A wide variety of elongated medical instruments are currently available that are adapted to be permanently or temporarily implanted in the mammalian body, usually the body of a human patient, or used to access a site in the body to facilitate introduction of a further implantable medical device or delivery of a therapeutic or diagnostic agent. Such elongated medical instruments have an instrument body extending between instrument body proximal and distal ends, and a distal segment of the instrument body is advanced to a remote site in the body.
In many cases, the introduction of such elongated medical instruments to a remote site in the body is effected through a skin incision accessing an incision into a blood vessel, whereby the instrument body is advanced through a pathway until the distal segment or the instrument body distal end are located at the remote site. Such advancement is often through a tortuous pathway having twists and turns requiring the capability to impart a curve or deflect the instrument body distal end to facilitate advancement. Therefore, the introduction of such elongated medical instruments through vascular pathways or other tortuous pathways in the body is facilitated by a wide variety of techniques and mechanisms that have been developed to impart curves in the distal segment of the instrument body or to deflect or steer the instrument body distal end.
Currently, endocardial cardiac pacing leads used in association with a pacemaker implantable pulse generator (IPG) and/or cardioversion/defibrillation leads used in association with implantable cardioverter/defibrillator (ICD) IPGs are introduced into a vein either via a cut down or percutaneous sheath introduction. Such cardiac leads are advanced under fluoroscopy into either the right atrium, right ventricle (or both in the case of a dual chamber pacemaker or ICD implantation) or into a cardiac vessel, e.g., the coronary sinus and great vein. Generally speaking, it is highly desirable that such cardiac leads be so flexible through their length that they are capable of flexing with the movement of the heart and other muscular movement so as to void the fracture of the lead body due to its cumulative stressing. Such cardiac lead bodies are generally too limp to be advanced axially on their own through the vascular pathway to the desired site in a heart chamber or vessel.
It has been commonplace for many years to employ thin wire, stiffening stylets extended down a lumen of the lead body to stiffen the entire assembly so that it can be pushed axially through the venous pathway. Then, the distal pace/sense electrodes or cardioversion/defibrillation electrodes (herein “cardiac electrodes”) must be fixed at the preferred site in the heart chamber or vessel to operate most efficaciously and to prevent dislodgement. The introduction and fixation of these cardiac leads is the most time consuming and difficult aspect of the implantation.
At the outset, a straight or slightly curved stiffening stylet wire is first extended into the lead body lumen within the cardiac lead in order to give the cardiac lead sufficient column strength and rigidity to be pushed through the tributary veins and typically into the subclavian vein. The stylet wire may be left straight or provided with a certain degree of curvature to facilitate the introduction through these veins and through the initial curvatures thereof. Thereafter, and from time to time, as the physician directs the distal tip of the cardiac lead in a tortuous path leading to the right heart through the superior vena cava (SVC), it may be necessary to withdraw the stylet and either substitute a new stylet wire or impart a different curvature to the distal portion of the stylet wire, reinsert the stylet wire, and advance the distal portion of the lead a bit further until another obstacle to advancement is encountered.
When the distal cardiac electrodes are to be placed in the right ventricle, the physician manually fashions a curve at the tip of another stylet wire that is inserted into the lead body lumen to advance the assembly through the tricuspid valve into the right ventricle. Most physicians continue advancing the lead with the curved tip stylet in place into the pulmonary artery outflow track to confirm right ventricle access and to rule out the possibility of entrance into the coronary sinus or coronary vein, which can mimic the appearance of a right ventricle placement under fluoroscopy. The conventional practice requires the physician to then remove the curved stylet and partially re-advance the original or another straight stylet into the lead body lumen, once the physician has confirmed that the lead is in fact in the pulmonary outflow track. The cardiac lead is then carefully pulled back under direct fluoroscopic observation until the lead body distal segment drops from the proximal portion of the pulmonary artery to the floor of the right ventricle. The physician then advances the stylet to its fully advanced position within the lead body lumen and advances the lead distal end into the right ventricular apex. Passive or active fixation mechanisms at the lead body distal end then effect fixation with the trabeculae or the myocardium to acutely maintain the cardiac electrode electrode(s) at the operative site.
In the case of atrial lead placement, the lead body distal end is typically lodged or affixed in the right atrial appendage which results in the lead body extending into the right atrium via the SVC and then bent through about a 180° or greater bend. Over the years, many atrial cardiac lead designs and atrial cardiac lead introduction tools and techniques have been proposed or clinically used to both achieve this orientation and to fix the cardiac lead body distal end within the atrial appendage and avoid dislodgement. Initially, such atrial cardiac leads were formed with a permanent “J”-shaped bend to facilitate both the positioning and the retention of the atrial electrode in the patient's atrial appendage as taught, for example, in U.S. Pat. No. 4,136,703. Insertion of these “J”-shaped leads is greatly facilitated through the use of a straight solid inner stylet which, in this case, straightens the bend normally fixed within the distal end of the lead itself to the extent that the stylet is advanced into or retracted from the lead body lumen. Such J-shaped atrial leads have largely been abandoned in favor of reduced diameter lead bodies that cannot accommodate shape-forming structures and the use of the straightening stylet as described above. Today, the small diameter cardiac lead body is normally straight, and the lead body distal end is typically aimed into the atrial appendage employing multiple insertions of relatively straight and curved stylets. The electrode bearing lead body distal end is fixed in the atrial appendage by means of an active fixation screw or passive fixation tines. However, dislodgements can occur before the fixation is effected when a stylet is withdrawn proximally as the stylet may bind against the lead body lumen in the region of the bend.
Similar techniques and multiple stylets are avoided to advance a cardiac lead distal segment into the coronary sinus and great vein.
Thus, there are multiple exchanges of straight stylet wires and curved stylet wires which have been bent according to the physician's choice in a typical cardiac lead implantation in the right atrium and ventricle. Stylets are typically formed of solid wire, typically about 0.014-0.018 inches in diameter. During handing, such stylets can easily become bent or kinked, and thereafter cause great difficulty when an attempt is made to reinsert them through the narrow inner diameter of the lead body lumen, which may only be 0.019 inch in the case of a stylet of 0.018 inch diameter, thereby providing no more than 0.0005 inch clearance around the circumference. The continual withdrawal and reintroduction of stylets is time consuming and offers the potential of damaging the lead in the process.
Moreover, it is undesirable to contaminate the lead body lumen with blood during this process because drying blood can form a strong adhesive bond between the stylet and the lumen wall, making stylet removal impossible and rendering the lead unusable. Because the surgeon is working through an open wound, even the most fastidious surgeon will have blood on his gloves that can be transferred to the stylet. The blood congeals, and because of the small clearance, even a few drops of blood are sufficient to causing jamming of the stylet inside the lead body lumen. When the stylet jams in the lead body lumen, kinking of the stylet within the lead can occur, which kinks, in turn, will create new jams or problems with the insertion and retraction of the stylet from the lead body lumen. In some cases, the jamming is so severe that the cardiac lead must be removed from the heart for fear of insulation puncture, discarded, and a new lead implanted, thereby at least doubling the lead cost used in the procedure as well as operative time. The overall result of such difficulties is that operative time is greatly increased, which results in increased time delay, associated cost, and prolonged X-ray exposure to the patient under continuous fluoroscopy as well as prolonged scattered X-ray exposure to the operating room staff due to procedural time delays. These problems with the use of multiple stiffening stylets have been recognized in the art and many proposals have therefore been advanced to reduce the number of stylets and the consequent number of times that stylet removal and re-insertion are needed.
In addition, the complexity of cardiac leads, the number of cardiac leads implanted in a common path, and the advancement of coronary sinus leads deep in a coronary vein have led to efforts to at least not increase and optimally to decrease the overall diameter of the cardiac lead body without sacrificing reliability and usability. More recently, it has been proposed to diminish the lead body by eliminating the lumen for receiving the stiffening stylet and by reducing the gauge and coil diameter of the coiled wire conductor or replacing it with highly conductive stranded filament wires or cables. In bipolar or multi-polar leads, each such cable extends through a separate lumen of the lead body to maintain electrical isolation.
Over the last 30 years, it has become possible to reduce endocardial lead body diameters from 10 to 12 French (3.3 to 4.0 mm) down to 2 French (0.66 mm) presently through a variety of improvements in conductor and insulator materials and manufacturing techniques. The lead bodies of such small diameter, 2 French, endocardial leads must possess little if any column strength that could cause the lead distal end fixation mechanism and electrode to perforate through the myocardium during implantation and if the lead body were to become axially force-loaded during chronic implantation. As a result, the small diameter lead bodies lack “pushability”, that is the ability to advance the lead distal end axially when the lead proximal end is pushed axially, particularly when the lead body extends through the tortuous transvenous pathway.
Commonly assigned U.S. Pat. Nos. 6,280,433 and 6,379,346 disclose steerable catheters that are employed to access a blood vessel through a percutaneous incision and to be advanced to a site within the vascular system or a heart chamber so that such a small diameter cardiac lead can be implanted through a delivery lumen of the catheter. A bilumen catheter body is disclosed that includes a relatively large diameter delivery lumen and a smaller diameter stylet lumen that is blocked at its distal end. The deflection mechanism in this case includes a stiffening stylet that can be selectively introduced into and removed from the stylet lumen from a proximal hub or handle. The stiffening stylet is advanced distally until the stylet distal end abuts the closed stylet lumen distal end to stiffen the catheter body to aid its introduction and advancement. The stylet distal end can be shaped when outside the stylet lumen opening to impart a curve to the catheter body when inserted into the lumen to assist in steering the catheter body distal end through the pathway. The stylet lumen is preferably lined with a wire coil sheath, and the handle and delivery lumen are preferably slittable by a slitting tool to aid in removing the introducer catheter from an electrical medical lead introduced through the delivery lumen. The delivery lumen exit port and the closed end of the stylet lumen are both located at the bitumen catheter body distal end.
A variety of deflectable or steerable stylets have been proposed and in some cases clinically introduced to aid in direct implantation of a cardiac lead having a lead lumen or to aid in the deflection and steering of a bilumen guide catheter. One approach has been to employ deflectable stylets wherein the stylet distal segment can be deflected or curved while within the lead body lumen from the proximal end thereof. Two-piece stylets that include a straight, tubular outer member and a curved inner member received within the outer member lumen enabling relative movement of the inner and outer members are disclosed in U.S. Pat. Nos. 4,136,703, 4,381,013 and 5,728,148. The outer tubular member of the '013 patent enables the transmission of torque applied by the implanting physician at the proximal end to be transmitted to a fixation helix located at the lead body distal end lead to screwed the helix into endocardial tissue. Alternatively, two-piece stylets comprising a curved outer member and a relatively straight inner member are also known to the art, as disclosed in U.S. Pat. Nos. 4,676,249 and 5,040,543. In such composite stylets, the relative position of the inner member with respect to the outer member determines the degree to which the curved member (inner or outer) is allowed to display its preset curvature.
A common approach to providing controllable deflection of the distal end segments of catheters, guidewires, and stylets employs a generally straight outer sheath or tube and a pull or push or push-pull wire extending through a lumen of the outer sheath to an attachment point at the sheath distal end. The wire is pushed or pulled on at its proximal end typically through a handle that is permanently or removably attached to the catheter or guidewire proximal end. The proximal retraction or distal advancement of the pull or push wire, respectively, causes at least a distal segment of the outer sheath to bend or deflect. Examples of such deflection mechanisms in catheters can be found in U.S. Pat. Nos. 4,815,478, 4,898,577, 4,940,062, 5,545,200 and 6,251,092. U.S. Pat. Nos. 4,815,478 and 4,940,062 disclose the use of push-pull wires extending through guidewire lumens for deflecting a guidewire distal end by manipulating a handle at the guidewire proximal end.
Deflectable stylets intended to be inserted into cardiac lead body lumens employing this type of deflection mechanism are disclosed in U.S. Pat. Nos. 5,170,787, 5,327,906, 5,439,006, 5,662,119, 6,027,462, 6,059,739, and 6,146,338. Such deflectable stylets include an elongated stylet body or tube extending from a stylet body proximal end to a stylet body distal end, a handle coupled to the stylet body proximal end, and a traction wire or pull wire extending through a stylet lumen of the stylet body of tube from the handle to the stylet body distal end. Many of these patents disclose steerable stylet handles at the stylet body proximal end that are manipulated by one hand operation to induce a bend in a distal segment of the stylet body.
Several embodiments of deflectable stylets are disclosed in the '119 patent that employ an elongated metal tube having a stylet tube lumen through which a traction or pull wire extends to a distal wire end. In each case, a distal portion of the pull wire is exposed along a like distal portion of the metal tube in such a way that traction applied to the pull wire proximal end causes the distal portion of the tube to bow or bend. The exposure of the distal portion of the pull wire extending alongside the metal tube is created in several ways.
In one embodiment depicted in FIGS. 1 and 2 (FIGS. 12 and 13 of the '119 patent), an elongated cutaway portion 11′ of the tube wall 15′ of tube 10′ is formed extending between cuts 18′ and 20′ proximal to tube distal end 22′. The cutaway portion 11′ extends about half way through the tube wall 15′ exposing a distal portion of the pull wire 12′ extending proximally and distally through the tube lumen 13′. The pull wire 12′ terminates in ball-shaped distal end element 23 having a diameter greater than the inside diameter of the tube lumen 13′ that bears against the tube distal end 22′ when pulled proximally.
In another embodiment depicted in FIG. 3 (FIG. 8 of the '119 patent), one side of the tube wall 15 is indented against the other side of the tube wall 15 between two openings 18 and 20 proximal to the tube distal end 22 to form a bendable indented portion 11 of the tube 10. The pull wire 12 is threaded out of the tube lumen 13 through the proximal opening 18 and back into the tube lumen 13 through the distal opening 20, whereby a distal portion of the pull wire 12 is exposed extending alongside the indented portion of the tube wall 15. The distal end of the pull wire 12 is crimped to the tube distal end 22 in this case.
The tube 10 is preferably formed of stainless steel hypodermic needle tubing having an outside diameter in the range of 0.012 to 0.016 inches. Pull wire 12 is preferably formed of high tensile strength stainless steel wire having a diameter in the range of 0.005 to 0.007 inches. The distance between the proximal openings 18, 18′ and the distal openings 20, 20′ is stated to be between 2 to 4 inches.
In a variation of these embodiments, a tubular retainer formed of a thin tube of polyimide, stainless steel or Nitinol is fitted over the tube 10 to extend across the indented portion 11 and the cutaway portion 11′ to restrict the outward movement of the pull wire 12 when it is tensioned, which could exert excessive friction against the coiled conductors defining the cardiac lead lumen that the tube 10 is inserted into.
In use, the tube 10 is normally relatively straight when the pull wire 12 is relaxed. A curve or bend is effected in the distal portion of the tube 10 at the indented portion 11 or the cutaway portion 11′ when the pull wire 12 is pulled proximally. The induced curve or bend stresses the tube wall 15 along the indented portion 11 and the cutaway portion 11′. It is stated in the '119 patent that prototypes of the cutaway tube embodiment of FIGS. 1 and 2 were found in testing to be inferior to the indented tube embodiment of FIG. 3 in resistance to kinking, and that the indented embodiment of FIG. 3 was easier to manufacture than the cutaway embodiment of FIGS. 1 and 2.
However, It can be difficult to indent thin wall stainless steel or shape memory alloy tubes to achieve the uniform indentation depicted in the '119 patent. It can also be difficult to precisely form uniform diameter proximal and distal holes as depicted in the '119 patent. Poor torque transmission and uneven bending characteristics can result particularly at the proximal end of the indented portion 11 that is weakened both by indentation and formation of the hole through the tube wall.
Thus, despite these improvements, there is still a perceived need for steerable stylet having a small diameter stylet body that is simple and inexpensive to manufacture, resists kinking, and that can be manipulated to control the deflection of and impart a wide degree of dynamic curvature in a distal segment of the stylet body.