Patent Publication Number: US-2020297993-A1

Title: Extravascular implantable electrical lead having undulating configuration

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/963,303, filed on Dec. 9, 2015, which claims the benefit of both U.S. Provisional Application No. 62/089,417, filed on Dec. 9, 2014 and U.S. Provisional Application No. 62/262,408, filed on Dec. 3, 2015, the entire content of each is incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present application relates to electrical stimulation leads and, more particularly, electrical stimulation leads having an undulating configuration for improved defibrillation, sensing, and/or pacing capabilities for use in extracardiovascular applications (e.g., subcutaneous or substernal applications). 
     BACKGROUND OF THE INVENTION 
     Malignant tachyarrhythmia, for example, ventricular fibrillation, is an uncoordinated contraction of the cardiac muscle of the ventricles in the heart, and is the most commonly identified arrhythmia in cardiac arrest patients. If this arrhythmia continues for more than a few seconds, it may result in cardiogenic shock and cessation of effective blood circulation. As a consequence, sudden cardiac death (SCD) may result in a matter of minutes. 
     In patients with a high risk of ventricular fibrillation, the use of an implantable cardioverter defibrillator (ICD) system has been shown to be beneficial at preventing SCD. An ICD system includes an ICD that is a battery powered electrical shock device, that may include an electrical housing electrode (sometimes referred to as a can electrode), that is coupled to one or more electrical lead wires placed within the heart. If an arrhythmia is sensed, the ICD may send a pulse via the electrical lead wires to shock the heart and restore its normal rhythm. Owing to the inherent surgical risks in attaching and replacing electrical leads directly within or on the heart, subcutaneous ICD systems have been devised to provide shocks to the heart without placing electrical lead wires within the heart or attaching electrical wires directly to the heart. 
     Electrical leads being utilized in subcutaneous systems typically include linear or curvilinear arrays of electrodes positioned on the lead body. Thus, the delivery of electrical stimulation therapy to the heart with current lead designs provides limited therapy vectors depending on the shape of the lead body, for which the electrical energy may impact the heart. 
     SUMMARY 
     This disclosure describes an implantable medical electrical lead and an ICD system utilizing the lead. The lead includes a lead body defining a proximal end and a distal portion, wherein at least a part of the distal portion of the lead body defines an undulating configuration. The lead includes a defibrillation electrode that includes a plurality of defibrillation electrode segments disposed along the undulating configuration spaced apart from one another by a distance. The lead also includes at least one electrode disposed between adjacent sections of the plurality of defibrillation sections. The at least one electrode is configured to deliver a pacing pulse to the heart and/or sense cardiac electrical activity of the heart. 
     In some instances, the plurality of defibrillation electrode segments are disposed along at least 80% of undulating configuration. In other instances, the plurality of defibrillation electrode segments are disposed along at least 90% of undulating configuration. The undulating configuration may include a plurality of peaks with a first portion of the plurality of peaks extending in a first direction away from a major longitudinal axis of the lead and a second portion of the plurality of peaks extending in a second, opposite direction away from the major longitudinal axis of the lead. The plurality of defibrillation electrode segments may, in some examples, be disposed along the first portion of the plurality of peaks and the at least one electrode may be disposed on the second portion of the plurality of peaks. In another example, the plurality of defibrillation electrode segments are disposed along at least one of the first and second portions of peaks and the at least one electrode is disposed along a segment of the undulating portion between peaks. 
     This application also provides an extravascular implantable cardioverter-defibrillator (ICD) system comprising an extravascular electrical stimulation lead and an ICD coupled to the extravascular electrical stimulation lead. The electrical stimulation lead includes a lead body defining a proximal end and a distal portion, wherein at least a part of the distal portion of the lead body defines an undulating configuration. The lead includes a defibrillation electrode that includes at least a first defibrillation electrode segment and a second defibrillation electrode segment disposed along the undulating configuration spaced apart from one another by a distance. The lead also includes at least one electrode disposed between the first and second defibrillation segments, the at least one electrode configured to, at least one of, deliver a pacing pulse to the heart and sense cardiac electrical activity of the heart. 
     This application also provides a method for implanting an extravascular electrical stimulation lead within a substernal location of a patient. The method includes creating an incision near a center of the torso of the patient, introducing an implant tool into the substernal location via the incision, and advancing the implant tool within the substernal location from the incision superior along a posterior of a sternum to form a substernal path. The method further includes introducing a distal portion of the lead into the substernal location. The lead includes a lead body defining a proximal end and the distal portion, wherein at least a part of the distal portion of the lead body defines a pre-formed undulating configuration, a defibrillation electrode that includes a plurality of defibrillation electrode segments disposed along the undulating configuration spaced apart from one another by a distance, and at least one electrode disposed between adjacent segments of the plurality of defibrillation segments, the at least one electrode configured to, at least one of, deliver a pacing pulse to the heart and sense cardiac electrical activity of the heart. The method includes advancing the distal portion of the lead through the substernal path, wherein the undulating configuration of the lead is in a relatively straight configuration when being advanced through the substernal path, and withdrawing the implant tool toward the incision to remove the implant tool from the body while leaving the lead in place along the substernal path. The distal portion of the lead takes its pre-formed undulating configuration within the substernal location as it exist the implant tool. The at least one electrode is disposed on the undulating configuration such that that undulating configuration pushes the at least one electrodes toward the left side of sternum compared to defibrillation electrode segments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a front view of a patient implanted with the extracardiovascular ICD system implanted intra-thoracically. 
         FIG. 1B  is a side view of the patient implanted with the extracardiovascular ICD system implanted intra-thoracically. 
         FIG. 1C  is a transverse view of the patient implanted with the extracardiovascular ICD system implanted intra-thoracically. 
         FIG. 2  is a front view of a patient implanted with the extracardiovascular ICD system implanted extra-thoracically. 
         FIG. 3A  is a schematic diagram illustrating an example lead constructed in accordance with the principles of the present application. 
         FIG. 3B  is a schematic diagram illustrating an side view of the distal portion of the example lead of  FIG. 3A . 
         FIG. 4  is a schematic diagram illustrating another example lead constructed in accordance with the principles of the present application. 
         FIG. 5  is a schematic diagram illustrating a further example lead constructed in accordance with the principles of the present application. 
         FIG. 6  is a schematic diagram illustrating another example lead constructed in accordance with the principles of the present application. 
         FIG. 7  is a schematic diagram illustrating another example lead constructed in accordance with the principles of the present application. 
         FIG. 8  is a schematic diagram illustrating another example lead constructed in accordance with the principles of the present application. 
         FIG. 9  is a schematic diagram illustrating another example lead constructed in accordance with the principles of the present application. 
         FIG. 10  is a functional block diagram of an example configuration of electronic components of an example ICD, such as the ICD of the system in  FIGS. 1A, 1C, and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, relational terms, such as “first” and “second,” “over” and “under,” “front” and “rear,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     Referring now to the drawings in which like reference designators refer to like elements, there is shown in  FIGS. 1A-C  and  FIG. 2  are conceptual diagrams illustrating various views of an exemplary extracardiovascular implantable cardioverter-defibrillator (ICD) system  8 . ICD system  8  includes an ICD  9  connected to a medical electrical lead  10  constructed in accordance with the principles of the present application.  FIG. 1A  is a front view of a patient implanted with the extracardiovascular ICD system  8 .  FIG. 1B  is a side view of the patient implanted with the extracardiovascular ICD system  8 .  FIG. 1C  is a transverse view of the patient implanted with the extracardiovascular ICD system  8 . 
     The ICD  9  may include a housing that forms a hermetic seal that protects components of the ICD  9 . The housing of the ICD  9  may be formed of a conductive material, such as titanium or titanium alloy, which may function as a housing electrode (sometimes referred to as a can electrode). In other embodiments, the ICD  9  may be formed to have or may include one or more electrodes on the outermost portion of the housing. The ICD  9  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 of lead  10  and electronic components included within the housing of the ICD  9 . As will be described in further detail herein, housing may house one or more processors, memories, transmitters, receivers, sensors, sensing circuitry, therapy circuitry, power sources and other appropriate components. The housing is configured to be implanted in a patient, such as the patient. 
     ICD  9  is implanted extra-thoracically on the left side of the patient, e.g., under the skin and outside the ribcage (subcutaneously or submuscularly). ICD  9  may, in some instances, be implanted between the left posterior axillary line and the left anterior axillary line of the patient. ICD  9  may, however, be implanted at other extra-thoracic locations on the patient as described later. 
       FIGS. 3A and 3B  are schematic diagrams illustrating various views of lead  10  in further detail. The lead  10  may include an elongated lead body  12  sized to be implanted in an extracardiovascular location proximate the heart, e.g., intra-thoracically (as illustrated in  FIGS. 1A-C ) or extra-thoracically (as illustrated in  FIG. 2 ). For example, the lead  10  may extend extra-thoracically under the skin and outside the ribcage (e.g., subcutaneously or submuscularly) from ICD  9  toward the center of the torso of the patient, for example, toward the xiphoid process of the patient. At a position proximate xiphoid process, the lead body  12  may bend or otherwise turn and extend superiorly. In the example illustrated in  FIGS. 1A-C , the lead body  12  extends superiorly intra-thoracically underneath the sternum, in a direction substantially parallel to the sternum. In one example, the distal portion  16  of lead  10  may reside in a substernal location such that distal portion  16  of lead  10  extends superior along the posterior side of the sternum substantially within the anterior mediastinum  36 . Anterior mediastinum  36  may be viewed as being bounded laterally by pleurae  39 , posteriorly by pericardium  38 , and anteriorly by the sternum  22 . In some instances, the anterior wall of anterior mediastinum  36  may also be formed by the transversus thoracis and one or more costal cartilages. Anterior mediastinum  36  includes a quantity of loose connective tissue (such as areolar tissue), adipose tissue, some lymph vessels, lymph glands, substernal musculature (e.g., transverse thoracic muscle), the thymus gland, branches of the internal thoracic artery, and the internal thoracic vein. In another example, e.g., illustrated in  FIG. 2 , the lead body  12  may extend superiorly extra-thoracically (instead of intra-thoracically), e.g., either subcutaneously or submuscularly above the ribcage/sternum. The lead  10  may be implanted at other locations, such as over the sternum, offset to the right of the sternum, angled lateral from the proximal or distal end of the sternum, or the like. 
     The lead body  12  may have a generally tubular or cylindrical shape and may define a diameter of approximately 3-9 French (Fr), however, lead bodies of less than 3 Fr and more than 9 Fr may also be utilized. In another configuration, the lead body  12  may have a flat, ribbon, or paddle shape with solid, woven filament, or metal mesh structure, along at least a portion of the length of the lead body  12 . In such an example, the width across the lead body  12  may be between 1-3.5 mm. Other lead body designs may be used without departing from the scope of this application. 
     The lead body  12  of lead  10  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 (not shown), however, the techniques are not limited to such constructions. The distal portion  16  may be fabricated to be biased in a desired configuration, or alternatively, may be manipulated by the user into the desired configuration. For example, the distal portion  16  may be composed of a malleable material such that the user can manipulate the distal portion into a desired configuration where it remains until manipulated to a different configuration. 
     The lead body  12  may include a proximal end  14  and a distal portion  16  which include an electrical stimulation therapy portion  18  configured to deliver electrical energy to the heart or sense electrical energy of the heart. The distal portion  16  may be anchored to a desired positioned within the patient, for example, substernally or subcutaneously by, for example, suturing the distal portion  16  to the patient&#39;s musculature, tissue, or bone at the xiphoid process entry site. Alternatively, the distal portion  16  may be anchored to the patient or through the use of rigid tines, prongs, barbs, clips, screws, and/or other projecting elements or flanges, disks, pliant tines, flaps, porous structures such as a mesh-like element and metallic or non-metallic scafolds that facilitate tissue growth for engagement, bio-adhesive surfaces, and/or any other non-piercing elements. 
     The lead body  12  may define a substantially linear portion  20  as it curves or bends near the xiphoid process and extends superiorly. As shown in  FIGS. 1-3 , at least a part of the distal portion  16  may define an undulating configuration  22  distal to the substantially linear portion  20 . In particular, the distal portion  16  may define an undulating pattern, e.g., (zig-zag, meandering, sinusoidal, serpentine, or other pattern) as it extends toward the distal end of the distal portion  16 . In other configurations, the lead body  12  may not have a substantially linear portion  20  as it extends superiorily, but instead the undulating configuration may begin immediately after the bend. 
     The undulating configuration  22  may include a plurality of peaks  24  along the length of the distal portion  16 . In an exemplary configuration, the undulating configuration  22  of lead  10  includes three peaks  24   a ,  24   b , and  24   c . In other configurations, however, the undulating configuration  22  may include any number of peaks  24 . For example, the number of peaks  24  may be fewer or greater than three depending on the frequency of the undulation configuration  22 . For example, a higher frequency undulating configuration  22  may include more peaks  24  (e.g., as illustrated in the examples illustrated in  FIGS. 6-9 ) while a lower frequency undulating configuration  22  may include fewer peaks  24  (e.g., as illustrated in the examples of  FIGS. 4 and 5 ). 
     The undulating configuration  22  may further define a peak-to-peak distance “d,” (shown in  FIG. 3 ), which may be variable or constant along the length of the undulating configuration  22 . In the configuration illustrated in  FIGS. 1-3 , the undulating configuration  22  defines a substantially sinusoidal configuration, with a constant peak-to-peak distance “d” of approximately 2.0-5.0 cm. The undulating configuration  22  may also define a peak-to-peak width “w,” (shown in  FIG. 3 ), which may also be variable or constant along the length of the undulating configuration  22 . In the configuration illustrated in  FIGS. 1-3 , the undulating configuration  22  defines a substantially sinusoidal shape, with a constant peak-to-peak width “w” of approximately 0.5-2.0 cm. However, in other instances, the undulating configuration  22  may define other shapes and/or patterns, e.g., S-shapes, wave shapes, or the like. 
     The distal portion  16  includes a defibrillation electrode  26  configured to deliver a cardioversion/defibrillation shock to the patient&#39;s heart. The defibrillation electrode  26  may include a plurality of sections or segments  28  spaced a distance apart from each other along the length of the distal portion  16 . The defibrillation electrode segments  28  may be a disposed around or within the lead body  12  of the distal portion  16 , or alternatively, may be embedded within the wall of the lead body  12 . In one configuration, the defibrillation electrode segments  28  may be a coil electrode formed by a conductor. The conductor may be formed of one or more conductive polymers, ceramics, metal-polymer composites, semiconductors, metals or metal alloys, including but not limited to, one of or a combination of the platinum, tantalum, titanium, niobium, zirconium, ruthenium, indium, gold, palladium, iron, zinc, silver, nickel, aluminum, molybdenum, stainless steel, MP35N, carbon, copper, polyaniline, polypyrrole and other polymers. In another configuration, each of the defibrillation electrodes segments  28  may be a flat ribbon electrode, a paddle electrode, a braided or woven electrode, a mesh electrode, a directional electrode, a patch electrode or another type of electrode configured to deliver a cardioversion/defibrillation shock to the patient&#39;s heart. 
     In the example illustrated in  FIGS. 1-3 , defibrillation electrode  26  includes two sections or segments  28   a  and  28   b , collectively  28 . The defibrillation electrode segments  28  extend along a substantial part of undulating portion  22 , e.g., along at least 80% of undulating portion  22 . The defibrillation electrode segments  28  may extend along more or less than 80% of the undulating configuration  22 . As another example, the defibrillation electrode segments  28  may extend along at least 90% of the undulating configuration  22 . The defibrillation electrode segment  28   a  extends along a substantial portion of undulation from the proximal end of undulating portion  22  to peak  24   b  (e.g., along a substantial portion of the first “wave” associated with peak  24   a ) and the defibrillation electrode segment  28   b  extends along a substantial portion of undulation from peak  24   b  to distal end of undulating portion  22  (e.g., along a substantial portion of the second “wave” associated with peak  24   c ). In the example illustrated in  FIGS. 1-3 , the only part of undulating portion  22  that defibrillation electrode  26  is not disposed on is the gap  30  on peak  24   b  where electrode  32   b  is disposed. 
     In one configuration, the defibrillation electrode segments  28  are spaced approximately 0.25-4.5 cm, and in some instances between 1-3 cm apart from each other. In another configuration, the defibrillation electrode segments  28  are spaced approximately 0.25-1.5 cm apart from each other. In a further configuration, the defibrillation electrode segments  28  are spaced approximately 1.5-4.5 cm apart from each other. In the configuration shown in  FIGS. 1-3 , the defibrillation electrode segments  28  span a substantial part of the distal portion  16 . Each of the defibrillation electrode segments  28  may be between approximately 1-10 cm in length and, more preferably, between 2-6 cm in length and, even more preferably, between 3-5 cm in length. However, lengths of greater than 10 cm and less than 1 cm may be utilized without departing from the scope of this disclosure. A total length of defibrillation electrode  26  (e.g., length of the two segments  28  combined) may vary depending on a number of variables. The defibrillation electrode  26  may, in one example, have a total length of between approximately 5-10 cm. However, the defibrillation electrode segments  24  may have a total length less than 5 cm and greater than 10 cm in other embodiments. In some instances, defibrillation segments  28  may be approximately the same length or, alternatively, different lengths. 
     The defibrillation electrode segments  28  may be electrically connected to one or more conductors, which may be disposed in the body wall of the lead body  12  or may alternatively be disposed in one or more insulated lumens (not shown) defined by the lead body  12 . In an exemplary configuration, each of the defibrillation electrode segments  28  is connected to a common conductor such that a voltage may be applied simultaneously to all the defibrillation electrode segments  28  to deliver a defibrillation shock to a patient&#39;s heart. In other configurations, the defibrillation electrode segments  28  may be attached to separate conductors such that each defibrillation electrode segment  28  may apply a voltage independent of the other defibrillation electrode segments  28 . In this case, ICD  9  or lead  10  may include one or more switches or other mechanisms to electrically connect the defibrillation electrode segments together to function as a common polarity electrode such that a voltage may be applied simultaneously to all the defibrillation electrode segments  28  in addition to being able to independently apply a voltage. 
     The distal portion  16  may define one or more gaps  30  between adjacent defibrillation segments  28 . The gaps  30  may define any length. In instances in which more than two defibrillation segments  28  exist, each gap  30  may define the same or substantially the same length as every other gap  30  or may define a different length than other gap  30  in the distal portion. In the example of  FIG. 3 , a single gap  30  exists between defibrillation electrode segments  28 . One or more electrodes  32  may be disposed within the respective gap  30 . In the configuration shown in  FIG. 1 , a single electrode  32   b  is disposed within the gap  30 . However, in other examples, more than one electrode  32  may exist within the gap  30  (e.g., as illustrated in the example of  FIGS. 4 and 8 ). In the configuration shown in  FIG. 1 , another electrode  32   a  is located proximal to defibrillation electrode segment  28   a . In other configurations, additional electrodes  32  may be disposed along the distal portion  16  of lead  10 , e.g., distal to defibrillation electrode segment  28   b  and/or proximal to electrode segment  28   a.    
     In one example, the distance between the closest defibrillation electrode segment  28  and electrodes  32  is greater than or equal to 2 mm and less than or equal to 1.5 cm. In another example, electrodes  32  may be spaced apart from the closest one of defibrillation electrode segments  28  by greater than or equal to 5 mm and less than or equal to 1 cm. In a further example, electrodes  32  may be spaced apart from the closest one of defibrillation electrode segments  28  by greater than or equal to 6 mm and less than or equal to 8 mm. 
     The electrodes  32   a  and  32   b  may be configured to deliver low-voltage electrical pulses to the heart or may sense a cardiac electrical activity, e.g., depolarization and repolarization of the heart. As such, electrodes  32  may be referred to herein as pace/sense electrodes  32 . In one configuration, the electrodes  32  are ring electrodes. However, in other configurations the electrodes  32  may be any of a number of different types of electrodes, including ring electrodes, short coil electrodes, paddle electrodes, hemispherical electrodes, directional electrodes, or the like. The electrodes  32  may be the same or different types of electrodes. The electrodes  32  may be electrically isolated from an adjacent defibrillation segment  28  by including an electrically insulating layer of material between the electrodes  32  and the adjacent defibrillation segments  28 . Each electrode  32  may have its own separate conductor such that a voltage may be applied to each electrode independently from another electrode  32  in the distal portion  16 . In other configurations, each electrode  32  may be coupled to a common conductor such that each electrode  32  may apply a voltage simultaneously. 
     In the configurations shown in  FIGS. 1-3 , each electrode  32  is substantially aligned along a major longitudinal axis (“x”). In one example, the major longitudinal axis is defined by a portion of the elongate body  12 , e.g., the substantially linear portion  20 . In another example, the major longitudinal axis is defined relative to the body of the patient, e.g., along the anterior median line (or midsternal line), one of the sternal lines (or lateral sternal lines), left parasternal line, or other line. The electrodes  32   a  and  32   b  may be disposed along the undulating configuration  22  such that each electrode  32   a  and  32   b  is substantially aligned or otherwise disposed along the major longitudinal axis “x.” In one configuration, the midpoint of each electrode  32   a  and  32   b  is along the major longitudinal axis “x,” such that each electrode  32   a  and  32   b  is at least disposed at substantially the same horizontal position when the distal portion is implanted within the patient. In other configurations, the electrodes  32  may be disposed at any longitudinal or horizontal position along the distal portion  16  disposed between, proximal to, or distal to the defibrillation electrode segments  28 , as described in other embodiments herein. In the example illustrated in  FIGS. 1-3 , the electrodes  32  are disposed along the undulating configuration  22  at locations that will be closer to the heart of the patient than defibrillation electrode segments  28  (e.g., at peak  24   b  that is toward the left side of the sternum). As illustrated in  FIG. 1A , for example, the electrodes  32  are substantially aligned with one another along the left sternal line. The defibrillation electrode segments  28  are disposed along the peaks  24   a  and  24   c  that extend toward a right side of the sternum away from the heart. This configuration places the pace/sense electrodes  32  at locations closer to the heart and thereby lower pacing thresholds and better sense cardiac activity of the heart. 
     As illustrated in longitudinal side view of distal portion  16  of  FIG. 3B , the pace/sense electrodes  32  and the defibrillation electrode segments  28  may further be disposed in a common plane when the distal portion  16  is implanted extracardiovasculalry. In particular, the undulating configuration  22  is substantially disposed in a plane defined by the longitudinal axis “x” and a horizontal axis (“y”), referred to herein as the horizontal plane (e.g., the x-y plane). In the example illustrated in  FIG. 3B , each defibrillation electrode segment  28  and each electrode  32  is at least partially disposed in the horizontal plane. Optionally, in other configurations, the undulating configuration  22  may not be substantially disposed in the horizontal plane. Instead, the electrical stimulation therapy portion  18  may be curved such that one or more the defibrillation electrode segments  28  or pace/sense electrodes  32  may be pressed inward toward the heart. For example, the electrical stimulation therapy portion  18  may define a concavity or a curvature to place the one or more of the defibrillation electrode segments  28  or the pace/sense electrodes  32  close to the heart. In such case, the undulating portion  22  may be viewed as being a 3-dimensional serpentine shape in which some of the peaks or portions of the peaks  24  extend in the z-direction, perpendicular to the horizontal plane and toward the heart. 
     The proximal end  14  of the lead body  12  may include one or more connectors  34  to electrically couple the lead  10  to the implantable cardioverter-defibrillator (ICD)  9  subcutaneously implanted within the patient, for example, under the left armpit of the patient. The ICD  9  may include a housing  38  that forms a hermetic seal which protects the components of ICD  9 . The housing  38  of ICD  9  may be formed of a conductive material, such as titanium or titanium alloy, which may function as a housing electrode for a particular therapy vector as illustrated by the arrows in  FIG. 1  between the housing  38  and the distal portion  16 . The ICD  36  may also include a connector assembly that includes electrical feedthroughs through which electrical connections are made between the one or more connectors  34  of lead  10  and the electronic components included within the housing  38 . The housing  38  may house one or more processors, memories, transmitters, receivers, sensors, sensing circuitry, therapy circuitry, power sources (capacitors and batteries) and/or other appropriate components. The components of ICD  9  may generate and deliver electrical stimulation therapy such as anti-tachycardia pacing, cardioversion or defibrillation shocks, post-shock pacing, bradycardia pacing, or other electrical stimulation. 
     The particular configuration of the undulating configuration  22  and the inclusion of the electrodes  32  between defibrillation electrode segments  28  provides a number of therapy vectors for the delivery of electrical stimulation therapy to the heart. For example, as shown in  FIGS. 1-3 , at least a portion of the defibrillation electrode  26  and one of the electrodes  32  may be disposed over the right ventricle, or any chamber of the heart, such that pacing pulses and defibrillation shocks may be delivered to the heart from the therapy portion  18 . The housing  38  may be charged with or function as a polarity different than the polarity of the one or more defibrillation electrode segments  28  and/or electrodes  32  such that electrical energy may be delivered between the housing  38  and the defibrillation electrode segment(s)  28  and/or electrode(s)  32  to the heart. Each defibrillation electrode segment  28  may have the same polarity as every other defibrillation electrode segment  28  when a voltage is applied to it such that a defibrillation shock may be delivered from the entirety of the defibrillation electrode  26 . In embodiments in which defibrillation electrode segments  28  are electrically connected to a common conductor within lead body  12 , this is the only configuration of defibrillation electrode segments  28 . However, in other embodiments, defibrillation electrode segments  28  may be coupled to separate conductors within lead body  12  and may therefore each have different polarities such that electrical energy may flow between defibrillation electrode segments  28  (or between one of defibrillation electrode segments  28  and one or pace/sense electrodes  32  or the housing electrode) to provide pacing therapy and/or to sense cardiac depolarizations. In this case, the defibrillation electrode segments  28  may still be electrically coupled together (e.g., via one or more switches within ICD  9 ) to have the same polarity to deliver a defibrillation shock from the entirety of the defibrillation electrode  26 . 
     Additionally, each electrode  32  may be configured to conduct electrical pulses directly to the heart, or sense a cardiac depolarization between adjacent defibrillation electrode segments  28 , whether disposed on the same defibrillation electrode segment  28  or on other defibrillation electrode segment  28 , and/or between proximate electrodes  32 . For example, the therapy vector lines shown in  FIG. 3  illustrate the flow of electrical energy between the electrodes  32   a  and  32   b  and adjacent defibrillation electrode segments  28 . The therapy vector lines illustrate potential vectors that can be generated to target specific areas of the heart for electrical stimulation therapy or to target different areas of the heart so as to be able to select a pacing and/or sensing vector with best performance (e.g., lowest pacing capture thresholds). Additionally electrodes  32  may conduct electrical pulses between one another, e.g., between one of electrodes  32  and an inferior and superior electrode  32 , between one of electrodes  32  and the housing electrode, or between a plurality of electrodes  32  (at the same polarity) and the housing electrode at the opposite polarity. As such, each electrode  32  may have the same polarity as every other electrode  32  or alternatively, may have different polarities such that different therapy vectors can be utilized to deliver pacing pulses to the heart. 
       FIG. 4  is a schematic diagram illustrating another example lead  40  constructed in accordance with the principles of the present application. Lead  40  can include one or more of the structure and/or functionality of lead  10  of  FIGS. 1-3  (and vice versa), including the electrode and lead body dimensions, spacings, materials, shapes, orientations, electrical conductor configurations, and the like. Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
     Lead  40  includes an undulating portion  42 . Undulating portion  42  is substantially similar to undulating portion  22  of lead  10 , but undulating portion  42  includes only two peaks  24 . However, undulating portion  42  may define a peak-to-peak distance “d” and peak-to-peak width “w” with similar dimensions described above with respect to  FIGS. 1-3 . 
     Lead  40  includes a defibrillation electrode  26  formed from two defibrillation electrode segments  28   a  and  28   b . The defibrillation electrode segments  28  extend along a substantial part of undulating portion  42 , e.g., along at least 80% of undulating portion  42 . The defibrillation electrode segment  28   a  extends along a substantial portion of undulation from the proximal end of undulating portion  42 , except for the part of undulating portion  42  that includes the gap  30  where electrode  32   b  is disposed. In the example illustrated in  FIG. 4 , the gap  30  and electrode  32   b  are located along the part of undulating portion  42  that transitions from peak  24   a  to peak  24   b , instead of at a peak as was the case in lead  10  of  FIGS. 1-3 . 
     Lead  40  also includes two pace/sense electrodes  32   a  and  32   b . The electrodes  32   a  and  32   b  are disposed along the undulating configuration  42  such that each electrode  32   a  and  32   b  is substantially aligned or otherwise disposed along the major longitudinal axis “x.” Unlike in lead  10  of  FIGS. 1-3 , however, the orientation of electrodes  32   a  and  32   b  are different even though they are substantially disposed at substantially the same horizontal position when the distal portion is implanted within the patient. Moreover, electrodes  32  are disposed along the undulating configuration  42  at locations such that the electrodes  32  will be substantially aligned with one another along the anterior median line instead of the left sternal line. In this case, the defibrillation electrode segment  28   a  is disposed along the peak  24   a  and will extend toward the left side of the sternum when implanted and defibrillation electrode segment  28   b  is disposed along the peak  24   b  and will extend toward the right side of the sternum when implanted. 
     Defibrillation electrode segments  28  and pace/sense electrodes  32  may include the structure and functionality described above with respect to  FIGS. 1-3 , including but not limited to the spacing between segments  28  and electrodes  32 , the size of segments  28  and  32 , electrode and lead body dimensions, spacings, materials, shapes, and the like. Additionally, as described above with respect to  FIGS. 1-3 , in some configurations defibrillation electrode segments  28  may each be connected to a common conductor such that a voltage may be applied simultaneously to all the defibrillation electrode segments  28  (and they function as a single polarity) to deliver a defibrillation shock to a patient&#39;s heart. In other configurations, the defibrillation electrode segments  28  may be attached to separate conductors such that each defibrillation electrode segment  28  may apply a voltage independent of the other defibrillation electrode segments  28 . In this case, ICD  9  or lead  40  may include one or more switches or other mechanisms to electrically connect the defibrillation electrode segments together to function as a common polarity electrode such that a voltage may be applied simultaneously to all the defibrillation electrode segments  28  in addition to being able to independently apply a voltage. 
       FIG. 5  is a schematic diagram illustrating another example lead  50  constructed in accordance with the principles of the present application. Lead  50  can include one or more of the structure and/or functionality of lead  10  of  FIGS. 1-3  (and vice versa) or lead  40  of  FIG. 4 , including the electrode and lead body dimensions, spacings, materials, shapes, orientations, electrical conductor configurations, and the like. Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
     Lead  50  includes an undulating portion  52 . Undulating portion  52  includes two peaks  24 , similar to undulating portion  42  of lead  40 , but undulating portion  52  includes a longer peak-to-peak width “w.” Lead  50  also includes three pace/sense electrodes  32  with two of them being disposed between defibrillation electrode segments  28 . Unlike the example leads illustrated in  FIGS. 1-4 , at least one of the pace/sense electrodes  32  is not substantially aligned or otherwise disposed along the major longitudinal axis “x.” 
       FIG. 6  is a schematic diagram illustrating another example lead  60  constructed in accordance with the principles of the present application. Lead  60  can include one or more of the structure and/or functionality of lead  10  of  FIGS. 1-3 , lead  40  of  FIG. 4 , and/or lead  50  of  FIG. 5  (and vice versa), including the electrode and lead body dimensions, spacings, materials, shapes, orientations, electrical conductor configurations, and the like. Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
     Lead  60  includes an undulating portion  62 . Undulating portion  62  is substantially similar to undulating portion  22  of lead  10 , but undulating portion  62  includes seven peaks  24   a - g  instead of three peaks. Undulating portion  62  defines a peak-to-peak distance “d” with similar dimensions described above with respect to  FIGS. 1-3 , but the peak-to-peak width “w” may be smaller than the peak-to-peak widths of undulating portions  22 ,  42 , or  52  due to the increased number of peaks  24 . 
     The defibrillation electrode also includes more defibrillation electrode segments  28  than the leads  10 ,  40  and  50 . The defibrillation electrode segments  28  extend along a substantial part of undulating portion  62 , e.g., along at least 80% of undulating portion  62 . The defibrillation electrode segments  28  extend along a substantial portion of undulation from the proximal end of undulating portion  62 , except for the part of undulating portion  62  that includes the gaps  30  where electrodes  32  are disposed. In the example illustrated in  FIG. 6 , the gaps  30  and electrodes  32   b  are located along the part of undulating portions  62  that transition from a peak  24  to adjacent peak  24  (at every other transition), instead of at a peak as was the case in lead  10  of  FIGS. 1-3 . 
     Lead  60  also includes three pace/sense electrodes  32   a - 32   c . The electrodes  32  are disposed along the undulating configuration  62  such that each electrode  32  is substantially aligned or otherwise disposed along the major longitudinal axis “x.” Unlike in lead  10 ,  40  and  50  of  FIGS. 1-5 , however, all electrodes  32  are located between adjacent defibrillation electrode segments  28 . In other instances, the lead  60  may also include one or more electrodes  32  proximal to the most proximal defibrillation electrode segment  28  or distal to the most distal defibrillation electrode segment  28 . Electrodes  32  are disposed along the undulating configuration  62  at locations such that the electrodes  32  will be substantially aligned with one another along the anterior median line. 
     Defibrillation electrode segments  28  and pace/sense electrodes  32  may include the structure and functionality described above with respect to  FIGS. 1-3 , including but not limited to the spacing between segments  28  and electrodes  32 , the size of segments  28  and  32 , electrode and lead body dimensions, spacings, materials, shapes, and the like. Additionally, as described above with respect to  FIGS. 1-3 , in some configurations defibrillation electrode segments  28  may each be connected to a common conductor such that a voltage may be applied simultaneously to all the defibrillation electrode segments  28  (and they function as a single polarity) to deliver a defibrillation shock to a patient&#39;s heart. In other configurations, the defibrillation electrode segments  28  may be attached to separate conductors such that each defibrillation electrode segment  28  may apply a voltage independent of the other defibrillation electrode segments  28 . In this case, ICD  9  or lead  60  may include one or more switches or other mechanisms to electrically connect the defibrillation electrode segments together to function as a common polarity electrode such that a voltage may be applied simultaneously to all the defibrillation electrode segments  28  in addition to being able to independently apply a voltage. 
       FIG. 7  is a schematic diagram illustrating another example lead  70  constructed in accordance with the principles of the present application. Lead  70  can include one or more of the structure and/or functionality of lead  10  of  FIGS. 1-3 , lead  40  of  FIG. 4 , lead  50  of  FIG. 5  and/or lead  60  of  FIG. 6  (and vice versa), including the electrode and lead body dimensions, spacings, materials, shapes, orientations, electrical conductor configurations, and the like. Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
     Lead  70  includes an undulating portion  72  that is substantially similar to undulating portion  62  of lead  60  of  FIG. 6  except that the electrode  32  may be sized to span the distance between two peaks  24  in the undulating configuration  72 . In this configuration, the electrodes  32  may be configured to sense a cardiac depolarization between an adjacent defibrillation electrode segment  28 . Moreover, the electrodes  32  are configured to deliver pacing pulses to the heart by conductive electrical energy between the electrodes  32  and an adjacent defibrillation electrode segment  28 . In such a configuration, the therapy vectors between a respective electrode  32  and an adjacent defibrillation electrode segment  28  may define a substantially rhomboid or diamond configuration to provide for a particular therapy vector. Electrodes  32  may also deliver electrical energy between respective ones of electrodes  32 . Repetitive description of like numbered elements described in other embodiments is omitted for the sake of brevity. 
       FIG. 8  is a schematic diagram illustrating another example lead  80  constructed in accordance with the principles of the present application. Lead  80  can include one or more of the structure and/or functionality of lead  10  of  FIGS. 1-3 , lead  40  of  FIG. 4 , lead  50  of  FIG. 5  lead  60  of  FIG. 6 , and/or lead  70  of  FIG. 7  (and vice versa), including the electrode and lead body dimensions, spacings, materials, shapes, orientations, electrical conductor configurations, and the like. Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
     Lead  80  includes an undulating portion  82  that may conform substantially to undulating portion  62  of lead  60  of  FIG. 6  and/or undulating portion  72  of lead  70  of  FIG. 7  except that two or more electrodes  32  may span the distance between two peaks  24  in the undulating configuration  62 . The electrodes  32  may be disposed in a single gap  30  between adjacent defibrillation electrode segments  28  or each electrode  32  may be disposed in a two gaps  30  and each gap  30  is separated by an electrically insulating section of the lead body  12 . In the configuration shown in  FIG. 8 , the electrodes  32  may be configured to sense a cardiac depolarization between each other or an adjacent defibrillation electrode segment  28 , depending on the polarity of each electrode  32 . Moreover, the electrodes  32  are configured to deliver pacing pulses to the heart with conductive electrical energy between the electrodes  32  and an adjacent defibrillation electrode segment  28  or between two of the electrodes  32 . For example, therapy vectors are shown in  FIG. 8  for a configuration in which, for example, electrodes  32   a  and  32   a ′ have the same polarity and the opposite polarity of an adjacent defibrillation electrode segment  28  to provide for a particular therapy vector. However, electrodes  32   a  and  32   a ′, and likewise  32   b  and  32   b ′ and  32   c , and  32   c ′ may be coupled to the same or different conductors such that the polarities between each electrode  32  may be the same or different depending on the application. Between each electrode  32   a  and  32   a ′, for example, may be a portion of the lead body  12  that is electrically insulating. Moreover, the gaps  30  may be sized to optimize particular electrical stimulation therapies. For example, the gap  30  size may range from approximately 8 mm-15 mm for between a pair of electrodes  32  configured to pace and/or sense a cardiac depolarization. Additionally, the size of the gaps  30  between an electrode  32  and a defibrillation electrode segment  28  may be approximately 3-10 mm in length or any of the lengths described above with respect to  FIGS. 1-3 . Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
       FIG. 9  is a schematic diagram illustrating another example lead  90  constructed in accordance with the principles of the present application. Lead  90  can include one or more of the structure and/or functionality of lead  10  of  FIGS. 1-3 , lead  40  of  FIG. 4 , lead  50  of  FIG. 5  and/or lead  60  of  FIG. 6 , lead  70  of  FIG. 7 , and/or lead  80  of  FIG. 8  (and vice versa), including the electrode and lead body dimensions, spacings, materials, shapes, orientations, electrical conductor configurations, and the like. Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
     Lead  90  includes an undulating portion  92  that may conform substantially to undulating portion  62  of lead  60  of  FIG. 6  except that electrodes  32  may be directional electrodes positioned to provide a therapy vector aimed at the heart and not skeletal muscle, such that only a portion of the lead body in which the electrodes  32  are disposed contain the electrode  32  and another portion includes the insulating portion of the lead body. The electrodes  32  would be arranged such that the electrodes are disposed on the posterior side of the lead (e.g., facing the heart) when implanted within the patient. In this configuration, the electrodes  32  may be configured to sense a cardiac depolarization between an adjacent defibrillation electrode segment  28 , between two of electrodes  32 , or between electrode(s)  32  and housing electrode. Moreover, the electrodes  32  are configured to deliver pacing pulses to the heart by conductive electrical energy between an adjacent defibrillation electrode segment  28 , between two of electrodes  32 , or between electrode(s)  32  and housing electrode. For example, therapy vectors are shown in  FIG. 6  for a configuration in which each electrode  32   a ,  32   b , and  32   c  are disposed on the superior portion of a lead body  62  section. In other configurations, for example, electrodes  32   a  and  32   c  may be facing electrode  32   b  to provide for particular therapy vectors. The arrangement of electrodes  32   a ,  32   b , and  32   c  may be such that electrical energy is directed toward the heart and not toward skeletal muscle or non-cardiac tissue to maximize the effectiveness of pacing pulses delivered to the heart. Repetitive description of like numbered elements described in other embodiments is omitted for sake of brevity. 
       FIG. 10  is a functional block diagram of an example configuration of electronic components of an example ICD  9 . ICD  9  includes a control module  100 , sensing module  102 , therapy module  104 , communication module  108 , and memory  110 . The electronic components may receive power from a power source  106 , which may be a rechargeable or non-rechargeable battery. In other embodiments, ICD  9  may include more or fewer electronic components. The described modules may be implemented together on a common hardware component or separately as discrete but interoperable hardware or software components. Depiction of different features as modules is intended to highlight different functional aspects and does not necessarily imply that such modules must be realized by separate hardware or software components. Rather, functionality associated with one or more modules may be performed by separate hardware or software components, or integrated within common or separate hardware or software components.  FIG. 10  will be described in the context of ICD  9  being coupled to lead  10  for exemplary purposes only. However, ICD  9  may be coupled to other leads, such as lead  40 ,  50 ,  60 ,  70 ,  80  or  90  described herein, and thus other electrodes. 
     Sensing module  102  is electrically coupled to some or all of electrodes  26  (or separately to segments  28   a  and/or  28   b ) and  32  via the conductors of lead  10  and one or more electrical feedthroughs, or to the housing electrode via conductors internal to the housing of ICD  9 . Sensing module  102  is configured to obtain signals sensed via one or more combinations of electrodes  26  (or separately to segments  28   a  and/or  28   b ) and  32  and the housing electrode of ICD  9  and process the obtained signals. 
     The components of sensing module  102  may be analog components, digital components or a combination thereof. Sensing module  102  may, for example, include one or more sense amplifiers, filters, rectifiers, threshold detectors, analog-to-digital converters (ADCs) or the like. Sensing module  102  may convert the sensed signals to digital form and provide the digital signals to control module  100  for processing or analysis. For example, sensing module  102  may amplify signals from the sensing electrodes and convert the amplified signals to multi-bit digital signals by an ADC. Sensing module  102  may also compare processed signals to a threshold to detect the existence of atrial or ventricular depolarizations (e.g., P- or R-waves) and indicate the existence of the atrial depolarization (e.g., P-waves) or ventricular depolarizations (e.g., R-waves) to control module  100 . 
     Control module  100  may process the signals from sensing module  102  to monitor electrical activity of the heart of the patient. Control module  100  may store signals obtained by sensing module  102  as well as any generated EGM waveforms, marker channel data or other data derived based on the sensed signals in memory  110 . Control module  100  may analyze the EGM waveforms and/or marker channel data to detect cardiac events (e.g., tachycardia). In response to detecting the cardiac event, control module  100  may control therapy module  104  to deliver the desired therapy to treat the cardiac event, e.g., defibrillation shock, cardioversion shock, ATP, post-shock pacing, or bradycardia pacing. 
     Therapy module  104  is configured to generate and deliver electrical stimulation therapy to the heart. Therapy module  104  may include one or more pulse generators, capacitors, and/or other components capable of generating and/or storing energy to deliver as pacing therapy, defibrillation therapy, cardioversion therapy, cardiac resynchronization therapy, other therapy or a combination of therapies. In some instances, therapy module  104  may include a first set of components configured to provide pacing therapy and a second set of components configured to provide defibrillation therapy. In other instances, therapy module  104  may utilize the same set of components to provide both pacing and defibrillation therapy. In still other instances, therapy module  104  may share some of the defibrillation and pacing therapy components while using other components solely for defibrillation or pacing. 
     Control module  100  may control therapy module  104  to deliver the generated therapy to the heart via one or more combinations of electrodes  26  (or separately to segments  28   a  and/or  28   b ) and  32  of lead  10  and the housing electrode of ICD  9  according to one or more therapy programs, which may be stored in memory  110 . In instances in which control module  100  is coupled to a different lead, e.g., lead  40 ,  50 ,  60 ,  70 ,  80 , or  90 , other electrodes may be utilized. Control module  100  controls therapy module  104  to generate electrical stimulation therapy with the amplitudes, pulse widths, timing, frequencies, electrode combinations or electrode configurations specified by a selected therapy program. 
     Therapy module  104  may include a switch module to select which of the available electrodes are used to deliver the therapy. The switch module may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple electrodes to therapy module  104 . Control module  100  may select the electrodes to function as therapy electrodes, or the therapy vector, via the switch module within therapy module  104 . In instances in which defibrillation segments  28   a  and  28   b  are each coupled to separate conductors, control module  100  may be configured to selectively couple therapy module  104  to either one of segments  28   a  and  28   b  individually or couple to both of the segments  28   a  and  28   b  concurrently. In some instances, the same switch module may be used by both therapy module  104  and sensing module  102 . In other instances, each of sensing module  102  and therapy module  104  may have separate switch modules. 
     In the case of pacing therapy being provided, e.g., ATP, post-shock pacing, and/or bradycardia pacing provided via electrodes  32  and/or defibrillation electrode segments  28   a  and  28   b  of lead  10 . In one example, therapy module  104  may deliver pacing (e.g., ATP or post-shock pacing) using an electrode vector that includes one or both defibrillation electrode segments  28   a  and  28   b . The electrode vector used for pacing may be segment  28   a  as an anode (or cathode) and one of electrodes  28   b ,  32  or the housing of ICD  9  as the cathode (or anode) or segment  28   b  as an anode (or cathode) and one of electrodes  28   b ,  32  or the housing of ICD  9  as the cathode (or anode). If necessary, therapy module  104  may generate and deliver a cardioversion/defibrillation shock (or shocks) using one or both of electrode segments  28  concurrently as a cathode and the housing electrode of ICD  9  as an anode. 
     Control module  100  controls therapy module  104  to generate and deliver pacing pulses with any of a number of shapes, amplitudes, pulse widths, or other characteristic to capture the heart. For example, the pacing pulses may be monophasic, biphasic, or multi-phasic (e.g., more than two phases). The pacing thresholds of the heart when delivering pacing pulses from the substernal space, e.g., from electrodes  32  and/or electrode segments  28  substantially within anterior mediastinum  36 , may depend upon a number of factors, including location, type, size, orientation, and/or spacing of electrodes  32  and/or electrode segments  28 , location of ICD  9  relative to electrodes  32  and/or electrode segments  28 , physical abnormalities of the heart (e.g., pericardial adhesions or myocardial infarctions), or other factor(s). 
     The increased distance from electrodes  32  and/or electrode segments  28  of lead  10  to the heart tissue may result in the heart having increased pacing thresholds compared to transvenous pacing thresholds. To this end, therapy module  104  may be configured to generate and deliver pacing pulses having larger amplitudes and/or pulse widths than conventionally required to obtain capture via leads implanted within the heart (e.g., transvenous leads) or leads attached directly to the heart. In one example, therapy module  104  may generate and deliver pacing pulses having amplitudes of less than or equal to 8 volts and pulse widths between 0.5-3.0 milliseconds and, in some instances up to 4 milliseconds. In another example, therapy module  104  may generate and deliver pacing pulses having amplitudes of between 5 and 10 volts and pulse widths between approximately 3.0 milliseconds and 10.0 milliseconds. In another example, therapy module  104  may generate and deliver pacing pulses having pulse widths between approximately 2.0 milliseconds and 8.0 milliseconds. In a further example, therapy module  104  may generate and deliver pacing pulses having pulse widths between approximately 0.5 milliseconds and 20.0 milliseconds. In another example, therapy module  104  may generate and deliver pacing pulses having pulse widths between approximately 1.5 milliseconds and 20.0 milliseconds. 
     Pacing pulses having longer pulse durations than conventional transvenous pacing pulses may result in lower energy consumption. As such, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of greater than two (2) milliseconds. In another example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of between greater than two (2) milliseconds and less than or equal to three (3) milliseconds. In another example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of greater than or equal to three (3) milliseconds. In another example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of greater than or equal to four (4) milliseconds. In another example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of greater than or equal to five (5) milliseconds. In another example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of greater than or equal to ten (10) milliseconds. In a further example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths between approximately 3-10 milliseconds. In a further example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths between approximately 4-10 milliseconds. In a further example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of greater than or equal to fifteen (15) milliseconds. In yet another example, therapy module  104  may be configured to generate and deliver pacing pulses having pulse widths or durations of greater than or equal to twenty (20) milliseconds. 
     Depending on the pulse widths, ICD  9  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. In other examples, the pacing pulse amplitudes may be greater than 20 volts. Typically the lower amplitudes require longer pacing widths as illustrated in the experimental results. Reducing the amplitude of pacing pulses delivered by ICD  9  reduces the likelihood of extra-cardiac stimulation and lower consumed energy of power source  106 . 
     For pacing therapy provided from the subcutaneous placement of lead  10  above the sternum and/or ribcage, pacing amplitudes and pulse widths may vary, e.g., be increased given the further distances from heart and the various anatomical features via which the energy must penetrate. 
     In the case of cardioversion or defibrillation therapy, e.g., cardioversion or defibrillation shocks provided by defibrillation electrode segments  28  (individually or together), control module  100  controls therapy module  104  to generate cardioversion or defibrillation shocks having any of a number of waveform properties, including leading-edge voltage, tilt, delivered energy, pulse phases, and the like. Therapy module  104  may, for instance, generate monophasic, biphasic or multiphasic waveforms. Additionally, therapy module  104  may generate cardioversion or defibrillation waveforms having different amounts of energy. As with pacing, delivering cardioversion or defibrillation shocks from the substernal space, e.g., from electrode segment(s)  28  substantially within anterior mediastinum  36 , may reduce the amount of energy that needs to be delivered to defibrillate the heart. When lead  10  is implanted in the substernal space, therapy module  104  may generate and deliver cardioversion or defibrillation shocks having energies of less than 65 J, less than 100 J, between 40-50 J, between 35-100 J, and in some instances less than 35 J. When lead  10  is implanted subcutaneously, ICD  9  may generate and deliver cardioversion or defibrillation shocks having energies around 65-80 J. 
     Therapy module  104  may also generate defibrillation waveforms having different tilts. In the case of a biphasic defibrillation waveform, therapy module  104  may use a 65/65 tilt, a 50/50 tilt, or other combinations of tilt. The tilts on each phase of the biphasic or multiphasic waveforms may be the same in some instances, e.g., 65/65 tilt. However, in other instances, the tilts on each phase of the biphasic or multiphasic waveforms may be different, e.g., 65 tilt on the first phase and 55 tilt on the second phase. The example delivered energies, leading-edge voltages, phases, tilts, and the like are provided for example purposes only and should not be considered as limiting of the types of waveform properties that may be utilized to provide substernal defibrillation via defibrillation electrode segment(s)  28 . 
     Communication module  108  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as a clinician programmer, a patient monitoring device, or the like. For example, communication module  108  may include appropriate modulation, demodulation, frequency conversion, filtering, and amplifier components for transmission and reception of data with the aid of antenna  112 . Antenna  112  may be located within connector block of ICD  9  or within housing ICD  9 . 
     The various modules of ICD  9  may include any one or more processors, controllers, digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or equivalent discrete or integrated circuitry, including analog circuitry, digital circuitry, or logic circuitry. Memory  110  may include computer-readable instructions that, when executed by control module  100  or other component of ICD  9 , cause one or more components of ICD  9  to perform various functions attributed to those components in this disclosure. Memory  110  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), static non-volatile RAM (SRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other non-transitory computer-readable storage media. 
     The leads and systems described herein may be used at least partially within the substernal space, e.g., within anterior mediastinum of patient, to provide an extravascular ICD system. An implanter (e.g., physician) may implant the distal portion of the lead intra-thoracically using any of a number of implant tools, e.g., tunneling rod, sheath, or other tool that can traverse the diagrammatic attachments and form a tunnel in the substernal location. For example, the implanter may create an incision near the center of the torso of the patient, e.g., and introduce the implant tool into the substernal location via the incision. The implant tool is advanced from the incision superior along the posterior of the sternum in the substernal location. The distal end of lead  10  (or other lead described herein, e.g., leads  40 ,  50 ,  60 ,  70 ,  80 , or  90 ) is introduced into tunnel via implant tool (e.g., via a sheath). As the distal end of lead  10  is advanced through the substernal tunnel, the distal end of lead  10  is relatively straight. The pre-formed or shaped undulating portion  22  is flexible enough to be straightened out while routing the lead  10  through a sheath or other lumen or channel of the implant tool. Once the distal end of lead  10  is in place, the implant tool is withdrawn toward the incision and removed from the body of the patient while leaving lead  10  in place along the substernal path. As the implant tool is withdrawn, the distal end of lead  10  takes on its pre-formed undulating configuration  22 . Thus, as the implant tool is withdrawn, the undulating configuration  22  pushes electrodes  32   a  and  32   b  toward the left side of sternum compared to electrodes  28   a  and  28   b . As mentioned above, the implanter may align the electrodes  32   a  and  32   b  along the anterior median line (or midsternal line) or the left sternal lines (or left lateral sternal line). 
     It will be appreciated by persons skilled in the art that the present application is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the application, which is limited only by the following claims.