Abstract:
A balloon catheter is disclosed including a “no-fold” balloon at a distal end thereof that surrounds a distal portion of a guidewire shaft having a compliant shaft or tubular section for selectively gripping a guidewire there within. Upon introduction of inflation fluid at low pressure values, the compliant shaft section of the guidewire shaft is radially compressed to “lock” onto the guidewire while an outer diameter of the no-fold balloon remains unchanged. The simultaneous compression of the compliant shaft section against a guidewire located within the guidewire lumen and the filling of the balloon with inflation fluid without expanding the balloon provides a clinician with a conjoined balloon catheter and guidewire ensemble that together may be pushed through a tight stenosis such as a chronic total occlusion (CTO).

Description:
FIELD OF THE INVENTION 
     The invention relates generally to a balloon catheter having improved pushability for crossing tight cardiovascular stenoses such as a chronic total occlusion. 
     BACKGROUND OF THE INVENTION 
     Cardiovascular disease, including atherosclerosis, is the leading cause of death in the United States. One method for treating atherosclerosis and other forms of arterial lumen narrowing is percutaneous transluminal angioplasty, commonly referred to as “angioplasty” or “PTA,” or “PTCA” when performed in the coronary arteries. The objective in angioplasty is to restore adequate blood flow through the affected artery, which may be accomplished by inflating a balloon of a balloon catheter within the narrowed lumen of the artery to dilate the vessel. 
     The anatomy of arteries varies widely from patient to patient. Often a patient&#39;s arteries are irregularly shaped, highly tortuous and very narrow. The tortuous configuration of the arteries may present difficulties to a clinician in advancement of the balloon catheter to a treatment site. In addition, in some instances, the extent to which the lumen is narrowed at the treatment site is so severe that the lumen is completely or nearly completely obstructed, which may be described as a total occlusion. If the occlusion has been established for a long period of time, the lesion may be referred to as a chronic total occlusion or CTO. Chronic total occlusions are often characterized by extensive plaque formation and typically include a fibrous cap surrounding softer plaque material. This fibrous cap may present a surface that is difficult to penetrate with a conventional medical guidewire such that one method of crossing a chronic total occlusion includes utilizing a stiffer guidewire to create a new channel through the occlusion. 
     Due to the fibrous cap of the total occlusion, a stiffer guidewire still may not be able to cross the occlusion and the distal end of the guidewire may buckle or prolapse within the vessel when force is applied. In addition, a clinician must take greater care to avoid perforation of the vessel wall when using a stiffer guidewire. Further, even if a stiffer guidewire can penetrate the proximal fibrous cap of the total occlusion, it may not be able to completely cross the occlusion due to multiple non-functional channels that often occur throughout the occlusion, which if entered by the guidewire lead to dead-end pathways and/or to the creation of false tracts within the occlusion and the problems attendant thereto. 
     Another challenge with the treatment of chronic total occlusions is that even after a guidewire successfully crosses the occlusion, the clinician may not be able to advance a dilatation balloon over the guidewire due to the fibrocalcific composition of the chronic total occlusion. In such situations, additional or alternative interventional devices may be needed to treat the occlusion further complicating the procedure. Accordingly, there exists a need in the art for improved devices and methods for treatment of a CTO. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments hereof are directed to a balloon catheter having an inflation lumen, a balloon in fluid communication with the inflation lumen, and a guidewire shaft disposed within at least a distal portion of the inflation lumen. The guidewire shaft defines a guidewire lumen for receiving a guidewire therethrough and has a compliant shaft section that is radially compressible upon delivery of an inflation fluid at an actuation pressure through the inflation lumen. An outer diameter of the balloon remains unchanged during delivery of the inflation fluid at the actuation pressure. The simultaneous compression of the compliant shaft section against a guidewire located within the guidewire lumen and the filling of the balloon with inflation fluid without expanding the balloon provides a clinician with a conjoined balloon catheter and guidewire ensemble that together may be pushed through a tight stenosis, such as a chronic total occlusion (CTO). 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing and other features and advantages of the invention will be apparent from the following description of the invention as illustrated in the accompanying drawings. The accompanying drawings, which are incorporated herein and form a part of the specification, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. The drawings are not to scale. 
         FIG. 1  is a side perspective view of a balloon catheter, wherein the balloon is in an unexpanded configuration, in accordance with an embodiment hereof. 
         FIG. 2  is a side perspective view of the balloon catheter of  FIG. 1 , wherein the balloon is in an expanded configuration. 
         FIG. 2A  is a cross-sectional view taken along line A-A of  FIG. 2 . 
         FIG. 3  is a longitudinal sectional view taken along line  3 - 3  of  FIG. 1 , wherein the balloon is in the unexpanded or delivery configuration and the compliant shaft section of the guidewire shaft is not compressed against the guidewire. 
         FIG. 4  is a sectional view taken along line  3 - 3  of  FIG. 1 , wherein the balloon is in the unexpanded or delivery configuration and the compliant shaft section of the guidewire shaft is compressed against the guidewire. 
         FIG. 5  is a sectional view taken along line  5 - 5  of  FIG. 2 , wherein the balloon is in its fully inflated or fully expanded configuration and the compliant shaft section of the guidewire shaft is compressed against the guidewire. 
         FIG. 6  is a compliance curve for a balloon according to an embodiment hereof. 
         FIGS. 7-10  illustrate the steps of a method of crossing a chronic total occlusion according to an embodiment hereof. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Specific embodiments of the present invention are now described with reference to the figures, wherein like reference numbers indicate identical or functionally similar elements. The terms “distal” and “proximal” are used in the following description with respect to a position or direction relative to the treating clinician. “Distal” or “distally” are a position distant from or in a direction away from the clinician. “Proximal” and “proximally” are a position near or in a direction toward the clinician. 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Although the description of the invention is in the context of treatment of blood vessels such as the coronary, carotid and renal arteries, the invention may also be used in any other body passageways where it is deemed useful. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
       FIGS. 1 ,  2  and  2 A depict a balloon catheter  100  according to an embodiment hereof. Balloon catheter  100  includes a proximal portion  102  that extends out of the patient and has a hub  116 . Distal portion  104  of catheter  100  is positionable at a target location within the vasculature and includes an inflatable balloon  108 , which is shown in an unexpanded or delivery configuration in  FIG. 1  and in an expanded or inflated configuration in  FIG. 2 . In embodiments hereof, catheter  100  may be used in balloon angioplasty procedures, as well as may form the basis of a stent delivery system, a graft delivery system, and/or a drug delivery system. 
     In the embodiment shown in  FIGS. 1 ,  2  and  2 A, balloon catheter  100  has an over-the-wire (OTW) catheter configuration with an inner guidewire shaft  128  that defines a guidewire lumen  130  extending substantially the entire length of the catheter for accommodating a guidewire  132 . More particularly, catheter  100  includes a tubular component or outer shaft  106  having a proximal end  110  coupled to hub  116  and a distal end  112  coupled to balloon  108 . Guidewire shaft  128  has a proximal end  134  coupled to a proximal guidewire port  138  of hub  116  and a distal end  136  terminating distally of balloon  108  and defining a distal guidewire port  140 . In an embodiment, guidewire shaft  128  may be a flexible tube of a polymeric material, such as, e.g., polyethylene tubing. 
     In the coaxial catheter construction of the illustrated embodiment, guidewire shaft  128  extends within outer shaft  106  such that an annular inflation lumen  114  is defined between an inner surface of outer shaft  106  and an outer surface of guidewire shaft  126 . Other types of catheter construction are also amendable to the invention, such as, without limitation thereto, a catheter shaft formed by multi-lumen profile extrusion. Inflation lumen  114  extends between proximal and distal ends  110 ,  112  of outer catheter shaft  106  to allow inflation fluid received through an inflation port  118  of hub  116  to be delivered to balloon  108 . As would be understood by one of ordinary skill in the art of balloon catheter design, hub  116  provides a luer hub or other type of fitting that may be connected to a source of inflation fluid and may be of another construction or configuration without departing from the scope of the present invention. 
     As will be explained in more detail below, balloon  108  is a “no-fold” or “zero-fold” balloon, which means that the balloon material is not folded prior to inflation but instead has a generally cylindrical or tubular shape in the unexpanded configuration shown in  FIG. 1 . An unexpanded no-fold balloon  108  generally has a reduced profile as compared with a more conventional balloon, which must be folded around the catheter in the unexpanded state during delivery, resulting in improved crossability and trackability. The term “crossability” refers to the ability of a balloon catheter to insert the deflated balloon into a targeted vessel narrowing to a generally axially centered position for performing angioplasty or balloon dilatation. The term “trackability” refers to the ability of a catheter to be advanced along a curved path, e.g. to “track” a catheter through tortuous blood vessels over a preplaced guidewire. In the unexpanded configuration of  FIG. 1 , balloon  108  has an outer diameter that is uniform along the full length of balloon  108 . In an embodiment, the unexpanded outer diameter of balloon  108  is approximately 0.6 mm. When radially expanded to the configuration depicted in  FIG. 2 , balloon  108  assumes a more conventional dilatation balloon shape having an inflated outer diameter that enlarges a lumen of the affected artery. In an embodiment, the expanded outer diameter of balloon  108  may be between approximately 1 mm to 3 mm. In some instances, the initial inflation of no-fold balloon  108  to its nominal diameter causes plastic deformation to occur in the balloon material such that, upon deflation of balloon  108 , wings or folds of balloon material are formed. 
     Balloon  108  includes a proximal neck  120  and a distal neck  122 . In the embodiment shown in  FIG. 3 , which is an enlarged longitudinal sectional view of distal portion  104  of catheter  100  taken along line  3 - 3  of  FIG. 1 , proximal neck  120  of balloon  108  is placed inside and joined to distal end  112  of outer shaft  106  in a joint transition area  124 . Encircling outer shaft distal end  112  over balloon proximal neck  120  accommodates the uniform cylindrical shape of unexpanded no-fold balloon  108 . Further, with proximal neck  120  of balloon  108  secured within outer shaft  106 , a smaller outer diameter at joint transition area  124  is achieved thus providing catheter  100  with a reduced profile. Further details regarding the joint between no-fold balloon  108  and outer shaft  106  are described in U.S. patent application Ser. No. 12/049,687 to McAndrew et al. entitled “Outer Catheter Shaft to Balloon Joint” filed Mar. 18, 2008, herein incorporated by reference in its entirety. Distal neck  122  of balloon  108  encircles and is joined to guidewire shaft  128  at joint  142 . Proximal and distal necks  120 ,  122  of balloon  108  may be joined to outer catheter shaft  106  and guidewire shaft  128 , respectively, in any conventional manner known to one of skill in the art of balloon catheter construction, such as by laser welding, adhesives, heat fusing, or ultrasonic welding. In another embodiment where no-fold balloon  108  has an inner diameter that is greater than or substantially equal to an outer diameter of outer shaft  106 , proximal neck  120  may be overlapped and secured around outer shaft distal end  112  by any of the aforementioned methods. 
     Referring now to  FIGS. 3-5 , guidewire shaft  128  includes a compliant shaft or tubular section  150  that is situated within an interior of balloon  108  such that an outer surface thereof may be contacted by inflation fluid  115 . In various embodiments, compliant shaft section  150  may be between 10 mm-20 mm in length. As shown in  FIG. 4 , when balloon  108  receives inflation fluid  115  at a predetermined actuation pressure value PA, compliant shaft section  150  is constructed to be radially compressed into a frictional engagement with or substantially “locked” to a guidewire  132  inserted within guidewire lumen  130  of guidewire shaft  128 . Guidewire  132  is shown somewhat schematically in  FIGS. 3-5  as only a single line for simplicity of illustration. In one embodiment, the predetermined actuation pressure PA is between 2 and 5 atmospheres (atm). At the actuation pressure PA, no-fold balloon  108  is constructed to remain unexpanded such that the outer diameter of balloon  118  remains unchanged. When compliant shaft section  150  is locked down onto guidewire  132 , conjoined balloon catheter  100  and guidewire  132  have an increased ability as an ensemble to cross the lesion when the clinician pushes both the guidewire and the balloon catheter. In addition, with inflation fluid  115  present at the actuation pressure PA within unexpanded balloon  108 , balloon  108  has increased stiffness that is additive to the overall pushability of conjoined balloon catheter  100  and guidewire  132  for crossing the lesion. The term “pushability” refers to the ability of a catheter distal tip to be pushed against resistance without having the catheter shaft buckle, e.g., longitudinal stiffness. 
     Once catheter  100  has successfully crossed the lesion, additional inflation fluid  115  is introduced to increase the inflation fluid pressure within balloon  108  to a predetermined expansion pressure PE that will cause balloon  108  to begin to radially expand to its fully inflated configuration shown in  FIG. 5 . In one embodiment, the predetermined expansion pressure PE at which balloon  108  begins to radially expand is between 6 atm and 15 atm. Upon removal of inflation fluid  115  with the attendant drop in inflation fluid pressure below PA, balloon  108  will deflate and compliant shaft section  150  will return to its original, non-compressed configuration shown in  FIG. 3   
     The materials, dimensions and processing of compliant shaft section  150  and balloon  108  are selected to provide compressibility of section  150  into frictional engagement with guidewire  132  at an inflation fluid pressure of at least predetermined actuation pressure PA, where PA is always less than predetermined expansion pressure PE, i.e., the inflation fluid pressure at which balloon  108  begins to inflate or, stated another way, first experiences an increase in outer diameter. At any inflation fluid pressures above predetermined actuation pressure PA, compliant shaft section  150  will remain substantially locked to guidewire  132 . 
     In one embodiment depicted in  FIGS. 3-5 , compliant shaft section  150  may be constructed to have a different stiffness than the remainder of guidewire shaft  128 . More particularly, compliant shaft section  150  may be formed from a first material having a first stiffness that permits the compliant shaft section to be compressed at the predetermined actuation pressure PA. Stiffness refers to the resistance of an elastic body to deflection or deformation by an applied force. Accordingly, the remainder of the guidewire shaft  128  may be formed from a second material having a second stiffness that is greater than the first stiffness, such that the remainder of guidewire shaft does not radially collapse or compress at any pressure of inflation fluid  115 . Thus, when the pressure applied by inflation fluid  115  around the exterior of compliant shaft section  150  of guidewire shaft  128  is equal to or exceeds the predetermined actuation pressure PA, the less stiff compliant shaft section  150  will be radially compressed and consequently locked against guidewire  132 . 
     Compliant shaft section  150  and the remainder of guidewire shaft  128  are sealingly coupled together to form a continuous guidewire shaft  128 . Any suitable coupling mechanisms or methods known to one of skill in the art of catheter construction may be employed for coupling compliant shaft section  150  to the remainder of the guidewire shaft  128  such as by laser welding, adhesives, heat fusing, or ultrasonic welding. In an embodiment, the first, less stiff material of compliant shaft section  150  may be PEBAX® 6333 polyethylene block amide (PEBA) copolymer from Arkema, Inc. of Philadelphia, Pa., and the second material forming the remainder of guidewire shaft  128  may include polyethylene with or without a PEBA outer layer for bondability. In another embodiment, compliant shaft section  150  may be made of the same material as the remainder of guidewire shaft  128 . To provide relatively less stiffness, compliant shaft section  150  has a wall thickness less than the wall thickness of the remainder of guidewire shaft  128 , as may be achieved e.g., by making two different extrusions of the same material. 
     The location of compliant shaft section  150  is not limited to the position illustrated in  FIG. 3 , such that in other embodiments hereof compliant shaft section  150  may be positioned proximal or distal of balloon  108  as long as compliant shaft section  150  is actuatable by an inflation fluid  115  as described above. 
     In another embodiment, manufacturing or processing steps may be employed in order to alter the stiffness, i.e., resistance to compression of a portion of the guidewire shaft material. More particularly, compliant shaft section  150  may be an integral portion of guidewire shaft  128 , such that guidewire shaft  128  is a unitary structure from proximal end  134  to distal end  136  that is initially formed from a single material having a first stiffness. A second stiffness less than the first stiffness, in compliant shaft section  150  is then achieved via a processing step, such as necking or thinning of the shaft wall in order to reduce the stiffness of compliant shaft section  150  with respect to the remainder of guidewire shaft  128 . In an embodiment suitable for necking or thinning, guidewire shaft  128  may be formed from an integral, seamless tube of a thermoplastic, such as PEBA or polyethylene with or without a PEBA outer layer. 
     As previously mentioned, balloon  108  is constructed to remain unexpanded at the predetermined actuation pressure PA such that the outer diameter of the no-fold balloon remains unchanged when compliant shaft section  150  of guidewire shaft  128  locks onto a guidewire. In an embodiment to achieve a resistance to expansion, balloon  108  may be constructed with a relatively greater wall thickness than a conventional balloon, with the greater wall thickness being consistent along the entire length of the balloon. Such a relatively thicker wall prevents balloon  108  from having any expansion until the inflation fluid pressure reaches predetermined expansion pressure PE. In one embodiment, the wall thickness is approximately 0.000775 inches. Balloon  108  may be formed from any material that is relatively elastic and deformable. Non-exhaustive examples of materials for balloon  108  include polymers such as polyethylene, PEBA, polyethylene terephthalate (PET), polyamide, and polyurethane. Further details regarding no-fold balloon technology are described in U.S. patent application Ser. No. 12/049,687, previously incorporated by reference in its entirety. In an embodiment, balloon catheter  100  may include a balloon  108  such as the 1.25 mm nominal diameter balloon with “Zero-Fold Technology”, which is available on the SPRINTER® Legend Balloon Catheter manufactured by Medtronic, Inc. of Minneapolis, MINN. Zero-Fold Technology includes a balloon featuring no wings or folded or wrapped material and no balloon shoulders thus facilitating crossing tightly occluded lesions. 
       FIG. 6  is an exemplary compliance curve for balloon  108  according to an embodiment hereof. As shown in  FIG. 6 , the outer diameter of balloon  108  remains unchanged at 0.60 mm during inflation fluid pressure values of zero to 5 atm. Thus, when the predetermined actuation pressure PA for collapsing compliant shaft section  150  of guidewire shaft  128  is between 1 atm and 5 atm, balloon  108  receives inflation fluid under pressure but remains unexpanded. When the inflation fluid is pressurized to the predetermined expansion pressure PE of 6 atm, balloon  108  begins to radially expand, or inflate. As the inflation fluid pressure increases to 15 atm, balloon  108  reaches its fully expanded configuration having an outer diameter of approximately 1.40 mm. 
     Outer shaft  106  may be formed of a polymeric material, non-exhaustive examples of which include polyethylene, PEBA, polyamide and/or combinations thereof, either blended or co-extruded. Optionally, outer shaft  106  or some portion thereof may be formed as a composite having a reinforcement material incorporated within a polymeric body in order to enhance strength and/or flexibility. Suitable reinforcement layers include braiding, wire mesh layers, embedded axial wires, embedded helical or circumferential wires, and the like. In one embodiment, for example, at least a proximal portion of outer catheter shaft  106  may be formed from a reinforced polymeric tube. 
     In another embodiment, catheter  100  may be modified to be of a rapid exchange (RX) catheter configuration without departing from the scope of the present invention such that guidewire shaft  128  extends within only distal portion  104 . In such an embodiment, a proximal portion of outer catheter shaft  106  may include a metal hypotube with a guidewire transition area having a proximal guidewire port being positioned proximal of balloon  108  approximately 20 cm to 25 cm. Guidewire shaft  128 , which in an embodiment may be of a polymeric tubing, would then extend within only distal portion  104  of catheter  100  surrounded by a distal portion of inflation lumen  114  and compliant shaft section  150  of guidewire shaft  128  would be positioned within balloon  108 , or just proximal or distal thereof, so as to be in contact with and actuatable by an inflation fluid as described above with reference to the OTW embodiment. 
       FIGS. 7-10  illustrate the steps of a method of crossing a chronic total occlusion according to an embodiment hereof. Although described in relation to crossing a chronic total occlusion, it should be understood that the methods and apparatus described herein may be used for crossing any tight stenoses and are not limited to total occlusions. Further, although described as advancing a balloon catheter over a previously positioned guidewire, it should be understood that the balloon catheter and guidewire may be simultaneously advanced to and through the target lesion. Typically, a guiding catheter is first inserted through an incision (not shown) and into a femoral artery of a patient. A guidewire  132  is typically pre-loaded into catheter  100  and the ensemble is then inserted into the guiding catheter and maneuvered through the vasculature to a treatment site, which in this instance is shown as a chronic total occlusion (CTO)  164  within lumen  162  of vessel  160 . In the method shown, guidewire  132  is advanced and navigated alone through occlusion  164 . As shown in  FIG. 7 , catheter  100  is positioned by a clinician such that the distal end of catheter  100  is proximal of occlusion  164 . Inflation fluid is then introduced into catheter  100  at the predetermined actuation pressure PA such that compliant shaft section  150  of guidewire shaft  128  is compressed against guidewire  132  (see also  FIG. 4 ). In one embodiment, the predetermined actuation pressure PA is less than 6 atm, and in another embodiment is between 2 atm and 3 atm. Balloon  108  receives inflation fluid but remains unexpanded such that the outer diameter of the no-fold balloon remains unchanged. 
     Referring to  FIG. 8 , with compliant shaft section  150  locked onto guidewire  132  and balloon  108  stiffened, but not expanded, by inflation fluid within the balloon at a pressure less than predetermined expansion pressure PE, the conjoined catheter  100  and guidewire  132  ensemble is pushed until the distal end of catheter  100  is pushed into occlusion  164 . In another embodiment, a channel (not shown) may be previously created within occlusion  164  by a ROTABLATOR® rotational atherectomy system, manufactured by Boston Scientific Corporation, in order to facilitate advancement of the distal end of catheter  100  within the CTO. Due to the frictional engagement of compliant shaft section  150  with guidewire  132 , the guidewire/balloon catheter ensemble provides enhanced pushability for crossing occlusion  164 . In addition, as mentioned above, unexpanded balloon  108  contains pressurized inflation fluid which makes balloon  108  stiffer and improves pushability thereof across the lesion. Catheter  100  and guidewire  132  are advanced as a unit until balloon  108  successfully “crosses” occlusion  164  to become longitudinally centered there within, as shown in  FIG. 9 . 
     Once balloon  108  is positioned within the lesion, additional inflation fluid is introduced such that the pressure in the inflation fluid is increased to the predetermined expansion pressure PE so that balloon  108  begins to radially expand. In one embodiment, the predetermined expansion pressure PE is 6 atm and in another embodiment PE is above 6 atm and may be, more particularly, between 7 atm and 10 atm. As the inflation fluid pressure is increased above the predetermined expansion pressure PE, balloon  108  will radially expand to its fully-inflated configuration to dilate occlusion  164  and thereby enlarge lumen  162  of vessel  160  at the lesion, as shown in  FIG. 10  (see also  FIG. 5 ). In an embodiment, as the inflation fluid pressure increases to between 12 atm and 15 atm, balloon  108  will reach an expanded outer diameter between 1.20 mm and 1.40 mm. Once the angioplasty procedure is completed, inflation fluid is withdrawn in order to deflate balloon  108 . Since the pressure in the inflation fluid will fall below actuation pressure PA when balloon.  108  is deflated, compliant shaft section.  150  will resume its original, non-compressed configuration, thus releasing guidewire  132 . Upon deflation, balloon  108  may form wings or folds of material around catheter  100  and catheter  100  may be retracted from the patient. If desired, another interventional catheter such as a balloon catheter having a larger balloon or a stent delivery system may be delivered over indwelling guidewire  132  to occlusion  164  in order to perform additional therapeutic procedures. 
     While various embodiments according to the present invention have been described above, it should be understood that they have been presented by way of illustration and example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the appended claims and their equivalents. It will also be understood that each feature of each embodiment discussed herein, and of each reference cited herein, can be used in combination with the features of any other embodiment. All patents and publications discussed herein are incorporated by reference herein in their entirety.