Abstract:
A medical lead for tissue stimulation and sensing is provided, the lead having an electrode with an active fixation mechanism. The active fixation mechanism using a conductive spring element or wave washer to maintain electrical contact after extension of the active fixation mechanism. The active fixation mechanism is engaged by rotating a piston within the electrode housing which advances the fixation mechanism distally.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to conductive medical leads including fixation electrodes. More particularly, this invention is directed to aspects related to maintaining more reliable electrical conductive contact in implantable fixation lead electrodes and urging such electrodes to remain in an extended, implanted position. 
     RELATED ART 
     Implantable leads form an electrical connection between a pulse generator or other electronic device and a tissue or structure in the body. For example, leads transmit electric signals used to stimulate cardiac or nerve tissue in one direction and signals generated by sensors placed in proximity to particular organs or tissues in the opposite direction. Cardiac leads are normally passed through the veins of a patient to form an electrical connection between a pulse generator or other electronic device and the heart. 
     Medical leads typically include one or more electrodes or sensors at the distal end of the lead. The electrodes are designed to form an electrical connection with a tissue or organ. A flexible conductor electrically connects the electrode to the pulse generator. The electrodes may be passively or actively attached to a target location within the patient. Passive fixation typically employs tines to interlock with the target tissue. Active fixation frequently utilizes a screw helix or similar structure to bore into the tissue to secure the lead. To allow the introduction of the lead to occur benignly without damaging tissue, the screw helix is typically retracted and maintained within a cavity in the electrode housing during insertion. The screw helix may function solely as an anchoring device or may additionally function as an electrode or sensor. When the screw helix functions as an electrode or a sensor, a need exists to maintain the integrity of the electrical connection between the helix and the conductors within the lead body. 
     Forming a secure electrical junction between the conductors and a screw helix has proven difficult and time consuming. Because of the need to rotate the helix during implantation, a rotatable connection must be formed within the electrode. Related devices have used the contact inherent in a threaded relationship between the piston base of the screw helix and the electrode housing to provide the necessary electrical connection. Thus, the threaded relationship of related art functioned to both advance the screw helix and provide an electrical connection between the screw helix and the conductors. The threaded elements may suffer from transient loss of the electrical contact between the base and the conductors due to the play necessary to allow relative rotation. The beating of the heart may further exacerbate this problem. That is, as the heart beats, the tip electrode is subjected to frequent forces which may result in transient electrical isolation of the screw helix. In application, the transient isolation can provide gaps or spikes in sensed data and interfere with the transmission of pacing stimuli. Therefore, a need exists for a more reliable rotatable connection that provides uninterrupted contact between the electrode and the screw helix. 
     In addition, the movement of the heart, in time, can cause the extended helix anchor that penetrates the tissue to rotate and retract or withdraw into the electrical assembly. Therefore, a need also exists for an electrode assembly that reliably maintains the helix in an extended orientation after implantation. 
     The present invention meets the above-described needs and provides additional advantages and improvements that will be evident to those skilled in the art upon further review of the disclosure. 
     SUMMARY OF THE INVENTION 
     By means of the present invention, many of the problems associated with prior leads are solved by the provision of an implantable medical lead of superior electrical contact security which, in addition, inhibits unwanted withdrawal of implanted electrode anchors. The present invention provides a secure uninterrupted electrical connection between a screw helix and a conductor. 
     The medical lead of the invention includes a medical electrode connected to the distal portion of the lead for use to electrically stimulate selected body tissues or to transmit signals from a sensor to a medical device. One illustrative or detailed embodiment of the invention generally involves a hot or conductive helical anchor electrode lead or fixation electrode for a cardiac pacing lead. The device includes an electrode base attached to the distal portion of the elongated lead body. A generally hollow electrode housing which defines an internal cavity is connected to the electrode base through the proximal end of the housing. A distal portion of the internal surface of the electrode housing is provided with helical threads. A piston member having matching external threads is rotatably mounted within the housing cavity and the internal housing threads to thereby move along said housing upon rotation relative thereto. A screw helix electrode-carrying anchor including a sharp distal tine is attached to the piston so as to rotate along with it. The piston and the screw helix electrode and anchor or fixation mechanism are of electrically conducting materials and a resilient conductor member is mounted between the piston and the electrode base to electrically connect the piston with the electrode base so that a positive electrical connection exists between the electrode anchor and the electrode base even when the piston is fully extended. 
     The electrical housing also may or may not be of a conductive material, depending on the nature of the application. In the case of conductive housings, the resilient conductor member may secure continuous conduction between the piston and the housing. 
     The resilient conductor member which electrically connects the piston and the electrode base or housing, not only provides a superior uninterrupted electrical contact, but also aids in preventing retraction of the piston once the electrode/anchor is in place in the heart muscle of the patient or other location by urging the piston to remain extended. Whereas other shapes and materials are contemplated, metal compression spring clips and memory-shaped or contact washers have been found useful. 
     Thus, it can be seen that the present invention provides a secure electrical connection between a screw helix electrode and anchor and a remote conductor in the manner which also aids in maintaining the position of an implanted electrode/anchor of the screw helix class. Other embodiments may occur to those skilled in the art upon familiarization with the contents of the specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates schematically a lead employing an electrode in accordance with the present invention; 
     FIG. 2A illustrates a side view partially in section of an electrode assembly in accordance with the invention with the screw helix retracted; 
     FIG. 2B illustrates a view similar to that of FIG. 2A with the screw helix extended; 
     FIG. 3A depicts a side view partially in section of an alternate embodiment of the electrode assembly of FIG. 2A with the screw helix retracted; 
     FIG. 3B illustrates a view substantially similar to FIG. 3A with the screw helix extended; 
     FIG. 3C illustrates a perspective view of a contact washer in accordance with the embodiments shown in FIGS. 3A and 3B; 
     FIG. 4A illustrates a side view partially in section of yet another embodiment of the electrode assembly shown with the screw helix retracted; 
     FIG. 4B illustrates a view similar to that in FIG. 4A with the screw helix extended; 
     FIG. 4C illustrates a perspective view of a spring clip in accordance with the embodiments shown in FIGS. 4A and 4B; 
     FIG. 5A illustrates a stylet as used to rotate a screw helix; and 
     FIG. 5B illustrates the stylet of FIG. 5A as partially withdrawn with the screw helix retracted. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to a variety of implantable medical leads used to electrically stimulate selected body tissues or to transmit signals from a sensor to a medical device. The invention is described generally in the context of a lead body for a cardiac pacing lead for illustrative purposes only. The appended claims are not intended to be limited to any specific end use, example or embodiment described in this specification. It will be understood by those skilled in the art that the lead body of the present invention may be used in a wide variety of implantable leads including, but not limited to, neurostimulation leads, pacing leads, cardiac sensing leads, defibrillation leads, and other leads as will be recognized by those skilled in the art. Further, the numbers are repeated throughout the figures where the individual elements are substantially identical to one another. 
     FIG. 1 schematically illustrates a lead  10  suitable for use in accordance with the present invention. Lead  10  is generally configured to conduct electric current between a medical device and a patient. Lead  10  typically includes a lead body  12 , a connector  14  and at least one electrode  16 . The lead body  12  also typically includes an internal passage or lumen  18 . Lumen  18  is generally configured to allow passage of a stylet during implantation or explantation. The stylet is typically configured to engage a screw helix  20  or other fixation mechanism, as better shown and described in relation to FIGS. 5A and 5B. The stylet is configured to facilitate rotation of the helix to secure the distal end of lead  10  to a target tissue. 
     FIGS. 2A and 2B illustrate an electrode  30  in accordance with the present invention. Electrode  30  includes an electrode housing  32  which defines a hollow interior chamber  33  and is provided with internal threads illustrated at  34  over a generally distal portion of its length. A piston member  36  is provided which has external threads  38  which match the internal threads  34  of the housing  32 . A screw helix, electrode and anchor or fixation device  40  has a proximal end  42  fixed to the distal end  43  of the piston member  36  and a generally hollow electrode base  44  is provided at the proximal end of the housing. In addition, electrode  30  may further include in certain embodiments a terminal electrode device  46  at the distal end of the electrode housing  32 . Electrode housing  32  further defines a cavity  48  into which screw helix  40  is withdrawn and resides during implantation and explantation. Electrode housing  32  may be constructed of a conductive or an insulating material in this embodiment. When composed of a conductive material, electrode housing  32  may also function as part of the circuit with the piston and screw helix or may itself act to confer an electrical stimulus to the patient or may function as a conductor to conduct an electrical current to terminal electrode device  46 . When the housing composed of an insulating material, screw helix  40  functions alone to confer the electrical stimulus to and receive electrical signals from the patient. In addition, electrode base  44  is provided for electrically connecting electrode  16  to the conductors insulated within lead body  12 . Electrode base  44  may be electrically continuous with electrode housing  32  when electrode housing  32  is constructed of a conductive material. 
     In the embodiment of FIGS. 2A and 2B, a compression spring  50  is mounted between the proximal end  52  of the piston  36  and the distal end  54  of the electrode base  44 . The spring  50  is attached to both the piston  36  and the electrode base  44  at  52  and  54 , respectively, so that a continuous electrical connection is provided therebetween regardless of the position of the piston in the housing. The spring  50  may be of any conductive material, such as spring steel, suitable for making such springs and one which is compatible with the other conductive parts of the system. The force exerted by the spring  50  even as the electrode including the screw helix  40  is fully extended is sufficient to inhibit retraction of the screw helix  40  for the duration of implantation in moving tissue as, for example, the tissue of a beating heart. FIG. 2B illustrates the electrode in the fully extended position with the screw helix  40  and the compression spring  50  in their nominal fully extended positions as might occur when the device is implanted. 
     An alternate embodiment illustrated in FIGS. 3A-3C includes a conductive housing  60  with internal threads  62  adapted to receive external threads  64  of a piston member  66 , also of a conductive material. In this embodiment, a contact washer  68  maintains the electrical continuity between the electrode housing  60  (and base  44 ) and the piston  66 . The contact washer  68  may be in the form of a wave washer or other uneven washer such as will be recognized by those skilled in the art. This can be seen in FIG.  3 B. The distal end of the piston  66  is flanged as at  70  to retain the washer  68  which, when the piston  66  is fully extended, assures good contact between the proximal end flange  70  of the piston  66  and the housing  60 . As indicated, this embodiment requires the housing  60  to be of an electrically conductive material. 
     FIGS. 4A-4C depict yet another embodiment of the implantable electroded lead system of the invention in which a spring clip  80  is utilized with a piston member  82  to assure contact between the side wall of an electrode housing  84  and the piston member  82 . Note that the spring clip  80  remains stationery with respect to the piston  82  throughout its travel and remains in continuous contact with the side wall of the electrode housing  84  in the manner of a piston ring. When the piston is fully extended, as shown in FIG. 4B, the spring clip  80  is also in full contact with the raised portion of the housing threads at  86  which aids in maintaining a positive electrical connection. The piston  82  may be provided with a minor circumferential receiving cavity for receiving and maintaining the washer  80  in position, if desired. 
     FIGS. 5A and 5B further depict a stylet  90  configured to engage and rotate a screw helix  40 . The stylet  90  is engaged at its proximal end  92  by a rotating device  94  located beyond the proximal end of hollow lead  10 . The stylet  90  further includes a flattened blade area  96  which is used to engage and rotate a helical screw member accessed through the generally hollow electrode base  44 . 
     With respect to any of the described embodiments, in use, a stylet as at  90  is inserted through the lumen  18  of lead  12  until the distal end of the stylet passes through a central opening in the electrode base  44  (not shown) and is received in a conforming cavity within the distal portion of the piston  36 ,  66 ,  82  (also not shown) where the blade portion is used to rotate the screw helix in the desired direction by means of rotating device  94 . In this manner, the proximal end of the stylet is then rotated. The rotational force is communicated through the length of the stylet to confer a rotational force on the distal end of the stylet. Utilizing the embodiment of FIGS. 2A and 2B as an example, the distal end of the stylet cooperates with the receiving cavity to rotate piston  36  within the cavity  33 . The rotation of piston  36  advances screw-helix  40  past the mouth of the cavity  48 . Concurrently, spring  50  extends longitudinally maintaining the electrical contact between electrode base  44  and the piston  36 . In the case of the embodiment of FIGS. 3A-3C, the contact washer  48  is brought into contact with the piston flange  70  and inner thread ridge  72  or, in the case of the embodiment of FIGS. 4A-4C, the spring clip maintains contact between the side wall of housing  84  and the piston  82  including the raised inner screw flange  86 . 
     All these embodiments form a robust contact which assures electrical continuity between the piston and the electrode base. It will be appreciated that the wave washers as at  68  and  80  also provide and maintain a force that discourages retraction of the respective piston  66 ,  82  during use. 
     This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the example as required. However, it is to be understood that the invention can be carried out by specifically different devices and that various modifications can be accomplished without departing from the scope of the invention itself.