Abstract:
A cardiac electrode termination pair system that is particularly compatible with Magnetic Resonance Imaging (MRI) procedures. The electrodes include tip and ring electrodes made of a body-compatible ceramic on which is applied via electroplating, sputtered or the like an ultra-thin conductive coating of platinum or titanium or other suitable metal which is compatible with conducting electrical impulses into cardiac tissue. The ring electrode may be adapted for connection to a photonic catheter carrying light signals or a non-photonic catheter carrying electrical signals. The ring (or the tip) may house pulse-delivering components and/or sensing components.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to pacemakers. More particularly, the invention concerns cardiac electrodes for use with pacemakers, and especially pacemakers designed to be compatible with MRI diagnostic apparatus.  
           [0003]    2. Description of Prior Art  
           [0004]    The metallic cardiac electrodes and leads used in conventional cardiac stimulation and monitoring devices have always been a problem. They tend to fatigue, corrode, and break. Their physical properties (corrosion, strength, chemical activity, etc.) limit the materials which can be used to titanium, platinum metals, their alloys, to certain stainless steels, and to special structures to limit fatigue (such as spring coils, metal cladding, multiple strands, etc.) With respect to metallic leads, a leaky interface is often produced between the metal and the insulating sheath that surrounds the leads.  
           [0005]    The problem of metallic leads has been addressed by applicant&#39;s assignee in an effort to provide a pacemaker that is compatible with MRI diagnostic imaging procedures. See copending Ser. Nos. 09/864,944 and 09,865,049, both filed on May 24, 2001, and copending Ser. Nos. 09/885,867 and 09/885,868, both filed on Jun. 20, 2001. In these copending patent applications, the contents of which are fully incorporated herein by this reference, MRI compatible/safe pacemakers are disclosed for both implantable and wearable use. The disclosed pacemakers feature photonic catheters carrying optical signals in lieu of metallic leads carrying electrical signals in order to avoid the dangers associated with MRI-generated electromagnetic fields.  
           [0006]    The devices of the copending applications also use only non-ferromagnetic materials and attempt to minimize the number of metal components of any kind. In accordance with these goals, the copending applications propose electrodes that are made from non-ferromagnetic metals such as titanium, platinum, and platinum-containing alloys. In addition, the copending applications advise that non-metals may also be used to provide the electrodes. It is the purpose and goal of the present invention to address such non-metallic electrodes and to propose specific nonmetallic electrode constructions that could be used to advantage in an MRI compatible/safe pacemaker, as well as in pacemakers and other electrical stimulation devices that are not necessarily designed for MRI compatibility and safety.  
         SUMMARY OF THE INVENTION  
         [0007]    The foregoing problems are solved and an advance in the art is provided by an electrode termination pair of novel construction for the distal end of a pacemaker catheter, and particularly a photonic pacemaker catheter. The electrode termination pair includes tip and ring structures that are made from a body-compatible ceramic material that is thinly coated, by electroplating, sputtering or other deposition technique, etc., with a suitable electrode metal such as platinum, titanium, or alloys thereof. The tip and ring structures may be formed on separate ceramic base structures or they may be integrated on a single ceramic base structure. If separate structures are used, the tip and ring can be separated by a short insulating stub structure having the same external diameter as the tip and ring. The stub structure can be made of silicone rubber, polyethylene, urethane, or some other material having suitable insulating properties and which is compatible with the human body. The ring structure is electrically connected to the positive electrical pulse output of the pacemaker, and the tip structure is electrically connected to the negative pulse output of the pacemaker. If the electrode termination pair is incorporated in a photonic pacemaker, the ring or the tip can be adapted to house an optical termination and an opto-electric transducer for stimulation pulse delivery to implanted cardiac tissue. The ring or the tip may also house an R-wave amplifier and an electro-optical transducer for pulse monitoring of implanted cardiac tissue. Additional functionality, such as a partial oxygen monitor, may also be provided.  
           [0008]    Accordingly, it is a principal object of the invention is to provide “tip and ring” electrode system with no magnetic materials and very little metallic content of any kind.  
           [0009]    A further object of the invention is to provide an electrode system for delivering stimulation impulses into cardiac tissue without adverse effect from MRI induced electromagnetic fields, and in a way that will not will not appreciably affect the accuracy of an MRI diagnostic record.  
           [0010]    A further object of the invention is to provide an enclosure for an opto-electrical transducer that converts light pulses from a photonic catheter into stimulating electrical impulses to drive the heart.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying Drawing in which:  
         [0012]    [0012]FIG. 1 is an exploded perspective view of an electrode termination pair constructed in accordance with a preferred embodiment of the present invention;  
         [0013]    [0013]FIGS. 2A and 2B are sectional views taken along the axial centerline of alternative electrode termination pairs constructed in accordance with the embodiment of FIG. 1.  
         [0014]    [0014]FIGS. 3A and 3B are exploded partial sectional views showing alternative electrode termination pairs constructed in accordance with the embodiment of FIG. 1 and mounted to the end of a photonic catheter and housing an opto-electrical transducer therein;  
         [0015]    [0015]FIGS. 4A and 4B are exploded partial sectional views showing alternative electrode termination pairs constructed in accordance with the embodiment of FIG. 1 and mounted to the end of a metallic lead catheter;  
         [0016]    [0016]FIG. 5 is a diagrammatic view of an implantable pacemaker comprising an electrode termination pair in accordance with the invention;  
         [0017]    [0017]FIG. 6 is a diagrammatic view of a wearable pacemaker comprising an electrode termination pair in accordance with the invention;  
         [0018]    [0018]FIG. 7 is a perspective view of an electrode termination pair constructed in accordance with a preferred embodiment of the present invention; and  
         [0019]    [0019]FIGS. 8A and 8B are sectional views taken along the axial centerline of alternative electrode termination pairs constructed in accordance with the embodiment of FIG. 9. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    Turning now to FIG. 1, a preferred embodiment of the invention is shown in the form of a electrode termination pair  2 . The electrode termination pair  2  includes a ceramic cup (tip)  4  and a ceramic ring  6 . The tip  4  and the ring  6  are both substantially cylindrical in shape, and preferably have the same wall thickness. Note that the tip  4  has a rounded nose portion and a base portion that is planar. The ring  6  has proximal and distal end portions that are both preferably planar.  
         [0021]    As shown in FIGS. 2A and 2B, the tip  4  includes a ceramic base structure  5  and an electrically conductive coating  8 . The ring  6  includes a ceramic base structure  7  and an electrically conductive coating  10 . The difference between FIGS. 2A and 2B is that all exposed surfaces of the ceramic base structures  5  and  7  are coated in FIG. 2A, whereas only the outer surface of the ceramic base structures  5  and  7  are coated in FIG. 2B.  
         [0022]    The material used to form the ceramic base structures  5  and  7  is preferably a suitable bio-compatible ceramic material such a ceramic of the type commonly used for joint prostheses. By way of example only, such material is available from Ceramic Components Inc. of Latrobe, Pa. To form the ceramic base structures  5  and  7 , a ceramic slurry is formed into the desired shapes and fired to bake the ceramic material.  
         [0023]    The electrically conductive coatings  8  and  10  are preferably formed by very thinly coating the ceramic base structures  5  and  7 , as by electroplating, sputtering or other deposition technique, etc., with a suitable metal. If MRI compatibility is desired, the metal preferably has low magnetic susceptibility, such as titanium, platinum, or alloys thereof. Preferably, if MRI compatibility is desired, the coatings  8  and  10  are applied as thin as possible to achieve the twin goals of efficient electrical interaction with an implanted heart while minimizing interaction with MRI induced electromagnetic fields. By way of example, the thickness of the coatings  8  and  10  may range from mono-molecular thickness to sub-micron or micron level thickness. An adhesion promoting sublayer (not shown) may be applied to the ceramic base structures  5  and  7  prior to application of the foregoing coatings.  
         [0024]    Turning now to FIGS. 3A and 3B, the electrode termination pair  2  of FIG. 1 may be configured in a tip/ring assembly  12  and mounted to the distal end of a photonic catheter  14  of the type disclosed in the copending applications referenced above. In FIG. 3A, the tip and ring structures  4  and  6  are coated on all surfaces with an electrically conductive coating. In FIG. 3B, only the outside surfaces of the tip  4  and ring  6  are coated.  
         [0025]    The tip and ring assembly  12  includes the tip  4 , the ring  6 , and a short intermediate stub  16  that is solid, generally cylindrical in shape, and made from silicone, polyurethane, polyethylene or other suitable bio-compatible electrically insulating material. The outside diameter of the stub  16  preferably equals the outside diameter of the tip  4  and the ring  6  in order to facilitate efficient implantation and removal in a patient. As described in more detail below, the interior of the stub  16  carries a metallic lead element that extends between the tip  4  and the ring  6 . The stub  16  includes a pair of end portions  18  that are preferably of reduced diameter so as to fit snugly inside the tip  4  and the ring  6  and thereby enhance connection integrity. The stub  16  can be implemented as a preformed element whose reduced diameter end portions  18  are respectively attached to the inside walls of the tip  4  and the ring  6  by way of bonding using a suitable medical adhesive. More preferably, however, the stub  16  is cast in place between the tip  4  and the ring  6  using a suitable bio-compatible material, such as silicone, polyurethane, polyethylene or the like. The reduced diameter end portions  18  would then be formed as a result of the stub material being forced into the respective interiors of the tip  4  and the ring  6  prior to hardening into a solid mass.  
         [0026]    The photonic catheter  14  is a generally cylindrical element whose exterior sheath  20  is made from silicone, polyurethane, polyethylene or other suitable bio-compatible electrically insulating material. The outside diameter of the sheath  20  is preferably the same as that of the ring  6  in order to facilitate efficient implantation and removal in a patient. As described in more detail below, the interior of the sheath  20  carries one or more optical conductors (e.g., fiber optic elements) that extend to the ring  6  from a photonic pacemaker pulsing unit or other medical device (not shown). The sheath  20  includes a distal end portion  22  that is preferably of reduced diameter so as to fit snugly inside the ring  6  and thereby enhance connection integrity. Like the stub  16 , the sheath  20  can be implemented as either a preformed element or can be cast in place. If the sheath  20  is a preformed element, its reduced diameter end portion  22  can be secured to the inside wall of the ring  6  by way of bonding using a suitable medical adhesive. If the sheath  20  is cast in place, the reduced diameter end portion  22  would be formed as a result of the sheath material being forced into the interior of the ring  6  prior to hardening into a solid mass  
         [0027]    An optical termination unit  24  is disposed within the ring  6  and is associated with an opto-electrical transducer  26  (described in more detail below). A positive electrical output of the opto-electrical transducer  26  connects to a short metallic lead  28  made from copper or other suitable electrically conductive material of low magnetic susceptance. The metallic lead  28  is electrically connected, as by soldering or the like, to the metallic coating  10  of the ring  6 . Note that in FIG. 3B, a small hole  30  can be made in the ring  6  to facilitate electrical connection of the lead  28  to the metallic coating  10 . Other connection schemes could also be employed. A negative electrical output of the opto-electrical transducer  26  connects to a longer metallic lead  32  that is also made from copper or other suitable electrically conductive material of low magnetic susceptance. The metallic lead  32  extends through the stub  16  (which is preferably molded around the lead  32  as indicated above) and is electrically connected, as by soldering or the like, to the metallic coating  8  of the tip  4 . Note that in FIG. 3B a small hole  34  is made in the tip  4  to facilitate electrical connection of the lead  32  to the metallic coating  8 . Again, alternative connection schemes could also be used.  
         [0028]    The tip/ring assembly  12  must be small enough to be implantable in a human heart. A diameter of about 5 millimeters or less and an overall length of about 4 centimeters or less should suffice. When the tip/ring assembly  12  is so implanted, the tip  4  will typically be embedded in the endocardial tissue of the heart, while the ring  6  will be situated in one of the chambers of the heart, such as the right ventricle, such that the ring  6  is placed in electrical contact with the endocardium via the patient&#39;s ventricular (or atrial) blood. During pacemaker operation, an optical pulse emanating from a photonic pacemaker pulsing unit or other medical device (not shown) is sent down at least one fiber optic element  36  of the photonic catheter  14 . The fiber optic element  36  passes into the interior of the ring  6  and is terminated at the optical termination unit  24 . The fiber optic element  36  delivers the optical pulse to the opto-electrical transducer  26 , which is preferably implemented as a photodiode array situated on or within the optical termination unit  24 . The opto-electrical transducer  26  produces an electrical pulse that negatively drives the tip  4  with respect to the ring  6  at a potential of about 3-4 volts and a current level of about 3 milliamperes for a total power output of about 10 milliwatts. The metallic coatings  8  and  10  respectively formed on the tip  4  and the ring  6  will provide highly efficient electrical contact with the heart for delivering the pulse while minimizing the use of metallic material that might otherwise result in complications during MRI imaging. Note that a sensing function for monitoring the heart&#39;s “R” wave signals could be added by introducing an R-wave amplifier and an electro-optical transducer (not shown) into the interior of the ring  6  and making appropriate electrical connections to the tip  4  and the ring  6 .  
         [0029]    Turning now to FIGS. 4A and 4B, the electrode termination pair  2  of FIG. 1 is configured in a tip/ring assembly  40  that is mounted to the distal end of a conventional pacemaker catheter  42 . In FIG. 4A, the ceramic base structures  5  and  7  of the tip  4  and the ring  6  are coated on all surfaces with respective electrically conductive coatings  8  and  10 . In FIG. 4B, only the outside surfaces of the tip and ring base structures  5  and  7  are coated.  
         [0030]    The catheter  42  comprises positive and negative metallic electrode leads  44  and  46 , preferably made of MP35 alloy or non-magnetic stainless steel. The leads  44  and  46  are surrounded by a cylindrical sheath  48  made from silicone, polyurethane, polyethylene or other suitable bio-compatible material. The outside diameter of the sheath  48  is selected so as to match the outside diameter of the ring  6 . A reduced diameter end portion  50  of the sheath  48  snugly engages the inside wall of the ring  6 .  
         [0031]    A stub  52  is used to interconnect the tip  4  and the ring  6 . The stub  52  can be formed in the same manner as the stub  16  of FIGS. 3A and 3B. The outside diameter of the stub  52  is selected so as to match the outside diameter of the tip  4  and the ring  6 . Reduced diameter end portions  54  of the stub  52  respectively engage the inside walls of the tip  4  and the ring  6 .  
         [0032]    The metallic leads  44  and  46  are respectively connected to the ring  6  and the tip  4 . The positive lead  44  extends into the interior of the ring  6  and is electrically connected, as by soldering or the like, to the metallic coating  10 . Note that in FIG. 4B, a small hole  56  can be made in the ring  6  to facilitate electrical connection of the lead  28  to the metallic coating  10 . Other connection schemes could also be employed. The negative lead  46  extends through the ring  6  and the stub  52  (which is preferably molded around the lead  46  as indicated above) and is electrically connected, as by soldering or the like, to the metallic coating  8  of the tip  4 . Note that in FIG. 4B a small hole  58  is made in the tip  4  to facilitate electrical connection of the lead  46  to the metallic coating  8 . Again, alternative connection schemes could also be used.  
         [0033]    Turning now to FIG. 5, an implantable pacemaker  102  is shown that may be constructed in accordance with the present invention. The pacemaker  102  includes a first (main) enclosure  104  that is connected to the proximal end  106  of a catheter  108 , which may be photonic or non-photonic. A distal end  110  of the catheter  108  mounts an electrode termination pair  112  constructed in accordance with a suitable one of the embodiments disclosed herein. Thus, the electrode termination pair  112  includes a coated ceramic tip  114  and a coated ceramic ring  116  separated by a short insulative spacer  118 .  
         [0034]    Turning now to FIG. 6, a wearable pacemaker  122  is shown that may be constructed in accordance with the present invention. The pacemaker  122  includes a first (main) enclosure  124  that is connected to the proximal end  126  of a catheter  128 , which may be photonic or non-photonic. A distal end  130  of the catheter  128  mounts an electrode termination pair  132  constructed in accordance with a suitable one of the embodiments disclosed herein. Thus, the electrode termination pair  132  includes a coated ceramic tip  134  and a coated ceramic ring  136  separated by a short insulative spacer  138 .  
         [0035]    Turning now to FIG. 7, another preferred embodiment of the invention is shown in the form of a modified electrode termination pair  142 . The electrode termination pair  142  includes a tip  144  and a ring  146 . As shown in FIGS. 8A and 8B, the tip  144  and the ring  146  are formed on a single ceramic base structure  148 . An electrically conductive coating  150  formed at the distal end of the base structure  148  provides the tip  144 . An electrically conductive coating  152  formed at the proximal end of the base structure  148  provides the ring  146 . The difference between FIGS. 8A and 8B is that both the inside and the outside surfaces of the ceramic base structure  148  are coated in FIG. 8A, whereas only the outer surface of the ceramic base structure  148  is coated in FIG. 8B.  
         [0036]    While various embodiments of the invention have been shown and described, it should be apparent that many variations and alternative embodiments could be implemented in accordance with the invention. For example, although the ring  6  has been described as housing components such as the optical termination unit  24  and the opto-electrical transducer  26 , these components could potentially be housed in the tip  4  if sufficient space is available within the tip interior. Other components, such as an R-wave amplifier and an electro-optical transducer for generating heartbeat monitoring signals could also be housed in the tip  4  if space permits. Another alternative would be to locate the optical termination unit  24  and the opto-electrical transducer  26  in the tip  4 , while placing an R-wave amplifier and an electro-optical transducer in the ring  6 . Additional functionality, such as a partial oxygen monitor, a core body temperature sensor, etc., may also be provided.  
         [0037]    It is understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.