Abstract:
The mechanical properties of various angioplastic surgical devices are altered in order to enhance their transportability through the vascular system by changing their operating temperature while situated within the body. Temperature increases are utilized to increase flexibility or cause devices to assume certain configurations while a temperature decrease serves to increase stiffness and rupture strength.

Description:
This application is a continuation-in-part of U.S. Ser. No. 08/242,410 filed May 13, 1994, now abandoned which is a continuation of U.S. Ser. No. 07/932,607 filed Aug. 20, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to angioplastic procedures and more particularly pertains to surgical devices that are introduced into and advanced through the vascular system during angioplastic procedures. 
     2. Description of the Prior Art 
     A variety of devices are employed in modern angioplastic techniques that are intended for introduction into and advancement through a patient&#39;s vasculature. Such devices include guidewires, various catheter type structures, retrieval devices, stents, etc. An important criterion inherent in the design and construction of such devices is the ability of the device to be easily advanced through vessels to the operative site and furthermore, to provide such &#34;transportability&#34; without compromising the ability of the device to perform its intended function once in position. The relatively small diameter of the vessels through which such devices are routinely passed and the potentially long and tortuous paths such devices may be forced to follow renders certain mechanical properties of such devices of critical importance. 
     The rigidity of an intravascular device is a representative example of a mechanical property of crucial import vis-a-vis the transportability of the device. The pushability, torqueability, and trackability of an elongated structure are all affected by the inherent rigidity of that structure. A guidewire or catheter must, for example, have sufficient rigidity to enable its distal end to be maneuvered by manipulation of its proximal end. Conversely, such device must be sufficiently flexible to enable it to follow a circuitous path and to prevent injury to the vessel walls. 
     The cross-sectional dimensions or profile of an intravascular device are dictated by the size of the vessels through which the device is to be advanced. In the case of balloon catheters, the mechanical properties of the materials employed in the device&#39;s construction play an important role in the ability of the device to be initially constrained to a sufficiently small profile in order to facilitate its advancement from the point of introduction to the operative site, to readily assume a substantially larger size upon expansion at the operative site, and to subsequently reassume the initial small profile for unrestricted extraction from the patient. It is therefore essential to select a material for the construction of a balloon that is flexible, strong and has a &#34;memory.&#34; 
     Some of these seemingly mutually exclusive requirements have been previously addressed by innovative approaches. With respect to the above-described requirements for guidewires or catheters to be rigid for some purposes yet simultaneously flexible for other reasons, it was recognized that a flexible distal end matched with a relatively rigid body element would substantially enhance the transportability of such a device. However, the utility of such an approach is nonetheless limited as situations inevitably will be encountered where the distal end of a particular device is either too stiff to be forced through a small radius bend, or too flexible to allow its position to be properly controlled. Furthermore, a very flexible tip may be necessary in order to reach a targeted stenosis, while substantially more stiffness may be required in order to cross through the stenosis if particularly occluded. 
     With regard to the above-described problems vis-a-vis the profile of balloon catheters, it has been found that by folding the balloon material into certain configurations a smaller deflated profile can be achieved for a given inflated size. However, the balloon&#39;s ultimate strength and the ability of a balloon to &#34;remember&#34; its folded shape typically are properties that are in conflict with one another, i.e. the stronger, more rupture-resistant the material, the weaker its memory. 
     The transportability of a prior art intravascular device may be hindered under certain conditions by the very mechanical properties of the materials employed in its construction that serve to enhance the device&#39;s transportability under a different set of conditions. Similarly, those mechanical properties of the materials that optimize the device&#39;s intended performance once in position may severely compromise its transportability. The prior art has provided for only limited improvements in this regard by typically employing either a specializing approach, i.e., by optimizing relevant parameters to fit a specific situation, or a compromising approach, i.e., providing somewhat compromised performance but for a wider range of situations. The capability for adapting a specific angioplastic device to different situations at different times has not been provided by the prior art. The present invention satisfies this need. 
     SUMMARY OF THE INVENTION 
     The present invention serves to substantially enhance the transportability of intravascular devices by providing for the controllable variation of some of the mechanical properties of such devices while situated within a patient&#39;s vasculature. This is achieved by employing carefully selected materials in the construction of critical components or regions of an intravascular device and by providing means therein for remotely varying its operating temperature. The materials are selected to have certain mechanical properties that change as their temperature is varied between limits tolerable within the body. The present invention encompasses both the cooling as well as the heating of the device by a variety of means while it is situated within the patient. Moreover, the invention is adaptable to many different intravascular devices. 
     A balloon or other expandable element constructed in accordance to the present invention will be somewhat distensible in at least its expanded diameter. The balloon diameter, when fully expanded under 6-12 atm. of pressure, will increase by 3% to 6%, or more, when heated to temperatures ranging from 37° C. to 70° C. The increase in balloon diameter is achieved without increasing pressure, thus reducing the likelihood of injuring the vessel or pressurizing the balloon up to its burst pressure. Further, upon deflation, the heated balloon more readily collapses to its folded, preinflated configuration and it is also far less likely to pancake or flatten upon deflation. In other words, the heated balloon of the present invention will fold back to its originally folded configuration much more easily than if it is unheated. 
     A guidewire, guiding catheter or catheter constructed according to the present invention has a distal end formed of material selected to become increasingly flexible with increased temperature. The material is further selected to display a desirable range of stiffness within the range of temperatures tolerable within the body. The present invention provides for a variety of ways to controllably increase the temperature of the distal end while situated within the vascular system. In a preferred embodiment, a lumen formed within certain angioplastic devices is employed to conduct heated fluid to and from selected regions of the device wherein an increased temperature is desired. As an alternative, light energy is conducted to the distal end via an optical fiber where it is absorbed by material forming the distal end. 
     In another preferred embodiment, a portion of tubing suitable for use as a catheter, balloon material, guiding catheter, guidewire, or other similar device, is made from a polymeric material such as PET (polyethylene teraphthalate), Nylon 12, LLDPE (linear low density polyethylene), HDPE (high density polyethylene), and surlyn ionomer. The tubing is heated in the range of 20° C. to 100° C. to selectively increase its flexibility at one or more points along its length, including at its distal tip. Thus, the present invention provides a catheter, for example, that can become more flexible while it is being inserted and routed through a tortuous vessel so that it can more easily traverse the vessel and reduce the likelihood of damage to the vessel wall. 
     The present invention additionally, optionally provides for the cooling of selected portions of surgical devices either to lower their operating temperature to below ambient or to more rapidly reduce their previously elevated temperature. This is accomplished either by conducting cooled fluid or gas to and from the selected region of the surgical device, allowing a liquid conducted thereto to undergo a phase change or allowing a compressed gas conducted thereto to expand. Thus, when pushing a balloon through a highly restricted coronary artery, the present invention provides a stiffer catheter (due to cooling) having greater pushability and torqueability, with somewhat less flexibility. These desired features are provided by selectively controlling the temperature of the tubing as described. 
     The temperature of the region of the device subject to heating and/or cooling is optionally monitored. A thermocouple, an optical sensor or a resistance device, for example, is readily adaptable to provide a real time, accurate indication of the temperature of a specific component or region of the surgical device while situated within the vascular system. The sensed temperature data is further optionally utilized in a closed loop control system to facilitate an automatic control of a desired set temperature. 
     During the angioplastic procedure, the progress of the distal end of the device through the vascular system is monitored in the conventional manner. In the case of a guidewire or catheter, when additional flexibility is required in order to negotiate a particularly circuitous path, the temperature of the device is increased. Conversely, when more pushability is required of the device, its temperature is allowed to decrease back down to ambient or is actively reduced by cooling means. In the case of a foldable balloon catheter, upon completion of the dilatation of the targeted artery, the balloon is collapsed and its temperature is increased to facilitate its folding after which the device is more easily extracted from within the vascular system. 
     These and other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments which, taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a balloon catheter assembly incorporating aspects of the invention. 
     FIG. 2 is a longitudinal, partial cross-sectional view of the catheter shaft of FIG. 1. 
     FIG. 3 is a longitudinal cross-sectional view of the distal end of the catheter assembly of FIG. 1, including the balloon portion. 
     FIG. 4 is a graph representing the test data of Table I. 
     FIG. 5 is an enlarged, longitudinal, cross-sectional view of a catheter device according to an alternative embodiment the present invention. 
     FIG. 6 is an enlarged cross-sectional view of the balloon portion of the catheter. 
     FIG. 7 is a cross-sectional view taken along lines 7--7 of FIG. 6 depicting the balloon in its folded state. 
     FIG. 8 is an enlarged longitudinal cross-sectional view of another embodiment of the catheter device. 
     FIG. 9 is an enlarged longitudinal cross-sectional view of an alternative embodiment of the catheter device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The Figures illustrate various embodiments of the present invention. Each embodiment comprises an medical device which is inserted into a patient, generally into the vascular system, maneuvered to the operative site, and ultimately removed at the conclusion of the procedure. The present invention provides for an enhancement of the transportability of such devices throughout the vascular system. 
     In keeping with the invention, as shown in FIGS. 1-3, a conventional angioplasty catheter assembly 11 is depicted. With this type of assembly, a typical angioplasty procedure can be performed, such as a percutaneous transluminal coronary angioplasty (PTCA). Such procedures are routine and are described in U.S. Pat. Nos. 4,771,778; 4,323,071; 4,468,224; and 4,573,470, which are incorporated herein by reference. 
     The catheter assembly 11 includes a catheter shaft 12 having an inner lumen 13. A guidewire 14 is slidably housed in inner lumen 13 and it assists in positioning the catheter shaft 12 in a patient&#39;s vessel. The catheter shaft 12 will slide over the in-place guidewire 14 to position balloon 15 located on the distal end of catheter shaft 12. This assembly is commonly referred to as an over-the-wire delivery system, however, the present invention is equally applicable to other known delivery systems such as fixed-wire, rapid-exchange, and monorail-type catheter systems. 
     In a typical PTCA procedure, the balloon 15 is positioned in the patient&#39;s coronary artery where a stenosed area exists. The balloon 15 is expanded using fluid pressure on the order of 8-15 atmospheres for short periods of time, i.e., 10-20 seconds. After the stenosed area is opened to blood flow, the balloon is deflated and catheter shaft 12 and balloon 15 are withdrawn from the patient, as is guidewire 14. 
     In keeping with the invention, catheter shaft 12 must be flexible enough to navigate through tortuous blood vessels and coronary arteries, yet must also be torqueable and pushable to cross narrowed and restricted arteries. Further, such catheters 12 must be able to track over a guidewire with minimal resistance. It is also important that the balloon deflate into a low profile shape that approximates its preinflated shape. The present invention solves the problems discussed. 
     In a preferred embodiment, catheter shaft 12 and balloon 15 are made from any combination of PET (polyethylene teraphthalate), Nylon 12, linear low density polyethylene (LLDPE), high density polyethylene (HDPE), or other polymeric materials such as polyvinyl chloride, polyester, polyimide, polyamides, nylon, polyurethane, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, cross-linked ethylene-vinyl acetate copolymer, polyamide elastomer, and surlyn ionomer. 
     By selectively heating portions of catheter shaft 12, its flexibility can be increased. Importantly, its flexibility can be changed at any point along catheter shaft 12 by increasing or decreasing its temperature. For example, if it is desired to make the distal third of catheter shaft 12 more flexible than the proximal two-thirds, a heated fluid can be injected from a fluid source through heated fluid lumen 17 so that the heated fluid contacts inner lumen 13 and raises the temperature of the distal one-third of catheter shaft 12. The heated polymeric material causes the distal one-third of catheter shaft 12 to become more flexible than the proximal two-thirds of the shaft, which is the desired effect. Similarly, reducing the temperature of catheter shaft 12 will result in less flexibility (more rigid) Ports 18 in heated fluid lumen 17 allow heated fluid to flow into contact with catheter shaft 12. Polymeric plugs 19 can be located at any point along lumen 13 to block the flow of heated fluid from areas where it is undesired. The heated fluid also is withdrawn from lumen 13 through port 18. 
     Numerous tests were conducted on tubing material to verify increased flexibility in the heated zones. Dynamic Mechanical Analysis measurements (DMA) were conducted on tubing samples to measure the viscoelastic properties of the materials. An oscillating strain was imposed on the test specimen and the resulting stress was measured. The materials&#39; viscoelastic properties include storage modulus (E&#39;), loss modulus (E&#34;), and damping modulus (Tan.sup.Δ) Storage modulus is a measure of the amount of energy stored in the material (e.g., elasticity). Loss modulus measures the energy dissipated per unit of stored energy (E&#34;/E&#39;). By using the temperature ramp method, the different properties mentioned were measured as functions of temperature. Test parameters such as frequency and strain amplitude were controlled at 6.28 radian/second and 0.05%, respectively. Heating rate was maintained at 5° C./min, from 20° C. to 100° C. temperature range. 
     The test results are reproduced in Table I for various polymeric materials. In order to more clearly demonstrate the increased flexibility of the tubing due to heating, the E&#39; values were plotted in Graph I of FIG. 4. 
     
                                           TABLE I__________________________________________________________________________Extrusion     Irradiation           Effective                 Frequency                      E&#39;, Dynes/sq cm(×10E8)MaterialNumber     Dose, mRad           Radius, mm                 Rad/sec                      37° C.                          50° C.                              70° C.__________________________________________________________________________PET  11-241-A      0    0.433 6.28 220.0                          192.0                              88.0LLDPE.sup.110-512-1     30    0.387 6.28 2.5 1.3 0.4LLDPE.sup.210-510-1     30    0.394 6.28 2.2 1.4 0.6HDPE.sup.110-518-1     30    0.383 6.28 6.5 3.5 1.9HDPE.sup.210-513-1     30    0.380 6.28 7.0 4.3 2.2__________________________________________________________________________ 
    
     Similar results in flexibility can be achieved by heating balloon 15 with a heated fluid delivered by heated fluid lumen 17 (see FIG. 3). In prior art catheters, it is generally desired to have non-distensible balloons that are pre-shaped. With the present invention, however, it is a preferred embodiment to provide balloon 15 that is non-distensible until it is heated, at which time it will increase in diameter without a corresponding increase in pressure. Thus, in order to finish opening a coronary artery, balloon 15 is inflated with a heated fluid so that its outer diameter increases thereby further increasing the lumen (artery) opening, but without increasing balloon pressures. Such increases in balloon diameter are referred to as balloon compliance which is a measurable characteristic. 
     Testing was conducted on balloons made from PE 600 to measure the effects of heating the balloons to various temperatures. PE 600 is a linear low density polyethylene material (LLDPE). Five PE 600 balloons were tested at 37° C., 50° C., and 70° C. Table II details the test results of atm. versus balloon outside diameter (OD) readings for various temperatures. 
     
                       TABLE II______________________________________                               Overall   37° C.            50° C.                        70° C.                               ChangeAtm     OD       OD          OD     in OD______________________________________ 6      2.734 mm 2.774 mm    2.962 mm                               .22810      3.070 mm 3.078 mm    3.248 mm                               .17812      3.188 mm 3.182 mm    3.325 mm                               .137______________________________________ 
    
     From the above data profile, there is a substantial change in balloon OD&#39;s at 70° C., when atmospheric pressure in the balloon is constant. Thus, there is an increase in the balloon OD&#39;s of 0.228 mm, 0.178 mm, and 0.137 mm as the temperature increases (pressure remaining constant). 
     Provided in Table III is the data profile of regression fit for the PE 600 balloon OD&#39;s at given temperatures and atms. 
     
                       TABLE III______________________________________Atm     37° C. 50° C.                          70° C.______________________________________ 6      2.789 mm      2.808 mm 3.026 mm10      3.092 mm      3.112 mm 3.360 mm12      3.244 mm      3.263 mm 3.527 mm______________________________________ 
    
     Other balloon materials were tested and similar results were achieved with respect to increased balloon OD&#39;s at higher temperatures. Five balloons having a surlyn ionomer composition were tested at 37° C. and 50° C. and the results are detailed in Table IV. 
     
                       TABLE IV______________________________________                          Overall   37° C. 50° C.                          Change inAtm     OD            OD       OD______________________________________ 6      2.514 mm      2.802 mm .288 mm10      2.842 mm      2.992 mm .154 mm12      2.926 mm      3.088 mm .162 mm______________________________________ 
    
     Table V provides the data profile of regression fit for the surlyn ionomer balloon OD&#39;s at given temperatures and atms. 
     
                       TABLE V______________________________________Atm           37° C.                  50° C.______________________________________ 6            2.557 mm 2.848 mm10            2.793 mm 3.101 mm12            2.911 mm 3.228 mm______________________________________ 
    
     Testing also was conducted on fine PET balloons (polyethylene teraphthalate) at 37° C. and 50° C., and the results are provided in Table VI. 
     
                       TABLE VI______________________________________                          Overall   37° C. 50° C.                          Change inAtm     OD            OD       OD______________________________________ 6      2.782 mm      2.806 mm .024 mm10      2.858 mm      2.880 mm .022 mm12      2.880 mm      2.902 mm .022 mm______________________________________ 
    
     Table VII provides the data profile of regression fit for the PET balloon OD&#39;s at given temperatures and atms. 
     
                       TABLE VII______________________________________Atm           37° C.                  50° C.______________________________________ 6            2.756 mm 2.767 mm10            2.840 mm 2.860 mm12            2.882 mm 2.907 mm______________________________________ 
    
     As can be seen by the test results, balloon diameter increased as the temperature of the PE 600 balloon material increased (while the pressure in the balloon remained constant). Importantly, the heated balloon will be more pliable as well, and when it is deflated it will more readily assume its low profile, folded configuration. A problem with some prior art balloons is that they will pancake or flatten when deflated which may result in the edges catching on the vessel walls when the catheter and balloon are withdrawn from the patient. The heated balloon 15 of the present invention, however, will readily deflate and assume its folded profile so that it may be more easily withdrawn from the patient. 
     As with catheter shaft 12, a decrease in flexibility (higher stiffness) in the balloon can be achieved by cooling it, with, for example, a cooling fluid. A cooling fluid would provide higher rupture strength during inflation, but would decrease flexibility and folding characteristics during deflation. Cooling fluid can be provided to balloon 15 in much the same manner as described for providing heating fluid. 
     In another embodiment, FIG. 5 illustrates a catheter 20 according to the present invention. Such catheter is advanceable along a previously emplaced guidewire to the operative site within a patient&#39;s vascular system. The catheter is formed of material that becomes substantially more flexible as its temperature is increased above the patient&#39;s ambient temperature (nominally 98.6° F. or 37° C.) to a temperature of 60° C. which has been found to be tolerable within the body for limited periods of time. In the embodiment illustrated in FIG. 5, an electrical conductor 21 is incorporated in the catheter wall 22. The two leads 23 of the conductor are inter-connectable to a power supply (not shown) near the catheter&#39;s proximal end and are linked to resistance wiring 24 coiled about distal end 25. Passing electric current through the wiring causes the temperature of the distal end to rise at a very controllable and predictable rate resulting in an increase in the flexibility of the distal end 25. 
     In use, the catheter 20 is advanced along an emplaced guidewire until its progress is impeded by an excessively tight bend in the guidewire. In order to facilitate further progress and prevent injury to the vessel walls, an electric current is passed through conductor 21 and resistance wiring 24 in order to increase the operating temperature of the distal end 25 and hence its flexibility. Once past the tight bend, the temperature of the distal end is reduced by decreasing power, which should correspondingly provide greater stiffness (less flexibility) to catheter 20. 
     FIG. 6 illustrates a balloon-type dilatation catheter 30. The device consists of a multiple lumen catheter body 31 having an inflatable balloon 32 near its distal end. While the balloon is in its collapsed state, the device is advanceable along a previously emplaced guidewire to the operative site. Dilatation of the targeted artery is accomplished upon inflation of the balloon via fluid pumped through one of the lumens. Deflation of the balloon at the conclusion of the operation allows the device to be withdrawn. 
     The catheter 30 shown in FIG. 6 is heated with the introduction of heated fluid just prior to the final deflation and withdrawal. The balloon 32 may be made of a material which is affected by changes in its operating temperature, for example cross-linked polymers or thermoplastics such as PET (polyethylene teraphthalate). The heated fluid serves to transfer heat to the balloon 32 which in turn renders the balloon more pliable and therefore more readily disposed to reassume its folded shape (FIG. 7). In the event enhanced rupture strength is required during dilatation, the balloon is expanded with cooled fluid. 
     FIG. 8 illustrates another alternative embodiment of a catheter 40. The distal end 41 of the surgical device is formed of material that becomes more flexible with an increase in temperature. Moreover, the material is selected to absorb light energy at a preselected wavelength. One or more optical fibers 42 are incorporated in the catheter wall to conduct light of the preselected wavelength from a laser (not shown) to the distal end 41 of the catheter 40. Energization of the laser causes the distal end 41 to heat up and become more flexible as previously described. 
     FIG. 9 illustrates an alternative embodiment wherein a plurality of Peltier-type devices 50 are incorporated near distal end 51 of a catheter 52. By energizing leads 53 with the appropriate voltage and polarity, the distal end 51 of the catheter is cooled. In use, when an increase in stiffness is necessary, as for example when it is necessary to push the catheter 52 through an occlusion in an artery, the Peltier-type devices 50 are energized to quickly and predictably lower the temperature and increase stiffness. Temperatures near 0° C. are tolerable within the vascular system for brief periods of time. 
     While a particular form of the invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. For example, a wide variety of materials will be adaptable for use in devices according to the present invention. Additionally, many different methods can be employed to bring about an increase or a decrease of the surgical device&#39;s operating temperature. Furthermore, it is not intended that the present invention be limited to the heating or cooling of the distal end of the various angioplastic surgical devices, as other regions of such devices may benefit from a change in their mechanical properties as brought about by the heating or cooling thereof. Accordingly, it is not intended that the invention be limited except as by the appended claims.