Patent Document

FIELD OF THE INVENTION 
   This invention relates to the field of body implantable medical device systems; and in particular, to a body implantable medical device system that includes an expandable helix and that is particularly designed for implantation into a vessel of the body. 
   BACKGROUND OF THE INVENTION 
   Modern electrical therapeutic and diagnostic devices for the heart, such as pacemakers, cardiovertors, and defibrillators, require a reliable electrical connection between the device and a region of the heart. Typically, a medical electrical lead is used for the desired electrical connection. 
   One type of commonly-used implantable lead is a transvenous lead generally taking the form of an elongated, substantially straight, flexible, insulated conductor. This type of lead is positioned through the venous system to attach to, and/or form an electrical connection with, the heart at the lead distal end. At the proximal end, the lead is typically connected to an implantable pulse generator. Because this type of lead may be placed through the venous system, electrical contact with the heart can be accomplished without requiring major thoracic surgery. 
   The specific design of a transvenous lead is generally dictated by the region of the heart in which it will be used. For example, U.S. Pat. No. 4,402,330 to Lindemans discloses a body implantable lead in which the lead body has a J-curve including a distal electrode with a permanent bend. This curve allows the lead to be readily positioned within, and connected to, the right atrium. 
   While the lead described in the &#39;330 patent has been found acceptable for pacing the right atrium, a need exists for a similar transvenous medical electrical lead adapted for use in the left atrium. Such leads have been difficult to develop for a number of reasons. For example, minor blood clots are often caused by implanted objects placed within the vascular system. Should lead implantation cause blood clots to develop within the left side of the heart or associated vasculature, the direction of blood flow could cause these clots to be carried to the brain, causing stroke and other tissue damage. Thus, at present, chronic transvenous leads may not be safely implanted within the left side of the heart. 
   Despite the difficulties with lead placement, there remains a great need to be able to electrically stimulate and/or sense the left side of the heart since it accounts for the majority of the heart&#39;s hemodynamic output. For this reason, various pathologies may be better treated through stimulation on the left side of the heart. For example, in patients with dilated cardiomyopathy, electrical stimulation of both the right and left sides of the heart has been shown to be of major importance to improve the patient&#39;s well-being and to manage heart failure. See, for example, Cazeau et al., “Four Chamber Pacing in Dilated Cardiomyopathy,” PACE, November 1994, pgs. 1974-79. See also Brecker and Fontainem, St. et al., “Effects Of Dual Chamber Pacing With Short Atrioventricular Delay In Dilated Cardiomyopathy,” Lancet November 1992 Vol. 340 p1308-1312; Xiao H B et al., “Effect Of Left Bundle Branch Block On Diastolic Function In Dilated Cardiomyopathy,” Br. Heart J 1991, 66(6) p 443-447; and Fontaine G et al, “Electrophysiology Of Pseudofunction,” C. I. Meere (ed.) Cardiac pacing, state of the art 1979, Pacesymp, 1979 Montreal. 
   At present, there are several techniques for implanting a lead to the left side of the heart. For example, a median sternotomy, an intercostals approach, or, in a more limited procedure, a sub-xiphod approach may be used to place a lead on the external surface of the heart. These procedures, however, involve major surgery, which may be painful and dangerous for the patient, as well as extremely costly. The sub-xiphod approach, moreover, only permits limited access to the anterolateral surface of the left ventricle as well as to the left atrium. 
   An alternative approach involves electrically accessing the left atrium through the coronary sinus. Many catheter designs are available to facilitate lead placement in the coronary sinus. For example, U.S. Pat. No. 5,423,772 to Lurie, et. al. discloses a coronary sinus catheter having three sections. Each section has varying degrees of flexibility, with the proximal reinforced section being stiffer than an intermediate section, and the intermediate section being stiffer than a softened tip section. The catheter includes a curve extending from the intermediate section and continuing into the softened tip section, where the radius of curvature decreases. One drawback to such a design, however, is that the particular shape of the curve is not ideally suited for electrically accessing the left atrium. In addition, such a catheter is relatively complicated to manufacture because of the braid or other means that is required to reinforce the proximal section. Finally, such a catheter does not permit introduction of a stylet to assist in the placement of the catheter into the coronary sinus. 
   Another design is disclosed in U.S. Pat. No. 6,161,029 to Spreigl, et al. This design utilizes a balloon-expandable or self-expanding stent-like electrode that is deployed within the coronary sinus to distribute the electrode surface area over a wide area and to maintain the distal lead end in place. However, this type of lead is difficult to re-position or remove, as may be necessary to improve thresholds, to increase intrinsic signal amplitudes, or to replace the lead in the case of chronic problems such as lead failure or infection. 
   Yet another lead design is discussed in U.S. Pat. No. 6,006,122 to Smits. The disclosed lead utilizes a bent fixation ring positioned adjacent to a distal coronary sinus electrode. The ring, which is formed of a pliable material, is adapted to wedge or fix the lead within the coronary sinus in such a manner that the electrode is pushed against the vessel wall without impeding blood flow through the vessel. One of the drawbacks of this particular design is the inability to re-position and/or remove the electrode as may be required for any of the reasons discussed above. 
   U.S. Pat. No. 5,129,394 to Mehra describes a method and apparatus for sensing in vivo blood pressure proportional to the left ventricular pressure for detecting ventricular tachyarrhythmias or the cardiovascular status in congestive heart failure, and/or for adjusting the rate of a pacemaker. A lead with a pressure sensor near its distal end is placed transvenously through the coronary sinus and located in the coronary vein. 
   When in place, an inflatable balloon proximal to the pressure sensor may be used to acutely occlude the coronary vein until the sensor position is stabilized by the growth of fibrous tissue. According to this mechanism, the sensor may not be used for approximately six weeks until fibrous tissue has formed. After that, the lead may not be easily re-positioned or removed. 
   Other types of lead systems employ a shape memory-metal or other super elastic material designed to make the leads easier to deploy and affix. For example, U.S. Pat. No. 4,913,147 to Fahlstrom, et. al. describes a lead including one or more components formed of a shape-memory metal. These components are designed to have a first shape when at body temperature, and a second shape when at a different predetermined temperature. Such a component may be disposed at the distal lead end to assist in providing a reliable mechanical and electrical connection to the heart when the component changes shape. For example, this type of component may be disposed in proximity to the electrode to assume a first shape permitting easy introduction of the lead through a vein, and a second shape such as a curve that is adapted to maintain the electrode at a predetermined position within the heart or vascular system. In one disclosed embodiment of the device, the lead includes an extendable, non-expanding helix that remains smaller than the inner diameter of the lead lumen following deployment. 
   U.S. Pat. No. 5,522,876 to Rusink describes a lead for use with a pacemaker in a pacing system, the lead having at least one electrode and a helical fixation member at the lead distal tip. The helical member, which is adapted to be affixed to heart tissue, is composed of shape-memory metal. The helix is encapsulated within a mannitol or other dissolvable member in a shrunken state so that the helix diameter is less than the diameter of the lead casing. When the dissolvable member is dissolved by body fluids, the helix is released to assume an expanded diameter that is greater than the electrode diameter. When the helix is embedded into the heart wall, the helical coils are displaced radially away from the outer edge of the tip electrode so that the damage to the heart tissue immediately proximate to the tip electrode is minimized. One disadvantage of this system is that the helix is not retractable once it is deployed. Moreover, the design is adapted for use in the right ventricular or right atrial cardiac wall. 
   What is needed, therefore, is an improved lead adapted for use in the coronary sinus, middle and great cardiac veins, or another vessel that is both easy to deploy, and that may be readily removed and/or re-positioned. 
   It is thus an object of the present invention to provide a medical electrical lead that is suitably shaped to provide an electrical connection through the coronary sinus to one or both of the left chambers of the heart. 
   A still further object of the present invention is to provide a medical electrical lead having an electrode positioned so that when the lead is implanted into the coronary sinus, the electrode is positioned against the coronary sinus wall. 
   A still further object of the present invention is to provide a medical lead having a fixation method that may be extended and retracted to allow positioning and re-positioning of the lead. 
   A still further object of the present invention is to provide a medical lead having a fixation helix constructed of shape memory metal or other super elastic material that, upon extension, increases in diameter to the vessel wall, securing the lead in position. 
   A still further object of the present invention is to provide a medical electrical lead having an electrode that may be positioned along a selected portion of the coronary sinus wall in a manner that minimizes the restriction of blood flow through the coronary sinus. 
   SUMMARY OF THE INVENTION 
   These and other objects are accomplished through the present invention. In one embodiment, the present invention comprises an implantable medical device (IMD) such as a transvenous lead or catheter specifically designed for implantation within the body. While the inventive IMD and associated method of positioning the IMD may be used within a chamber of the heart such as the right ventricle, the invention is particularly suited for use in any body vessel, including the coronary sinus and cardiac great vein. The lead of the present invention includes an expandable fixation member such as a helix that may function as a pacing/sensing or a defibrillation electrode. The fixation member may be constructed of a shape memory metal or other super elastic material, and functions to wedge or fix the device within the coronary sinus or other vessel so that the fixation member is pushed against the vessel wall. The fixation member includes a central lumen so that the flow of blood through the vessel is not impeded. In alternative embodiments, the fixation member may be used for positioning only, as with defibrillation electrodes and/or sensors that are carried on a lead body. Alternatively, the current expandable fixation member may be usefully employed with any elongated implantable medical device, including catheters. 
   In one embodiment, the fixation member may be advanced using a stiffening member such as a stylet. The stiffening member is rotated to impart rotation to a helix in a manner that advances or retracts the helix. In another embodiment, the helix is coupled at a proximal end to a coiled conductor. Rotation of a proximal end of the conductor serves to extend or retract the helix. According to yet another aspect of the invention, a helix lumen including a flexible fluid-tight seal may be utilized to house the helix when it is in the retracted position. Additional aspects of the current invention will become apparent from the detailed description of the invention and the drawings. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a perspective view of a lead according to the present invention implanted in a heart. 
       FIG. 2  is a plan view of the lead of  FIG. 1 , and further illustrates an implanting stylet assembly. 
       FIG. 3  is a cutaway side view of the inventive lead illustrating the helix in a deployed position. 
       FIG. 4  is a cross-sectional view of the lead of  FIG. 1  illustrating an alternative embodiment of a helix deployment assembly. 
       FIG. 5  is a plan view illustrating the lead of  FIG. 1  located within a vessel when the helix is extended and expanded. 
       FIG. 6  is a plan view of a lead deployed within a vessel, wherein the helix electrode tapers from a larger diameter to a smaller diameter moving distally away from the lead. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a perspective view of lead  10  according to the present invention, which is shown implanted in a heart  4 . As seen in this embodiment, an implantable pulse generator  2  is coupled to a lead  10  by connector  5  as is well known in the art. Pulse generator  2  may be any model capable of sensing and stimulating two or more chambers of the heart  4  with at least one situated on the left side of heart  4 . As further seen, in this embodiment, lead  10  may feature include one or more ring electrodes disposed along the lead body. In the illustrated embodiment, two ring electrodes  7  (right atrium) and  9  (left atrium) are shown. This embodiment further includes a helix  28  which extends beyond the distal end of the lead  10 . Upon deployment, the helix expands and wedges within the coronary sinus, cardiac veins, or another vessel. This ensures excellent contact is maintained between the electrode  28  and the heart while also maintaining the lead  10  at the predetermined site of implant. The present invention permits the lead  10  to reliably pace and/or sense the right atrium, the left atrium, the left ventricle or any combination thereof by the electrodes  7 ,  9 , and  28  respectively. 
     FIG. 2  is a plan view of one embodiment of the inventive lead. The bipolar lead  10  of this embodiment is stylet-activated, and includes an active fixation mechanism. Lead  10  further includes a flexible, elongate lead body  12  covered by an insulative sleeve, such as polyurethane or silicone rubber. Terminal assembly  14  is provided at the proximal end for coupling lead  10  to an implantable pulse generator  2  (FIG.  1 ). Terminal assembly  14  has sealing rings  16  and terminal pin  18 , all of a type known in the art. An anchoring sleeve  20  (shown partially in cross-section) may also be provided for suturing lead body  12  to body tissue. Anchoring sleeve  20  and terminal assembly  14  are preferably fabricated from silicone rubber, although they may also be constructed of any other suitable biocompatible material known in the art. 
   The lead  10  of  FIG. 2  is further shown to include stylet guide  22  and stylet assembly  24  coupled to terminal pin  18 . The stylet assembly  24  imparts stiffness to lead  10  during placement. The stylet further actuates fixation helix  28  in a manner described below. Stylet guide  22  and stylet assembly  24  are typically discarded after use and before connection of terminal pin  18  to pulse generator  2  (FIG.  1 ). Other types of stiffening members as known in the art may be used in the alternative for this purpose. 
   With continued reference to  FIG. 2 , an electrode and fixation assembly designated generally as  26  is disposed at the distal end of lead body  12 . In the illustrated embodiment, lead  10  is of the multi-polar, single pass configuration as may be adapted for placement in the coronary sinus or another vessel. The assembly  26  includes a distal helix electrode  28 , and a ring electrode  9  positioned proximal to the distal end. As will be appreciated by those of ordinary skill in the art, helix electrode  28  and ring electrode  9  are coupled to separate, insulated lead conductors (not shown in  FIG. 2 ) that extend substantially the length of lead body  12 . Lead conductors are preferably configured as concentric multi-filar coils of a platinum-iridium alloy or any other suitable alloy, such as MP35N. This configuration allows for a longitudinal lumen that extends along the length of lead body  12  and that is adapted to receive stylet assembly  24 . The lead may include one or more additional electrodes such as right atrium electrode  7  (FIG.  1 ). 
     FIG. 3  illustrates a cutaway side view of the fixation assembly  26  of lead  10  with helix  28  deployed. In this embodiment, fixation assembly  26  includes a stylet socket  62  that is slidably disposed in a piston-like fashion within the cylindrical interior of helix sleeve  46 . Stylet socket is preferably made of hard plastic, which is molded to retain helix  28 , which extends axially outward from stylet socket  62 . Stylet socket  62  includes a coupling member such as axially-oriented slot  66  formed therein, which may include a flared opening  68  corresponding to screwdriver tip  25  of stylet  24 . 
   The embodiment of  FIG. 3  further includes a conductor coil  42  which defines a lumen for stylet  24 . Conductor  42  terminates at helix sleeve  46 . In one embodiment, helix sleeve  46  is made of machined polysulfone, and is provided with a rectangular slot  48  which allows the diameter of the coil defined by inner conductor  42  to increase at a “dog-leg” point designated generally as  90 . Distally from “dog-leg” point  90 , inner conductor  42  coils around the outer surface of helix sleeve  46  toward the distal end of helix sleeve  46 , and is electrically coupled, e.g., by spot or laser welding, to helix  28 . 
   Fixation assembly  26  may be retained within helix sleeve  46  by means of a substantially cylindrical helix seal  70  which may be formed of molded silicone rubber. Helix seal  70  is provided with a helical lumen or channel  72  extending from front to back, through which helix  28  is able to pass. Helical lumen  72  in helix seal  70  may be initially sealed at the distal end  76 . When helix  28  advanced into helix seal  70  from the back, helix seal  70  guides helix  28  forward, such that the pointed tip of helix  28  pierces point  76  of lumen  72 . When helix  28  is screwed back out, the resiliency of silicone rubber seal  70  is such that lumen  72  effectively seals itself. This self-sealing arrangement is believed to be advantageous in that it prevents body fluids from entering electrode and fixation assembly  26 . 
   Helix  28  may be advanced and retracted using an embodiment of stylet  24  having a flattened tip  25 . Rotation of the stylet imparts rotation to the helix assembly, causing the straightened helix to advance or retract. Upon exiting the distal end of the lead, the helix expands into a helical shape that makes solid contact with the vessel wall. The embodiment of fixation assembly and stylet illustrated in  FIG. 3  are substantially described in U.S. Pat. Nos. 5,522,874 and 5,522,875 respectively to Gates, and which are hereby incorporated by reference in their entireties. 
   Transvenous implantation of lead  10  may be accomplished using conventional lead introduction techniques. During the implantation procedure, stylet  24  is used to provide stiffness to lead body  12 , facilitating manipulation of lead  10  through the patient&#39;s venous system. Helix  28  is maintained in its most retracted position until the distal end of lead  10  including fixation assembly  26  is brought into contact with the desired coronary sinus, great vein, or other stimulation site. Since helix  28  is contained completely within the electrode and fixation assembly  26 , it is prevented from damaging tissue as lead  10  is advanced through the venous system. 
   Once the desired electrode positioning is achieved, stylet  24  is rotated in the appropriate direction to cause helix  28  to advance through helical channel  72  within helix seal  70 , eventually piercing a sealed portion at the distal end of helical channel  72  in the manner discussed above. Continued rotation of helix  28  will cause further advancement, so that helix  28  extends to its full diameter, engaging the endocardial vessel wall. In this way, electrode and fixation assembly  26 , and in particular, helix electrode  28 , is secured in contact with the desired stimulation site. To later remove or re-locate the lead, helix  28  can be released without significantly damaging the cardiac tissue by rotating stylet  24  in the opposite direction. Helix  28  re-assumes a compressed configuration when retracted within the lead inner lumen. 
     FIG. 4  is a cross-sectional view illustrating an alternative embodiment of the inventive lead. In this embodiment, fixation helix  28  is coupled to coiled conductor  100 . The helix  28  is advanced or retracted out of a chamber or inner recess  102  in the distal end of the lead body  106  by rotation of the coiled conductor  100 . This type of coil mechanism is disclosed in commonly assigned U.S. Pat. No. 4,106,512 to Bisping with improvements thereto disclosed in commonly assigned U.S. Pat. No. 4,311,153 to Smits, in U.S. Pat. No. 4,886,074 to Bisping, and in U.S. Pat. No. 5,837,006 to Ocel, all hereby incorporated by reference in their entireties. In these “Bisping” leads and the commercial embodiments thereof, rotation of the proximal end of a fixed pin  104  or other helix guide structure is utilized to impart rotational motion to the proximal end of coiled conductor  100 . This rotation is, in turn, translated into axial advancement and retraction of the helix out of, and into, distal chamber  102 . Upon exiting distal chamber  102  of lead  10 , helix  28  expands into a helical shape wherein the helix is forced against a vessel wall such as the wall of the coronary sinus. 
   Yet another embodiment of a helix as may be employed with the current invention is shown in commonly-assigned U.S. Pat. No. 4,570,642 to Kane et al. hereby incorporated by reference in its entirety. According to this embodiment, the helix is fixed on a member slidably located within a chamber of the distal end of the pacing lead. A cylindrical stylet is employed to distally advance the slidable member within the chamber. This, in turn, exposes the fixation helix so that it may be screwed into the myocardium by rotation of the entire lead body. 
     FIG. 5  is a plan view of fixation assembly  26  at distal end  12  of lead  10  implanted in a vessel such as the coronary sinus. As shown, helix  28  is sized to wedge against the coronary sinus walls  6  when deployed. The helix defines a lumen that permits the unimpeded flow of blood, as depicted by arrow  8 . This uninterrupted blood flow prevents the formation of thrombosis and possible necrosis of the tissue, which may, in turn, cause stagnation in the vessel such that the health of the surrounding tissue is negatively impacted. 
     FIG. 6  is a plan view of yet another embodiment of helix  28  wherein the helix has a decreasing diameter. This embodiment may be adapted for use in a portion of a vessel wherein the vessel is decreasing in size, for example. In the alternative, a helix having an increasing diameter towards the helix distal end may be utilized. Any other type of size variations along the length of the helix may likewise be utilized, such that only a portion of the helix diameter exceeds the diameter of the lead body. 
   Although the lead of  FIG. 1  is shown as a pacing lead, one skilled in the art will appreciate that the current invention may be employed with many other embodiments of implantable leads, catheters, or other elongated implantable medical devices (IMDs) that are to be maintained chronically at a desired implant site. For example, a defibrillation lead connected to a defibrillator  2 , as substantially described in U.S. Pat. No. 5,549,642 to Min, et al. and incorporated herein by reference in its entirety, may usefully employ the current invention. Alternatively, a helix formed of a non-conductive or conductive material may be employed to affix the distal end of a drug-delivery lead within the coronary sinus or another vessel. In yet another invention, the distal end of the fixation helix may be utilized to attach the associated IMD to tissue within a cardiac chamber such as the right ventricle. Many other alternative embodiments are also contemplated within the scope of the current invention. 
   In yet another embodiment of the current lead system, the lead body carries a sensing device such as sensor  110  ( FIG. 5 ) to be placed in the coronary sinus or great vein. This is as substantially described in U.S. Pat. No. 5,129,394 to Mehra and incorporated herein by reference in its entirety. According to the described embodiment, a method and apparatus is provided for sensing in vivo blood pressure proportional to the left ventricular pressure. The measured pressure can be used to detect ventricular tachyarrhythmias or the cardio vascular status associated with congestive heart failure. Pressure measurements can also be used to adjust the rate at which pacing pulses are delivered. A lead with a pressure sensor near its distal end is placed transvenously through the coronary sinus and located in the coronary vein or great vein. The pressure that is sensed in that location is proportional to the left ventricular pressure. Values representing the left ventricular pulse, systolic and diastolic pressures, as well as the differentiated rate of change, dP/dt, gross rate of change, ΔP/Δt, and mean or average pressure values are all, or selectively, developed by software algorithms and implemented in microprocessor-based control circuitry. In one preferred embodiment, one or more of the values are utilized in software-implemented algorithms to cause a pacemaker to pace the heart at a required rate to achieve a desired cardiac output. Alternatively, these left ventricular pressure-related values may be employed to confirm the absence of mechanical pumping action of the heart, which, in conjunction with other cardiac signals, confirm the existence of a tachyarrhythmia requiring anti-tachy pacing, cardioversion or defibrillation. 
   Many other types of sensors may be carried on a lead configured according to the current invention. Such sensors may measure O 2  saturation, temperature, flow, impedance, stroke volume, pH, and/or any of the other types of physiologic measurements known in the art. These sensors may be deployed, positioned and firmly anchored in the coronary sinus, middle and/or great cardiac vein, or another vessel using the current invention. 
   Although a specific embodiment of the invention has been disclosed, this is done for purposes of illustration only, and is not intended to be limiting with regards to the scope of the invention. It is contemplated various substitutions, alterations and/or modifications may be made to the disclosed embodiment without departing from the spirit and scope of the invention. Such modifications may include substituting elements or components that perform substantially the same function in substantially the same way to achieve substantially the same result as those described herein.

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