Abstract:
An body implantable stimulation lead is provided including tip electrode patterns and configurations producing low noise, clean sensed signals devoid of far field components, such sensed signals being generated irrespective of the direction of the incident depolarization wavefront. The invention also provides high pacing impedances and advantageous anode-to-cathode surface area ratios. Further, implantable leads utilizing the features of the present invention are particularly suitable for left side stimulation therapies.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to body implantable leads, such as bipolar endocardial pacing and sensing leads, and more particularly to improved electrode arrangements for such leads.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Far Field Signals  
           [0003]    Unipolar or bipolar cardiac pacemaker lead systems fulfill two functions. The first function is to provide an electrical conduit by which a pacemaker output pulse is delivered to stimulate the local tissue adjacent to the distal tip of the lead. The second function is to sense local, intrinsic cardiac electrical activity that takes place adjacent to the distal end of the lead.  
           [0004]    One of the problems with body implantable pacing and sensing lead systems is their inability to suppress or attenuate the voltage levels of far field electrical signals. These signals are generated by depolarizations of body tissues in areas remote from the local sensing site and are manifested as propagated voltage potential wavefronts carried to and incident upon the local sensing site. For example, a far field signal may comprise the intrinsic signal originating from the chamber of the heart opposite the one in which the lead electrode is located. Thus, where the lead electrode is implanted in the atrium, the ventricular QRS-wave comprises a far field signal; in contrast, for a ventricular implanted electrode, the atrial P-wave is the far field signal. The sensing electrodes detect or sense the voltages of these signals and interpret them as depolarization events taking place in the local tissue when such polarizations are above the threshold sensing voltage of the system. When far field signal voltages surpassing the threshold voltage are applied to the sensing circuitry of the pulse generator or pacemaker, activation of certain pacing schemes or therapies can be erroneously triggered.  
           [0005]    With the development of universal stimulation/sensing systems, that is, three and four chamber combination pacemaker/cardioverter/defibrillators, accurate sensing of cardiac signals has become even more critical, and management, suppression, and/or elimination of far field signals is vitally important to allow appropriate device algorithms to function without being confused by the undesirable far field signals. As noted, an error in sensing can result in either a wrongfully delivered therapy or a wrongfully withheld therapy.  
           [0006]    Approaches to the problem of far field signal sensing include configuring the circuitry of the pacemaker to attenuate far field signals, and introducing a blanking period long enough to prevent the sensing of unwanted signals. These solutions are described in U.S. Pat. No. 4,513,752 assigned to the owner of the present invention.  
           [0007]    2. Net Signal Amplitude Between Sensing Electrodes  
           [0008]    U.S. Patent No. 5,306,292 teaches the use of multiple small electrodes on a lead tip for pacing and/or sensing. Each electrode has its own dedicated conductor and pacemaker connector terminal. The &#39;292 patent discloses a scheme for selecting the best combination of electrodes for pacing and/or sensing. However, if two electrodes are selected for sensing a problem arises: For any two electrodes selected an orthogonal wavefront impinging on the two electrodes would result in a null output signal, that is, a net sensed signal having an amplitude of virtually zero volts which therefore would not be sensed by the device&#39;s circuitry.  
           [0009]    U.S. Pat. No. 6,064,905 discloses a multi-element temporary mapping catheter including a plurality of small electrodes disposed about a tip section. As in the &#39;292 patent, the electrodes in the &#39;905 patent are each connected to a separate conductor. Accordingly, as in the &#39;292 patent, a depolarization wavefront orthogonally incident on any pair of electrodes can result in a substantially zero net voltage signal. This is true also of the sensing electrode arrangement disclosed in U.S. Pat. No. 4,365,639 in which the electrodes are carried about the side surface of the lead body.  
           [0010]    3. Ratio of Anode-to-Cathode Surface Areas  
           [0011]    As illustrated by U.S. Pat. No. 5,476,496, it is known that in a bipolar pacing and sensing lead, the indifferent electrode or anode, typically in the form of a conductive ring disposed proximally of the tip electrode (which serves as the cathode), should have a large active surface area compared to that of the cathode. The objects of such an areal relationship are to reduce the current density in the region surrounding the anode so as to prevent needless or unwanted stimulation of body tissue around the anode when a stimulation pulse is generated between the cathode and anode, and to minimize creation of two focal pacing sites, one at the cathode and one at the anode which could promote arrhythmia. Typically, the total surface area of the anode is selected so as to be about two times to about six times that of the cathode.  
           [0012]    4. Pacing Impedance  
           [0013]    The design of a stimulation electrode typically carried at the distal tip of a body implantable lead must satisfy various requirements. An essential requirement is that a high impedance be provided at the tissue/electrode interface so as to decrease the current necessary for stimulation and consequently to increase the life span of the pulse generator battery without being electrically inefficient. A simple way to efficiently increase the interface impedance is to reduce the area of the active surface of the stimulation electrode. A relatively high impedance, for example, about 1,000 ohms, is a typical target value. (See, for example, U.S. Pat. No. 6,181,972.)  
           [0014]    5. Left Side Stimulation and Sensing  
           [0015]    The advantages of providing pacing therapies to both the right and left heart chambers are well established. For example, in four chamber pacing systems, four pacing leads, typically bipolar leads, are positioned for both pacing and sensing in the respective heart chambers. To provide left side stimulation and sensing, leads are transvenously implanted in the coronary sinus region, for example, in a vein such as the great vein, the left posterior ventricular (LPV) vein, or other coronary veins, proximate the left ventricle of the heart. Such placement avoids the risks associated with implanting a lead directly within the left ventricle which can increase the potential for the formation of blood clots which may become dislodged and then carried to the brain where even a small embolism could cause a stroke. As used herein, the phrase “coronary sinus region” refers to the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other coronary vein accessible by way of the coronary sinus.  
           [0016]    The tip electrode of a lead implanted in the coronary sinus region can pace and sense left side ventricular activity. When such a lead includes a second electrode proximal of the tip electrode and residing in the coronary sinus above the left ventricle closely adjacent to the left atrium of the heart, pacing and sensing of left atrial activity is made possible. Moreover, the lead may include one or more electrodes for the delivery of electrical shocks for terminating tachycardia and/or fibrillation. Such cardioverting/defibrillating electrodes may be used by themselves or may be combined with pacing and/or sensing electrodes.  
         SUMMARY  
         [0017]    Broadly, the present invention provides tip electrode patterns and configurations producing low noise, clean sensed signals devoid of far field components, such sensed signals being generated irrespective of the direction of the incident depolarization wavefront. The invention also provides high pacing impedances and advantageous anode-to-cathode surface area ratios. Further, implantable leads utilizing the features of the present invention are particularly suitable for left side stimulation therapies.  
           [0018]    In accordance with one specific, exemplary embodiment of the invention, there is provided an implantable stimulation lead for transmitting electrical signals between an implantable medical device and selected body tissue, the lead comprising a proximal end carrying a connector assembly connectable to the implantable medical device; a distal end; and at least one electrode of a first polarity carried by the distal end of the lead and adapted to electrically communicate with the selected body tissue. The lead further comprises at least two electrodes of a second polarity carried by the distal end of the lead and adapted to electrically communicate with the selected body tissue. A housing of insulating material couples the proximal and distal ends of the lead and a first electrical conductor enclosed within the housing electrically couples the at least one electrode of the first polarity with a first terminal on the connector assembly. A second electrical conductor enclosed within the housing electrically couples the at least two electrodes of the second polarity with a second terminal on the connector assembly.  
           [0019]    In accordance with another aspect of the invention, the at least one electrode of the first polarity and the at least two electrodes of the second polarity comprise electrodes for sensing local electrical activity manifested by an incident depolarization wavefront. Further, the at least one electrode of the first polarity and the at least two electrodes of the second polarity are arranged on the distal end in a non-aligned pattern whereby an output voltage signal of sufficient amplitude to be acknowledged by the implantable medical device is generated between the at least one electrode of the first polarity, on the one hand, and the at least two electrodes of the second polarity, on the other hand, irrespective of the direction of an incident depolarization wavefront. All of the electrodes are preferably arranged in a closely spaced cluster so as to sense local electrical events within the selected body tissue and not far field signals. Still further, the at least one electrode of the first polarity may comprise a cathode and the at least two electrodes of the second polarity may collectively comprise an anode. Preferably, the at least two anode electrodes have a total surface area greater than the surface area of the least one cathode electrode, and the electrodes have surface areas providing an impedance of at least about 1,500 ohms.  
           [0020]    In accordance with yet another aspect of the invention, the distal end of the lead carries an extendable/retractable screw-in helix for anchoring the distal end. Preferably, the screw-in helix has an electrically conductive portion, a third electrical conductor coupling the screw-in helix to a third terminal on the connector assembly. Where the selected body tissue comprises a heart, the distal end of the lead may be configured to passively anchor the distal end within a coronary vessel overlying the left side of the heart.  
           [0021]    Pursuant to another specific, exemplary embodiment of the invention, there is provided a body implantable lead for transmitting electrical signals between an electrical connector at a proximal end of the lead and selected body tissue, the electrical connector being adapted to be received by a receptacle in an implantable medical device. The lead comprises a distal end including a tip and a side surface, the side surface carrying at least two parallel-connected electrodes jointly functioning as an anode, and further carrying at least one electrode functioning as a cathode, the anode and cathode electrodes being positioned proximally of the tip and adapted to electrically communicate with the selected body tissue. A first conductor electrically couples the at least two parallel-connected anode electrodes with a first contact on the connector assembly and a second conductor electrically couples the at least one cathode electrode with a second contact on the connector assembly.  
           [0022]    Preferably, the anode and cathode electrodes on the side surface of the distal end are configured and positioned to generate a net voltage output in response to an incident depolarization wavefront, the net voltage output being sufficient to be acknowledged by the implantable medical device. Further, the anode and cathode electrodes may be arranged on the side surface so as to generate the net voltage output irrespective of the direction of the incident depolarization wavefront. Still further, the at least two anode electrodes preferably have a total active surface area greater than the total active surface area of the at least one cathode electrode, and all of the electrodes are preferably arranged in a closely spaced cluster so as to sense local electrical events within the selected body tissue and not far field signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The foregoing and other objects, features and advantages of the invention will be evident to those skilled in the art from the detailed description below, taken together with the accompanying drawings, in which:  
         [0024]    [0024]FIG. 1 is side view of a bipolar endocardial pacing and sensing lead system incorporating the present invention;  
         [0025]    [0025]FIG. 2 is a perspective view of a portion of the distal end of a bipolar endocardial pacing and sensing lead including a distal tip carrying an electrode array in accordance with one specific exemplary embodiment of the present invention;  
         [0026]    [0026]FIG. 3 is a front elevation view of the lead of FIG. 2;  
         [0027]    [0027]FIG. 4 is a perspective view of a portion of the distal end of a bipolar endocardial pacing and sensing lead having a distal tip incorporating an electrode array in accordance with another embodiment of the present invention;  
         [0028]    [0028]FIG. 5 is a front elevation view of the lead shown in FIG. 4;  
         [0029]    [0029]FIG. 6 is a perspective view of the distal end portion of a multipolar lead in accordance with yet another embodiment of the present invention;  
         [0030]    [0030]FIG. 7 is a front elevation view of the lead shown in FIG. 6;  
         [0031]    [0031]FIG. 8 is a perspective view of a portion of the distal end of a bipolar endocardial pacing and sensing lead having a distal tip incorporating an electrode array in accordance with yet another embodiment of the present invention;  
         [0032]    [0032]FIG. 9 is a front elevation view of the lead shown in FIG. 8;  
         [0033]    [0033]FIG. 10 is a side view of the distal end of the lead shown in FIG. 8;  
         [0034]    [0034]FIG. 11 is a perspective view of a portion of the distal end of a bipolar endocardial pacing and sensing lead having a distal tip incorporating an electrode array in accordance with still another embodiment of the present invention;  
         [0035]    [0035]FIG. 12 is a side view of the distal end of the lead shown in FIG.  
         [0036]    [0036]FIGS. 13, 14 and  15  are front elevation views of the distal tips of bipolar endocardial pacing and sensing leads showing further variations of electrode arrays in accordance with the present invention;  
         [0037]    [0037]FIG. 16 is a side view of the distal end of a bipolar endocardial pacing and sensing lead including an electrode array disposed on the side surface of the lead in accordance with yet another embodiment of the present invention;  
         [0038]    [0038]FIG. 17 is a side view of an alternative embodiment of the electrode arrangement on the lead shown in FIG. 16;  
         [0039]    [0039]FIG. 18 is a perspective view of a portion of the distal end of a bipolar endocardial pacing and sensing lead having a distal tip incorporating an electrode array in accordance with another embodiment of the present invention;  
         [0040]    [0040]FIG. 19 is a side view of the distal end of the lead shown in FIG. 18; and  
         [0041]    [0041]FIG. 20 is a side view of the distal end of a bipolar endocardial pacing and sensing lead in accordance with an alternative embodiment of the invention particularly useful for implantation in the coronary sinus region of the heart for left side stimulation and/or sensing.  
     
    
     DETAILED DESCRIPTION  
       [0042]    The following description presents preferred embodiments of the invention representing the best mode contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention whose scope is defined by the appended claims. Moreover, the context in which the invention is principally shown and described herein, that is, bipolar pacing and sensing leads, is illustrative only; it will be understood by those skilled in the art that the invention has applicability to a wide variety of body implantable lead types.  
         [0043]    With reference to FIGS.  1 - 3 , there is shown a bipolar endocardial pacing and sensing lead  10  in accordance with a preferred embodiment of the present invention. The lead  10  includes a lead body  12  comprising a distal end  14  and a proximal end  16  joined by a tubular sheath or housing  18  made of an insulating, biocompatable, biostable material such as silicone rubber or polyurethane.  
         [0044]    The distal end  14  of the lead body  12  includes a distal tip  19 , comprising a tip surface  20  incorporating a pacing/sensing electrode array to be described. Optionally, disposed proximally of the tip surface  20  along the distal end of the lead body is a pacing and/or sensing ring electrode  22 .  
         [0045]    The proximal end  16  of the lead  10  incorporates a connector assembly  24  for connecting the lead body  12  to a pacemaker and/or ICD  26 . The connector assembly  24  includes a tubular, rotatable pin terminal  28  and a pair of ring terminal contacts  30 ,  32  electrically coupled to the electrodes on the tip surface  20 . The connector assembly  24  of the lead is received within a receptacle of the pacemaker/ICD  26  and to prevent ingress of body fluids into the receptacle the connector assembly is provided with spaced sets of seals  34  in accordance with well known arrangements in the art. In accordance with well known implantation techniques, a stylet or guidewire for delivering and steering the distal end  14  of the lead body during implantation is inserted into a lumen of the lead body  12  through the tubular connector pin  28 . Further, in accordance with well known techniques, the lead body housing  18  may have a lubricious coating on most or all of its outer surface to facilitate its movement through a delivery introducer and the patient&#39;s vascular system.  
         [0046]    The distal tip  19  of the lead body includes a fixation means in the form of a helix  36  extended or retracted by means of a rotatable actuator such as a coil  38  coupling the helix  36  with the rotatable tubular pin terminal  28 . The lumen of the helix coil  38  may provide a passage for a stylet or guidewire for steering and positioning the distal tip  19  during implantation. In the specific example under consideration, the helix  36  is passive in the sense that it is not electrically active, functioning only to provide for fixation and secure contact between the distal tip surface  20  and the adjacent tissue of the heart. Alternatively, as will be described below, the helix may be conductive along its entire length or along only a portion thereof.  
         [0047]    The tip surface  20  carries three “dot” electrodes  40 ,  42  and  44  having active surface areas and spaced closely together in a clustered array about a central aperture  46  through which the anchoring helix  36  projects when extended. Electrodes  40  and  42  collectively function as an anode; as shown in FIG. 3, these two electrodes are connected in parallel to a single conductor  48  extending the length of the lead body to one of the contacts  30  and  32  on the connector assembly  24 . The third electrode  44  on the tip surface  20  functions as a cathode; this electrode is connected to a second conductor  50  extending the length of the lead body and coupled to the other of the contacts  30  and  32 . The ring electrode  22  may be used as an anode in combination with the cathode  44  for cardiac pacing or to sense electrical impulses produced by the heart tissue. The cross-sectional configuration of the lead body housing  18  may accommodate various combinations of coil and/or cable conductor combinations including, for example, bipolar coaxial coils or bipolar cables or multilumen combinations of coils and/or cables.  
         [0048]    The electrodes  40 ,  42  and  44  have small active surface areas and are closely spaced so as to simultaneously provide immunity from far field signals and a high pacing impedance. For example, the two anode electrodes  40  and  42  may each have an active surface area of 0.5 mm 2  in which case the cathode electrode  44  may have an active surface area of 0.3 mm 2 . It will thus also be seen that the total surface area of the anode would be about three times that of the cathode which, as indicated, is known in the art to be a desirable anode-to-cathode active surface area ratio. The small electrode active surface areas, such as those mentioned above, can provide a high pacing impedance in the range of 1,500 to 3,000 ohms. The spacing between electrodes may be about 0.3 mm. The electrode “dots” need not be circular. They can have any geometrical shape suitable for use as a pacing and/or sensing electrode, including but not limited to semicircular, square, rectangular, hexagonal, oval, annular, and so forth.  
         [0049]    It will be seen that the electrodes  40 ,  42  and  44  on the tip surface  20  of the lead body are not arranged in a straight line. This nonlinear pattern assures that regardless of the direction of an approaching depolarization wavefront, the cathode electrode  44  and at least one of the anode electrodes  40 ,  42  will be intercepted in succession so as to produce a robust sensed voltage above the threshold needed to allow appropriate device algorithms to function without confusion. The amplitude of these signals far exceeds that of any unwanted far field signals. Accordingly, a clean, low noise cardiac signal is generated with minimal or no far field signals.  
         [0050]    [0050]FIGS. 4 and 5 show the distal end  60  of a bipolar pacing and sensing body-implantable lead in accordance with another embodiment of the present invention. The distal end  60  of the lead includes a distal tip  62  terminating at a tip surface  64  carrying five electrodes  66 ,  68 ,  70 ,  72  and  74 . The four electrodes  66 ,  68 ,  70  and  72  collectively function as an anode while the remaining electrode  74  serves as a cathode. As shown in FIG. 5, the four anode electrodes are connected in parallel to a single electrical conductor  76  which, as in the first embodiment, may take the form of a small diameter, closely wound coil of fine wire or, preferably, a multistrand or braided cable, coupling the anode electrodes to a contact on a connector assembly at the proximal end of the lead as previously described. The cathode electrode  74  is connected to a separate, single conductor  78  which also may take the form of either a coil or a multistrand cable conductor.  
         [0051]    In the specific embodiment shown in FIGS. 4 and 5, the surface areas of the five electrodes  66 ,  68 ,  70 ,  72  and  74  are all the same; the total active surface area of the four anode electrodes would therefore be about four times that of the active surface area of the single cathode electrode which, as explained, is within the range of desirable anode-to-cathode surface area ratios of about 2:1 to about 6:1. The small surface area, closely spaced electrodes would not generate any significant voltages due to far field artifacts but would produce between the anode electrodes on the one hand and the cathode electrode on the other hand a clean, high amplitude potential from the sensing of local cardiac events, the term “local” meaning in the immediate vicinity of the tip surface.  
         [0052]    In addition, as in the first embodiment, given the nonlinear electrode arrangement, an output voltage would invariably be generated regardless of the direction of an approaching depolarization wavefront since the cathode electrode and at least one of the anode electrodes would always be intercepted in succession by the wavefront. In addition, given the small surface areas of the electrodes, high pacing impedances exceeding 1000 ohms, and preferably in the range of 1500 to 3000 ohms, are provided.  
         [0053]    As in the first embodiment, the lead of the second embodiment includes an electrically passive fixation helix  80  actuated by a rotatable coil  82 . It will be evident that the four anode electrodes could have different surface areas, as could the cathode electrode. Alternatively, three of the five dot electrodes could comprise anodes with the remaining two serving as cathodes; other possible combinations will be apparent to those skilled in the art. Still further, it will be evident to skilled artisans that the total number of electrodes, instead of five, could be three, four, six or even more, connected in various anode and cathode combinations.  
         [0054]    With reference now to FIGS. 6 and 7, there is shown the distal end  90  of a bipolar pacing and sensing lead in accordance yet another embodiment of the present invention. The distal end  90  includes a distal tip  92  including a segmented collar  94 . In the specific embodiment under consideration, the collar  94  comprises three conductive segments  96 ,  98  and  100  separated by intervening insulative segments  102 ,  104  and  106 . Two of the conductive segments  96  and  98  function collectively as an anode electrode while the third conductive segment  100  serves as a cathode. As before, the anode electrodes  96  and  98  are parallel-connected to a single conductor  108  coupled to a contact on the connector assembly at the proximal end of the lead. The cathode electrode  100  is similarly connected to a single conductor  110 , separate from the first mentioned conductor, and connected to another contact on the connector assembly. As before, the electrical conductors  108 ,  110  may comprise small coiled conductors of fine, closely wound wire, or cable conductors preferably of the multistrand or braided type.  
         [0055]    In the specific embodiment shown in FIGS. 6 and 7, three equiangularly spaced, segment electrodes  96 ,  98  and  100  subtending equal angles are illustrated. By way of example only and not by way of limitation, the surface area of each of the electrodes  96 ,  98  and  100  may range from about 0.3 square mm to about 1.5 square mm and the interelectrode spacings may range from about 0.2 mm to about 0.4 mm. It will be obvious, however, that the electrodes may be configured to subtend different angles, and that more than three segmented electrodes may be used. The embodiment shown in FIGS. 6 and 7 may also include a rotatable fixation helix  112  and an associated helix actuator coil  114  along the lines already described. Also, optionally included, is a ring pacing or sensing electrode  116  coupled to a contact assembly by means of a conductor  118 .  
         [0056]    FIGS.  8 - 10  show a variation of the bipolar pacing and sensing lead of the embodiment of FIGS. 6 and 7. The lead of FIGS.  8 - 10  includes a distal end  120  having a distal tip  122  incorporating a segmented collar  124  comprising three conductive segments  126 ,  128  and  130  separated by intervening insulating segments  132 ,  134  and  136 . The three conductive segments jointly function as an anode and in this respect, as shown in FIG. 9, the three conductive segments  126 ,  128  and  130  are parallel-connected to a single coil or cable conductor  138  coupled to a terminal contact on the connector assembly (not shown). The distal tip  122  further includes a central aperture  140  through which a rotatable helix  142  may be extended to anchor the distal end  120  of the lead to adjacent tissue. Unlike the helix in the embodiment of FIGS. 6 and 7, the helix  142  is electrically active, functioning as the cathode of the bipolar lead. In this connection, the helix  142  includes an uninsulated or bare mid-section  144  interposed between insulated distal and proximal portions  146  and  148 . With the helix  142  extended to anchor the distal end  122  of the lead body in the adjacent heart tissue, electrical contact will be established between the bare mid-section  144  of the helix  142  and the surrounding tissue. It will be evident that instead of a mid-section of the helix being electrically active, an uninsulated distal end section of the helix may be made to comprise the electrically active surface, with the remaining, proximal portion of the helix being electrically insulated. In either case, the helix  142  is mechanically and electrically connected to a coil conductor  150  in turn coupled to a rotatable pin on a connector assembly at the proximal end of the lead, the pin in this case serving also as an electrical terminal contact. By way of example and not limitation, the electrically active surface of the helix  142  may have an area ranging from about 2 square mm to about 8 square mm, and the total surface area of the three anode electrodes  126 ,  128  and  130  may range from about 4 square mm to about 15 square mm.  
         [0057]    [0057]FIGS. 11 and 12 show another variation of the bipolar pacing and sensing lead of the embodiment of FIGS. 6 and 7. The lead of FIGS.  11  and  12  includes a distal end  152  having a distal tip  153  incorporating a segmented collar  154  comprising, in this specific example, three electrically conductive segments  155 - 157  separated by intervening insulating segments  158 - 160 . The two conductive segments  155  and  156  together function as an anode electrode and accordingly may be parallel-connected to a single coil or cable conductor  161  coupled to a terminal contact on a proximal connector assembly (not shown). The conductive segment  157  serves as a cathode coupled to a terminal contact on the connector assembly of the lead by means of a coil or cable conductor  162 . In accordance with one specific example, the anode pair  155 ,  156  and the cathode  157  may be used to sense local cardiac electrical activity.  
         [0058]    The distal tip  153  further carries a rotatable helix  163  which is extendable by means of a rotatable actuator coil  164  to anchor the distal tip  153  to adjacent body tissue. The helix  163  is electrically active, functioning by way of example as a cathode. In this connection, the helix  163  includes an uninsulated or bare mid-section  165  interposed between insulated helix sections  166  and  167 . Electrical pulses applied across the electrically active helix  163  and an anode ring electrode  168  proximal of the segmented collar  154  provide pacing stimuli to the adjacent body tissue. The ring electrode  168  is connected to a terminal contact on the connector assembly via an electrical conductor  169 .  
         [0059]    It will be evident that instead of the segmented electrodes  155 - 157 , dot electrodes arranged across the distal tip may be used. Further, the helix  163  may be designed so that a distal section thereof, instead of the mid-section  165 , is uninsulated so as to present an electrically conductive surface to the surrounding tissue. Desirable anode-to-cathode area ratios, as already described, can be readily provided.  
         [0060]    [0060]FIGS. 13 through 15 illustrate still further variations of bipolar lead electrode arrays in accordance with the present invention. FIG. 13 shows a tip surface  170  carrying three electrodes  172 ,  174 , and  176  having the same surface area. Two of the electrodes  172  and  174  jointly function as an anode, and these electrodes are connected in parallel to a single conductor  178 , along the lines already described. The third electrode  176  functions as the cathode and it is connected to a separate conductor  180 . A passive helix  182  may be provided for anchoring.  
         [0061]    [0061]FIG. 14 shows a lead tip surface  190  in accordance with the present invention in which two anode electrodes  192  and  194  commonly connected to a conductor  196 , and a third, cathode electrode  198  connected to a conductor  200  occupy a portion of the tip surface. The lead includes an off-center, passive anchoring helix  202 . In all other respects, the lead is the same as those already described.  
         [0062]    [0062]FIG. 15 shows yet another embodiment of the present invention in which a tip surface  210  carries three arcuate electrodes  212 ,  214  and  216 , two of which ( 212  and  214 ) jointly function as an anode and the remaining one of which ( 216 ) functions as a cathode. For redundancy, each electrode is attached to two multistrand or braided cable conductors. Thus, the anode electrode  212  is connected to a pair of conductors  218 ,  220 ; the anode electrode  214  is connected to a pair of conductors  222 ,  224 ; and the cathode electrode  216  is connected to a pair of conductors  226 ,  228 . The electrical conductors  218 ,  220 ,  222  and  224  connected to the anode electrodes are all connected in parallel to the same terminal contact on a connector assembly on the proximal end of the lead. Similarly, the redundant cathode electrode conductors  226  and  228  are coupled to another terminal contact on the connector assembly. The embodiment of FIG. 15 may also include a fixation helix  230  which may be either electrically active or electrically passive, as already described. Although three arcuate electrodes are illustrated in FIG. 15, it will be obvious that other numbers of electrodes may be utilized so long as the aforedescribed preferable area ratio is observed. Further, although the arcuate electrodes shown in FIG. 15 are arranged concentrically about a central, longitudinal axis of the lead body, it will be evident that this need not be the case.  
         [0063]    [0063]FIG. 16 shows a distal end  240  of a bipolar pacing and sensing lead in accordance with yet another embodiment of the present invention. The distal end  240  terminates in a tip surface  242  and further has a side surface  244 , which will typically be substantially cylindrical although it will be evident that the lead body outer surface need not be limited to any particular geometry. The side surface  244  along the distal end of the lead carries an array of three electrodes  246 ,  248  and  250  which may function as sensing electrodes. The electrodes  246  and  248  serve as anodes and are commonly connected to a single conductor  252  in turn connected to a terminal contact on a connector assembly at the proximal end of the lead. The remaining electrode  250  which can be a dot electrode, as shown, or a full ring electrode, functions as a cathode and is connected by means of a separate conductor  254  to another contact on the connector assembly. In the particular electrode pattern illustrated in FIG. 16, the anode electrodes are disposed along a common transverse plane  256  while the cathode electrode is positioned distally of the plane  256  occupied by the anode electrodes. In this way, a sensed potential will always be generated irrespective of the direction of an incident depolarization wavefront, in the manner already described. Preferably, the surface area of the anode electrodes exceeds that of the cathode electrode also as previously described.  
         [0064]    [0064]FIG. 17 shows a variation of the electrode pattern illustrated in FIG. 16. In the embodiment of FIG. 17, four electrodes  270 ,  272 ,  274  and  276 , together serving as an anode, are arranged about a side surface  278  of the distal lead end  280 . The four electrodes lie along a transverse plane  282 . These electrodes are connected in parallel to a single conductor  284 . A fifth electrode  286 , which may comprise a dot electrode, as shown, or a full ring electrode, serves as the cathode and is disposed distally of the plane  282  of the anode electrode array. As before, the cathode is connected to a separate, single conductor  288 .  
         [0065]    [0065]FIGS. 18 and 19 show the distal end  290  of a bipolar pacing and sensing lead in accordance with yet another specific, exemplary embodiment of the present invention. The distal end  290  includes a distal tip  291  incorporating a non-segmented, that is, a one-piece, electrically conductive collar  292  connected to terminal contact on a connector assembly (not shown) at the proximal end of the lead by means of an electrical coil or cable conductor  293 . The distal tip  291  further carries an electrically active fixation helix  294  having a conductive mid-section  295  and coupled by a coil conductor/actuator  296  to a rotatable terminal contact pin forming part of the connector assembly at the proximal end of the lead. As noted earlier, the electrically conductive portion of the helix may be provided along the distal end thereof instead of along the mid-section  295 . The distal end  290  of the lead may include, as an option, a conductive ring electrode  297  proximal of the distal tip  291 ; an electrical conductor  298  connects the ring electrode  297  to another terminal contact on the lead&#39;s connector assembly. By way of example and not limitation, pacing and sensing may be performed between the collar  292 , functioning as an anode, and the smaller electrically active area of the helix  294  serving as a cathode. When provided, the optional ring electrode  297  may function as an additional, area-increasing anode whose conductor  298  may be connected in parallel with the collar conductor  293 .  
         [0066]    With reference to FIG. 20, there is shown the distal end  300  of a lead in accordance with yet another embodiment of the invention particularly suitable for left side placement within a vessel  301  overlying the left side of the heart. For left side placement, a softer, more flexible distal end is preferred with a length corresponding to the coronary sinus and its associated coronary vessels overlying the left side of the heart. In a case in which a sensing electrode such as a ring electrode  302  is provided, it is desirable to have the distance between the tip surface  304  and the ring sensing electrode  302  sufficiently small to allow both of these electrodes to be placed in a target coronary vessel such as the LPV vein. Such placement of the tip and ring electrodes ensures achieving capture of the left ventricle.  
         [0067]    The tip surface  304  may include any of the various tip and/or side “dot”, collar and/or ring electrode arrays and configurations already described, the essential difference being that the embodiment of FIG. 20 would not include a helix fixation means. Instead, the distal end  300  includes an alternative passive fixation means to help anchor the distal portion of the lead body within a target vessel of the coronary sinus region. The passive fixation or anchoring means may comprise one or more preformed humps, spirals, S-bends or other configurations manufactured into the distal portion of the lead body. In the specific exemplary embodiment shown in FIG. 20, the distal portion of the lead body has a single S-bend  306  so that when the distal end of the lead body is in place within the target coronary vessel  301 , there will be biased contact between the S-shaped bend  306  and the inner wall of the vessel  301  so as to create wedging forces sufficient to anchor the lead and prevent its displacement or dislodgment. Ideally, as illustrated in FIG. 20, the distal end  300  is positioned within the vessel  301  so that the ring electrode  302  and the electrode(s) on the tip surface  304  are in intimate electrical communication with the inner wall of the vessel  301 . Alternatively, the passive fixation means may comprise—either by itself or in combination with humps, spirals, bends, or the like—one or more soft, flexible protuberances that also tend to wedge the distal portion of the lead body in the target coronary vein. In either case such passive fixation means biases the distal portion against the vessel wall. As further shown in FIG. 20, the passive fixation means can further include texturization  308  of the distal end  300  of the lead body to promote rapid blood clotting and resulting fibrotic tissue growth about the distal portion to help anchor that portion in place.  
         [0068]    While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. For example, it will be obvious that instead of a single cathode electrode as provided in several of the described embodiments, two or more such electrodes may be included. Further, although the tip surfaces shown in certain of the aforedescribed embodiments each lies generally in a plane perpendicular to a central, longitudinal lead axis, it will be obvious that the tip surfaces may be curved, for example, in the shape of a hemispherical surface or other configuration to assure optimal electrical engagement with the cardiac tissue to be stimulated with minimal risk of perforating the tissue engaged by the tip surface. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.