Abstract:
Disclosed is an expandable transluminal sheath, for introduction into the body while in a first, low cross-sectional area configuration, and subsequent expansion of at least a part of the distal end of the sheath to a second, enlarged cross-sectional configuration. The sheath is configured for use in the vascular system. The access route is through the inferior vena cava to the right atrium, where a trans-septal puncture, followed by advancement of the catheter is completed. The distal end of the sheath is maintained in the first, low cross-sectional configuration during advancement through the atrial septum into the left atrium. The distal end of the sheath is expanded using a radial dilator. In one application, the sheath is utilized to provide access for a diagnostic or therapeutic procedure such as electrophysiological mapping of the heart, radio-frequency ablation of left atrial tissue, placement of atrial implants, valve repair, or the like.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority to U.S. application Ser. No. 60/709,240, filed Aug. 18, 2005, U.S. application Ser. No. 60/674,226, filed Apr. 22, 2005, U.S. application Ser. No. 60/660,512, filed on Mar. 9, 2005, and U.S. Provisional Application Ser. No. 60/608,355, filed on Sep. 9, 2004, the entirety of which are hereby incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to medical devices and, more particularly, to methods and devices for accessing the cardiovascular system.  
         [0004]     2. Description of the Related Art  
         [0005]     A wide variety of diagnostic or therapeutic procedures involves the introduction of a device into the vasculature through a percutaneous incision at an access site. Such regions of the vasculature, preferred for access, include both the arteries and veins, typically at peripheral locations in the body. Typical access sites include the jugular vein, the subclavian artery, the subclavian vein, the brachial artery, the femoral arteries and the femoral veins. Techniques commonly known for such vascular access include the Seldinger technique. The Seldinger technique involves using a hollow needle to puncture the skin and gain access to the selected artery or vein. A guidewire is next placed through the hollow needle into the selected region of vasculature. The guidewire may be advanced to a target location in the vasculature, often more than 100 cm away from the access site. The needle is removed and a tapered dilator with a sheath and a central lumen in the dilator is advanced over the guidewire into the vasculature. The dilator is next removed and a guide catheter is advanced through the sheath over the guidewire. The guide catheter can be advanced all the way, or part way, to the target site. The guide catheter, following, or without, removal of the guidewire can be used for directing therapeutic or diagnostic catheters to regions of the vasculature and central circulation, including external and internal structures of the heart. A general objective of access systems, which have been developed for this purpose, is to minimize the cross-sectional area of the access lumen, while maximizing the available space for the diagnostic or therapeutic catheter placement therethrough. These procedures are especially suited for coronary angioplasty, stent placement, cerebrovascular coil placement, diagnostic cardiac catheterization, and the like.  
         [0006]     Electrophysiology (EP) mapping and cardiac tissue ablation procedures are examples of diagnostic or therapeutic interventional procedures that are commonly performed on the heart. The procedure involves the steps of inserting a hollow needle, with a hemostasis valve affixed to its proximal end, into the femoral vein via a percutaneous puncture. A guidewire is next inserted through the hemostasis valve and the central lumen of the needle into the femoral vein. The guidewire is routed, under fluoroscopic control, cranially toward the heart until it reaches the right atrium via the inferior vena cava. The hollow needle is removed and a sheath with a tapered tip central obturator further including a central guidewire lumen, termed a dilator, is routed over the guidewire, through the skin puncture, through the wall of the femoral vein, and into the central lumen of the femoral vein. The central obturator or dilator is next removed. A Mullins catheter is next routed through the sheath, over the guidewire, and advanced to the right atrium. The guidewire is removed and a Brockenbrough™ (Trademark of C. R. Bard, Inc.)-type needle is inserted through the proximal end of the Mullins™ catheter and routed to the right atrium. The Mullins catheter is positioned, under fluoroscopic guidance, so that its distal end is located in the Foramenal valley, a feature in the septal wall of myocardium that divides the right atrium from the left atrium. The Foramenal valley is the remains of a communication between the right and left atrium, which exists prior to birth, but which closes following birth due to the pressures imposed by the beating heart of the newborn infant. The Brockenbrough needle is next advanced through the atrial septum in the general region of the Foramenal valley. The Mullins catheter is next advanced over the Brockenbrough needle until its distal end resides within the left atrium. Hemostatic valves at the proximal end of all hollow devices permit sealing around catheters and devices inserted therethrough with corresponding prevention or minimization of blood loss and the entry of air.  
         [0007]     The procedure continues with the Brockenbrough needle being withdrawn and replaced with a 0.032 to 0.038 inch diameter guidewire, generally of the stiff variety. This guidewire may have a bifurcated distal end to prevent inadvertent retraction once the guidewire has been advanced and expanded into the left atrium. The Mullins catheter is next withdrawn and replaced with a guide catheter having internal dimensions generally around 8 French and a tapered, removable obturator. The guide catheter is advanced into the right atrium and across the atrial septum, following which the obturator is removed. At this time, diagnostic and therapeutic catheters can be advanced into the left atrium so that appropriate EP mapping and ablation can occur. However, problems sometime arise, when trying to pass the guide catheter across the atrial septum, in that the tract generated by the Brockenbrough needle and Mullins catheter closes too tightly to allow passage of the guide catheter. At this point, a balloon catheter is advanced over the guidewire and through the guide catheter. The balloon catheter is advanced so that its dilatation balloon traverses the atrial septum. The balloon catheter is next inflated to stretch the tissues surrounding the atrial septal puncture. At this time, the guide catheter can have its dilator re-inserted and the entire assembly advanced over the guidewire through the atrial septum and into the left atrium.  
         [0008]     Current therapeutic techniques may involve advancing an EP mapping catheter through the guide catheter and positioning the EP mapping catheter at various locations within the left atrium. Electrocardiogram signals are sensed by the EP mapping catheter. These signals are conducted or transmitted from the distal tip to the proximal end over electrical lines routed along the length of the EP catheter. The signals are analyzed by equipment electrically connected to the proximal end of the EP mapping catheter. Catheter guidance is generally accomplished using X-ray fluoroscopy, ultrasound imaging such as ICE, TEE, and the like. Therapy generally involves radio-frequency (RF) electromagnetic wave generation by external equipment electrically connected to an EP therapeutic catheter. The EP therapeutic catheter is advanced into the left atrium into regions of foci of electrical interference of the hearts normal electrical conduction. Application of such radio-frequency energy at the tip of the EP therapeutic catheter, which is brought into contact with the myocardium, causes tissue ablation and the elimination of the sources of these spurious signals or re-entry waveforms. A primary area targeted for RF tissue ablation is the area surrounding the origin of the pulmonary veins. Often a ring-type electrode is beneficial in performing this procedure. Such tissue ablation can be performed using RF energy to generate heat, but it can also be performed using microwaves, Ohmic heating, high-intensity focused ultrasound (HIFU), or even cryogenic cooling. The cryogenic cooling may have certain advantages relative to heating methodologies in that tissue damage is lessened. Although a single atrial septal puncture may be adequate for electrophysiological mapping of the left atrium, therapeutic systems, including RF ablation devices often require that two atrial septal punctures be performed. A risk of atrial septal punctures includes potentially perforating the aorta, a high-pressure outlet line, which resides quite close to the atrial septum.  
         [0009]     Provision is generally made to deflect instrumentation through substantial angles, between 20 and 90 degrees, within the right atrium to gain access to the atrial septum from a catheter routed cranially within the inferior vena cava. To address this situation, the Brockenbrough needle, the Mullins catheter, or both, are substantially curved and significant skill is required, on the part of the cardiologist or electrophysiologist to negotiate the path to the atrial septum and into the left atrium.  
         [0010]     One of the primary issues that arise during electrophysiology procedures in the heart is the need to remove and replace multiple instruments multiple times, which is highly expensive and adds substantial time to the conduct of the procedure. A reduction in the number of catheter and guidewire passes and interchanges would reduce procedure time, reduce the risk of complications, improve patient outcomes, reduce procedural cost, and increase the number of cases that could be performed at a given catheterization lab. Current procedures involving multiple atrial septal penetrations would be reduced in frequency or become less time consuming and less risky if only a single atrial septal penetration was necessary. Additional benefit could be derived if larger catheters could be used, thus enabling the use of more sophisticated, powerful, and accurate instruments to improve patient outcomes. The limitations of current systems are accepted by physicians but the need for improved instrumentation is clear. Furthermore, placement of implants within the left atrium, such as the Atritech Watchman™ or the Microvena PLAATO™ would be facilitated if a larger working channel could be made available.  
         [0011]     Further reading related to the diagnosis and treatment of atrial fibrillation (AF) includes Hocini, M, et al., Techniques for Curative Treatment of Atrial Fibrillation, J. Cardiovasc Electrophysiol, 15(12): 1467-1471, 2004 and Pappone, C and Santinelli, V, The Who, What, Why, and How-to Guide for Circumferential Pulmonary Vein Ablation, J. Cardiovasc Electrophysiol, 15(10): 1226-1230, 2004. Further reading on RF ablation includes Chandrakantan, A, and Greenberg, M, Radiofrequency Catheter Ablation, eMedicine, topic 2957 Oct. 28, 2004. Further reading regarding catheter approaches to treating pathologies of the left atrium include Ross, et al, Transseptal Left Atrial Puncture; New Technique for the Measurement of Left Atrial Pressure in Man, Am J. Cardiol, 653-655, May 1959 and Changsheng M, et al., Transseptal Approach, an Indispensable Complement to Retrograde Aortic Approach for Radiofrequency Catheter Ablation of Left-Sided Accessory Pathways, J. HK Coll Cardiol, 3: 107-111,1995.  
         [0012]     A need, therefore, remains for improved access technology, which allows a device to be percutaneously or surgically introduced, endovascularly advanced to the right atrium, and enabled to cross the atrial septum by way of a myocardial puncture and Dotter-style follow-through. The device would further permit dilation of the myocardial puncture in the region of the atrial septum so that the sheath could pass relatively large diameter instruments or catheters, or multiple catheters through the same puncture. Such large dilations of the tissues of the atrial septum need to be performed in such a way that the residual defect is minimized when the device is removed. It would be beneficial if a cardiologist or hospital did not need to inventory and use a range of catheter diameters. It would be far more useful if one catheter or sheath diameter could fit the majority of patients or devices. Ideally, the catheter or sheath would be able to enter a vessel or body lumen with a diameter of 3 to 12 French or smaller, and be able to pass instruments through a central lumen that is 14 to 30 French. The sheath or catheter would be capable of gently dilating the atrial septum using radially outwardly directed force and of permitting the exchange of instrumentation therethrough without being removed from the body. The sheath or catheter would also be maximally visible under fluoroscopy and would be relatively inexpensive to manufacture. The sheath or catheter would be kink resistant, provide a stable or stiff platform for atrial septum penetration, and minimize abrasion and damage to instrumentation being passed therethrough. The sheath or catheter would further minimize the potential for injury to body lumen or cavity walls or surrounding structures. The sheath or catheter would further possess certain steering capabilities so that it could be negotiated through substantial curves or tortuosity and permit instrument movement within the sheath.  
       SUMMARY OF THE INVENTION  
       [0013]     Accordingly, an embodiment of the present invention comprises an expandable endovascular access sheath for providing minimally invasive access to a left atrium. An axially elongate sheath tube includes a proximal end, a distal end, and a central through lumen. The sheath has a distal region which is expandable in circumference in response to outward pressure applied therein. A hub is coupled to the proximal end of the sheath tube. The hub is configured to facilitate the passage of instrumentation. An obturator extends through the central lumen and is configured to occlude the central lumen of the sheath during insertion. The obturator comprises an obturator hub that is releasably coupled to the hub of the sheath. A guidewire lumen is within the obturator. The obturator is a balloon dilator capable of expanding the distal region of the sheath from a collapsed configuration to an expanded configuration.  
         [0014]     Another embodiment of the present invention is a method of instrumenting a left atrium of a patient. A guidewire is routed into the right atrium from a peripheral vein. A sheath is inserted with a collapsed distal region and a pre-inserted dilator into the patient over the guidewire. The sheath is advanced to a treatment or diagnostic site within the right atrium of the heart. A trans-septal puncture is made between the right and left atrium. The collapsed distal region is advanced through the puncture into the left atrium. The distal region of the sheath is dilated so that the distal region of the sheath is expanded. The dilator is collapsed and removed from the sheath. Instrumentation is inserted through the lumen of the sheath into the left atrium. A therapy or diagnosis procedure is performed with the instrumentation. The sheath is removed from the patient.  
         [0015]     Another embodiment of the invention is a sheath adapted for insertion into the right or left atrium of the heart. The sheath has a diametrically collapsed distal end and means for tracking the sheath over an already placed guidewire to a target treatment site in the right or left atrium of the heart. The sheath also includes means for articulating the distal end of the sheath, means for dilating at least a portion of the distal end of the sheath, and means for removal of the sheath from the patient.  
         [0016]     In another embodiment, a radially expanding access sheath is used to provide access to the left atrium by way of a tran&#39;s-septal puncture and advancement in the atrial septum dividing the right and left atriums. In an particular embodiment, the sheath can have an introduction outside diameter that ranges from 3 to 12 French with a preferred range of 5 to 10 French. The diameter of the sheath can be expandable to permit instruments ranging up to 30 French to pass therethrough, with a preferred range of between 3 and 20 French. The sheath can have a working length ranging between 40-cm and 200-cm with a preferred length of 75-cm to 150-cm. The ability to pass the traditional electrophysiology therapeutic and diagnostic catheters and instruments as well as larger, more innovative, instruments through a catheter introduced with a small outside diameter is derived from the ability to atraumatically expand the distal end of the catheter or sheath to create a larger through lumen to access the cardiac chambers. The ability to pass multiple catheters through a single sheath with a single septal penetration is inherently safer and less time-consuming than a multiple septal puncture procedure. The expandable distal end of the catheter can comprise between 5% and 95% of the overall working length of the catheter. The proximal end of the catheter is generally larger than the distal end to provide for pushability, torqueability (preferably approximately 1:1 torqueability), steerability, control, and the ability to easily pass large diameter instruments therethrough. In an embodiment, the sheath can be routed to its destination over one or more already placed guidewires with a diameter ranging from 0.010 inches up to 0.040 inches and generally approximating 0.035 inches in diameter. An advantage of approaching the treatment site by the veins, instead of the arteries, is that the venous pressure is lower than that in the arterial system, thus reducing the potential for catastrophic hemorrhage during the procedure.  
         [0017]     Another embodiment of the invention comprises an endovascular access system for providing minimally invasive access to atrial structures of the mammalian heart. The system includes an access sheath comprising an axially elongate tubular body that defines a lumen extending from the proximal end to the distal end of the sheath. At least a portion of the distal end of the elongate tubular body is expandable from a first, smaller cross-sectional profile to a second, greater cross-sectional profile. In an embodiment, the first, smaller cross-sectional profile is created by making axially oriented folds in the sheath material. These folds may be located in only one circumferential position on the sheath, or there may be a plurality of such folds or longitudinally oriented crimps in the sheath. The folds or crimps may be made permanent or semi-permanent by heat-setting the structure, once folded. In an embodiment, a releasable or expandable jacket is carried by the access sheath to restrain at least a portion of the elongate tubular structure in the first, smaller cross-sectional profile during insertion and up to or during inflation of the distal region. In another embodiment, the jacket is removed prior to inserting the sheath into the patient. In an embodiment, the elongate tubular body is sufficiently pliable to allow the passage of objects having a maximum cross-sectional size larger than an inner diameter of the elongate tubular body in the second, greater cross-sectional profile. The adaptability to objects of larger dimension is accomplished by pliability or re-shaping of the cross-section to the larger dimension in one direction accompanied by a reduction in dimension in a lateral direction. The adaptability may also be generated through the use of malleable or elastomerically deformable sheath material. This re-shaping or non-round cross-section can be beneficial in passing two or more catheters through a single sheath with a minimum lateral cross-sectional area.  
         [0018]     In another embodiment of the invention, a transluminal access sheath assembly for providing minimally invasive access comprises an elongate tubular member having a proximal end and a distal end and defining a working inner lumen. In this embodiment, the tubular member comprises a folded or creased sheath that can be expanded by a dilatation balloon. The dilatation balloon, if filled with fluids, preferably liquids and further preferably radiopaque liquids, at appropriate pressure, can generate the force to radially dilate or expand the sheath. The dilatation balloon is removable to permit subsequent instrument passage through the sheath. Longitudinal runners may be disposed within the sheath to serve as tracks for instrumentation, which further minimize friction while minimizing the risk of catching the instrument on the expandable plastic tubular member. Such longitudinal runners are preferably circumferentially affixed within the sheath so as not to shift out of alignment. In yet another embodiment, the longitudinal runners may be replaced by longitudinally oriented ridges and valleys, termed flutes. The flutes, or runners, can be oriented along the longitudinal axis of the sheath, or they can be oriented in a spiral, or rifled, fashion.  
         [0019]     In many of the embodiments, the proximal end of the access assembly, apparatus, or device is preferably fabricated as a structure that is flexible, resistant to kinking, and further retains both column strength and torqueability. Such structures include tubes fabricated with coils or braided reinforcements and preferably comprise inner walls that prevent the reinforcing structures from protruding, poking through, or becoming exposed to the inner lumen of the access apparatus. Such proximal end configurations may be single lumen, or multi-lumen designs, with a main lumen suitable for instrument, guidewire, endoscope, or obturator passage and additional lumens being suitable for control and operational functions such as balloon inflation. Such proximal tube assemblies can be affixed to the proximal end of the distal expandable segments described heretofore. In an embodiment, the proximal end of the catheter includes an inner layer of thin polymeric material, an outer layer of polymeric material, and a central region comprising a coil, braid, stent, plurality of hoops, or other reinforcement. It is beneficial to create a bond between the outer and inner layers at a plurality of points, most preferably at the interstices or perforations in the reinforcement structure, which is generally fenestrated. Such bonding between the inner and outer layers causes a braided structure to lock in place. In another embodiment, the inner and outer layers are not fused or bonded together in at least some, or all, places. When similar materials are used for the inner and outer layers, the sheath structure can advantageously be fabricated by fusing of the inner and outer layer to create a uniform, non-layered structure surrounding the reinforcement. The polymeric materials used for the outer wall of the jacket are preferably elastomeric to maximize flexibility of the catheter. The polymeric materials used in the composite catheter inner wall may be the same materials as those used for the outer wall, or they may be different. In another embodiment, a composite tubular structure can be co-extruded by extruding a polymeric compound with a stent, braid, or coil structure embedded therein. The reinforcing structure is preferably fabricated from annealed metals, such as fully annealed stainless steel, titanium, or the like. In this embodiment, once expanded, the folds or crimps can be held open by the reinforcement structure embedded within the sheath, wherein the reinforcement structure is malleable but retains sufficient force to overcome any forces imparted by the sheath tubing.  
         [0020]     In an embodiment of the invention, it is beneficial that the sheath comprise a radiopaque marker or markers. The radiopaque markers may be affixed to the non-expandable portion or they may be affixed to the expandable portion. Markers affixed to the radially expandable portion preferably do not restrain the sheath or catheter from radial expansion or collapse. Markers affixed to the non-expandable portion, such as the catheter shaft of a balloon dilator can be simple rings that are not radially expandable. Radiopaque markers include shapes fabricated from malleable material such as gold, platinum, tantalum, platinum iridium, and the like. Radiopacity can also be increased by vapor deposition coating or plating metal parts of the catheter with metals or alloys of gold, platinum, tantalum, platinum-iridium, and the like. Expandable markers may be fabricated as undulated or wavy rings, bendable wire wound circumferentially around the sheath, or other structures such as are found commonly on stents, grafts, stent-grafts, or catheters used for endovascular access in the body. Expandable radiopaque structures may also include disconnected or incomplete surround shapes affixed to the surface of a sleeve or other expandable shape. Non-expandable structures include circular rings or other structures that completely surround the catheter circumferentially and are strong enough to resist expansion. In another embodiment, the polymeric materials of the catheter or sheath may be loaded with radiopaque filler materials such as, but not limited to, bismuth salts, or barium salts, or the like, at percentages ranging from 1% to 50% by weight in order to increase radiopacity. The radiopaque markers allow the sheath to be guided and monitored using fluoroscopy.  
         [0021]     In order to enable radial or circumferential expansive translation of the reinforcement, it may be beneficial not to completely bond the inner and outer layers together, thus allowing for some motion of the reinforcement in translation as well as the normal circumferential expansion. Regions of non-bonding may be created by selective bonding between the two layers or by creating non-bonding regions using a slip layer fabricated from polymers, ceramics or metals. Radial expansion capabilities are important because the proximal end needs to transition to the distal expansive end and, to minimize manufacturing costs, the same catheter may be employed at both the proximal and distal end, with the expansive distal end undergoing secondary operations to permit radial or diametric expansion.  
         [0022]     In another embodiment, the distal end of the catheter is fabricated using an inner tubular layer, which is thin and lubricious. This inner layer is fabricated from materials such as, but not limited to, FEP, PTFE, polyamide, polyethylene, polypropylene, Pebax, Hytrel, and the like. The reinforcement layer comprises a coil, braid, stent, or plurality of expandable, foldable, or collapsible rings, which are generally malleable and maintain their shape once deformed. Preferred materials for fabricating the reinforcement layer include but are not limited to, stainless steel, tantalum, gold, platinum, platinum-iridium, titanium, nitinol, and the like. The materials are preferably fully annealed or, in the case of nitinol, fully martensitic. The outer layer is fabricated from materials such as, but not limited to, FEP, PTFE, polyamide, polyethylene, polypropylene, polyurethane, Pebax, Hytrel, and the like. The inner layer is fused or bonded to the outer layer through holes in the reinforcement layer to create a composite unitary structure. The structure is crimped radially inward to a reduced cross-sectional area. A balloon dilator is inserted into the structure before crimping or after an initial crimping and before a final sheath crimping. The balloon dilator is capable of forced radial, or diametric, expansion of the reinforcement layer, which provides sufficient strength necessary to overcome any forces imparted by the polymeric tubing, thus controlling the cross-sectional shape of the polymeric tubing. The dilator is also capable of overcoming any forces imparted by tissues, including atrial or even ventricular myocardial tissue, through which the sheath is inserted.  
         [0023]     Another embodiment of the invention comprises a method of providing endovascular access to the left atrium. The method first comprises percutaneously placing a hollow needle into the femoral vein, inserting a guidewire through the hollow needle into the vein, withdrawing the needle, and inserting a sheath with a tapered obturator into the puncture site and into the vein over the guidewire. The guidewire is next withdrawn, as is the tapered obturator and a 0.032 to 0.035-inch stiff guidewire is advanced into the vein and to the level of the right atrium or superior vena cava (SVC) through the inferior vena cava (IVC). A radially expandable sheath is next advanced into the femoral vein and advanced to the right atrium over the guidewire. The expandable sheath is articulated at its distal end so that it is turned toward and positioned against the Foramen Ovale of the atrial septum. The guidewire is next withdrawn and replaced with a Brockenbrough-type needle, which is advanced through the guidewire lumen of the expandable sheath. The Brockenbrough-type needle is advanced through the atrial septum into the left atrium while maintaining the expandable sheath in position against the septal wall, either by normal cardiac movement or by mechanical forward force on the Brockenbrough needle. The expandable sheath is next advanced axially through the septal wall, over the Brockenbrough-type needle and the needle, affixed to its control wire is withdrawn from the proximal end of the expandable sheath. A dilator, positioned within the expandable-sheath is next radially expanded causing the distal end of the sheath to expand radially so as to dilate the hole in the tissues of the atrial septum. The dilator is, next, deflated and removed form the sheath, leaving a large central lumen for the passage of instruments into the left atrium. The expanded sheath is capable of holding a single instrument or multiple instruments of, for example, 8 to 10 French diameter. Suitable hemostatic and anti-reflux valves and seals are affixed the distal end of all devices except guidewires to ensure maintenance of hemostasis and prevention of air entry into the vasculature. Following therapeutic or diagnostic procedures, or both, the sheath is withdrawn from the patient allowing the septal puncture to close, thus preventing communication of blood between the right and left atrium. Throughout substantially the entire procedure, heparinized saline or other anti-thrombogenic solution is infused through an infusion port operably connected to an infusion line at the proximal end of the sheath. The infusion line is operably connected to the central lumen of the sheath, generally in the region of the hub, and the infused fluid flows out through the distal end of the sheath. In another embodiment, the distal end of the expandable portion of the sheath comprises a plurality of fenestrations or holes so that the infused fluid can exit the sheath through these holes. The infusion of this fluid is beneficial in minimizing thrombosis within or about the sheath as well as minimizing the occurrence of thromboemboli that might be generated by a sheath in the venous or arterial circulation.  
         [0024]     In many embodiments, the expandable access sheath is configured to bend, or flex, around sharp corners and be advanced into the right atrium so that the longitudinal axis of its distal end is perpendicular to the atrial septal wall. Provision can optionally be made to actively orient or steer the sheath through the appropriate angles of between 20 to 120 degrees or more and to bend in one or even two planes of motion. The steering mechanism, in various embodiments, can be a curved guidewire and straight catheter, curved catheter and straight guidewire, a movable core guidewire, or a combination of the aforementioned. The expandable sheath also needs to be able to approach the right atrium from a variety of positions. In one embodiment, radial expansion of the distal end of the access sheath from a first smaller diameter cross-section to a second larger diameter cross-section is next performed, using a balloon dilator. The balloon dilator is subsequently removed from the sheath to permit passage of instruments that may not normally have been able to be inserted into the atrium of the heart. Once the sheath is in place, the guidewire may be removed or, preferably, it may be left in place. The atrial septum is gently dilated with radial force, preferably to a diameter of 10 mm or less, rather than being axially or translationally dilated by a tapered dilator or obturator. In most embodiments, the use of the expandable trans-septal sheath eliminates the need for multiple access system components.  
         [0025]     In another embodiment of the invention, the expandable sheath comprises steerable members that eliminate the need for a 0.038-inch guidewire to be placed prior to sheath insertion and advancement. In another embodiment, the Brockenbrough-type needle, or septal penetrator, is integrated into the expandable sheath so that it can be used to puncture the atrium but does not need to be advanced and withdrawn through the sheath. The integral septal penetrator is actuated by the operator at the proximal end of the sheath. The controls at the proximal end of the sheath are operably connected to the septal penetrator at the distal end of the sheath by linkages, pressure lumens, electrical lines, or the like, embedded within the sheath and routed from the proximal end to the distal end. The septal penetrator is capable of bending with the articulating sheath distal region. In yet another embodiment, a reversible fixation device, or safety cushion, is provided at the distal end of the expandable sheath. The reversible fixation device is actuated by the operator at the proximal end of the sheath. The controls at the proximal end of the sheath are operably connected to the fixation device at the distal end of the sheath by linkages, pressure lumens, electrical lines, or the like, embedded within the sheath and routed from the proximal end to the distal end. The reversible fixation device can be an inflatable structure such as a balloon, a moly-bolt expandable structure, an expandable mesh, an umbrella, or the like, preferably positioned to expand within the left atrium. In an embodiment, the structure of the catheter or sheath is such that it is able to maintain a selectively rigid operating structure sufficient to provide stability against the atrial septum to support the advancement of trans-septal needles or penetrators. The sheath can be selectively stiffened, at least at its distal end, to provide a non-deflecting platform for support of instrumentation, such as the septal penetrator, which is passed therethrough.  
         [0026]     In another embodiment of the invention, the proximal end of the expandable sheath comprises hemostasis or backflow check seals or valves to prevent blood loss and retrograde flow of air into the circulatory system. The hub of the sheath comprises such hemostasis seal. The seal comprises an annular soft elastomeric gasket that seals against catheters, instruments, and the dilator, inserted therethrough. The seal can further comprise a valve such as a stopcock, one-way valve such as a duckbill or flap valve, or the like to prevent significant blood loss and air entry when an instrument or catheter is removed from the lumen of the expandable sheath. The soft annular seal can further comprise a mechanism to compress the inner diameter of the seal radially inward, such as the mechanisms found on Tuohy-Borst valves. The hub further comprises one or more sideport for injection of contrast media such as Omnipaque, Renografin, or other Barium-loaded solutions, for example, or anticoagulant solutions such as heparin, coumadin, persantin, or the like, or for the measurement of pressure at or near the distal end of the sheath. The dilator hub comprises a central lumen with a Tuohy-Borst valve and one or more sideports for balloon inflation, said sideports operably connected to lumens in the dilator catheter for injection or withdrawal of fluids from a balloon at the distal end of the dilator and optionally for measurement of pressure at or near the dilator distal end. The dilator hub can also comprise a slide knob, a trigger, or other lever to actuate a septal puncture device at the distal end of the dilator. The dilator hub, the sheath hub, or both, can also comprise a handle, lever, or trigger mechanism to enable steering mechanisms at the distal end of the dilator, the sheath, or both, respectively.  
         [0027]     The expandable sheath, in an embodiment, comprises radiopaque markers to denote the beginning and end of the expandable region, and the middle of the expandable region. The middle of the expandable region is useful in that it can be aligned with the atrial septum during the sheath expansion procedure. The sheath can comprise radiopaque materials such as gold wire, platinum wire, tantalum wire, or coatings of the aforementioned over a malleable, stainless steel, deformable reinforcing layer. Such complete radiopaque markings are especially useful for sheath dilation insofar as they allow the operator to more clearly visualize the extent to which the sheath has been dilated once the dilator is activated. In a preferred embodiment, a radiopaque marker band is affixed to the dilator substantially near the distal tip of the dilator so that the position of the distal tip can be observed and controlled relative to the wall of the left atrium or other cardiac structures. This radiopaque marker band can be a non-expandable, axially elongate tubular structure that is adhered to the non-expandable dilator shaft. Another non-expandable radiopaque marker band can be adhered to the dilator shaft at a position substantially corresponding to the proximal most dilating portion of the dilator or sheath. Another non-expandable radiopaque marker band can be adhered to the dilator shaft at a position substantially corresponding to the distal most dilating portion of the dilator or sheath. Thus, the atrial septum can be positioned with confidence between the two dilator radiopaque markers and dilation will be assured. The radiopaque marker bands can further be configured to appear different under fluoroscopy, for example by making the distal tip marker a single band, the distal dilation marker two bands, and the proximal dilator marker, three bands. Yet another configuration of radiopaque marker bands can be achieved by using malleable wire windings of gold, tantalum, platinum alloys, or the like, which are embedded within the folded and expandable sheath, preferably at or near the distal end of the sheath and, optionally, at or near the proximal end of the expandable portion of the sheath. These wire windings can expand with the sheath and can help show the extents of the sheath even after the dilator has been removed.  
         [0028]     Since in many embodiments the hub of a Trans-Septal sheath requires many hemostasis valves and fluid input connectors or ports, the hub can be a longer structure than that on current guide catheters. Therefore, it may be required that a longer Brockenbrough needle is used to allow sufficient working length to provide for maneuverability within the cardiac anatomy. It may be beneficial to use Brockenbrough needles, which are longer than the standard 60-71 cm length, preferably those of 80 to 90 cm in length. Furthermore, the sheath hub length can be advantageously foreshortened by use of tightly grouped ports and minimum length Tuohy-Borst valves as well as “Y” connectors that are integrated into the hub, rather than being separately attached. Thus, the working length of the entire system is between 50 and 90 cm and preferably between 60 and 80 cm. The sheath hub length, including the length of the dilator hub, is between 3 and 15 cm and preferably between 4 and 8 cm, the preferred length being appropriate if a shorter 70-cm or 71-cm long Brockenbrough needle is used. In an embodiment, the hub of the dilator comprises a “Y” or “T” connector operably connected to the guidewire lumen of the hub. The guidewire access port is controlled by and comprises a hemostasis valve or seal such as Tuohy-Borst connector. The side port is generally a luer port, luer lock port, or similar and is controlled by a stopcock, valve, Tuohy-Borst valve or other device to prevent unwanted fluid (including air) flow into or out of the guidewire port. The side port is beneficial in that it can be affixed to and operably connected to an infusion line for infusion of heparinized saline or other antithrombogenic material into the guidewire lumen of the sheath dilator. Such infusion of antithrombogenic fluid into the guidewire port during the procedure can help minimize thrombosis and thromboemboli generation.  
         [0029]     In order to facilitate maneuvering the expandable trans-septal sheath into the right atrium and through the atrial septum, as well as for support of the sheath during catheter passage therethrough, it is beneficial in many embodiments to impart a curve into the trans-septal sheath, and optionally through the dilator. This curve is preferably a bend of between 20 to 120 degrees and preferably between 30 and 90 degrees. The bend can be in one plane or it can be in two orthogonal planes. An exemplary bend is to bend the sheath approximately 45 degrees out of plane  1  and approximately 50 degrees out of line in plane  2 , which is orthogonal to plane  1 . The radius of the curve can range between 2-cm and 12-cm and preferably between 3-cm and 10-cm in each of the two directions. Another example is a single plane curve of 90 degrees with a radius of around 3-cm to 12-cm. These bends are preferably imparted to the distal region of the non-expandable sheath tubing, just proximal to the expandable region. The bends can also be imparted through the expandable region but maintaining those bends in the expandable region may further require the use of a bent or curved shaped balloon, a resilient longitudinal support within the expandable region, a bent or curved dilator shaft, or both. The bending can be imparted to the tubing by placing the tubing over a curved mandrel and then heat-setting the tubing while over the mandrel. The tubing needs to be heated above glass-transition temperature, which is preferably above body temperature (37 degrees centigrade) for the heat set to be optimal. Materials used in the heat settable region can include, but not be limited to, polyethylene, PEN, PET, polyamide, polyimide, PEBAX, Hytrel, and the like. The expandable region of a trans-septal sheath need not be long and ranges between 0.5-cm and 20-cm with a preferred length of between 1-cm and 10-cm. By keeping the expandable region short, the region of the sheath comprising the bend, a bend, which makes the sheath have properties similar to those of a guiding catheter, is not in the expandable region, although methodologies of maintaining a bend within the expandable region are disclosed herein.  
         [0030]     In yet another embodiment, the exterior of the sheath, and optionally the internal lumen of the sheath, can be coated with a lubricious coating comprising materials such as, but not limited to, silicone oil or a hydrophilic hydrogel comprising polyethylene glycol, polyether polyurethane, or the like.  
         [0031]     In another embodiment, the proximal end of the sheath comprises a non-circular interior cross-section. The interior cross-section of the sheath can be oval, or it can comprise two or more completely walled off or partially walled off separate lumens. The sheath hub, which is affixed to the non-expandable proximal end of the sheath, can comprise two or more separate instrumentation ports, each of which are operably connected to a lumen or partial lumen within the sheath and which can advantageously comprise hemostasis valves. The instrumentation ports are especially useful for passage of, for example, multiple electrophysiology catheters, a mapping catheter and a therapeutic catheter, a ring catheter and an ablation catheter, or the like. Segregation of the multiple instrumentation can be useful to prevent binding or interference between the multiple catheters or instruments passed through the sheath. In yet another embodiment, the proximal end of the sheath has a non-circular cross-section that minimizes the overall cross-sectional area or circumference of a sheath configured to accept two or more catheters. This non-circular cross-section can be an oval, ellipse, rounded triangle, or the like. The non-circular cross section can, for example, reduce an 18 French OD catheter to around 15.5 French, using the same wall thickness and still retain the capability to accept two 8 French catheters within its internal lumen or lumens. Reduction in exterior cross-section is clearly useful in making the procedure as minimally invasive as possible and may make a procedure, which normally takes a cutdown, a percutaneous procedure.  
         [0032]     For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention are described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. These and other objects and advantages of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements.  
         [0034]      FIG. 1  is a front view schematic representation of the human venous circulatory system including the heart and the great veins;  
         [0035]      FIG. 2  is a front view schematic representation of the human venous circulatory system with a guidewire routed from the femoral vein into the right atrium;  
         [0036]      FIG. 3  is a front view schematic representation of the human venous circulatory system with an expandable sheath advanced into the right atrium, according to an embodiment of the invention;  
         [0037]      FIG. 4  is a cross-sectional illustration of the heart with the expandable sheath articulated and positioned within the right atrium and the guidewire removed, according to an embodiment of the invention;  
         [0038]      FIG. 5  is a cross-sectional illustration of the heart with the expandable sheath positioned at the atrial septum and the septal penetrator advanced across the atrial septum into the left atrium, according to an embodiment of the invention;  
         [0039]      FIG. 6  is a cross-sectional illustration of the heart with the expandable sheath advanced into the left atrium across the atrial septum and the septal penetrator withdrawn into the dilator of the expandable sheath, according to an embodiment of the invention;  
         [0040]      FIG. 7  is a cross-sectional illustration of the heart with the expandable sheath dilated at its distal end by the dilator, according to an embodiment of the invention;  
         [0041]      FIG. 8  is a cross-sectional illustration of the heart with the expandable dilator withdrawn from the sheath leaving a large central lumen for instrument passage into the left atrium, according to an embodiment of the invention;  
         [0042]      FIG. 9  is a cross-sectional illustration of the heart with an electrophysiology therapeutic catheter advanced through the central lumen of the expanded sheath into the left atrium, according to an embodiment of the invention;  
         [0043]      FIG. 10  is a cross-sectional illustration of the heart with an atrial septal plug delivery catheter advanced through the central lumen of the expanded sheath into the left atrium, according to an embodiment of the invention;  
         [0044]      FIG. 11  is a cross-sectional illustration of the heart with a collapsible mitral valve prosthesis delivery catheter advanced through the central lumen of the expanded sheath into the left atrium, according to an embodiment of the invention;  
         [0045]      FIG. 12  is a cross-sectional illustration of the heart with the expandable sheath traversing into the left atrium and secured in place with a left atrial anchor system, according to an embodiment of the invention;  
         [0046]      FIG. 13  is a cross-sectional illustration of the expandable sheath showing the proximal sheath and dilator hubs along with various hemostasis valves, actuators, and seals, according to an embodiment of the invention;  
         [0047]      FIG. 14  is a cross-sectional illustration of the expandable sheath showing a deflection mechanism, according to an embodiment of the invention;  
         [0048]      FIG. 15  is a cross-sectional illustration of the expandable sheath showing a distal anchor mechanism, according to an embodiment of the invention;  
         [0049]      FIG. 16  is a cross-sectional illustration of the expandable sheath showing an atrial septal penetrator integral to the dilator, according to an embodiment of the invention;  
         [0050]      FIG. 17A  illustrates a side view of a collapsed, non-expanded trans-septal sheath, according to an embodiment of the invention;  
         [0051]      FIG. 17B  illustrates a side view of an expanded trans-septal sheath, according to an embodiment of the invention;  
         [0052]      FIG. 17C  illustrates a side view of an expanded trans-septal sheath with the dilator removed, according to an embodiment of the invention;  
         [0053]      FIG. 18A  illustrates a lateral cross-section of the proximal region of the expandable trans-septal sheath, according to an embodiment of the invention;  
         [0054]      FIG. 18B  illustrates a lateral cross-section of the distal region of the expandable trans-septal sheath in its non-expanded configuration, according to an embodiment of the invention;  
         [0055]      FIG. 19  illustrates a partial breakaway side view of a proximal end of a trans-septal sheath comprising multiple instrumentation ports on its hub, according to an embodiment of the invention;  
         [0056]      FIG. 20  illustrates a side view of a distal end of a trans-septal sheath and dilator comprising curvature near its distal end to facilitate trans-septal puncture, according to an embodiment of the invention;  
         [0057]      FIG. 21A  illustrates an embodiment of a lateral cross-sectional profile of a proximal end of a sheath comprising a non-circular outer profile and a dual partial lumen inner profile, according to an embodiment of the invention; and  
         [0058]      FIG. 21B  illustrates an embodiment of a lateral cross-sectional profile of the folded, compressed, expandable distal region of the sheath, according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0059]     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.  
         [0060]     In the description herein the terms catheter or sheath will be used to refer to being an axially elongate hollow tubular structure having a proximal end and a distal end. The structure can have any cross-sectional shape but in most embodiment the structure has a circular cross-sectional shape. The axially elongate structure further has a longitudinal axis and has an internal through lumen that extends from the proximal end to the distal end for the passage of instruments, fluids, tissue, or other materials. The axially elongate hollow tubular structure is generally flexible and capable of bending, to a greater or lesser degree, through one or more arcs in one or more directions perpendicular to the main longitudinal axis. As is commonly used in the art of medical devices, the proximal end of the device is that end that is closest to the user, typically a cardiologist, surgeon, or electrophysiologist. The distal end of the device is that end closest to the patient or that is first inserted into the patient. A direction being described as being proximal to a certain landmark will be closer to the user, along the longitudinal axis, and further from the patient than the specified landmark. The diameter of a catheter is often measured in “French Size” which can be defined as 3 times the diameter in millimeters (mm). For example, a 15 French catheter is 5 mm in diameter. The French size is designed to approximate the circumference of the catheter in mm and is often useful for catheters that have non-circular cross-sectional configurations. While the original measurement of “French” used π (3.14159 . . . ) as the conversion factor between diameters in millimeters (mm) and French, the system has evolved today to where the conversion factor is 3.0.  
         [0061]      FIG. 1  is a schematic frontal (anterior) illustration (looking posteriorly) of a human patient  100  comprising a heart  102 , a descending aorta  104 , an inferior vena cava  106 , a superior vena cava  108 , a right jugular vein  110 , a left jugular vein  112 , a subclavian vein  114 , a right femoral vein  116  and a left femoral vein  118 . In this illustration, the left anatomical side of the body of the patient  100  is toward the right of the illustration.  FIG. 1  primarily illustrates components of the venous circulation.  
         [0062]     Referring to  FIG. 1 , the heart  102  is a pump, the outlet of which is the aorta, including the descending aorta  104 , which is a primary artery in the systemic circulation. The circulatory system, which is connected to the heart  102  further comprises the return, or venous, circulation. The venous circulation comprises the superior vena cava  108  and the inferior vena cava  106 , which return blood from the upper extremities and lower extremities, respectively. The right and left jugular veins,  110  and  112 , respectively, and the subclavian vein  114  are smaller venous vessels with venous blood returning to the superior vena cava  108 . The right and left femoral veins,  116  and  118  respectively, return blood from the legs to the inferior vena cava  106 . The veins carry blood from the tissues of the body back to the right heart, which then pumps the blood through the lungs and back into the left heart. Pressures within the venous circulation generally average 20 mm Hg or less. The arteries of the circulatory system carry oxygenated blood (not shown) from left ventricle of the heart  102  to the tissues of the body. The pressures within the arteries for a normal person undulate, with a modified triangle waveform, between a diastolic pressure of around 80 mm Hg to a systolic pressure of around 120 mm Hg. A hypotensive person may have arterial pressure lower than 120/80 mm Hg and a hypertensive person may have arterial pressures higher than 120/80 mm Hg. Systolic arterial pressures of 300 mm Hg can occur in extremely hypertensive persons.  
         [0063]      FIG. 2  is a schematic frontal illustration, looking posteriorly from the anterior side, of the patient  100 . A vascular introduction sheath  204  has been inserted into the right femoral vein  116  via a percutaneous puncture or incision. A guidewire  200  has been inserted through the introduction sheath  204  and routed, cranially, up the inferior vena cava  106  to the right atrium  202 , one of the chambers of the heart  102 . In this illustration, the left anatomical side of the patient  100  is toward the right. The guidewire  200  has been placed so that it can be used to track therapeutic or diagnostic catheters into a region of the heart  102 .  
         [0064]     Referring to  FIG. 2 , the venous circulation, through which the guidewire  200  has been routed, is generally at lower pressure between 0 and 20 mm Hg than is the systemic circulation, of which the descending aorta is a part. The pressure within the systemic circulation may range from 60 to over 300 mm Hg depending on the level of hypertension or hypotension existent in the patient. By accessing the heart through the venous circulation, the chance of hemorrhage from the catheter insertion site is minimized, as is the demand on the hemostasis valves built into any catheters used on the patient.  
         [0065]      FIG. 3  is a frontal illustration, looking posteriorly from the anterior side, of the patient  100 . The vascular introduction sheath  204  of  FIG. 2  has been removed from the right femoral vein  116  and a larger Trans-Septal Expandable Sheath  300  having certain features and advantages according to the present invention has been inserted into the venous circulation over the guidewire  200  and routed through the inferior vena cava  106  into the right atrium  202  of the heart  102 . The expandable trans-septal sheath  300  further comprises a dilator  306 , the proximal most part of which is shown in  FIG. 3 . The expandable trans-septal sheath  300  further comprises a proximal non-expandable region  304  and a distal expandable region  302 .  
         [0066]     Referring to  FIG. 3 , the venous circulation is filled with blood (not shown) that is somewhat depleted of oxygen and enriched with carbon dioxide as a result of interaction with body tissues. In the illustrated embodiment, the expandable region  302  of the expandable trans-septal sheath  300  is smaller in diameter than the proximal non-expandable region  304 .  
         [0067]      FIG. 4  is a cross-sectional illustration of the heart  102 , further comprising the descending aorta  104 , the inferior vena cava  106 , the superior vena cava  108 , the right atrium  202 , a right ventricle  400 , a left ventricle  402 , a left atrium  404 , and a left atrial appendage  406 . The heart  102  also comprises an aortic arch  408 , a ventricular septum  410 , a mitral valve  412 , an aortic valve  414 , a pulmonary valve  416 , a tricuspid valve  418 , and a pulmonary artery  420 . The expandable region  302  of the sheath  300  is visible in the right atrium  202  and the proximal non-expandable region  304  of the expandable trans-septal sheath  300  is visible in the inferior vena cava  106 .  
         [0068]     Referring to  FIG. 4 , the expandable distal region  302  has been articulated or deflected in an arc so that its distal end rests against the atrial septum (not shown), the wall of myocardium that divides the right atrium from the left atrium. In this illustration, the atrial septum is obscured by the ascending aorta  602  ( FIG. 6 ), that region of aorta between the aortic arch  408  and the aortic valve  414 , as well as the pulmonary artery  420  and the pulmonary valve  416 . The distal end of the distal sheath region  302  is positioned so that it rests within the Foramenal valley of the atrial septum, a naturally thin area of the atrial septum and a preferred landmark for continuing the procedure. The distal region  302  can be articulated, in an embodiment, with the use of an integral or removable internal steering mechanism. The distal region  302 , in another embodiment, can be articulated using a movable core guidewire or a bent guidewire (not shown) inserted through the central lumen of the distal region  302  of the sheath  300 .  
         [0069]      FIG. 5  is a cross-sectional illustration of the heart  102 , showing the atrial septum  504 . The ascending aorta  602  ( FIG. 6 ), aortic valve  414 , pulmonary artery  420 , and pulmonary valve  416  of  FIG. 4  have been removed from this illustration for clarity and to show the atrial septum  504 . The distal expandable region  302  of the sheath  300 , substantially located within the right atrium  202 , is shown with its long axis perpendicular to the atrial septum  504 . The proximal end  304  of the sheath  300  is shown resident within the inferior vena cava  106 . A septal penetrator  500  is shown extended through a puncture  502  in the atrial septum  504  and is routed into the left atrium  404 .  
         [0070]     Referring to  FIG. 5 , the septal penetrator  500  is a needle or axially elongate structure with a sharp, pointed distal end. The septal penetrator  500  is resident within the guidewire lumen of the dilator  306  ( FIG. 3 ), which is removably resident within the distal expandable region  302 . The septal penetrator  500  is actuated at the proximal end of the sheath  300 . The septal penetrator  500  is operably connected to a control mechanism such as a button, lever, handle, trigger, etc., which is affixed, permanently or removably, at the proximal end of the dilator  306  by way of a linkage, pusher rod, electrical bus, or the like that runs the length of the dilator  306 . The penetrator  500  can also be integrated into the sheath  300  but the removable dilator  306  is more advantageous. Care must be taken not to have the septal penetrator  500  pierce the wall of the left atrium  404  opposite the atrial septum  504  so length control and advance control are important as is guidance, either by fluoroscopy, MRI, ultrasound, or the like. Further care must be taken not to inadvertently pierce the aorta in the region upstream or anatomically proximal to the aortic arch  408  ( FIG. 4 ). The distal expandable region  302  is bent, deflected, or articulated through an angle of between 30 and 120 degrees to achieve approximate perpendicularity with the atrial septum  504 . The septal penetrator  500  can be solid, it may be hollow like a hypodermic needle, or it may have a “U” or “C”-shaped cross-section. The center or core of a hollow, “C”, or “U”-shaped septal penetrator can be filled with a guidewire or other core element to prevent incorrect tissue penetration. The septal penetrator  500  can be rigid or it can be flexible but retain column strength. Such flexible configurations can comprise cutouts in the wall of the penetrator  500  or guidewire-like construction. The septal penetrator  500  can be initially straight or it can be initially curved. The septal penetrator  500  can be fabricated from shape memory material such as nitinol and heat treated to cause curving once the material is heated from martensitic to austenitic temperatures. Such heating can be performed using electrical heating, hot water injection, or the like. Preferred temperatures for the austenite finish temperature, in this application range from 25 degrees to around 42 degrees centigrade. Higher temperatures require more heating and rely on hysteresis to minimize the return to martensite when the heating temperature is removed.  
         [0071]      FIG. 6  illustrates a cross-sectional view of the heart  102  showing the distal expandable region  302  having been advanced across the atrial septum  504  from the right atrium  202  and into the left atrium  404 . The tapered tip  600  of the dilator  306  leads the distal end of the expandable region  302  through the septal puncture  502  created by the penetrator  500 . That region of the ascending aorta  602  that does not obscure this anterior view of the atrial septum  504  is shown. The proximal non-expandable region  304  has advanced, to follow the advancing distal expandable region  302 , so that the proximal region  304  is located not only in the inferior vena cava  106  but also within the right atrium  202 .  
         [0072]     Referring to  FIG. 6 , the expandable access sheath  300  is pre-assembled with its internal dilator  306 . The dilator  306  is, in an embodiment, a catheter with a dilatation balloon (not shown) affixed to a dilator shaft. The dilatation balloon is preferably an angioplasty-type, non-elastomeric balloon and is fabricated from materials such as, but not limited to, PET, polyamide, cross-linked polyolefins, or the like. The dilator shaft is terminated at its proximal end with an inflation port that is operably connected to a lumen within the dilator shaft. The lumen within the dilator shaft is operably connected to the interior of the balloon by way of scythes or other openings. The tapered tip  600  is affixed to the distal end of the dilator  306  and is fabricated from thermoplastic elastomer such as, C-Flex or from elastic polymers such as silicone elastomer, polyurethane, or the like. The tapered tip  600  can have a general funnel shape tapering from small at the distal end to large at the proximal end. In another embodiment, the tapered dilator tip  600  can have a complex taper with two or more angles and can also include intermediate cylindrical, non-tapered, regions. The tapered tip  600  can be made to expand with the distal end of the balloon and then shrink down with the balloon when it is deflated, facilitating withdraw through the lumen of the expanded distal region  302  of the sheath  300 . The tapered tip  600  can be asymmetric to substantially match the cross-sectional configuration of an expandable sheath section that is folded and has inherently axial asymmetry. The tapered tip  600  can, in an embodiment, be elastomeric or resilient, to expand and compress with the balloon.  
         [0073]      FIG. 7  illustrates a cross-sectional view of the heart  102  showing the distal expandable region  302  having been radially expanded while placed across the atrial septum  504  between the right atrium  202  and the left atrium  404 . The distal expandable region  302  is now generally of the same diameter as the proximal region  304 . The transition zone  700  is that region connecting the distal region  302  and the proximal region  304 . The dilator balloon resides within the transition zone  700  as well as the distal expandable region  302 . The puncture  502  in the atrial septum  504  has now been dilated using radial dilation means and the distal end of the sheath  302  is resident within the left atrium  404 . The dilator tip  600  remains within the left atrium  404 . The use of radial dilation is considered beneficial and superior to translation dilation by tapered axially translating dilators with regard to tissue healing and wound closure. The radial dilation allows the septal transit to be performed with relatively small expandable tips in the range of 7 to 10 French. Following transit of the septum through the perforation created by the penetrator, a small sheath with a smooth, tapered, distal transition can be advanced readily through the penetration. The expandable region  302  can then be dilated radially, opening up the septal penetration to any size from 12 to 30 French. Such radially dilated openings are known to heal more completely, following removal of the instrument. In another embodiment, the expandable region  302  can be expanded by forcing an inner dilator (not shown) distally along the long axis of the sheath  300  to force the expandable region  302  to dilate diametrically. Such axial translation dilation can be generated by way of a pusher affixed to the inner dilator at its distal end and a handle or mechanical lever at the proximal end of the sheath  300 . The expandable region  302  can be elastomeric or comprise one or more longitudinal folds, which cause the circumference, and thereby the diameter, to be small until dilated.  
         [0074]      FIG. 8  illustrates a cross-sectional view of the heart  102  wherein the distal end of the distal expandable region  302  is resident within the left atrium  404  and is located across the atrial septum  504 . The tip  600  ( FIGS. 6 and 7 ) of the dilator  306  (not shown) has been removed and withdrawn from the proximal end of the sheath  300 . In this configuration, the sheath  300  retains a large, central lumen capable of passing instrumentation, catheters, or the like into the left atrium  404 . The size of the sheath  300  is substantially the same whether in the distal expandable region  302  or the proximal non-expandable region  304 . The central lumen of the sheath is exposed to pressure within the left atrium  404 , said left atrial pressures being 20 mm Hg or less. This large sheath  300  is capable of delivering one, two, or more catheters into the left atrium  404  without the need for more than one atrial septal puncture  502 .  
         [0075]      FIG. 9 a  cross-sectional view of the heart  102  wherein the distal end of the distal expandable region  302  is resident within the left atrium  404  and is located across the atrial septum  504 . Two of the outlets for the pulmonary veins  902  are shown within the left atrium  404 . The tissue around the pulmonary veins  902  is often a site for re-entrant waveforms that cause atrial arrhythmias. Ablation of this tissue using heat or extreme cold temperatures (cryogenics) can alleviate the arrhythmias. In the illustrated embodiment, an electrode  904  that emits Radiofrequency (RF) energy has been introduced at the end of an electrophysiology catheter  900  into the right atrium  404  through the expandable sheath  300 . The electrode  904  shown is a round electrode called a lasso electrode and is capable of heating and ablating a ring of tissue in a single operation. Single point electrodes  904  can create line or ring ablations but must be drawn slowly along the tissue to ablate the desired pattern. Such electrode movement is difficult to achieve at the end of a curved 100-cm long, or longer, catheter being monitored by fluoroscopy or ultrasound. The heating electrodes can deliver energies such as microwaves, radio frequencies, high-intensity focused ultrasound (HIFU), and the like. Because these ring electrodes  904  are large in diameter, they may be advantageously placed through very large sheaths such as the expandable trans-septal sheath  300 .  
         [0076]      FIG. 10  illustrates a cross-sectional view of the heart  102  wherein the distal end of the distal expandable region  302  of the expandable trans-septal sheath  300  is resident within the left atrium  404  and is located across the atrial septum  504 . A delivery catheter  1000  for an implantable device  1002  is routed through the expandable sheath  300 . In this embodiment, the implantable device  1002  is an expandable plug capable of closing off the opening between the left atrium  404  and the left atrial appendage  406 . The implantable device  1002  is releasably affixed to the distal end of the catheter  1000  by a releasable coupler  1004 , activated by a linkage extending between the distal coupler  1004  and the proximal end of the delivery catheter  1000 . Such left atrial appendage  406  plugs or filters have been shown to reduce emboli generation by the left atrial appendage  406  in conditions where the left atrium  404  is in a state of atrial fibrillation, or uncoordinated muscle contraction. Atrial fibrillation, while not life threatening, results in reduced cardiac output and exercise tolerance. It is also associated with a high rate of cerebrovascular embolic stroke. Left atrial appendage implants  1002  are radially collapsible during delivery. They are generally delivered through 14 French or larger catheters and a radially expandable delivery sheath would be advantageous. The trans-septal sheath  300  further comprises a plurality of ports, holes, fenestrations, or scythes  1006  near the distal end of the sheath  300 . The holes  1006  penetrate through the sheath  300  from the outside to the inside and operably connect the central lumen (not shown) of the expandable region  302  to the environment outside the sheath  300 . Fluid administered through the proximal end of the sheath  300  can exit these holes  1006  as well as through the open distal end of the sheath  300 . It is beneficial to infuse fluids such as heparinized saline or other antithrombogenic fluid so as to minimize the risk of thrombus buildup within or around the sheath  300  as well as to minimize the risk of thromboemboli generation within the cardiovascular system. In another embodiment, the holes  1006  can be located anywhere on the sheath  300 , including the non-expandable proximal end.  
         [0077]      FIG. 11  illustrates a cross-sectional view of the heart  102  wherein the distal end of the distal expandable region  302  of the expandable trans-septal sheath  300  is resident within the left atrium  404  and is located across the atrial septum  504 . A mitral valve implant delivery catheter  1100  is routed through the expandable sheath  300 . The catheter  1   100  is controllably, releasably, affixed to the inlet side of a collapsible, mitral valve prosthesis  1102  by a coupler  1104 . The coupler  1104  is operably connected to the proximal end of the delivery catheter  1100  by a linkage. The delivery catheter  1100  is required to articulate to reach the mitral valve orifice to place the mitral valve prosthesis  1102 . The mitral valve prosthesis  1102  is expanded so that it engages the remnants of the diseased mitral valve leaflets  1106  so that it is secured in place. Such a prosthesis is necessarily large, (up to 35 mm diameter fully expanded) and requires a very large trans-septal catheter (20 to 30 French), even for a radially collapsed device. The expandable trans-septal catheter  300  would allow placement of such large.devices with minimal damage to the atrial septum  504 .  
         [0078]      FIG. 12  illustrates a cross-sectional view of the heart  102  wherein the distal end of the distal expandable region  302  of the expandable trans-septal sheath  300  is resident within the left atrium  404  and is located across the atrial septum  504 . A distal anchor  1200  is shown inflated within the left atrium  404  for the purpose of stabilizing the sheath  300  so that its expandable region  302  cannot be inadvertently pulled out of the left atrium  404 . Two electrophysiology catheters  1202  are shown extending into the left atrium  404  out the distal end of the expandable region  302 . An inflation lumen  1204  is illustrated riding on the surface of the sheath  300  in both the proximal region  304  and the distal expandable region  302 . The inflation lumen  1204  is operably connected to an inflation port and valve at the proximal end of the sheath  300  and is operably connected to the interior of the distal anchor  1200 . The distal anchor, in this embodiment, is a balloon. The balloon can be either non-compliant like an angioplasty balloon or compliant like a Foley balloon, the latter of which is fabricated from silicone elastomer, latex rubber, polyurethane, or the like. Non-compliant balloons can be made from cross-linked polyethylene or polypropylene or from stretch blow molded polyethylene terephthalate, polyamides, or the like. In another embodiment, a second balloon  1506  ( FIG. 15 ) can be placed so that it expands within the right atrium  202  against the atrial septum  504 . Inflation of the second balloon  1506  can be performed through the same inflation lumen  1204  as that used for the distal anchor  1200 . Such inflation through the same inflation lumen  1204  would be substantially simultaneous with the distal anchor  1200 . The second balloon  1506  would prevent distal migration of the sheath  300 . In another embodiment, a dumbbell shaped balloon would replace the two separate balloons. The small diameter part of the dumbbell balloon is configured to reside within the puncture site  502 . Such dumbbell balloon is preferably fabricated as a non-compliant balloon. The distal anchor could also be fabricated as a moly-bolt, umbrella, expandable braid, or other expandable structure activated by a linkage to the proximal end of the sheath. Fabrication of the distal anchor is achieved using materials such as, but not limited to, polyolefins such as polyethylene or polypropylene, polyamide, polyurethane, polyester, elastomeric materials, Hytrel, Pebax, or the like.  
         [0079]      FIG. 13  illustrates a longitudinal cross-section of the proximal end  1300  of an embodiment of an expandable trans-septal sheath system that can be used as described above. The proximal end  1300  comprises a dilator shaft  1302 , a sheath shaft  1304 , an anchor inflation line  1306 , a fluid infusion line  1308 , an anchor line stopcock  1310 , a fluid infusion valve  1312 , a sheath hub  1314 , a sheath valve  1316 , a dilator inflation port  1318 , a dilator hub  1320 , a dilation stopcock  1322 , a guidewire port valve  1324 , a penetrator shaft  1326 , a penetrator knob  1328 , a penetrator spring (not shown), a penetrator access port  1330 , an anchor inflation lumen  1332 , and a penetrator linkage  1334 .  
         [0080]      FIG. 14  illustrates a longitudinal cross-section of an articulating expandable trans-septal sheath  1400 . The articulating, expandable sheath  1400  further comprises a proximal region  1402 , a distal expandable region  1404 , a sheath hub  1406 , a transition zone  700 , a central lumen  1412 , a steering linkage lumen  1424 , an anchor inflation line  1306 , a fluid infusion line  1308 , a compression cap  1414 , a variable valve element  1316 , a lever support  1418 , a steering lever  1420 , a steering linkage  1422 , and a steering linkage distal fixation point  1426 . In this embodiment, the articulation is generated by tension or compression force in the steering linkage  1422  being applied to the fixation point  1426  affixing the steering linkage  1422  to the distal end of the distal expandable region  1404 . The distal expandable region is flexible and can be made preferentially more flexible in the region just proximal to the distal fixation point  1426 . The lever  1418  provides mechanical advantage and can be used with ratchets, locks, friction elements, or the like to restrict movement of the lever  1418  and consequently the linkage  1422  when manual pressure is removed. The distal end of the sheath  1400  is shown bent, or articulated, into an arc and the lever  1420  is correspondingly moved forward, relative to the hub  1406 , to cause tension in the linkage  1422 . A second lever support  1418 , steering lever  1420 , steering linkage  1422 , distal fixation point  1426  and steering linkage lumen  1424  can be added, in another embodiment, to permit articulation of the distal region  1404  in a second direction.  
         [0081]      FIG. 15  illustrates a longitudinal cross-section of an articulating expandable sheath  1400  further comprising a distal anchor  1508 , a proximal anchor  1506 , and a plurality of anchor bonds  1510 . The sheath  1400  further comprises an anchor inflation lumen  1332 , a plurality of scythes  1504 , an anchor inflation manifold  1502 , an anchor inflation line  1306 , an anchor inflation valve  1312 , a hub  1406  and a central sheath lumen  1412 . The distal anchor  1508  and the proximal anchor  1506  are shown as balloons that are inflated with fluid, preferably saline, water, or radiopaque contrast media. Inflation occurs through the anchor inflation valve  1312 , the anchor inflation line  1306 , the anchor inflation manifold  1502  within the hub  1406 , and the anchor inflation lumen  1332 , which are all operably connected. Fluid pressure is added or removed to the balloons  1506  and  1508  through the scythes  1504 , which are holes or ports in the wall of the sheath  1400  that expose the region inside the distal anchor  1508  and proximal anchor  1506  to the fluid pressure of the anchor inflation lumen  1332 .  
         [0082]      FIG. 16  illustrates a longitudinal cross-section of a dilator  1600  suitable for use with the expandable trans-septal sheath  1400  ( FIG. 15 ). The dilator  1600  further comprises a dilator hub  1320 , a guidewire port with valve  1324 , a penetrator access port  1330 , a penetrator shaft  1326 , a penetrator knob  1328 , a penetrator spring (not shown), a penetrator linkage  1334 , a penetrator  1302 , a penetrator coupler  1606 , a penetrator port closure  1602 , an inner dilator tube  1610 , an outer dilator tube  1612 , a dilatation balloon  1604 , a plurality of balloon bonds  1608 , a distal fairing  1630 , and a plurality of radiopaque markers  1620 ,  1622 ,  1624 ,  1626 . The penetrator linkage  1334  and the penetrator  1302  can be solid, coiled, hollow tubes, or C-shaped. The C-shaped embodiment is capable of further accepting a guidewire in the guidewire lumen  1614  at the same time as the penetrator  1302  and penetrator linkage  1334 . The spring (not shown) can be located between the penetrator knob  1328  and the penetrator port closure  1602  and allows the penetrator  1302  to be advanced temporarily and then retracted to its safety position automatically. The guidewire can serve the function of plugging a central hole or hollow within the penetrator  1302 . The penetrator  1302  can be a curved or a straight needle, or it may be fabricated from shape memory materials such as nitinol and be configured to be inserted straight but bend upon exposure to Ohmic heating, body temperature, hot water flushed therethrough, or the like. The dilator balloon  1604  is preferably an angioplasty-type unfurling balloon with bonds at its proximal and distal end. The balloon  1604  is fabricated from high-strength materials such as, but not limited to, PET, polyamide, cross-linked polymers, polyethylene, and the like. The balloon  1604  and dilator  1600  can be fabricated to generate pressures of up to about 20 atmospheres without leakage or failure.  
         [0083]     Referring to  FIG. 16 , the radiopaque markers  1620 ,  1622 ,  1624 , and  1626  are all of the non-expandable type and are affixed to catheter or balloon tubing using adhesive, compression fit, interference fit, potting, overmolding, or the like. The radiopaque markers  1620 ,  1622 ,  1624 , and  1626  are fabricated as short, axially elongate hollow cylinders using materials such as, but not limited to, platinum, gold, tantalum, iridium, barium, bismuth, or the like. The distal tip radiopaque marker  1620  is affixed over the balloon bond  1608  for ease of assembly and is generally covered by a distal shroud or fairing  1630 . The radiopaque markers  1622 ,  1624  and  1626  are affixed to the inner tubing  1610  prior to attachment of the dilator balloon  1604 . The radiopaque marker  1622  delineates the approximate distal end of the full diameter region of the dilatation balloon  1604 . The radiopaque marker  1626  delineates the approximate proximal end of the full diameter region of the dilatation balloon  1604 . The marker  1626  can also be positioned to correspond to the proximal end of the fully expandable portion of the sheath (not shown). The marker  1624  is generally optional and corresponds with the approximate center of the balloon  1604  or the expandable portion of the sheath (not shown). The inclusion of the radiopaque markers  1622 ,  1624 ,  1626  facilitates fluoroscopic visualization of the expandable portion of the sheath (not shown) or the dilatation balloon  1604  across the atrial septum to ensure correct positioning during sheath expansion. The distal marker  1620  facilitates fluoroscopic visualization of the distal tip of the dilator  1600  to ensure that it does not impinge on, perforate, or damage cardiac or other tissue structures within the body and that it follows the desired path within the patient. The distal fairing or shroud  1630  forms a gentle taper from the distal tip of the dilator and shields the distal edge of the sheath (not shown) so that the distal edge of the sheath does not hang up on tissue when it is advanced distally. The distal shroud  1630  is preferably fabricated from elastomeric materials such as, but not limited to, thermoplastic elastomer, silicone elastomer, polyurethane elastomer, and the like. The proximal end of the distal fairing  1630  affixed over the distal end of the balloon  1604  and may ride up over the tapered part of the balloon  1604 . When the balloon  1604  expands, the distal fairing  1630  can expand therewith. When the balloon  1604  is re-collapsed, the distal fairing  1630  re-compresses and can be withdrawn proximally through the expanded sheath tubing (not shown).  
         [0084]      FIG. 17A  illustrates a radially expandable sheath system  1700 , shown in its radially compressed configuration, comprising a dilator  1600  and an expandable trans-septal sheath  1400 . The sheath  1400  further comprises a proximal anchor  1506 , a distal anchor  1508 , a sheath radiopaque marker  1702 , a chevron transition zone  1704 , a plurality of distal infusion ports or holes  1006 , and a fold line  1714 . The dilator  1600  further comprises a dilatation balloon  1604 , an inner dilation tube  1610 , and a penetrator  1302 . The penetrator  1302  is shown extended beyond the distal end of the inner dilator tubing  1610 . The dilator  1600  comprises the dilator hub  1320  ( FIG. 13 ), which is affixed to the dilator shaft  1302 . The dilator hub  1320 , in an embodiment, further comprises anti-rotation elements (not shown) to prevent it from rotating relative to the sheath hub  1406  ( FIG. 14 ). In an embodiment, such anti-rotation elements can include tabs on the dilator hub  1320  and slots on the sheath hub  1406 , or visa versa, which can disengage by simple axial retraction of the dilator hub  1320  proximally away from the sheath hub  1406 . The anti-rotation elements can prevent inadvertent distortion of the sheath system  1700  during insertion and manipulation inside the patient. The dilator  1600  can further comprise a fairing or distal shroud (not shown) that prevents the distal edge of the folded sheath tubing  1404  from catching on tissue as it is being advanced distally. This distal shroud serves as a shoehorn to ensure that the sheath  1400 -dilator  1600  combination  1700  can be smoothly advanced through a tissue puncture or endovascular lumen without becoming caught or hung up. The distal shroud is preferably elastomeric to expand with the dilatation balloon  1604  and is affixed at its distal end to the dilatation balloon  1604  or inner dilator tubing  1610 , or both. The distal shroud retracts distally away from the expandable distal section  1404  of the sheath  1400  because it is affixed to the dilator  1600 . The infusion ports or holes  1006  are distorted and folded along with the distal end of the sheath  1400 . The infusion holes or ports  1006  can be distal to, or proximal to, the distal anchor  1508 .  
         [0085]      FIG. 17B  illustrates the sheath system  1700  in its radially or diametrically expanded configuration. The sheath system  1700  comprises the dilator  1600  and the sheath  1400 . Also shown in  FIG. 17B  are the chevron transition zone  1704 , the proximal balloon anchor  1506 , the distal balloon anchor  1508 , a plurality of drainage or infusion holes or ports  1006 , the anchor inflation line  1332 , the steering linkage lumen  1424 , and the sheath radiopaque marker  1702 . The dilatation balloon  1604  is shown in its expanded, inflated configuration over the inner dilator tubing  1610 . When the dilator balloon  1604  is deflated, the distal shroud (not shown) collapses diametrically and can be easily pulled proximally through the expanded tubing  1404  as the dilator  1600  is being withdrawn. The infusion holes  1006  are shown expanded and non-distorted following expansion of the expanded tubing  1404 . In this embodiment, the dilator  1600  comprises a hub  1320 , which is affixed to a “T” or “Y” fitting  1720 . The “T” or “Y” fitting  1720  is operably connected to the guidewire lumen of the dilator  1600 . The “T” or “Y” fitting  1720  further comprises a stopcock  1724  or other valve affixed to the sideport and a Tuohy-Borst fitting  1722  affixed to and operably connected to the guidewire port. The “T” or “Y” fitting  1720  permits infusion of antithrombogenic liquid or fluid or radiopaque contrast media into the guidewire lumen thus minimizing the risk of thrombus forming or the generation of thromboemboli between the guidewire and the guidewire lumen walls within the dilator  1600 . The stopcock  1724  allows the infusion port or sideport to be closed off, thus preventing fluid flow through the sideport.  
         [0086]      FIG. 17C  illustrates the sheath  1400  after removal of the dilator  1600  ( FIGS. 17A and 17B ). The sheath  1400  further comprises the sheath hub  1406 , the lever  1420 , the proximal tubing  1402 , the distal tubing  1404 , the proximal anchor  1506 , the distal anchor  1508 , a plurality of infusion or drainage holes or ports  1006 , the sheath radiopaque marker  1702 , and the transition zone  1704 . The sheath  1400  is fully expanded at its distal end  1404  and the proximal and distal anchors  1506  and  1508  are deflated. The proximal tubing  1402 , the distal tubing  1404 , or both can be fabricated using composite construction comprising a lubricious&#39; inner layer, a reinforcing layer, and an outer lubricious layer. Suitable materials for use in fabricating the inner layer and the outer layer include, but are not limited to, polyurethane, polyethylene, polypropylene, Hytrel, PEBAX, polyamide, and the like. Wall thicknesses of these layers can range from 0.0005 to 0.025 inches and preferably between 0.001 and 0.010 inches. In another embodiment, an elastomeric layer can be disposed outside the reinforcing layer and under the outer layer. In yet another embodiment, an elastomeric layer can be disposed between the reinforcing layer and the inner lubricious layer. The elastomeric layer can be fabricated from materials such as, but not limited to, thermoplastic elastomer, silicone elastomer, polyurethane elastomer, C-Flex, or the like. The proximal tubing  1402  in another embodiment, can be configured with a plurality of lumens to control the motion of multiple catheters that can be inserted therethrough. In an exemplary embodiment, the proximal tubing  1402  comprises two lumens that can each accept an 8 French catheter, or smaller, inserted therethrough. The lumens can be discreet or the separator wall can be removed at least in part to minimize catheter size. In the multiple lumen embodiment of the proximal region, the dilator  1600  can be inserted through one of the lumens. The cross-sectional shape of the proximal tubing  1402  can further be configured as non-circular to minimize the cross-sectional area while two round catheters, such as EP ablation or diagnostic catheters, are inserted therethrough. The distal region  1404  can be similarly ovalized or non-round but, because of its malleable nature, the distal region  1404  can be made capable of simply deforming to accept the two or more catheters. The sheath hub  1406  can further be configured with dual hemostasis valves and further include “Y” guides to facilitate placement of dual (or more) catheters therethrough.  
         [0087]      FIG. 18A  illustrates a cross-sectional view of the sheath proximal end  1402 . The proximal region  1402  further comprises the sheath tubing  1800 , the outer dilator tubing  1802 , the inner dilator tubing  1610 , the guidewire  200 , the penetrator linkage  1334 , the steering linkage lumen  1424 , and the anchor inflation lumen  1332 . The sheath tubing  1800  is, in an embodiment, a composite tube with an inner layer of lubricious material, an outer layer, and an intermediate reinforcing layer fabricated from a coil or braid. The coil or braid in the proximal region  1402  possesses spring characteristics and is fabricated from stainless steel, titanium, nitinol, cobalt-nickel alloys, or the like. The coil or braid can also be fabricated from polymers such as PET, PEN, polyamide, HDPE, or the like. In an exemplary embodiment, the reinforcing layer is a braid of PEN. The coil configuration can be fabricated from flat wire or from round wire. The coil or braid can be coated with radiopaque materials such as gold, tantalum, platinum, or the like, to enhance radiopacity. More than one steering linkage lumen  1424  can be used to achieve push-pull action, if separated by 180 degrees, or two axis steering if separated by 90 degrees, or 120 degrees, for example.  
         [0088]      FIG. 18B  illustrates a cross-sectional view of the sheath distal region  1404  in its collapsed configuration. The sheath  1404  further comprises the distal expandable tubing  1810 , the collapsed dilatation balloon  1604 , the anchor inflation lumen  1332 , the guidewire  200 , the penetrator  1302 , the inner dilator tube  1610 , one or more longitudinal folds  1714 , and the steering linkage lumen  1424 . The distal expandable tubing  1810  is, in an embodiment, a composite structure with an inner layer, an outer layer, both of which are formed from polymers similar to those used in the proximal region  1402 , and an intermediate malleable reinforcing layer, preferably fabricated from annealed metals such as, stainless steel, gold, platinum, tantalum, or the like. In an exemplary embodiment, the malleable reinforcement comprises a coil of stainless steel  304 , which has been substantially annealed. The stainless steel is formed into a flat wire with a thickness of 0.002 to 0.004 inches and a width of 0.010 to 0.040 inches. The flat wire is formed into a coil with a spacing substantially the same as the width of the flat wire. The stainless steel wire can be coated with a layer of gold to a thickness of 100 angstroms or more. Enhanced radiopacity can be gained by winding coils of gold wire alongside the stainless steel wire. The configuration, in this embodiment is of a double helical spring where the helical elements run parallel to each other. Typical gold wire suitable for such use has a diameter of 0.001 inches to 0.009 inches. The gold wire can be round or it can be flat wire. It is important that the gold wire not fill the entire space between the stainless steel wires so that some polymer can fill the space between substantially most of the coils.  
         [0089]      FIG. 19  illustrates a side view of a proximal end of a trans-septal sheath  1900  comprising multiple instrumentation ports  1902 ,  1904 , and  1906  on its hub  1908 , a length of proximal sheath tubing  1910 , a straight through hub lumen  1912 , an upper hub lumen  1914 , a lower hub lumen  1916 , a fluid infusion line  1308 , and a fluid infusion line stopcock  1312 . The hub  1908  is shown in partial breakaway view to better illustrate interior details. The trans septal sheath  1900  is capable of accepting two or more catheters simultaneously therethrough. One catheter is typically routed through the upper instrumentation port  1904 , the upper hub lumen  1914  and finally into the lumen of the proximal sheath tubing  1910 . Another catheter can be routed, at the same time, through the lower instrumentation port  1906 , the lower hub lumen  1916  and into the lumen of the proximal sheath tubing  1910 . The straight through hub port  1902  and the straight through hub lumen  1912  are used for insertion of guidewires and for insertion of a dilator such as the one shown in  FIG. 16 . The straight through hub lumen  1912  can align with the center of the through lumen of the sheath tubing  1910  or it can be offset to correspond to the center of a separate lumen or cross-sectional orifice. The ports  1902 ,  1904 , and  1906  are preferably hemostatic valves such as a Tuohy-Borst device, ball valve, duckbill valve, or similar.  
         [0090]      FIG. 20  illustrates a side view of a distal end of a trans-septal sheath  2000  comprising curvature  2008  near its distal end to facilitate trans-septal puncture. The sheath  2000  further comprises a non-expandable region  2002 , an expandable region  2004 , and a transition zone  2006  that separates the expandable region  2004  from the non-expandable region  2002 . The curvature is imparted in a first plane in the illustrated embodiment. The curvature  2008  is imparted onto the non-expandable region  2002  but may also be imparted onto at least a portion of the expandable region  2004 . In an exemplary embodiment, the curvature  2008  is imparted onto both a portion of the distal end of the non-expandable region  2002 , the entire transition zone  2006 , and a proximal portion of the expandable region  2004 . The radius of curvature is approximately 7.5 cm and a proximal portion of the curved zone  2008  is curved outward away from the direction in which the distal end of the sheath  2000  points. In another embodiment, there can also be curvature  2008  out of the first plane into a second plane. The curvature can be such that a right-handed or left-handed twist or spiral configuration results. The retrograde curvature is advantageous in stabilizing the distal end of the sheath  2000  within the cardiovascular system while other instrumentation are being passed therethrough.  
         [0091]      FIG. 21A  illustrates an embodiment of a lateral cross-sectional profile of a proximal end of a sheath comprising a non-circular outer profile and a dual partial lumen inner profile. In this embodiment, the proximal tubing  2100  is suitable for conveying a catheter through each of its two lumens  2102 . Said catheter passage may be simultaneous, sequential, or one at a time. The proximal tubing  2100  comprises a separation wall  2104 , a plurality of side walls  2108 , and rounded end walls  2106 . The cross-section of a round sheath  2110  is illustrated with dashed lines to show the additional area required to maintain the same two catheter lumens. In this embodiment, a sheath with the rounded cross-section would exceed 18 French to carry two 8-French catheters and have 0.013-inch thick walls while the oval embodiment would approximate 15.5 French to 16 French for the same carrying capacity. The reduced cross-sectional area is beneficial to reduce the size of the wound necessary for arterial or venous access and could make the difference between a cutdown and a perclitaneous procedure. In another embodiment, the separation wall  2104  could be eliminated to further reduce cross-sectional size. In yet another embodiment, the separation wall  2104  can only partially protrude inward from the side walls  2108  and thus serve as a catheter guide without completely separating the lumens. Such configurations are beneficial in preventing two catheters from twisting or interfering with each other while both are being placed. The partially or completely separated lumens  2102  can extend partially or completely along the proximal non-expandable region of the sheath. In yet another embodiment, the interior of the sheath  2100  is oval and approximately matches the exterior geometry of the sheath.  
         [0092]      FIG. 21B  illustrates an embodiment of a lateral cross-sectional profile of a distal, expandable region  2150  of a sheath in the region of curvature  2008  (refer to  FIG. 20 ). The distal expandable region  2150 , shown in the collapsed state, comprises a malleable reinforced wall  2152 , a plurality of folded regions  2154 , and a plurality of fold edges  2156 . The dilator is shown inserted through the distal region  2150  and the inner tube  1610  and the folded balloon  1604  are illustrated. The fold edges  2156  are preferably oriented along the inside of the curve  2008 . If the fold edges  2156  are oriented differently, the structure has a greater chance of buckling or kinking. If a single fold  2154 , rather than two folds  2154  are used, the single fold  2154  is oriented in the direction of the inside of the curvature  2008 .  
         [0093]     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. For example, the sheath may include instruments affixed integrally to the interior central lumen of the sheath, rather than being separately inserted, for performing therapeutic or diagnostic functions. The hub may comprise tie downs or configuration changes to permit attaching the hub to the mouth, nose, or face of the patient. The dilatation means may be a balloon dilator as described in detail herein, it may rely on axial compression of a braid to expand its diameter, or it may be a translation dilator wherein an inner tube is advanced longitudinally to expand an elastomeric small diameter tube. Dilation may also occur as a result of unfurling a thin-film wrapped tube or by rotation of a series of hoops so that their alignment is at right angles to the long axis of the sheath. The embodiments described herein further are suitable for fabricating very small diameter catheters, microcatheters, or sheaths suitable for cardiovascular or neurovascular access. Various valve configurations and radiopaque marker configurations are appropriate for use in this device. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.