Patent Publication Number: US-2011071609-A1

Title: Biasing and fixation features on leads

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/548,838, filed Oct. 12, 2006, which is a continuation in part of U.S. application Ser. No. 11/424,440, filed Jun. 15, 2006, all of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to medical devices and devices for securing a lead. More specifically, the invention relates to devices and methods for positioning and fixing a lead within a vessel of the heart. 
     BACKGROUND 
     Implantable medical devices for treating irregular contractions of the heart with electrical stimuli are well known in the art. Some of the most common forms of such implantable devices are defibrillators and pacemakers. Various types of electrical leads for defibrillators and pacemakers have been suggested in the prior art. 
     A broad group of leads may be characterized by the fact that they are placed transvenously. These leads are introduced into the patient&#39;s vasculature at a venous access site and travel through veins to the locations where the leads&#39; electrodes will implant in or otherwise contact coronary tissue. One large subfamily of the group of transvenously-placed leads are those that are implanted in the endocardium (the tissue lining the inside of the heart) of the right atrium or ventricle. Another subfamily of the group of transvenously-placed leads are those that are placed in the branch vessels of the coronary venous system to stimulate the left ventricle. 
     The treatment of heart failure often requires left ventricular stimulation either alone or in conjunction with right ventricular stimulation. For example, cardiac resynchronization therapy (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 advance a lead endovenously into the coronary sinus and then advance the lead through a branch vein onto the surface of the left ventricle so as to stimulate the myocardium of the heart. Although methods and tools have been developed to navigate the lead through the vasculature, and in particular to direct the lead into a selected branch vessel of the coronary sinus, it can be difficult to orient the electrodes to face and stimulate the myocardium. If the electrodes come into contact with the pericardial wall portion of the branch vessel, rather than the myocardial wall portion, a degraded site for sensing and pacing may result. 
     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 branch vessel. Over time, the electrodes may lose contact with the heart muscle, or move from their original location and orientation. 
     There is a need for an improved lead and method of implantation for orienting the lead into the coronary sinus branch vessels such that the lead electrodes contact the myocardium, and also to provide controlled fixation and removal of the lead. 
     SUMMARY 
     In one embodiment, the present invention is a lead assembly for placement in a coronary vessel of the heart. The coronary vessel has a pericardial wall portion and a myocardial wall portion. The lead assembly comprises a lead body extending from a proximal end adapted for coupling to a pulse generator to a distal end adapted for implantation in the heart, an electrode positioned at the distal end of the lead body, and a loop biasing feature located at the distal end of the lead body. The loop biasing feature includes a resilient loop positioned to bias a portion of the electrode towards the myocardial wall portion of the coronary vessel by exerting a force against the pericardial wall portion. The loop biasing feature further includes a collar for coupling the loop biasing feature to the lead body. 
     In another embodiment, the present invention is a lead assembly for placement in a coronary vessel of the heart. The coronary vessel has a pericardial wall portion and a myocardial wall portion. The lead assembly comprises a lead body extending from a proximal end adapted for coupling to a pulse generator to a distal end adapted for implantation in the heart. The lead body includes a lumen extending from the proximal end to the distal end. An electrode is positioned at the distal end of the lead body. A loop biasing feature is located at the distal end of the lead body. The loop biasing feature includes a resilient loop positioned to bias a portion of the electrode towards the myocardial wall. A cord is coupled to the loop and extends to the proximal end of the lead body. A tensile force applied to the cord causes the loop to flatten towards the lead body and a portion of the loop to slide into the lumen. 
     In yet another embodiment, the present invention is a method of implanting a lead in a coronary vessel of the heart. The coronary vessel has a pericardial wall portion and a myocardial wall portion. The method comprises providing a lead body extending from a proximal end adapted for coupling to a pulse generator to a distal end adapted for implantation in the heart, an electrode positioned at the distal end of the lead body, and a loop biasing feature located at the distal end of the lead body. The loop biasing feature includes a resilient loop. The resilient loop is compressed towards the lead body by inserting the lead into a guide catheter. The distal end of the lead body is advanced into the coronary vessel to a fixation location. The electrode is biased towards the myocardial wall portion of the coronary vessel by engaging the loop biasing feature. 
     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 in relation to a heart. 
         FIG. 2  shows a side view of the distal end of a lead assembly according to one embodiment of the present invention in relation to a branch vessel of the coronary sinus. 
         FIG. 3  shows a side view of the distal end of the lead assembly of  FIG. 2  detailing tissue in-growth. 
         FIG. 4  shows a side view of the distal end of the lead assembly of  FIG. 3  partially removed from the branch vessel. 
         FIGS. 5A-5D  show a loop biasing feature according to alternative embodiments of the present invention. 
         FIG. 6  shows a side view of the distal end of a lead assembly according to another embodiment of the invention. 
         FIG. 7  shows a side view of the distal end of a lead assembly according to yet another embodiment of the invention. 
         FIGS. 8A-8B  show a loop biasing feature according to yet another alternative embodiment of the present invention. 
         FIGS. 9A-9C  show a loop biasing feature according to an alternative embodiment of the present invention. 
         FIG. 10  shows a side view of a distal end of a lead assembly according to one embodiment of the present invention positioned within a guide catheter. 
         FIG. 11  shows a side view of a distal end of a lead assembly according to another embodiment of the present invention positioned within a guide catheter. 
         FIG. 12A  shows a side view of a lead assembly according to yet another embodiment of the invention. 
         FIG. 12B  shows a cross-sectional view of the lead assembly of  FIG. 12A  taken along line  12 A- 12 A. 
         FIG. 13  shows a side view of a distal end of a lead assembly according to another embodiment of the invention. 
         FIG. 14  shows a side view of a distal end of a lead assembly according to another embodiment of the present invention. 
         FIG. 15  shows a side view of a distal end of a lead assembly 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  is a schematic drawing of a cardiac rhythm management system  5  including a pulse generator  8  coupled to an exemplary lead assembly  10  deployed in a patient&#39;s heart  12  from a superior vena cava  13 . As shown, the heart  12  includes a right atrium  15  and a right ventricle  16 , a left atrium  17  and a left ventricle  18 , a coronary sinus ostium  19  in the right atrium  15 , a coronary sinus  21 , and various cardiac vessels including a great cardiac vein  23  and other branch vessels of the coronary sinus  21  including an exemplary branch vessel  25 . 
       FIG. 2  shows a portion of the lead assembly  10  according to one embodiment of the present invention. The lead assembly  10  is shown implanted in the branch vessel  25 . The branch vessel  25  has a myocardial wall portion  26   a  nearer to a myocardium  26  of the heart  12  and a pericardial wall portion  27   a  nearer to a pericardium  27  of the heart  12 . The lead assembly  10  includes a lead body  28  extending from a proximal end  29  (see  FIG. 1 ) adapted for coupling to the pulse generator  8  to a distal end  30  adapted for insertion into the heart  12 . An electrode  31  is positioned at the distal end  30  of the lead body  28  for pacing and sensing electrical stimuli. While the lead assembly  10  is shown as a monopolar-type lead having a single electrode  31 , it is also contemplated that one or more electrodes may be positioned on the lead body  28  to allow for unipolar, bipolar or multi-polar pacing and sensing. It is also contemplated that one or more electrodes may be positioned on the lead body  28  to allow pacing and sensing at a selected electrode in a preferred position. 
     The lead assembly  10  further includes a loop biasing feature  32  at the distal end  30  of the lead body  28 . In the embodiment generally shown in  FIG. 2 , the loop biasing feature  32  includes a resilient loop  34  of material protruding from the lead body  28 . As shown in  FIG. 2 , the loop  34  forms a closed curve with the lead body  28 . The size of the lead biasing feature  32  and the loop  34  may vary with respect to the size of the lead assembly  10 . In one embodiment, the loop  34  extends a distance between approximately 0.003 and 0.250 inches from the lead body  28 . In one embodiment, the loop  34  has a radius between approximately 0.025 and 10.0 inches. The loop  34  may have an included angle between approximately 10 and 179 degrees. 
     The loop  34  is resilient. It rebounds or springs back into shape after bending or being compressed. The resilient loop  34  allows the loop biasing feature  32  to exert a force against the pericardial wall portion  27   a . In one embodiment, the loop biasing feature exerts a force between 1 and 800 grams when fully compressed. The force exerted by the loop biasing feature  32  when the loop  34  is extended is a function of the vessel size and the loop shape and size. The loop biasing feature  32  may be formed from a material having a predetermined shape. The loop biasing feature  32  may be made of a variety of materials, including, for example, molded or extruded silicone rubber, polyurethane or other polymeric materials. The loop biasing feature  32  may also be made of a flexible coil, cable or wire, coated or uncoated with a material as described above. In other embodiments, the loop biasing feature  32  can be made of any material and have any shape that is capable of exerting a force against the pericardial wall portion  27   a.    
     In the embodiment illustrated in  FIG. 2 , the loop biasing feature  32  further includes a collar  38  coupling the loop  34  to the lead body  28 . The collar  38  is initially slidable along the lead body  28  so as to selectively position the loop  34  at various locations along the lead body  28 , as well as to permit addition and/or removal of the loop biasing feature  32  from the lead body  28 . In the illustrated embodiment, a groove  47  is formed in the lead body  28  for receiving the collar  38  and retaining the collar  38  in position. The groove  47  may be sized as shown such that the collar  38  is isodiametric with the remainder of the lead body  28 . In other embodiments, an adhesive or other fixator (not shown) may be employed to fix the collar  38  to the lead body  28 , or the loop biasing feature  32  may be otherwise mechanically coupled to the lead body  28 , or integrally formed with the lead body  28 . 
     The loop biasing feature  32  protrudes from the lead body  28  in such a manner as to frictionally engage the pericardial wall portion  27   a  of the branch vessel  25  as shown in  FIG. 2 . The loop biasing feature  32  thus biases the electrode  31  away from the pericardial wall  27   a  and towards the myocardial wall  26   a  of the branch vessel  25 . In addition, the loop biasing feature  32  increases the frictional force between the lead body  28  and the branch vessel  25 , thus helping to fix the lead body  28  within the branch vessel  25 . 
     In the illustrated embodiment, the loop biasing feature  32  is positioned adjacent to the electrode  31  so as to bias a portion of the electrode  31  opposite the loop  34  towards the myocardial wall  26   a . In other embodiments, however, one or more loop biasing features  32  may be positioned at various locations on the lead body  28 , not necessarily adjacent to the electrode  31 , so as to bias the one or more electrodes  31  towards the myocardial wall  26   a . In other embodiments, the loop biasing features  32  may be positioned at the same distal location on the lead body  28  (as shown in FIGS.  8  and  9 A- 9 C) or may be staggered along the length of the lead body  28  (not shown). The position of the loop biasing feature  32  may therefore be selected to take advantage of the complex shape of the branch vessel  25  so as to bias and fix the electrode  31  towards the myocardial wall  26   a . In other embodiments, a loop biasing feature  32  may include one or more loops  34 , one or more collars  38 , or any variation in the number of loops  34  and collars  38 . 
     The loop biasing feature  32  defines a tissue in-growth area  48  between the lead body  28  and the loop  34 . The tissue in-growth area  48  is an open region into which scar tissue or clotting material may grow upon implantation, further fixing the lead assembly  10  in place.  FIG. 3  shows tissue  49 , which has grown into the tissue in-growth area  46 . In some embodiments, a pharmaceutical agent, such as a clotting agent, or other therapeutic treatment  52  is embedded or coated onto the loop biasing feature  32 , as shown in  FIGS. 2 and 3 , or nearby on the lead body  28 , to facilitate tissue in growth on and about the tissue in-growth area  48 . One such exemplary clotting agent is the QuikClot® brand hemostatic agent (available from Z-Medica Corporation of Wallingford, Conn.). The loop  34  can be coated with or encapsulated by a drug so as to be a delivery mechanism for delivering drugs or other therapeutic treatments such as steroids to the heart  12  (not shown). In  FIG. 3 , the tissue  49  is shown completely extending from the pericardial wall  27   a  to the lead body  28  in the area of the loop  34 , but in other embodiments, the tissue  49  need not extend completely from the pericardial wall  27   a  to the lead body  28 . The tissue  49 , for example, may partially encompass the loop  34 . In other embodiments, the lead assembly  10  may further include a drug collar  56  on the lead body  28  for delivering drugs or other therapeutic treatments such as steroids to the heart  12 . 
     In the illustrated embodiment, the loop biasing feature  32  includes an optional necked down region  46  of the loop  34  connecting the loop  34  to the collar  38 . As shown in  FIGS. 2 and 3 , the necked down region  46  of the loop  34  is thinner than the remainder of the loop  34 , so as to break at a pre-determined axial load. In one embodiment, the predetermined axial load can be between 1 and 800 grams. In another embodiment, the predetermined axial load can be 100 grams. By breaking the loop  34 , as illustrated in  FIG. 4 , the biasing and frictional force between the lead assembly  10  and the branch vessel  25  is reduced and the loop  34  can be pulled out of the tissue  49  surrounding the loop  34 . This can be used to disengage the loop biasing feature  32  from the branch vessel  25  to facilitate removal of the lead assembly  10 . In the embodiment generally illustrated in  FIGS. 2-4 , the necked down region  46  is at an end of the loop  34 . In other embodiments, the necked down region  46  may be located anywhere on the loop biasing feature  32 , including, for example, in the center of the loop  34 . The loop  34  can pull off of the lead body  28  or out of the lead body  28 . The loop  34  can remain connected to the lead body  28  after the necked down region  46  has been broken. 
       FIGS. 5A-5D  show alternative embodiments of the loop biasing feature  32 . In the embodiment shown in the side view of  FIG. 5A , the necked down region  46  has the same dimensions as the loop  34 , but has been weakened by heat, radiation, or any other suitable means for weakening the necked down region  46 . Alternatively, the loop biasing feature  32  can also include a hole  47  in the necked down region  46 , as shown in the top view of  FIG. 5B . This hole  47  weakens the loop  34 , thus reducing the axial force necessary to break the loop  34 . The side view of  FIG. 5C  illustrates an alternative embodiment of the loop biasing feature  32  where the necked down region  46  includes a notch  49 . The top view shown in  FIG. 5D  illustrates an embodiment where the necked down region  46  is thinner in a plane parallel to the top of the loop biasing feature  32 . Any combination of holes  47 , notches  49 , and necked down regions  46  can be used to alter the axial force required to break the loop  34 . In an alternative embodiment, the loop  34  does not include a necked down or weakened region  46 , and the cross-sectional area of the loop  34  controls the axial force needed to break the loop  34 . 
       FIGS. 6 and 7  show additional embodiments of the loop biasing feature  32  where the loop biasing feature  32  includes a fixation structure. For example, the loop  34  may be formed with a tine  60 , as shown in  FIG. 5 , or scales  64 , as shown in  FIG. 6 , to increase fixation or friction between the loop biasing feature  32  and the branch vessel  25 . These features may reduce unintended dislodgement of the lead assembly  10  from a selected fixation location. 
       FIGS. 8A and 8B  show yet another alternative embodiment of the loop biasing feature  32  where the loop biasing feature  32  includes two loops  34 . In some circumstances, the groove along the heart  12  where the vessel  25  lies may be somewhat oval. As a result, certain positions of loops  34  around the circumference of the lead body  28  may preferentially orient the lead assembly  10  and loop biasing feature  32  into a position where the electrode  31  faces towards the heart  12  and the loops  34  face away from the heart  12 . This can result in better electrical contact with the heart  12 . The location of the loop or loops  34  around the circumference of the lead body  28  may therefore be critical.  FIGS. 8A-8B  depict an embodiment for such orientation. Although two loops  34  are shown in  FIGS. 8A and 8B , the loop biasing feature  32  could include any number of loops  34 . 
       FIGS. 9A-9C  show another alternative embodiment of the loop biasing feature  32  where the loop biasing feature  32  includes more than one loop  34 . In this embodiment, the loop biasing feature  32  includes two loops  34   a  and a loop  34   b  interposed between the loops  34   a . The loops  34   a ,  34   b  are similar to the tines  60 . The loops  34   a  resist motion of the lead tip  30  in the proximal direction and the loop  34   b  resists motion of the lead tip  30  in the distal direction. As shown in  FIG. 9C , the loops  34  folds down into down onto the lead body  28  so that it can more easily slide thru a catheter (not shown) and vessels  25 , or deploy in a more predictable manner. The loop biasing feature  32  may optionally include holes  47  located in the necked down region  46  to facilitate removal of the lead assembly  10 . Although the embodiments illustrated in  FIGS. 8A-8B  and  9 A- 9 C show two loops  34 , the loop biasing feature  32  could have additional loops  34  having any desired shape and optionally including a necked down region  46 , a hole  47 , or a notch  49 . 
     The lead assembly  10  may be delivered into the branch vessel  25  with a variety of techniques as are known in the art, including through the use of a guide catheter and/or stylet.  FIG. 10  shows an embodiment of the lead assembly  10  positioned for delivery into the branch vessel (not shown) through a lumen  70  of a guide catheter  72 . A stylet, guidewire, or another catheter (not shown) may be used to advance the lead assembly  10  through the lumen  70 . As the lead assembly  10  passes through the lumen  70 , the loop  34  flattens down against the lead body  28 . Thus, the diameter of the guide catheter  72  may be sized smaller than the combined diameter of the lead body  28  and the loop  34 . In one embodiment, as illustrated in  FIG. 11 , a groove  74  is formed into an outer surface  76  of the lead body  28  to receive the flattened loop  34 . This further reduces the diameter of the lead assembly  10  and of the guide catheter  72 . In the illustrated embodiment, the groove  74  extends over the electrode  31 . In other embodiments, the loop biasing feature  32  may be positioned on the lead body  28  such that the loop  34  and/or the groove  74  do not pass over the electrode  31  (not shown). 
       FIGS. 12A and 12B  show a lead assembly  110  according to another embodiment of the invention. The lead assembly  110  includes a lead body  128  extending from a proximal end  129  adapted for coupling to a pulse generator (not shown) to a distal end  130  adapted for insertion into the heart  12 . An electrode  131  is positioned at the distal end  130  of the lead body  128 . 
     The lead assembly  110  further includes a loop biasing feature  132  at the distal end  130  of the lead body  128 . The loop biasing feature  132  includes a loop  134  of cord, filament or cable material protruding from the lead body  128 . In the illustrated embodiment, the loop  134  has a distal end  135  fixed to the lead body  128  by, for example, adhesive, a crimp tube, compressive fit, or any other suitable manner (not shown). A proximal end  136  of the loop  134  is coupled to a cord  180  extending through a cord lumen  184  formed in the lead body  128 . The proximal and distal ends  136 ,  135  of the loop  134  exit and enter the cord lumen  184  through a pair of ports  188  in the lead body  128 . A proximal end  192  of the cord  180  is coupled to an optional grasp feature  196 . A pocket  198  is provided in an outer surface  176  of the lead body  128  for receiving the grasp feature  196 . A slidable tube  193  is positioned over the lead body  128  for covering the proximal end  192  of the cord  180  and the pocket  198 . In one embodiment, as illustrated in  FIG. 11 , a groove  174  is formed in the outer surface  176  of the lead body  128  for receiving the flattened loop  134 . In an alternative embodiment where the loop distal end  135  is fixed to the lead body  128  through an adhesive, crimp tube, compressive fit, or any other suitable manner, a tensile force applied to the cord  180  causes the loop distal end to detach, thus facilitating removal of the lead assembly  110 . 
     In some embodiments, the loop  134  and the cord  180  are not separate components, but rather the loop  134  is formed of a portion of the cord  180  which is sufficiently rigid to bias the lead body  128  towards the epicardium  27  and the myocardium  26 . As previously discussed, in some embodiments, pharmaceutical agents  152  can be added to the lead assembly  110  to facilitate tissue in-growth into a tissue in-growth area  148  defined between the loop  134  and the lead body  128 . Alternatively, the lead body  128  could include a surface treatment such as a plasma treatment. 
     The loop  134  protrudes from the lead body  128  in such a manner as to engage the pericardial wall  27   a  as previously described. The loop biasing feature  132  thus biases the electrode  131  towards the myocardial wall  26   a  and also helps to fix the lead body  128  within the branch vessel  25  (not shown). A proximally directed force exerted on the grasp feature  196 , as illustrated by arrow  197 , tensions the cord  180 , thereby flattening the loop  134  towards the lead body  128  for implanting, revising or removing the lead assembly  110 . Stated another way, the cord  180  permits the loop biasing feature  132  to be neutralized by reducing a distance d between the loop  134  and the exterior surface  176  of the lead body  128 . In other embodiments, the loop  134  twists or bends on itself so as to flatten towards the lead body  128  during implantation. 
     The cord  180  is typically formed of a lubricious material such as ethylene tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE) or expanded polytetrafluoroethylene (ePTFE) or other strong polymer such as polyester, aramid, Kevlar®, or is coated with such a material so as to reduce friction between the cord  180  and the cord lumen  184 . Examples of other suitable materials include coextruded PTFE/Kevlar®, and can be polymer coated cable or nitinol wire. This reduces the axial force necessary to tension the cord  180  so as to neutralize the loop biasing feature  132  and reposition or extract the lead assembly  110 . 
       FIG. 12B  shows a cross-sectional view of the lead body  128 , showing the cord  180  and cord lumen  184 . As illustrated in  FIG. 12B , the lead body  128  may include additional lumens  199  for uses such as delivery of payloads or receiving a conductive member. The cord  180  and/or the electrode  131  can be radio-opaque to allow the implanter to visualize deployment and electrode orientation with respect to the myocardial wall  26   a  to provide optimal or desirable orientation. 
     In the embodiment illustrated in  FIG. 12A , the loop  134  is preformed with the curvature shown. The loop  134  is thus biased outwardly into the loop shape protruding from the lead body  128  by virtue of the preformed curvature. In other embodiments, the loop  134  may be preformed with different shapes than that shown in  FIG. 12A . For example, the loop  134  may be preformed with multiple curvatures (as shown in  FIG. 13 ), or may have a variable cross-section along the length of the loop  134  (not shown). In alternative embodiments, the loop  34  can have a thickness such that the loop  34  shields or otherwise insulates a portion of the electrode  31  to prevent stimulation of the nerves located near the pericardium  27  and coronary vessels or the diaphragm (not shown). 
     In yet another embodiment, the loop  134  is biased outward by a spring mechanism  195 , as illustrated in  FIG. 14 . The spring mechanism  195  is shown in a window view in  FIG. 14  for illustrative purposes only. The loop  134  is connected to the spring mechanism  195  such that the loop  134  is biased distally, or outwardly, from the lead body  128 . The spring  195  can optionally be coupled to a cord  180  extending through the lumen  184 . In the embodiment of  FIG. 14 , a proximally directed tensioning force on the cord  180  flattens the loop  134 , thus aiding in removal of the lead assembly  110  from the vessel  25 . 
     In the embodiment shown in  FIG. 15 , the spring  195  is located at the distal tip  130  of the lead assembly  110  and surrounding a portion  196  of the loop  134 . The spring  195  pushes the loop  134  distally or outwardly. In the illustrated embodiment, the loop  134  can optionally be coupled to a cord  180  extending through the lumen  184 . As discussed with respect to  FIG. 14 , application of a proximally directed tensioning force to the cord  180  flattens the loop  134 , thereby aiding in removal of the lead assembly  110  from the vessel  25 . In an alternative embodiment, the lead assembly does not include the spring  195 . The loop biasing feature  132  itself can be stiff enough to provide a biasing force. In this embodiment, the loop  134  can be optionally coupled to a cord  180  extending through a lumen  184  for lead removal as described with respect to  FIG. 14 . The lead assembly  110  may include multiple loops  134  and the loops  134  may be preformed with multiple curvatures. The loop biasing features  132  may include any combination of necked down regions  146 , holes  147 , or notches  149 . 
     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.