Patent Publication Number: US-7904178-B2

Title: Medical electrical lead body designs incorporating energy dissipating shunt

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention is a continuation-in-part of U.S. application Ser. No. 10/909,518, filed Aug. 2, 2004, now U.S. Pat. No. 7,783,365 now published as 20050004643, entitled “Implantable medical device conductor insulation and process for forming,” which is a continuation-in-part of U.S. application Ser. No. 10/407,653, filed Apr. 4, 2003, now abandoned now published as 20030216800, entitled “Implantable medical device conductor insulation and process for forming,” which is a utility application filed off of U.S. Provisional Application Ser. No. 60/371,995, filed Apr. 11, 2002. 
    
    
     TECHNICAL FIELD 
     The present invention pertains to medical electrical leads and more particularly to medical electrical leads including energy dissipating shunts. 
     BACKGROUND 
     The technology explosion in the medical device industry has resulted in a variety of innovative diagnostic and therapeutic devices and methods. Many implantable medical devices (IMDs), for example, including pacemakers, cardioverter-defibrillators and neural stimulators, are operatively coupled to electrodes, which are joined to elongate lead wires that extend from the devices to a target site either on or within a body of a patient. The electrodes may deliver stimulation therapy and/or sense electrical signals from the patient, for example cardiac depolarization signals, which are used to guide or dictate therapy delivery. 
     Patients, in which such leads are implanted, may be exposed to a substantial amount of radio frequency (RF) energy, for example, when subject to magnetic resonance imaging (MRI) or radio diathermy processes. An implanted lead wire can act as an antenna during exposure to these RF signals and an appreciable amount of current may thereby be generated in the lead wire, resulting in high current concentration at a surface of a tissue-contacting electrode, for example, implanted to provide pacing stimulation. Much of this current produces heat, due to energy dissipated in a resistance of the electrode-to-tissue interface, which may result in tissue damage in proximity to the electrode. Leads that include an energy dissipating shunt component have been described, for example, in co-pending and commonly-assigned patent application Ser. No. 11/426,207, but there is still a need for novel lead body designs that incorporate more effective energy dissipating shunts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are illustrative of particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. 
         FIG. 1  is a plan view including a partial section view of a medical electrical lead, according to some embodiments of the present invention. 
         FIG. 2  is a cross-section view through a conductor of  FIG. 1 . 
         FIG. 3  is a plan view including a partial section view of a coaxially constructed medical electrical lead, according to some embodiments of the present invention. 
         FIG. 4  is a plan view including a partial section view of a multi-conductor medical electrical lead, according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical illustrations for implementing exemplary embodiments of the present invention. Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements, and all other elements employ that which is known to those of skill in the field of the invention. Those skilled in the art will recognize that many of the examples provided have suitable alternatives that can be utilized. 
       FIG. 1  is a plan view including a partial section view of a medical electrical lead  10 , according to some embodiments of the present invention.  FIG. 1  illustrates lead  10  including an elongate lead body  11  terminated at a proximal end by a connector pin  150  and terminated at a distal end by a stimulation electrode  15 . Those skilled in the art will understand that connector pin  150  may be coupled to an implantable pulse generator, for example, a pacemaker device, and that electrode  15  may act as both a sensing and stimulation electrode when coupled to myocardial tissue. According to the illustrated embodiment, electrode  15  is coupled to connector pin  150  by a conductor  104  formed into a coil  114 ; coil  114  includes a first portion  141 , which extends within an outer insulation sheath  112 , and a second portion  142 , which extends outside sheath  112 , to form an energy dissipating shunt when lead  10  picks up radio frequency (RF) energy, for example, during an magnetic resonance imaging (MRI) procedure. According to an alternate embodiment, coil  114  includes more than one conductor  104 , being what is known in the art as a multi-filar coil, and the multiple filars or conductors  104  may extend along an entire length of coil  114  between connector pin  150  and electrode  15 .  FIG. 1  further illustrates another outer insulation sheath  113  extending over coil  114  between second portion, or shunt  142  and electrode  15 . Insulation sheaths  112 ,  113  are formed from insulative material. Exemplary insulative materials include silicone, polyurethane, a combination thereof etc. 
       FIG. 2  is a cross-section view through conductor  104 .  FIG. 2  illustrates conductor  104  including a wire  14  having an outer jacket of insulation  24  extending about an entire circumference thereof. Wire  14  may have an outer diameter in a range from about 0.003 inch to about 0.006 inch. Outer jacket of insulation  24  extends at least along a length of conductor  104  that forms second portion or shunt  142 . According to embodiments of the present invention, insulation  24  provides a high enough impedance, between wire  14  and an implant environment of lead  10 , so that relatively low frequency stimulation pulses may pass through conductor  104  to electrode  15  without current leakage along shunt  142 , and provides for a capacitive coupling with the environment external to lead  10  for the current induced by relatively high frequency RF energy, which may be picked up by conductor  104 , so that a high current concentration at the relatively small tissue-contacting surface of electrode  15  is prevented. It should be noted that a length of shunt  142 , and a position of shunt  142 , when lead  10  is implanted, are such that the energy of the relatively high-frequency current will be dissipated over a relatively large surface area, resulting in a reduced local energy density and, thus, a reduced temperature rise, thereby avoiding tissue damage. Insulation  24  may be any dielectric, biostable and biocompatible material, for example an oxide, a polymer, or a ceramic, but is preferably one that can sustain repeated flexing imposed by a cardiac implant environment, either endocardial or epicardial, without sustaining breaches. Insulation jacket  24  may have a thickness in a range from about 0.0001 inch to about 0.001 inch. 
     According to some embodiments of the present invention, jacket  24  extends along an entire length of conductor  104  to form a primary insulation for sensing and stimulation between connector pin  150  to electrode  15 ; according to some alternate embodiments, jacket  24  only extends along a length of conductor  104 , which forms second portion  142  of coil  114 , to act as a primary insulation for only second portion  142 , while outer insulation sheaths  112 ,  113  act as a primary insulation for first portion  141  of coil and that portion extending between second portion  142  and electrode  15 , respectively. According to preferred embodiments of the present invention, conductor  104  includes a single conductor wire, for example, wire  14 , which is continuous from a junction with connector pin  150  to a junction with electrode  15 . A length of second portion  142  may be between about 3 centimeters and about 7 centimeters, or even approaching an entire length of lead  10 , for example, between about 30 centimeters and about 110 centimeters; and a length of second insulation sheath  113 , providing a spacing between second portion  142  and electrode  15 , is greater than about 3 millimeters, and, preferably, less than about 6 to 8 centimeters. 
       FIG. 2  further illustrates wire  14  including an optional ‘low-resistance’ core  29 , for example, formed from silver, tantalum or gold, to decrease a resistance of wire  14 . Such a decrease may be desirable in order to increase a number of turns of coil  114 , thereby increasing an inductance of conductor  104  without significantly increasing a resistance thereof. Careful selection of increased inductance (e.g. increased number of turns in a coil, etc.) of conductor coil  104  reduces the current induced by the MRI. By increasing the inductive impedance, minimal or no energy is delivered to the tip of the electrode by increasing the high-frequency (i.e. 21 megaHertz (Mhz) to 128 MHz) impedance of conductor  104  to electrode  15 . With reference back to  FIG. 1 , according to a preferred embodiment, an outer diameter of coil  114  is increased along second portion  142  in order to be about flush with an outer diameter of adjacent insulation sheaths  112  and  113 .  FIG. 1  further illustrates a filler material  124 , for example, formed by silicone rubber, between turns of second portion  142  of coil  114 , which may lend some mechanical stability to portion  142  and may prevent tissue in-growth between the turns. Silicone rubber is commercially available from Silicone Specialties Fabricators located in Elk Rapids, Mich. According to some alternate embodiments, coil  114 , either just along second portion  142 , or along an entire length thereof, may be mounted on a flexible insulative core, which may or may not include a longitudinally extending lumen. Embodiments including such a core may also include filler material  124  along second portion  142 . According to yet further alternate embodiments, first portion  141  of coil  114  may be embedded in sheath  112 . 
     According to an exemplary embodiment of the present invention, coil  114  of lead  10  is bi-filar, including two of conductors  104  wound coaxially side-by-side; and key dimensions for lead  10  are as follows: an overall length of lead  10  is about 68 cm; wire  14 , preferably formed from silver-cored MP35N alloy, has a diameter of about 0.003 inch; insulation jacket  24 , preferably formed from one of polytetrafluoroethylene (PTFE), tetrafluorethylene hexafluoropropylene vinylidene fluoride (THV), a fluorinated terpolymer, ethylene tetrafluorethylene (ETFE), polyvinylidene fluoride (PVD) THV 400, THV 600, and Si polyimide. Insulation jacket  24  has a thickness of about 0.0005 inch; insulation sheath  112 , preferably formed from polyurethane, has an outer diameter of about 0.055 inch; second portion  142  of coil  114  has an outer diameter of about 0.055 inch and a length of about 6 cm; a pitch of coil  114 , at least along second portion  142  is about 0.012 inch; and insulation sheath  113 , preferably formed from polyurethane, has a length of about 1.5 cm. With respect to the Si polyimide, it is preferable to use hydrolytically stable polyimide. According to this exemplary embodiment, a capacitance of shunt  142  is between about 400 and 500 picoFarads. It should be noted that the Si polyimide insulative jacket may be as thin as about 0.0002 inch to further increase the capacitance. Alternate constructions of leads including shunts that are integral along a length of an electrode conductor thereof, similar in nature to shunt  142 , will be described below. 
       FIG. 3  is a plan view including a partial section view of a coaxially constructed medical electrical lead  10 ′, according to some embodiments of the present invention.  FIG. 3  illustrates lead  10 ′ including conductor coil  114  coupling connector pin  150  to electrode  15 , as previously described for lead  10 , wherein coil  114  includes first portion  141  extending within outer insulation sheath  112  and second portion, or shunt  142  extending outside sheath  112 . According to the illustrated embodiment, a lead body  11 ′ of lead  10 ′ further includes another conductor  404  formed into a coil  414  extending about coaxially about coil  114  and isolated therefrom by an inner insulation sheath  412 , for example formed from silicone or polyurethane; conductor  404  couples another electrode  47 , which is disposed proximal to shunt  142 , to a connector ring  470 , which is disposed in proximity to connector pin  150 . Those skilled in the art will appreciate that electrode  47  in conjunction with electrode  15  may form a bipolar pair for sensing and stimulation, however, it should be noted that electrodes  47  and  15  may function independently of one another. According to the illustrated embodiment, inner insulation sheath  412  extends distal of electrode  47  where an outer diameter thereof is increased to be about flush with an outer diameter of electrode  47 ; an outer diameter of shunt  142  is preferably about flush with the outer diameter of sheath  412  distal to electrode  47 , which may be about equal to an outer diameter of sheath  112 . Although  FIG. 3  illustrates a ring component forming electrode  47 , it should be understood that such a component is not necessary and an exposed portion of coil  414 , distal to sheath  112 , may form electrode  47 . 
       FIG. 4  is a plan view including a partial section view of a multi-conductor medical electrical lead  30 , according to some embodiments of the present invention.  FIG. 4  illustrates lead  30  including an elongate lead body  31  terminated at a proximal end by a connector pin  350  and terminated at a distal end by a first electrode  35 ; lead  30  further includes a second electrode  37  disposed proximal to first electrode  35  and a connector ring  370  disposed in proximity to pin  350 . According to the illustrated embodiment, first electrode  35  is coupled to pin  350  by a first conductor  304  formed into a coil  314 , and second electrode  37  is coupled to ring  370  by a second conductor  306  also formed into coil  314 .  FIG. 4  further illustrates coil  314  including a first portion  341 , which extends within an outer insulation sheath  312 , a second portion  342 , which extends outside sheath  312  to form an energy dissipating shunt when lead  30  picks up RF energy, and another outer insulation sheath  313  extending over coil  314  between second portion, or shunt  342  and electrode  35 . Second conductor  306  is shown terminated in proximity to a distal end  345  of coil first portion  341  to join with electrode  37 , and first conductor  304  is shown extending within a bore of electrode  37 , past distal end  345  of first coil portion  341 , to second coil portion, or shunt  342 . Like shunt  142  ( FIG. 1 ), shunt  342  is shown having an outer diameter about equal to an outer diameter of outer insulation sheath  312 . 
     With reference back to  FIG. 2 , conductor  304  includes wire  14 , which may include the illustrated low-resistance core  29 , as previously described, and insulation jacket  24 , which extends about an entire circumference thereof to electrically isolate conductor  304  from conductor  306 , along first portion  341  of coil  314 , and from the environment external to lead  30 , along coil second portion, or shunt  342 , during transmission of relatively low frequency stimulation pulses from connector pin  350  to electrode  35 . According to alternate embodiments, second conductor  306  includes jacket of insulation  24  along first portion  341  of coil  314 , and first conductor may only include jacket of insulation  24  along second portion  342  of coil  314 . As previously described for conductor  104 , insulative jacket  24  around wire  14  of first conductor  304  along shunt  342  provides capacitive coupling for the current induced by relatively high frequency RF energy, thereby preventing a high current density at the relatively small tissue-contacting surface of electrode  35 ; and a length and disposition of shunt  342  safely dissipates energy of the relatively high frequency current along shunt  342 . According to preferred embodiments of the present invention, wire  14  of first conductor  304  is continuous from connector pin  350  to electrode  35 . 
     In the foregoing detailed description, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims.