Patent Publication Number: US-7218971-B2

Title: Implantable lead with dissolvable coating for improved fixation and extraction

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   This application is a continuation of U.S. patent application Ser. No. 09/906,571, filed on Jul. 16, 2001, now U.S. Pat. No. 6,584,363 which is a continuation of U.S. patent application Ser. No. 09/280,096, filed on Mar. 29, 1999, now issued as U.S. Pat. No. 6,304,786, the specifications of which are incorporated herein by reference. 

   TECHNICAL FIELD 
   This invention relates generally to electrical leadwires and particularly, but not by way of limitation, to a cardiac rhythm management system providing an endocardial cardiac rhythm management lead with an at least partially dissolvable coating on at least portions of an insulating lead body for improved fixation and extraction. 
   BACKGROUND 
   When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body&#39;s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart. 
   Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. 
   Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn&#39;t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias. 
   One aspect of typical cardiac rhythm management systems is providing an appropriate endocardial lead coupling an implantable cardiac rhythm management device to the heart for sensing intrinsic electrical heart activity signals and/or delivering electrical therapy such as pacing stimulations or defibrillation countershocks. In conventional cardiac rhythm management systems, the lead typically includes a distal electrode that is transvenously guided into the right atrium or right ventricle. The electrode is carefully positioned to contact adjacent cardiac tissue for sensing and/or providing therapy. The preferred location provides adequate amplitude sensed intrinsic electrical heart activity signals and low threshold energy requirements for effectively delivering pacing and/or defibrillation therapy. To ensure that the electrode&#39;s location does not change over time in spite of continuous heart contractions, a fixation device may be used to anchor the distal end of the leadwire in position within the heart. 
   Even if the position of the electrode is stabilized, stimulation thresholds may vary over time as a result of interactions between the electrode and the adjacent tissue. Fibrotic scar tissue may form during the recovery and healing process as the body reacts to the presence of the electrode. The growth of fibrotic tissue results in chronic stimulation energy thresholds that exceed the acute energy thresholds obtained immediately after implant. As a result, higher stimulation energies are required, thereby shortening the usable life of the battery-powered implantable cardiac rhythm management device. By providing a steroid at the electrode and/or its associated fixation device, modified fibrotic tissue is formed and lower chronic stimulation thresholds are obtained, as disclosed in Heil, Jr. et al., U.S. Pat. No. 4,819,661, which is assigned to the assignee of the present application, and which is incorporated herein by reference in its entirety. 
   Use of a fixation device to anchor the lead typically obtains consistent chronic sensing and stimulation thresholds, but such fixation devices complicate removal of the lead such as, for example, in the event of an infection or lead failure. For example, removing a lead with a corkscrew or barb fixation device risks damage to the surrounding tissue to which the fixation device is firmly attached. Where such surrounding tissue is fibrotic scar tissue resulting from lead placement and fixation in conjunction with the normal healing process, separation of the lead from the surrounding scar tissue is even more difficult because the lead will likely have become firmly incorporated within the body. Moreover, many such fixation devices are adapted only for securing the distal tip of the lead; they may not adequately secure ring electrodes that are disposed at a slight distance away from the distal tip of the lead. Furthermore, most conventional fixation techniques are directed toward anchoring leads in the right side of the heart. Such techniques may not be as well suited for anchoring leads elsewhere. Thus, there is a need for providing a cardiac rhythm management lead that is capable of both adequate fixation and removal. There is also a need for providing a cardiac rhythm management lead that is capable of placement and fixation in other regions of the heart, such as within fragile vascular structures. 
   SUMMARY 
   This document describes, among other things, a cardiac rhythm management system providing an endocardial cardiac rhythm management lead with an at least partially dissolvable coating on at least portions of the lead body at or near its distal end, which promotes tissue ingrowth to secure the lead in place within fragile vascular structures or elsewhere. In one embodiment, dissolution of the coating releases a therapeutic agent. In a further embodiment, the therapeutic agent includes a steroid that modifies the fibrotic scar tissue component of tissue ingrowth, such that the resulting bond between the tissue and the lead is weak, such that the lead can be easily extracted if desired. 
   In one embodiment, the lead includes an insulating elongate body having a proximal and a distal end and a peripheral surface. At least one elongate electrical conductor, having a proximal end and a distal end, is carried within the elongate body. The conductor extends longitudinally along substantially the entire length between the proximal and distal ends of the elongate body. At least one electrode is located at or near the distal end of the elongate body. The electrode is coupled to the distal end of the conductor. The lead also includes an at least partially dissolvable coating on at least portions of the insulating peripheral surface at or near the distal end of the elongate body. The coating provides at least one of a rough surface, a porous surface, and a swollen surface after being exposed to an aqueous substance. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. 
       FIG. 1  is a schematic drawing illustrating generally one embodiment of portions of a cardiac rhythm management system and one environment in which it is used. 
       FIG. 2  is a schematic drawing illustrating generally one embodiment of a disposition of distal portions of a lead in a coronary sinus and/or a great cardiac vein. 
       FIG. 3A  is a schematic drawing illustrating one embodiment of portions of a lead. 
       FIG. 3B  is a schematic drawing illustrating the portions of the lead of  FIG. 3A  after exposure to an aqueous substance. 
       FIG. 4A  is a schematic drawing illustrating generally another embodiment of portions of a lead. 
       FIG. 4B  is a schematic drawing illustrating the portions of the lead of  FIG. 4A  after exposure to an aqueous substance. 
       FIG. 5A  is a schematic drawing illustrating a further embodiment of portions of a lead. 
       FIG. 5B  is a schematic drawing illustrating the portions of the lead of  FIG. 5A  after exposure to an aqueous substance. 
       FIG. 6  is a graph of steroid absorbance vs. time, illustrating generally test results for a prototype embodiment of a lead. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents. In the drawings, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. 
   GENERAL OVERVIEW 
   This document describes, among other things, a cardiac rhythm management system providing an endocardial cardiac rhythm management lead with an at least partially dissolvable coating on at least portions of an insulating lead body, for improved fixation and extraction. 
     FIG. 1  is a schematic drawing illustrating generally, by way of example, but not by way of limitation, one embodiment of portions of a cardiac rhythm management system  100  and one environment in which it is used. In  FIG. 1 , system  100  includes an implantable cardiac rhythm management device  105 , which is coupled via an intravascular endocardial lead  110  to a heart  115  of a human or other patient  120 . System  100  also includes an external programmer  125  providing wireless communication with device  105 , such as by using a telemetry device  130 . Lead  110  includes a proximal end  135 , which is coupled to device  105 , and a distal end  140 , which is coupled on or about one or more portions of heart  115 . 
     FIG. 2  is a schematic drawing illustrating generally, by way of example, but not by way of limitation, one embodiment of an exemplary disposition of distal portions of lead  110 .  FIG. 2  illustrates chambers of heart  115 , including a right atrium  200 , a right ventricle  205 , a left atrium  210 , and a left ventricle  215 . In the embodiment illustrated in  FIG. 2 , distal end  140  of lead  110  is transvenously guided into right atrium  200 , through a coronary sinus  220 , and into a great cardiac vein  225 . This example disposition of lead  110  is useful for delivering pacing and/or defibrillation energy to the left side of heart  115 , such as for treatment of congestive heart failure (CHF) or other cardiac disorders requiring therapy delivered to the left side of heart  115 . Other possible dispositions of distal portions of endocardial lead  110  include insertion into right atrium  200  and/or right ventricle  205 , or transarterial insertion into the left atrium  210  and/or left ventricle  215 . 
     FIG. 3A  is a schematic drawing illustrating generally, by way of example, but not by way of limitation, one embodiment of portions of lead  110 , including its distal end  140  and its proximal end  135 . In this embodiment, intravascular endocardial lead  110  includes a biocompatible flexible insulating elongate body  300  (e.g., including a polymer such as medical grade silicone rubber) for translumenal (i.e., transvenous or transarterial) insertion and access within a living organism such as patient  120 . In one embodiment, slender elongate body  300  is tubular and has a peripheral outer surface of diameter d that is small enough for translumenal insertion into coronary sinus  220  and/or great cardiac vein  225 . An elongate electrical conductor  305  is carried within insulating elongate body  300 . Conductor  305  extends substantially along the entire length between the distal end  140  and proximal end  135  of lead  110 , and this length is long enough for lead  110  to couple device  105 , which is implanted pectorally, abdominally, or elsewhere, to desired locations within heart  115  for sensing intrinsic electrical heart activity signals or providing pacing/defibrillation-type therapy thereto. Elongate body  300  forms an insulating sheath covering around conductor  305 . Conductor  305  is coupled to an electrode  310  at or near distal end  140  of elongate body  300 . Conductor  305  is coupled to a connector  315  at or near proximal end  135  of elongate body  300 . Device  105  includes a receptacle for receiving connector  315 , thereby obtaining electrical continuity between electrode  310  and device  105 . 
   Electrode  310 , or at least a portion thereof, is not covered by the insulating sheath of elongate body  300 . The embodiment of  FIG. 3A  includes a ring or ring-like electrode  310  that provides an exposed electrically conductive surface around all, or at least part of, the circumference of lead  110 . In one embodiment, the exposed surface of electrode  310  is smooth. In another embodiment, the exposed surface of electrode  310  is not smooth, thereby allowing tissue ingrowth into interstitial spaces of electrode  310 . In one example, electrode  310  is a coiled wire electrode that is wound around the circumferential outer surface of lead  110 . Lead  110  also includes other configurations, shapes, and structures of electrode  310 . 
   Lead  110  includes a biocompatible coating  320  on at least one insulating portion of the peripheral surface of elongate body  300  at or near distal end  140 . In one embodiment, as illustrated in  FIG. 3A , coating  320  extends circumferentially completely (or at least partially) around the tubular outer peripheral surface of lead  110 . In the embodiment of  FIG. 3A , coating  320  occupies one or more nonelectrode regions (i.e., insulating portions of the lead body) extending along a length l from the tip of distal end  140  of lead  110 , where length l is approximately equal to a distance within coronary sinus  220  and great cardiac vein  140  into which distal end  140  of lead  110  is inserted. In a further embodiment, coating  320  also overlaps one or more portions of one or more electrodes, such as electrode  310 , providing an insulating covering that reduces the exposed surface area of the electrode  310 . 
   According to one aspect of the invention, coating  320  is at least partially dissolvable when exposed to an aqueous substance such as blood or bodily fluids. In one embodiment, coating  320  provides a substantially smooth low friction outer surface before dissolution of portions of coating  320 . After being exposed to an aqueous substance, however, portions of coating  320  dissolve. According to one aspect of the present system, this dissolution initially makes distal end  140  of lead  110  more slippery, making it easier to insert into patient  120 . After portions of coating  320  dissolve, remaining substrate portions of coating  320  provide a rough and/or porous surface (as illustrated in  FIG. 3B ) that promotes tissue ingrowth into resulting interstitial regions. Such tissue ingrowth anchors and/or secures lead  110  to adjacent cardiac tissue, such as within coronary sinus  220  and/or great cardiac vein  225 , or elsewhere in or about heart  115 . In one embodiment, portions of coating  320  undergo osmotic swelling after being exposed to the aqueous substance. This further assists in securing lead  110  within a vascular region such as, for example, within coronary sinus  220  and/or great cardiac vein  225 . Thus, in one embodiment, coating  320  includes a plurality of substantially soluble particles dispersed in a substantially insoluble medium that is adhered to portions of elongate body  300  at or near distal end  140 . When exposed to an aqueous environment, the substantially soluble particles dissolve, leaving behind a rough and/or porous surface that results in tissue ingrowth and/or osmotic swelling, which promotes lead fixation that is particularly well-suited for vascular structures. 
   In another embodiment, coating  320  includes a therapeutic agent such as, by way of example, but not by way of limitation, a drug, a steroid, a corticosteroid, an antibiotic, and/or an antirejection agent. In one example, the therapeutic agent is provided by the substantially soluble particles that are dispersed in the substantially insoluble medium. In one example, coating  320  includes substantially soluble particles of a steroid such as dexamethasone acetate. When coating  320  is exposed to an aqueous environment, the substantially soluble steroid elements dissolve, providing sustained release into the surrounding tissue. This delays local protein synthesis and tissue healing, which modifies the formation of normal fibrotic scar tissue during the recovery and healing process. As a result, a lesser amount of strongly-bound fibrotic tissue is formed during tissue ingrowth into interstices provided by the rough and/or porous surface of the remaining insoluble portion of coating  320 . The resulting ingrown tissue promotes adequate anchoring and fixation of lead  110 . Because the ingrown tissue includes modified fibrotic scar tissue, however, it is easier to separate lead  110  from the surrounding tissue by pulling and/or turning the lead  110  during removal. Stated differently, steroid release from the structure receiving tissue ingrowth on an insulating portion of elongate body  300  results in friable tissue ingrowth that is more easily disrupted or separated from lead  110  upon extraction and removal of lead  110 . Thus, lead  110  offers advantages for both its fixation and removal. Furthermore, reduced pacing and defibrillation threshold energies may be obtained by providing the therapeutic agent near electrode regions, as discussed above. 
   In a further embodiment, coating  320  provides a therapeutic agent including more substantially soluble dexamethasone sodium phosphate particles. These particles are dispersed in a substantially insoluble medium, such as biocompatible silicone rubber medical adhesive, other polymer, or other suitable biocompatible adhesive substance. Silicone rubber medical adhesive is permeable by water vapor. As a result, water vapor can reach interior dry pockets of the dispersed soluble particles, allowing such particles to dissolve and be released from coating  320  into the surrounding tissue. Other biocompatible, water vapor permeable, and substantially insoluble adhesive or polymeric media can also be used. 
   In another embodiment, coating  320  provides a therapeutic agent including more substantially soluble dexamethasone sodium phosphate elements dispersed in the substantially insoluble silicone rubber medical adhesive. This embodiment advantageously provides both friable tissue encapsulation and osmotic swelling. 
   In yet another embodiment, smooth coating  320  includes a combination of the therapeutically active agents, such as dexamethasone acetate and dexamethasone sodium phosphate, dispersed in the substantially insoluble medium (e.g., silicone rubber). In this embodiment, upon exposure to bodily fluids, rapid initial release of the dexamethasone drug substance occurs predominantly in the form of the more soluble dexamethasone sodium phosphate. Release of the dexamethasone drug substance in the form of dexamethasone acetate also occurs upon exposure to bodily fluids, but does so more slowly. Dissolution and release of the dexamethasone sodium phosphate phase will also result in the development of porous surface structures into which tissue ingrowth can occur. Release of the combined forms of the steroid will also serve to modify the tissue ingrowth producing friable encapsulation. 
   In yet another embodiment, smooth coating  320  includes a combination of the therapeutically active agent dexamethasone acetate and a more substantially soluble nontherapeutic inert agent, such as mannitol or glycerol. In this embodiment, upon exposure to bodily fluids, both the active and inert agents dissolve. The more substantially soluble inert agent first aids in the initial implantation, by lowering lead friction and making the lead more slippery. Then the inert agent quickly dissolves to generate porous features on the surface of the lead body. Release of the dexamethasone drug substance in the form of dexamethasone acetate also occurs upon exposure to bodily fluids, but does so more slowly, as described above. Release of these combined forms of active and inert additives again serves to modify the tissue ingrowth, providing friable encapsulation. 
   During manufacture, at least one insulating portion of elongate body  300  is coated with coating  320 . The coating  320  cures such that it adheres to elongate body  300 . Increasing the percentage of soluble particles dispersed in the insoluble medium results in increased roughening and/or porosity after lead  110  is exposed to an aqueous substance during use. However, the percentage of soluble particles dispersed in the insoluble medium is typically not so great as to diminish the adhesive properties of coating  320  or to diminish the integrity of coating  320  after its partial dissolution during its use. In a first embodiment, coating  320  includes up to 40% soluble particles (e.g., dexamethasone, dexamethasone acetate or dexamethasone sodium phosphate) combined with or dispersed in the insoluble medium (e.g., silicone rubber medical adhesive). In a second embodiment, coating  320  includes up to 35% soluble particles (e.g., dexamethasone, dexamethasone acetate or dexamethasone sodium phosphate) combined with or dispersed in the insoluble medium (e.g., silicone rubber medical adhesive). In a third embodiment, coating  320  includes up to 30% soluble particles (e.g., dexamethasone, dexamethasone acetate or dexamethasone sodium phosphate) combined with or dispersed in the insoluble medium (e.g., silicone rubber medical adhesive). In a fourth embodiment, coating  320  includes between 30%-40% soluble particles (e.g., dexamethasone, dexamethasone acetate or dexamethasone sodium phosphate) combined with or dispersed in the insoluble medium (e.g., silicone rubber medical adhesive). 
     FIG. 4A  is a schematic drawing illustrating generally, by way of example, but not by way of limitation, another embodiment of lead  110 , which is similar in many respects to the embodiment of lead  110  described with respect to  FIG. 3A . The embodiment of  FIG. 4A  includes a plurality of electrodes, such as ring electrodes  310 A–C, and tip electrode  400  at distal end  140  of lead  110 . The electrodes  310 A–C and  400  are coupled to proximal end  135 , such as by a corresponding plurality of conductors  305 A–D interconnecting electrodes  310 A–C and  400  to individual connectors  315  that are received at corresponding receptacles on device  105 . Alternatively, one or more of electrodes  310 A–C is tied to one or more others of electrodes  310 A–C using one or more shared/common conductors  305 A–D. 
   Interelectrode portions of insulating elongate body  300  of lead  110  are covered with coatings  320 A–C, which are at least partially dissolved during use as described above, as illustrated in  FIG. 4B . In one embodiment, coatings  320 A–C elute steroids or other therapeutic agents during dissolution, as discussed above. In a further embodiment, one or more of coatings  320 A–C also overlap one or more portions of one or more adjacent electrodes, providing an insulating covering that reduces the exposed surface area of the partially covered electrode. The embodiment of  FIGS. 4A–B  also optionally include one or more additional passive or active fixations devices, such as one or more tines  405 A–B extending laterally outward from a portion of elongate body  300  at or near tip electrode  400 . In one example, tines  405 A–B are formed of the same insulating material as elongate body  300 . 
     FIG. 5A  is a schematic drawing illustrating generally, by way of example, but not by way of limitation, another embodiment of lead  110 , which is similar in many respects to the embodiment of lead  110  described with respect to  FIG. 4A . The embodiment of  FIG. 5A  includes one or more electrodes, such as coil electrodes  310 A–B, which each include wires wound around the peripheral circumference of elongate body  300 . Coil electrodes  310 A–B provide a nonuniform, rough peripheral surfaces that allow tissue ingrowth into interstitial regions between the individual coil windings for providing improved fixation. Interelectrode coating  320 A partially dissolves after implantation, as discussed above, which also allows tissue ingrowth into the substantially insoluble substrate portion of interelectrode coating  320 A, as illustrated in  FIG. 5B . In one embodiment, coating  320 A releases a therapeutic agent during dissolution, such as a steroid that modifies the fibrotic scarring content of the tissue ingrowth into the rough/porous substrate portion of coating  320 A. The steroid elution from coating  320 A also modifies the fibrotic scar tissue content of the tissue that grows into the nearby interstices of electrodes  310 A–B. The resulting ingrown tissue is friable. It provides fixation of lead  110 , but allows easier lead extraction. In a further embodiment, coating  320 A also overlaps one or more portions of one or more electrodes, providing an insulating covering that reduces the exposed surface area of the partially covered electrode.  FIG. 5A  also illustrates an embodiment of lead  110  that includes optional other fixation mechanisms, such as one or more of mesh tip electrode  400  or a helical anchor  500  extending longitudinally outward from tip electrode  400 . It is understood that one or more of these or other like fixation mechanisms can be used either alone, or in combination with each other, as desired. 
     FIG. 6  is a graph of steroid absorbance vs. time, illustrating generally, by way of example, but not by way of limitation, test results for a prototype embodiment of lead  110 , which is similar to that illustrated in  FIG. 4A , but including a single tip electrode and 4 ring electrodes. The electrodes were separated by polymeric interelectrode regions coated with silicone rubber medical adhesive that included approximately 10% dexamethasone acetate. Approximately 1.8 milligrams of dexamethasone acetate was released over a period of approximately one week, with some drug release still continuing at one week after exposure to an aqueous solution as indicated by the slight positive slope of the curve in  FIG. 6  at that time. In one embodiment, the time period during which the therapeutic agent is released is optimized by proper selection of its aqueous solubility, or by otherwise selecting the appropriate combination of the substantially soluble therapeutic agent, the substantially insoluble binder medium, and an appropriate inert additive. 
   CONCLUSION 
   The above-described system provides, among other things, a cardiac rhythm management system providing an endocardial cardiac rhythm management lead with an at least partially dissolvable coating on insulating portions of the lead body at or near its distal end, which promotes tissue ingrowth to secure the lead in place. In one embodiment, dissolution of the coating releases a therapeutic agent. In a further embodiment, the therapeutic agent includes a steroid that modifies the fibrotic scar tissue content of tissue ingrowth, such that the resulting bond between the tissue and the lead is weak, such that the lead can be easily extracted if desired. 
   Although the present cardiac rhythm management system is described above with respect to a lead that is disposed within the vasculature of a coronary sinus and/or a great cardiac vein, it is understood that the endocardial lead can be positioned elsewhere within the heart, such as within the right or left atria or ventricles, within other blood vessels, or elsewhere. 
   It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.