Abstract:
A sheath is provided to introduce electrophysiology catheters into the heart. The sheath has controlled geometric and physical properties and may be used to control ablation therapy.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electrophysiology equipment and more particularly to an apparatus and method for using the apparatus for positioning electrophysiology instruments within the heart of a patient. 
     BACKGROUND OF THE INVENTION 
     Electrophysiologic mapping and cardiac ablation are two procedures which are typically performed inside of a beating heart. The mapping procedure is diagnostic and it is intended to reveal the location of regions of ectopic electrical activity within the heart which can give rise to tachyarrhythmias. Once an ectopic site has been localized, it is common to ablate tissue in that region of the heart to prevent conduction of electrical signals in that portion of the cardiac tissue. The ablation of tissue is a therapeutic and has been demonstrated to eliminate some tachyarrythmias. 
     Both brachial and femoral approaches to the interior of the heart are commonly used to introduce catheters. Approaching the heart through the thorax is not widely employed at the present time. Consequently the ability to quickly and reliably exchange instruments within the heart chamber is desirable. However it is difficult to exchange diagnostic and therapeutic catheters given current therapeutic approaches. 
     SUMMARY OF THE INVENTION 
     The present invention includes an over-tube or sheath which is guided into a heart chamber by a steerable catheter. This sheath is left in place in the heart during the procedures and it is used to permit the rapid exchange of diagnostic and therapeutic catheters through the central lumen of the sheath. The physical and mechanical properties as well as the geometry of the sheath are important properties of the device. 
     In some embodiments of the sheath a lateral window is provided in the distal tip to facilitate the creation of certain types of ablation lesions. Additionally a distal foot member may be used with the sheath. This distal foot may be retracted or articulated to bring therapeutic catheters into contact with the heart wall and to permit the formation of multiple lesions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structures shown in the various figures are illustrative and exemplary and should not be regarded as limiting the form or appearance of the invention. Throughout the several figures identical reference numerals depict identical structure wherein: 
     FIG. 1 is a schematic representation of the ablation sheath in use; 
     FIG. 2 is a schematic diagram of the ablation sheath juxtaposed with an ablation catheter; 
     FIG. 3A is a schematic diagram of the ablation catheter within the ablation sheath; 
     FIG. 3B is a schematic diagram of the ablation catheter within the ablation sheath; 
     FIG. 4 is schematic diagram of an alternate embodiment of the ablation sheath proximate cardiac tissue; 
     FIG. 5A is a schematic diagram showing relative sheath and electrode locations for ablation; 
     FIG. 5B is a schematic diagram showing relative sheath and electrode locations for ablation; 
     FIG. 5C is a schematic diagram showing relative sheath and electrode locations for ablation. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic view which shows a patient heart  12  in isolation. The ablation sheath  10  has been inserted into a peripheral vessel  14  and advanced into a heart chamber  16  such as a ventricle. In the drawing a companion ablation catheter  18  has been advanced through the ablation sheath  10  and moved into contact with the cardiac tissue at site  22 . This drawing shows the proximal end  28  of the ablation sheath  10  located outside the patients body where it may be manipulated by an attending physician. Similarly the proximal end  26  of the ablation catheter  18  is located outside the patients body for manipulation by the attending physician. It is generally preferable to use ablation catheters with a diameter less than about ten French and preferably equal to about seven French. The perfusion apparatus  34  is also coupled to the ablation sheath  10  and operated by the physician. In use the ablation sheath is advanced into position in the heart by advancing the ablation sheath over a steerable catheter. In this context the term steerable requires both deflectability and torqueability. It is expected that an ablation catheter will be used for this portion of the procedure however alternative catheter structures may be used. 
     FIG. 2 shows the companion ablation catheter  18  withdrawn from the central lumen  32  of the ablation sheath  10 . This figure also shows the illustrative perfusion apparatus  34  in more detail. The perfusion apparatus  34  includes a reservoir  35 , along with a suitable pump  38  and tubing  40 . When used for therapeutic ablation it is expected that the physician will want to perfuse the central lumen  32  of the ablation sheath  10  with a fluid  36  such as normal saline (NaCl) or the like. As seen in the figure the reservoir  35  holds the perfusion fluid  36  and is coupled to a suitable pump  38  which supplies the solution though suitable tubing  40  connected to the central lumen  32  through an off-axis sidearm. 
     In FIG. 3 the total length of the ablation sheath  10  is shown in the diagram by length  42 . In general, the ablation sheath will be one meter in length, or longer. The body  44  of the sheath is divided into two distinct regions, which are shown in the figure as distal tip portion  46  and proximal body section  48 . 
     In all embodiments the distal tip portion  46  will be short and on the order of two to nine inches depicted in the FIG. 2 by length  45 . For lengths shorter than about nine inches it is critical that this distal tip portion  46  be substantially more flexible than the proximal body section  48 . Typically, the proximal body section  48  will include a woven reinforcing braid which permits the structure to transmit torque, which is important for guiding and positioning the ablation sheath  10  within the patient&#39;s heart. The distal tip section should exhibit a moment (or stiffness) “E*I”, which is less than 0.2 lbs-inch squared. Although any of a variety of materials can be utilized to fabricate both the proximal section  48  and the distal tip portion  46 , it is preferred to have the outer body diameter constant throughout the length of the ablation sheath  10 . It is also considered advantageous to have a constant diameter inner central lumen  32 . To achieve the required variation in stiffness, it is preferred to adopt a material such as Pebax (a nylon-acetal material) for the distal tip portion  46  and to reinforce this material with a fabric represented in the figure by fibers typified by fiber  47 , for the proximal section  48 . 
     As shown in FIG. 2, saline solution can emerge from the intersticial area  52  between the ablation catheter  18  and the distal opening  51  of the ablation sheath  10 . This on-axis distal opening  51  is indicated in the figure near the flow arrow  50  which illustrates fluid flow through the intersticial area  52 . It is preferred to have the distal opening have a circular cross section. 
     As seen in FIG. 3A it may be useful to insert a incremental motion catheter  52  into the central lumen  32  the ablation sheath  10 . It should be appreciated that an additional lumen dedicated to this incremental motion catheter is contemplated as well. This incremental motion catheter  52  may take the form of a 0.010 inch diameter wire which includes a blunt distal loop  54 , which forms a foot member that can be urged into position against cardiac tissue to control the motion of the ablation catheter  18  and ablation sheath  10  by sequentially advancing and retracting the incremental motion catheter  52  in and out of the sheath opening  51 . The tip of the ablation catheter  18  may be reliably and accurately moved along a cardiac surface by manipulating the deflectable ablation catheter in conjunction with incremental motion catheter  52 . 
     FIG. 3B shows the companion incremental motion catheter  52  in isolation juxtaposed to the ablation sheath  10 , which includes a ablation catheter  18  and a incremental motion catheter  52  in the operating position. Depending on the particular section of the heart to be ablated, the incremental motion catheter  52  may take various shapes and may be made of plastically deformable material such as stainless steel to facilitate reshaping. 
     FIG. 4 represents an alternate embodiment of the ablation sheath  10 , with a lateral window  56  positioned in the distal tip portion  46  of the ablation sheath body. This lateral window is cut into the side wall of the sheath so that an opening is formed which communicates with the central lumen  32 . In use, the lateral window directs the flow of saline fluid toward the cardiac tissue  58 . This window  56  also occludes or masks the electrode  59  at the distal tip of the ablation catheter  18 . By occluding the electrode and flooding it with saline the size of the lesion produced in the cardiac tissue  58  can be controlled. The ability to occlude a segment of the ablation catheter electrode by relative motion between the catheter and the ablation sheath  10 , as indicated by arrow  60 , is an important feature permitting the physician to control the therapy supplied through the ablation sheath  10 . 
     FIG. 5 includes several panels represents the use of the sheath in several configurations against a segment of cardiac tissue  58 . In this instance, the sheath has been steered and guided to have nearly perpendicular abutment with the interior surface of the heart wall. In panel A, the sheath has been retracted to permit the maximum flow of saline past the ablation electrode  59 , as indicated by flow arrow  64 . In panel B, sheath  10  has been abutted directly against the cardiac tissue  58 , and only a modest flow of saline indicated by flow arrow  62  can emerge from the sheath. In this instance, the fluid is primarily used to cool the electrode, where in panel A, the fluid flow is sufficiently removed from the electrode, and its primary function is to flood the ablation sight to exclude blood from the electrode surface. In panel C, the distal tip portion  46  has been abutted against the cardiac tissue  58 , and the ablation electrode  59  has been withdrawn several millimeters from the tissue by relative motion along path arrow  66 . In this instance, the saline solution within the distal volume of central lumen  32  acts as the electrode conducting radio frequency ablation emerging directly from the electrode to the cardiac tissue  58 . In this fashion, the sheath in combination with an ablation electrode can be used to ablate tissue by contact with a metallic electrode, as shown in panel B, or saline electrode, as shown in panel C. In a similar fashion, the metal electrode can be cooled by the saline, or saline can be used to exclude blood from the ablation sight, as seen in panel A. 
     The demands of the sheath, presented by panel FIG. 5C, are at odds with other criteria for the sheath. However, it has been found that the ablation sheath  10  operates optimally in a critical fashion with stiffness values below 0.2 lbs-inch squared. 
     The preferred method of performing a measurement to determine the distal stiffness of the ablation sheath maybe determined by supporting a section of the sheath of length L from a cantilever support, and applying a small weight (1.07 grams) to the distal tip. In this instance, a deflection from approximately 1.1 ⅛ millimeters should occur for a length of L=39 millimeters. Stiffness may be computed from this arrangement through the expression EI=W.L 3 /3*D. Measurements of this type are difficult to make, since the thinness of the tubing can cause it to buckle. Consequently, measurements need to be made with weights, selected such that the deflection angle is less than 3°. The measurements made during the course of developing exemplary versions of the catheter have an accuracy of approximately plus, or minus 5%. Although the preferred sheath construction includes a distal segment with reduced stiffness, as compared to the body section, there are a number of mechanisms which be used to achieve this result. The preferred method is to introduce reinforcing structure in the proximal body section to improve its “torque ability”. However, material selection alone can be used to control the relative stiffness. Additionally, it should be recognized that both tapered lumens and tapered outer sheath diameters can also be utilized to provide the relative change in stiffness. 
     Although the invention has been described in connection with the preferred embodiment, and certain variations on the preferred embodiment, it should be recognized that alternate geometries and structures can be used to carry out the invention.