Patent Publication Number: US-2021178151-A1

Title: Systems and methods for implanting a medical electrical lead

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/257,519, filed on Apr. 21, 2014 which claims the benefit of priority from U.S. Provisional Application No. 61/820,014, filed on May 6, 2013, the contents of both applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The disclosure relates generally to implantable medical devices of the type for performing monitoring of a physiologic state and/or therapy delivery. In particular, the disclosure pertains to tools for implanting medical electrical leads for the physiologic state monitoring and/or therapy delivery. 
     BACKGROUND 
     Implantable cardiac defibrillator (ICD) systems are used to deliver high energy electrical pulses or shocks to a patient&#39;s heart to terminate life threatening arrhythmias, such ventricular fibrillation. Traditional ICD systems include a housing that encloses a pulse generator and other electronics of the ICD and is implanted subcutaneously in the chest of the patient. The housing is connected to one or more implantable medical electrical leads that are implanted within the heart. 
     Traditional ICD systems that utilize transvenous leads may not be the preferable ICD system for all patients. For example, in some patients, difficult vascular access precludes placement of transvenous leads. As another example, children and other younger patients may also be candidates for non-transvenous ICD systems. Moreover, transvenous leads may become fibrosed in the heart over time, making lead revision and extraction procedures challenging. 
     A subcutaneous ICD system may be preferred for some patients. A subcutaneous ICD system includes a lead (or leads) that are implanted subcutaneously in the patient, i.e., between the skin and the ribs and/or sternum of the patient. As such, the subcutaneous ICD may eliminate the need for transvenous leads being within the heart. A need exists for tools and methods for delivery of non-transvenous leads to implant locations other than to the heart. 
     SUMMARY 
     Devices and methods for implantation of an implantable medical lead are disclosed. Exemplary implantation devices include a pliable sheath having an inner lumen, an elongate tool having a proximal end and a distal end configured to be slidingly disposed within the inner lumen, wherein the elongate tool includes a pre-biased curvature that is oriented to form a bend at a distal portion of the tool, and a handle coupled to the proximal end of the elongate tool. 
     In accordance with embodiments of this disclosure, the method for placement of an implantable medical lead in a patient&#39;s body includes forming an access point at a first location of the body, providing an implant tool including a distal end having a pre-biased curve, inserting the distal end through the access point into the substernal space, and utilizing the implant tool to advance the implantable medical lead into an implant location within the substernal space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of the compositions and methods according to the invention will be described in detail, with reference to the following figures wherein: 
         FIG. 1A  is a front view of a patient implanted with implantable cardiac system; 
         FIG. 1B  is a side view the patient implanted with implantable cardiac system; 
         FIG. 1C  is a transverse view of the patient implanted with implantable cardiac system; 
         FIG. 2  depicts a perspective view of an embodiment of a delivery system for implanting a medical electrical lead; 
         FIG. 3A  depicts a side cross-sectional view of an embodiment of a delivery system for implanting a medical electrical lead; 
         FIG. 3B  shows a transverse sectional view of an embodiment of a delivery system for implanting a medical electrical lead; 
         FIG. 4  depicts a perspective view of an alternative embodiment of a delivery system for implanting a medical electrical lead; 
         FIG. 5  depicts a perspective view of an alternative embodiment of a delivery system for implanting a medical electrical lead; 
         FIG. 6  depicts a perspective view of an alternative embodiment of a delivery system for implanting a medical electrical lead; 
         FIG. 7  illustrates a side cross-sectional view of an alternative embodiment of a portion of a delivery system; 
         FIG. 8  illustrates a side cross-sectional view of an alternative embodiment of a portion of a delivery system; 
         FIG. 9  illustrates a side cross-sectional view of an alternative embodiment of a portion of a delivery system; 
         FIG. 10  illustrates a transverse cross-sectional view of an alternative embodiment of a portion of a delivery system; 
         FIG. 11  illustrates a transverse cross-sectional view of an alternative embodiment of a portion of a delivery system; 
         FIG. 12  illustrates a transverse cross-sectional view of an alternative embodiment of a portion of a delivery system; 
         FIG. 13  depicts an alternative embodiment of a delivery system for implanting a medical electrical lead; 
         FIG. 14  depicts a partial side cross-sectional view of a portion of the delivery systems in accordance with some embodiments; 
         FIG. 15A  shows a transverse sectional view of a portion of the delivery systems in accordance with some embodiments; 
         FIG. 15B  shows a transverse sectional view of a portion of the delivery systems in accordance with some embodiments; 
         FIG. 16  is a flow chart depicting a method of implanting a lead according to an embodiment of the disclosure; 
         FIGS. 17-19  are partial perspective views that illustrate the method of implanting a lead of  FIG. 16 . 
     
    
    
     The following detailed description is to be read with reference to the figures, in which like elements in different figures have like reference numerals. The figures, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the present teachings. Skilled artisans will recognize the examples provided herein have many useful alternatives and fall within the scope of the present teachings. 
     DETAILED DESCRIPTION 
     In this disclosure, techniques, components, assemblies, and methods for delivery of a lead into a targeted delivery site within a substernal space are described. The lead may be delivered through a surgical incision created on the skin/tissue adjacent to or below the xiphoid process (also referred to as “subxiphoid”) to form an access point to the substernal space, and advancing the lead with the aid of a delivery system through which the lead is inserted into the substernal space. The access point may also be formed at the notch (not shown) that connects the xiphoid process to the sternum. In other embodiments, the substernal space may also be accessed through the manubrium. 
     In this disclosure, “substernal space” refers to the region defined by the undersurface between the sternum and the body cavity but not including the pericardium. In other words, the region is posterior to the sternum and anterior to the ascending aorta. The substernal space may alternatively be referred to by the terms “retrosternal space” or “mediastinum” or “infrasternal” as is known to those skilled in the art and includes the region referred to as the anterior mediastinum. The substernal space may also include the anatomical region described in Baudoin, Y. P., et al., entitled “The superior epigastric artery does not pass through Larrey&#39;s space (trigonum sternocostale).” Surg. Radiol. Anat. 25.3-4 (2003): 259-62 as Larrey&#39;s space. For ease of description, the term substernal space will be used in this disclosure, it being understood that the term is interchangeable with any of the other aforementioned terms. 
     In this disclosure, the term “extra-pericardial” space refers to region around the outer heart surface, but not within the pericardial sac/space. The region defined as the extra-pericardial space includes the gap, tissue, bone, or other anatomical features around the perimeter of, and adjacent to the pericardium. 
       FIGS. 1A-C  are conceptual diagrams of a patient  12  implanted with an example implantable cardiac system  10 .  FIG. 1A  is a front view of patient  12  implanted with implantable cardiac system  10 .  FIG. 1B  is a side view patient  12  with implantable cardiac system  10 .  FIG. 10  is a transverse view of patient  12  with implantable cardiac system  10 . 
     Implantable cardiac system  10  includes an implantable cardiac defibrillator (ICD)  14  connected to a first lead  16  and a second lead  18 . The first lead  16  and the second lead  18  may be utilized to provide an electrical stimulation therapy such as pacing or defibrillation. For example, lead  16  may provide defibrillation therapy while lead  18  may provide pacing therapy, or vice versa, while in other embodiments, both lead  16  and lead  18  may provide pacing therapy or defibrillation therapy. In the example illustrated in  FIGS. 1A-C  ICD  14  is implanted subcutaneously on the left midaxillary of patient  12 . ICD  14  may, however, be implanted at other subcutaneous locations on patient  12  as described later. 
     Lead  16  includes a proximal end that is connected to ICD  14  and a distal end that includes one or more electrodes. Lead  16  extends subcutaneously from ICD  14  toward xiphoid process  20 . At a location near xiphoid process  20 , lead  16  bends or turns and extends subcutaneously superior, substantially parallel to sternum  22 . The distal end of lead  16  may be positioned near the second or third rib. However, the distal end of lead  16  may be positioned further superior or inferior depending on the location of ICD  14  and other factors. Although illustrated as being offset laterally from and extending substantially parallel to sternum  22  in the example of  FIGS. 1A-C , lead  16  may be implanted over sternum  22 , offset from sternum  22 , but not parallel to sternum  22  (e.g., angled lateral from sternum  22  at either the proximal or distal end). 
     Lead  16  includes a defibrillation electrode  24 , which may include an elongated coil electrode or a ribbon electrode, toward the distal end of lead  16 . Lead  16  is placed such that a therapy vector between defibrillation electrode  24  and a housing or can electrode of ICD  14  is substantially across the ventricle of heart  26 . 
     Lead  16  may also include one or more sensing electrodes, such as sensing electrodes  28  and  30 , located toward the distal end of lead  16 . In the example illustrated in  FIGS. 1A-C , sensing electrode  28  and  30  are separated from one another by defibrillation electrode  24 . ICD  14  may sense electrical activity of heart  26  via a combination of sensing vectors that include combinations of electrodes  28  and  30  and the housing or can electrode of ICD  14 . For example, ICD  14  may obtain electrical signals sensed using a sensing vector between electrodes  28  and  30 , obtain electrical signals sensed using a sensing vector between electrode  28  and the conductive housing or can electrode of ICD  14 , obtain electrical signals sensed using a sensing vector between electrode  30  and the conductive housing or can electrode of ICD  14 , or a combination thereof. In some instances, ICD  14  may even sense cardiac electrical signals using a sensing vector that includes defibrillation electrode  24 . 
     Lead  18  includes a proximal end that is connected to ICD  14  and a distal end that includes one or more electrodes. Lead  18  extends subcutaneously from ICD  14  toward xiphoid process  20 . At a location near xiphoid process  20 , the lead  18  bends or turns and extends superior upward in the substernal space. In one example, lead  18  may be placed in the mediastinum  36  and, more particularly, in the anterior mediastinum. The anterior mediastinum is bounded laterally by pleurae  40 , posteriorly by pericardium  38 , and anteriorly by sternum  22 . Lead  18  may be implanted within the mediastinum such that one or more electrodes  32  and  34  are located over a cardiac silhouette of the ventricle as observed via fluoroscopy. In the example illustrated in  FIGS. 1A-C , lead  18  is located substantially centered under sternum  22 . In other instances, however, lead  18  may be implanted such that it is offset laterally from the center of sternum  22 . Although described herein as being implanted in the substernal space, the mediastinum, or the anterior mediastinum, lead  18  may be implanted in other extra-pericardial locations. 
     Lead  18  includes electrodes  32  and  34  located near a distal end of lead  18 . Electrodes  32  and  34  may comprise ring electrodes, hemispherical electrodes, coil electrodes, helical electrodes, ribbon electrodes, or other types of electrodes, or combinations thereof. Electrodes  32  and  34  may be the same type of electrodes or different types of electrodes. In the example illustrated in  FIGS. 1A-C  electrode  32  is a hemispherical electrode and electrode  34  is a ring or coil electrode. 
     ICD  14  may deliver pacing pulses to heart  26  via a pacing or therapy vector that includes any combination of one or both of electrodes  32  and  34  and a housing electrode or can electrode of ICD  14 . For example, ICD  14  may deliver pacing pulses using a pacing or therapy vector between electrodes  32  and  34 , deliver pacing pulses using a pacing or therapy vector between electrodes  32  and the conductive housing or can electrode of ICD  14 , deliver pacing pulses using a pacing or therapy vector between electrodes  34  and the conductive housing or can electrode of ICD  14 , or a combination thereof. In some instances, ICD  14  may deliver pacing therapy via a therapy vector between one of electrode  32  (or electrode  34 ) and defibrillation electrode  24 . In still further instances, ICD  14  may deliver pacing therapy via a therapy vector between one of electrode  32  (or electrode  34 ) and one of sensing electrodes  28  or  30 . ICD  14  may generate and deliver the pacing pulses to provide anti-tachycardia pacing (ATP), bradycardia pacing, post shock pacing, or other pacing therapies or combination of pacing therapies. In this manner, ATP therapy or post shock pacing (or other pacing therapy) may be provided in an ICD system without entering the vasculature or the pericardial space, nor making intimate contact with the heart. 
     ICD  14  may generate and deliver pacing pulses with any of a number of amplitudes and pulse widths to capture heart  26 . The pacing thresholds of heart  26  when delivering pacing pulses substernally using lead  18  may depend upon a number of factors, including location of electrodes  32  and  34 , location of ICD  14 , physical abnormalities of heart  26  (e.g., pericardial adhesions), or other factors. The pacing thresholds needed to capture heart  26  tend to increase with shorter pulse widths. In the case of ATP, ICD  14  may deliver pacing pulses having longer pulse widths than conventional ATP pulses to reduce the amplitude of the pacing pulses. For example, ICD  14  may be configured to deliver pacing pulses having pulse widths or durations of greater than or equal to one (1) millisecond. In another example, ICD  14  may be configured to deliver pacing pulses having pulse widths or durations of greater than or equal to ten (10) milliseconds. In a further example, ICD  14  may be configured to deliver pacing pulses having pulse widths or durations of greater than or equal to fifteen (15) milliseconds. In yet another example, ICD  14  may be configured to deliver pacing pulses having pulse widths or durations of greater than or equal to twenty (20) milliseconds. Depending on the pulse widths, ICD  14  may be configured to deliver pacing pulses having pulse amplitudes less than or equal to twenty (20) volts, deliver pacing pulses having pulse amplitudes less than or equal to ten (10) volts, deliver pacing pulses having pulse amplitudes less than or equal to five (5) volts, deliver pacing pulses having pulse amplitudes less than or equal to two and one-half (2.5) volts, deliver pacing pulses having pulse amplitudes less than or equal to one (1) volt. Typically the lower amplitudes require longer pacing widths as illustrated in the experimental results. Reducing the amplitude of pacing pulses delivered by ICD  14  reduces the likelihood of extracardiac stimulation. 
     ICD  14  may sense electrical activity of heart  26  via a combination of sensing vectors that include combinations of electrodes  32  and  34  and the housing or can electrode of ICD  14 . For example, ICD  14  may obtain electrical signals sensed using a sensing vector between electrodes  32  and  34 , obtain electrical signals sensed using a sensing vector between electrode  32  and the conductive housing or can electrode of ICD  14 , obtain electrical signals sensed using a sensing vector between electrode  34  and the conductive housing or can electrode of ICD  14 , or a combination thereof. In some instances, ICD  14  may sense electrical activity of heart  26  via a sensing vector between one of electrode  32  (or electrode  34 ) and electrodes  24 ,  28  and  30  of lead  16 . ICD  14  may deliver the pacing therapy as a function of the electrical signals sensed via one or more of the sensing vectors of lead  18 . Alternatively or additionally, ICD  14  may deliver the pacing therapy as a function of the electrical signals sensed via the one or more of the sensing vectors of lead  16 . 
     ICD  14  also analyzes the sensed electrical signals from one or more of the sensing vectors of lead  18  and/or one or more of the sensing vectors of lead  16  to detect tachycardia, such as ventricular tachycardia or ventricular fibrillation. In some instances, ICD  14  delivers one or more ATP therapies via the one or more pacing or therapy vectors of lead  18  in response to detecting the tachycardia in an attempt to terminate the tachycardia without delivering a defibrillation shock. If the one or more ATP therapies are not successful or it is determined that ATP therapy is not desired, ICD  14  may deliver one or more defibrillation shocks via defibrillation electrode  24  of lead  16 . 
     The configuration described above in  FIGS. 1A-1C  is directed to providing ventricular pacing via lead  18 . In situations in which atrial pacing is desired in addition to or instead of ventricular pacing, lead  18  may be positioned further superior. A pacing lead configured to deliver pacing pulses to both the atrium and ventricle may have more electrodes. For example, the pacing lead may have one or more electrodes located over a cardiac silhouette of the atrium as observed via fluoroscopy and one or more electrodes located over a cardiac silhouette of the ventricle as observed via fluoroscopy. A pacing lead configured to deliver pacing pulses to only the atrium may, for example, have one or more electrodes located over a cardiac silhouette of the atrium as observed via fluoroscopy. In some instances, two substernal pacing leads may be utilized with one being an atrial pacing lead implanted such that the electrodes are located over a cardiac silhouette of the atrium as observed via fluoroscopy and the other being a ventricle pacing lead being implanted such that the electrodes are located over a cardiac silhouette of the ventricle as observed via fluoroscopy 
     ICD  14  may include a housing that forms a hermetic seal that protects components of ICD  14 . The housing of ICD  14  may be formed of a conductive material, such as titanium. ICD  14  may also include a connector assembly (also referred to as a connector block or header) that includes electrical feedthroughs through which electrical connections are made between conductors within leads  16  and  18  and electronic components included within the housing. As will be described in further detail herein, housing may house one or more processors, memories, transmitters, receivers, sensors, sensing circuitry, therapy circuitry and other appropriate components. Housing  34  is configured to be implanted in a patient, such as patient  12 . 
     Leads  16  and  18  include a lead body that includes one or more electrodes located near the distal lead end or elsewhere along the length of the lead body. The lead bodies of leads  16  and  18  also contain one or more elongated electrical conductors (not illustrated) that extend through the lead body from the connector assembly of ICD  14  provided at a proximal lead end to one or more electrodes of leads  16  and  18 . The lead bodies of leads  16  and  18  may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. However, the techniques are not limited to such constructions. 
     The one or more elongated electrical conductors contained within the lead bodies of leads  16  and  18  may be coupled to one or more of electrodes  24 ,  28 ,  30 ,  32 , and  34 . In one example, each of electrodes  24 ,  28 ,  30 ,  32 , and  34  is electrically coupled to a respective conductor within its associated lead body. The respective conductors may electrically couple to circuitry, such as a therapy module or a sensing module, of ICD  14  via connections in connector assembly, including associated feedthroughs. The electrical conductors transmit therapy from a therapy module within ICD  14  to one or more of electrodes  24 ,  28 ,  30 ,  32 , and  34  and transmit sensed electrical signals from one or more of electrodes  24 ,  28 ,  30 ,  32 , and  34  to the sensing module within ICD  14 . 
     The examples illustrated in  FIGS. 1A-C  are exemplary in nature and should not be considered limiting of the techniques described in this disclosure. In other examples, ICD  14 , lead  16 , and lead  18  may be implanted at other locations. For example, ICD  14  may be implanted in a subcutaneous pocket in the right chest. In this example, lead  16  may be extend subcutaneously from the device toward the manubrium of the sternum and bend or turn and extend subcutaneously inferiorly from the manubrium of the sternum, substantially parallel with the sternum and lead  18  may extend subcutaneously from the device toward the manubrium of the sternum to the desired location and bend or turn and extend substernally inferiorly from the manubrium of the sternum to the desired location. 
     In the example illustrated in  FIGS. 1A-C , system  10  is an ICD system that provides pacing therapy. However, these techniques may be applicable to other cardiac systems, including cardiac resynchronization therapy defibrillator (CRT-D) systems, cardioverter systems, or combinations thereof. 
     In addition, it should be noted that system  10  may not be limited to treatment of a human patient. In alternative examples, system  10  may be implemented in non-human patients, e.g., primates, canines, equines, pigs, bovines, ovines, and felines. These other animals may undergo clinical or research therapies that may benefit from the subject matter of this disclosure. 
       FIGS. 2, 3A and 3B  illustrate an embodiment of a delivery system  100  for implanting a medical electrical lead in a substernal space of a patient. The delivery system  100  may be utilized to create a pathway through the body of patient  12  to access an implant location within the substernal space. The delivery system  100  will be discussed in conjunction with  FIGS. 2, 3A, and 3B , where  FIG. 2  depicts a perspective view,  FIG. 3A  depicts a side cross-sectional view, and  FIG. 3B  shows a transverse sectional view. 
     The delivery system  100  includes a sheath  102 , an elongate tool  104  and a handle  106 . The sheath  102  includes a continuous lumen through which the elongate tool  104  is disposed. The continuous lumen may extend between openings at a proximal end and a distal end of the sheath  102  such that, in use, the sheath  102  is slidingly-disposed over the elongate tool  104  during axial advancement of the elongate tool  104  through patient  12  to facilitate an implant procedure. 
     In some embodiments, the sheath  102  may include a slit segment  118  that is formed proximate to the proximal end. The slit segment  118  may extend partially through or entirely along a length of the wall of sheath  102 . For example, the slit segment  118  may be formed as perforations that extend from the inner to outer surface along the side wall of sheath  102 . The slit segment  118  facilitates the slitting of the sheath  102  during the implant procedure. In use, the lead  18  will be advanced to the target site via the lumen of sheath  102 . After placement of the lead  18 , the sheath  102  may be separated from the lead so as to withdraw the sheath  102  from the patient  12  by slitting the side walls of the sheath  102  at the slit segment  118 . 
     The inventors of the present disclosure have discovered that it may be desirable to implant the lead  18  such that it overlies the cardiac silhouette of the heart  26  as visualized through an imaging technique for effective therapy delivery by the lead  18 . Yet, it may be desirable not to place the lead  18  in direct contact with the heart tissue. Therefore, the present disclosure addresses techniques for implanting the lead  18  in the substernal space underneath the sternum. 
     Accordingly, one embodiment of the elongate tool  104  includes a pre-biased curvature  108  that is formed along a length of the body of elongate tool  104  proximate to a distal end  110  of the elongate tool. As shown in  FIG. 2 , the pre-biased curvature  108  is configured such that the segment of the elongate body  104  adjacent to the distal end  110  is curved to orient the distal portion in a non-parallel plane relative to the plane defined by the proximal portion. The angle of curvature of the pre-biased curvature is predicated on orienting the section of the elongate tool  104  that is proximal to distal end  110  at an angle that is substantially perpendicular to the sternum of patient  12  while the rest of the elongate tool  104  is generally parallel to the sternum of patient  12 . For example, the pre-biased curvature  108  is configured having a bend that orients the distal end  110  at an angle that is greater than 5 degrees relative to a first plane, with the first plane being defined along a central axis of the proximal portion of the elongate tool  104 . 
     The distal end  110  is configured to provide a tactile signal in response to contact with tissue, bone or other anatomical features along a pathway from the access point into the substernal space of patient  12  to a desired implant location. For example, the pre-biased curvature  108  may be oriented such that the distal end  110  is placed in contact with the sternum, or more particularly the sternebrae. 
     Continuing with the example, the distal end  110  contacts the various bones along the ribcage or at the fusion point between the ribs and the sternum or with the sternum itself as the elongate tool  104  is advanced during the implantation. Responsive to the contact between the elongate tool  104  and the patient  12 , distal end  110  creates a tactile signal that provides an indication of the position of the distal end  110  relative to the patient  12 . 
     An additional benefit of the pre-biased curvature  108  is that it positions the distal end  110  away from the body cavity and the organs underneath the sternum by orienting the distal end  110  towards the sternum during navigation of the elongate tool with the substernal space. 
     Sheath  102  may be formed from a pliable material such as bio-compatible plastic including polyaryletheretherketone (PEEK) thermoplastic, PARYLENE® polyxylylene polymers, or other suitable polymer material. The elongate tool  104  may be formed from a rigid material such as a metal including, titanium or stainless steel. In other embodiments, the elongate tool  104  material is a biocompatible rigid material such, for example, as TECOTHANE® thermoplastic polyurethanes that may have elastic “memory” properties. 
     The handle  106  facilitates maneuvering of the elongate tool  104 . As such, the handle  106  is coupled to the proximal end  112  of the elongate tool  104 . The handle  106  may be formed from materials that are similar to those of the elongate tool  104  or from a dissimilar material. Handle  106  further includes a directional indicator  116  that provides an indication of the orientation of the pre-biased curvature  108  of the distal end  110 . As will be discussed below, the handle  106  may alternatively be formed in a predefined shape, such that the shape of the handle will provide an indication of the orientation of the pre-biased curvature  108 . 
     The directional indicator  116  provides a visual indicator of the orientation of distal end  110  positioned within the body of patient  12  from the exterior of the patient  12 . In addition, the directional indicator  116  will facilitate re-orientation of the distal end  110  during navigation of the delivery system  100  within the body of patient  12 , such, for example, as the navigation to the substernal space. 
     The delivery system  100  may deliver a fluid through a port or an opening to tissue adjacent to the port or opening. As will be discussed below in conjunction with embodiments of  FIGS. 4, 5 and 6 , the fluid may be held in a reservoir of the delivery system  100 , or delivered from an external reservoir through the delivery system  100 . 
     In one embodiment, elongate tool  104  may be provided with a lumen(s) and a fluid dispersion port(s) (not shown) for passage of the fluids through the lumen to be dispensed through the opening or port along the length of the elongate tool  104 . Such a lumen is configured to dispense the fluid through an opening at the distal end  110 . The lumen may facilitate delivery of a fluid such as a therapeutic solution, such as antibiotics or antimicrobial agents, or any other fluid solution (e.g., a contrast solution) during an implantation procedure of a medical electrical lead into the substernal space. For example, the fluid may be a medical anesthetic substance that is delivered into the tissue adjacent to the implant pathway as the elongate tool  104  is advanced through the patient. 
     Alternatively, or in addition, the fluid may be a contrast solution that facilitates visualization of the elongate tool  104  to verify the location of the distal end  110 . 
     In some embodiments, a radiopaque marker element  114  may be disposed on the elongate tool  104  and/or sheath  102 . In the illustrative embodiment of  FIG. 2 , for instance, the element  114  is depicted overlaying a segment of the distal end  110 . Nevertheless, it should be understood that the element  114  may overlay or coat any other section or sections of the elongate tool  104  or may alternatively overlay the entire elongate tool  104 . Element  114  may be formed from a band of radiopaque material that is coupled to the distal end  110  through any suitable mechanism. In other embodiments, the distal-most portion of the elongate tool  104  may be formed from a radiopaque material. The radiopaque material may include a compound, such as barium sulphate, that is visible through a fluoroscopic imaging procedure. In use, the marker element  114  can provide a visual depiction or image of the distal end  110 . 
     In other embodiments, one or more mapping electrodes  130  may be positioned on the sheath  102  or the elongate tool  104 . The mapping electrodes  130  may be used in conjunction with, or as a substitute for the radiopaque marker element  114  to facilitate mapping of the location of the delivery system  100  within the substernal implant location. The mapping electrodes  130  are electrically coupled to a location mapping unit such as that disclosed in U.S. Pat. No. 7,850,610 issued to Ferek-Petric, which is incorporated herein by reference in its entirety. 
     In one embodiment, the elongate tool  104  and sheath  102  may be sized such that the dimensions of the lumen of sheath  102  will permit insertion of elongate tool  104  and/or the lead  18  therethrough. In an example, sheath  102  may suitably be formed having a lumen having a diameter in the range of 4 French (Fr) to 12 Fr, and preferably a 10.5 Fr diameter and having a length ranging from between 6 inches and 24 inches, it being understood that the length may further be customized outside those dimensions to cater for the variation of the human anatomy from patient-to-patient. It should be appreciated that the length of the elongate tool  104  is dimensioned to be slightly longer, for example 2 inches longer, than the sheath  102 . This relative difference will ensure that the distal-most portion of the tool  104 , including distal end  110 , is exposed distally of the distal opening of the sheath  102 . For illustrative purposes, it should be appreciated that the length of the elongate tool  104  is dimensioned having a length that enables the distal end  110  of the elongate tool  104  to be positioned adjacent to the first rib within the substernal space and extend to an incision performed on the skin adjacent to the xiphoid process of patient  12 , with the proximal end  112  being located external to the patient  12 . 
       FIGS. 4-6  depict alternative embodiments of delivery systems for implanting a medical electrical lead in a substernal space of a patient.  FIGS. 7, 8, 9 and 10, 11, and 12  illustrate cross sectional views of alternative embodiments of an elongate tool. In particular,  FIG. 7  depicts a side cross-sectional view of any one of the elongate bodies depicted in  FIGS. 4-6 , and  FIG. 10  shows the corresponding transverse sectional view.  FIG. 8  depicts a side cross-sectional view of any one of the elongate bodies depicted in  FIGS. 4-6 , and  FIG. 11  shows the corresponding transverse sectional view.  FIG. 9  depicts a side cross-sectional view of any one of the elongate bodies depicted in  FIGS. 4-6 , and  FIG. 12  shows the corresponding transverse sectional view. 
     Each of the delivery systems  200   a - c  (collectively, “delivery system(s)  200 ”) includes an elongate tool  204  and a handle  206   a - d  (collectively, “handle(s)  206 ”). A distal portion of the elongate tool  204  of the delivery systems  200  includes a pre-biased curvature that may correspond to the pre-biased curvature  108  described in conjunction with  FIG. 2 . A fluid insertion port  222  is provided on any of the handles  206   a - d  that may be in fluid communication with one or more fluid lumen(s)  224  disposed within the elongate tool  204 . One or more fluid dispersion ports or openings (not shown) are provided in fluid communication with the fluid lumens  224  for delivery of the fluid. 
     The handle  206   a  is formed with a directional indicator  220   a  that is integrally formed with the handle and that can be visualized on the external surface. The handle  206   a  is configured to provide an indication of the orientation of the distal end of elongate tool  204 . The directional indicator  220   a  may comprise a projection formed on a portion of the handle  206   a  that is shaped as a prominently visible protrusion. The directional indicator  220   a  such as a detent, that is directed towards a plane that is parallel to the plane of the curved portion of the distal end of the elongate tool  204 . 
     The handle  206   b  illustrated in  FIG. 5  includes a reservoir (not shown) that may be configured to hold a fluid for delivery through the lumen  224  of elongate tool  204 . A plunger  226  may be provided to control the injection of fluid through the lumen  224 . 
     The handle  206 C illustrated in the alternative embodiment of  FIG. 6  is coupled to an external reservoir that holds a fluid that is delivered through the elongate tool  204 . 
       FIG. 13  depicts another embodiment of a delivery system  250  for implanting a medical electrical lead in a substernal space of a patient. The delivery system  250  includes a sheath  252 , an elongate tool  254 , a handle  256 , and a sealing assembly  258 . System  250  will be discussed in conjunction with  FIGS. 14, 15A, and 15B , where  FIG. 14  depicts a partial side cross-sectional view of the sheath  252  and sealing assembly  258 , and  FIGS. 15A and 15B  show transverse sectional views of the sealing assembly  258 . 
     Sheath  252  may be constructed with the distal terminal end having a tapered profile. Providing the tapered distal end reduces the trauma caused to patient  12  during advancement of the system  250  in an implant procedure. The sheath  252  includes a continuous lumen  260  through which the elongate tool  254  is disposed. The lumen  260  (shown in dashed lines) may extend distally from a proximal opening to a distal end  262  to facilitate axial advancement of the elongate tool  254  therethrough during an implant procedure. 
     A slit segment  264   a  may be provided along the wall of sheath  252  to enable slitting of the sheath  252  during the implant procedure. The slit segment  264   a  may be provided at the proximal end of the sheath  252 . 
     In accordance with an embodiment, the elongate tool  254  is constructed with a pre-biased curvature  266  that extends proximally from the distal end  262 . The pre-biased curvature  266  forms a bend at a location situated about 1 to 4 inches from the distal end  262 . The angle of curvature of the pre-biased curvature may vary from between 1 degree to 20 degrees relative to an imaginary axial line formed by the proximal portion of the elongate tool, so as to orient the distal end  262  towards a different plane relative to the axial plane defined by the proximal portion of the elongate tool  254 . 
     In use, the distal end  262  may provide a tactile signal as described in conjunction with the distal end  110  responsive to contact with tissue, bone or other anatomical features along a pathway from the access point into the substernal space of patient  12  to a desired implant location. The pre-biased curvature  266  also positions the distal end  262  away from the body cavity and the organs underneath the sternum by orienting the distal end  262  towards the sternum during navigation of the elongate tool  254  with the substernal space. 
     Sheath  252  may be formed from a pliable material such as bio-compatible plastic including polyaryletheretherketone (PEEK) thermoplastic, PARYLENE® polyxylylene polymers, a polyether block amide such as Pebax®, a polyolefin such as Pro-fax, or other suitable polymer material. The distal end  262  of sheath  252  may be constructed from an elastomer such as polyether block amide, or polyamide  12  and/or with a hydrophilic coating or any other material that facilitates gliding of the distal end over the elongate tool  254 . The elongate tool  254  may be formed from a rigid material such as a metal including, titanium or stainless steel. In other embodiments, the material for elongate tool  254  is a bio-compatible rigid material such, for example, as TECOTHANE® thermoplastic polyurethanes that may have elastic “memory” properties. 
     The handle  256  is coupled to the proximal end  274  of the elongate tool  254 . The handle  256  facilitates maneuvering of the elongate tool  254 . Embodiments of the handle  256  may resemble the handles  106 , or  206 . The handle  256  is depicted having a directional indicator  268  that facilitates visualization of the orientation of distal end  262 . In addition, the directional indicator  268  will facilitate re-orientation of the distal end  262  during navigation of the delivery system  250  within the body of patient  12 , such, for example, as the navigation to the substernal space. 
     The delivery system  250  may further include a fluid lumen for delivery of a fluid through a port or an opening to tissue adjacent to the port or opening as discussed in conjunction with embodiments of  FIGS. 7-12 . 
     In other embodiments, a radiopaque marker element  270  may be disposed on the elongate tool  254 . In the illustrative embodiment of  FIG. 13 , for instance, the element  270  is depicted overlaying two segments of the distal portion. Element  254  may be formed as a band of radiopaque material that is coupled to the elongate tool through coating or any other any suitable mechanism. The material of the radiopaque marker element  270  may include a compound, such as barium sulphate, that is visible through a fluoroscopic imaging procedure. In use, the marker element  270  can provide a visual depiction or image of the distal portion of elongate tool  254  within the patient  12 . In other embodiments, the marker element  270  may be coupled to the sheath  252  instead of or in addition to being coupled to the elongate tool  254 . 
     As described above, the elongate tool  254  and the sheath  252  may be sized such that the dimensions of the sheath  252  will permit insertion of a lead  18  therethrough. 
     It may be desirable to prevent air from being pushed into the body cavity of the patient  12  during the implant procedure of the lead  18 . Preventing the introduction of air within the body cavity facilitates the effectiveness of therapy delivery to the patient. This may further assist in establishing the threshold parameters for the patient  12  during the implant procedure. 
     Accordingly, the sheath  252  is provided with the sealing assembly  258  that prevents or reduces the amount of air that is pushed into the body cavity during the implant procedure. Thus, the sealing assembly  258  may be disposed proximal to a proximal opening into the lumen  260  to provide a seal into the lumen  260  of sheath  252 . The sealing assembly  258  defines a passage  272  therethrough that is substantially aligned with the lumen  260 . As used herein, substantially aligned refers to the central axis of the lumen  260  and the central axis of the passage  272  being adjacent to each other such that the lumen  260  and passage  272  are in fluid communication. In addition, substantially aligned refers to the alignment of the passage  272  with a portion of the lumen  260  especially because of the dimensional differences as will be discussed below. 
     As shown in the illustrations of  FIGS. 15A and 15B , the sealing assembly  258  is configured such that passage  272  defines a first diameter D 1  prior to introduction of an accessory device such as the elongate tool  254  or lead  18 , and a second diameter D 2  responsive to insertion of the accessory device. Hence, the diameter D 1  is less than the diameter D 2 . In an embodiment, the sealing assembly  258  tapers distally towards the intersection of the passage  272  with the lumen  260 . For example, the passage  272  may expand up to 100% of the diameter of the lumen  260  or as little as 0.1% of the diameter of the lumen  260 . The sealing assembly  258  is constructed such that the diameter D 1  is tailored to accommodate passage of the accessory devices, e.g., elongate tool  254  and the lead  18 , while providing an interference seal to prevent ingress of air around the outer circumference of the accessory device. In order to accommodate variations in the diameters of the accessory devices, the sealing assembly  258  is formed from materials that have the necessary elongation properties to prevent permanent deformation during insertion and passage of the accessory devices. Such a material may include a relatively soft and resilient material, for example, a liquid silicone rubber (LSR) material or a thermoplastic elastomer (TPE) material, as compared to the material that forms the sheath  252 . 
     The sealing assembly  258  may also be formed having a slit segment  264   b  that extends from an exterior surface of the sidewall to the interior surface. The slit segment  264   b  is formed such that it is continuous with the slit segment  264   a  to create a continuous slit path. As has been described above, the slit segment  264   b  in conjunction with slit segment  264   a  will enable the sheath  252  to be separate from the lead  18  during an implant procedure. 
     Although the sealing assembly  258  is depicted being positioned proximate to the proximal end  274 , it should be understood that the sealing assembly  258  may suitably be positioned anywhere along a length of the sheath  252 .  FIG. 16  is a flow chart  300  of a method of implanting a lead according to an embodiment of the present invention.  FIGS. 17-19  are partial perspective views that illustrate the method  300  of implanting the lead  18  at a suitable implant location within a substernal space  6 .  FIGS. 18-19  depict a schematic view of the ribcage  4  of patient  12 . The sternum  22  is a flat, narrow bone comprising three segments: the manubrium, the body, and the xiphoid process. 
     At task  302 , an incision  2  is made on the skin/tissue adjacent to or below the xiphoid process (also referred to as “subxiphoid”) to form an access point sized for passage of a delivery system and/or a lead ( FIG. 17 ) to the substernal space. The access point may also be formed at the notch (not shown) that connects the xiphoid process to the sternum. In other embodiments, the substernal space may also be accessed through the manubrium.  FIG. 17  illustrates the exemplary anterior or pectoral incision  2  on patient  12 . The incision location provides access to the substernal space  6  underneath the ribcage  4  and is sized to allow insertion of a delivery system for navigation of the lead. The lead e.g., lead  18 , may be coupled to an implantable medical device  14  that is implantable or implanted in a subcutaneous location. As such a portion of the lead  18  may be tunneled through subcutaneous tissue from the device  14  to the incision location. 
     A delivery system  1000  is provided for facilitating the lead implant ( 304 ). The delivery system  1000  may be embodied as any of the aforementioned delivery systems  100 ,  200 ,  250 , or combinations thereof, described in conjunction with  FIGS. 2-15B  that include a sheath, an elongate tool, a catheter and optionally a sealing assembly. The delivery system  1000  will be provided with the elongate tool being disposed within the lumen of the sheath. 
     At task  306 , delivery system  1000  is inserted through the incision. As described above, the exemplary delivery systems include an elongated body having a pre-biased curvature at the distal end. At task  308 , the curved distal portion is oriented such that the distal end is pointed towards the sternum ( FIG. 18 ). A directional indicator on the handle of the delivery system may be utilized to assist in placement or to confirm the proper orientation of the distal end. The directional indicator may resemble any one of those described in conjunction with the preceding figures. 
     At task  310 , the elongated body of the delivery system is advanced within the substernal space underneath the sternum in a generally axial direction from the xiphoid process towards the jugular notch. During the advancing of the delivery system, the distal end is navigated in direct contact or close proximity with the sternum. In some embodiments, a fluid may be delivered during the advancing of the delivery system into the substernal space at task  312 . For example, the delivery system may deliver an analgesic agent or a contrast solution or any other suitable fluid. Optionally, a signal is generated that is indicative of the location of the distal end of the delivery system ( 314 ). The signal may be a tactile signal such as a sensation or sound that is generated in response to the interaction of the distal end with various segments of the sternum or ribs of the ribcage connected to the sternum. Alternatively, or in addition, an imaging procedure may be performed to obtain an image of a segment of the delivery system. To that end, the radiopaque marker elements described above may be utilized in conjunction with fluoroscopy during the advancing of the delivery system  1000  to obtain a visual indication of the directional orientation of the distal portion of the delivery system within the patient. With the aid of the signal(s), the delivery system is navigated such that a distal portion is positioned at a target implant location of the distal end of the lead ( FIG. 19 ). 
     Upon confirmation that the delivery system has been positioned at the appropriate location, the elongate tool is then removed from the sheath ( 316 ). Subsequent to withdrawing the elongate tool from the lumen of the sheath, the lead is then advanced through the body along the length of the lumen of the sheath ( 318 ). 
     It is during this exchange of the elongate tool with the lead body that potential air can be trapped in the tubing and pushed into the body cavity of the patient in the substernal space  6 . As such, a delivery system such as that disclosed in  FIGS. 14-15B  may be utilized in accordance with some embodiments of the method. The sealing mechanism of such a delivery system will seal the distal opening of the sheath to prevent air from filling the lumen. When such a catheter is used, advancing of the lead into the sheath will not push air (or will push only a minimal amount of air) into the substernal space  6 . 
     At task  320 , the lead is advanced to the implant location through the delivery system. In one embodiment, the elongated body of the delivery system may be retracted from the sheath, leaving the sheath positioned within the substernal space. In other embodiments, the delivery system may have a lumen for insertion of the lead through the lumen. The lead may be preloaded within the lumen of the delivery system, in some examples, prior to insertion of the delivery system into the substernal space. At task  322 , the lead is positioned at the appropriate implant location. In some embodiments, the positioning may include orienting the electrodes to provide a targeted stimulation therapy and/or fixation of the lead to the tissue at the implant site. 
     At task  324 , the sheath is withdrawn from the patient  12  and the lead remains within the substernal space. In accordance with some embodiments, a slittable sheath such as those described above may be utilized. Slitting of the sheath may be performed in accordance with conventional techniques, for example, utilizing a slitting tool. The slittable sheath facilitates withdrawal of the sheath by separating the body of the sheath from the lead to ensure that the lead placement is not impacted as the sheath is pulled distally away from the incision  2 . At task  326 , the lead may be coupled to a stimulation pulse generator, such as ICD  14 . In other embodiments the lead may be tunneled from the access point to the ICD  14  that is positioned subcutaneously on the left midaxillary of patient  12 . 
     As described herein, delivery systems in accordance with various embodiments are provided that facilitate implantation of a lead in the substernal space. In alternative implementations, the delivery systems may be utilized for delivery of a lead in locations other than the substernal space including but not limited to the aforementioned extra-pericardial space. 
     Various examples have been described. It is contemplated that the features described in the different embodiments may be combined to create additional embodiments. All such disclosed and other examples are within the scope of the following claims.