CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/360,171, filed on Jun. 30, 2010, entitled “LEAD HAVING COIL ELECTRODE WITH PREFERENTIAL BENDING REGION,” which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to implantable medical devices and relates more particularly to leads for cardiac rhythm management (CRM) systems. 
       BACKGROUND 
       [0003]    Various types of medical electrical leads for use in cardiac rhythm management (CRM) and neurostimulation systems are known. For CRM systems, such leads are typically extended intravascularly to an implantation location within or on a patient&#39;s heart, and thereafter coupled to a pulse generator or other implantable device for sensing cardiac electrical activity, delivering therapeutic stimuli, and the like. The leads frequently include features to facilitate securing the lead to heart tissue to maintain the lead at its desired implantation site. 
       SUMMARY 
       [0004]    Example 1 is an implantable lead assembly that includes a flexible lead body extending between a proximal region including a proximal end and a distal region including a distal end. A distal shocking coil is disposed about the flexible lead body, the distal shocking coil located within the distal region and being configured to enable the flexible lead body to preferentially buckle within a predetermined buckle region within the shocking coil. A connector assembly is secured to the proximal end for coupling the lead to an implantable medical device, the connector assembly including a terminal pin rotatable relative to the lead body, a coil conductor disposed longitudinally within the lead body, the coil conductor rotatable relative to the lead body and coupled to the terminal pin, an electrode base rotatably disposed within the lead body, the electrode base having a proximal end and a distal end, the proximal end connected to the coil conductor, and a helical electrode fixedly secured to the electrode base, the electrode base rotatably engaged with the terminal pin via the coil conductor such that rotation of the terminal pin causes the electrode base to rotate. 
         [0005]    In Example 2, the implantable lead of Example 1 in which the distal shocking coil includes an expanded polytetrafluoroethylene coating disposed thereon. 
         [0006]    In Example 3, the implantable lead of Example 1 or 2 in which the distal shocking coil has a non-uniform coil pitch. 
         [0007]    In Example 4, the implantable lead of any of Examples 1-3 in which the distal shocking coil has a non-uniform filar angle. 
         [0008]    In Example 5, the implantable lead of any of Examples 1-4 in which the distal shocking coil includes a first coil and a second coil spaced apart from the first coil, the first and second coils electrically connected via a cable conductor extending through the flexible lead body. 
         [0009]    In Example 6, the implantable lead of any of Examples 1-4 in which the distal shocking coil includes a multifilar coil having a reduced number of filars within the predetermined buckle region. 
         [0010]    In Example 7, the implantable lead of Example 6 in which the distal shocking coil has two filars proximate the predetermined buckle region and three filars on either side of the predetermined buckle region. 
         [0011]    In Example 8, the implantable lead of any of Examples 1-7, further including a proximal shocking coil disposed about the flexible lead body within the proximal region of the lead body, the proximal shocking coil being configured to have a flexibility that is substantially equivalent to that of the flexible lead body on either side of the proximal shocking coil. 
         [0012]    In Example 9, the implantable lead of Example 8 in which the proximal shocking coil has a spaced-apart coil pitch. 
         [0013]    Example 10 is an implantable lead that includes a flexible lead body extending between a proximal region including a proximal end and a distal region including a distal end, a coil electrode disposed about the flexible lead body, the coil electrode configured to enable the flexible lead body to preferentially buckle within a predetermined region within the coil electrode, a connector assembly secured to the proximal end for coupling the lead to an implantable medical device, and one or more electrical conductors extending through the flexible lead body, the one or more electrical connectors providing electrical communication between the connector assembly and each of the coil electrodes. 
         [0014]    In Example 11, the implantable lead of Example 10 in which the lead is an active fixation lead. 
         [0015]    In Example 12, the implantable lead of Example 10 in which the lead is a passive fixation lead. 
         [0016]    In Example 13, the implantable lead of any of Examples 10-12 in which the coil electrode has a non-uniform coil pitch or a non-uniform filar angle. 
         [0017]    In Example 14, the implantable lead of any of Examples 10-13 in which the coil electrode includes a multifilar coil having a reduced number of filars within the predetermined buckle region. 
         [0018]    Example 15 is an implantable lead assembly that includes a flexible lead body extending between a proximal region including a proximal end and a distal region including a distal end, a lumen extending from the proximal end to the distal end. A distal shocking coil is disposed about the flexible lead body and is located within the distal region and configured to enable the flexible lead body to preferentially buckle within a predetermined lead buckle region. A connector assembly is secured to the proximal end for coupling the lead to an implantable medical device. One or more electrical conductors extend through the flexible lead body and provide electrical communication between the connector assembly and each of the shocking coils. A stylet is disposable within the lumen, the stylet including a stylet buckle region corresponding to the predetermined lead buckle region within the distal shocking coil when the stylet is extended through the predetermined lead buckle region. 
         [0019]    In Example 16, the implantable lead assembly of Example 15 in which the stylet buckle region includes a reduced-diameter portion of the stylet. 
         [0020]    In Example 17, the implantable lead assembly of Example 16 in which the stylet includes a distal taper and the reduced-diameter portion of the stylet is disposed proximal of the distal taper. 
         [0021]    In Example 18, the implantable lead of any of Examples 15-17 in which the distal shocking coil has a non-uniform coil pitch or a non-uniform filar angle. 
         [0022]    In Example 19, the implantable lead of any of Examples 15-18 in which the distal shocking coil includes a multifilar coil having a reduced number of filars within the predetermined buckle region. 
         [0023]    In Example 20, the implantable lead of any of Examples 15-19, further including a proximal shocking coil disposed about the flexible lead body within the proximal region thereof, the proximal shocking coil having a non-uniform pitch to provide a flexibility substantially equivalent to a lead body flexibility on either side of the proximal shocking coil. 
         [0024]    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 
         [0025]      FIG. 1  is a combined cutaway and perspective view of an implantable medical device and lead in accordance with an embodiment of the present invention. 
           [0026]      FIG. 2  is a side elevation view of the lead of  FIG. 1 . 
           [0027]      FIG. 3  is a partial cross-sectional view of the lead of  FIG. 1 . 
           [0028]      FIG. 4  is a cross-sectional view of a shocking coil in accordance with an embodiment of the present invention. 
           [0029]      FIG. 5  is a cross-sectional view of a shocking coil in accordance with an embodiment of the present invention. 
           [0030]      FIG. 6  is a cross-sectional view of a shocking coil in accordance with an embodiment of the present invention. 
           [0031]      FIG. 7  is a cross-sectional view of a shocking coil in accordance with an embodiment of the present invention. 
           [0032]      FIG. 8  is a cross-sectional view of a shocking coil in accordance with an embodiment of the present invention. 
           [0033]      FIG. 9  is a partial side elevation view of a lead and a stylet in accordance with an embodiment of the present invention. 
       
    
    
       [0034]    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 
       [0035]      FIG. 1  is a perspective view of an implantable cardiac rhythm management (CRM) system  10 . The CRM system  10  includes a pulse generator  12  and a cardiac lead  14 . The lead  14  operates to convey electrical signals between the heart  16  and the pulse generator  12 . The lead  14  has a proximal region  18  and a distal region  20 . The lead  14  includes a lead body  22  extending from the proximal region  18  to the distal region  20 . The proximal region  18  is coupled to the pulse generator  12  and the distal region  20  is coupled to the heart  16 . In some embodiments, the lead  14  is an active fixation lead and the distal region  20  may, as illustrated, include an extendable/retractable fixation helix  24  which as will be discussed in greater detail below locates and/or secures the distal region  20  within the heart  16 . In some embodiments, the lead  14  may be a passive fixation lead. 
         [0036]    The pulse generator  12  is typically implanted subcutaneously within an implantation location or pocket in the patient&#39;s chest or abdomen. The pulse generator  12  may be any implantable medical device known in the art or later developed, for delivering an electrical therapeutic stimulus to the patient. In various embodiments, the pulse generator  12  is an implantable cardioverter/defibrillator (ICD), a cardiac resynchronization (CRT) device configured for bi-ventricular pacing, and/or includes combinations of pacing, CRT, and defibrillation capabilities. 
         [0037]    The lead body  22  can be made from any flexible, biocompatible materials suitable for lead construction. In various embodiments, the lead body  22  is made from a flexible, electrically insulative material. In one embodiment, the lead body  22  is made from silicone rubber. In another embodiment, the lead body  22  is made from polyurethane. In various embodiments, respective segments of the lead body  22  are made from different materials, so as to tailor the lead body characteristics to its intended clinical and operating environments. In various embodiments, the proximal and distal ends of the lead body  22  are made from different materials selected to provide desired functionalities. 
         [0038]    As is known in the art, the heart  16  includes a right atrium  26 , a right ventricle  28 , a left atrium  30  and a left ventricle  32 . It can be seen that the heart  16  includes an endothelial inner lining or endocardium  34  covering the myocardium  36 . In some embodiments, as illustrated, the fixation helix  24 , located at the distal region  20  of the lead, penetrates through the endocardium  34  and is imbedded within the myocardium  36 . In one embodiment, the CRM system  10  includes a plurality of leads  14 . For example, it may include a first lead  14  adapted to convey electrical signals between the pulse generator  12  and the right ventricle  28  and a second lead (not shown) adapted to convey electrical signals between the pulse generator  12  and the right atrium  26 . 
         [0039]    In the illustrated embodiment shown in  FIG. 1 , the fixation helix  24  penetrates the endocardium  34  of the right ventricle  28  and is embedded in the myocardium  36  of the heart  16 . In some embodiments, the fixation helix  24  is electrically active and thus can be used to sense the electrical activity of the heart  16  and/or to apply a stimulating pulse to the right ventricle  28 . In other embodiments, the fixation helix  24  is not electrically active. Rather, in some embodiments, other components of the lead  14  are electrically active. 
         [0040]      FIG. 2  is an isometric illustration of the lead  14  according to one embodiment. A connector assembly  40  is disposed at or near the proximal region  18  of the lead  14  while a distal assembly  42  is disposed at or near the distal region  20  of the lead  14 . Depending on the functional requirements of the CRM system  10  (see  FIG. 1 ) and the therapeutic needs of a patient, the distal region  20  may include one or more electrodes. In the illustrated embodiment, the distal region  20  includes a distal shocking coil  44  and a proximal shocking coil  45  that can function as shocking electrodes for providing a defibrillation shock to the heart  16 . 
         [0041]    In various embodiments, the lead  14  may include only a single shocking coil. In various other embodiments, the lead  14  includes one or more ring electrodes (not shown) along the lead body  22  in lieu of or in addition to the shocking coils  44 ,  45 . When present, the ring electrodes operate as relatively low voltage pace/sense electrodes. In short, a wide range of electrode combinations may be incorporated into the lead  14  within the scope of the various embodiments of the present invention. In some embodiments, the distal shocking coil  44  and/or the proximal shocking coil  45  may be configured to preferentially bend or buckle in a predetermined buckle region in order to reduce the lead&#39;s column strength. 
         [0042]    The connector assembly  40  includes a connector  46  and a terminal pin  48 . The connector  46  is configured to be coupled to the lead body  22  and is configured to mechanically and electrically couple the lead  14  to a header on the pulse generator  12  (see  FIG. 1 ). In various embodiments, the terminal pin  48  extends proximally from the connector  46  and in some embodiments is coupled to a conductor member (not visible in this view) that extends longitudinally within the lead body  22  and which is rotatable relative to the lead body  22  such that rotating the terminal pin  48  (relative to the lead body  22 ) causes the conductor member to rotate within the lead body  22  as well. 
         [0043]    In some embodiments, the terminal pin  48  includes an aperture extending therethrough, and the conductor member defines a longitudinal lumen in communication with the aperture. When present, the aperture and/or conductor lumen are configured to accommodate a guide wire or an insertion stylet for delivery of the lead  14 . 
         [0044]    The distal assembly  42  includes a housing, within which the fixation helix  24  is at least partially disposed. In some embodiments, the housing includes or accommodates a mechanism that enables the fixation helix  24  to move distally and proximally relative to the housing. In some embodiments, as the lead  14  may be a passive fixation lead and thus may not include the fixation helix  24 . 
         [0045]    In some embodiments, the housing may accommodate or include structure that limits distal travel of the fixation helix  24  (relative to the housing). As noted above, the fixation helix  24  operates as an anchoring means for anchoring the distal region  20  of the lead  14  within the heart  16 . In some embodiments, the fixation helix  24  is electrically active, and is also used as a pace/sense electrode. In some embodiments, the fixation helix  24  is made of an electrically conductive material such as Elgiloy, MP35N, tungsten, tantalum, iridium, platinum, titanium, palladium, stainless steel, as well as alloys of any of these materials. In some embodiments, the fixation helix  24  is made of a non-electrically conductive material such as PES (polyethersulfone), polyurethane-based thermoplastics, ceramics, polypropylene and PEEK (polyetheretherketone). 
         [0046]      FIG. 3  illustrates an embodiment of a lead including a distal assembly in accordance with one embodiment of the present invention. In  FIG. 3 , the fixation helix  24  is illustrated in a retracted position. In the illustrated embodiment, the fixation helix  24  is electrically active so as to be operable as a pace/sense electrode. 
         [0047]    As shown in  FIG. 3 , the distal assembly  42  includes a distal region  52  and a proximal region  54 . The distal assembly  42  is, in general, relatively rigid or semi-rigid. In some embodiments, the distal assembly  42  is made of an electrically conductive material such as Elgiloy, MP35N, tungsten, tantalum, iridium, platinum, titanium, palladium, stainless steel, as well as alloys of any of these materials. In some embodiments, the distal assembly  42  is made of a non-electrically conductive material such as PES, polyurethane-based thermoplastics, ceramics, polypropylene and PEEK. 
         [0048]    In the illustrated embodiment, a drug eluting collar  56  is disposed about an exterior of the distal assembly  42  within the distal region  52 . In various embodiments, the drug eluting collar  56  is configured to provide a time-released dosage of a steroid or other anti-inflammatory agent to the tissue to be stimulated, e.g., the heart tissue in which the electrically active fixation helix  24  is implanted. While not illustrated, in some embodiments the distal assembly  42  may include a radiopaque element disposed under the drug eluting collar  56 . 
         [0049]    As shown, the distal assembly  42  includes an electrode base  58 . In some embodiments, the electrode base  58  is made of an electrically conductive material such as Elgiloy, MP35N, tungsten, tantalum, iridium, platinum, titanium, palladium, stainless steel, as well as alloys of any of these materials. In some embodiments, the electrode base  58  is made of a non-electrically conductive material such as PES (polyethersulfone), polyurethane-based thermoplastics, ceramics, polypropylene and PEEK (polyetheretherketone). 
         [0050]    In some embodiments, the electrode base  58  is configured to move longitudinally and/or rotationally with respect to the distal assembly  42 . As illustrated, the electrode base  58  includes a distal portion  60  and a proximal portion  62 . As shown, the fixation helix  24  is connected to the distal portion  60  of the electrode base  58 . In some embodiments, as illustrated, the distal portion  60  may have a relatively smaller diameter that is configured to accommodate the fixation helix  24 . In some embodiments, the proximal portion  62  of the electrode base  58  may be configured to accommodate a seal (not illustrated). 
         [0051]    A conductor coil  64  is secured to the proximal portion  62  of the electrode base  58 , and extends proximally through the lead body  22  to the connector assembly  40 . In some embodiments, the conductor coil  64  is welded or soldered to the proximal portion  62  of the electrode base  58 . In some embodiments, the conductor coil  64  includes or is otherwise formed from a metallic coil. 
         [0052]    In the connector assembly  40 , the conductor coil  64  is coupled to the terminal pin  48  such that rotation of the terminal pin  48  causes the conductor coil  64  to rotate. As the conductor coil  64  rotates, the electrode base  58  and the fixation helix  24  will also rotate. In some embodiments, the fixation helix  24  is rotated via a stylet that is inserted through an aperture that may be formed within the terminal pin  48  ( FIG. 2 ). 
         [0053]    In some embodiments, as illustrated, the electrode base  58  includes a threaded portion  66  that interacts with a corresponding threaded portion  68  secured within the distal assembly  42 . It will be appreciated that relative rotation between the threaded portion  66  and the threaded portion  68  will cause the electrode base  58  to translate or move axially relative to the distal assembly  42 . For example, the threaded portion  66  and the threaded portion  68  may be configured such that rotating the terminal pin  48  in a clockwise fashion will cause the electrode base  58  and the fixation helix  24  to move outwards towards an extended position while counter clockwise rotation of the terminal pin  48  may retract the electrode base  58  and the fixation helix  24 . 
         [0054]    The particular arrangement illustrated in  FIG. 3  facilitating extension and retraction of the fixation helix  24  is exemplary only. In other words, any arrangement, whether now known or later developed, for providing the extendable/retractable functionality of the fixation helix  24  can be utilized in connection with the various embodiments of the present invention. In one embodiment, the lead  14  includes structures such as those described and illustrated in co-pending and commonly assigned U.S. Provisional Patent Application 61/181,954, the disclosure of which is incorporated by reference herein in its entirety. In other embodiments, a different arrangement for extending and retracting the fixation helix  24  is utilized. 
         [0055]    In some embodiments, the distal shocking coil  44  and/or the proximal shocking coil  45  may include a coating that inhibits tissue ingrowth yet permits electrical charges to pass from the shocking coil  44 ,  45  to the surrounding heart tissue. In some embodiments, the coating is a porous polymer coating that inhibits tissue ingrowth yet permits ions to pass through. An exemplary coating is an expanded polytetrafluoroethylene. In some embodiments, inclusion of the coating may stiffen the coil by limiting relative movement between adjacent filars. In some embodiments, the shocking coil may be modified or configured to bend or buckle within a predetermined buckle region. 
         [0056]    In some embodiments, the distal shocking coil  44  may be configured to preferentially bend or buckle within a predetermined buckle region. In some embodiments, the predetermined buckle region may correspond to a point within the distal shocking coil  44 . In some embodiments, the proximal shocking coil  45  may be configured to have a flexibility that more closely matches a flexibility of the lead body  22  on either side of the proximal shocking coil  45 .  FIGS. 4-8  provide illustrative examples of how the distal shocking coil  44  and/or the proximal shocking coil  45  may be configured to provide desired bending and/or flexibility characteristics. 
         [0057]      FIG. 4  is a partial cross-section view of a coil  70  that may be used as the distal shocking coil  44  and/or the proximal shocking coil  45 . While the coil  70  is illustrated as including a single filar  72 , the coil  70  may, in some embodiments, be formed as a multi-filar coil. In some embodiments, the coil  70  may be defined at least in part based upon its pitch. Pitch refers to the spacing between adjacent filar turnings. For example, a tight pitch may be defined as referring to a coil or a portion of a coil in which adjacent filar turnings are touching or slightly spaced apart. In contrast, an expanded pitch may be defined as referring to a coil or a portion of a coil in which adjacent filar turnings are spaced farther apart. 
         [0058]    The coil  70  has a distal region  74  having a tight coil pitch, a proximal region  76  having a tight coil pitch and a predetermined buckle region  78  in which the coil  70  has an expanded coil pitch. As an illustrative but non-limiting example, assume that coil  70  is formed of three adjacent filars  72 , each having a diameter of 0.008 inches. If the filars  72  are in close contact, the coil  70  would be considered as having a pitch of 0.024 inches (3 times 0.008 inches). In this example, the distal region  74  and the proximal region  76  may each have a pitch that is in the range of about 0.024 inches to about 0.028 inches while the predetermined buckle region  78  may have a pitch that is in the range of about 0.028 inches to about 0.040 inches. It should be noted that these ranges are merely illustrative and may vary, depending upon the number of filars  72  and the diameters of the filars  72 . 
         [0059]      FIG. 5  is a partial cross-section view of a coil  80  that is similar to the coil  70 , but has a less drastic pitch change. The coil  80  may be used as the distal shocking coil  44  and/or the proximal shocking coil  45 . In some embodiments, as illustrated, the coil  80  is formed from a single filar  72 . In other embodiments, the coil  80  may be formed as a multi-filar coil. The coil  80  has a distal region  82  having a tight coil pitch, a proximal region  84  having a tight coil pitch and a predetermined buckle region  86  in which the coil  80  has an expanded coil pitch. In the illustrated embodiment, the distal region  82  and the proximal region  84  both have a tight coil pitch in which adjacent filar turnings are in contact. In some embodiments, the distal region  82  and/or the proximal region  84  may have a coil pitch in which adjacent filar turnings are slightly spaced apart, but not as spaced apart as the adjacent filar turnings within the predetermined buckle region  86 . 
         [0060]      FIG. 6  is a partial cross-section of a coil assembly  88  that may be used as the distal shocking coil  44  and/or the proximal shocking coil  45 . The coil assembly  88  includes a distal coil  90 , a proximal coil  92  and an intervening predetermined buckle region  94  in which there is no coil filars extending therethrough. The distal coil  90  and the proximal coil  92  are each, as illustrated, formed of a single filar  72 , although in some embodiments the distal coil  90  and/or the proximal coil  92  may be multi-filar coils. 
         [0061]    The distal coil  90  and the proximal coil  92  are electrically connected together by being secured to a cable conductor  96  that extends between the distal coil  90  and the proximal coil  92 . In some embodiments, the cable conductor  96  extends through the lead body  22  back to the connector assembly  40 . In some embodiments, a fitting (not illustrated) may be welded to at least one of the distal coil  90  and the proximal coil  92 , and the cable conductor  96  can be crimped into the fitting to provide electrical contact as well as a secure attachment between the coil  90  or  92  and the cable conductor  96 . 
         [0062]      FIG. 7  is a partial cross-section of a coil  98  that may be used as the distal shocking coil  44  and/or the proximal shocking coil  45 . The coil  98  is illustrated as being formed from a single filar  72 , although in some embodiments the coil  98  may be configured as a multi-filar coil. The coil  98  includes a distal region  100  having a first filar angle alpha measured with respect to a longitudinal axis  99 , a proximal region  102  having the same filar angle alpha and an intermediate predetermined buckle region  104  having a second filar angle beta. 
         [0063]    In some embodiments, beta may be an acute angle that is greater than alpha. In some embodiments, the relative stiffness of the predetermined buckle region  104  may be altered by increasing or decreasing the filar angle beta within the predetermined buckle region  104 . In some embodiments, the coil pitch as well as the filar angle may be altered in the predetermined buckle region  104 . 
         [0064]    In some embodiments, the filar angle alpha may be in the range of about 45 degrees to about 80 degrees while the filar angle beta may be in the range of about 70 degrees to about 90 degrees. In some embodiments, these values may vary, depending on various factors such as the coil size and filar diameter. In some embodiments, the relative values for alpha and beta may not be as important as the difference between alpha and beta. 
         [0065]      FIG. 8  is a partial cross-section of a coil  106  that may be used as the distal shocking coil  44  and/or the proximal shocking coil  45 . The coil  106  includes a distal region  108 , a proximal region  110  and a predetermined buckle region  112  disposed between the distal region  108  and the proximal region  110 . In this embodiment, the distal region  108  and the proximal region  110  include a two-filar configuration having a first filar  114  and a second filar  116  while the predetermined buckle region  112  only includes the first filar  114 . In some embodiments, the second filar  116  may be removed from the predetermined buckle region  112  while in other embodiments the predetermined buckle region  112  may be formed with only a single filar. 
         [0066]    In some embodiments, other filar counts may also be used. For example, the distal region  108  and the proximal region  110  may include two, three, four or more filars while the predetermined buckle region  112  may include one or more fewer filars than used in the distal region  108  and the proximal region  110 . In some embodiments, the entire coil  106  may have a constant filar count of one, two, three or more filars, but the filar(s) may have a non-constant diameter. For example, in some embodiments, the filar(s) may have a relatively larger diameter within the distal region  108  and the proximal region  110  and a relative smaller diameter within the predetermined buckle region  112 . 
         [0067]    In some embodiments, the lead  14  may be deployed using a stylet to stiffen the lead  14  and/or provide better steerability to the lead  14 . In some embodiments, and as illustrated in  FIG. 9 , a stylet may be configured to have a stylet buckle location that corresponds to the predetermined buckle location of the distal shocking coil  44 . While  FIG. 9  illustrates a passive fixation lead, it will be appreciated that a similar stylet may be used in combination with an active fixation lead such as that described above. 
         [0068]      FIG. 9  is a partial cross-section of a distal region of a passive fixation lead  118 . The passive fixation lead  118  includes a plurality of fixation wings  120  and a distal electrode  122 . A lumen  124  extends through the lead  118  and includes a terminus  125  that is configured to accommodate a distal end of a stylet. The lumen  124  is defined at least in part by a first polymeric layer  126 . A coil  128  that as discussed above may be a single filar coil or a multi-filar coil is wound around the first polymeric layer  126 . A second polymeric layer  130  extends around the first polymeric layer  126  and provides the lead  118  with a constant outer diameter. The coil  128  includes a distal region  132 , a proximal region  134  and an intervening predetermined buckle region  136  in which the coil  128  has an increased pitch (distance between adjacent turnings). 
         [0069]    A stylet  138  extends through the lumen  124 . The stylet includes a distal region  140  including a distal end  141 , a proximal region  142  and an intervening stylet buckle region  144 . In some embodiments, as illustrated, the intervening stylet buckle location  144  includes a narrowed portion  146  within a stylet shaft  148 . The narrowed portion  146  provides a reduced stiffness region that corresponds to the predetermined buckle region  136  of the coil  128 . 
         [0070]    In some embodiments, the stylet buckle region  144  may instead have an increased diameter or otherwise be configured to be stiffer than the rest of the stylet  138 . In some embodiments, a relatively stiffer stylet buckle region  144  may counteract the flexibility of the predetermined buckle region  136  of the coil  128  in order to improve maneuverability during lead implantation. 
         [0071]    The coil configurations described and illustrated herein have been described as shocking coils. In some embodiments, these coil configurations may be used as other types of coil electrodes in a variety of different lead types including brady leads, pacing leads and the like. 
         [0072]    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.
An implantable lead may have a distal shocking coil that is configured to include a predetermined buckle region in order to limit potential cardiac damage that might otherwise occur if the implantable lead is too stiff. An implantable lead may have a proximal shocking coil that is configured to have a flexibility that more closely matches the flexibility of the lead body on either side of the proximal shocking coil.