulting in targeted electrical contact with the tissue of the heart via the electrode poles. 
         [0044]    In addition to the electrode poles  22  and  24 , which are typically used for stimulation (in this case, ventricular) of the cardiac tissue, the electrode line  20  has two large-surface electrode poles  30  and  32 , which are used as defibrillation electrodes and are formed by at least one bare helically wound wire. 
         [0045]    It is pointed out that within the scope of this exemplary embodiment the invention is explained with reference to a right ventricular cardiac pacemaker and defibrillator. As a medical device within the meaning of at least one embodiment of the invention, however, in principle an ablation electrode line may also be used, which in the application likewise extends into the heart of a patient and is controlled by a device located outside the patient, and for this purpose is connected to the device. 
         [0046]      FIG. 2  illustrates a typical temperature curve  100  of a conventional pacemaker/ICD electrode in magnetic resonance imaging (MRI). The temperature increases rapidly when the high-frequency alternating field in the MRI unit is switched on at time  110 , the steepness of the increase and the maximum achievable temperature being greatly dependent on the electrode position relative to the high-frequency alternating fields of the MRI. When the high-frequency alternating field is switched off at time  120 , the electrode tip quickly cools due to its relatively low heat capacity. 
         [0047]      FIG. 3  schematically shows an electrode pole in the form of a tip electrode  210 , and two electrode poles in the form of annular electrodes  220  and  230 , each of which is connected to a function conductor (supply line) ZL 1 , ZL 2 , and ZL 3 , respectively. The schematically illustrated electrode poles are typically located at the distal end of an electrode line, which also has further components, for example an insulating sheath, which are not shown in the schematic illustration. It is known that the function conductors ZL 1 , ZL 2  or ZL 3  may assume various shapes, and may be designed as cable feeds or also as helically shaped feeds.  FIG. 3  shows the manner in which a field-generating electrode pole, in this case the tip electrode  210 , cooperates with sensing electrode poles in the form of annular electrodes  220  and  230  when the function conductors ZL 1 , ZL 2 , ZL 3  are exposed to intense interference injection, for example as the result of externally acting high-frequency alternating fields in a magnetic resonance imaging (MRI) unit. The electrical field around the tip electrode  210  is indicated by arrows and the letter E. The product of E and the distance dl (E*dl) results in the voltage which is present between the annular electrodes  220  and  230  due to the field, having field strength E, emanating from the tip electrode  210 . Thus, a measure of the field strength of the electrical field E around the tip electrode  210  may be derived from the difference in potentials between the annular electrodes  220  and  230 . 
         [0048]    The tip electrodes and annular electrodes  310  and  320 , respectively, and their respective supply lines (function conductors) ZL 1  and ZL 2  are schematically illustrated in  FIGS. 4A and 4B , similarly as in  FIG. 3 . 
         [0049]      FIGS. 4A and 4B  show that the current intensity emanating at the electrode pole is of different magnitudes, depending on the strength of the acting electromagnetic fields. This is correlated with a change in the strength of the electrical field E as well as the heating which is produced. For coaxial electrode lines (the predominant design in current clinical practice), for example the degree of heating (and therefore the electrical field) of the annular electrode(s) is usually much lower than that of the tip electrode. Therefore, it may be ignored as an approximation. However, the annular electrode(s)  320  is/are located in the field of the tip electrode  310 , and are therefore at a different potential as a function of the field strength. The corresponding voltage is calculated from the integral ∫E(l)dl. This signal reaches the IPG via function line ZL 2  (the supply line for the annular electrode  320 ), where it may be evaluated. The fact that the annular electrode(s) is/are at a given potential in the field of the tip electrode may be determined in relation to other reference potentials (thus, for example, the tip itself, another ring, or the IPG housing). 
         [0050]      FIGS. 5A and 5B  each show a block diagram of the cardiac stimulator  10 , which in the present case is referred to as an implantable pulse generator (IPG). The IPG has a sensing device for evaluating an electrical signal of the electrode which is correlated with the heating of an electrode pole. 
         [0051]    The IPG housing  400  of the cardiac stimulator  10  is schematically indicated in  FIGS. 5A and 5B . The components which are relevant to the present exemplary embodiment are illustrated as blocks in the IPG housing  400 . Therefore, the possibility of further components of a typical cardiac stimulator being present in addition to the illustrated components is not excluded. An electrode line  405  is connected to the IPG housing  400 . The electrode line  405  is likewise schematically illustrated in the sense of the diagram in  FIG. 3 , and in the illustrated exemplary embodiment has an electrode pole in the form of a tip electrode, and two electrode poles in the form of annular electrodes, each of which is connected via its own supply line (function conductor)  421 ,  422 ,  423 , respectively, to an optional filter  420 . A mixer  430  is connected downstream from the optional filter  420 , and downstream from the mixer  430  is a threshold value detector  440 . The output signal of the threshold value detector  440  is delivered to an evaluation unit  450 , which in turn acts on the remaining electronics system of the cardiac pacemaker, referred to in general as the IPG electronics system  410 . The arrow  411  indicates that the IPG electronics system  410  is externally programmable. 
         [0052]    The function conductor (supply lines)  421 - 423  of the electrode line  405  extend to the interior of the IPG, and arrive at that location. The optional filter  420  results in only signals in specific frequency ranges, for example greater than 1 MHz, or bands having bandwidths of less than 10 MHz around typical magnet resonance HF frequencies such as 42, 64, 128 MHz, for example, being further processed. 
         [0053]    The IPG housing  400  may likewise be used as an electrode pole, for example as a reference electrode, and for this purpose is connected via a line  424  to the filter  420 . 
         [0054]    These signals are added on a weighted basis in the mixer  430 , and this result is added to the threshold value detector  440 . When the threshold conditions are met, the threshold value detector  440  generates a corresponding detector output signal. This detector output signal is further processed in the evaluation unit  450 , and a control signal is sent to the IPG electronics system  410 . The IPG electronics system  410  determines the particular function conductors which are to be connected to the filter as input lines. This may once again be externally programmed (see arrow  411 ). 
         [0055]    In the exemplary embodiments illustrated in  FIGS. 5A and 5B , the threshold value detector  440  and the evaluation unit  450  together form an evaluation device  460  which responds to an exceedance of the threshold value. 
         [0056]    The evaluation device  460  may also be designed in such a way that it determines and evaluates the cumulative effect of the signal by integration, averaging, or determining the effective value of the signal, and outputs an output signal as a function of the magnitude of the cumulative effect. 
         [0057]    The threshold value detector  440  may be designed in such a way that it responds to the maximum value of its input signal (the output signal of the mixer  430 ), and in this sense functions as a maximum value detector. 
         [0058]    The IPG electronics system  410 , as part of the control device of the cardiac stimulator  10 , may be designed in such a way that it always switches the cardiac stimulator  10  to a safe operating mode in the event of an intense electromagnetic field effect, as a function of the output signal of the evaluation unit  450  or of the evaluation device  460 , and does so, for example, for a specified, preprogrammed period of time. 
         [0059]    The evaluation device  460  may also be connected to a telemetry unit  470  to allow the output signal of the evaluation device to be transmitted to an external device, and thus to provide information for a medical practitioner via a corresponding central service center. 
         [0060]    A converter  480  may be mounted at the distal end of the electrode line  405  in the immediate proximity of the particular sensing electrode pole, and the converter converts the signals (differences in potential, for example) detected by the sensing electrode poles in such a way that the signals may be transmitted to the evaluation unit without interference. Such a converter  480  is shown as an optional component in  FIGS. 5A and 5B , and is connected to the evaluation unit  450  via a signal line SL. 
         [0061]    The converter  480  may be a transformer, for example, which transforms the signal detected by the sensing electrode poles to a higher value in order to conduct the signal to the evaluation unit  450  via high-impedance lines (signal line SL) with immunity to interference from high-frequency alternating fields. 
         [0062]    Alternatively, the converter  480  may be a piezoelectric converter which converts the signals detected by the sensing electrode poles into mechanical, specifically acoustic, output signals, for example, which may be conducted to the evaluation unit  450  via a suitable signal line. The evaluation unit  450  may also have an acoustoelectric converter which converts acoustic signals back to electrical signals. 
         [0063]    According to another alternative, the converter  480  may be a rectifier or demodulator whose output signal is a direct current signal, so that any high-frequency interferences of this direct current signal may be easily filtered out by the evaluation unit  450 . 
         [0064]    According to another alternative, the converter  480  may be an electrooptical converter, for example a light-emitting diode (LED), which converts the signal detected by the sensing electrode poles to a light signal, which may be conducted without interference as a signal line SL to the evaluation unit  450  via an optical fiber. In this case the evaluation unit  450  has a complementary converter, for example a photodiode, which converts the light signals back to electrical signals. 
         [0065]    The converter for converting the acoustic signal or the light signal to an electrical signal may also be provided at the proximal end of the electrode line  405 . This is indicated in  FIG. 5B  by the converter  490 , which is connected to the evaluation unit  450  via an electrical signal line SL&#39;. 
         [0066]    It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching. The disclosed examples and embodiments are presented for purposes of illustration only. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention.