Abstract:
An electrical lead equipped with cathode and anode active succession electrodes for positioning in the vicinity of the His bundle tissue. The lead includes a lead body for carrying conductors coupled between electrodes located at or near the distal lead end and a connector assembly located at the proximal lead end for connecting to an implantable pacemaker. The electrode is shaped, at the distal end, for positioning and attachment in the His bundle and branches thereof, cathode and anode electrodes co-extensive with the lead body. The cathode and anode electrodes may be helical screw-in type or equivalent electrodes adapted for secure fixation deep within the His bundle tissue or the tissue in the vicinity of the His bundle.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to medical electrical leads and particularly to a cardiac pacing lead for stimulating the His bundle.  
           [0002]    BACKGROUND OF THE INVENTION  
           [0003]    The frequency, pathway of conduction and rate of propagation of action potentials through the heart, which cause the heart to beat in an efficient manner, are controlled by specialized groups of cardiac cells which form the cardiac conduction system. This special conduction system includes the sinoatrial node (SA node), the atrial internodal tracts, the atrioventricular node (AV node), the His bundle, and the right and left bundle branches.  
           [0004]    The SA node, located at the junction of the superior vena cava and right atrium, normally acts as the natural pacemaker, generating action potentials, which are conducted through the rest of the heart. When normal conduction pathways are intact, an action potential generated in the SA node is conducted through the atria and to the AV node via the atrial internodal tracts. The conduction through the AV nodal tissue is takes longer than through the atrial tissue, resulting in a delay between atrial contraction and the start of ventricular contraction.  
           [0005]    The AV node, located in the central fibrous body, conducts the action potential to the His bundle, located under the annulus of the tricuspid valve. The His bundle splits into the left and right bundle branches, which are formed of specialized fibers called “Purkinje fibers.” The Purkinje fibers rapidly conduct the action potential down the ventricular septum, spreading the depolarization wavefront quickly through the remaining ventricular myocardium, producing a coordinated contraction of the ventricular muscle mass.  
           [0006]    Conduction abnormalities may cause slowed or disrupted conduction anywhere along this conduction pathway. For example, the SA node may not generate action potentials at a fast enough rate resulting in too slow of heart rate, or bradycardia. AV block may prevent conduction of the action potential from the atria to the ventricles. These and other conduction abnormalities may be treated by an external or implantable pacemaker.  
           [0007]    Pacemakers are typically coupled to the heart via one or more leads, carrying one or more electrodes for stimulating the heart and for sensing the intrinsic electrical signals associated with a conducted action potential.  
           [0008]    Electrodes are commonly placed on the endocardial surface using a transvenous approach. For example a right ventricular lead may be advanced into the right ventricle and placed such that an electrode is positioned at or near the right ventricular apex. Low capture thresholds and stable lead positioning have made the right ventricular apex a preferred ventricular stimulation site.  
           [0009]    However, ventricular pacing at the location of the right ventricular apex does not mimic the normal ventricular conduction pathway. Both experimental and clinical studies have shown that septal pacing can improve various indices of cardiac function compared to apical pacing. See, for example, Kolettis T M, et al., Chest, 2000;117:60-64, Rosenquvist M., et al, PACE 1996;19:1279-86, and Takagi Y, et al., PACE 1999;22:1777-81. Direct myocardial stimulation can cause remodeling of the ventricular myocardium, including myofibrilar disarray and local hypertrophy away from the electrode. See, for example, Karpawich P P et al., PACE. 1999;22( 0 ): 1372 - 7 .  
           [0010]    The most normal physiological approach to pacing the ventricles when normal AV nodal conduction fails may be to deliver electrical stimulation pulses directly to the His bundle. Depolarization of the His bundle tissue may be conducted normally through the ventricular conduction pathway, down the left and right bundle branches and to the remainder of the ventricular myocardium. The resulting ventricular contraction, which is more rapid, resulting in a narrow QRS complex and a more vigorous, normal contraction, may produce a better-coordinated contraction for achieving efficient heart pumping action.  
           [0011]    The His bundle, however, is surrounded by non-excitable tissue, normally resulting in unacceptably high thresholds for depolarizing, or “capturing” the His bundle tissue. The current field produced around a conventional unipolar or biopolar stimulation electrode may not effectively reach the relatively deep His bundle tissue or penetrate the surrounding non-excitable tissue. Increasing the stimulation energy in an attempt to capture the His bundle can result in capturing other surrounding, excitable tissue. Two leads could be placed adjacent each other near the His bundle so that the tip electrodes could be used as a bipolar pacing pair. However, placement of two leads would require longer surgical time, may not allow for controlled inter-electrode spacing and would increase the number of lead bodies running through a patient&#39;s veins. A medical lead is needed, therefore, for delivering stimulation pulses for effectively capturing the His bundle and thereby recruiting the normal ventricular conduction pathway for more physiological contraction dynamics and potentially improved hemodynamics during cardiac pacing.  
           [0012]    SUMMARY OF THE INVENTION  
           [0013]    The present invention is directed at providing a cardiac pacing lead designed for effectively stimulating the His bundle. The present invention is realized in an electrical lead equipped with cathode and anode active fixation electrodes that may be positioned in or near the His bundle tissue. The lead includes a lead body, which may be a multi-lumen lead body, for carrying conductors coupled between electrodes located at or near the distal lead end and a connector assembly located at the proximal lead end for connecting to an implantable pacemaker.  
           [0014]    In one embodiment, the distal lead end is bifurcated such that cathode and anode electrodes are located on separate branches of the lead. The cathode and anode electrodes may be helical, “screw-in” type, electrodes such that they may be fixed deep within the His bundle tissue. The electrodes may be positioned at a desired interelectrode distance by separating the two branches. In one embodiment the branches are separated and positioned by first deploying the lead with a guide catheter and positioning the electrode on a first branch, retracting the guide catheter so that the branches may be separated, then advancing a stylet into the lumen of the second branch and positioning the second electrode at a desired location relative to the first electrode. In another embodiment, the desired interelectrode distance may be achieved by pre-forming the lead branches.  
           [0015]    A His bundle pacing lead may also be equipped with sensing ring electrodes located on one or both of the branches carrying the pacing anode and cathode electrodes. Alternatively, one or more sensing ring electrodes may be located on the main lead body, proximal to the bifurcation of the lead.  
           [0016]    In another embodiment, the cathode and anode electrodes for His bundle stimulation are positioned along a single lead body that is preformed in a generally “S” shape. The generally “S” shape allows each electrode to be positioned against the annulus of the tricuspid valve, adjacent the His bundle and then fixed in or near the His bundle tissue.  
           [0017]    In yet another embodiment, a His bundle pacing lead may be constructed by assembling two small diameter leads, each having an active fixation tip electrode, and then inserting the two leads into the lumen of larger diameter tubing. The larger diameter tubing has a length that is somewhat shorter than the two small diameter leads such that the two small-diameter leads extend from the distal end of the tubing. The portion of the two small diameter leads extending from the large-diameter tubing may then be separated such that the electrodes located at the distal end of each lead may be positioned at a desired location a given distance from each other. At the proximal end, the connector assemblies of each small diameter lead extend from the larger diameter tubing so that one lead may be connected to a cathode terminal and the second lead may be connected to an anode terminal of a pacemaker.  
           [0018]    A His bundle pacing lead and method for deploying such a lead, as provided by the present invention, is expected to provide a more natural conduction and synchronized contraction response during cardiac pacing that will be more beneficial to the patient than pacing at other cardiac sites. Direct myocardial stimulation, which can cause remodeling of the ventricular myocardium, including myofibrilar disarray and local hypertrophy away from the electrode, is avoided. Furthermore, aspects of the present invention allow His bundle pacing to be achieved at lower pacing energies by stimulating deep within tissue containing the His bundle. Optimal cathode and anode placement for His bundle pacing creates a current field that is spread within the excitable tissue of the His bundle rather than through the non-excitable tissue layer surrounding the His bundle. The optimal cathode and anode placement achieved using aspects of the present invention, therefore, is expected to efficiently depolarize the His bundle tissue resulting in a more favorable myocardial conduction and contraction pattern.  
           [0019]    BRIEF DESCRIPTION OF THE DRAWINGS  
           [0020]    [0020]FIG. 1 is a plan view of one embodiment of a lead intended for stimulating the His bundle.  
           [0021]    [0021]FIG. 2A is a cross-sectional view of the main lead body included in the lead of FIG. 1.  
           [0022]    [0022]FIGS. 2B and 2C are alternate embodiments of FIG. 2A.  
           [0023]    [0023]FIG. 3 is a side cut-away view of the distal branches of the lead shown in FIG. 1.  
           [0024]    [0024]FIG. 4 is a partially cut-away view of the human heart illustrating the placement of the lead of FIG. 1 when it is used for stimulating the His bundle.  
           [0025]    [0025]FIG. 5A is an illustration of the current field produced during bipolar stimulation using a conventional bipolar lead.  
           [0026]    [0026]FIG. 5B is an illustration of the current field produced during bipolar stimulation using the branched lead shown in FIG. 1, in accordance with the present invention.  
           [0027]    [0027]FIG. 6 is a plan view of the distal end of an alternative embodiment of a His bundle lead in accordance with the present invention.  
           [0028]    [0028]FIG. 7 is a side, cut-away view of the distal end of the lead of FIG. 6.  
           [0029]    [0029]FIG. 8A is a plan view of an alternative embodiment of a branched lead in accordance with the present invention.  
           [0030]    [0030]FIG. 8B is an end view of the lead shown in FIG. 8A.  
           [0031]    [0031]FIG. 9 is an alternative embodiment of a His bundle lead wherein the distal end of the lead is provided in a generally “S” shaped configuration.  
           [0032]    [0032]FIG. 10 is a side, cut-away view of the distal end of the lead of FIG. 9.  
           [0033]    [0033]FIGS. 11A and 11B are plan views illustrating a method for implanting the lead of FIG. 9 using a stylet.  
           [0034]    [0034]FIG. 12 is yet another embodiment of a branched lead according to the present invention wherein the branched lead is constructed by enclosing two, small diameter leads in a larger diameter tube.  
           [0035]    [0035]FIG. 13 is an illustration of the lead of FIG. 12 as it may be implanted for stimulating the His bundle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    [0036]FIG. 1 is a plan view of one embodiment of a lead intended for stimulating the His bundle. Lead  10  is provided with a lead body  22  for carrying at least two conductors between electrodes, located at or near the distal end of lead  10 , and corresponding connectors on a connector assembly  30  at the proximal end of the lead. Lead  10  is shown having a distal bifurcation  20  and two distal branches  14  and  18 . Each branch  14  and  18  is provided with an active fixation member, shown as helices  12  and  16 , that may be inserted into the central fibrous body in the vicinity of the His bundle. Helices  12  and  16  preferably serve as tip electrodes for use as a bipolar pacing electrode pair. Helices  12  and  16  may alternatively be provided as other types of active fixation electrodes, such as a barb or hook type electrode. Ring electrodes may optionally be provided for use in sensing cardiac signals and may be located on the main lead body  22  and/or branches  14  and  16 , spaced proximally from helices  12  and  16 . In FIG. 1, a ring electrode  24  is shown located on branch  14  and a ring electrode  26  is shown located on the main lead body  22 .  
         [0037]    Branch  14  extends at an angle  62  with respect to the main lead body central axis, indicated by dashed line  60 . Branch  18  extends at an angle  64  with respect to axis  60 . Angles  62  and  64  are shown to be approximately equal and each approximately 30 degrees, resulting in approximately a 60 degree angle between branches  14  and  18 . Angles  62  and  64 , however, may be any, preferably acute, angle which provides an appropriate distance between helices  12  and  16  for securing into His bundle stimulation sites. A preferable interelectrode distance for optimal His bundle stimulation is expected to be on the order of 4 to 10 mm. Branches  14  and  18  may also be curved so that helices  12  and  16  are presented to the tissue surface at a right angle rather than an oblique angle. Branches  14  and  18  may alternatively emerge from the main lead body  22  in parallel. The main lead body  22  and branches  14  and  18  are preferably formed from a resilient, biocompatible polymer such as polyurethane or silicone rubber. Bifurcation  20  and branches  14  and  18  may be preformed in a desired shape during molding processes.  
         [0038]    A bifurcated proximal connector assembly  30  located at the proximal end of lead  10  is provided with a connector associated with each electrode. Connector assembly branch  31  includes a pin connector  32  corresponding to helix  12 , and a ring connector  34  corresponding to ring electrode  24 . Connector assembly branch  35  includes a pin connector  36  corresponding to helix  16  and a ring connector  28  corresponding to ring electrode  26 . Stylets  40  and  41  are shown exiting pin connectors  32  and  36 , respectively, and may be used to position the distal branches  14  and  18  at desired implant locations.  
         [0039]    After implanting lead  10 , connector assembly  30  may be connected to an implantable pacemaker having at least two connector ports. One pin connector may be connected to an anode terminal in the pacemaker connector block and the other pin connector may be connected to a cathode terminal in the pacemaker connector block such that the two helices serve as the anode-cathode pair during pacing. The ring connectors  34  and  38  may be coupled to ring terminals such that the ring electrodes  24  and  26  may be used, in combination with each other, one or both helices  12  and  16  or the pacemaker housing, for sensing cardiac signals or evoked responses.  
         [0040]    [0040]FIG. 2A is a cross-sectional view of the main lead body  22  of lead  10  in FIG. 1. Lead body  22  may be provided with multiple lumens for carrying multiple conductors between electrodes located at the distal lead end to the corresponding connectors at the proximal lead end. Lead body  22  may be provided as a multilumen lead body as generally described in U.S. Pat. No. 5,584,873 issued to Shoberg et al., incorporated herein by reference in its entirety. In FIG. 2, lead body  22  is shown having four lumens  42 ,  44 ,  46  and  48 . Lumens  42  and  44  are provided for carrying separate conductors to helices  12  and  16 . Lumens  46  and  48  are provided for carrying separate conductors to ring electrodes  24  and  26 . Alternatively, the lead body  22  may be provided with two lumens for carrying conductors arranged concentrically, as disclosed in U.S. Pat. No. 4,355,646 issued to Kallok et al., also incorporated herein by reference in its entirety.  
         [0041]    A multi-lumen lead body is shown in FIG. 2 to demonstrate the possibility of including one or more ring electrodes or other sensors on lead  10 , in addition to helices  12  and  16 . In a preferred embodiment including only helices  12  and  16 , a bitumen lead body could be provided for carrying two conductors, one to each helix. If an optional ring electrode is included, two concentrically-arranged conductors could be provided extending through one lumen to one helix and the optional ring electrode and a third conductor may extend through a second lumen to the second helix.  
         [0042]    [0042]FIG. 2B depicts an alternate configuration of lumens  42 ,  44  encased in a common lumen and set in adjacent placement therein. Similarly, conductor carrying lumens  48 ,  46  are encased in a common lumen.  
         [0043]    [0043]FIG. 2C is yet another alternate embodiment in which lumen  42  and conductor lumen  46 , and lumen  44  and conductor lumen  48  are encased in a common lumen, respectively.  
         [0044]    [0044]FIG. 3 is a side cut-away view of the distal branches of the lead  10  shown in FIG. 1. Helices  12  and  16  are preferably rotatable relative to lead body branches  14  and  18 . A conductor  60 , which is preferably a coiled conductor, extends through lumen  44  of main lead body  22  and continues into branch  14 . Conductor  62  extends through lumen  42  of main lead body  22  and continues into branch  16 .  
         [0045]    Helix  12  is mounted on a drive shaft  50 . Drive shaft  50  is provided with an electrically conductive stem  52 . Conductor  60  is electrically and mechanically coupled to stem  52  at its distal end and to pin connector  32  (shown in FIG. 1) at its proximal end. Pin connector  32  may be rotated with respect to connector assembly  30  to cause rotation of coiled conductor  52  with respect to lead body  22 . Rotation of coiled conductor  62  will cause advancement of helix  12  on guide  58 . Similarly, helix  16 , mounted on drive shaft  54  is advanced on guide  59  by rotating coiled conductor  62 , which is coupled to drive shaft  54  via stem  56 . A rotatable helix may be provided as generally described in U.S. Pat. No. 4,106,512 issued to Bisping, U.S. Pat. No. 4,886,074 to Bisping, and U.S. Pat. No. 5,837,006 to Ocel et al., all of which patents are incorporated herein by reference in their entirety.  
         [0046]    Ring electrode  24  is coupled to a conductor  45  extending through lumen  46 . Ring electrode  26  is coupled to a conductor  49  extending through lumen  48 . Conductors  45  and  49  may be provided as cabled or stranded conductors. An example of a stranded conductor that may be used in the present invention is disclosed in U.S. Pat. No. 5,246,014 issued to Williams, et al., incorporated herein by reference in its entirety. Ring electrodes  24  and  26  may be used for sensing intrinsic cardiac signals or evoked responses for verifying capture following delivery of a pacing pulse.  
         [0047]    Both helices  12  and  16  are shown in a partially extended position in FIG. 3. During implantation, both helices  12  and  16  are initially in a fully retracted position to prevent snagging or catching on anatomical structures as lead  10  is advanced through a venous pathway. Lead  10  may be advanced with the aid of a guide catheter or stylet. Once positioned at the final implant site, a first helix, for example helix  12 , is advanced into a tissue site. The second branch  18  is then spaced a desired distance from the first branch  14  and the second helix  18  is advanced into a second tissue site.  
         [0048]    In an alternative embodiment, a first helix may be fixedly attached at the distal end of a first lead branch, and a second helix may be rotatable with respect to a second lead branch, as will be further described below. Lead  10  may then be provided with an in-line connector assembly rather than a bifurcated connector assembly as shown in FIG. 1. The first helix, coupled via a conductor to a connector ring on an in-line connector assembly, may be screwed into a tissue site by rotating the main lead body at the proximal end while the second helix remains retracted within the distal branch. The second helix may be coupled via a conductor to connector pin of the in-line connector assembly. After fixing the first helix at a tissue site, the second helix may be advanced into a second, adjacent tissue site by rotating the connector pin.  
         [0049]    Lead branches  14  and  18  may be provided with fluid-tight seals,  64  and  66 , respectively, to prevent body fluids from entering lumens  42  and  44 . Seals  64  and  66  may encircle a portion of the drive shaft, as generally disclosed in U.S. Pat. No. 5,984,015, issued to Hess, et al., incorporated herein by reference in its entirety. Alternatively, seals may be located at or near the distal end of the lead branches such that an advancing helix pierces through the seal as disclosed in U.S. Pat. No. 4,217,913 issued to Dutcher or U.S. Pat. No. 4,311,153, issued to Smits, both patents incorporated herein by reference in their entirety.  
         [0050]    [0050]FIG. 4 is a partially cut-away view of the human heart illustrating the placement of lead  10  when it is used for stimulating the His bundle. The lead  10  is advanced into the right atrium (RA) to the annulus of the tricuspid valve, near the atrial septum where an intrinsic His potential can be mapped. Helices  12  and  16  are fixed in the central fibrous body, in close proximity, or preferably into, the His bundle. When a patient is diagnosed with a conduction abnormality superior to the His bundle, for example conduction abnormalities associated with the AV node, commonly referred to as “AV block,” stimulation of the His bundle may produce more efficient ventricular contractions by recruiting the normal ventricular conduction pathways through the left and right bundle branches.  
         [0051]    [0051]FIG. 5A is an illustration of the current field produced during bipolar stimulation using a conventional bipolar lead. A bipolar lead  92 , having a helical tip electrode  94  and ring electrode  96  is shown implanted in the central fibrous body  4 , over the His bundle  6 . The His bundle is surrounded by fibrous, non-excitable tissue  7 . The stimulating current field, indicated by dashed lines, will spread from the tip electrode  94  to the ring electrode  96 . This current field is generally directed away from the His bundle tissue and may not even adequately penetrate the His bundle tissue to capture the cells. Higher pacing amplitude may produce a current field that penetrates the His bundle tissue, however other surrounding tissue may be captured and earlier depletion of the pacemaker battery becomes an issue.  
         [0052]    [0052]FIG. 5B is an illustration of the current field produced during bipolar stimulation using the bifurcated lead shown in FIG. 1, in accordance with the present invention. Helices  12  and  16  are implanted in the central fibrous body  4 , over the His bundle  6 . For a given pacing pulse energy, the current field, indicated by dashed lines, is expected to spread more deeply than the current field produced using the prior art lead shown in FIG. 5A. The current field is expected to pass through the fibrous, non-excitable tissue layer  7  and into the His bundle  6 . An acceptable pacing pulse energy that does not excessively deplete battery charge may be used to effectively capture the His bundle and thereby recruit the normal ventricular conduction pathway, through the left and right bundle branches and Purkinje fiber system.  
         [0053]    The depth of the current field produced will depend on the length of the helices, the distance between the two helices, and the delivered pulse energy. In some embodiments, the helices may be provided with insulation on a proximal portion of the helix such that the stimulating current field is generated from a distal portion of the helix creating a current field directed more exclusively toward the His bundle tissue. The active surface area of the distal, uninsulated portion of the helices must be large enough that it does not impede the stimulating current.  
         [0054]    [0054]FIG. 6 is a plan view of the distal end of an alternative embodiment of a His bundle lead in accordance with the present invention. In FIG. 6, one distal branch  114  extends linearly from the main lead body  122 , and a second distal branch  118  extends at an angle, which may be approximately 90 degrees or less with respect to the main lead body  122 . Branch  118  is curved so that distal fixation helix  116  may be aligned parallel to helix  112  against the central fibrous body over the His bundle. The branches of the lead  100  may be pre-shaped in a number of geometries wherein the branches  114  and  118  may be straight or having varying degrees of curvature and the branches may extend at varying angles from the bifurcation  108 .  
         [0055]    Branch  114  is shown having a tip ring electrode  120 . In addition or alternatively to ring electrodes that are positioned more proximally on a lead branch or main lead body as shown on lead  10  of FIG. 1, a tip ring electrode may be provided on one or both branches and may be used for sensing intrinsic cardiac signals and/or evoked responses. Ring tip electrode  120  may resemble the electrode disclosed in U.S. Pat. No. 5,342,414 issued to Mehra, also incorporated herein by reference in its entirety.  
         [0056]    [0056]FIG. 7 is a side, cut-away view of the distal end of lead  100  of FIG. 6. Lead  100  is provided with one rotatable helix  116  and one non-rotatable helix  112 . Helix  112  is provided with a conductive stem  111  that is electrically coupled to a conductor  130 . Conductor  130  is preferably provided as a coiled conductor with insulation  131  to ensure electrical isolation between conductor  130  and tip ring electrode  120 . Insulation  131  may be formed from an insulating polymer tubing, such as polytetrafluorethylene (PTFE) or ethylene tetraflouoretheylene (ETFE). Insulation  131  may be provided as generally described in U.S. Pat. No. 6,052,625 issued to Marshall, incorporated herein by reference in its entirety. Helix  112  may also be provided with an insulating coating  113  along a proximal segment of helix  112  to ensure electrical isolation between helix  112  and tip ring electrode  120 . Tip ring electrode  120  is electrically coupled to a conductor  134 , which may be provided as a coiled, cabled or stranded conductor.  
         [0057]    In an alternative embodiment, helix  112  may serve only as a fixation device and tip ring electrode  120  may be used in combination with helix  116  for stimulating the His bundle tissue. In yet another embodiment, ring tip electrodes may be provided at the distal end of both lead branches and used as the pacing electrode pair with helices or other fixation members at the end of each lead branch used only for anchoring the branches. However, this embodiment may result in higher stimulation thresholds than embodiments that include at least one electrode inserted at a depth within the central fibrous body, in the vicinity of the His bundle.  
         [0058]    A stylet  138  is shown extending through the lumen of coiled conductor  130  carried in lumen  124  of multi-lumen lead body  122 . Stylet  138  may be used to aid in advancing lead  100  to a desired implant site, and positioning helix  112  over the His bundle. Lead body  122  may then be rotated at its proximal end to fix helix  112  in the central fibrous body, in or over the His bundle.  
         [0059]    Helix  116  carried by distal branch  118  is mounted on drive shaft  128 , which is further coupled to a coiled conductor  132  in a manner described previously in conjunction with FIG. 3. Helix  116  may be advanced into the central fibrous body, adjacent to helix  112 , by rotating coiled conductor  132  at its proximal end. Helix  116  may be positioned at a desired distance from helix  112  with the use of a stylet advanced through lumen  126 , down the center of coiled conductor  132 . Branch  118  may also be preformed to naturally extend helix  116  a predetermined distance away from helix  112 . Branch  118  could alternatively be provided with a shape memory material, such as Nitinol or a shape memory polymer, near its distal end that may be activated, by thermal or electric energy, to acquire a desired shape.  
         [0060]    Lead  100  may alternatively be provided with a rotatable helix on branch  114  and a non-rotatable helix on curved branch  118 . A guide catheter may be used for delivering lead  100  to an implant site. When branches  114  and  118  are contained within a guide catheter, curved branch  118 , having a greater length than branch  114 , may extend from the distal end of the guide catheter while branch  114  remains in the guide catheter. Lead  100  may be rotated to engage the non-rotatable helix on curved branch  118 . The guide catheter could then be withdrawn to allow the distal end of branch  114  to be positioned against the central fibrous body such that the rotatable helix may be fixed in the vicinity of the His bundle.  
         [0061]    [0061]FIG. 8A is a plan view of an alternative embodiment of a branched lead in accordance with the present invention. In FIG. 8A, a separating membrane  90  is provided between two distal branches  82  and  84  extending from a bifurcation  83  of a main lead body  80 . Membrane  90  maintains a desired distance between branches  82  and  84 , thereby providing a desired interelectrode distance between helices  86  and  88 . Membrane  90  may be provided as a thin, flexible member, formed from a biocompatible polymer such as polyurethane or silicone rubber, that may be ribbed or otherwise provided with relatively more rigid reinforcing structures  91  to aid in maintaining a desired distance between branches  82  and  84 . Reinforcing structures  91  could also be provided as compressible structures that could be compressed to allow insertion into a guide catheter and would then expand when the guide catheter is removed to cause branches  82  and  84  to spread apart. Compressible reinforcing structures may fold or coil such that they do not produce too much friction between the lumen of the guide catheter and the distal branches  82  and  84 , allowing the lead to still be easily passed through the guide catheter.  
         [0062]    [0062]FIG. 8B is an end view of the lead shown in FIG. 8A. Branches  82  and  84  are shown connected by membrane  90  extending between the two branches, holding the branches at a predetermined distance from each other. Membrane  90  is preferably flexible enough to be folded to allow the lead to be inserted into a guide catheter during a lead implant procedure. Upon withdrawing the guide catheter, membrane  90  will regain its natural shape, spreading branches  82  and  84  to a desired distance.  
         [0063]    [0063]FIG. 9 is an alternative embodiment of a His bundle lead wherein the distal end of the lead is provided in a generally “S” shaped configuration. Lead  200  includes a lead body  202  that is preformed into a general “S” shape at its distal end  204 . A fixation member, shown as helix  206  extends from the first, more proximal, curve of the “S” shape. A second fixation member, shown as helix  208 , extends from the distal lead end. Helices  206  and  208  are thus positioned such that they may be fixed adjacent to each other in the central fibrous body. The spacing between helices  206  and  208  is predetermined by the preformed shape of distal end  204 .  
         [0064]    One or more ring electrodes may optionally be provided in addition to helices  206  and  208 . Ring electrodes may be located any where along the lead body  202 , proximal to and/or in between helices  206  and  208 . A ring tip electrode could also be included at the distal end of lead  200 .  
         [0065]    An in-line connector assembly  210  is provided at the proximal end of lead  200 . Connector assembly  210  includes a pin connector  214  associated with helix  206  and a ring connector  212  associated with helix  208 . A stylet  216  is shown entering the distal end of pin terminal  214 .  
         [0066]    [0066]FIG. 10 is a side, cut-away view of the distal end  204  of lead  200  of FIG. 9. Lead body  202  is provided with at least two lumens  220  and  222  for carrying conductors  224  and  226  to helices  206  and  208 , respectively. Helix  206  is preferably a rotatable helix, mounted on a drive shaft  230  having a conductive stem  232  coupled to coiled conductor  224 . Rotation of coiled conductor  224  at its proximal end via pin terminal  214  will cause advancement of helix  206  in a manner as described previously. Helix  208  includes a conductive stem  228  which is electrically coupled to coiled conductor  226 . Helix  208  is fixedly attached at the distal lead end  204 . Helix  208  may alternatively be provided as a rotatable helix.  
         [0067]    [0067]FIG. 11A is a plan view illustrating a method for implanting lead  200 . A stiffening stylet  216 , indicated by dashed line, may be advanced through lumen  222 , through the center of coiled conductor  226  to distal lead end  204 . Stylet  216  will straighten the preformed distal end  204  allowing lead  200  to be advanced easily through a venous pathway. The distal end of stylet  216  may be bent or curved to allow lead  200  to be maneuvered to the annulus of the tricuspid valve in a position over the His bundle  252 . Lead  200  may be rotated, by rotating the proximal lead end, to secure helix  208  into the central fibrous body  250 , in or over the His bundle  252 .  
         [0068]    In FIG. 11B, the stylet  216  has been withdrawn from the distal lead end  204 . Withdrawal of stylet  216  allows the distal lead end  204  to regain its natural, generally “S” shape. Stylet  216  may be used to ensure helix  206  is positioned over the His bundle. The preformed shape of distal end  204  will determine the distance between helix  206  and helix  208 . Helix  206  may then be advanced into the central fibrous body  250  by rotating the proximal end of conductor  224 . Stylet  216  may then be fully withdrawn leaving lead  200  in place. Alternatively or additionally, a guide catheter may be used to aid in placing lead  200  over the His bundle.  
         [0069]    [0069]FIG. 12 is yet another embodiment of a branched lead according to the present invention wherein the branched lead is constructed by enclosing two, small diameter leads in a larger diameter tube. Lead  300  includes two small diameter leads each equipped with active fixation electrodes, shown as helices  308  and  310 , coupled to conductors  312  and  314 , respectively. Conductors  312  and  314  are preferably carried in insulating tubing  302  and  304 , respectively. Insulating tubes  302  and  304  are preferably constructed as thin-walled, flexible, biocompatible polymeric tubes formed, for example, from silicone rubber, polyurethane, PTFE, or ETFE. Conductors  312  and  314  are preferably coiled conductors such that a stylet may be advanced through lumens  316  and  318  to aid in implanting lead  300 .  
         [0070]    Insulating tubes  302  and  304  extend from an outer insulating tube  306 . Tube  306  has a length somewhat shorter than tubes  302  and  304  such that tubes  302  and  304  extend out of tube  306  and may be parted to allow helices  308  and  310  to be implanted at a desired interelectrode distance. A distal seal  320 , which may be formed by injecting a medical grade adhesive between insulating tubes  302  and  304 , may be provided to seal the distal end of outer insulating tube  306 .  
         [0071]    [0071]FIG. 13 is an illustration of the lead of FIG. 12 as it may be fixed to the central fibrous body  350 . The two small diameter leads are shown having proximal connector assemblies  342  and  346  exiting the outer insulating tube  306 . A proximal seal  340 , which may be provided by injecting a medical grade adhesive between proximal connector assemblies  342  and  346 , may be provided to seal the proximal end of outer insulating tube  306 . Pin connectors  344  and  348  allow the two small diameter leads to be connected to anode and cathode terminals of a pacemaker such that helices  308  and  310  may be used as an anode-cathode pair for pacing the His bundle  352 . Stylets  330  and  332 , indicated by dashed line, enter the hollow pin terminals  344  and  348  and are advanced within lumens  316  and  318  for use in positioning helices  308  and  310  at a desired distance from each other, against the central fibrous body  350 , over the His bundle  352 . Helices  308  and  310  may be provided as rotatable helices, as described previously, such that they may be advanced into the central fibrous body  350 , into or over the His bundle  352 , by rotating the proximal pin connectors  344  and  348 .  
         [0072]    Thus, a medical lead is provided for directing stimulating current more deeply into a targeted tissue site than conventional bipolar or unipolar pacing leads. While the various embodiments described herein are particularly suitable for delivering cardiac pacing pulses to the His bundle, various aspects of the present invention may be incorporated in medical leads used for other applications requiring delivery of a stimulation pulse at a depth within a targeted tissue. Numerous variations of the embodiments described herein may exist. The descriptions provided herein, therefore, should be considered exemplary and not limiting in regard to the following claims.