Abstract:
A device for steering a cardiac cryoablation catheter through the vasculature of a patient includes separate first and second deflection members that connect a tip member to the distal end of a catheter tube. Importantly, the first deflection member has a first flexural modulus, and the second deflection member has a second flexural modulus that is greater than the first flexural modulus. Also included is a control wire for pulling the tip member toward the catheter tube. In response, the first and second deflection members differentially deflect, according to their respective flexural moduli, to move the tip member for steering the catheter through the vasculature.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to mechanisms for steering catheters through the vasculature of a patient. More particularly, the present invention pertains to steering mechanisms that rely on a differential bending of separate structures to deflect the distal tip of a catheter for purposes of steering the catheter. The present invention is particularly, but not exclusively, useful as a steering mechanism for a cardiac cryoablation catheter. 
     BACKGROUND OF THE INVENTION 
     Steerability, among several attributes, is an important consideration in the manufacture and operation of an invasive catheter. In particular, when the operation of a catheter requires that it be advanced through portions of a patient&#39;s vasculature, the ability to steer the catheter along tortuous paths, and into selected branches of the vasculature, is of crucial importance. Further, in addition to having good steering properties, it may also be important to conform the catheter to a particular configuration as it is positioned in the vasculature. In either case, the steering and configuring of an invasive catheter requires that the distal tip of the catheter be articulated in a safe, predictable and controllable manner. 
     Several devices have been previously suggested for the purpose of steering a catheter through the vasculature of a patient. In the earlier mechanisms, such as the one disclosed in U.S. Pat. No. 1,060,665, that issued to Bell on May 6, 1913, for an invention entitled “Catheter”, the steerability of the catheter was provided for by using a pre-bent stiffening member in the catheter&#39;s distal end. Subsequently, more complex devices have relied on a pull-wire to deflect the catheter tip. In general, these mechanisms have variously included concentric or eccentric pull-wires that generate an eccentrically applied force on the tip of the catheter. For example, U.S. Pat. No. 4,456,017, which issued to Miles for an invention entitled “Coil Spring Guide with Deflectable Tip” incorporates a concentric core wire for this purpose. On the other hand, U.S. Pat. No. 4,586,923, which issued to Gould et al., uses an eccentric wire for the same purpose. Further, devices have also been proposed which will bias the deflection of a catheter tip in a predetermined plane. An example of such a device is disclosed in U.S. Pat. No. 4,886,067, which issued to Palermo. In the Palermo patent, such a bias is established by flattening the core wire. 
     Heretofore, as indicated by the examples given above, the steerability of a catheter tip has been primarily engineered by determining the direction in which a deflecting force should be applied to the tip. Accordingly, these earlier devices did not specifically incorporate structural aspects into the construction of a catheter&#39;s distal portion with a view toward using this construction as a functional aspect for tip deflection. Such a consideration, however, becomes more significant when, in addition to steerability, the configurability of a catheter in the vasculature of a patient is an important consideration. 
     In accordance with well known engineering applications, structures will predictably bend according to their shape of the structure and according to particular properties of the material, such as its modulus. By definition, a modulus is the ratio of stress to strain and, for a given material, is constant up to an elastic limit. Importantly, a modulus can be used as a measure of the deflection a material will experience under stress. Also, by definition, stress is the force per unit area acting on a material and tending to change its dimensions, i.e. cause a strain. With this in mind, it is evident to the skilled artisan that when two different materials are subjected to the same force, the materials will experience different strains according to their respective moduli. Further, when two different materials are incorporated into the same structural component of a system, a differential modulus is created for the component by the respective moduli that biases, or favors, a bending of the component according to the dictates of the material having the higher (flexural) modulus. 
     In light of the above, it is an object of the present invention to provide a device for steering a cardiac cryoablation catheter through the vasculature and in and around the heart of a patient that can be both steered and configured, as desired, while the catheter is in the vasculature and heart of a patient. Another object of the present invention is to provide a device for steering a cardiac cryoablation catheter through the vasculature and heart of a patient that relies on a differential modulus in the structure of the catheter&#39;s distal portion to steer and reconfigure the catheter. Still another object of the present invention is to provide a device for steering a cardiac cryoablation catheter through the vasculature and heart of a patient that is relatively easy to manufacture, is simple to use, and is comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     A device for steering a cardiac cryoablation catheter through the vasculature and heart of a patient includes a resilient, cylindrical-shaped articulation segment that is connected to the distal end of a catheter tube. The articulation segment defines a longitudinal axis for the device and, further, the segment is formed with a lumen that extends between its proximal and distal ends. A tip member is affixed to the distal end of the articulation segment, and a flexible spine extends within the lumen between the proximal and distal ends of the segment. Importantly, the flexible spine is off-set from the axis of the articulation segment, and is oriented substantially parallel thereto. 
     One end of a control wire is attached to the tip member, while the control wire itself extends from the tip member, through the lumen of the articulation segment and through the catheter tube. As intended for the present invention, the control wire is connected to the tip member at an attachment point that lies between the axis of the articulation segment and a location with some opposition to the spine (e.g. diametrically opposite). Further, the device of the present invention includes a mechanism that is engaged with the control wire at the proximal end of the catheter tube for axially pulling on the control wire. 
     As intended for the present invention, the articulation segment has a first flexural modulus, and the spine has a second flexural modulus. More specifically, for the device of the present invention, the second flexural modulus of the spine is greater than the first flexural modulus of the articulation segment. Consequently, in combination with each other, the spine and the articulation segment establish a differential modulus. Thus, in response to a pulling of the control wire in a proximal direction, the differential modulus allows the tip member to be deflected for the purpose of steering or configuring the catheter in the vasculature and heart of a patient. Additionally, due to the relative location of the spine on the articulation segment, a direction for the deflection of the tip member can be established. 
     For a specific application of the present invention, the tip member is made of a material having a relatively high thermal conductivity. Additionally, the device produces a fluid which is in a fully saturated liquid state at the operational pressure used for the system. A transfer tube, that extends from the refrigeration source and passes through the catheter tube and through the lumen of the articulation, interconnects the refrigeration source in fluid communication with the tip member. With this connection, the cooled fluid can be released within the tip member during a cardiac cryoablation procedure. The spent refrigerant can then be removed through the lumen of the articulation segment and the catheter tube. 
     In the manufacture of the device for steering a cardiac cryoablation catheter through the vasculature of a patient, the articulation segment is made with a helical spring which defines the axis and forms the lumen. An inner tube is positioned in the lumen of the helical spring, and an outer tube is positioned against the helical spring opposite the inner tube. The outer tube is then bonded to the inner tube to embed the helical spring therebetween or this whole segment can be made by a continuous or intermittent heat extrusion process. Preferably, both the inner tube and the outer tube can be made of a Pebax material or other suitable material, such as a polyurethane. In any event, the resultant cylindrical-shaped articulation segment will have a first flexural modulus. 
     Once the articulation segment has been made, the flexible spine is positioned in its lumen and fixedly attached to the articulation segment to extend between its proximal and distal ends. Importantly, as mentioned above, the spine is oriented on the articulation segment off-set from the axis and substantially parallel thereto. As also mentioned above, it is important that the spine have a second flexural modulus that is greater than the first flexural modulus of the articulation segment. In this combination, the tip member is affixed to the distal end of the articulation segment. 
     A mechanism for controlling the deflection of the tip member is provided by attaching a control wire to the tip member. Specifically, the control wire is attached to the tip member at an attachment point on the tip member. Preferably, the attachment point lies between the axis of the articulation segment and a location in some opposition to the spine (e.g. diametrically opposite), but this need not necessarily be so. In any case, it is the intent of the present invention that, due to the difference in the respective flexural moduli of the articulation segment and the spine, whenever the control wire is pulled, the tip member will predictably bend through an arc in a predetermined plane for the purposes of steering and configuring a catheter in the vasculature and heart of a patient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a perspective view of a catheter incorporating the device of the present invention, as it is being advanced into the vasculature of a patient for an invasive procedure; 
         FIG. 2  is a segmented, perspective view of the device of the present invention when it is incorporated into a cardiac cryoablation catheter; 
         FIG. 3  is a cross sectional view of the segments of the device of the present invention as seen at the distal end portion of a catheter along the line  3 — 3  in  FIG. 2 ; 
         FIG. 4  is a free body diagram of forces acting on the tip member of the device of the present invention as the tip member of the device is being deflected for steerage of the catheter through the vasculature of the patient; and 
         FIG. 5  is a side plan view of the device of the present invention in a fully deflected configuration. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , a cardiac cryoablation catheter (device) in accordance with the present invention is shown and is designated  10 . In  FIG. 1 , the device  10  of the present invention is shown as it is being positioned in the vasculature and into the heart of a patient  12 . Importantly, the device  10  includes a tip member  14  that is located at the distal end of the device  10 . Further, the device  10  includes an articulation segment  16  that is attached proximal to the tip member  14 . Still further, a catheter tube  17  is attached proximal to the articulation segment  16 . 
     Referring now to  FIG. 2 , it will be seen that both the articulation segment  16  and the catheter tube  17  are formed with a contiguous lumen  18  that essentially extends through the length of the device  10 . Further,  FIG. 2  indicates that a control wire  20  extends through the lumen  18  from an extracorporeal control mechanism  22  to the tip member  14 . For example, the control mechanism  22  can include a pivot arm  24  which can be rotated about the pivot point  26  by an operator (not shown) to exert a proximally directed force on the control wire  20 . It will be appreciated by the skilled artisan, however, that the control mechanism  22  shown in  FIG. 2  is only exemplary. Any device known in the pertinent art for generating an axial force on the control wire  20  is suitable for the present invention. Further, it will be appreciated that the control mechanism  22  may be attached directly to the catheter tube  17 . 
     Still referring to  FIG. 2 , the device  10  is shown to include a refrigeration source  28  which is to be used for the purpose of cooling a fluid. Also shown is a transfer tube  30  that extends from the refrigeration source  28  through the lumen  18  of both the catheter tube  17  and the articulation segment  16 . For the device  10 , this transfer tube  30  connects the refrigeration source  28  in fluid communication with the tip member  14 . Thus, a fluid which is in a fully saturated liquid state at the operational pressure used for the system can be delivered to the tip member  14 . Additionally,  FIG. 2  shows that, within the articulation segment  16  there is a spine  32  that is positioned between the tip member  14  and the catheter tube  17 . The details of the articulation segment  16 , and its interactive components, will perhaps be best appreciated with reference to  FIG. 3 . 
     In  FIG. 3 , it can be seen that the articulation segment  16  includes an inner wall  34 , an outer wall  36 , and a helical spring  38  that is embedded between the inner wall  34  and the outer wall  36 . As intended for the present invention, both the inner wall  34  and the outer wall  36  are made of a Pebax material or other suitable material, such as a polyurethane. Thus, the inner wall  34  can be bonded with the outer wall  36  in any manner known in the pertinent art, such as by thermal bonding, or by the use of an appropriate glue or cement or by an extrusion process. In any case, it is important that an effective flexural modulus is established for the articulation segment  16  (i.e. collectively, the walls  34 ,  36  and the helical spring  38 ). Importantly, this modulus of the articulation segment  16  must be less than the modulus of the spine  32  when it is positioned within the lumen  18  of the articulation segment  16 . 
     When considered together, because they individually have different moduli, the articulation segment  16  and the spine  32  effectively establish a differential (flexural) modulus for the device  10 . With this difference in mind, it should be noted that the articulation segment  16  and the spine  32  are, preferably, co-extensive. Stated differently, they essentially have the same effective length. This is accomplished by having both of the components, articulation segment  16  and spine  32 , positioned between the tip member  14  and the distal end of the catheter tube  17 . Insofar as the spine  32  is specifically concerned,  FIG. 3  shows that the spine  32  is positioned between the tip member  14  and the catheter tube  17  to urge against or is attached to an abutment  40  that is formed as part of the catheter tube  17 . 
     For a discussion of the operation of the device  10  of the present invention, changes in its configuration are perhaps best described relative to the axis  42 . More specifically, for this purpose the axis  42  can be generally considered as being the longitudinal axis, or centerline, of the device  10 . From this reference, it is then necessary to identify the interactive forces that are involved in the operation of the device  10 , and the locations where these forces act on the device  10 . This is best accomplished by cross-referencing  FIG. 3  with  FIG. 4 . 
     Referring first to  FIG. 3 , it will be seen that the control wire  20  is attached to the base  44  of tip member  14  at an attachment point  46 . Also,  FIG. 3  shows that the spine  32  is positioned to effectively urge against the tip member  14  at a point  48 . Relative to the axis  42 , the attachment point  46  is preferably diametrically opposite the point  48 , though the points  46  and  48  need not necessarily be at a same radial distance from the axis  42 . In fact, as shown in  FIG. 4 , for purposes of discussion, the attachment point  46  is considered to be at a radial distance “a” from the axis  42 , while the point  48  is at a radial distance “d” from the axis  42 . 
     A free body diagram of the forces acting on tip member  14  (represented by its base  44 ) during an operational deflection of the tip member  14 , are shown in  FIG. 4 . Though only tip member  14  is being specifically considered, it will be appreciated by the skilled artisan that the reaction of the articulation segment  16 , and the deflection of the tip member  14  in response to the application of a force, F w , on the tip member  14  by the control wire  20  is the important result. 
     For the static equilibrium of a body or structure, such as the device  10 , it is well known that the summation of forces in all given directions (e.g. an axial direction) must equal zero (ΣF=0). It is also well known that another condition for static equilibrium is that the summation of moments around a point must equal zero (ΣM=0). With this in mind, consider the forces acting in an axial direction on the device  10 , and the summation of moments about the point  48 . For this consideration, the force exerted by the control wire  20  at attachment point  46  is represented by F w , the force exerted by the spine  32  at point  48  is represented by F s , and the resultant forces exerted by the articulation segment  16  on each side of the axis  42  are represented by F A1  and F A2 . For purposes of this discussion, it will be assumed that the transfer tube  30  exerts no effective forces on the base  44 . Accordingly:
 
 ΣF=F   A1   −F   w   +F   s   +F   A2 =0
 
 ΣM=M   T +( f ) F   A2 +( d+a ) F   w −( d+a+g ) F   A1 =0
 
     Several observations can be made from the above equations. To do so, however, recall that the moduli for the articulation segment  16  are less than the corresponding moduli for the spine  32 . Thus, for a given deflection or compression, F s  will be greater than either F A1  or F A2  (F s &gt;F A1 ≅F A2 ). Consequently, when a force (F w ) is applied at the attachment point  46  by a pull of the control wire  20  in the proximal direction, the spine  32  gives the most resistance. Further, because the force F w  on the control wire  20  is operationally variable, the moment (d+a)F w  can be made greater than the resistive moment (d+a+g)F A1 . Due to these relationships, the result here is that the articulation segment  16  of the device  10  will deflect in a plane that is generally defined by the spine  32  and the axis  42 . More specifically, as best seen in  FIG. 5 , this deflection will result in a radius of curvature  50  for the spine  32  (only partially shown in  FIG. 5 ) that is greater than a radius of curvature  52  for the axis  42 . Further, depending on the magnitude of the force F w  and the resultant movement of the control wire  20 , the arc through which the articulation segment is deflected (identified in  FIG. 5  by the arrows  54 ) may be greater than about two radians. 
     While the particular Wire Reinforced Articulation Segment as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.