Patent Publication Number: US-7899554-B2

Title: Intravascular System and Method

Description:
PRIORITY 
     This is a divisional of U.S. application Ser. No. 10/862,113 now U.S. Pat. No. 7,529,589, filed Jun. 4, 2004, which is a continuation-in-part of U.S. application Ser. No. 10/454,223 now U.S. Pat. No. 7,082,336, filed Jun. 4, 2003, and claims the benefit of U.S. Provisional Application No. 60/515,746, filed Oct. 30, 2003, U.S. Provisional Application No. 60/516,026, filed Oct. 31, 2003, U.S. Provisional Application No. 60/525,332, filed Nov. 26, 2003, U.S. Provisional Application No. 60/525,336, filed Nov. 26, 2003, and U.S. Provisional Application No. 60/543,260, filed Feb. 10, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to devices, systems, and methods for diagnosing and treating the heart. In particular, the invention provides methods and systems for implanting medical devices into the patient&#39;s vasculature and using the devices for sensing electrical activity and/or electrically stimulating the heart 
     BACKGROUND OF THE INVENTION 
     Pacemakers, defibrillators and implanted cardioverter defibrillators (“ICDs”) have been successfully implanted for years for treatment of heart rhythm conditions. 
     Pacemakers are implanted in patients who have bradycardia (slow heart rate). The pacemakers detect periods of bradycardia and deliver electrical stimuli to increase the heartbeat to an appropriate rate. 
     ICDs are implanted in patients who may suffer from episodes of fast and irregular heart rhythms called tachyarrhythmias. An ICD can cardiovert the heart by delivering electrical current directly to the heart to terminate an atrial or ventricular tachyarrhythmia, other than ventricular fibrillation. An ICD may alternatively defibrillate the heart in a patient who may suffer ventricular fibrillation (VF), a fast and irregular heart rhythm in the ventricles. During a VF episode, the heart quivers and can pump little or no blood to the body, potentially causing sudden death. An ICD implanted for correction of ventricular fibrillation will detect a VF episode and deliver an electrical shock to the heart to restore the heart&#39;s electrical coordination. 
     Another type of implantable defibrillation device treats patients who may suffer from atrial fibrillation (AF), which is a loss of electrical coordination in the heart&#39;s upper chambers (atria). During AF, blood in the atria may pool and clot, placing the patient at risk for stroke. An electrophysiological device implanted for correction of atrial fibrillation will detect an AF episode and deliver an electrical shock to the atria to restore electrical coordination. 
     Pacemakers and ICDs are routinely implanted in the pectoral region either under the skin (subcutaneous) or under the pectoral muscle. The leads are placed at appropriate locations within or on the heart. Because of this complexity, a cardiologist identifying a heart rhythm condition may be required to refer his or her patient to sub-specialists or surgeons for implantation of a pacemaker or ICD—thus delaying implantation of the device in a patient who urgently needs it. It is thus desirable to simplify these devices and the procedures for implanting them so as to permit their implantation by a broader range of physicians. 
     SUMMARY OF THE INVENTION 
     The present application describes an intravascular implantable electrophysiological system that may carry out cardioversion, pacing and/or defibrillation of a human heart. The described system includes a pulse generator that is implantable within a blood vessel and/or the heart and electrodes coupled to the pulse generator. During implantation, the pulse generator is introduced into a patient&#39;s vasculature, advanced to a desired vessel and anchored in place within the vessel. The electrode(s) are positioned within the heart or surrounding vessels as needed to deliver electrical pulses to the appropriate location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective illustration showing human cardiac anatomy. 
         FIG. 2A  is a plan view generally showing components of one form of intravascular electrophysiological system which utilizes a lead on the inferior portion of the device body. 
         FIG. 2B  is a plan view generally showing components of a second form of intravascular electrophysiological system, which utilizes a lead on the superior portion of the device body. 
         FIG. 2C  is a plan view generally showing components of a third form of intravascular electrophysiological system, which has a bifurcated configuration. 
         FIGS. 2D and 2E  are side elevation views of distal portions of the leads of the system of  FIG. 2C . 
         FIG. 2F  is a plan view generally showing components of a fourth form of intravascular electrophysiological system which utilizes leads on the inferior and superior portions of the device body. 
         FIG. 3A  is a plan view showing a first embodiment of an intravascular electrophysiological device of a type which may be used with the systems shown in  FIGS. 2A-2F . 
         FIG. 3B  is a plan view similar to  FIG. 3A  showing a second embodiment of an intravascular electrophysiological device of a type which may be used with the system shown in  FIGS. 2A-2F . 
         FIG. 3C  is a plan view showing a third embodiment of an intravascular electrophysiological device of a type which may be used with the system shown in  FIGS. 2A-2F . 
         FIG. 3D  is a plan view similar to  FIG. 3C  illustrating bending of the device. 
         FIG. 3E  a plan view showing the mechanical features of a fourth embodiment of an intravascular electrophysiological device of a type which may be used with the system shown in  FIGS. 2A-2F . 
         FIG. 4A  is a perspective view illustrating the coupler and rod of the embodiment of  FIG. 3E .  FIG. 4B  is a perspective view illustrating the coupler and rod assembly of  FIG. 4A  in combination with a pair of device enclosures. 
         FIGS. 5A-5E  are a sequence of figures illustrating formation of the electrical and mechanical connections within a device enclosure of the type shown in  FIG. 3E .  FIG. 5A  is an end view showing a device component and end cap, and  FIG. 5B  is a cross-sectional side view taken along the plane designated  5 B- 5 B in  FIG. 5A .  FIGS. 5C and 5D  are similar to  FIGS. 5A and 5B , respectively, but show the component and end cap combined with a flex circuit, enclosure and coupler.  FIG. 5E  is similar to  FIG. 5D  but adds the conductor assembly, rod and elastomer. 
         FIGS. 6A and 6B  are perspective views showing a pair of enclosures with a conductor assembly extending between them. 
         FIG. 7  is a perspective end view of an enclosure showing an alternate conductor assembly extending from the enclosure for coupling to associated components in a second enclosure. 
         FIG. 8A  is a plan view showing a fifth embodiment of an intravascular electrophysiological device of a type that may be used with the systems shown in  FIGS. 2A-2F . 
         FIG. 8B  is a plan view showing the ribbon portion of the fifth embodiment. 
         FIG. 9A  is a perspective view schematically illustrating use of an anchor to anchor an intravascular electrophysiological device within a vessel. 
         FIG. 9B  is cross-sectional perspective view showing a portion of the anchor of  FIG. 9A . 
         FIG. 9C  is a perspective view similar to  FIG. 9A  but further illustrating use of a liner within the vessel. 
         FIG. 10A  is a perspective view of an anchor suitable for use with the systems of  FIGS. 2A-2F . 
         FIG. 10B  is a perspective view showing the anchor of  FIG. 10A  attached to an implantable electrophysiological device and in the expanded position. 
         FIG. 10C  is a cross-sectional end view of the device shown in  FIG. 10B . 
         FIG. 10D  is a side elevation view of a device showing the anchor of  FIG. 10B  positioned on a device and compressed by a sheath. 
         FIG. 10E  is similar to  FIG. 10D  but shows retraction of the sheath to permit expansion of the anchor within a blood vessel. 
         FIGS. 11A-11F  are a sequence of drawings schematically illustrating implantation of the system of  FIG. 2A . 
         FIGS. 12A-12E  are a sequence of drawings schematically illustrating implantation of the system of  FIG. 2B . 
         FIGS. 13A-13C  are a sequence of drawings schematically illustrating implantation of the system of  FIG. 2C .  FIGS. 13D-13I  show a modification to the implantation method of  FIGS. 13A-13C  to include steps for implanting separately-implantable retention anchors and liners. 
         FIGS. 14A-F  are a sequence of drawings schematically illustrating implantation of the system of  FIG. 8A . 
         FIG. 15A  is a plan view of a device similar to the devices of  FIGS. 3A-3D  and  8 A- 8 B but slightly modified to include a cuff on the lead for receiving a guidewire. 
         FIG. 15B  is a plan view of a device similar to the devices of  FIGS. 3A-3D  and  8 A- 8 B but slightly modified to include a bore in the device for receiving a guidewire. 
         FIG. 15C  is a plan view similar to  FIG. 15B  showing an alternative configuration for receiving a guidewire. 
         FIG. 15D  is a plan view similar to  FIG. 15A  showing an alternative use of the  FIG. 15A  device and lead. 
         FIG. 15E  is a cross-section view of the lead of  FIG. 15D  taken along the plane designated  15 E- 15 E in  FIG. 15D . 
         FIGS. 16A-20  schematically illustrate various applications of intravascular electrophysiological systems. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Cardiac Anatomy 
       FIG. 1  shows the cardiac anatomy of a human, including the heart and major vessels. The following anatomic locations are shown and identified by the listed reference numerals: 
                                                Right Subclavian   2a           Left Subclavian   2b           Superior Vena Cava (SVC)   3a           Inferior Vena Cava (IVC)   3b           Right Atrium (RA)   4a           Left Atrium (LA)   4b           Right Atrial Appendage (RAA)   5            Coronary Sinus Ostium (CS Os)   6            Right Ventricle (RV)   7a           Left Ventricle (LV)   7b           Aortic Arch   8            Descending Aorta   9                         
System Components
 
     Generally speaking, the present disclosure describes intravascular electrophysiological systems that may be used for a variety of functions. These functions include defibrillation, pacing, and/or cardioversion. In general, the elements of the systems described below include at least one device body and typically, but optionally, at least one lead coupled to the body. One or more retention devices may facilitate retention of the device body and/or leads or other elements within the vasculature. Also described are components such as mandrels, stylets and/or guidewires used to facilitate implantation of the system. 
       FIGS. 2A through 2F  illustrate systems well suited for use as defibrillators used in the treatment of tachyarrhythmias. Although the description of these systems focuses on their use in the treatment of ventricular tachycardia, systems such as these, or modifications thereof, may be used for various other electophysiologic applications, some of which are described in connection with  FIGS. 16A through 19 . 
     One configuration of an electrophysiological system  10   a  is shown in  FIG. 2A . The elements of the  FIG. 2A  system  10   a  include an elongate device body  12   a , lead  14   a , retention device  16   a , a sleeve  17 , a positioning mandrel  18  and an introducer sheath  19 . It should be understood that certain of these elements may be eliminated, or others added to the system, without departing from the spirit of the invention. 
     Device  12   a  houses components known in the art to be necessary to carry out the system functions. For example, device  12   a  may include one or more pulse generators, including associated batteries, capacitors, microprocessors, and circuitry for generating electrophysiological pulses for defibrillation, cardioversion and/or pacing. Device also includes detection circuitry for detecting arrhythmias or other abnormal activity of the heart. The specific components to be provided in the device will depend upon the application for the device, and specifically whether the device is intended to perform defibrillation, cardioversion and/or pacing along with its sensing functions. 
     The device  12   a  is proportioned to be passed into the vasculature and to be anchored within the patient&#39;s vasculature with minimal obstruction to blood flow. Suitable sites for the device  12   a  may include, but are not limited to the venous system using access through the right or left femoral vein or the subclavian or brachiocephalic veins, or the arterial system using access through one of the femoral arteries. Thus, the housing of device  12   a  preferably has a streamlined maximum cross sectional diameter which may be in the range of 3-15 mm or less, with a most preferred maximum cross-sectional diameter of 3-8 mm or less. The cross-sectional area of the device in the transverse direction (i.e. transecting the longitudinal axis) should be as small as possible while still accommodating the required components. This area is preferably in the range of approximately 79 mm 2  or less, and more preferably in the range of approximately 40 mm 2  or less, or most preferably between 12.5-40 mm 2 . 
     The cross-section of the device (transecting the longitudinal axis) may have a circular cross-section, although other cross-sections including crescent, flattened, or elliptical cross-sections may also be used. It is highly desirable to provide the device with a smooth continuous contour so as to avoid voids or recesses that could encourage thrombus formation on the device. 
     A first array of electrodes  22   a  is positioned on a superior region of the device body  22   a , and a second array of electrodes  24   a  is positioned on an inferior region. Individual electrodes may be used in place of the arrays. Electrodes  22   a ,  24   a  are preferably positioned on the surface of the device  12   a . For example, electrodes  22   a ,  24   a  may take the form of conductive elements attached to the non-conductive housing of the device  12   a . Alternatively, if the device includes a conductive housing to which an insulating material is to be applied, the electrodes may be formed by selectively applying the coating or removing portions of the coating to leave one or more exposed electrode regions on the surface of the device  12   a . As yet another alternative, the retention device  16   a  in this and the other embodiments may include conductive elements and function as an electrode. 
     A proximal portion of the device includes a connector  25  for receiving the distal end of positioning mandrel  18 , which may be used to steer the device  12   a  (by pushing, pulling and/or torquing) through the patient&#39;s vasculature as described below. The connector  25  may take the form of a threaded bore for receiving a threaded screw member at the distal end of the mandrel  18 , or it may have any other type of configuration for detachably engaging the distal end of the mandrel. 
     Mandrel  18  may serve purely mechanical purposes, or it may also be a “smart mandrel” that provides electrical and/or fluid connections. Such connections can be used to couple the device (via an instrument cable) for electrical, electronic, and/or fluid communication between the device and instrumentation located outside the body. This communication may be used several purposes, including device testing, initiation and/or programming during implantation, and/or recharging of the device battery. If the device is to be used for drug delivery, the mandrel may be used for re-filling a reservoir in the device with pharmaceutical agents that may be deliverable by the device to a patient. 
     Lead  14   a  is attachable to the inferior end of device  12   a  as will be described in detail in the “Implantation” section, although the lead  14   a  may be instead integrally connected to device. Lead  14   a  includes one or more defibrillation and/or pacing electrodes  26   a  and may also be equipped to sense electrical activity of the heart. Monitoring of the heart&#39;s electrical activity is needed to detect the onset of an arrhythmia. Activity sensed by the sensing electrode(s) is used by the device electronics to trigger delivery of a defibrillation shock. Additional leads may be provided if desired. 
     The lead  14   a  may be a conventional defibrillation/pacing lead, although alternative lead configurations may be desirable if warranted by the desired placement of the device  12   a  and lead within the body. For example, the physician will preferably want to select a location for the device within a chosen vessel (e.g. the inferior or superior vena cava) that will prevent the device from blocking significant peripheral vessels extending from that vessel. An optimal lead will preferably give the physician implanting the device flexibility to position the device at an appropriate location in the chosen vessel without concern that the leads extending from the device will not reach their intended location. Thus, for some patients it may be necessary to use a lead that is slightly longer than conventional leads, or the lead may include a coiled section (see coiled section  166  of  FIG. 16B ) that is similar to the configuration of a coiled telephone cord. A coiled section can allow elongation of the effective length of the lead when tension is applied to the coil. The coiled section or any alternate type of yieldable lead section may be a plastically deformable metal or polymer that will retain its extended configuration after it has been stretched to that configuration Other configurations that will allow additional lead length to pay out from the device if needed may also be used. 
     For leads that are to be positioned within a chamber of the heart, the leads may be the helical screw-in or tined variety for fixation to the cardiac tissue, and/or they may have steroid-eluding tips to facilitate tissue in-growth for fixation purposes. As illustrated in  FIG. 2A , a detachable screw-in lead tip  9  may be detachable from the lead  14   a . This allows the lead tip  9  to be left within the chamber of the heart when the remainder of the lead  14   a , so as to prevent damage to the heart tissue as could occur upon extraction of the helical tip. Tip  9  preferably includes a torque socket  11   a  which mates with a corresponding wire torque element  11   b  on the lead body  14   a  to optimize torque transmission when the lead tip  9  is screwed into the heart tissue. 
     The leads may include non-thrombogenic and/or non-proliferative surfaces or coatings as also described above in connection with the Device Configuration section below. For example, the leads may include a coating that is anti-thrombogenic (e.g. perfluorocarbon coatings applied using supercritical carbon dioxide) so as to prevent thrombus formation on the lead. It is also beneficial for the coating to have anti-proliferative properties so as to minimize endothelialization or cellular ingrowth, since minimizing growth into or onto the lead will help minimize vascular trauma when the device is explanted. The coating may thus also be one which elutes anti-thrombogenic compositions (e.g. heparin sulfate) and/or compositions that inhibit cellular in-growth and/or immunosuppressive agents. 
     It should also be noted that the lead may be attachable to the device  12   a  in situ or prior to implantation, or it may be permanently attached to the device, or it may be integral with the device as an elongate extension of the device itself. Thus it should be appreciated that in this disclosure the term “lead” is used to mean an element that includes conductors and electrodes and that thus may be positioned somewhat remotely from the circuitry that energizes the electrodes. Thus, leads may include elements that are simply extensions or tapers of the device  12   a  itself (such as the portion of the device  12   a  at which electrodes  22   a  are located) as well as more conventional leads. 
     A second embodiment of a system  10   b  is shown in  FIG. 2B  and differs from the  FIG. 2A  embodiment primarily in that its lead  14   b  is attachable (or integrally attached) to the superior end of device  12   b.    
     The third embodiment of  FIG. 2C  includes two leads  14   c ,  15   c , both extending from the superior end of the device  12   c . Either or both of the leads may be attachable or detachable from the device  12   c , permanently attached to the device, or integral with the device as an elongate extension of the device itself. Lead  15   c  preferably includes one or more defibrillation electrodes  22   c  and lead  14   c  preferably includes at least one defibrillation electrode (not shown). Either or both of the leads may also be equipped to sense electrical activity of the heart so as to identify onset of an arrhythmia. 
     Because the leads extend from one end of device  12   c , the leads  14   c ,  15   c  will be positioned side-by-side within a blood vessel at some point, at least during implantation of the system. Thus, the diameters of the leads are proportioned to permit continued blood flow through the vessel even when the leads are side-by-side. In the shown embodiment, lead  15   c  is longer than lead  14   c , and includes a narrow section  28  along the portion of the lead  15   c  that is adjacent to lead  14   c . Thus, the combined diameters of narrow section  28  and lead  14   c  must be small enough to fit through the vessels through which they will be passed, and preferably do not exceed the maximum diameter of device  12   c . In one example, lead  15   c  includes a diameter of 1-10.0 mm except at narrow section  28  which has a diameter of 0.5-9.5 mm; lead  14   c  has a diameter of 1-10 mm; and device  12   c  has a diameter of 3-15 mm. It should also be noted that a breakaway retention means  30  might be provided for coupling the narrow section  28  of lead  15   c  with the lead  14   c  during advancement of the device  12   c  through the vasculature. 
     The leads may include non-thrombogenic, non-proliferative and/or anti-inflammatory surfaces or coatings as also described above in connection with the device  12   a.    
     Each lead  14   c ,  15   c  includes a guidewire lumen to aid in implantation of the lead. Referring to  FIG. 2D , lead  15   c  includes guidewire lumen  32  which extends between opening  34  and opening  36 . Likewise, as shown in  FIG. 2E , a guidewire lumen  38  in lead  14   c  extends between openings  40  and  42 . Naturally, the leads may be provided with alternative ways of receiving guidewires, many of which are known in the art and/or described below. As shown in  FIG. 2C , the system may include guidewires  43   a ,  43   b  for use in implanting the leads  14   c ,  15   c.    
       FIG. 2F  shows a fourth embodiment of a system  10   d , which also includes a pair of leads  14   d ,  15   d  but which differs from the system  10   c  of  FIG. 2C  in that the leads extend from opposite ends of the device  12   d . As with the previous embodiments, the leads may be attachable/detachable to/from the device  12   d , permanently attached to the device, or integral with the device as an elongate extension of the device itself. The retention device  16   d  differs from the retention devices  16  of the systems of  FIGS. 2A ,  2 B and  2 C in that it is provided as a separate component rather than being integral with the device  12   d . Moreover, an additional retention device  16   e  is provided for anchoring the lead  15   d . Details concerning the retention devices are set forth below in the section entitled “Retention Devices” describing  FIGS. 9A-10E . 
     Device Configuration 
     Given the minimal space allowed for components, it is desirable to arrange the device components so as to make efficient use of the available space. Examples of devices having space efficient arrangements of their contents are shown in  FIGS. 3A ,  3 B,  3 C,  3 E, and  8 A. The features of these devices are applicable to any of the systems described herein. 
     A first example is identified by reference numeral  12   e  in  FIG. 3A . Device  12   e  includes an elongate enclosure  20  shown in cross-section in  FIG. 3A  to allow the components housed within it to be seen. Enclosure  20  is a rigid, semi-rigid or flexible housing preferably formed of a material that is biocompatible, capable of sterilization and capable of hermetically sealing the components contained within the enclosure  20 . The housing may be formed of a molded compound. Alternatively, a conductive material such as titanium, stainless steel, or other materials may be used. 
     The housing is preferably covered by a layer or coating  21 , which may be electrically insulative particularly if the enclosure  20  is conductive. One example of such a coating is ePTFE. It is desirable to provide a coating that is anti-thrombogenic (e.g. perfluorocarbon coatings applied using supercritical carbon dioxide) so as to prevent thrombus formation on the device. It is also beneficial for the coating to have anti-proliferative properties so as to minimize endothelialization or cellular ingrowth, since minimizing growth into or onto the device will help minimize vascular trauma when the device is explanted. The coating may thus also be one which elutes anti-thrombogenic compositions (e.g. heparin sulfate) and/or compositions that inhibit cellular in-growth and/or immunosuppressive agents. If the enclosure  20  is conductive, this layer or coating may be selectively applied or removed to leave an exposed electrode region  60  on the surface of the enclosure  20 . 
       FIG. 3A  illustrates one means for detachably connecting a lead  14   e  to the device  12   e . In this embodiment, device  12   e  includes a header  44  having a socket  46 . To attach lead  14   e  to the device  12   e , a pin  48  at the proximal end of lead  14   e  is inserted into socket  46 . A series of o-ring seals  50  surround the pin  48  within the socket  46  to prevent body fluids from passing into the device  12   e . A set screw  52  tightens against the pin  48  to secure the pin within the socket. 
     Within the enclosure  20  are the electronic components  54   a ,  54   b  that govern operation of the device  12   e . For example, in the  FIG. 3A  embodiment, components  54   a  are associated with delivery of a defibrillation pulse via lead  14 , whereas components  54   b  are associated with the sensing function performed using sensing electrodes on the defibrillation lead or on a separate lead (not shown). Isolating components  54   a  from components  54   b  may be desirable if noise generated by the high voltage defibrillation circuitry  54   a  during charging might interfere with performance of the sensing circuitry  54   b.    
     Device  12   e  further includes one or more batteries  56  for supplying power to the device, and one or more capacitors  58  for storing an electrical charge and for delivering stored charge to the defibrillation lead(s)  14   e  and/or exposed electrode  60  on the enclosure  20 . A circuit interconnect  62  provides the electrical coupling between the electronic components  36   a ,  36   b , lead  14   e , electrode  60 , batteries  56  and capacitors  58 . Contacts  64  couple these components to the interconnect  62 . 
     As shown in  FIG. 3A , the components of device  12   e  may be arranged in series with one another to give the device  12   e  a streamlined profile. Because the device  12   e  is intended for implantation within the patient&#39;s vasculature, some flexibility may be desired so as to allow the elongate device to be easily passed through the vasculature. Flexibility may be added by segmenting the device, such as by forming one or more breaks  66  in the enclosure  20 , and by forming one or more articulations  68  at each break  23  by connecting the segments using silicone rubber filler. The articulations  68  thus form living hinges, which bend in response to passage of the device  12   e  though curved regions of the vasculature. It should be noted that in this embodiment it is desirable to form interconnect  62  as a flex circuit so that it will not prevent bending at the articulations. 
     As discussed previously, the proximal portion of the device  12   e  may include a connector  25  for receiving the distal end of positioning mandrel  18  ( FIG. 2A ), which may optionally be used to push the device  12   e  through the patient&#39;s vasculature as described below. The connector  25  may take the form of a threaded bore for receiving a threaded screw member at the distal end of the mandrel  18 , or it may have any other type of configuration for detachably engaging the distal end of the mandrel. 
     A second example of an arrangement of components for the intravascular electrophysiological device is shown in the device identified by reference numeral  12   f  in  FIG. 3B . Many of the components are the same as those shown in the  FIG. 3A  embodiment and will not be discussed again in connection with  FIG. 3B . This second embodiment differs from the first embodiment primarily in that the electronic components  54  are included within a single area of the enclosure  20 . This configuration may be used, for example, when the device is intended only for performing pacing functions (and thus lacks the relatively noisy charging circuitry found in the defibrillation circuitry), or if isolation of the type shown in the  FIG. 3A  embodiment is not necessary to prevent noise from the charging circuit from interfering with the sensing circuits. 
     One variation on the  FIGS. 3A and 3B  embodiments is the device  12   g  shown in  FIGS. 3C and 3D . In device  12   g , each segment may be separately enclosed by its own enclosure  20   a ,  20   b ,  20   c  or partial enclosure formed of titanium or other suitable material. The components within the enclosures  20   a ,  20   b , and  20   c  are electrically connected by flex circuits  62   a , and the enclosures are connected using a flexible material such as silicone rubber filler to form articulations  68   a .  FIG. 3D  illustrates bending of the device  12   g  at one of the articulations. Many of these enclosures may be “strung together” to form the device body. This configuration is particularly desirable for embodiments incorporating particularly long device bodies, such as the devices of the  FIGS. 2A and 2B  embodiments. For these embodiments, which may have device bodies of approximately 10-60 cm in length with individual segments ranging from approximately 2-28 cm in length, flexibility of the device may be essential for movement and positioning of the device within the vasculature with minimal damage to the blood vessels. 
       FIG. 3E  illustrates an alternative mechanical assembly of individual segments to form a device  12   h . The mechanical components used to connect the segments are optimally designed such that axial, flexural and torsional forces imparted to the device  12   h  are transmitted by the mechanical components rather than by the electrical conductors that extend between the segments and the associated pins and feed-through components that collectively provide electrical coupling between the components in the device&#39;s segments. 
     The drawing shows the device  12   h  in partially-constructed form and without the electrical and electronic components, so that the mechanical elements can more easily be seen. Each segment comprises a tubular enclosure  20   h , which may take the form of a hollow tube having open ends  70  as shown. Enclosures  20   h  may vary between 2 mm and 13 cm in length, depending on the nature of the elements to be housed within the enclosures. Collectively, a device  12   h  may range in length from 10-60 cm, and in most instances between 25-55 cm. 
     Couplers  72  are secured (e.g. by welding or similar techniques) within the enclosures  20   h , near the ends  70 . Hinge regions  80  lie between the enclosures  20   h  and are filled with elastomer to seal the enclosures against body fluids. 
       FIG. 4A  shows the couplers  72  separate from the tubular enclosures. Each coupler  72  includes a central bridge  74  and may include radial spokes  76  or an alternative structure that leaves open spaces for passage of conductors around the coupler as described in greater detail with respect to  FIG. 5E . One or more stiffening rods  78  are joined to the coupler  72 . Each such rod  78  extends between two couplers  72  as shown in  FIG. 4A  to form a mechanical assembly that mechanically links a pair of adjacent enclosures  20   h  as shown in  FIGS. 3E and 4B . In the embodiment shown, rod  78  is coupled to the central bridge  74  of the coupler  72  and is secured in place using welding techniques or alternative methods. 
     The rod and coupler materials may be selected from materials that will transmit axial, flexural and torsional forces imparted to the device  12   h , but that will allow flexion of the device at hinge regions  80 . The rod  78  may thus be formed of a solid core wire, tubing, coil, or mesh braid of materials such as titanium, nitinol, stainless steel, or polymers such as nylon or polyurethane. Exemplary materials for the coupler  72  include titanium, nitinol, stainless steel, polymers, and Kevlar. Forming all or a portion coupler  72  of a flexible material or a spring-like material may also provide the needed flexibility. Alternatively, the coupler  72  may be fairly rigid and the rod  78  may be somewhat flexible. As another alternative, both the coupler  72  and the rod  78  may have some flexibility. It should be mentioned at this point that the coupler/rod assembly are but one example of assemblies that may be used for mechanically linking the enclosures  20   h.    
       FIGS. 5A through 5E  illustrate one example of a sequence of steps that may be used for assembling components into the segments  20   h  and for electrically coupling components between segments  20   h  of  FIG. 3E . 
       FIG. 5B  shows a component  82  that is to be housed within a segment  20   h  ( FIG. 3E ) of the device  12   h . Components  82  will include batteries, capacitors, circuitry, electronics, etc. The components may have the cylindrical shape as shown, or any other shape that can be inserted into the enclosure  20   h . Component  82  is fitted with a cap  84  formed of ceramic or other insulative material. Cap  84  may include ring  86  that seats against the component  82  as shown. A connector pin  88  extends through a bore hole in the cap  84  and is electrically coupled to the component  82 . One or more conductive pins  90  (seven are shown) are isolated within blind holes in the cap  84 . Cap  84  and pins  88 ,  90  may be integral with the component  82 , or they may be a separate component that is positioned in contact with the component  82  such that pin  88  is in registration with a corresponding contact on the component. The connector pin  88  may be angular as shown such that its free end is within the circumferential arrangement of the other pins  90  as best shown in  FIG. 5A . 
     Referring to  FIG. 5D , the component  82 /cap  84  assembly of  FIG. 5B  is positioned within the segment enclosure  20   h , with a flex circuit  92  wrapped at least partially around the component  82  as shown. Other conductive elements may be used in place of the flex circuit, including an array of conductors embedded in polymer and molded into a sheet or extruded into a tube. 
     Flex circuit  92  includes conductor tabs  94  folded over into contact with the pins  88 ,  90  as shown. After the tabs  94  are positioned in contact with the pins  88 ,  90 , coupler  72  (also shown separately in  FIG. 4A ) is introduced into the enclosure  20   h  and is secured in place using, for example, welding or a mechanical interlock. Although the rod  78  (also shown separately in  FIG. 4A ) is not shown in  FIGS. 5C and 5D , the rod  78  may be integral with coupler or it may be pre-connected (by welding or mechanical connection) to the coupler before or after the coupler  72  is placed in the enclosure  20   h .  FIG. 5E  shows the assembly with the rod  78  in place. Referring again to  FIG. 4B , the rod  78  extends between adjacent segment enclosures  20   h , each of which is assembled as described above. 
     Referring to  FIG. 6A , a conductor assembly  98  completes the electrical connection between the segments  20   h . The conductor assembly may comprise wires  100  arranged in a configuration (such as the illustrated helical configuration) that will prevent damage to or disconnection of the conductors when the device flexes at the hinge regions  80 . As another alternative, the conductor assembly may be comprised of wires  102  extending through a flexible insulated ribbon  104  positioned around the rod  78  as shown in  FIG. 7 . In other embodiments, flexibility may be added to the conductor assembly by incorporating loops, coils, or sinusoidal bends into the wires to allow slight elongation of the net length of the wires when the device  12   h  is flexed. 
     Referring again to  FIG. 5E , the ends of the wires  100  are coupled to corresponding ones of the pins  88 ,  90 . This connection may be made by various methods, including soldering or employment of a mechanical jack-type connector (not shown) with corresponding mating components on the wires  100  and conductors  88 ,  90 . Once electrical coupling is achieved between wires  100  and conductors  88 ,  90 , the gap  80  ( FIGS. 4B and 6A ) between neighboring enclosures  20   h  is filled with an elastomeric material such as silicone, polyurethane, perfluoroethers, or epoxies to create a sealed barrier  106  ( FIG. 5E ). The barrier  106  prevents body fluids from entering the enclosures  20   h . Application of the barrier  106  may be preceded by application of a paralene pre-coating or other redundant barrier to the conductors and other components within and extending between the enclosures  20   h.    
     The elastomeric barrier material preferably fills the ends of the enclosures  20   h  as well as the space between the enclosures. As shown in  FIG. 5E , the ends  108  of the enclosure  20   h  may be swaged into the elastomeric barrier  106  to facilitate retention of the barrier  106 , to improve sealing, and to minimize the chance for delamination of the elastomeric material. It may also be desirable to roughen the interior surface of the enclosure  20   h , or to form holes around the end circumference of the enclosure  20   h  to create a mechanical or interference fit between the enclosure and the elastomer. 
     The method described in connection with  FIGS. 5A through 7  is but one example of the many methods available for connecting the enclosures  20   h.    
     Another arrangement of device components is found in the intravascular device identified by reference numeral  12   i  and shown in  FIG. 8A . Many of the components are the same as those shown in the  FIGS. 3A and 3B  embodiments and will not be discussed again. The  FIG. 8A  embodiment differs from the prior embodiments largely in the configuration of the capacitor  58   a , which takes the form of a coiled ribbon  112  mechanically coupled to the proximal end of the device  12   i  (or to a more distal location) and electrically coupled to the circuit interconnect  62 . The coiled ribbon may take the form of a flex circuit of the type described in connection with  FIG. 5B  below, or it may be formed of layers of capacitor material overlaying one another to form the ribbon itself. 
     Prior to implantation, the ribbon  112  is compressible to a streamlined condition for introduction into the body. For example, it may be placed within a delivery sheath or it may be retained in a streamlined position by winding the ribbon against the mandrel and retaining it with a shorter sleeve, suture or wire etc. As yet another example, proximal tension may be imparted on the ribbon by pulling the ribbon in the longitudinal direction, thereby elongating the ribbon while reducing its overall width, much like pulling on a coiled telephone wire. Once positioned within the vessel at the appropriate site for implantation, the capacitor is released from the compressed position and springs to an expanded position within the vessel, as further discussed in the section entitled “System Implantation” below. 
     Although the ribbon is described as being a capacitor, it should be appreciated that a different subset of the device components may be provided in the form of a ribbon-like structure or circuit. For example, the capacitor may be similar to the capacitors  58  shown in  FIGS. 3A and 3B , and the device&#39;s battery may instead be formed in the coiled ribbon configuration. In yet another variation, the coiled ribbon may instead be an antenna for transmitting signals alerting a physician to the occurrence of an arrhythmia, and both the capacitor and battery may take the forms shown in  FIGS. 3A and 3B , or some alternate form. 
       FIG. 8B  is an enlarged view of the ribbon  112  used for capacitor  58   a  of  FIG. 8A . The ribbon  112  is a coiled flex circuit electrically connected to the rest of the device  12   i  by tab  114 . Discrete capacitor segments  116  are preferably arranged in a stepped pattern on the ribbon surface and may be applied using spray-on/lithographic techniques or other means. Segments  116  have terminals  118  that may be connected in parallel using parallel connections  120 , or in series using series connections  122  as needed. The segments  116  may be on the exterior surface of the ribbon  112 , and/or there may be additional segments or related components  124  (including integrated circuit components, passive circuitry components, microprocessor components etc.) on the interior surface of the coil. 
     It should also be noted that the entire device (including the capacitors, batteries, microprocessor, electronics, etc) may take the form of a coiled ribbon flex circuit, with the components being located on the exterior or interior surface of the ribbon and with the leads coupled to the ribbon. 
     Any one of the devices described herein is preferably able to communicate via wireless telemetry to an instrument outside of the patient&#39;s body. This is commonly referred to as device interrogation and/or programming and allows the physician to monitor the state and performance of the device. It also allows the physician to reconfigure the device in the case of programmable settings. 
     The circuitry used for device interrogation and/or programming can be included in any of the device embodiments, with the device telemetry antenna either encapsulated within the device enclosure(s) or as part of a ribbon component set of the type shown in  FIG. 8A . The circuitry may include a circuit that will respond in the presence of a magnetic field, which is a feature also known in the implantable device industry. These types of communication means are intended to allow the device to communicate the device&#39;s status to the physician. For example, the status information may include the state of the battery system, and whether or not a therapeutic energy delivery had occurred or not. The communication might also identify the parameters the device used, including a stored electrogram, to allow reconstruction of the delivery episode by the instrument. The telemetry feature may also be used to program certain features governing function of the device, such as the threshold heart rate in beats per minute which, when detected by the device, will cause the device to provide appropriate energy therapy. 
     Retention Devices 
     The intravascular system further includes a mechanism for retaining the device in the patient&#39;s vasculature, such as in the superior vena cava  3   a , inferior vena cava  3   b , or the left or right subclavian  2   a ,  2   b  (see  FIG. 1 ). Although various means may be used to retain the device within the vasculature, one example of a retention device is the tubular retention sleeve or anchor  16   d  of the type illustrated with device  12   d  in  FIG. 2F  and as shown in greater detail in  FIGS. 9A and 9B . The retention device is described as a separate component from the device  12   d , but it will be appreciated that the anchor  16   d  or other retention device may be integral with the device  12   d.    
     The anchor  16   d  may include features that give some structural stability to cause the anchor to radially support the device against a vessel wall. For example, a mesh, band or other framework  126  ( FIG. 9B ) formed of shape memory (e.g. nickel titanium alloy, nitinol or shape memory polymer) elements or stainless steel, Eligoy, or MP35N wires or structures may be used. The anchor  16   d  is preferably provided with a smooth polymeric barrier  128  that is both anti-proliferative and anti-thrombogenic and that thereby prevents endothelial growth and thrombus formation on the anchor. Examples of materials for the polymeric barrier include, but are not limited to ePTFE, or other fluoropolymers, silicone, non-woven nylon, or biomimetic materials. 
     Layers of barrier material on the interior and exterior surfaces of the framework preferably form the polymeric barrier  128 , although it will be appreciated that the framework  126  and barrier  128  may be combined in a variety of ways to prevent thrombus formation and endothelialization on the anchor walls. As one alternative (or in addition to the polymeric barrier), the anchor material could include surfaces for eluting non-coagulative, anti-platlet (e.g. IIBIIIA glycoprotein receptor blockers), anti-proliferative, and/or anti-inflammatory substances. 
     The framework  126  may extend through the entire length of the anchor, or it may be included in only a portion of the anchor, such as at the proximal and distal end regions as shown in  FIG. 9A , leaving the intermediate region  130  between them with no structural reinforcement. This arrangement may be preferable in that is allows the intermediate region to conform to the surface of the device  12   d  during use. As another alternative, the intermediate region may include some structural reinforcement, but less than is provided in the more rigid proximal and distal regions  126  so as to allow some conformability of the anchor to the device surface. 
     During implantation, the anchor  16   d  is compressed to a streamlined positioned for passage through the vasculature. The anchor  16   d  may be inserted into a positioning sheath to facilitate movement through the vasculature. 
     Typically the anchor will be deployed after the device has been positioned at a desired location within the vessel, although if the anchor and device are integral components they will be implanted simultaneously. The anchor is advanced to a position adjacent the device, released from the sheath (if used) and expanded to a radially expanded position as shown in  FIG. 9A . The anchor may self-expand and/or it may be expanded using an inflation tool such as a balloon passed into the anchor&#39;s central lumen and subsequently inflated. When the anchor is expanded, its radial force engages the device  12   d  and secure the device  12   d  against the vessel wall. As shown, the force of the anchor against the device may cause the vessel to distend outwardly due to the vessel&#39;s compliance. Blood flowing through the vessel passes through the tubular interior of the anchor as indicated by arrows in  FIG. 9A . Because the device  12   d  occupies the distension in the vessel, the presence of the device causes minimal (if any) obstruction to blood flowing through the vessel. 
     It is desirable to minimize passage of blood between the anchor  16   d  and the device  12   d  so as to minimize the chance of thrombus formation and endothelialization around the device  12   d . For this reason, the rims  132   a ,  132   b  surrounding the anchor&#39;s proximal and distal openings are preferably designed to make sealing contact against the surrounding vessel tissue (and against the lead  15   d ) as shown in  FIG. 9A  so as to direct all blood flow into the interior of the anchor. For example, rims  132   a ,  132   b  may be formed of a thicker and more pliable material such as silicone or polyurethane-siloxane, or the rims may be supplemented with compliant members that seal against the lead and surrounding tissue. As another example, a swellable hydrogel which expands when placed in contact with fluids including blood, may be included on the anchor&#39;s ends to optimize sealing. Ideally, these barriers will form a seal with the adjacent tissue, however it is sufficient that the barriers prevent a substantial amount of blood from passing between the exterior of the anchor and the device, without necessarily forming an impermeable seal. 
     As will be described below, additional anchoring devices such as anchor  16   e  ( FIG. 2F ) similar to the anchor  16   d  may also be used to anchor leads within the vasculature. 
     As discussed, it is desirable to minimize endothelial growth onto the anchor, since endothelial growth onto the anchor  16   d  can make it difficult to separate the anchor and device  12   d  from the vessel tissue during explantation. Referring to  FIG. 9C , a tubular liner  134  may be deployed within the vessel prior to implantation of the device  12   d  and anchor  16   d . Liner  134  may be similar in design to the anchor  16   d , but is preferably longer than either the device  12   d  or anchor  16   d  so that the liner contacts the vessel wall but the device and anchor  16   d  do not. If used with the  FIG. 8A  embodiment of the device  12   i , which includes coiled ribbon  112 , the liner  134  is preferably longer than the combined length of the device enclosure and coil  112 . The liner  134  helps to reduce the risk of trauma to the vessel tissue during explantation of the device and/or anchor  16   d.    
     During implantation, the liner  134  is deployed in the desired anatomic location before the device is moved into place. The steps for deploying the liner  134  may be similar to those described above for deploying the anchor  16   d . Once the liner  134  is in place, the device is deployed, followed by the anchor  16   d , in the same manner as described elsewhere. Over time the liner may become endothelialized, particularly at its edges. However, the endothelial growth is self-limiting to the edge or rim of the liner due to increasing distance from a sustaining blood supply and should not reach the inner retaining anchor  16   d . Thus, when it is necessary to explant the device  12   d  for servicing (such as to replace a battery for example) the inner anchor  16   d  may be grabbed by a surgical instrument with the outer liner  134  acting as a protective layer for the vessel. The liner  134  may be left in place following removal of the anchor  16   d  and device  12   d . If the device  12   d  (or a replacement) is to be later re-implanted, it may be returned to its original location within the liner  134 . 
     In an alternative implantation method using the liner  134 , the device  12   d  may be “sandwiched” between the liner  134  and anchor  16   d  before implantation by placing the device inside the liner, then placing the anchor in a compressed position within the liner, and then expanding the anchor to engage the device between the sleeve and anchor. The three components are then compressed into a positioning sheath and introduced as described elsewhere. 
       FIGS. 10A  though  10 E illustrate an alternative anchor  16   a  of the type shown with the systems of  FIGS. 2A through 2C . The anchor  16   a  is beneficial in that it is implanted integrally with the device, and thus does not require a separate implantation step. 
     Referring to  FIG. 10A , anchor  16   a  includes structural features that allow the anchor to radially engage a vessel wall. For example, a band, mesh or other framework formed of one or more shape memory (e.g. nickel titanium alloy, nitinol, thermally activated shape-memory material, or shape memory polymer) elements or stainless steel, Elgiloy, or MP35N elements may be used. The anchor may include anti-proliferative and anti-thrombogenic coatings, although in this embodiment the anchor structure  16   a  is preferably provided to promote tissue ingrowth to as to enhance anchor stability within the vessel. The anchor may also have drug delivery capability via a coating matrix impregnated with one or more pharmaceutical agents. 
       FIG. 10B  shows one anchor  16   a  attached to a device  12   a , although naturally one, two or more such anchors may alternatively be used. In one embodiment, anchor  16   a  is attached to the implant  12   a  by a collar  136 , or other suitable connection. The implant  12   d  may include a recessed portion  138  that allows the exterior of the anchor to sit flush with the exterior of the implant  12   a  when the anchor is its compressed position. The recessed portion should have smooth contours in order to discourage thrombus formation on the device. 
     The anchor  16   a  and device  12   a  may be detachably connected to the recessed portion using methods that allow the anchor  16   a  and the implant  12   a  to be separated in situ, for permanent or temporary removal of the implant  12   a . A detachable connection between the anchor  16   a  and implant  12   a  may utilize a snap fit between the collar  136  and implant  12   a . As shown in  FIG. 10C , both the collar  16   a  and the recessed portion  138  of the implant may include an elliptical cross-section. If it becomes necessary to remove the medical implant from the patient&#39;s body, the medical implant may be torqued about its longitudinal axis, causing the body of the implant to cam the edges of the collar  136  to a slightly opened position, thereby allowing the implant to be passed between the edges  140  of the collar  136 . In an alternative embodiment, a clevis pin-type connection may be made between the anchor  16   a  and the device  12   a . Such a connection would be provided with a remotely actuated mechanism for releasing the clevis pin connection to thus permit separation of the device and the anchor. 
     The anchor may be configured such that the device  12   a  and anchor  16   a  share a longitudinal axis, or such that the axes of device  12   a  and anchor  16   a  are longitudinally offset. 
     Referring to  FIG. 10D , a retractable sheath  142  may be slidably positioned over the anchor  16   a  and implant  12   a  so as to retain the anchor in its compressed position. Retraction of the sheath as indicated in  FIG. 10E  allows the anchor  16   a  to expand into contact with the surrounding walls of the vessel, thereby holding the medical implant in the desired location. Once deployed, the anchor  16   a  is preferably intimate to the vessel wall, which is distended slightly, allowing the vessel lumen to remain approximately continuous despite the presence of the anchor and thus minimizing turbulence or flow obstruction. 
     Implantation Methods 
     Several methods for implanting intravascular electrophysiological systems are shown in  FIGS. 11A through 15E . These implantation methods are preferably carried out under fluoroscopic visualization. Although the methods described in connection with  FIGS. 11A through 15E  introduce the device into the venous system via the femoral vein, the device and components may alternatively be introduced into the venous system via that subclavian vein or the brachiocephalic veins, or into the arterial system using access through one of the femoral arteries. Moreover, different components of the intravascular systems may be introduced through different access sites. For example, a device may be separately introduced through the femoral vein and a corresponding lead may be introduced via the subclavian vein. 
     First Exemplary Method 
       FIGS. 11A through 11F  illustrate a method for implanting the system  10   a  of  FIG. 2A . First, a small incision is formed in the femoral vein and the introducer  19  is inserted through the incision into the vein to keep the incision open during the procedure. Next, the device  12   a  is passed into the introducer  19 , and pushed in a superior direction through the inferior vena cava  3   b  (“IVC”), through the right atrium  4   a  towards the superior vena cava  3   a  (“SVC”). With an end of the device  12   a  still remaining outside the body, mandrel  18  and lead  14   a  are attached to the exposed end of the device  12   a  as shown in  FIG. 11B . Pressure is applied against the mandrel  18  to advance the device  12   a  into the left subclavian vein (“LSV”)  2   b.    
     Referring to  FIG. 11C , once the device  12   a  is in the target position, the anchor  16   a  is expanded into contact with the walls of the inferior vena cava  3   b . The mandrel  18  is detached from the device  12   a  and removed from the body. 
     A steerable guidewire or stylet  144  is attached to the free end  146  of the lead  14   a  or inserted into a lumen in the lead  14   a  and is used to carry the free end  146  of the lead through the introducer  19  and into the IVC  3   b  such that the lead  14   a  folds over on itself as shown in  FIG. 11E . The free end  146  is steered into the right ventricle  7   a  (“RV”) using the stylet  144  and is fixed in place using a helical screw member at the free end  146  or another attachment feature. The stylet  144  is removed, leaving the lead  14   a  positioned in the right ventricle  7   a  as shown in  FIG. 11F . As an alternative, the free end  146  of lead  14   a  may be steered into the middle cardiac vein. 
     Second Exemplary Method 
       FIGS. 12A through 12E  illustrate implantation of the device  12   b  of  FIG. 2B . As with the first exemplary method, this method positions a portion of the device in the left subclavian vein  2   b  and a lead in the right ventricle  7   a  (or, alternatively, the middle cardiac vein). However, the method of  FIGS. 12A through 12E  orients the device  12   b  of  FIG. 2B  such that the lead  14   b  is positioned at the superior end of the device  12   b  as opposed to the inferior end of the device. 
     Referring to  FIG. 12A , lead  14   b  is first passed into the introducer  19  and steered into the right ventricle  7   a  using steerable stylet  144 . The lead  14   b  is then rotated by torquing its free end  150  to fix a helical tip (not shown) on the lead  14   b  into tissue of the right ventricle as shown in  FIG. 12B . A handle  148  may be attached to the free end  150  for this purpose. 
     The device  12   b  is then attached to the free end  150  of lead  14   b , which is positioned outside the body. Next, the device is advanced into the vasculature as shown in  FIG. 12C . Mandrel  18  is attached to the inferior end of the device  12   b  and is used to advance the device  12   b  fully into the vasculature as shown in  FIG. 12D . Since the lead  14   b  will be provided with extra length be ensure that there will be sufficient slack in the lead, some of the lead slack may remain in the IVC  3   b . It may thus be necessary to advance the device  12   b  beyond its target position to drive any slack in the lead  14   b  beyond the target location. Once the lead has advanced beyond the target anchor location, the mandrel  18  is withdrawn slightly to retract the device  12   b  into its intended position. The anchor  16   a  is deployed and the mandrel  18  is removed from the body, leaving the device and lead in place as shown in  FIG. 12E . 
     Third Exemplary Method 
       FIGS. 13A through 13C  illustrate implantation of the bifurcated system of  FIG. 2C . As with prior methods, a small incision is first formed in the femoral vein and the introducer sheath  19  is inserted through the incision into the vein to keep the incision open during the procedure. Next, guidewires  43   a ,  43   b  are passed through the sheath  19  and into the inferior vena cava  3   b . Guidewire  43   a  is steered under fluoroscopy into the left subclavian vein  2   b  and guidewire  43   b  is guided into the right ventricle  7   a  of the heart. 
     Next, as shown in  FIG. 13B , the lead  15   c  is threaded over guidewire  43   a  and lead  14   c  is threaded over guidewire  43   b . Positioning mandrel  18  is attached to the proximal end of the device  12   c . The leads  14   c ,  15   c  and then the device  12   c  are then passed through the sheath  19  and into the IVC  3   b . The leads are sufficiently rigid that pushing on the mandrel  18  to advance the device causes advancement of the leads over their respective guidewires. Advancement of the mandrel  18  is continued until the lead  15   c  is disposed in the desired position within the LSV  2   b , and the lead  14   c  is within the right ventricle  7   a  as shown in  FIG. 13C . 
     Finally, the device  12   c  is anchored in place by releasing the anchor  16   a  to its expanded position as shown in  FIG. 13C . The anchor expands into contact with the surrounding vessel wall, thereby preventing migration of the device  12   c . If desired, lead  15   c  may be anchored in the LSV  2   b  using another suitable anchor. The mandrel  18  is detached from the device  12   c , and the mandrel  18  and introducer sheath  19  are withdrawn from the body. 
     A variation on the third exemplary method uses a system that uses a separately deployable anchor  16   d  rather than an integrated anchor to retain the device  12   d . Referring to  FIG. 13D , a delivery catheter  29   a  is provided for carrying the anchor  16   d  through the vasculature. A compressive sheath (similar to sheath  142  shown in  FIG. 10D ) may be used to maintain the retention device  16   d  in the streamlined or compressed position for implantation and is removable to release the sleeve to the expanded position. If a retention device is also to be used for the LSV lead  15   c , a second delivery catheter  29   b  may be provided for introducing the second retention device  16   e.    
     Optional liners  134   a,b  are provided for minimizing endothelial growth onto the retention devices by forming a lining between the vessel tissue and the retention sleeves  16   d ,  16   e . As described above in connection with  FIG. 9C , each liner may have a design similar to that of the retention devices but it is preferably long enough prevent the implant device, retention device, or lead from contacting the vessel wall. Delivery catheters  29   c ,  29   e  are provided for introducing the liners  134   a ,  134   b  into the vessels. 
     Referring to  FIG. 13E , according to this variation, small incisions are formed in each femoral vein and the introducer sheaths  19   a ,  19   b  are inserted through the incisions. Next, guidewire  43   c  is passed through the sheath  19   b  in the right femoral vein, and into the left subclavian vein  2   b . Delivery catheter  29   d  is passed over the guidewire  43   c  and guided under fluoroscopy into the left subclavian vein (“LSV”)  2   b  as shown in  FIG. 13F . Liner  134   b  is expanded and released from the catheter, and the catheter  29   d  is withdrawn. Next, the catheter  29   c  is passed over the guidewire  43   c  and guided into the inferior vena cava  3   b . Liner  134   a  is released and expanded within the IVC to the position shown in  FIG. 13G . 
     Next, guidewires  43   a ,  43   b  are inserted into introducer  19   a . Guidewire  43   b  is under fluoroscopy into the LSV  2   b  and guidewire  43   a  is guided into the heart, through the right ventricle and into the pulmonary vein as shown in  FIG. 13G . The leads  15   c ,  14   c  are threaded over the guidewires  43   a ,  43   b  as described above, and the mandrel  18  is attached to the device  12   c . The mandrel  18  is advanced until the lead  15   c  is disposed in the desired position within the LSV, and the lead  14   c  has tracked guidewire  43   a  into the pulmonary vein. If a breakaway retention mechanism  30  ( FIG. 2C ) is used to hold the leads  15   c ,  14   c  in a streamlined configuration, its components release at this point, due to the divergent paths of the respective guidewires and leads. 
     The next step involves backing the lead  14   c  out of the pulmonary vein and directing it onto the right ventricle. This is accomplished by withdrawing the mandrel  18  ( FIG. 13H ) to retract the system slightly until the lead  14   c  and guidewire  43   b  slip out of the pulmonary vein and drop into the right ventricle  7   a . The mandrel  18  is again advanced or rotated as described previously to seat the lead  14   c  within the right ventricular apex. 
     Next, if an anchor is to be used for the LSV lead  15   c , guidewire  43   c  is passed through introducer sheath and into the LSV  2   b , and delivery catheter  29   b  ( FIG. 13D ), with retention device  16   e  on it, is passed over the guidewire  43   c  and used to position the sleeve  16   e  adjacent to the lead  15   c . The sleeve is expanded and released from the catheter  29   b , leaving the lead  15   c  sandwiched between the liner  134   b  and retention sleeve  16   e  as shown in  FIG. 13J . The retention device  16   d  is positioned in similar fashion by threading delivery catheter  29   a  over guidewire  43   a , and advancing the retention device  16   d  into position adjacent to device  12   c  as shown in  FIG. 13I . The retention device  16   d  is released and expanded into contact with the vessel wall, thereby retaining the device  12   c.    
     Over time the liners may become endothelialized, particularly at their edges. However, the endothelial growth is self-limiting to the edge or rim of the liner due to increasing distance from a sustaining blood supply and should not reach the retaining sleeves. Thus, if it becomes necessary to explant the device  12   d  permanently or for servicing (such as to replace a battery for example) the retention sleeve  134   a  may be grabbed by a surgical instrument with the outer liner acting as a protective layer for the vessel. The liner may be left in place following removal of the retention sleeve and device  12   d . If the device  12   d  (or a replacement) is to be later re-implanted, it may be returned to its original location within the liner. 
     Fourth Exemplary Method 
     Implantation of the device  12   i  of  FIG. 8A  will next be described with reference to  FIGS. 14A through 14F . Prior to implantation, positioning mandrel  18  is attached to the proximal end of the device  12   i  and the ribbon coil  112  is wrapped around the mandrel  18 . At least a portion of the device, and particularly the ribbon coil  112 , is enclosed within a sleeve  142  (shown in a partially withdrawn position in  FIG. 14A ) to compress the coil  112  to the streamlined position for passage through the vasculature. The device is advanced through an introducer sheath (see sheath  19  of  FIG. 2A ) and pushed using mandrel  18  into the vasculature to the desired location. 
     Turning to  FIG. 14B , once the device  12   i  has been advanced by mandrel  18  to the desired position, the sleeve  142  is withdrawn, allowing the ribbon coil  58   a  to spring to its expanded condition in contact with the vessel walls. The expanded coil may take the form shown in  FIG. 14B , or it may spiral into overlapping layers to shorten its longitudinal dimension. A balloon catheter may be introduced into the vessel and expanded within the coil if needed for full expansion. 
     At this point in the procedure, the device is anchored at the target location. A steerable guidewire  154  is threaded through the lead  14   d  near the lead&#39;s free end as shown in  FIG. 15A  and is passed through the introducer sheath into the vein and steered to the desired location. The lead preferably includes a cuff  156  for receiving the guidewire for this purpose. A pusher  158  is then threaded over the guidewire and advanced into contact with the cuff  156 . Because cuff  156  is attached to the lead  14   d , advancing pusher  158  pushes the lead  14   d  to the target site. 
     If the target lead location is within a vessel such as the left subclavian vein  2   b  as shown in  FIG. 16A , and anchoring of the lead is desired, the lead is held in place while a sheath (similar to sheath  152  of  FIG. 14D ) having an anchor (see  16   d  of  FIG. 14D ) positioned inside it is moved into position in parallel with a distal portion of the lead. The sheath is withdrawn, releasing anchor  16   d  into the vessel. The anchor self-expands or is expanded, causing the anchor to radially compress the lead against the vessel wall. 
     If the target lead location is within a chamber of the heart, it may be secured at the target site using conventional securing means such as a helical fixation tip or tines on the distal end of the lead. 
     If a second lead is to be deployed, the procedure is repeated for that lead. 
     If further anchoring of the device  12   i  is desired beyond that provided by coil  58   a , an integral anchor similar to anchor  16   a  of  FIGS. 10E and 10E  may be used, or a separate anchor  16   d  of the type shown in  FIG. 9A  may be used.  FIGS. 14C-14F  illustrate one method for anchoring the device using anchor  16   d . Although these figures illustrate anchoring of device  12   i , they are equally applicable to deployment of other devices within the vasculature, including the devices  12 ,  12   a , and  12   b  and  12   c  as well as leads. 
       FIG. 14C  shows the device  12   i  of  FIG. 14A  (attached to mandrel  18 ) after it has been advanced into the vessel and after the ribbon coil  58   a  has been released to the expanded position. Once the device  12   i  is in the desired position, a sheath  152  with the anchor  16   d  inside it is positioned in parallel with the device  12   i  while the device  12   i  is held in place using the mandrel  18 . The sheath  152  is withdrawn as shown, releasing anchor  16   d  into the vessel. As discussed in connection with  FIG. 9A , although the sheath  152  facilitates placement of the anchor, it should be considered optional. 
     The anchor self-expands or is expanded using an expansion device such as a balloon (not shown) inflated within the anchor&#39;s central lumen, causing the anchor to radially engage the device  12   i  against the vessel wall. See  FIG. 14E . Once the anchor is deployed, the mandrel  18  is detached from the device  12   i  and withdrawn from the body, leaving the device  12   i  and anchor  16   d  in the vessel as shown in  FIG. 14F . 
     Fifth Exemplary Method 
     According to a yet another implantation method, implantation of the device (e.g. device  12   d  of  FIG. 2F  or the device  12   i  of  FIG. 8A ) involves first positioning the lead(s) at the desired location (i.e. in a vessel or in a chamber of the heart) and then positioning the device at the appropriate position. As with the method described with respect to  FIG. 15A , this method of lead implantation preferably uses over-the-wire techniques that are widely used for cardiac lead placement. Using the over-the-wire procedure, an introducer sheath is inserted into the femoral vein (of elsewhere in the vasculature) and a steerable guidewire is inserted into the introducer sheath. With the aid of fluoroscopy, the physician guides the wire to the intended lead location. For example, for positioning a system in the configuration shown in  FIG. 16A , the guidewire would be directed to the patient&#39;s left subclavian vein  2   b , whereas for positioning in the configuration of  FIG. 17B , the guidewire would be directed to the right ventricle  7   a.    
     Next, the lead (e.g. lead  14   d  or  15   d  of  FIG. 2F , or lead  15   d  of  FIG. 16A , or lead  14   d  of  FIG. 17B  is threaded over the wire and pushed by the physician to the desired location. The lead is anchored at the desired location as described in connection with the first exemplary method. If a second lead is to be implanted, the process is repeated for the second lead. 
     Implantation of the device  12   d  begins once the distal end of the lead has been placed or anchored at the target location. At this point the proximal end of the lead preferably extends outside the body from the introducer sheath, which remains in the open vein. If the lead is provided as a separate component from the device, the lead is next attached to the device  12   d.    
     Next, a positioning (e.g. mandrel  18  of  FIG. 2F ) is attached to the proximal end of the device  12   d . Device  12   d  is advanced into the introducer sheath, and pushed using mandrel  18  (preferably under fluoroscopic visualization) to the desired location. Once at the desired location, device  12   d  is anchored in place using anchor  16   d  ( FIG. 2F ) as described in connection with the prior embodiments. 
     The positioning sheath, mandrel, and introducer sleeve are withdrawn from the patient. 
     Sixth Exemplary Method 
     The next example of an implantation method is similar to the prior example, but differs in that the leads and device are simultaneously advanced using the over-the-wire technique. As such, the sixth example is particularly useful for devices having pre-attached leads. 
     For this example, the lead and/or device is modified to allow the lead to be advanced over a guidewire even though it is attached to the device. Thus, as shown in  FIG. 15B  the body of the device  12   j  may be provided with a bore  160  that receives the same guidewire  154  that also extends through the lead  15   d . Alternatively, a channel  162  may extend through a portion of the device  12   k  as shown in  FIG. 15C . In this configuration, the guidewire  154  extends through the lead, into the channel  162 , and then runs along the exterior of the device  12   k . As yet another example, shown in  FIGS. 15D and 15E , the lead  15   d  is modified to include a cuff  164  that receives the guidewire  154  externally of the lead, allowing the guidewire to run alongside the device  12   l . It should be noted that although  FIGS. 15A and 15B  show o-ring seals  50  and a set screw  52 , these features may be eliminated if the lead and device are provided to be integral with one another. 
     According to the sixth example, an introducer sheath is inserted into the femoral vein and a steerable guidewire  154  is inserted into the introducer sheath. The physician guides the guidewire to the intended lead location as described above. 
     Next, the lead  15   d  is threaded over the guidewire. If the  FIG. 15B  configuration is used, the proximal end of the guidewire  154  is threaded through the lead  15   d  and then passes through the bore in device  12   j . If the  FIG. 15C  configuration is used, the proximal end of the guidewire passes from the lead into channel  162  in the device header, and then exits the device  12   k . In either case, the lead is passed into the introducer sheath. The mandrel  18  (not shown in  FIGS. 15A through 15D ) is preferably attached to the device body and used to push the device and lead over the guidewire through the vasculature. Once the lead has reached the desired location, it is anchored at the desired location as described in connection with the first exemplary method. The mandrel  18  is used to maneuver the device to the target device position, and the device is anchored in place using the anchor as described above. 
     If the  FIG. 15D  configuration is used, the distal portion of the guidewire is threaded through cuff  164 , and the mandrel (not shown) is attached to the device. The lead is passed into the introducer sheath. A pusher  158  is likewise threaded over the guide wire and advanced into contact with cuff  164 . The pusher  158  is further advanced to push the lead to the desired location, at which time the lead is anchored as described. The mandrel is used to advance the device to the appropriate device position, and the device is then anchored in place. 
     Seventh Exemplary Method 
     A seventh example of an implantation method utilizes steps from prior examples and is useful for device configurations such as the  FIG. 2F  configuration in which leads  15   d ,  14   d  extend from opposite ends of the device. According to the seventh example, the superior lead  15   d  and device  12   d  are first implanted using the procedure of the sixth example ( FIGS. 15B , C and D), thus leaving the inferior lead  14   d  extending out the incision in the femoral vein. Lead  14   d  is then carried into the vein and pushed to the desired position using the procedure illustrated in  FIG. 15A  and described as part of the fourth exemplary method. 
     Eighth Exemplary Method 
     An eighth example may be used to implant a device having a pre-attached lead. First, incisions are formed in the patient&#39;s subclavian  2   b  and in the femoral vein and introducer sheaths are inserted into each vessel. A guidewire is passed into the introducer sheath in the subclavian, through the right atrium to the left superior vena cava  3   a  and out the introducer sheath in the femoral vein. The end of the guidewire extending out the femoral vein is attached to the lead, and is then withdrawn at the subclavian incision, thereby pulling the lead into the subclavian and drawing the device that is attached to the lead into the inferior vena cava. The mandrel  18  may be used as described above to facilitate “fine-tuning” of the device position. The lead and/or device are anchored as described above. 
     Ninth Exemplary Method 
     A “leadless” embodiment shown in  FIG. 17C  may be provided with a bore or similar means for receiving a guidewire, in which case it may be implanted by first directing a guidewire through the subclavian or inferior vena cava to the right ventricle, threading the device over the guide wire, and then pushing the device (e.g. using mandrel  18 ) to the ventricle. Alternatively, the device may be advanced through a hollow catheter having its distal end positioned in the right ventricle. 
     Applications 
     Intravascular electrophysiological systems of the type described herein are adaptable for use in a variety of applications, including single chamber atrial or ventricular pacing, dual chamber (atrial and ventricular) pacing, bi-atrial pacing for the suppression of atrial fibrillation, bi-ventricular pacing for heart failure patients, cardioversion for ventricular tachycardia, ventricular defibrillation for ventricular fibrillation, and atrial defibrillation. The system may be adapted to perform multiple functions for use in combinations of these applications. The system may be implanted for permanent use, or it may be implanted for temporary use until more permanent interventions can be used. 
     In general, the system is responsive to fast and/or irregular heartbeats detected using sensing electrodes positioned on the device body and/or leads. Typically, at least two primary sensors will be positioned across the heart so as to provide a macroscopic view of the electrical activity of the heart. Common locations for these primary sensors will include a position below the heart such as the inferior vena cava  3   b , and a position above the heart such as the superior vena cava  3   a  or the left subclavian vein  2   b . Data obtained from these sensors may be optionally supplemented with localized data from more closely spaced sensors at particular areas of interest, such as the right atrium. This data can bring into focus the nature of the abnormal activity detected by the primary sensors, and can allow the system to be programmed to differentiate between electrical activity requiring delivery of corrective defibrillation or pacing pulses, and electrical activity that can resolve without intervention. 
     The system should be programmed to deliver sufficient energy to disrupt the aberrant electrical activity and restore the heart to its normal rhythm. Energy pulses of approximately 1 J to 50 J may be used for ventricular defibrillation, whereas pulses in the range of 0.1 J to 40 J may be needed for atrial defibrillation. Pacing pulses may be delivered in the range of 0.1 to 10 Volts, with 0.1 to 2.0 millisecond pulse widths. The system may be programmed to deliver a specific amount of energy or to determine the appropriate energy level. 
       FIGS. 16A through 20  illustrate some of these applications, some configurations of the system that are suitable for each application, and shock vectors deliverable to the heart as a result of the configurations. The intravascular electrophysiological device embodiments and associated anchors and other components may be used for the described applications, although numerous alternative forms of electrophysiological devices and anchoring mechanisms may also be used without departing from the scope of the invention. 
     The applications that follow reference placement of the device in the venous system, although the device and/or electrodes may alternatively be placed within the arterial system (such as to allow generation of defibrillation vectors from the aortic arch to the descending aorta) if warranted. Moreover, while this section describes certain electrode combinations that can produce shock vectors across the heart, these combinations are given by way of example and are not intended to limit the scope of the claims. Generally speaking, the system may be implanted to include electrodes in any vessel and/or chamber of the heart arranged to distribute energy through the heart in a manner sufficient to control the aberrant electrical activity of the heart. 
     More specifically,  FIGS. 16A through 20  show electrodes in various combinations positioned in the left subclavian vein, inferior vena cava, left ventricle, right ventricle, right atrium, middle cardiac vein, and coronary sinus, however defibrillation electrodes may be positioned within other vessels, including but not limited to the pulmonary vein, hepatic vein, renal vein, axillary vein, lateral thoracic vein, internal thoracic vein, splenic vein. These locations may provide particularly good substitutes for lead placement in the right ventricle for several reasons. For example, when used in combination with electrodes in the left subclavian and in the inferior vena cava, electrodes in these alternate locations result in shock vectors that satisfactorily surround the heart. Additionally, electrode placement within a vein can be more stable, even in the absence of an anchor, than electrode placement in the heart, and so avoiding lead placement within the heart can thus simplify the implantation procedure. 
       FIGS. 16A and 16B  show components of the  FIG. 2F  system  10   d  as used as an implanted cardioverter defibrillator (ICD) for treatment of ventricular fibrillation. In this configuration, device  12   d  is anchored in the inferior vena cava  3   b  using anchor  16   d.    
     A defibrillation lead  15   d  is positioned and optionally anchored within the patient&#39;s left subclavian. An anchor  16   e  similar to the anchor  16   d  may be used for this purpose. Anchor  16   e  may be smaller than anchor  16   d  since the lead  15   d  it anchors is relatively lighter than the device anchored by anchor  16   d , and because the anchor  16   a  is positioned within the smaller-diameter left subclavian. 
     As discussed previously, the lead  15   d  may include a coiled section  166  as shown in  FIG. 16B  to permit elongation of the effective length of the lead in response to longitudinal tension. 
     Referring again to  FIG. 16A , lead  15   d  includes a high voltage electrode surface  168  through which the defibrillation pulse is delivered. During defibrillation, the defibrillation shock vector flows between electrode surface  168  on the lead and the electrode surface  170  on the device  12   d  as indicated by arrows. Orienting the electrode  170  towards the heart as shown in  FIG. 16A  contributes to focusing of the defibrillation current. Moreover, because the anchor  16   d  functions as an insulator, it helps to minimize conduction of the high voltage current away from the heart and thus also facilitates current focusing. Configuring the system to focus the current can reduce the amount of defibrillation energy needed to defibrillate the patient, since less energy is lost to surrounding tissue, and allows a smaller capacitor to be used within the system. This is beneficial in that it reduces the overall size of the device  12   d  and further ensures that the device profile will not interfere with blood flow within the vessel. 
     Although electrode surface  170  is shown positioned towards one side of the device, it may take other forms. For example, the electrode surface may instead extend around the device body to form a band. Focusing is facilitated in this embodiment by positioning the anchor  16   d  against the side of the device that is furthest from the heart (as is also done in the  FIG. 16A  application), so as to thereby minimize conduction of the high voltage current from the electrode  170  away from the heart. 
     In the  FIG. 16A  application, electrical activity of the heart may be sensed between the high voltage electrode  168  or another electrode on the lead  15   d  and the electrode  170  on device  12   d . Device  12   d  may alternatively include one or more separate sensing electrodes (not shown) on its surface for detecting electrical activity of the heart. 
       FIG. 16C  illustrates a second application for the  FIG. 2F  system. This application is similar to the first application in that it uses the device  12   d  as an implantable cardioverter defibrillator (“ICD”), but it further includes a pacing or defibrillation lead  14   d  positioned in the right ventricle  7   a . Pacing lead  14   d  may be a conventional lead, which includes one or more sensing and pacing electrodes  172 . For example, a bi-polar lead of a type commonly used for ICD&#39;s and pacemakers may be used, in which case sensing could be carried out between two spaced-apart electrodes on the lead. If a smaller lead is desired, it may be provided with a single sensing electrode and sensing could be accomplished between the single electrode and an exposed electrode (see electrode  170 ,  FIG. 16A ) on device  12   d . It should be noted that although  FIG. 16C  shows the defibrillation lead  15   d  and the sensing lead  14   d  extending from opposite ends of the device  12   d , both leads may instead extend from one end of the device. 
     For defibrillation, the  FIG. 16C  arrangement may be configured such that the shock vector applied to the heart extends from the defibrillation electrode  168  and a location on the device  12   d  as indicated by arrows. Alternatively, a high voltage lead may be used as the lead  14   d , in which case the device could also be configured to apply the shock vector between electrode  168  and the electrode  172  on lead  14   d.    
       FIG. 16D  illustrates shock vector patterns that may be delivered using the  FIG. 2C  system. As shown, shock vectors may be delivered between electrodes  168   a  on lead  15   c  in the LSV and electrodes  170   a  on device  12   c , and between LSV electrodes  168   a  and RV electrodes  172   a . Shock vectors may also be applied between the RV electrodes  172   a  and the electrodes  170   a  on device  12   c . All, or any subset, of the illustrated shock vectors may be applied simultaneously, sequentially, or in various combinations. For example, the system may be programmed to deliver energy only between the electrodes  168   a  in the left subclavian and the electrodes  172   a  in the right ventricle. Naturally, other embodiments including those of  FIGS. 11F and 12E  can be used to obtain similar shock vectors. As discussed previously, the RV electrodes  172   a  (in this as well as the other embodiments utilizing RV electrodes) may instead be positioned in the middle cardiac vein, hepatic vein, renal vein, axillary vein, lateral thoracic vein and splenic vein. Moreover, the device body  12   c  may be positioned in the superior vena cava  3   a  rather than the inferior vena cava  3   b , allowing for delivery of alternative shock vectors between its electrodes  170   a  and the electrodes  168   a ,  172   a  in the left subclavian and right ventricle, respectively. 
     A fourth application is shown in  FIG. 16E . The fourth application is largely similar to the third application, but uses a device  12   m  that is divided into two separate housings  174   a ,  174   b . For example, housing  174   a  may contain the components needed for defibrillation (e.g. the electronics, capacitors and the batteries) while housing  174   b  contains the components associated with the sensing function (e.g. the electronics and associated batteries). 
     Dividing components into separate packages may provide several advantages. First, it allows for use of an anchor having a shorter longitudinal dimension, which facilitates placement of the anchor in a location where it will not obstruct blood flow into/from peripheral vasculature. The separate packages can be anchored by a single anchor  16   d  as shown in  FIG. 16E , or the packages may be positioned in series in the vessel and separate anchors may be used for each. 
     Second, battery life may be optimized by using separate batteries for pacing and defibrillation—thereby supporting each function with a battery type most suitable for the function and optimizing battery life. For example, one or more batteries having chemistries of the type used for pacemaker batteries (typically lithium iodide batteries which can come in very small sizes due to their high energy density) may be used for the low current pacing and sensing function. Batteries having chemistries similar to typical implantable defibrillator batteries, which are faster charging and which can produce larger current surges than pacing batteries, (e.g. LSOV) may be separately used for the defibrillation function. Third, as discussed previously, physically isolating the sensing components from the defibrillation components can improve electrical sensing performance during device charging. 
     An inter-device cable  176  provides communication between the components of the housings  174   a ,  174   b , although other forms of communication (e.g. wireless RF, infrared, acoustic, telemetric) might also be used. As yet another alternative, the structural framework of the anchor  16   d  may be used as a conductor or antenna for this purpose. 
       FIG. 16F  shows device  12   a  of  FIG. 11F  as modified to include an additional array  23  of defibrillation electrodes positionable near the superior vena cava  3   a , as well as sensing electrodes  27  positioned to detect supraventricular (or atrial) tachycardia. Supraventricular tachycardia is significantly less life threatening than ventricular tachycardia, and does not require treatment using the large shocks needed to treat ventricular tachycardia. The presence of sensing electrodes  27  in the right atrium gives the system a local reading of electrical activity within the atria. This allows the system to differentiate between supraventricular tachycardia and episodes of ventricular tachycardia that will be detected using the primary sense electrodes positioned above and below the heart, such as in the inferior vena cava  3   b  and the superior vena cava  3   a  or the left subclavian vein  2   b . When abnormal electrical activity is detected by the primary sense electrodes, data from the electrodes  27  reflecting supraventricular tachycardia will trigger the system to forgo delivery of corrective shocks (or to trigger lower energy pulses to bring the heart out of the supraventricular tachycardia), thereby preserving battery life and preventing the discomfort that the patient might experience if the system were to treat the arrhythmia as a ventricular tachycardia and thus deliver a higher energy shock. In the event a ventricular tachycardia is detected, energy may be delivered along shock vectors extending between the LSV electrodes  22   a  and the RF electrodes  26   a , and/or between the RV electrodes  26   a  and the IVC electrodes  24   a , and/or the LSV electrodes  22   a  and the IVC electrodes  24   a , and/or between the SVC electrodes  23  and the RV electrodes  26   a.    
       FIG. 16G  shows an alternative configuration of a device  12   n  that is similar to the bifurcated device  12   c  of  FIG. 2C  but that is inverted such that the bifurcation is on the inferior end of the device  12   n . Shock vectors similar to those shown in  FIGS. 16D and 16F  may be attained using the device  12   n . As with prior devices, the device  12   n  may be introduced superiorly such as through the brachiocephalic vein or subclavian, or inferiorly through the inferior vena cava.  FIGS. 16H and 16I  shows additional configuration of devices designated  12   o  and  12   p , respectively, which may be used to achieve similar shock vectors. In the  FIG. 16I  embodiment, some of the electrodes are shown in contact with the septal wall. 
       FIG. 16J  shows the FIG.  12 E/ FIG. 2B  device  12   b  as modified to include a second electrode lead  21  positionable in the middle cardiac vein, and to include an array  23  of electrodes  23  positionable within the superior vena cava  3   a  or within the left subclavian vein  2   b . The electrodes  23  may be on the device  12   b  or on either of the leads  21 ,  14   b . Shock vectors include those described with respect to prior embodiments, as well as vectors extending between middle cardiac vein lead  21  and the IVC electrodes  24   b  and/or the LSV electrodes  22   b . It should be noted that the  FIG. 16J  configuration may be further modified to eliminate the RV lead  14   b.    
     In yet another alternative shown in  FIG. 16K , lead  14   b  may be positioned in the coronary sinus of the heart, thus enabling use of shock vectors extending between the electrodes  26   b  on the coronary sinus lead  14   b  and the IVC electrodes  24   b  on the device body  12   b . A second lead may be connected to the device and placed in the right ventricle  7   a  to allow application of a shock vector between the coronary sinus and the right ventricle. 
       FIG. 17A  shows an alternative application in which the  FIG. 2F  system is used for single-chamber ventricular pacing. As shown, device  12   d  is anchored by anchor  16   d  in the superior vena cava  3   a . The distal end of pacing lead  14   d  is positioned in the right ventricle  7   a . As shown in  FIG. 17B , the device  12   d  and anchor  16   d  may alternatively be positioned within the inferior vena cava  3   b . As yet another variation on this application shown in  FIG. 16C , a leadless device  12   q  having a surface pacing electrode  178  is itself positioned within the right ventricle  7   a.    
       FIG. 18A  shows an alternative application in which the  FIG. 2F  system is used to treat atrial fibrillation, with device  12   d  anchored by anchor  16   d  in the superior vena cava  3   a , and a sensing/pacing lead  14   d  having electrode  172  extending through the coronary sinus. Using this embodiment, the shock vector extends between an exposed electrode on device  12   d  and electrode  172  within the coronary sinus.  FIG. 18B  shows an alternative to the  FIG. 18A  configuration for atrial fibrillation, in which the device  12   d  may be positioned in the inferior vena cava  3   b  and a high voltage electrode lead  180  placed in the superior vena cava  3   a . Lead  180  may optionally be retained by an anchor  16   f . In this variation, the cardioversion shock vector extends between a distal electrode on pacing lead  172  and a high voltage electrode  182  on lead  180 . Other applications utilizing a coronary sinus electrode may employ shock vector patterns between the coronary sinus electrodes and electrodes in the right ventricle  7   a , inferior vena cava, middle cardiac vein, the pulmonary vein, left hepatic vein, renal vein, axillary vein, lateral thoracic vein, internal thoracic vein, and splenic vein. 
       FIGS. 19A and 19B  shows use of the  FIG. 2F  system as a dual chamber pacer having two pacing leads  14   f ,  14   g . Device  12   d  is anchored in the inferior vena cava  3   b  using anchor  16   d . The  FIG. 19A  embodiment is shown positioned for pacing the right and left ventricles. Ventricular pacing is performed by one of the pacing leads  14   f  which is positioned in the right ventricle  7   a  as shown, and atrial pacing is performed using another pacing lead  14   g  that is positioned in the right atrium  4   a  in contact with the intra-atrial septum. The  FIG. 19B  embodiment is shown positioned for bi-ventricular pacing, with each of the pacing leads  14   f ,  14   g  positioned in one of the ventricles  7   a ,  7   b . Access to the left ventricle  7   b  may be attained trans-septally with a puncture or through a naturally occurring septal defect. A similar approach may be used to access the left atrium  4   b  in other applications requiring left atrial electrodes. 
       FIG. 20  shows use of the  FIG. 2F  system for atrial pacing. In this application, an atrial J-lead  14   h  is coupled to the device  12   d  and is positioned in contact with the intra-atrial septum. 
     Alternative Applications 
     It should be pointed out that many of the device configurations, components, retention devices and methods, implantation methods and other features are equally suitable for use with other forms of intravascular implants. Such implants might include, for example, artificial pancreas implants, diagnostic implants with sensors that gather data such as properties of the patient&#39;s blood (e.g. blood glucose level) and/or devices that deliver drugs or other therapies into the blood from within a blood vessel. More particularly, fully implantable intravascular systems may be used for administering drugs including hormones, chemotherapeutic agents, pharmaceuticals, synthetic, recombinant or natural biologics, and other agents within the body. Generally speaking, the systems include drug reservoirs and associated components (e.g. batteries, electronics, motors, pumps, circuitry, telemetric components, sensors) that are anchored in the vasculature and programmed to administer drugs into the bloodstream or directly into certain organs or tissues. Drug delivery microtubules may extend from the device body and into surrounding vessels in a similar way that the leads in the embodiments described above extend from the device body. These microtubules may be positioned within the vasculature to deliver drugs directly into the bloodstream, and/or they may extend from the device through the vascular into or near a body organ. For example, by directing drugs to a particular aortic branch (e.g. hepatic artery, renal artery, etc), an intravascular delivery device can achieve target delivery of therapeutic drugs to specific organs including the brain, liver, kidneys etc. 
     In some embodiments, such intravascular drug delivery systems may be controlled remotely using telemetry or via internal intelligence that may be responsive to in-situ sensing of biological, physical or biochemical parameters. 
     Various embodiments of systems, devices and methods have been described herein. These embodiments are given only by way of example and are not intended to limit the scope of the present invention. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, implantation locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the invention.