Patent Publication Number: US-2007106358-A1

Title: Tissue stimulating lead and method of implantation and manufacture

Description:
TECHNICAL FIELD  
      The present invention relates to medical devices and methods for accessing an anatomical space of the body. More specifically, the invention relates to minimally-invasive devices and methods for implanting an electrode in a myocardium of a heart.  
     BACKGROUND  
      Cardiac rhythm management systems are used to treat heart arrhythmias. Pacemaker systems are commonly implanted in patients to treat bradycardia (i.e., abnormally slow heart rate). A pacemaker system includes an implantable pulse generator and leads, which form the electrical connection between the implantable pulse generator and the heart. An implantable cardioverter defibrillator (“ICD”) is used to treat tachycardia (i.e., abnormally rapid heart rate). An ICD also includes a pulse generator and leads that deliver electrical energy to the heart.  
      The leads coupling the pulse generator to the cardiac muscle are commonly used for delivering an electrical pulse to the cardiac muscle, for sensing electrical signals produced in the cardiac muscle, or for both delivering and sensing. The leads are susceptible to categorization according to the type of connection they form with the heart. An endocardial lead includes at least one electrode at or near its distal tip adapted to contact the endocardium (i.e., the tissue lining the inside of the heart). An epicardial lead includes at least one electrode at or near its distal tip adapted to contact the epicardium (i.e., the tissue lining the outside of the heart). Finally, a myocardial lead includes at least one electrode at or near its distal tip inserted into the heart muscle or myocardium (i.e., the muscle sandwiched between the endocardium and epicardium). Some leads have multiple spaced apart distal electrodes at differing polarities and are known as bipolar type leads. The spacing between the electrodes can affect lead performance and the quality of the electrical signal transmitted or sensed through the heart tissue.  
      The lead typically consists of a flexible conductor surrounded by an insulating tube or sheath that extends from the electrode at the distal end to a connector pin at the proximal end. Endocardial leads are typically delivered transvenously to the right atrium or ventricle and commonly employ tines at a distal end for engaging the trabeculae.  
      The treatment of congestive heart failure (“CHF”), however, often requires left ventricular stimulation either alone or in conjunction with right ventricular stimulation. For example, cardiac resynchronization therapy (“CRT”) (also commonly referred to as biventricular pacing) is an emerging treatment for heart failure, which requires stimulation of both the right and the left ventricle to increase cardiac output. Left ventricular stimulation requires placement of a lead in or on the left ventricle in the lateral or posterior-lateral aspect/region of the heart. One technique for left ventricular lead placement is to expose the heart by way of a thoracotomy. The lead is then positioned so that the electrodes contact the epicardium or are embedded in the myocardium. Another method is to advance an epicardial lead endovenously into the coronary sinus and then advance the lead through a lateral vein of the left ventricle. The electrodes are positioned to contact the epicardial surface of the left ventricle.  
      The epicardium is a tough, fibrous tissue layer. It may be difficult to penetrate the epicardium when manipulating a tool from a distance, as when minimally invasive surgical techniques are used. In addition, repeated or failed attempts to penetrate the epicardium may result in increased trauma to the heart. Furthermore, the left ventricle beats forcefully as it pumps oxygenated blood throughout the body. Repetitive beating of the heart, in combination with patient movement, can sometimes dislodge the lead from the myocardium. The electrodes may lose contact with the heart muscle, or spacing between electrodes may alter over time.  
      There is a need for an improved pacing lead suitable for chronic implantation and a minimally invasive delivery system and method for implanting such a lead into the myocardium.  
     SUMMARY  
      In one embodiment, the present invention is a cardiac lead for stimulating a myocardium of a heart. The cardiac lead includes an anchor and a tether. The anchor has first and second electrically active areas spaced apart and electrically isolated from one another. The tether has a distal end mechanically coupled to the anchor and a proximal end adapted for coupling to a cardiac rhythm management device. The tether includes first and second electrically conductive cables electrically isolated from one another and electrically coupled to the first and second electrically active areas, respectively.  
      In another embodiment, the present invention the present invention is a cardiac lead for stimulating a myocardium of a heart. The cardiac lead includes an anchor and a tether. The anchor has a first electrically active area. The tether has a distal end mechanically coupled to the anchor and a proximal end adapted for coupling to a cardiac rhythm management device. The tether includes a first electrically conductive cable electrically isolated coupled to the first electrically active area. An outer diameter of the tether from the proximal end to the distal end is substantially isodiametric.  
      In yet another embodiment, the present invention is a method of implanting a cardiac lead into a myocardium of a heart, wherein the lead includes an anchor and a tether. The anchor has first and second electrically active areas spaced apart and electrically isolated from one another. The tether has a distal end mechanically coupled to the anchor and a proximal end adapted for coupling to a cardiac rhythm management device. The tether includes first and second electrically conductive cables electrically isolated from one another and electrically coupled to the first and second electrically active areas, respectively. The anchor is engaged to a distal end of an insertion tool and the distal end of the insertion tool is advanced to the heart. The anchor is injected into a myocardium of the heart by ejecting the anchor from the insertion tool so that the first and second electrically active areas are located within the myocardium. The tool is withdrawn proximally and the anchor is deployed to a configuration adapted to resist migration by tensioning the tether.  
      While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an exemplary implantable medical device including a lead attached to a heart according to an embodiment of the present invention.  
       FIG. 2  shows a cross-sectional view of the lead of  FIG. 1 .  
       FIG. 3  shows a cross-sectional view of a distal end of a lead according to another embodiment of the invention in which the tether is coupled closer to a first end of the anchor than a second end.  
       FIG. 4  shows a cross-sectional view of a distal end of a lead according to another embodiment of the invention in which the anchor is provided with a notch to facilitate removal.  
       FIG. 5  shows a cross-sectional view of a distal end of a lead according to another embodiment of the invention in which the anchor has a curved configuration.  
       FIG. 6  shows a side plan view of a distal end of a lead according to another embodiment of the present invention.  
       FIG. 7  shows a perspective view of a distal end of a lead according to another embodiment of the invention in which the lead is adapted for unipolar use.  
       FIG. 8  shows a perspective view of a distal end of a lead according to another embodiment of the present invention in which the electrodes are located proximally from the anchor.  
       FIG. 9A  shows a perspective view of a distal end of a lead according to another embodiment of the invention.  
       FIG. 9B  shows a perspective view of a distal end of a lead according to another embodiment of the invention.  
       FIG. 10A  shows a cross-sectional view of a distal end of a lead in a collapsed configuration according to another embodiment of the invention.  
       FIG. 10B  shows a cross-sectional view of the distal end of the lead of  FIG. 10A  in an expanded configuration.  
       FIG. 11  shows a cross-sectional view of a distal end of a lead according to another embodiment of the invention.  
       FIG. 12  shows a cross-sectional view of a distal end of a lead according to another embodiment of the invention.  
       FIG. 13  shows a lead including a retention feature in relation to the anatomical layers of the heart according to an embodiment of the invention.  
       FIG. 14A  shows a cross-sectional view of a lead and a tool for injecting the lead into the heart in relation to the anatomical layers of the heart according to an embodiment of the invention.  
       FIG. 14B  shows a cross-sectional view of the lead of  FIG. 14A  deployed within the heart.  
       FIG. 15A  shows a cross-sectional view of a tool for injecting a lead into the heart according to another embodiment of the present invention.  
       FIG. 15B  shows a cross-sectional view of the tool of  FIG. 15A  taken along line B-B.  
       FIG. 16  shows a cross-sectional view of a tool for injecting a lead into the heart according to another embodiment of the present invention.  
      While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  illustrates an implantable medical device  10  for sensing or pacing of a heart  14 . The implantable medical device  10  includes a cardiac rhythm management device such as a pulse generator  18  implanted in the chest or abdomen and a lead  22 . The lead  22  has a distal end  26  implanted in a myocardium  30  of the heart  14  between an inner endocardium  34  and an outer epicardium  38 . A proximal end  42  of the lead  22  is electrically couplable to the pulse generator  18 .  
       FIG. 2  shows the lead  22  in greater detail according to one embodiment of the present invention. The lead  22  includes a distal tissue anchor  44  coupled to a flexible tether  48 . The anchor  44  serves as an electrode for sensing or pacing the heart  14 , while electrical signals are transmitted to and from the pulse generator  18  via the tether  48 . The anchor  44  may take many different configurations suitable for implantation into the heart  14 . In general, however, the lead  22  has a reduced outer diameter at both the anchor  44  and tether  48  as would permit injection of the lead  22  into the heart  12 .  
      In one embodiment, the anchor  44  is deployable from a first, collapsed configuration to a second, expanded configuration. By collapsed it is meant that the anchor  44  has a profile sized, shaped and otherwise adapted to traverse the myocardium  30  with minimal trauma thereto in conjunction with a minimally invasive surgical procedure. By expanded, it is meant that the anchor  44  has a profile configured to resist traversing the myocardium  30  and migrating or displacing from an implantation site within the heart  14 . In some embodiments, the anchor  44  is merely repositioned when deploying from the first configuration to the second configuration. In other embodiments, the size and/or shape of the anchor  44  changes when deploying from the first configuration to the second configuration.  
      The anchor  44  is sized and shaped to anchor or fix the electrodes  56 ,  60  in the heart  14 , such that when the anchor  44  is implanted in the heart  14 , the-electrodes  56  and  60  are in contact with the heart  14 . In one embodiment, as is shown in  FIG. 2 , the anchor  44  is elongated and shaped like a bar or cylinder having a longitudinal axis X. However, the anchor  44  may take many shapes, examples of which are described further below.  
      The anchor  44  as shown includes a separation member  52  formed of a non-conductive material which electrically isolates a first electrically active surface or electrode  56  of the anchor  44  from a second electrically active surface or electrode  60  of the anchor  44 . In the present embodiment, the lead  22  is capable of bipolar pacing and sensing in which one of the electrodes  56 ,  60  is an anode and the other is a cathode. In other embodiments, however, the lead  22  includes a single electrode  56  for unipolar pacing and sensing. Alternately, the lead  22  includes a greater number of electrodes. The electrodes may be positioned on the lead  22  to stimulate or sense different layers of the heart  12 . In one embodiment, for example, the lead  22  includes four electrodes (not shown). The electrodes  56 ,  60  may also be referred to as electrically active areas or surfaces.  
      The body of the anchor  44  may be formed of a variety of biocompatible materials, including silicon rubber and polyurethane, and may be rigid or flexible. In one embodiment, the separation member  52  is formed of a non-conductive tubular structure, for example, PEEK (polyetheretherketone) tubing. The electrodes  56 ,  60  may be held in place adjacent the separation member  52  by variety of attachment means, including crimping, welding/fusing techniques, and adhesives, including, for example, epoxy, polyurethane, and silicone adhesives.  
      The anchor  44  is sized to be as small as possible yet still have a sufficient surface area at the electrodes  56  and  60  to make contact with the heart  14  for efficient sensing and pacing. In one embodiment, the anchor  44  has an outer diameter a of from about 1 French to about 6 French (0.333 to about 2 mm). In another embodiment, the outer diameter a of the anchor  44  is from about 1.5 French to about 2.5 French (0.5 to about 0.833 mm). In one embodiment, each electrode  56 ,  60  has a length of from about 0.5 mm to about 1.5 mm. In another embodiment, each electrode has a length of about 1 mm. In one embodiment, the electrodes  56 ,  60  are spaced apart from one another by from about 5 mm to about 9 mm. In another embodiment, the electrodes  56 ,  60  are spaced apart from one another by about 7 mm.  
      The electrodes  56 ,  60  may include surface treatments, coatings or other means for minimizing charge injection, maximizing sensing capabilities and/or helping to reduce pacing thresholds. Such surface treatments or coatings may include IROX (iridium oxide-coated titanium) or other compatible treatments. The electrodes  56 ,  60  may also be provided with a surface treatment or other means to increase electrode surface area.  
      In one embodiment, at least a portion of the anchor  44  is provided with a coating  62  of a therapeutic treatment or drug such as a steroid. The treatment or drug is thus delivered to the tissues of the heart  14  in the immediate locale in which sensing and pacing occurs, and also in the immediate locale in which the heart  14  may suffer trauma during implantation. Such steroids or other therapeutic treatments can therefore more effectively reduce inflammation and/or provide drug therapy.  
      In one embodiment, at least a portion of the anchor  44  is formed of a material chosen to reduce tissue ingrowth. One such exemplary material is ePTFE (expanded polytetrafluoroethylene). Reduced tissue ingrowth facilitates removal and repositioning of the lead  22 . In another embodiment, a portion of the anchor  44  is formed of a material chosen to enhance tissue ingrowth. Enhanced tissue ingrowth may provide improved fixation to the heart  14  and thus enhanced lead stability. In another embodiment, the anchor  44  is formed with features to facilitate mechanical attachment of the lead  22  to the heart  14 . For example, as is shown in  FIG. 2 , a channel  63  may extend through the anchor  44 . Following implantation, tissue will tend to invade the channel  63 , increasing fixation of the anchor  44  to the heart  14 . In other embodiments, the anchor  44  may be provided with tines or ridges increasing the amount of surface area to which tissue may attach (not shown).  
      The tether  48  is formed of an electrically conductive material and thereby communicates electrical signals from the pulse generator  18  to the electrodes  56  and  60  and vice versa. The proximal end  42  of the lead  22  and tether  48  is adapted for coupling to the pulse generator  18 . The distal end  26  of the tether  48  is mechanically coupled to the anchor  44  and electrically coupled to the electrodes  56  and  60 . This may be accomplished in a variety of ways. In one embodiment, as is shown in  FIG. 2 , the separation member  52  is provided with an access aperture  65  through which the tether  48  passes. This allows connection of the tether  48  to the electrodes  56 ,  60  inside of the anchor  44 . In other embodiments, the tether  48  is coupled to the anchor  44  and electrodes  56 ,  60  by, for example, crimping the tether  48  to the anchor  44  adjacent the electrodes  56 ,  60 .  
      The tether  48  is coupled to the anchor  44  between a first end  76  of the anchor  44  and a second end  78  of the anchor  44 . In this configuration, tension exerted on the tether  48  will tend to rotate or swing the anchor  44  from the first configuration in which anchor axis X is aligned with the tether  48  to the second configuration in which the anchor axis X is transverse to the tether  48 . This tendency facilitates anchoring of the anchor  44  in the myocardium  30  or against a surface such as the endocardium  34  or the epicardium  38 .  
      In the present embodiment, the tether  48  is formed of first and second cables  64  and  68  electrically isolated from one another and electrically coupled to the first and second electrodes  56  and  60 , respectively. The first and second cables  64 ,  68  may be separate, or, as is shown in  FIG. 2 , may be encased in an outer sheath  72  such that the tether  48  has a unitary construction. In one embodiment, the cables  64  and  68  are twisted cables. Twisted cables may have a reduced size and mass in comparison to the more common spiral- or coiled-type leads.  
      In one embodiment, the tether  48  has an outer diameter t of from about 0.8 French to about 3 French (0.267 to about 1 mm). In another embodiment, the outer diameter t of the tether  48  is about 1.5 French (0.5 mm). In one embodiment, as shown in  FIG. 2 , the outer diameter t of the tether  48  is no greater than one half of the outer diameter of the tether  48 . In one embodiment, as shown in  FIG. 2 , the outer diameter t of the tether  48  is substantially isodiametric, or has substantially the same outer diameter, along the length of the tether  48  from the distal end  26  of the tether  48  adjacent the anchor  44  to the proximal end  42  of the tether  48  (excluding any features provided on the proximal end of the tether  48  for coupling the lead  22  to a cardiac rhythm management device (not shown). Thus, substantially the entire lead  22  has a reduced outer diameter.  
      A lead having a reduced outer diameter provided several benefits. For example, the size of any passageway through the tissues of the heart  14  formed by the lead  22  is reduced, potentially reducing trauma to the patient during insertion. A lead  22  having a reduced outer diameter has a smaller outer surface area which reacts to the body, potentially reducing the formation of scar tissue about the lead  22 . A lead  22  having a reduced outer diameter, and in particular having a cable construction as shown in the accompanying figures has a reduced mass. Reduced mass, in conjunction with reduced outer diameter, provides a lead which has a reduced impact on blood flow and which is impacted less by blood flow. The lead  22  may be advanced into smaller vessels during implantation and may be implanted using smaller, less invasive tools in conjunction with minimally invasive surgical procedures. Tether  48  may also be more flexible because of the reduced outer diameter and simplified construction.  
      The cables  64  and  68  may be formed of a variety of conductive materials, including platinum-clad tantalum. In one embodiment, as is shown in  FIG. 2 , the cables  64 ,  68  include insulation, for example, a non-conductive coating so that the cables  64 ,  68  are electrically isolated from one another and from the patient&#39;s tissues. In other embodiments, however, the cables  64  and  68  may be positioned in isolated lumens of the sheath  72  or may be provided with other means for electrically isolating from one another and from the patient&#39;s tissues.  
      The anchor  44  may include features to facilitate implantation into the heart  14 . In one embodiment, a first end  76  of the anchor  44  is pointed and/or has a sharpened edge to dissect tissue. In other embodiments, the first end  76  of the anchor  44  is blunt or a combination of blunt and pointed (i.e., bullet shaped). The first end  76  of the anchor  44  may be shaped to puncture or pierce the epicardium  38  in such a way that trauma to the epicardium  38  and the amount of force needed to puncture or pierce the epicardium  38  is reduced.  
      A second end  78  of the anchor  44  may be adapted to cooperate with a tool maneuverable to implant the lead  22  in the heart  14 . In one embodiment, the anchor  44  includes a first tool engaging area  80  including a recess extending into the second end  78  of the anchor  44 . The second end  78  of the anchor  44  may also include a second tool engaging area forming a shoulder  86 . The tool engaging shoulder  86  may be sized and shaped such that the second end  78  of the anchor  44  can be inserted into the distal end of a tool. The anchor  44  may also include other types of tool engaging features, including, for example, surface irregularities, grooves, recesses, ridges, threads or other means for grasping by insertion tools.  
      It may be desirable to remove or revise the lead  22  after implantation. In one embodiment, the lead  22  includes a detachment means for detaching the distal end  26  of the tether  48  from the anchor  44 . In one embodiment, the tether  48  has a weak region  97  proximally adjacent to the anchor  44 . Upon the exertion of sufficient tension on the tether  48  by the surgeon, the weak region  97  fails and the tether  48  separates from the anchor  44 , leaving the anchor  44  safely encapsulated within the myocardium  30 .  
      In one embodiment, as is shown in  FIG. 3 , the connection between the anchor  44  and tether  48  is modified to facilitate detaching the tether  48  from the anchor  44  after implantation. Instead of or in addition to the inclusion of weak region  97  as is shown in  FIG. 2 , the tether  48  is coupled to the anchor  44  closer to the second  78  than the first end  76 , or vice versa. The tether  48  is still positioned relative to the first and second ends  76  and  78  such that tension exerted on the tether  48  tends to deploy the anchor  44  from the first configuration to the second configuration (i.e., the anchor is rotated from a longitudinally oriented position to a transverse position). The tension, however, is transferred from the tether  48  to the anchor  44  slightly unevenly, reducing the minimum tension required to separate the tether  48  from the anchor  44 .  
      In one embodiment, as is shown in  FIG. 4 , the detachment means is a notch  81  is formed in the anchor  44  between the first and second ends  76 ,  78  and aligned with connection of the tether  48  to the anchor  44 . The notch  81  provides a fold or stress line to facilitate removal or revision of the anchor  44 . Upon sufficient tension being exerted on the tether  48 , the anchor  44  will tend to buckle along the notch  81 , allowing for easier removal of the anchor  44  from the heart  14 .  
      In one embodiment, as is shown in  FIG. 5 , the anchor  44  is curved. The degree of curvature may be chosen to facilitate insertion through the tissues of the heart  14 . The degree of curvature may also be chosen to conform to the geometry of a surface of the heart  14 .  
       FIGS. 6-12  show the lead  22  according to various embodiments of the present invention. Similar parts in relation to the embodiments shown in  FIGS. 2-5  are given similar numbering with the addition of “a”, “b”, “c”, etc. In general, a lead according to the present invention may employ any combination of some or all of the features discussed herein, and the invention is not limited to any particular combination as shown or discussed. In each of these embodiments, as further discussed below, the anchor  44  may be implanted in the myocardium  30  or positioned to engage the epicardium  38  or the endocardium  34 .  
       FIG. 6  shows a lead  22   a  according to another embodiment of the present invention. As is shown in  FIG. 6 , the cables  64   a ,  68   a  may first be threaded through an aperture  65   a  in the anchor  44   a  and then coiled around opposite ends  76   a  and  78   a  of the anchor  44   a . The distal tips of the cables  64   a  and  68   a  are secured to the anchor  44   a  to securely couple the tether  48   a  to the anchor  44   a . The ends of the cables  64   a ,  68   a  which travel around the anchor  44   a  to form coils are exposed, thus serving as the electrodes  56   a  and  60   a . A portion of the cables  64   a ,  68   a  thus form the electrodes  56   a ,  60   a . Furthermore, the coils may be arranged to provide increased electrode surface area. Because the cables  64   a  and  68   a  are at least partially exposed, they should be formed of a biocompatible material. Exemplary materials include, but are not limited to, platinum-clad tantalum.  
       FIG. 7  shows a lead  22   b  according to another embodiment of the present invention. Lead  22   b  is similar to the lead  22   a  shown generally in  FIG. 6  in that exposed coils the cable  64   b  forms an electrode  56   b . In addition, the anchor  44   b  is provided with a recessed area  85   b  around which the exposed region of the cable  64   b  is coiled. Furthermore, lead  22   b  is configured as a unipolar lead and includes a single electrically active area or electrode  56   b.    
       FIG. 8  shows a lead  22   c  according to another embodiment of the present invention. As is shown in  FIG. 8 , a first electrode  56   c  and a second electrode  60   c  are located on the tether  48   c  proximal to the anchor  44 . The tether  48   c  includes a first cable  64   c  mechanically connected to the anchor  44   c  and electrically coupled to the first electrode  56   c . The tether  48   c  further includes a second cable  68   c  electrically coupled to a second electrode  60   c  proximal to the first electrode  56   c . The second electrode  60   c  includes a through-hole  79   c  through which the first cable  64   c  extends. The first and second cables  64 ,  68  are insulated so that the first cable  64   c  is not electrically coupled to the second electrode  60   c  as it passes through the through-hole  79   c.    
      In one embodiment, as is shown in  FIG. 8 , an outer sheath  72   c  is disposed around the tether  48   c  proximal to the second electrode  60   c  and between the first and second electrodes  56   c ,  60   c  and between the first electrode  56   c  and the anchor  44   c . The sheath  72   c  may also be mechanically coupled to the anchor  44   c  to improve the strength of the connection of the tether  48   c  to the anchor  44   c . The anchor  44   c  may be deployed onto a surface of the heart  14  such as the endocardium  34  or the epicardium  38  or within the myocardium  30  such that the electrodes  56   c ,  60   c  are positioned within the myocardium  30 . The spacing between the anchor  44   c  and the first electrode  56   c  and between the first electrode  56   c  and the second electrode  60   c  may be chosen to position the electrodes  56   c  and  60   c  within a particular region of the myocardium  30  or to accommodate a myocardium  30  having an unusual thickness.  
       FIGS. 9A and 9B  show a lead  22   d  according to another embodiment of the present invention in which electrodes  56   d  and  60   d  are located on the tether  48   c  proximal to the anchor  44   c . In the present embodiment, the cables  64   d ,  68   d  that are electrically coupled to the respective electrodes remain separate from one another. The electrodes  56   d  and  60   d  may be coupled to the cables  64   d  and  68   d , as shown in  FIG. 9A . Alternately, as shown in  FIG. 9B , the electrodes  56   d  and  60   d  may be no more than exposed regions of the cables  64   d  and  68   d . The electrodes  56   d  and  60   d  are spaced apart from one another along the length of the respective cables  64   d ,  68   d  so as to avoid contacting one another and, if desired, to pace or sense different areas of the myocardium  30 .  
       FIGS. 10A and 10B  show a lead  22   e  according to another embodiment of the present invention. An anchor  44   e  of the lead  22   e  has at least a first movable wing or tab  87   e , and optionally has two wings  87   e  as shown in  FIGS. 10A and 10B . The wings  87   e  are deployable from a first configuration in which the wings  87   e  lie flat or are aligned with a longitudinal axis x of the anchor  44   e , as is shown in  FIG. 10A , to a second or expanded configuration in which the wings  87   e  are spread outwardly or away from the anchor  44   e , as is shown in  FIG. 10B . The tether  48   e  is coupled to a second end  78   e  of the anchor  44   e  such that upon tensioning the tether  48   e  in a proximal direction, the anchor  44   e  slides slightly proximally, causing the wings  87   e  to catch on the tissues of the heart  14  and deploy outwardly. When in the second configuration, the wings  87   e  prevent proximal migration of the anchor  44   e  within the heart  14 . Electrodes  56   e  and  60   e  may be positioned on the anchor  44   d  proximal to the wings  87   e  so as to contact the heart  14  when the anchor  44   e  is deployed. In other embodiments, the electrodes  56   e  and  60   e  may be located on the wings  87   e  or on the tether  48   e.    
      In other embodiments of the invention, the lead  22   e  may include other fixation means such as spines, barbs, tabs. These and any other fixation means may be provided in greater numbers and at different locations on the anchor  44   e.    
       FIG. 11  shows a lead  22   f  according to another embodiment of the present invention in which the wings  87   f  are biased to an outwardly protruding configuration. The bias increases the likelihood of the anchor  44   f  successfully deploying to the second configuration. In one embodiment, the wings  87   f  are coupled to a tensioning device  89   f  extending proximally and accessible outside of the body following implantation of the lead  22 . The tensioning device  89   f  may be coupled to one or more of the wings  87   f  such that tensioning of the tensioning device  89 f causes the attached wings  87   f  to collapse to the first, collapsed configuration. The tensioning device  89   f  may be employed to reposition, re-deploy or remove the anchor  44   f . The lead  22   f  may further include a webbing or mesh  91   f  attached to the anchor  44   f  and to the wing or wings  87   f . The mesh  91   f  folds in on itself when the attached wings  87   f  are in the first collapsed configuration and stretch between the anchor  44   f  and the attached wings  87   f  when the anchor  44   f  is in the second, expanded configuration. When folded, the mesh  91   f  may reduce tissue ingrowth at the wing  87   f , which might interfere with collapse of the wings  87   f . The mesh could be made of, for example, rubber or ePTFE.  
       FIG. 12  shows a lead  22   g  according to another embodiment of the present invention in which the anchor  44   g  is adapted to be flexible. As shown in  FIG. 12 , the anchor  44   g  is constructed of an inner coiled member  90   g  located in an outer tube  92   g . The coil structure provides flexibility to the anchor  44   g  to facilitate deployment and any subsequent repositioning of the anchor  44   g . The anchor  44   g  further includes a tool-receiving recess  80   g  for receiving a tool, such as a stylet, to facilitate implantation. The tool may provide additional rigidity to the anchor  44   g  during implantation. The anchor  44   g  may also include other features as previously discussed, including a dissecting or cutting edge at a first end  76   g  of the anchor  44   g , a pair of electrodes  56   g  and  60   g , and a tether  48   g.    
      Where the anchor  44  is intended to be located on a surface of the heart  14 , such as the endocardium  34  or the epicardium  38 , the lead  22  may also include means for chronically engaging the anchor  44  against the surface. This may be accomplished by fixing the tether  48  to the heart  14 .  
       FIG. 13  shows the lead  22  implanted such that the anchor  44  engages the endocardium  34 . The lead  22  is further provided with a retention feature  94  for fixing the anchor  44  into position. In the present embodiment, the retention feature  94  is slidable over the tether  48  so as to abut the epicardium  38 . The retention feature  94  may have various configurations, including a button-like feature or a cylinder, and may be formed of various materials, including, for example, silicone rubber. The distal end of the tether  48  may include an engaging feature  96  such as tines or ridges for engaging the retention feature  94 . The engaging features  96  facilitate a secure connection between the tether  48  and the retention feature  94  and reduce slippage between the tether  48  and the retention feature  94 .  
      The retention feature  94  may be advanced over the tether  48  using a catheter or other tool. The tether  48  is tensioned and the retention feature  94  is fixed to the tether  48  adjacent the epicardium  38  to retain the portion of the tether  48  between points A and B in a tensioned state. The tension exerted by the tether  48  causes the anchor  44  to engage the endocardium  34  at point A, reducing lead migration.  
      The lead  22  may include other retention features or means for retaining the anchor  44  against a surface of the heart  12 . For example, in other embodiments, the tether  48  may be sutured to a surface of the heart  12  to hold the anchor  44  tensioned against the heart  12 . In still other embodiments, the lead  22  is pre-shaped to resist migration. For example, the distal end of the tether  48 , i.e., that portion of the tether  48  that would be located within the myocardium  30 , may be crimped, bent or otherwise shaped so as to resist migration (not shown).  
       FIGS. 14A and 14B  show another embodiment of the present invention, in which a tool  100  is provided for implanting the lead  22  into the heart  14 . The tool  100  includes an injection mechanism  102  and an elongated tool body  104 . The tool body  104  includes a nest  106  provided at a distal end  108  of the tool body  104  into which the anchor  44  is at least partially inserted. As previously described, the anchor  44  may include a tool engaging shoulder  86  or other means adapted for seating the anchor  44  within the nest  106 . The tether  48  may extend outside of the tool body  104 , as is shown in  FIG. 14A , or may be threaded within a lumen  110  extending through the tool body  104  proximally from the nest  106 .  
      The injection mechanism  102  may be a stylet or other plunger-like device that is inserted into the lumen  110  of the tool body  104  so that a distal end  112  is positioned proximally adjacent to the nest  106 . The injection mechanism  102  may be inserted into the tool body  104  prior to or during the implantation procedure to provide the tool body  104  with additional support, or may be inserted into the tool body  104  later so that the tool body  104  remains flexible during the implantation procedure.  
      To implant the lead  22  into the heart  14 , the anchor  44  is inserted into the nest  106 . The distal end  108  of the tool body  104  is then advanced to the heart  14 , using the first end  76  of the anchor  44  to cut or dissect the heart  14 . The anchor  44  creates a tract  114  through the heart  14  within which the tool body  104  and the tether  48  extend proximally from the anchor  44 .  
      When the anchor  44  has been placed in an appropriate site for sensing and pacing, the injection mechanism  102  is actuated to eject the anchor  44  from the nest  106  and inject the anchor  44  into the heart  14 . In the present embodiment, the injection mechanism  102  is actuated when the surgeon slides the stylet distally through the lumen  110  relative to the tool body  104  and engages the anchor  44  at the tool engaging area  80 . Doing so displaces the anchor  44  from the nest  106 , injecting the anchor  44  into the heart  14 . The tool  100  is then withdrawn proximally, leaving the anchor  44  in the implantation site and the tether  48  extending proximally through the tract  114 . Tension may be exerted on the proximal end  42  of the tether  48  to deploy the anchor  44  from the first, collapsed configuration in which the anchor  44  is positioned longitudinally within the tract  114  to the second, or expanded configuration in which the anchor  44  is positioned transverse to the tract  114  (See  FIG. 14B ). The withdrawal of the tool  100  may be sufficient to tension the tether  48 , particularly if the tether  48  is threaded through the tool body lumen  110 . However, the surgeon may also manually tension the tether  48  by grasping the proximal end  42  of the tether  48  and pulling gently.  
      In the second configuration the anchor  44  is wider than the tract  114 , helping to secure the anchor  44  in position and resist repositioning and migration. Following implantation of the anchor  44 , the tether  48  is coupled to the pulse generator  18  to provide sensing and pacing of the heart  14  (See  FIG. 1 ).  
       FIGS. 15A and 15B  show a tool  200  for injecting the anchor  44  into the heart  14  according to another embodiment of the present invention. In the present embodiment, the tool  200  is adapted for automated injection of the anchor  44  into the heart  14 . By automatic it is meant that the tool  200  allows the surgeon to inject the anchor  44  into the heart  14  with a pre-determined force and/or distance rather than by “feel” or estimation alone.  
      In the present embodiment, the tool  200  includes an injection mechanism  202  operably coupled to an elongated tool body  204  for holding and maneuvering the lead  22  into the heart  14 .  
      The tool body  204  has a proximal end  203  adapted for grasping and manipulation by the surgeon, and may include a proximal handle or other grip  205 . The tool body  204  has a distal end  208  provided with a nest  206  sized and shaped to securely hold at least a portion of the anchor  44 . In one embodiment, as is shown in  FIG. 15A , the nest  206  is sized so that a tool engaging shoulder  86  of the anchor  44  is inserted into the nest  206  while the first end  76  of the anchor  44  protrudes from the nest  206 . Also shown in the  FIG. 15A  is an embodiment of the anchor  44  in which the first end  76  of the anchor  44  is blunted or made dull to facilitate dissection rather than cutting as the anchor  44  passes through the heart  14 . In other embodiments, the tool  200  may include a cutting, needle-like or dissecting edge of its own and the anchor  44  may be fully received in the nest  106  and/or may lack a cutting or dissecting feature of its own.  
      The tool body  204  has a first lumen  210  extending proximally from the nest  206  in which the injection mechanism  202  is located. As is shown in  FIG. 15B , the tool body  204  optionally has a second lumen  216  extending through the tool body  204  for holding the tether  48 . A distal end of the second lumen  216  is slit open to the first lumen  210  to accommodate the connection between the tether  48  and the anchor  44 .  
      The tool body  204  optionally includes additional or auxiliary lumens adapted for providing visualization or endoscopic tools, fluids, or other devices access to the heart  14 . In the embodiment shown generally in  FIGS. 15A and 15B , the tool body  204  includes first and second auxiliary lumens  218  and  220 .  
      The injection mechanism  202  is positioned within the first lumen  210  proximal to the nest  206  and includes a coiled member  222  coupled to a plunger  224 . In other embodiments, the coiled member  232  may be replaced with one or more elastic members capable of stretching and recoiling. A distal end  226  of the coiled member  222  is fixed to the tool body  204  proximal to the nest  206  with a coil ring  228 . A proximal end  230  of the coiled member  222  is free to move within the first lumen  210  relative to the tool body  204 . The plunger  224  extends through the center of the coiled member  222  and is coupled to the proximal end  223  of the coiled member  222 . A proximal end  232  of the plunger  224  extends from the proximal end  203  of the tool body  204  and may have a gripping region  234  for easy manipulation by the surgeon.  
      The tool  200  is operable to inject the anchor  44  into the heart  14  as follows. First, the anchor  44  is loaded into the nest  206  with the tether  48  either extending outside of the tool body  204  or threaded into the second lumen  216 . The surgeon then manipulates the proximal end  203  of the tool body  204  to advance the distal end  208  of the tool body  204  to the heart  14 .  
      To inject the anchor  44  into the heart  14 , the surgeon grasps the grip  234  at the proximal end  232  of the plunger  224  and withdraws the plunger  224  proximally, causing the coiled member  222  to stretch between the coil ring  228  and the proximal end  232  of the plunger  224 , becoming tensioned or loaded. Upon a sudden release of the plunger  224 , the coiled member  222  relaxes, sliding the plunger  224  through the tool body  204  distally through the lumen  210 . The plunger  224  engages the anchor  44 , ejecting the anchor  44  out of the nest  206  and injecting the anchor  44  into the heart  14  distal to the tool  200 . The tool  200  is then withdrawn proximally over the tether  48 . Doing so tensions the tether  48 , causing the anchor  44  to deploy from the first configuration to the second configuration. The anchor  44  is now resistant to further proximal dislocation, so that as the tool  200  is withdrawn fully the tether  48  is extracted from the second lumen  216 .  
      The size and tensioning force of the coiled member  222  may be chosen to provide sufficient injection force so as to cause the anchor  44  to puncture, for example, the epicardium  38 , which is known to be a particularly tough tissue. It is believed that by puncturing the epicardium  38  in a single, somewhat forceful action, trauma to the epicardium  38  and to the heart  14  as a whole may be reduced. The shape of the first end of the anchor  44  may also be adapted to facilitate accessing tissues of the heart  14 , as described previously. In addition, the diameter of the anchor  44  is such that the size of any opening through the epicardium  38  or other tissues of the heart  14  is reduced as well.  
      In one embodiment, the tool  200  includes an injection limiter  240  for preventing over injection of the anchor  44  into the heart  14 . In the present embodiment, the injection limiter  240  is a cooperating flange structure located on the plunger  224  and the tool body  200  that functions to prevent excessive loading of the coiled member  222  and forward movement of the plunger  224  beyond a pre-determined location. In this manner, the plunger  224  may only be withdrawn as far proximally as a proximal flange  242  and may only advance distally as far as a distal flange  244 .  
      In one embodiment, the tool  200  may also include gradations or other indicia  246  to guide the surgeon. The indicia  246  may relate to the length over which the plunger  224  is withdrawn proximally, potential injection force of the coiled member  222 , the distance the anchor  44  is likely to be injected, or other factors that may help the surgeon to implant the anchor  44  in a chosen location.  
       FIG. 16  shows a tool  300  for injecting the lead  22  into the heart according to another embodiment of the present invention. In the present embodiment, the tool  300  includes an injection mechanism  302  operably coupled to an elongated tool body  304  for holding a lead  22 .  
      The tool body  304  has a proximal end  303  adapted for grasping and manipulation by the surgeon, and may include a proximal handle or other grip  305 . The tool body  304  has a distal end  308  provided with a nest  306  sized and shaped to securely hold at least a portion of the anchor  44 .  
      The tool body  304  has a first lumen  310  extending proximally from the nest  306  in which the injection mechanism  302  is located. The injection mechanism  302  is positioned within the first lumen  310  proximal to the nest  306  and includes a coiled member  322  coupled to a plunger  324 . In contrast to the previous embodiment, the coiled member  322  is configured such that compression of the coiled member  322  causes the coiled member  322  to become loaded.  
      A proximal end  230  of the coiled member  322  is connected to the grip  305 . The grip  305  is provided with cleats  350  for engaging detents  352  formed on the proximal end  303  of the tool body  304  and a hasp  354  for securing the proximal end  42  of the tether  48 . Each detent  352  may be provided with indicia for indicating the distance separating them or indicating the distance or force through which the anchor  44  will be injected.  
      The distal end  326  of the coiled member  322  is free to slide within the first lumen  310  and terminates in a plunger  329 . A tension wire  356  extends from the plunger  329  through the center of the coiled member  322  to the handle  305  and is sized to retain the coiled member  322  in a loaded state.  
      To implant the lead  22  into the heart  14 , the anchor  44  is inserted into lumen  310  at the proximal end  303  of the tool body  304 . The coiled member  322  is inserted into the first lumen  310  proximal to the anchor  44  and advanced distally to push the anchor  44  into the nest  306 . The cleats  350  are inserted into a proximal detent  352  to secure the handle  305  to the tool body  304 . The tether  48  is placed in the hasp  354  to secure the anchor  44  to the tool  100 , but with a slight amount of slack.  
      The tool  300  is then inserted into the body and maneuvered to position the nest  306  at the desired implant location. The handle  305  is then pushed distally relative to the tool body  304  into a more distal cleat  352 . The plunger  329  engages the anchor  44 , ejecting the anchor  44  out of the nest  306  with sufficient force to inject the anchor  44  into the heart.  
      The spacing between the cleats  352  may be chosen to correspond to the injecting distance and may be regular or irregular. Furthermore, two or more distal cleats  352  may be provided to allow for different injection distances. Finally, the tether  48  is freed from the hasp  354  and the tool  300  is withdrawn proximally. The tether  48  is tensioned and the anchor  44  is deployed to the second configuration.  
      Because the injection mechanism  302  employs a flexible coiled member  322  rather than a solid plunger  329  to inject the anchor  44  from the nest  306 , the tool  300  remains flexible and is operable to inject the anchor  44  even when the tool body  304  is bent or curved.  
      The previously described tools are operable in combination with a lead according to any embodiment of the present invention. Furthermore, the tool may be inserted into the myocardium  30  and the anchor  44  injected therefrom. Alternately, the tool may be positioned at or near a surface of the heart  14  such as the epicardium  38  and the anchor  44  injected into the myocardium  30  from that surface.  
      The lead  22  may be injected out of a tool into the endocardium  34  or epicardium  38  by transvenous, thoracoscopic, pericardioscopic or other transthoracic means. The above described procedure may be performed in a minimally invasive procedure such that the anchor  44  is injected into the heart  14  from the outside of the heart  14 . The tool body  300  may be correspondingly sized and shaped to access the heart  14  from, for example, a sub-xiphoid insertion location. The tool body may be rigid or semi-rigid to provide sufficient support while advancing the tool towards the heart  14 . Furthermore, the tool may be employed in conjunction with a dilator to provide access to the heart  14 .  
      The anchor  44  may be injected into the heart  14  such that the anchor  44  passes through the epicardium  38  and is positioned within the myocardium  30 . As shown in  FIGS. 14A and 14B , the anchor  44  is injected into the myocardium- 30  so that both electrodes  56  and  60  are disposed within the myocardium  30  for stimulating the myocardium  30 . In one embodiment, the anchor  44  is injected a minimum distance I of one and a half times a length L of the anchor  44  into the myocardium  30 .  
      In others embodiments, the anchor  44  may be injected still further through the myocardium  30  such that the anchor  44  traverses the endocardium  34  and is positioned within a chamber of the heart  14 , such as the left ventricle. Upon tensioning of the tether  48 , the anchor  44  is forced into contact with the endocardium  34  for endocardial stimulation. According to still other embodiments, the anchor  44  may be injected through the epicardium  38 , the myocardium  30  and back out of the epicardium  38  for epicardial pacing.  
      The above described procedure may also be employed in a transvenous approach to the heart  14 . In this embodiment, the tool body is correspondingly sized and shaped to allow the surgeon to manipulate the tool body through a series of vessels into the heart  14  from a more distal insertion location, such as the subclavian vein, into the coronary sinus. The tool may then be advanced into a lateral vein of the coronary sinus and the lead  22  injected through the epicardium  38 . When employed in a transvenous approach to the heart  14 , the tool body may be flexible. In one embodiment, the tool body is a catheter.  
      Therapeutically, the scope of the apparatus and method described herein may be employed for implantation of device specialized for resynchronization therapy and bradycardia therapy, and all heart chambers may be implanted with various embodiments. Furthermore, the present invention is not limited to implantation of a lead within the heart  14 . Rather, the apparatus and methods generally described herein may also be used for neural applications.  
      Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.