Patent Publication Number: US-2021187285-A1

Title: Device, system, and method for delivery of an implantable cardiac lead and associated active agent delivery component

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 62/949,601 filed Dec. 18, 2019, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure is directed to implantable medical devices such as, for example, cardiac rhythm management pacing and defibrillation leads. More specifically, the present disclosure is directed to systems and methods for implantation of devices for delivery of active pharmaceutical ingredients at implantation sites of implantable medical devices. 
     BACKGROUND 
     Implantable cardiac rhythm management devices (ICRMD), such as pacemakers with implantable cardiac leads and leadless pacemakers, are generally used for pacing and/or defibrillation of the heart to address various cardiac pathologies. Such devices may include implantable elements may further employ components for delivering an active agent to implantation sites of the implantable elements. In the case of implantable leads and leadless pacemakers, such delivery components may include monolithic controlled release device (MCRD), which are generally positioned near the distal tip of the implantable lead or leadless pacemaker. The MCRD carries an active agent and is positioned such that the MCRD is held in close proximity to, if not in actual physical contact with, the cardiac tissue at the implantation site following implantation of the lead. Following implantation of the lead, the MCRD elutes the active agent (which may include but is not limited to anti-inflammatory medications, steroids, and other therapeutic agents) to the surrounding tissue. In pacing applications, for example, the active agent may be selected to reduce myocardial inflammation at the implantation site in order to keep pacing thresholds low, thereby improving operation and efficiency of the implanted medical device. 
     Accordingly, there is a need in the art for electrode designs, MCRD designs, and lead designs, generally, that improve MCRD yield, and provide consistent delivery of the active agent at the implantation site. 
     SUMMARY 
     In one aspect of the present disclosure, a system for implanting an implantable medical device lead is provided. The system includes a lead including a lead body defining a lead lumen and a distal lead end adapted to engage tissue, the lead lumen extending through the distal lead end. The system further includes a delivery stylet insertable through the lead lumen. The delivery stylet includes a stylet body having a distal stylet end and an active agent delivery component detachably coupled to the distal stylet end. The delivery stylet is insertable through the lead lumen such that the distal stylet end protrudes from the distal lead end, thereby enabling insertion of the active agent delivery component into the tissue to which the distal lead end is engaged. 
     In certain example implementations the lead is a cardiac pacing lead and the distal lead end further includes a pacing electrode. 
     In certain example implementations the delivery stylet may include a first feature configured to abut a second feature disposed within the lead lumen. By doing so, each of distal movement of the delivery stylet within the lead lumen and protrusion of the delivery stylet from the distal lead end may be limited. In certain implementations, the first feature may be a distally facing surface of the delivery stylet while the second feature may be a proximally facing surface disposed within the lead lumen. 
     In certain example implementations the delivery stylet may include an outer sleeve and an inner stylet. The inner stylet is movable within the outer sleeve and includes the distal stylet end and the active agent delivery component. 
     In certain example implementations the stylet body defines a stylet lumen configured to receive at least one of a push tool for applying a force on the active agent delivery component and a retention tool extending through the stylet body and coupled to the active agent delivery component. 
     In certain example implementations the active agent delivery component may include a steroid. 
     In another aspect of the present disclosure, a method of delivering active agents to tissue at an implantation location of a lead is provided. The lead includes a lead body defining a lead lumen and a distal lead end engaging the tissue at the implantation location, the lead lumen extending through the distal lead end. The method includes the steps of inserting a delivery stylet through the lead lumen, the delivery stylet including a stylet body having a distal stylet end and an active agent delivery component detachably coupled to the distal stylet end. The method further includes extending the distal stylet end from the distal lead end such that the active agent delivery component is inserted into the tissue at the implantation location. The method also includes detaching the active agent delivery component from the distal stylet end and within the tissue at the implantation location. 
     In certain example implementations the delivery stylet includes a distally facing surface and the lead comprises a proximally facing surface disposed within the lead lumen. In such implementations, extending the distal stylet end from the distal lead end comprises abutting the distally facing surface with the proximally facing surface such that the active agent delivery component is extended a predetermined distance from the distal lead end. 
     In certain example implementations the delivery stylet may include a distally facing surface and the lead may include a proximally facing surface disposed within the lead lumen. The delivery stylet may further include an outer tubular body including the distally facing surface and an inner stylet movable within the outer tubular body, the inner stylet including the distal stylet end. In such implementations, extending the distal stylet end from the distal lead end may include moving the inner stylet distally relative to the outer tubular body after abutting the distally facing surface and the proximally facing surface. 
     In certain example implementations detaching the active agent delivery component from the distal stylet may include at least one of retracting the stylet body after insertion of the active agent delivery component in the tissue at the implantation site, inserting a push tool through the stylet body to apply a proximal force on the active agent delivery component, retracting a retention tool extending through the stylet body and coupled to the active agent delivery component, and dissolving a bond between the active agent delivery component and the distal stylet end. 
     In certain example implementations the method further includes, prior to inserting the delivery stylet through the lead lumen, implanting the distal lead end into the tissue at the implantation site. 
     In certain example implementations the active agent delivery component includes a steroid. 
     In still another aspect of the present disclosure, a stylet for use in delivery of active agents to an implantation site of an implantable medical lead is provided. The stylet includes a stylet body having a distal stylet end and an active agent delivery component detachably coupled to the distal stylet end. 
     In certain example implementations the stylet body further includes an outer sleeve and an inner stylet movable within the outer sleeve, the inner stylet comprising the distal stylet end and the active agent delivery component. 
     In certain example implementations the stylet body includes a distally facing surface disposed proximal the distal stylet end. 
     In certain example implementations the active agent delivery component includes a radiopaque marker. 
     In certain example implementations the active agent delivery component is coupled to the distal stylet end by a dissolvable bond. 
     In certain example implementations the stylet body defines an inner lumen and the active agent delivery component is detachable from the distal stylet end by inserting an elongate tool through the inner lumen and applying a distal force on the active agent delivery component. 
     In certain example implementations the stylet body defines an inner lumen and further includes an elongate retention tool disposed within the inner lumen and coupled to the active agent delivery component, the active agent delivery component being detachable from the distal stylet end by retracting the elongate retention tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional side view of a distal region of a lead including an electrode and an example monolithic controlled release device (MCRD) positioned in a chamber of the electrode. 
         FIG. 1B  is a cross-sectional side view of the electrode  108  abutting the myocardium of a heart with a stylet used to advance the electrode. 
         FIG. 1C  is a cross-sectional side view of the electrode abutting the myocardium of the heart showing an active agent eluting from the MCRD and interfacing with blood and tissue from the myocardium. 
         FIG. 2A  is a plan view of a lead in accordance with the present disclosure that is connectable with a pulse generator, wherein an active fixation anchor of the lead is shown in an extended or deployed state. 
         FIG. 2B  is a partial cutaway view of the distal region of the implantable cardiac lead of  FIG. 2A , the distal region having a helical active fixation anchor. 
         FIG. 3  is a side view of a delivery stylet coupled with an active agent delivery component, such as an MRCD. 
         FIGS. 4A-4C  are, respectively, cross-sectional side views of an electrode positioned against the myocardium with a placement stylet positioned therein, a delivery stylet positioned therein, and the active agent delivery component positioned within the myocardium with the delivery stylet retracted. 
         FIGS. 5A and 5B  are, respectively, a cross-sectional side view of a conventional electrode with a distal chamber for housing an active agent delivery component, and a cross-sectional view of an electrode with a lumen fully passing through the electrode. 
         FIGS. 6A-6F  are cross-sectional side views of various shapes and configurations of active agent delivery components in accordance with the present disclosure. 
         FIGS. 7A-7D  are cross-sectional side views of a delivery stylet illustrating various structures for releasably coupling an active agent delivery component to a distal stylet end of the delivery stylet. 
         FIG. 8A  is a side view of a delivery stylet coupled with an active agent delivery component, such as an MRCD. 
         FIGS. 8B-8E  are cross-sectional side views of a distal region of a lead with an active fixation anchor/electrode in various stages of delivery of an active agent delivery component using a delivery stylet. 
         FIGS. 9A and 9B  are, respectively, cross-sectional side views of a two-part delivery stylet coupled with an active agent delivery component in a retracted state and a deployed state. 
         FIGS. 10A-10D  are cross-sectional side views of a distal region of a lead with an active fixation anchor/electrode in various stages of delivery of an active agent delivery component using a two-part delivery stylet. 
         FIG. 11  is a flowchart showing an exemplary method of delivering an implantable cardiac lead and an active agent delivery component into tissue at an implantation site. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is directed to implantable medical devices, such as cardiac rhythm management devices (ICRMD), employing active agent delivery components, such as monolithic controlled release devices (MCRD), that carry an active agent. In certain example applications, the active agent delivery component may be an MCRD formed from a blend of steroid powder (or other active agent powder) and silicone rubber. Also in certain example applications, the ICRMD may be in the form of an implantable cardiac lead for pacing, sensing, and/or defibrillation. As discussed below in further detail, such leads may be configured for passive or active fixation. 
     Conventionally, as seen in  FIG. 1A , which is a cross-sectional side view of a distal region  102  of an implantable lead  100 , a tubular lead body  104  (which may include outwardly extending fixation members  106  in the form of tines) partially encases an electrode  108  which extends from the end of the tubular lead body  104 . The electrode  108  is physically and electrically connected to a conductor  110 , e.g., a tubular coiled conductor, that extends proximally along the length of the implantable lead  100 . As seen in  FIG. 1A , in at least certain conventional applications, the electrode  108  includes a proximal opening  112  forming a lumen that continues through the coiled conductor  110 . The electrode  108  also includes a chamber  114  that opens to a distal opening  116 . In conventional applications, an MCRD  118  is generally housed in the chamber  114 . 
     The distal region  102  of the lead  100  is delivered to the myocardium  120  of the heart, as seen in  FIG. 1B , which shows the electrode  108  by itself in a cross-sectional side view abutting the myocardium  120  and being delivered using a placement stylet  150 . In this position, the fixation members  106  (shown in  FIG. 1A ) would be anchored to the epicardium (not shown) or similar cardiac tissue. As seen in  FIG. 1B , a distal tip  122  of the electrode and the distal opening  116  may be made to contact the myocardium  120 . 
     Following implantation, blood and other bodily fluids infiltrate the chamber  114 , as seen in  FIG. 1C , which is a cross-sectional side view of the electrode  108  abutting the myocardium  120 , which causes the release of an active agent from the MCRD  118  into the tissue and blood. The active agents may be chosen for various purposes; however, in at least some implementations, the active agents may be chosen to reduce inflammation of the tissue disturbed by the introduction of the electrode  108 , fixation members  106 , and the lead  100 . 
     Various disadvantages are associated with conventional techniques in which the MCRD  118  is housed in the chamber  114 . For example, if replacement of the MCRD  118  is required (e.g., in conjunction with repositioning the lead after an initial implantation) the lead  100  must generally be fully removed from the heart in order to replace the MCRD  118 . Then, following replacement of the MCRD  118 , the lead may be reinserted and implanted in the tissue. Alternatively, in at least certain conventional applications in which the MCRD  118  is not removable, the entire lead must generally be replaced. 
     As discussed in detail below, implementations of the present disclosure facilitate delivery of an active agent delivery component, such as an MCRD, directly into tissue at an implantation site. Such implementations generally include a lead that may be implanted at an implantation site and that include a lumen extending from a proximal end through a distal tip of the lead. Following implantation of the lead, a delivery stylet having an MCRD or similar active agent delivery component disposed on a distal end may be inserted through the lumen of the lead such that the active agent delivery component extends through the distal tip of the lead. Such extension enables the MCRD to be inserted/implanted directly into the tissue at the implantation site. 
     Notably, implementations of the present disclosure facilitate delivery of additional MCRDs or similar active agent delivery components without full removal of the lead. For example, following initial implantation, an additional MCRD or active agent delivery component may be delivered through the lumen of the lead without requiring detachment and removal of the lead from the implantation site. Similarly, if the lead tip is to be repositioned, the lead may be detached from the initial implantation site and implanted at a second implantation site without substantially removing the lead from the patient and/or the lead tip from an area of interest (e.g., a heart chamber). Following implantation at the second site, a new MCRD or other active agent delivery component may be delivered through the lead for implantation at the second implantation site. 
     Moreover, without requiring a dedicated chamber for housing an MCRD at a lead tip, the rigid length of the lead tip may be shortened, resulting in a more flexible and more maneuverable lead. 
     All of the foregoing combines to provide improved active agent delivery, easier modification to lead placement, and easier access to the implantation site, among other things. 
     I. Implantable Cardiac Lead Including an MCRD 
     In some instances, aspects of the present disclosure may include an implantable MCRD for use with an implantable cardiac lead. A non-limiting example of such a lead  100  is depicted in  FIG. 2A , which is a plan view of an embodiment of a lead  100  that is connectable with a pulse generator  311 . As illustrated in  FIG. 2A , in one embodiment, the lead  100  has a fixation anchor  326 , which is shown in an extended or deployed state. In general, the fixation anchor  326  implants into tissue to fix a distal end of the lead  100  therein. The fixation anchor  326  may be active such that it also provides pacing and/or sensing functionality or passive, such that the fixation anchor  326  anchors the lead to the tissue only. In implementations including a passive fixation anchor, pacing and/or sensing is generally facilitated by one or more additional electrodes disposed at a distal end of the lead  100 . Fixation member  106  of  FIG. 1A , for example, is a passive fixation member  106  in which electrode  108  provides sensing/pacing functionality. 
     The lead  100  may be designed for intravenous insertion and contact with the endocardium, or the lead  100  may be designed for placement external to the heart, for example, in the pericardial space. As indicated in  FIG. 2A , the lead  100  is provided with an elongated lead body  312  that extends between a proximal region  314  and distal region  102  of the lead  100 . 
     The proximal region  314  of the lead  100  includes a connector assembly  318 , which may include sealing rings  320  and at least one or more electrical connectors in the form of ring contacts  322 , pin contacts  324 , and the like. The connector assembly  318  is configured to be plugged into a receptacle  305  of the pulse generator  311 , the sealing rings  320  forming a fluid-tight seal to prevent the ingress of fluids into the receptacle  305  of the pulse generator  311 . When the connector assembly  318  is plugged into the pulse generator receptacle  305 , the contacts  322 ,  324  electrically connect with the circuitry of the pulse generator such that electrical signals can be administered and sensed by the pulse generator via the electrical pathways of the lead  100 . 
     The connector assembly  318  is constructed using known techniques and is preferably fabricated from silicone rubber, polyurethane, silicone-polyurethane-copolymer (“SPC”), or other suitable polymers. The electrical contacts  322 ,  324  are preferably fabricated of stainless steel or other suitable electrically conductive material that is biocompatible. 
     As can be understood from  FIG. 2A , in some embodiments, the distal region  102  of the lead  100  includes the fixation anchor  326  distally extending from a distal tip end  328  of the lead  100 . In certain implementations, the fixation anchor  326  may be transitioned between a retracted/non-deployed state and a deployed state, with  FIG. 2A  illustrating the fixation anchor in the deployed state. The fixation anchor  326  may be transitioned to the non-deployed state via retraction of the anchor  326  into the confines of the distal region  316  of the lead  100 . Although illustrated in the form of a helical wire/coil, the fixation anchor  326  may have any suitable shape and configuration that permits anchoring to tissue and, in implementations in which fixation anchor  326  is active, communication of electrical signals (e.g., pacing signals, sensing signals) to and/or from the tissue. 
     As previously noted, when active, the fixation anchor  326  is configured to act as an electrode in addition to providing fixation to heart tissue. Where the anchor  326  is also configured to act as an electrode, depending on the dictates of the pulse generator  311 , the anchor  326  may be employed for sensing electrical energy and/or administration of electrical energy (e.g., pacing). The anchor  326  is electrically coupled to the pin contact  324  of the connector assembly  318  via an electrical conductor extending through the lead body  312  and the connector assembly  318 . In implementations in which the anchor  326  is passive, similar electrical connections may be established between an electrode at or near the distal tip end  328  of the lead and the various components of the connector assembly  318 . 
     In certain implementations, the distal region  102  of the lead  100  may include a ring electrode  330  proximally offset from the distal tip end  328  of the lead  100 . Depending on the dictates of the pulse generator  311 , this ring electrode  330  may be employed for sensing electrical energy and/or administration of electrical energy (e.g., pacing). The ring electrode  330  is electrically coupled to one of the ring contacts  322  of the connector assembly  318  via an electrical conductor extending through the lead body  312  and the connector assembly  318 . 
     As depicted in  FIG. 2A , the lead  100  may include a fixation sleeve  334  slidably mounted around the lead body  312 . Among other things, the fixation sleeve  334  serves to stabilize the pacing lead  100  at the site of venous insertion. 
     Where the lead  100  is equipped for defibrillation as shown in  FIG. 2A , a shock coil  336  may also be supported on the lead body  312  proximal the ring electrode  330  and distal the fixation sleeve  334 . The shock coil  336  is electrically coupled to one of the ring contacts  322  of the connector assembly  318  via electrical conductors extending through the lead body  312 . 
     The foregoing description of the lead  100  is only as an example only and should not be considered limiting to the present disclosure. The present disclosure is not limited to any specific leads or implantable medical devices provided that the lead facilitates the active agent delivery systems and methods discussed below. Stated differently, leads and implantable devices in accordance with the present disclosure may vary from the specific example discussed above and may still be within the scope of this disclosure and, more specifically, the disclosure related to active agent delivery systems and methods provided below. Among other things, the design of the anchor, the placement and configuration of the electrical components, the type of the implantable device (e.g., pulse generator), lead construction, and other aspects of the system may vary from that which is disclosed above while still being within the scope of this disclosure. 
     Referring now to  FIG. 2B , an enlarged partial cutaway view of the distal region  102  of the implantable lead  100  of  FIG. 2A  is provided that illustrates conventional placement of the MCRD  118  within the distal region  102  of the implantable lead  100 . As illustrated in  FIG. 2B , the MCRD  118  is conventionally positioned within a housing structure  370  proximal the distal tip end  328  and the fixation anchor  326 . As discussed above in the context of  FIGS. 1A-1C , following implantation of the fixation anchor  326 , blood and other bodily fluids infiltrate the chamber  114  through a distal opening  116 , which causes the release of an active agent from the MCRD  118  into the adjacent tissue and blood. 
     II. Implantable Cardiac Lead System Employing an Implantable Active Agent Delivery Component, a Delivery Stylet, and a Pass-Through Lead Tip 
     A. Delivery Stylets 
     To begin a detailed discussion of an example cardiac lead and active agent delivery system, reference is made to  FIG. 3 , which depicts a side view of a delivery stylet  500  coupled to an active agent delivery component. For purposes of this disclosure, the active agent delivery component is also and interchangeably referred to as an MCRD  502 . As can be understood from  FIG. 3 , the delivery stylet  500  may include a stylet body  504  in the form of a cylindrical rod, a distal stylet end  506  for coupling with the MCRD  502 , and shoulder  508  having a distal facing surface  510 . The distal stylet end  506  may include a cylindrical body of a lesser diameter than a diameter of the stylet body  504 . The shoulder  508  joins the cylindrical bodies of different diameters of the distal stylet end  506  and the stylet body  504 . The distal stylet end  506  may include a distal bore  512  for receiving a portion of the MCRD  502 . The distal bore  512  may be defined centrally within the cylindrical body of the distal stylet end  506 . 
     As seen in  FIG. 3 , in one implementation of the present disclosure, the MCRD  502  may include a distal head  514  in the form of a conical distal surface that terminates at a piercing tip  516 . And a tang  518  extending proximally from the distal head  514  and that is received in the distal bore  512  of the distal stylet end  506  of the delivery stylet  500 . Various shapes and configurations of the MCRD  502 , as well as mechanisms to facilitate coupling and release of the MCRD  118  to/from the distal stylet end  506 , are contemplated and will be described subsequently. 
     In certain instances, the MCRD  502  may be injection molded from a blend of resorbable polymer and an active agent. In at least certain implementations, the active agent may be a steroid (e.g. dexamethasone acetate or dexamethasone). Resorbable polymers include polylactic acid (PLA), polyglycolic acid (PGA), and polycaprolactone (PCL) or a related co-polymer. By controlling the ratio of monomers in the polymer, the resorption time can be well controlled, allowing customization of active agent release to optimize the physiological effect. The resorbable polymers may be relatively rigid and may be formulated to be highly compatible with dexamethasone derivatives or other steroids so that a homogenous and highly concentrated blend can be achieved. Also, these polymers can be injection molded with high precision, allowing MCRD  502  designs smaller than conventional designs, such as the MCRD  118  illustrated in  FIG. 2B . In at least certain other implementations, the resorbable polymer may be substituted with an alternative material that is biocompatible and may be blended or impregnated with an active ingredient for subsequent release when exposed to tissue or bodily fluid. In at least certain implementations, the MCRD  502  may include a radiopaque marker. For example, and without limitation, the radiopaque marker may include a bead, ring, strip, stick, or similar object formed of a radiopaque material and embedded or coupled to a body of the MCRD  502 . Alternatively, the MCRD  118  may be molded, at least in part, using a material having radiopaque particles, powder, or other similar material mixed in. An example of a radiopaque bead  503  is illustrated in  FIG. 6A , which is discussed below in further detail. 
     B. Delivery Using a Lead Tip with Passive Fixation 
       FIG. 4A  is a cross-sectional side view of an electrode  408  of a lead  400  abutting the myocardium  120  with a placement stylet  520  positioned therein. The delivery stylet  500  and MCRD  502  of  FIG. 3  may be used with the lead tip or electrode  408  illustrated in  FIG. 4A . Although a passive fixation member is not included in  FIGS. 4A-4C  for clarity, the lead  400  may include a passive fixation member, such as the fixation members  106  of  FIG. 1A  for purposes of engaging and coupling the distal portion of the lead to the myocardium  120  and maintaining the electrode  408  in contact with the myocardium  120 . 
     As shown in  FIGS. 4A-4C , the electrode  408  is disposed at a distal end of the lead  400 . A tubular lead body  404  of the lead  400  surrounds and secures the electrode  408  within the lead  400  and is shown in broken line to clearly depict the electrode  408 . As can be understood from  FIG. 4A , the electrode  408  may include a generally cylindrical body of various diameters including a proximal cylindrical body  522 , a central cylindrical body  524 , a distal cylindrical body  526 , and a distal tip  528  in the form of a partially spherical surface tapering inward to a distal opening  530 . A proximally facing surface  532  separates the proximal cylindrical body  522  and the central cylindrical body  524 . And a distal facing surface  534  separates the central cylindrical body  524  and the distal cylindrical body  526 . A proximally facing surface  536  separates the distal tip  528  and the distal cylindrical body  526 . The tubular lead body  104  grips the body of the electrode  408  so as to secure it in position without permitting fluid to enter a space between the electrode  408  and the lead body  404 . 
     The electrode  408  further includes a proximal opening  538  and an inner surface  540  defining a lumen  542  that extends to the distal opening  530 . That is, the lumen  542  extends fully through the electrode  408 . The inner surface  540  of the electrode  408  includes a proximal cylindrical section  544 , a distal cylindrical section  546 , and a shoulder  548  having a proximally facing surface  550  separating the two sections  544 ,  546 . The distal opening  530  expands as it extends and transitions to the distal tip  528 . 
     As seen in  FIG. 4A , the placement stylet  520  includes a stylet body  552  that may terminate in a ball end  554 . The stylet body  552  may extend proximally through the lead and may be manipulated by the user to position the lead within the heart. As seen in  FIG. 4A , the placement stylet  520  may be positioned within the lumen  542  of the electrode  408  such that the ball end  554  of the stylet  520  abuts the proximally facing surface  550  of the shoulder  548  on the inner surface  540  of the electrode  408 . That is, the stylet  520  does not fully pass through the electrode  408 . In this way, the placement stylet  520  may be used to deliver the lead through the heart and in particular to advance the lead tip or electrode  408  to a desired position within the heart (e.g., abutting the myocardium) as seen in  FIG. 4A . 
     Following placement of the electrode  408  at an implantation site, such as the myocardium  120 , the placement stylet  520  may be withdrawn from the tubular lead body  404  of the lead  400 . Next, as seen in  FIG. 4B , the delivery stylet  500  coupled with the MCRD  502  may be inserted into the tubular lead body  404  of the lead  400 , and advanced until the distal facing surface  510  of the shoulder  508  of the delivery stylet  500  (or a similar feature, each of the distal facing surface  510  and the shoulder  508  being indicated in  FIG. 3 ) abuts the proximally facing surface  550  of the shoulder  548  on the inner surface  540  of the electrode  408  (or a similar feature). In this position, the distal stylet end  506  extends through the distal cylindrical section  546  of the electrode  108  with the MCRD  502  extending distally beyond a distal-most point of the distal tip  528  of the electrode  408  and into the tissue of the myocardium  120 . It is noted that an insertion depth may be limited by the length of the distal stylet end  506  as the stylet  500  can only extend up until it makes contact with the proximally facing surface  550  of the shoulder  548  on the inner surface  540  of the electrode  408 . 
     Next, as seen in  FIG. 4C , which is the same cross-sectional side view of the electrode  408  and myocardium  120  of  FIGS. 4A and 4B , the delivery stylet  500  may be proximally retracted, leaving the MCRD  502  implanted in the tissue of the myocardium  120 . Following implantation, the MCRD  502  elutes an active agent, such as a steroid, to surrounding tissue as indicated by the arrows surrounding the MCRD  502  in  FIG. 4C . 
     Although not fully illustrated in  FIGS. 4A-4C , it should be understood that the lead body  404  generally defines a lumen extending from a proximal portion of the lead  400  to the proximal opening  538  of the electrode  408  through which the delivery stylet  500  may be inserted. The lead body  404  may further include coiled wire or other electrical transmission means disposed along or about the lumen to enable electrical connection between the electrode  408  and other electrical components of the lead  400  (e.g., a shock/defibrillation coil, sensing coils, etc.) and a connector assembly of an implantable device, such as a pulse generator. 
     A physician may opt for a MCRD  502  with a particular shape, configuration, and/or material for the particular procedure. As an example, the physician may elect a MCRD  502  made from a resorbable polymer with a particular ratio of monomers in the polymer that yield a resorption time that is suitable for the procedure and optimizes the physiological effect. Additionally, or alternatively, the physician may elect a MCRD  502  with a particular shape, such as the shapes and configurations shown and described with reference to  FIGS. 6A-6F . 
     In certain instances, the MCRD  502  may be utilized independently of the electrode  408 . For example, a physician may opt for delivery of an electrode  408  without subsequent delivery of a MCRD  502 . Such instances may include, without limitation, cases where the patient may have an intolerance to a particular active agent. The electrode  408  may nevertheless permit a subsequent implantation of an MCRD  502  in a subsequent procedure. 
     In another instance, the electrode  408  may be repositioned after initial implantation. For example, following an initial implantation procedure including delivery of a first MCRD, a physician may determine that adequate pacing is not being achieved or that there is some other factor affecting the performance of the lead. Such cases may not be detected until well after (e.g., days or weeks) the initial implantation of the lead. With a conventional electrode design, such as shown in  FIGS. 1A-1C , the entire lead must generally be removed from the patient in order to replace the original MCRD (which would be partially or entirely depleted) with a new MCRD. Alternatively, in cases where the MCRD is sealed within the lead and cannot be removed/replaced, the initial lead may need to be replaced in its entirety with a new lead including a new MCRD. In either case, the lead including a new MCRD or a new lead must then be reinserted into the heart for implantation at a new pacing location. 
     In contrast to the foregoing conventional approach, implementations of the present disclosure enable relocation of the electrode  408  and provision of a new MCRD  502  without requiring removal of the lead  400  from the patient. More specifically, the electrode  408  may be dislodged from the initial implantation site and relocated to a new implantation site. Once at the new implantation site, a new delivery stylet  500  including a new MCRD  502  may be used to deliver a new MCRD  502  to the new implantation location. Accordingly, the electrode  408  may be relocated and a new MCRD  502  may be provided at the new implantation site without the time, complexity, and costs associated with fully removing a lead, replacing an MCRD of the lead (or the entire lead if the MCRD cannot be removed/replaced), and delivering the lead to the new implantation location, among other things. 
     At least certain implementations of the present disclosure may facilitate a reduced rigid length of the implantable lead as compared to conventional leads. By reducing the rigid length—particularly at the tip of the lead—the lead becomes easier to manipulate and place within the heart (or other implantation location) and, as a result, may lead to improved implantation accuracy, reduced implantation time, and other similar benefits. Moreover, the reduced rigid length of the lead may enable implantation of the lead in areas of the heart that are better for pacing but ultimately unreachable or unavailable when using conventional leads having longer rigid lengths. 
     The foregoing improvement is illustrated in further detail in  FIGS. 5A and 5B . More specifically,  FIGS. 5A and 5B  illustrate a conventional electrode  108  and an electrode  408  in accordance with the present disclosure, respectively. The conventional electrode  108  of  FIG. 5A  includes a distal chamber  114  for housing an MCRD  118 . The conventional electrode  108  further defines a proximal opening  112  that terminates proximal the distal chamber  114  and into which a placement stylet  150  may be inserted to facilitate delivery and implantation of the conventional electrode  108 . In contrast, the electrode  408  of  FIG. 5B  defines a lumen  542  fully passing through the electrode  408 . As previously discussed in the context of  FIG. 4A , during delivery and implantation of the electrode  408 , the placement stylet  520  is made to abut a shoulder  548  disposed just proximal the distal opening  530  of the electrode  408 . As shown in  FIG. 5B , the shoulder  548  also provides a stop for the delivery stylet  500  during delivery of the MCRD  502 . As seen in  FIG. 5A , a length L 1  of the conventional electrode  108  from a distal tip  152  to a proximal end  154  is longer than a length L 2  of the electrode  408  of  FIG. 5B  extending between the distal tip  528  and the proximal end  556 . More specifically, the electrode  108  of  FIG. 5A  slidingly couples with the placement stylet  150  only through a portion of the proximal end of the electrode  108  since the distal chamber  114  is sealed from the distal opening  112 . Accordingly, the length of the conventional electrode  108  is at least as long as the length of the distal chamber  114  and the proximal opening  112 , combined. In contrast, the electrode  408  of  FIG. 5B , includes a lumen  542  fully extending through the electrode  408  for an instrument (e.g., placement stylet, delivery stylet) to be positioned up to the shoulder  548  on the inner surface  540  of the electrode  408 . Accordingly, the length of the electrode  408  is generally based on providing sufficient electrode length to be received and retained within a lead body (e.g., the lead body  404  of  FIGS. 4A-4C ) and/or to provide sufficient contact surface between the surface of the lumen  542  and the delivery stylet  500  so as to provide stability for the delivery stylet  500  during MCRD delivery. Stated differently, the omission of at least the distal chamber  114  from the electrode  408  allows for the overall length of the electrode  408  to be reduced, resulting in the various benefits, including the specific benefits noted above. 
     C. Example Active Agent Delivery Component Configurations 
       FIGS. 6A-6F  illustrate six exemplary cross-sectional side views of active agent delivery components/MCRDs for use with the cardiac lead system described herein. In each figure, the MCRD  502 A- 502 F is coupled with the distal stylet end  506  of the delivery stylet  500  in an interference fit or friction fit arrangement whereby the MCRD  502 A- 502 F is capable of sliding out from within the distal bore  512  upon the MCRD  502 A- 502 F being anchored to the tissue of the heart. Each MCRD  502 A- 502 F of  FIGS. 6A-6F  includes a distal head  514  designed to anchor into the tissue of the heart and a tang  518  extending proximally from the distal head  514 . The MCRD  502 A of  FIG. 6A  includes a distal head  514  in the form of a conical surface that terminates at a penetrating tip  516 . The distal head  514  also includes a proximal radial edge  558  and a planar proximally facing surface extending from the radial edge  558  to the tang  518 . The MCRD  502 A of  FIG. 6A  also includes a radiopaque marker in the form of an embedded radiopaque bead  503  embedded therein. As previously discussed, such markers may be included in any MCRD disclosed herein and may take various forms including, without limitation, embedded beads, strips, bands, or similar objects or radiopaque additives included when molding the MCRD. 
     The MCRD  502 B of  FIG. 6B  includes a bulbous distal head  514  in the form of an ellipsoid that terminates at a penetrating tip  516 . A widest portion of the bulbous head  514  is about halfway along its length with the head  514  tapering inwardly in both distal and proximal directions. The MCRD  502 C of  FIG. 6C  is similar to the MCRD  502 A of  FIG. 6A , except the MCRD  502 C of  FIG. 6C  includes an intermediate cylindrical body  560  positioned between the tang  518  and the distal head  514 . In this way, the proximal radial edge  558  is spaced apart from the end of the distal stylet end  506  to permit additional anchoring between the MCRD  502 C and the tissue of the heart. The MCRD  502 D of  FIG. 6D  includes a distal head  514  in the form of a conical surface that terminates at a penetrating tip  516 . The distal head  514  is coupled to a cylindrical body  562  that is about the same diameter as the distal stylet end  506 . The cylindrical body  562  includes a proximal radial edge  558  and a planar proximally facing surface extending from the radial edge  558  to the tang  518 . 
     The MCRD  502 E of  FIG. 6E  is similar to the MCRD  502 A of  FIG. 6A , except the MCRD  502 E of  FIG. 6E  includes barbs  564  on the conical surface of the distal head  514  to facilitate anchoring to the tissue of the heart. The MCRD  502 F of  FIG. 6F  includes a generally cylindrical body  566  and a helical thread  568  wrapping around the body  566 . The cylindrical body  566  terminates at a distal tip  516 . The tang  518  may be keyed so as to be non-rotational within the distal bore  512 . In this way, the delivery stylet  500  may be rotated within the lead body  104  so as to rotationally advance the MCRD  502 F into the tissue of the heart. 
     Each of MCRD  502 A- 502 F should be regarded as examples of active agent delivery components/MCRDs that may be used in implementations of the present disclosure as well as illustrating certain features that may be included or combined in an MCRD. Accordingly, features of the foregoing MCRD designs may be used alone or in combination. Moreover, while specific examples of MCRDs are provided herein, implementations of the present disclosure are not limited to any specific shapes or configurations disclosed herein. More generally, any MCRD shape or configuration may be used in implementations of the present disclosure provided that the shape/configuration facilitates each of coupling to and deployment from a distal end of a delivery stylet, including implantation in tissue to be treated using the MCRD. 
     D. Example Coupling and Release Mechanisms 
       FIGS. 7A through 7D  depict four exemplary ways for decoupling of the MCRD  502  and the distal stylet end  506  of the delivery stylet  500 . These figures do not show the electrode  408  or other components of the lead system for clarity. To begin,  FIG. 7A  depicts a side cross-sectional view of the delivery stylet  500  and the MCRD  502 , which is positioned within the myocardium  120 . In this instance, the delivery stylet  500  is proximally retracted from the MCRD  502  via pulling after implantation of the MCRD  502 . The pulling overcomes the friction-fit arrangement between the tang  518  of the MCRD  502  and the distal bore  512  of the distal stylet end  506  of the delivery stylet  500 , thereby releasing the MCRD  502  from the delivery stylet  500 . 
       FIG. 7B  depicts a side cross-sectional view of the delivery stylet  500  decoupling with the MCRD  502 . As seen in the figure, the distal bore  512  of the stylet  500  is a full through-bore extending the length of the delivery stylet  500 . A push stylet  570  may be distally advanced within the delivery stylet  500  to push against the tang  518  of the MCRD  502  and to force the MCRD  502  into the myocardium  120 . Pushing of the MCRD  502  by the push stylet  570  overcomes the friction-fit arrangement between the tang  518  of the MCRD and the distal bore  512  of the distal stylet end  506 . In certain implementations, the friction-fit may be further or alternatively overcome by retracting the delivery stylet  500  while simultaneously pushing against the MCRD  502  or maintaining the MCRD  502  in position using the push stylet  570 . 
       FIG. 7C  depicts a side cross-sectional view of the delivery stylet  500  decoupling with the MCRD  502 . As seen in the figure, a retention member  572  in the form of a breakable wire (e.g., formed of the MCRD  502  material or otherwise) is coupled to the tang  518  of the MCRD  502 . Upon implantation of the MCRD  502  in the myocardium  120  the delivery stylet  500  and the retention member  572  may be retracted proximally. The proximal force will then sever the retention member  572  leaving the MCRD  502  anchored within the myocardium  120 . As with the delivery stylet  500  of  FIG. 7B , the stylet  500  in  FIG. 7C  may include a full through-bore within which the retention member  572  may be disposed. 
       FIG. 7D  depicts a side cross-sectional view of the delivery stylet  500  coupled with the MCRD  502 , which is positioned in the myocardium  120 . As seen in the figure, the annular tip of the distal stylet end  506  is coupled to the proximally facing surface of the MCRD  118  via a biocompatible chemical bonding material  574 . The chemical bonding material  574  may dissolve upon interaction with the blood and/or bodily tissue/fluids in the myocardium  120 . In at least certain implementations, the diameter of the distal bore  512  may be slightly larger than that of the tang  518 , relying primarily on the chemical bonding material  574  to retain the MCRD  118  within the distal stylet end as opposed to the friction-fit arrangement in the previously described embodiments. 
     E. Delivery Using a Lead Tip with Active Fixation 
     Section II.B., above, described a cardiac lead system utilizing a lead tip or electrode  408  that was fixed to the myocardium via passive fixation members, e.g., fixation members that were not part of the electrode  408  but that were part of the tubular lead body  404  of the lead  400 . In this section, the cardiac lead system described herein includes active fixation members, i.e., fixation members that both fix the lead to the myocardium and provide sensing and/or stimulation functionality. 
     To begin, reference is made to  FIGS. 8A-8E .  FIG. 8A  depicts a cross-sectional side view of a delivery stylet  700  coupled to an active agent delivery component, such as an MCRD  702 . As can be understood from  FIG. 8A , the delivery stylet  700  may include a proximal stylet body  704  in the form of a cylindrical rod and a distal stylet end  706  in the form of a cylindrical rod that couples with the MCRD  702 . As shown, the distal stylet end  706  may include a cylindrical body of a lesser diameter than a diameter of the stylet body  704 . As a result, the delivery stylet  700  includes a shoulder  708  having a distal facing surface  710  defined by the junction of the distal stylet end  706  and the stylet body  704 . The distal stylet end  706  may include a distal bore  712  for receiving a portion of the MCRD  702 . The distal bore  712  may be defined centrally within the cylindrical body of the distal stylet end  706 . 
     As seen in  FIG. 8A , in one embodiment, the MCRD  702  may include a distal head  714  in the form of a conical distal surface that terminates at a piercing tip  716 . And a tang  718  extending proximally from the distal head  714  for receiving in the distal bore  712  of the distal stylet end  706  of the delivery stylet  700 . As previously discussed and as further discussed below, various shapes and configurations of the MCRD  702  are contemplated as are mechanisms to facilitate coupling and decoupling of the MCRD  702  to the distal stylet end  706 , including mechanisms to facilitate deployment/implantation in cardiac tissue. 
       FIGS. 8B-8E  depict cross-sectional side views of a distal region  802  of an implantable cardiac lead  800 . As illustrated in  FIG. 8B , the distal region  802  includes a fixation anchor  826  that further acts as an electrode  808 . The fixation anchor  826  is coupled to and supported within the lead  800  by a conductive post  876  or similar structure that is in turn coupled to conductive wires  878  extending along the lead  800 , e.g., from a proximal lead connector assembly, such as the connector assembly  318  of  FIG. 2A . Accordingly, an electrical pathway exists between the proximal end of the lead  800  (e.g., the connector assembly  318 ) and the fixation anchor/electrode  826 ,  808 , thereby facilitating delivery of electrical impulses to the anchor/electrode  826 ,  808  and/or sensed electrical impulses from the anchor/electrode  826 ,  808 . 
     During implantation, the lead  800  is inserted into and navigated to an implantation site, e.g., an implantation site within or on an exterior surface of a patient&#39;s heart. The anchor/electrode  826 ,  808  is then rotated (e.g., by rotating the lead  800  as a whole or an independently rotatable shaft within the lead  800  (not shown)) to engage the anchor/electrode  826 ,  808  with the cardiac tissue  890 , such as shown in  FIG. 10C . Once initially engaged, additional rotation of the anchor/electrode  826 ,  808  may be performed to further embed the anchor/electrode  826 ,  808  into the tissue  890 . 
     As further illustrated in  FIG. 8C , following implantation of the lead  100  using the anchor/electrode  826 ,  808 , the delivery stylet  700  including the MCRD  702  may be inserted into and distally advanced within the lead  800 . Alternatively, the delivery stylet  700  may be disposed within the lead  800  during implantation of the anchor/electrode  826 ,  808  and subsequently moved distally following implantation of the anchor/electrode  826 ,  808 . To facilitate movement of the delivery stylet  700  within the lead  800 , the lead  800  generally defines a lumen  842  extending through the conductive wires  878 , the conductive post  876 , and the anchor/electrode  826 ,  808 . The delivery stylet  700  and coupled MCRD  702  may therefore be advanced through the lumen  842  until the shoulder  708  contacts a proximal end of the conductive post  876 , as shown in  FIG. 8D . In this position, the MCRD  702  extends past the distal annulus  880  of the lead  800 , and into the tissue  890  at the implantation site. Next, the delivery stylet  700  may be proximally retracted and removed, as seen in  FIG. 8E , leaving the MCRD  702  within the tissue  890 . 
     F. Delivery Stylet with Movable Inner Stylet 
     In certain implementations, the delivery stylet  700  of  FIG. 8A  (or any other delivery stylet disclosed herein) may be alternatively constructed with an outer sleeve  782  and an inner stylet  784 , as seen in  FIGS. 9A and 9B . As seen in  FIG. 9A , which is a cross-sectional side view of the delivery stylet  700 , the inner stylet  784  may slide within the outer sleeve  782 , and may include a distal bore  712  defined therein for releasably coupling to the MCRD  702 .  FIG. 9B , which is also a cross-sectional side view of the delivery stylet  700  of  FIG. 9A , illustrates how the inner stylet  784  may extend beyond the annulus of the outer sleeve  782  so as to extend the MCRD  702 . 
     Use of the delivery stylet  500  of  FIGS. 9A and 9B  is illustrated in  FIGS. 10A-10D , which depict cross-sectional side views of a distal region  802  of the implantable cardiac lead  800  previously discussed in the context of  FIGS. 8A-8E .  FIG. 10A  depicts the distal region  802  of the cardiac lead  800  prior to introduction of the delivery stylet  700 . In  FIG. 10B , the outer sleeve  782  of the delivery stylet  700  is shown as being advanced within the lumen  842  of the lead  800  until the distal end of the outer sleeve  782  contacts a proximal portion of the conductive post  876 . In at least certain instances, in this configuration, the outer sleeve  782  may be used to advance the distal region  802  of the lead  800 , similar to the placement stylets discussed previously herein. 
     Next, as seen in  FIG. 10C , the inner stylet  784  may be advanced through the outer sleeve  782 , through the lumen  842  of the conductive post  876 , and out of the lead  800 . Doing so extends the MCRD  702  out of the distal end of the lead, e.g., into the tissue at the implantation site. Next, the inner stylet  784  and the outer sleeve  782  may be proximally retracted, as seen in  FIG. 10D , leaving the MCRD  702  in place in the tissue at the implantation site. 
     Although illustrated in  FIGS. 10A-10D  as being used with an active fixation lead, delivery stylets including multiple sleeves, such as the delivery stylet  700  of  FIGS. 9A and 9B  may also be configured for use with passive fixation leads, such as those discussed in the context of  FIGS. 4A-5D . For example, referring to  FIG. 4B , following implantation of the lead and during delivery of the MCRD  702 , the outer sleeve  782  of the delivery stylet  700  may be made to abut the proximally facing surface  550  of the shoulder  548  of the electrode  408 . Subsequently, the inner stylet  584  may be translated distally to deliver/implant the MCRD  702 . Alternatively, the outer sleeve may be made to abut any other proximally facing surface of the electrode prior to distal translation of the inner stylet  584 . 
     III. Exemplary Method of Implanting a Cardiac Lead Tip and Active Agent Delivery Component 
       FIG. 11  is a flowchart showing an exemplary method  1100  of implanting a cardiac lead tip into tissue at an implantation site, including implanting an active agent delivery component/MCRD at the implantation site. 
     As seen in  FIG. 11 , the method  1100  may include, at step  1102 , implanting a distal lead end of a lead body into tissue, such as the myocardium. The lead body generally includes a distal lead and defines a lead lumen extending through the lead body to the distal lead end. In certain implementations, the distal lead end may be an active fixation distal lead end, such as a helical fixation anchor that also functions as an electrode and discussed above in the context of  FIGS. 8B-8E and 10A-10D . Alternatively, the distal lead end may include a passive fixation anchor (e.g., tines, barbs jutting out from the tubular lead body) with a separate electrode fitted to the lead body and that abuts against the tissue when implanted, e.g., as seen in  FIGS. 4A-4C , and  5 B. 
     Next, at step  1104 , the method  1100  may include inserting a delivery stylet through the lead lumen. The delivery stylet may include a stylet body having a distal stylet end and an active agent delivery component (e.g., an MCRD) detachably coupled to the distal stylet end. The delivery stylet may be a single piece or multi-piece stylet, as described above. 
     Step  1106  of the method  1100  may include extending the distal stylet end distally from the distal lead end such that the active agent delivery component is inserted into the tissue at the implantation site. Step  1108  of the method  1100  may include detaching the active agent delivery component from the distal stylet end and within the tissue at the implantation site. Detaching of the active agent delivery component may be done a variety of ways. For example, and without limitation, any of the release mechanisms and techniques described above in the context of  FIGS. 7A-7D, 8E, and 10D  may be used to detach the active agent delivery component within the tissue. 
     At step  1110 , the delivery stylet may be fully retracted and removed from the lead body. Subsequent to removal of the delivery stylet, a proximal connector assembly of the lead may be inserted into or otherwise connected to an IPG or similar implantable device to electrically connect the electrode of the lead to the internal implantable device circuitry, thereby enabling pacing and/or sensing via the lead. 
     The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present invention. From the above description and drawings, it will be understood by those of ordinary skill in the art that the particular embodiments shown and described are for purposes of illustrations only and are not intended to limit the scope of the present invention. References to details of particular embodiments are not intended to limit the scope of the invention.