Patent Publication Number: US-2005119682-A1

Title: Vascular exclusion catheter

Description:
This is a non-provisional application claiming the benefits of Patent Convention Treaty Application Serial No. PCT/US02/28830 filed on Sep. 10, 2002 and entitled “Vascular Exclusion Catheter”; and U.S. Provisional Application Ser. No. 60/339,901 filed on Oct. 30, 2001 and entitled “Vascular Exclusion Catheter”, both of which are fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates generally to apparatus and methods for at least partially occluding flow within a body conduit.  
      2. Discussion of the Relevant Art  
      Body conduits commonly provide for a flow of fluid from one location in the body to another location in the body. Typical of these fluid conduits are arteries and veins of the vascular system which provide a flow of blood between the heart and the organs of the body. When a particular procedure requires that the vessel be accessed, the flow of blood can be expected to exit the conduit through any access hole. This not only results in a loss of fluid such as blood, but also invades the general surgical environment with the fluid. In one such procedure, it is desirable to harvest the saphenous vein from the leg and to connect that vein to the ascending aorta in a Coronary Artery Bypass Grafting (CABG) procedure.  
      In the past, surgeons used an occlusion catheter to stop the flow of blood through the conduit or vessel. This catheter was provided with a spherical balloon which, when inflated, would totally obstruct the blood flow within the vessel. Particularly in the case of blood vessels, this is undesirable as an uninterrupted flow of blood is necessary to maintain the tissues of the body.  
      In order to avoid total occlusion, another procedure has been developed whereby the blood is totally removed upstream of the operative site and introduced down-stream of the operative site. In this procedure, commonly referred to as an “on-pump” (OPCABG) procedure, there is continuous uninterrupted beating of the heart. Nevertheless, this procedure requires management of blood flow from the aortotomy in order to create a viable proximal anastomosis. It is for this reason that many CAPD procedures are still performed off-pump.  
      Presently the surgeon&#39;s primary tool to accomplish sensation of blood flow from the aortotomy is a Partial Occluding Clamp. In these off-pump procedures, the Partial Occlusion Clamp is often used to engage the conduit or vessel exteriorly and thereby isolated a small portion of the vessel from the ongoing fluid flow.  
      While the partial occluding clamp is relatively simple to use, it is perceived by many to be very traumatic. Its use has been reported to cause secondary complications such as the fracturing of plague and resultant Transient Ischemic Attack or Cerebral Vascular Accident, with both local and global consequences. The partial occluding clamp also consumes much of the procedural area not only with its jaws on the aorta, but also with its clamp handles in the surgical field.  
     SUMMARY OF THE INVENTION  
      These deficiencies of the past are overcome with the present invention which includes a catheter with a dilation assembly capable of maintaining fluid flow through a conduit while excluding a portion of the conduit from this fluid flow. Importantly, this catheter is non-invasive and is inserted endoluminally so that it does not require major space in the surgical environment. The dilation assembly of the catheter is capable of maintaining fluid flow within the conduit while producing an exclusion cavity that isolates a portion of the conduit from this fluid flow.  
      In one aspect the invention, a fluid-control device is adapted for disposition in a body conduit for controlling a flow of body fluids in the body conduit. The device includes a wall of separation having a first surface and an opposing second surface. The first surface defines a flow passage facilitating the flow of body fluids within the body conduit; and the second surface of the wall defines an exclusion chamber sealed from the flow passage and the flow of body fluids through the body conduit.  
      In another aspect of the invention, a catheter is adapted for disposition in a body conduit and includes a shaft which extends along an axis between a proximal end and a distal end. A dilation assembly is disposed at the distal end of the shaft and includes a first dilator operable to move between a high-profile state and a low-profile state. A second dilator is included in the assembly and is operable to move generally independently of the first dilator between the high-profile state and the low-profile state. A sleeve is carried by the first dilator and the second dilator between the high-profile state and the low-profile state.  
      In another aspect of the invention, a catheter is adapted for disposition in a body conduit. The catheter includes a shaft and a dilation assembly disposed at a distal end of the shaft. The dilation assembly has a low-profile state facilitating insertion of the assembly into the body conduit and a high-profile state facilitating operation of the assembly within the body conduit. The shaft includes an inner member which is disposed in a telescoping relationship with an outer member. A dilator has a first end carried by the outer member and a second end carried by the inner member. These first and second ends have a generally proximate relationship when the dilator is in the high-profile state and a generally spaced relationship when the dilator is in the low-profile state.  
      In another aspect of the invention, the dilation assembly includes a balloon that is inflatable to move the balloon to the high-profile state. In this state, first portions of the balloon define a fluid flow path to facilitate a flow of fluids in the body conduit.  
      In a further aspect, the invention includes an endovascular method for restricting blood flow along a predetermined area of a vessel without occluding blood flow through the vessel. This method includes the step of providing a catheter with a dilation assembly having a wall movable between a high-profile state and a low-profile state. The assembly is inserted into the vessel to an operative site in the low-profile state. At the operative site, the assembly is dilated to move the wall to the high-profile state where the wall defines with the predetermined area of the vessel an occlusion cavity isolated from the blood flow within the vessel.  
      These and other features and advantages of the invention will be further discussed with reference to preferred embodiments of the invention and reference to the associated drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an axial cross-section of a catheter having a dilation assembly in accordance with the present invention;  
       FIG. 2  is an axial cross-section view similar to  FIG. 1  and illustrating the dilation assembly in a low-profile state;  
       FIG. 3  is an axial cross-section view showing the dilation assembly in a high-profile state and disposed within a body conduit;  
       FIG. 4  is an end view of the dilation assembly taken along lines  4 - 4  of  FIG. 3 ;  
       FIG. 5  is an axial cross-section view similar to  FIG. 3  and illustrating movement of the dilation assembly between the high-profile state and the low-profile state;  
       FIG. 6  is an axial cross-section view similar to  FIG. 5  and illustrating a sleeve carried by dilators in a body conduit having a variable diameter;  
       FIG. 7  is an axial cross-section view similar to  FIG. 6  and illustrating formation of a flow passage and in an exclusion cavity in accordance with the present invention;  
       FIG. 8  is an axial cross-section view of an additional embodiment wherein the exclusion cavity has an annular circumferential configuration;  
       FIG. 9  is a perspective view showing the embodiment of  FIG. 8  in a body conduit;  
       FIG. 10  is an axial cross-section view showing use of the dilation assembly to occlude a secondary conduit without occluding a primary conduit;  
       FIG. 11  is a side-elevation view of a further embodiment of the invention;  
       FIG. 12  is a radial cross-section view taken along lines  12 - 12  of  FIG. 11 ;  
       FIG. 13  is a perspective view of the embodiment of  FIG. 11  showing an inflatable dilator in a high-profile state;  
       FIG. 14  is a perspective view of the dilator illustrated in  FIG. 13  disposed in a body conduit;  
       FIG. 15  is a perspective view similar to  FIG. 14  and showing dilation assembly of  FIG. 11 ;  
       FIG. 16  illustrates placement of two sheets of material to form the inflatable dilator;  
       FIG. 17  illustrates the formation of heat seals to form seams of the balloon;  
       FIG. 18A  illustrates a step for forming a seal line to define a lateral recess;  
       FIG. 18B  is a radial cross-section view of the balloon taken along lines  18 B- 18 B of  FIG. 18A ;  
       FIG. 19  is a perspective view of a further embodiment of the invention including circumferential connection lines;  
       FIG. 20  is a radial cross-section view taken along lines  20 - 20  of  FIG. 19 ;  
       FIG. 21  is a further embodiment of the invention, including diagonal connection lines; and  
       FIG. 22  is a radial cross-section view taken along lines  22 - 22  of  FIG. 21 .  
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS AND BEST MODE OF THE INVENTION  
      An exclusion catheter apparatus is illustrated in  FIG. 1  and designated generally by the reference numeral  10 . This particular apparatus  10  is adapted to exclude a segment of a body conduit while facilitating flow through the remainder of the conduit. The apparatus  10  comprises a handle assembly  20  with a hand piece  22  and an axially movable thumb slide  24 . The thumb slide  24  is coupled to an inner elongate member  26  of a tube assembly  31 . In the preferred embodiment, the inner elongate member  26  comprises a tube with a hollow core or lumen  27 . Alternatively, the inner elongate member  26  may have a solid core and, thus, comprise a wire, for example.  
      The handle assembly  20  is coupled to the tube assembly  31 , which in this embodiment comprises a first proximal outer tube  33  coupled to a distal portion  35  of the handle assembly  20 . The inner elongate member  26  is disposed within the proximal outer tube  33 , and extends distally outwardly from a distal tip  37  of the outer tube  33 . A second floating outer tube  39  is disposed distally of the proximal outer tube  33  and is slidingly carried by the inner member  26 . A third distal outer tube  42  is disposed distally of the floating outer tube  39  and secured to a distal portion  44  of the inner elongate member  26 .  
      The tube assembly  31  includes a first proximal dilator  46  and a second distal dilator  48  which are movable between a low profile state, as illustrated in  FIGS. 1 and 2 , and a high profile state as illustrated in  FIG. 3 . The dilators  46 ,  48 , are each provided with a permeable configuration in order to facilitate fluid flow through the dilators  46 , 48  in the high-profile state. In a preferred embodiment, each dilator  46 ,  48  comprises a braided tube which may be composed of a mesh of wires configured in a diamond or crisscross pattern as shown in  FIG. 4 . As best illustrated in the detail of  FIG. 3 , the proximal dilator  46  comprises a first dilator proximal end  51  secured to the proximal outer tube  33 , and a first dilator distal end  53  secured to a proximal portion of the floating outer tube  39 .  
      The distal dilator  48  comprises a second dilator proximal end  55  secured to a distal portion of the floating outer tube  39 , and a second dilator distal end  57  secured to the distal outer tube  42 . The ends of each dilator  46 ,  48  are configured to move with respect to each other in order to facilitate transition between a spaced-apart relationship, associated with the low profile state, and a proximate relationship, associated with the high profile state. It follows that the distance between the ends of each of the dilators  46 ,  48  determines the profile state of that dilator.  
      A sleeve  60  is coupled to the proximal dilator  46  and the distal dilator  48 . The sleeve  60  surrounds the floating tube  39  and adjacent portions of the dilators  46 ,  48 . In a preferred embodiment the sleeve  60  is composed of a thin-walled elastomeric material which is coupled to the dilators  46 ,  48  through a heat-sealing process. As fluid passes through the sleeve  60 , the resulting fluid pressure expands the wall of the sleeve  60 . An indented or recessed side portion  66  of the sleeve  60  is adapted to form an isolated exclusion chamber or recess  67  when the sleeve  60  is expanded to the high profile state. In an alternative embodiment, the sleeve  60  may omit the recess  67  and thus comprise an axially uniform cylinder.  
      In order to effect a low-profile state in the embodiment of  FIGS. 1 and 2 , the thumb slide  24  can be moved in a distal direction along the handpiece  22  causing the inner elongate member  26  to extend distally. Accordingly, the distal outer tube  42  is spaced apart from the floating outer tube  39  which is in turn spaced apart from the proximal outer tube  33 . As these gaps are formed between the outer tubes  42 ,  39 ,  33 , spaced-apart relationships are facilitated between first dilator proximal end  51  and the first dilator distal end  53 , as well as between the second dilator proximal end  55  and the second dilator distal end  57 .  
      The low-profile state of the dilators  46 ,  48  enables smoother introduction and removal of the apparatus  10  through body conduits, thereby minimizing trauma to the patient. Furthermore, the dilators  46 ,  48 , and the sleeve  60  can be coated with an antithrombin agent and/or a hydrophilic coating to eliminate any potential thrombogenic response from the body conduit.  
      To effect a high-profile state of the dilators  46 ,  48 , the thumb slide  24  is moved in a proximal direction along the handpiece  22 , causing the inner elongate member  26  to move proximally. Fixed to the inner elongate member  26 , the distal outer tube  42  also moves proximally carrying with it the second dilator distal end  57 . The proximally directed force may also move the floating tube  39  in a proximal direction toward the proximal outer tube  33 . As a result, the first dilator distal end  53  and the first dilator proximal end  51  move closer together. Similarly, the second dilator distal end  57  and the second dilator proximal end  55  also move closer together. Maximum dilation of the dilator  46 ,  48  may be achieved when the distal outer tube  42  is directed proximally to abut the floating tube  39 , and when the floating tube is directed proximally to abut the proximal outer tube  33 . In this configuration, the distal ends  53 ,  57  of the dilators  46 ,  48  are moved closely adjacent to the respective proximal ends  51 ,  55 , as shown in  FIG. 3 . An incremental locking mechanism (not shown) may be provided on the thumb slide  24  to releasably lock each of the dilators  46 ,  48  to a preferred expanded diameter.  
       FIG. 5  illustrates two additional features which may be associated with the present invention. First, it will be noted that the elongate member  26  can be provided with the axial lumen  27  to facilitate insertion of the catheter apparatus  10  over a guidewire  61 . Second,  FIG. 5  illustrates that the catheter apparatus  10  can be used in a conduit which is smaller than the maximum diameter which can be achieved by the dilators  46  and  48 .  
      In  FIG. 5  it will be noted that these dilators,  46 ,  48  have expanded to meet the body conduit portion  68  and to carry the sleeve  60  into contact with this body conduit portion  68 . This desirable result is achieved even though the dilators  46  and  48  have not been expanded to their maximum diameter as discussed with reference to in  FIG. 4 .  
      When the dilator  48  has a diameter less than its maximum diameter, it will also have an increased width along the axis of the catheter apparatus  10 . This increased width is associated with a greater separation between the distal end of the floating outer tube  39 , and the proximal end of the distal outer tube  42 . Similarly, when the dilator  46  has a diameter less than its maximum diameter, it will have an increased width along the axis of the catheter apparatus  10  and greater separation between the distal end of the proximal outer tube  33  and the proximal end of the floating outer tube  39 .  
      In  FIGS. 6 and 7 , the catheter apparatus  10  is illustrated in the body conduit  64 . However in this case the conduit  64 , more realistically, has a variable rather than a constant diameter. This is illustrated more specifically by the diameter D1 in proximity to the dilator  46 , and the larger diameter D2 in proximity to the dilator  48 . With the intent of maximizing flow through the sleeve  60 , the catheter apparatus  10  is operable to move the sleeve  60  into contact with the inner wall  62  even when the conduit  64  has a variable diameter.  
      In operation, the elongate member  26  is moved proximally which initiates the process of expanding the dilators  46  and  48  as previously discussed. It is likely that only one of the dilators  46 ,  48  will expand until it contacts the inner wall  62 . This will fix the floating outer tube  39  so that further proximal movement of the elongate member  26  will expand the diameter of the other dilator. In  FIG. 6 , the elongate member  26  is moved proximally along with the distal outer tube  42  and the floating outer tube  39 . This closes the spacing between the proximal outer tube  33  and the floating outer tube  39 , and accordingly increases the diameter of the dilator  46 . When the dilator  46  reaches the diameter D1 of the inner wall  62 , the movement of the floating outer tube  39  stops. The continued proximal movement of the elongate member  26  brings the distal outer tube  42  into closer proximity with the floating outer tube  39  thereby increasing the diameter of the dilator  48 . The diameter of the dilator  48  will increase until it reaches the diameter D2 associated with the inner wall  62  at that location.  
      With reference to  FIG. 7 , it will be noted that the sleeve  60  is brought into contact with the inner wall  62 , notwithstanding the variable diameters of the body conduit  64 . Notably, this highly desirable feature is achieved because the dilators  46  and  48  can be provided with individual diameters that are independent of each other.  
      It will also be appreciated that full expansion of both dilators  46 ,  48  is accomplished when the force exerted against a first adjacent body wall by the first dilator  46  is equal to the force exerted against a second adjacent body wall by the second dilator  48 . Therefore, in any body conduit wherein the diameters of the conduit portions adjacent to the dilators are not uniform, the self-adjusting characteristics of the apparatus  10  enable each dilator  46 ,  48  to expand to contact the respective adjacent portions with the same force.  
      As previously noted, the recessed sleeve portion  66  is radically spaced from the isolated body conduit portion  68  between the dilators  46 ,  48 . The permeable dilators  46 ,  48  enable fluid to pass through the sleeve  60  with a resulting fluid pressure which distends the sleeve  60  to contact the inner wall  62  of the body conduit  64 . Thus, the sleeve  60  facilitates flow through the body conduit  64  while isolating the particular body conduit portion  68 . As a result, an isolated exclusion chamber  67  is defined by the recessed sleeve portion  66  and the isolated body conduit portion  68 .  
      This optimizes the flow of fluid passing by the selected surgical site while the isolated portion  68  of the conduit remains excluded. Thus drugs, such as therapeutics, and fluids, such as irritants, may be delivered to or aspirated from the exposed conduit portion without risk of leakage into the isolated conduit portion  68 . Tissue biopsy could also be obtained via the lateral access recess  66 . An anastamosis or repair of the conduit portion  68  could also be performed while body fluid continues to flow through the remainder of the conduit  64 . In particular, the body conduit portion  68  may be accessed exteriorly via a puncture, for example. Blood loss is minimized since only the volume contained in the isolated chamber  67  would be subject to loss. The sleeve  60  directs the passing fluid through the body conduit  62  and thus prevents any fluid communication between the flow channel of the sleeve  60  and the isolated chamber  67 .  
      An additional embodiment of the invention is illustrated in  FIGS. 8 and 9  where structural elements similar to those previously described are designated by the same reference numeral followed by the lower case letter “b.” Thus, in the embodiment of  FIGS. 8 and 9 , an alternative sleeve  60   b  is provided. The elongate member  26   b  in this embodiment includes the proximal outer tube  33   b  and the distal outer tube  42   b  which telescopes within the proximal outer tube  33   b . A skeletal structure  70  is formed by a plurality of bendable members such as wires  72 , each having two ends, one fixed to the outer proximal tube  33   b  and the other fixed to the distal outer tube  42   b . With this construction, the distal outer tube  42   b  can be moved relative to the proximal outer tube  33   b  to provide the skeletal structure  70  with both a low-profile state and a high-profile state.  
      For example, if the distal outer tube  42   b  is moved distally of the proximal outer tube  33   b , the ends of the wires  72  are widely separated. This causes the wires  72  to move into close proximity with the elongate member  26   b  in the low-profile state. However, when the distal outer tube  42   b  is moved proximally relative to the proximal outer tube  33   b , the ends of the wires  72  become closely spaced. This causes the wire  72  to move generally radially to a high profile state as illustrated in  FIGS. 8 and 9 .  
      In order to form the sleeve  60   b , a cover  74  is disposed over the skeletal structure  70 . This cover  74  is typically formed of a distensible or elastomeric material and provided with a tubular configuration so that it at least partially covers the skeletal structure  70 . At a central portion or waist  83 , the cover  74  is provided with a collar or belt  85  which maintains the waist  83  at a reduced diameter in the high-profile state. As a result, the combination of the cover  74  and belt  85  provide the sleeve  60   b  with an hour-glass configuration. On either side of the belt  85 , the cover  74  is free to expand to a relatively large diameter with the wires  72 . However, at the central portion of the waist  83 , the belt  85  limits this expansion to a reduced diameter.  
      Thus, the belt  85  provides the sleeve  60   b  with the recess  67   b  which in this case is formed circumferentially between the dilators  46   b  and  48   b . When operatively disposed as illustrated in  FIG. 9 , the sleeve  60   b  isolates the body conduit portion  68   b  which in this case comprises a full 360 degree or circular portion of the body conduit  64   b . Notwithstanding this isolated conduit portion  67   b , the sleeve  60   b  is capable of continuing fluid flow within the body conduit  64   b . Thus, a surgeon may exteriorly remove or puncture any part of the full circular conduit portion  68   b  without disrupting fluid flow through the remainder of the conduit  64   b.    
      In some cases, it may not be necessary to operate on the isolated body conduit portion  68 , but only to isolate the body conduit portion  68  from the flow in the main body conduit  64 . In these instances, an embodiment such as that illustrated in  FIG. 10  may be appropriate. In  FIG. 10 , elements of structure similar to those previously disposed are designated with the same reference numeral followed by the lower-case letter “c”. Thus, the sleeve  60   c  in this embodiment comprises an axial uniform cylinder which omits any recessed portion. In this case, the cylindrical sleeve  60   c  completely isolates a body conduit portion  91  which includes a branch conduit  93 , for example. With the intent of merely isolating this branch conduit  93  from the flow in the main conduit  94   c , there is no need for a recess such as that designated by the reference numeral  67   b  in the embodiment of  FIG. 9 .  
      From the foregoing description, it will be apparent that the dilators  46  and  48  may comprise a variety of structures. In the embodiment of  FIGS. 11-15 , elements of structure similar to those previously discussed or designated with the same reference numeral followed by the lower-case letter “d.” 
      In  FIGS. 11-13 , the vascular exclusion catheter apparatus  10   d  includes a single inflatable dilator or balloon  112 , that also serves as a dilating sleeve  113 . This dilating sleeve  113  is coupled to a catheter shaft  114  that extends from the handle assembly  20   d . The shaft  114  comprises an outer tube  116  which in this case defines a relatively large through-lumen  118 . The through-lumen  118  is sized and configured to receive a standard guidewire which can be used to place the catheter apparatus  10   d  and to otherwise orient the dilating sleeve  112  at the operative site.  
      The handle assembly  20   d  includes a stopcock  119  which controls access to the through-lumen  118 . An inner tube  121  having an inflation lumen  122  accessible through an inflation port  123 , is coupled to a proximal portion of the outer tube  116 . In a preferred embodiment, the inner tube  121  extends only partially along the through-lumen  118  terminating within the through-lumen  118  in proximity to the dilating sleeve  112 . Thus, an inflation gas exiting from the inflation lumen  122  is directed through the through-lumen  118  into the dilating sleeve  112 .  
      In this manner, gas from the inflation lumen  123  inflates the balloon or dilating sleeve  112  to a high profile state. In this state, the sleeve is circumferentially inflated but defines an axial flow passage shown by the arrows  124  in  FIG. 13 .  
      A preferred method of constructing the balloon  112  is illustrated in  FIGS. 16, 17 ,  18   a  and  18   b . In accordance with this method, the balloon  112  is formed of two layers,  125  and  126 , of thermoplastic material, each sealed or otherwise joined together, for example, along seams  127 ,  128  and  129 . The layers  125  and  126  can also be spot welded at a plurality of layer-joining connection points  130 . With this construction, the balloon  112  is formed between the layers  125  and  126  and bounded by the seams  127 - 129 . The catheter shaft  114  can be inserted between, and sealed to the seams  127  and  128 . This gives the catheter shaft  114 , and particularly the inflation lumen  122  access to the interior of the balloon  112  between the layers  125  and  126 . With this construction, the balloon  112  can be formed into the cylindrical configuration of the sleeve  113  by rolling the layers  125  and  126  back on themselves and attaching the seam  127  to the seam  129  as illustrated in  FIG. 18   b . In this embodiment, if the recess  66   d  is desired, it can be formed by removing a portion  138  as illustrated in  FIG. 18   a  and forming a seal of  139  to join the four edges of the layers  125  and  126 .  
      As illustrated in  FIG. 18   b , the catheter shaft  114  can be formed with multiple lumens, namely, the through-lumen  118  and the inflation lumen  122 . With this construction, at least one skive  131  can be cut in the shaft  114  to access the inflation lumen  122 . In operation, the inflation gas will pass from the inflation lumen  122  through the skive  131  and into the balloon  112  between the layers  125  and  126 .  
      With further reference to  FIGS. 14 and 15 , it will be noted that the dilating sleeve  113  facilitates maximum fluid flow while excluding or isolating a specific area  144  of a body conduit  146  to form an isolated chamber  148 . The lateral recess  66   d  facilitates access to a portion of the body conduit for fluid or therapeutic administration, tissue biopsy, anastomosis procedure, or for repairing damage while body fluid continues to flow through the conduit. As with previous embodiments, the apparatus  10   d  may be introduced to the surgical site through either percutaneous or direct access.  
      In an alternate embodiment shown in  FIGS. 19 and 20 , elements of structure similar to those previously disclosed are designated with the same reference numeral followed by the lower-case letter “e.” Thus, this embodiment includes the tube assembly  31   e , the balloon  112   e , the outer tube  116   e , and the inflation lumen  122   e . However, in this case additional tubes  151 ,  153  are disposed within the tube  116 . These additional tubes  151 ,  153  provide further lumens  155 ,  157 , respectively, for fluid administration. Also, the dilating sleeve  113   e  comprises an inner balloon layer joined to the outer balloon layer via transverse connection lines  159 , instead of the connection points  130  shown in the embodiment of  FIGS. 16-18 .  
      In a further embodiment, illustrated in  FIGS. 21 and 22 , elements of structure similar to those previously disclosed are designated with the same reference numeral followed by the lower-case letter “f.” Thus, this embodiment includes the tube assembly  31   f , the balloon  112   f  and the connection lines  159   f . In this case, however, the dilating sleeve  112   f  may be formed without any recess  67  ( FIG. 5 ) and thus may comprise an axially uniformed cylinder.  
      Furthermore, the outer layer  126   f  of the balloon  112   f  may be provided with a lesser thickness than the inner layer  125   f  thereof. This difference in layer thickness facilitates expansion of the balloon  112   f  toward the thinner area upon inflation. Thus, as the outer layer  127   f  is expanded, the inner layer  129   f  is uniformly pulled along with the outer layer.  
      With the specific disclosure of the foregoing embodiments, it will be apparent that many alterations and modifications can be made without departing from the spirit and scope of the invention. It is for this reason that the illustrated embodiments are set forth only as examples and should not be taken as limiting the invention.  
      The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification the generic structure, material or acts of which they represent a single species.