Abstract:
A system implantable in the coronary venous system, including a pacing lead with an anchoring screw is disclosed. The system includes a stimulation lead ( 10 ) for stimulating a left heart cavity of a patient, and a removable catheter ( 26 ) for implanting the lead. The lead ( 10 ) has at least one stimulation electrode having an anchoring screw ( 14 ) that penetrates into the epicardial tissue of the patient. The catheter tube ( 26 ) is a pre-shaped tube with two curvatures in the absence of stress. The two curvatures are inscribed in two separate surfaces ( 38, 40 ) for self-orientating the distal end of the catheter tube into the target vein and maintaining the axis of the anchoring screw towards the epicardial wall during the screwing of the lead head.

Description:
FIELD 
       [0001]    The present invention relates to “active implantable medical devices” as defined by the Jun. 20, 1990 Directive 90/385/EEC of the European Communities, particularly to implantable devices that continuously monitor a patient&#39;s heart rhythm and, if necessary, deliver at the patient&#39;s heart electrical pulses for stimulation or resynchronization, and more particularly to cardiac pacing leads implanted in the coronary network of the patient&#39;s heart to allow pacing of a left (ventricle or atrium) cavity. 
       BACKGROUND 
       [0002]    Unlike the right cavities of the heart, for which it is sufficient to implant endocardial leads via the right peripheral venous network, the implantation of permanent leads into a left heart cavity involves significant operating risks, e.g., passing bubbles to the cerebral vascular network located downstream of the left ventricle. For this reason, when it is desirable to stimulate the left cavity, a stimulation catheter is introduced via the coronary network, rather than in the cavity. The lead is equipped with an electrode to be applied against the wall of the epicardium and to steer it toward the left ventricle or left atrium, as appropriate. 
         [0003]    The introduction of such a lead is made through the coronary sinus opening into the right atrium, thus by an endocardial approach. It is then guided and driven along the network of coronary veins to the chosen stimulation site. This intervention is particularly difficult given the peculiarities of the venous system and of its access paths, particularly because of the valves and tortuosities, and because of the gradual reduction in diameter of the vessel as the lead progresses in the selected coronary vein. Furthermore, the position of the stimulation site, a good electrical contact of the electrode with the tissue of the epicardium, and the correct orientation of the stimulating electrode, are particularly important aspects to consider when implanting the lead into the heart cavity. 
         [0004]    For example, in the case of a “multisite” pacemaker, preferably for the interventricular resynchronization, the two points of stimulation of the left ventricle/right ventricle should be as far apart as possible, so as to optimize the resynchronization of all the cardiac chambers. It is also important to reduce the risk of phrenic nerve stimulation during or after implantation. 
         [0005]    These aspects must be evaluated not only at the time of implantation, but it is also necessary that the quality and accuracy of the contact in the site of stimulation are not significantly changed in the medium term, for example, because of patient movements such as changes in posture, wide movements of arms and heavy breathing. 
         [0006]    EP 0993840 A1 and its US counterpart U.S. Pat. No. 6,385,492 (ELA Medical, now known as Sorin CRM S.A.S) describe a pre-shaped lead adapted for implantation in the coronary venous system. The lead head includes a sector electrode and has a double curvature to ensure a self-orientation of the lead tip so that the sector electrode is directed toward the epicardium when the lead head has reached its final position. This pre-shaped lead is used in combination with a stylet that allows the operator to more or less reform the curvature during implantation to facilitate the progression of the lead head to the heart in the peripheral venous system, then the search of the coronary sinus, and finally, the progression of the lead head in the coronary system to the selected site of stimulation. A commercial embodiment of this type of lead is marketed by Sorin CRM under the Situs LV lead brand. 
         [0007]    The pre-shaped leads are sometimes difficult to advance, as they move along according to the principle of least energy. At an applied force to insert the lead or torque to rotate the lead, the lead moves or rotates in the direction and orientation where the least energy is required. In addition, their extraction capacity remains unknown, with a risk of damaging the coronary network veins during explantation of the lead. 
         [0008]    Finally, one of the classical drawbacks of conventional pre-shaped leads is their tendency to restore to their original shape (due to the principle of least energy), which causes postoperative micro-displacements affecting the quality of the thresholds and/or the resynchronization. 
         [0009]    Another problem is the difficulty in finding a good stimulation site, to achieve a good electrical contact of the electrode against the tissue of the epicardium, and maintain this contact despite variations or various solicitations in time. It is also essential to avoid any phrenic stimulation, both during and after the implantation. 
         [0010]    To overcome these difficulties, it was proposed to have multiple electrodes along the lead body to increase the chances of an acceptable compromise, possibly by giving the lead body a particular lead conformation. The operator can thus choose, among the various electrodes present on the lead body, the one providing the best electrical efficiency. On the other hand, to reduce risks of variation in the medium term of the electrical contact and of later phrenic stimulation, manufacturers have introduced the concept of “electronic repositioning” to direct or redirect the electric field between different electrodes placed along the stimulation lead of the left ventricle and/or with one of the electrodes of the stimulation lead from the right ventricle, in order to avoid a re-intervention. The counterpart of this solution is a growing complexity of the structure of the lead, an increase in the number of electrodes causing an increase in the number of components and electrical wiring, or the use of multiplexing circuits for selecting from among the various electrodes on the same lead. 
         [0011]    The WO-A-02/04062 A2 (Medtronic) describes a technique of “telescopic approach” usually used in various applications (notably in angioplasty) to make a cannulation either of the ostium of the coronary sinus or a lateral vein. This technique is based on the use of two catheters, one inserted inside the other, with longitudinal and circular relative movements possible, the movements being controlled by the physician on the proximal side. It is theoretically possible to orient the termination of the inner catheter to direct the termination of its distal curve to the epicardium but, as in the targeted application the catheter is “partitioned/adjusted” in the venous network, the expected equilibrium position generates a lot of counter torque that must be opposed. As a result, the expected equilibrium position is relatively unstable. 
         [0012]    Several phenomena complicate the task of the physician, for example, the fact that the fine control of the orientation is subject to the quality of the torque transmission, which is particularly linked to: (i) the diameter of the catheter, which is expected to be as small as possible and (ii) the length of the catheter, which is in the order of 70 cm. The small diameter and long length of the catheter are unfavorable for good control. This fine control is also perturbed by the cardiac movements which render the equilibrium of the system unstable, as the screwing operation can occur over a duration in the order of ten seconds. Keeping the catheter in place while performing the screwing operation may require two operators. 
         [0013]    The implantation of a catheter is consequently “operator dependent” because it is difficult to obtain a secured positioning of the anchoring helical screw of the catheter. 
         [0014]    The anchoring helical screw needs to be oriented in an appropriate direction during screwing. Indeed, the fixation qualities of a screw lead are broadly known in the field of cardiac pacing leads, but this technique is not used as of today in the coronary sinus because the risks of dissection of the coronary sinus is too high. Nevertheless, the fixation remains the main problem when implanting the left ventricular pacing leads via the coronary sinus. 
         [0015]    The U.S. Pat. No. 5,837,007 (Pacesetter) proposes a solution to this problem by means of a retractable screw. But this technique is complex, because it requires at least three components (up to six components) with the use of a soluble element or a component having mechanical characteristics that significantly change after impregnation by corporeal fluids. Furthermore, the torque transmission is performed via a stylet that directly engages the screw. This requires an additional accessory and complicates the operation, which is already complex and delicate to implement. 
         [0016]    The present invention aims to remedy the above difficulties, in particular by overcoming problems associated with the use of leads equipped with multiple electrodes. 
       SUMMARY 
       [0017]    It is therefore an object of the present invention to enable an accurate choice and screening of a stimulation site, and to ensure an optimum and durable stability of the electrode placement on the stimulation site. 
         [0018]    If the electrode can be fixed in place at the stimulation site where it was originally placed, subsequent movements of the lead after the fixation are of no concern thus it would be unnecessary to overcome the consequences of such movements by sophisticated techniques such as electronic repositioning or the selection among multiple electrodes. 
         [0019]    The present invention proposes to use as the stimulation lead for the left ventricle a screw anchor similar to the technique for attaching stimulation leads in the right cavities. 
         [0020]    The screw anchor stimulation leads for the right cavities are endocardial leads that are introduced to the apex of the right ventricle. The lead tip is pushed to the bottom of the cavity (ventricular apex) where it abuts, and the physician assumes that the correct permanent position of the lead is reached. The lead head is rotated and screwed into the tissue of the myocardium in a reversible manner. When the lead needs to be removed or relocated, the physician can unscrew the lead and redo the fixation operation. 
         [0021]    The direct implantation of such a lead in the coronary system, however, presents several difficulties. Firstly, the handling of a fixed or retractable screw lead into the sinus would create high risks, with damage levels varying up to the complete dissection of the coronary sinus. On the other hand, once the lead head is positioned at the stimulation site, the screw tends to be oriented in a direction parallel to the general orientation of the vein, not in the direction of the epicardial wall, despite the electrode must precisely be in close contact with the tissue of the epicardium for fixation. 
         [0022]    To solve both of these problems (safe access to the desired site and orientation of the lead head to the epicardium), the present invention combines the lead with a screw setting tool specifically ensuring optimum safety and providing an adapted angle of screwing. 
         [0023]    In one embodiment of the present invention, the screw setting tool is a preformed guide catheter with a double curve similar to that described for a lead in the aforementioned EP 0993840 A1 and its US counterpart U.S. Pat. No. 6,385,492. The bi-curve shape is applied to the lead itself, because it has a sector electrode. Means for keeping the lead against the wall of the vein at the stimulation site is provided to ensure that the electrode is directed toward the epicardium. 
         [0024]    In one embodiment of the present invention, the bi-curve shape is applied to the setting tool to the guide catheter alone. The sector lead is replaced with an anchoring screw lead with all the inherent advantages of this type of lead, including the reduction or complete elimination of the risk of displacement after fixation, the excellent electrical performance in contact with a myocardial tissue, and the ease of extraction. 
         [0025]    The present invention uses the local anatomical configuration of the coronary sinus and its tributaries (a left turn) to self-orient and stabilize the distal end of the catheter. The local self-orientation provides a sufficient torque to orient the support curvature in the desired position, allowing a precise positioning of the screw head at the time to start screwing into the tissue. This technique is not operator-dependent, and the operator has only to push the catheter into the target vein to self-orient after several beats in the desired position, freeing his hands from the screwing operation. 
         [0026]    More specifically, the present invention is directed to an implantable system of the general type notably described in WO 02/04062 A2 cited above, more particularly to an implantable system implanted in the coronary venous system including a pacing lead and a removable catheter for the implantation of the pacing lead. The pacing lead includes: a lead body with a sheath in a deformable material; at the distal end, a lead head with at least one stimulation electrode coming into contact with a region facing the epicardium of the wall of a targeted vein of the coronary network, and on the proximal side, means of coupling to a generator of an active implantable medical device. The catheter includes a hollow tube open at both ends with a lumen. The lead is inserted within the catheter lumen and moved in translation inside the catheter, first to a retracted position where the lead head is positioned at the distal end of the catheter lumen, for example, flush with the distal end, or recessed relative to the catheter lumen distal end, and is then moved to a deployed position where the lead head with the electrode emerges from and extends beyond the opening of the hollow tube. The hollow tube of the catheter is elastically deformable and relatively more rigid than the sheath of the lead body. 
         [0027]    In accordance with one embodiment of the present invention, the lead is a screw lead that is equipped with a projecting anchoring helical screw axially extending from the lead head, and penetrates the tissue of the epicardium under the action of a screw motion imparted to the lead head. The sheath of the lead body preferably is not pre-shaped at its distal end. The catheter includes a single tube that is pre-shaped having, in the absence of any stress, two curves in two separate respective planes, said two curves auto-orienting the distal catheter tube into the target vein with the lead head in the retracted position, and maintaining the axis of the anchoring screw towards the epicardium during a combined movement of screwing and moving of the lead head towards the extended position. 
         [0028]    In accordance with one embodiment, the two distinct surfaces defining the respective two curves have a first surface corresponding to a curvature of an orientation curve defined by a pre-shaping of the catheter tube and a second surface corresponding to a curvature defined by a natural curve of the distal end of the catheter tube. 
         [0029]    In one embodiment, the anchoring screw is a screw extending axially of the lead head. In another embodiment, the anchoring screw is a retractable mobile screw into a housing of the lead head. The anchoring screw may be an active screw forming said stimulation electrode, or a passive screw, electrically decoupled from said stimulation electrode. 
         [0030]    The sheath of the lead body has sufficient rigidity in torsion to transmit a rotating movement over its length from the proximal end of the lead to screw the anchoring screw. Alternatively, if the rigidity in torsion of the sheath of the lead body is insufficient to transmit a rotating movement over the length from the proximal end of the lead, a removable stylet is introduced in a lumen of the lead body, movable in translation within the lumen up to the lead head and including means of coupling in rotation with the lead head. The stylet has a sufficient rigidity in torsion to transmit a rotation movement over its length to the means of coupling from the proximal end of the stylet to screw the anchoring screw. 
         [0031]    In a preferred embodiment the diameter of the lead body is less than or equal to 5 French (1.65 mm). 
         [0032]    In one embodiment of the present invention, the anchoring screw includes a helical end portion adapted to penetrate the tissue of the epicardium and connect to the lead head by a transition portion mechanically deformable in flexion, more preferably a part of the helical screw with adjacent turns. More preferably, the anchoring screw includes an helix end portion penetrating the tissue of the epicardium and including an active region of an electrically conductive tip extending through an electrically insulated part, with an axial length of the active region of electrically conductive tip that is less or equal to 1 mm. 
         [0033]    In yet another embodiment, the system includes a removable guide wire. The removable guide wire, after having been introduced into the coronary venous system to the target vein, receives the catheter and allows by sliding the guiding of the catheter to this target vein. 
         [0034]    The catheter with pre-shaped tube is preferably a sub-catheter for vein selection. The system also includes a main catheter including a hollow tube open at both ends with a lumen within which the catheter is inserted and moved in translation and rotation within the catheter to advance into the main venous system until it reaches the coronary target vein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    Further features, advantages and characteristics of the present invention will become apparent to a person of ordinary skill in the art in view of the following detailed description of preferred embodiments of the invention, made with reference to the drawings annexed, in which like reference characters refer to like elements, and in which: 
           [0036]      FIG. 1  is a sectional view of the lead head of the system, according to the present invention; 
           [0037]      FIG. 2  schematically illustrates a three dimensional perspective view of a system in accordance with a preferred embodiment of the present invention; 
           [0038]      FIG. 3  is a sectional view of the lead of  FIG. 1  after it has been implanted in a target vein of the coronary network, with the screw being anchored in the wall of the epicardium; 
           [0039]      FIG. 4  is an end view according to section AA of  FIG. 2 ; and 
           [0040]      FIG. 5  is a top view according to section BB of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    Examples of an implementation of the present invention will now be described with reference to the drawings  FIGS. 1-5 . In the figures, reference  10  generally refers to the screw lead of the system according to the present invention. The lead  10  comprises a lead body  12  which structure is in itself known, usually a polyurethane sheath to reduce friction when the catheter is inserted into a guide catheter, and to provide better sensitivity and better transmission of torque torsion. The diameter of the sheath of the lead body  12  is chosen to be thin enough to be compatible with the sub-selection catheters of veins of the coronary network, typically less than or equal to 5 French (1.65 mm). 
         [0042]    The lead  10  is terminated at its distal end by a helical anchoring screw  14  made of a conductive material, connected through a metal tip  16  to an inner conductor  18  such as a spiral conductor providing electrical continuity between the anchoring screw  14  that is an electrode for collection and stimulation and a generator located at the proximal end (not shown) of the lead  10 . It should be understood that lead body  12  made of a polyurethane sheath provides rigidity in torsion to transmit torque from the proximal end to the distal end and to rotate the screw  14  to make it penetrate into the heart tissue. 
         [0043]    The conductive material of the screw is advantageously a NiTi alloy (nitinol), which has a capacity to transmit a sufficient torque for the intended application. The main advantage of this material is its extreme fatigue endurance. The disadvantage of nitinol is its relative high electrical resistance, but this drawback may be compensated by a bi-material structure comprising a silver core (for electrical conductivity) wrapped or coated by nitinol (for properties of resistance to mechanical stress) such that the less mechanically tough material (silver) is encapsulated in a sheath of nitinol. 
         [0044]    For bipolar stimulation, the lead is also provided with a ring electrode  20  connected to the generator by a separate conductor (not shown) in a manner well known to persons of ordinary shell in the art. 
         [0045]    The anchoring screw  14  is advantageously carried out with a distal portion  22  formed of non-touching turns over a length of about 1.5 to 2 mm. The distal portion  22  is connected to the lead body  12  via a mechanical transition portion  24  having flexibility in flexion, for example, a part formed by adjacent turns in the absence of stress of the screw. 
         [0046]    The purpose of this transition portion  24  is to introduce between the screw  14  itself (the part that will penetrate into the tissues) and the lead body  12  an elastic function to limit the mechanical action of the distal part of the lead  10  on the cardiac tissues and/or the veins. Advantageously, this elastic feature (i) does not alter the torque transmission between the lead body  12  and the screw  14  under the two aspects of efficacy and safety (coring effect); (ii) does not alter the transmission of the electrical pulse, and (iii) is extremely resistant to flexion/compression events. 
         [0047]    On the other hand, the screw  14  is advantageously insulated over its length, for example, by a coating of parylene, except on the last millimeter of the distal part, which is the only electrically active part of the screw  14 . This structure reduces the stimulation surface and thereby reduces the risk of phrenic nerve stimulation. This electrically active part will also be buried deep into the wall of the epicardium, thereby concentrating the electric flow to the target tissue and stimulate a deep and more physiological zone. 
         [0048]    A long screw (in the order of 10-15 mm) penetrates deeply into the ventricular wall and performs a localized endocardial stimulation ensuring during stimulation a faster wave of depolarization from the endocardium to the epicardium. To avoid the risk of coring, the internal lumen of the flexible portion of the screw  14  is equipped with a silicone cartridge (possibly filled with a steroid) to maximize the effect of abutment to the transmission of torque. 
         [0049]    In the illustrated drawings, the screw  14  is an active screw playing (at least at its distal end) the role of a stimulation electrode. 
         [0050]    Alternatively, the screw  14  is an electrically passive screw used for anchoring the lead  10  against the wall of the epicardium. The lead  10  may be provided at its end with a distal electrode in the shape of a ring electrode, a second electrode, or other suitable configurations. 
         [0051]    For the implantation of the lead  10  in the chosen stimulation site, in accordance with a preferred embodiment of the present invention, a guide catheter  26  with a double curvature is used. With reference to  FIG. 2 , the guide catheter  26  is illustrated with the lead  10  inserted inside. The distal portion  28  of the guide catheter  26  is open at its end, so as to bring out the distal end of the lead  10  and its anchoring screws  14  by relative axial movement of the lead body  12  inside the guide catheter  26 . 
         [0052]    In addition, a stylet  30  is inserted inside an inner lumen  32  of the lead  10  so as to stiffen it and straighten the natural curvature of the catheter  26  by more or less axially sliding the stylet  30  within the lead  10 . 
         [0053]    As indicated above, the catheter  26  has at its distal end  28  a double curvature, each curvature being inscribed in a separate area  38  and  40 . The curved surface  38  is an orientation curvature that follows the natural curvature of the coronary veins during the progression of the catheter  26  into the coronary sinus, while the curved surface  40  is a curve for supporting the orientation of the distal portion of the lead  10  once the site of stimulation is reached. More specifically, this supporting curvature  40  has the effect of directing the axis of the anchoring screw  14  not in line with the target vein  34 , but instead, as shown in  FIG. 3  (after removal of the catheter  26 ), to the wall facing the epicardium  36  of the target vein. According to this method, the anchoring of the screw  14  and the subsequent screwing of the screw  14  along a direction D makes an angle a with the axial general direction of the lead  10 , roughly corresponding to the direction of progression of target vein  34 . 
         [0054]    The implantation procedure of the screw  14  at the selected site will now be described. This procedure is described in its most complete aspect, but it should be understood by persons of ordinary skill in the art that certain steps or the use of certain elements may be omitted, adapted, or modified without deviating from the scope of the present invention. 
         [0055]    According to one embodiment, a technique called OTW (Over-The-Wire) is used. OTW involves introducing into the coronary sinus and then in the coronary network a very thin guide wire provided at its distal end with a flexible termination that is not traumatic. Previously, the practitioner arranges a main catheter to reach the outlet of the coronary sinus, insert the guide wire into the catheter, and pushed into the coronary venous system. 
         [0056]    The practitioner inserts the guide catheter  26  according to the invention. The guide catheter  26  may be used as a sub-selection catheter to choose, under fluoroscopy, the path of the venous network that will allow reaching the target vein corresponding to the chosen stimulation site. The self-orientation of the catheter  26  during this phase of positioning results from the principle of least energy, especially during the “left turn” in the area of intersection of the great cardiac vein and a lateral vein. 
         [0057]    The self-orientation of the orientation curve locally generates enough torque to force the orientation of the support curvature in the desired position. The predominance of the torque effect of the orientation curvature relatively to the support curvature is related to the fact that, in terms of dimensions:
       the radius of curvature of orientation is greater than the support radius curvature, and   the length of the orientation curvature is greater than the support curvature length.       
 
         [0060]    The distance between the area generating the drive torque (orientation curvature) and the area to be controlled (support curvature) is thus extremely limited (a few millimeters). 
         [0061]    It should be noted that the present invention requires a single catheter to determine the relative positions of the two curvatures. 
         [0062]    Thus, as stated above, the lead solution of the present invention is not operator-dependent: the practitioner just pushes catheter into the target vein for the catheter  26  to self-orient in the desired position after a few beats, frees both hands for the screwing operation, while ensuring a secure and accurate positioning of the screw  14  and holding it in this position during the screwing phase that follows. 
         [0063]    According to one embodiment, a hollow dilator catheter is used in the sub-region to establish a gradual transition between the guide wire and the tip of the sub-catheter, particularly to prevent the catheter tip, while sliding on the guide wire, from crashing against the wall of the vein, for example, where a curvature is met at a blocking point in the course of traversing the tortuous vein. This dilator catheter is advantageously preformed in its distal part to facilitate cannulation of the lateral veins. This option allows to compensate the self-positioning of the sub-catheter into the left atrium as it moves into the large vein (because of the principle of least energy, but implemented in the great cardiac vein). This behavior of the dilator catheter enables placement of a left atrial lead even in the proximal part of the great cardiac vein. 
         [0064]    Once the desired stimulation site is reached, the practitioner slides the lead  10  inside the catheter  26  until the distal end of the lead  10  and its anchoring screws  14  emerge from the corresponding end of the catheter  26  (configuration illustrated in  FIG. 2 ). Because of the double curvature described above, the distal end of catheter  26 , the distal orifice of the catheter  26  is directed toward the epicardium  36  making an angle a compared to the general direction of the target vein. The catheter  26  is introduced into the lead  10  until it emerges from the catheter housing and its anchoring screw  14  comes in contact with the epicardium. An initial mapping is conducted to electrically test the contact point(s) and validate the chosen stimulation site. If the position is not satisfactory, the practitioner moves the catheter along the vein and test a new site until a suitable location is found. 
         [0065]    The final anchoring is obtained by imparting an axial rotation to the lead body in the case of a fixed screw lead. For a pin-driven lead, the axial rotation is imparted to the connector plug, where at the proximal side the connection plug is secured to a conductor extending axially within the lead body. The surgeon holds in one hand the proximal end of the lead body and turns a pin at the proximal end with the other hand, directly or through the intermediary of a tool. The pin is secured to an axial conductor extending within the lead body, and this conductor is free in rotation and is connected at its distal end to the connector plug deployment mechanism of the screw. Another possibility is to introduce a specific screwing stylet into the lumen  32  of the lead body, especially in case when the sheath does not have a sufficient torsional rigidity to drive the screw directly from the proximal end. 
         [0066]    In an alternative embodiment, a retractable screw is used instead of a fixed screw. In this case, the rotation first deploys the screw out of its slot and subsequently penetrates the screw into the wall of the epicardium. 
         [0067]    After the screw  14  is anchored into the wall of epicardium, the catheter  26  is removed. The removal of the catheter  26  is performed according to a standard procedure of cutting through a slitter tool, as described, for example in, EP 2039390 A1 and its US counterpart U.S. Published Application 2009/0071012 (Sorin CRM S.A.S. formerly known as ELA Medical). 
         [0068]    The final and definitive installed configuration is illustrated in  FIG. 3 . 
         [0069]    Advantages of using the present invention include the following improvements over the prior known coronary venous leads:
       the quality of the fixation for anchoring screws;   the stability of the electrical contact with the tissue regardless of the size of the vein;   the ability to map large portions of the vein before the final fixation;   the possibility to expand the exploitable part of the vein, particularly towards the proximal part of the venous system, known to be the least exposed to the risk of phrenic nerve stimulation, but having with traditional leads, the disadvantage of a lower stability due to a larger diameter;   the concentration of the electrical stimulation in a deep region of the epicardium, decreasing the risk of phrenic nerve stimulation;   the improved extraction capacity, by a simple unscrewing of the distal end;   the mechanical simplicity of the system, a low manufacturing cost and a high reliability.       
 
         [0077]    One skilled in the art will appreciate that the present invention can be practiced by other than the embodiments disclosed herein, which are provided for purposes of illustration and not of limitation.