Abstract:
The present invention reduces patient risks associated with RF-induced thermogenic tissue damage and with pulsed gradient-field-induced arrhythmias by using a defibrillator lead having a self-healing dielectric material that prevents induced voltages from MRI equipment from damaging an ICD or causing unintended defibrillation shocks to a patient. Another aspect of the present invention utilizes a sliding contact arrangement to prevent induced voltages from MRI equipment from being electrically coupled to an ICD thereby reducing patient risks associated with RF-induced thermogenic tissue damage and with pulsed gradient-field-induced arrhythmias.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/946,411 filed Jun. 27, 2007. 
     
    
     FIELD OF THE PRESENT INVENTION 
       [0002]    The present invention is directed to a defibrillator electrode. More particularly, the present invention is directed to a defibrillator electrode that can be used in a magnetic resonance imaging environment. 
       BACKGROUND OF THE PRESENT INVENTION 
       [0003]    Various approaches have been taken to reduce or eliminate the risks associated with patients having implanted medical devices who need magnetic resonance imaging (MRI) examinations. 
         [0004]    However, the specific characteristics and requirements of defibrillator systems create unique challenges. Unlike pacemaker, drug pump, and neurostimulation devices, implantable cardioverter defibrillators (ICDs) not only sense and pace the heart in a manner similar to a pacemaker, but also may release electrical energy in pulses of up to 40 Joules and at an excess of 800 volts and 10 amps if ventricular fibrillation (VF) or other anomalous conditions are sensed. 
         [0005]    While this may occur very rarely, prior art solutions to thermogenic tissue risks associated with the radio frequency (RF) fields used in magnetic resonance imaging in some cases utilize small electrical components that can be damaged in the presence of electrical potentials and currents of this magnitude. Specifically, the miniature inductive, capacitive, and semiconductor components that may be packaged in the electrode assembly of a pacemaker lead are typically rated for potential and current levels far below those used in defibrillation. 
         [0006]    Thus, it is desirable to provide a defibrillation electrode that enables highly reliable operations over the life of the ICD implant in a patient, but such that when the patient is placed in the bore of a magnetic resonance imaging system, all sources of RF-induced energy and gradient-field-induced energy that could harm the patient are totally isolated electrically, thus providing complete safety for the patient. 
       SUMMARY OF THE PRESENT INVENTION 
       [0007]    One aspect of the present invention utilizes a self-healing dielectric material that prevents induced voltages from MRI equipment from damaging an ICD or causing unintended defibrillation shocks to a patient thereby reducing patient risks associated with RF-induced thermogenic tissue damage and with pulsed gradient-field-induced arrhythmias. 
         [0008]    Another aspect of the present invention utilizes a sliding contact arrangement to prevent induced voltages from MRI equipment from being electrically coupled to an ICD thereby reducing patient risks associated with RF-induced thermogenic tissue damage and with pulsed gradient-field-induced arrhythmias. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  illustrates a patient having an implantable cardioverter defibrillator utilizing a defibrillator lead in accordance with the present invention; 
           [0010]      FIG. 2  illustrates a preferred embodiment for the defibrillator lead of  FIG. 1 ; and 
           [0011]      FIG. 3  illustrates another preferred embodiment for the defibrillator lead of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]      FIG. 1  illustrates a patient  1  having a cardioverter defibrillator  2  implanted in the right shoulder area  3 . A lead  4  is shown extending into the right atria of the patients heart  6 , while another lead  5  is shown extending into the right ventricle of the heart  6 . The ventricular lead  5  comprises a pacing/defibrillation lead capable of pacing the heart  6  in the event an intrinsic heart beat is not detected. The lead  5  can also deliver a defibrillation pulse (shock) to the patient in the event that the defibrillator  2  determines that a life threatening arrhythmia is detected. 
         [0013]      FIG. 2  shows a preferred embodiment of the tip of the defibrillator lead  15  that uses a self-healing dielectric material, such as disclosed in published US Patent Application, publication number 2006-0271138A1, entitled Electromagnetic Interference Immune Pacing/Defibrillation Lead. Published US Patent Application, publication number 2006-0271138A1, describes various means to employ self-healing dielectric materials in the manufacture of the long conductive structure connecting the implantable cardioverter defibrillator (ICD)  2  to the electrode that rests within the patient&#39;s heart. The entire content of published US Patent Application, publication number 2006-0271138A1, is hereby incorporated by reference. 
         [0014]    Referring to  FIG. 2 , electrode assembly  10  has an electrode sheath  12  in contact with cardiac tissue, or in contact with circulating blood in the case of the defibrillation lead in an ICD. Electrode core  14  is centered within electrode sheath  12 , the cavity between them being filled with self-healing dielectric material  16  such as described in published US Patent Application, publication number 2006-0271138A1. 
         [0015]    As illustrated in  FIG. 2 , the self-healing dielectric material is located in the electrode assembly itself. By having the self-healing dielectric material located in the electrode assembly, the self-healing dielectric material is not located in the remainder of the lead, thereby simplifying manufacturing and increasing reliability in an implant environment. 
         [0016]    Under normal or “standby” conditions, the dielectric material  16 , having thickness  30  between the electrode sheath  12  and electrode core  14 , resists the flow of electrical current, so unwanted energy associated with RF and gradient field sources is not conducted to the electrode sheath and thus not to the patient&#39;s body. Upon application of a potential at a level associated with a defibrillation pulse, the dielectric material  16  breaks down and conducts, permitting delivery of the defibrillation pulse from the ICD to the patient. 
         [0017]    The threshold for dielectric breakdown is an inherent feature that depends on the material itself, and the thickness  30 ; this can be chosen to be far above the levels associated with RF-induced and gradient-field-induced energy, but far below the level employed during defibrillation. Thus, the threshold should be designed to be significantly above 10 volts and significantly below 800 volts. A nominal choice may be made at 100 volts, but may be anywhere between 20 and 600 volts. 
         [0018]    Electrode core  14  is held by insulator  18 , and is hermetically sealed to it at juncture  26 . Filar conductor  22  is also physically captured by insulator  18 , providing strain relief and for a reliable solder or weld connection  24  that does not undergo mechanical fatigue from repeated bending. Insulator  18  is also hermetically sealed on its periphery  28  to electrode sheath  12 , so that the dielectric material  16  is never exposed to water or ions that would otherwise migrate from blood or body tissues into it and degrade its electrical properties. 
         [0019]    The filar lead  22  is encased in jacket material  20 , and care is taken to maintain jacket thickness  34  so that the jacket material  20  will not break down and conduct under the high voltage defibrillation pulse. 
         [0020]    In order to avoid “corona-like hot spots” that may be the focus of repeated dielectric breakdown, care is taken to design electrode core  14  with a rounded distal end, and to provide adequate axial spacing  32  so that breakdown at the distal end can be avoided and breakdown can rather occur at various locations in the cylindrical volume between the electrode core  14  and the electrode sheath  12 . 
         [0021]    Various additional features such as the grooves shown on the exterior of insulator  18  to enhance bonding with jacket material  20 , and other features relating to the range of defibrillator lead designs currently used to optimize electrophysiological performance (e.g. coil shapes rather than ‘bullet’ shapes) may be used in concert with the invention described herein. 
         [0022]      FIG. 3  illustrates an approach to magnetic resonance imaging safety for an ICD lead that takes advantage of the intense static magnetic field used in magnetic resonance imaging in order to open an otherwise closed contact, and completely isolate the electrode exterior from any RF-induced or gradient-field induced energy during the magnetic resonance imaging examination. 
         [0023]    Electrode assembly  40  includes an electrode tip  42  that delivers the defibrillation pulse to the patient when appropriate. It should be noted, as stated above, that various alternative approaches to optimizing electrical contact with the patient&#39;s body tissues (e.g. coils vs. ‘bullet’ shapes) may be readily adapted to the invention disclosed herein. 
         [0024]    Electrode tip  42  is connected to insulating sheath  44  by way of a hermetic seal at juncture  46 . In like manner, base contact  48  is retained and hermetically sealed to insulating sheath  44  at juncture  50 , providing an interior volume that is completely isolated from water or ions in the patient&#39;s blood and body tissues. While electrode tip  42  is manufactured from an electrically conductive, hermetically sealable, ferromagnetic, biocompatible material (e.g. cobalt chromium), base contact  48  is manufactured from an electrically conductive, non-magnetic, hermetically sealable, biocompatible material (e.g. titanium or nitinol). 
         [0025]    As in the previous design, filar  52  is encased in a jacket  54  having insulating properties and thickness  70  sufficient to withstand the defibrillation pulse without breakdown, and the filar  52  is soldered or welded to base contact  48  at one or more locations  68 , thus providing structural support and eliminating fatigue due to repeated bending. 
         [0026]    Sliding contact  56  is manufactured from the same or similar electrically conductive, hermetically sealable, ferromagnetic, biocompatible material as is electrode tip  42 , and is designed to slide axially within insulating sheath  44  upon application of an axial force. Sliding contact  56  is connected to electrode tip  42  by spring  58 . Solder or weld joints at locations  60  and  62  create a path for electrical current flow; spring  58  and solder/weld joints are made from materials and designed to pass pulses of current having magnitudes well beyond those typically used in an ICD (e.g. spring and joints designed with a 3× safety factor over the typical 10 amp pulse will withstand 30 amp pulses). 
         [0027]    The spring  58  is designed to provide a modest force, capable of maintain physical contact between base contact  48  and sliding contact  64  under any foreseeable acceleration associated with operation as an implant. Thus, outside of an applied magnetic field, contact gap  64  will essentially be closed, or zero in size. It should be noted that some high-energy switches are “wetted” with metallic liquid mercury, or other contact-enhancing material; a similar approach may be taken with this design but is not required for reliable operation. 
         [0028]    When a small magnetic field is applied to the electrode assembly  60 , but a field that is less than that found in a magnetic resonance imaging system, a very small attractive force is developed between electrode tip  42  and sliding contact  56  due to the fact that both are ferromagnetic. 
         [0029]    Spring  58  is designed to provide sufficient compressive force so that gap  64  is always closed, under both acceleration loads and in the presence of terrestrial magnetic fields that are orders of magnitude smaller than those used in magnetic resonance imaging. The compressive force designed into spring  58 , in concert with the materials and masses of tip electrode  42  and sliding contact  56 , are chosen so that when the electrode assembly  40  is place within a static magnetic field exceeding 0.05 Tesla, the attractive force created between tip electrode  42  and sliding contact  56  overcomes the compressive force provided by spring  58 , resulting in contact gap  64  opening to a spacing equivalent to rest gap  66 . As long as the static magnetic field is applied, contact gap  64  will remain open, and the electrode tip  42  will remain electrically isolated from RF-induced and gradient-field-induced sources of energy. 
         [0030]    In a first embodiment, rest gap  66  may be designed so that when it is closed by the static magnetic field and contact gap  64  opens, the spacing is sufficient to withstand a 1000 volt applied potential without arcing. In this first embodiment, the ICD will be unable to deliver a defibrillation pulse even if the need is detected; this approach may be appropriate if there is a significant probability that the logic circuits within the ICD pulse generator can be disrupted by the magnetic resonance imaging system and deliver one or more defibrillation pulses that are actually not appropriate, and that the risk to the patient resulting from this is higher than the risk associated with not being defibrillated in the event it is actually needed during the magnetic resonance imaging procedure. 
         [0031]    In a second embodiment, rest gap  66  may be designed to have a dimension such that when closed by the magnetic resonance imaging static magnetic field contact gap  64  is opened to a dimension that is large enough so that electrical potentials associated with RF-induced and gradient-field-induced energies will not conduct or arc between sliding contact  56  and base contact  48 , but this dimension is small enough that when the electrical potential between sliding contact  56  and base contact  48  is at a level typically used in a defibrillation pulse, and an electrical arc conducts this electrical pulse to the tip electrode  42 , permitting defibrillation of the patient&#39;s heart. It can be seen that in this second embodiment, the ICD may retain its full functionality while the patient is protected from risks associated with RF-induced thermogenic tissue damage and gradient-field-induced arrhythmia. 
         [0032]    In either the first or second embodiments described above, the volume interior to the hermetic seals may under vacuum, or filled to an appropriate pressure with dry nitrogen, or one or more gases chosen for their electrical properties. 
         [0033]    As set forth above, two fundamental approaches provide patient safety for an individual having an implanted defibrillator lead, in the presence of magnetic resonance imaging-related electromagnetic fields. 
         [0034]    While limited examples and embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that the spirit and scope of the present invention are not limited to the specific description and drawings herein, but extend to various modifications and changes.