Abstract:
A method and apparatus of providing endovascular therapy. The steps include arranging optical fibers within a catheter, the catheter having a tip whose length is at least 1 cm and whose diameter of less than 1 millimeter, connecting an excimer laser to the optical fibers; and delivering laser energy from the excimer laser in excess of 60 fluence at 40 Hertz through the optical fibers. The delivering of the laser energy may be to non-calcified or calcified deposits of an atherosclerotic lesion to ablate the same. The method also includes the step of inserting the catheter through an artery by pushing the same until the tip is in within laser energy striking distance of the atherosclerotic lesion.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a catheter used to deliver laser energy from an excimer laser to a lesion within an arterial system or to a body organ. The catheter is particularly suited for use in endovascular therapy to ablate calcified or non-calcified deposits in atherosclerotic lesions. 
     2. Discussion of Related Art 
     U.S. Pat. Nos. 5,429,604 and 4,844,062 reveal that fiber optic catheter assemblies have increasingly been used for probing and clearing obstructions in various vessels such as arteries. The size of the vessel and the distance from the insertion point to the critical region in the vessel determine the characteristics of the catheter to be used. For example, in some situations it is desirable to insert a very thin catheter a considerable distance into a vessel. The catheter must be quite flexible so that it may be steered considerable distances through winding vessel passages. 
     The excimer laser enables the disintegration of targeted tissue and can yield relatively pure disintegration without excessive thermal damage to otherwise healthy tissue. The excimer laser is a combination of argon fluoride or krypton chloride and a rare earth gas. This combination forms a laser beam having a very short wavelength and hence photons of very high energy. 
     Excimer catheters for endovascular therapy are presently produced by the Spectranetics Corporation in the United States. Presently approved laser catheters in the United States for endovascular therapy range in diameter from 1.4 mm to 2.2 mm. The energy output for ablating atherosclerotic tissue in coronary or peripheral arteries is 60 fluence, 40 Hertz. 
     The ablation of atherosclerotic lesions by excimer laser energy transforms the lesions into water and carbon dioxide. A gas bubble forms at the laser catheter tip and expands rapidly, which stretches the vessel wall. The abrupt stretch can result in dissection and even occlusion of the vessel. 
     Calcium can not be ablated with commercially available catheters. Spectranetics has recently developed an excimer laser with more densely packed optical fibers, which results in a decrease of dead space and a higher energy output of 80 fluence or more, 40 Hertz or more. This recently developed excimer laser enables laser catheters to ablate calcium, but the higher energy output increases the likelihood of dissections. Also, existing laser catheters are relatively stiff, which precludes them from negotiating acute bends, which are common in arterial systems or in body organs. 
     It is desired that an excimer laser catheter be provided that is capable of being used to ablate hard, calcified areas and which may negotiate acute bends in the arterial system. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the invention resides in an apparatus and method for providing endovascular therapy. The apparatus includes an elongated catheter containing optical fibers arranged in a concentric manner, the catheter having a tip whose diameter is smaller than a remainder of the catheter, the tip having a length of at least 1 cm and a diameter of less than 1 millimeter. The apparatus preferably also includes an excimer laser connected to the optical fibers capable of delivering laser energy through the optical fibers. The method also includes the step of inserting the catheter through an artery by pushing the same until the tip is in within laser energy striking distance of a lesion for ablating the same. For treatment of heavily calcified lesions, the excimer laser should be capable of delivering laser energy at higher energy levels in excess of 60 fluence and in excess of 40 Hertz. For treatment of non-calcified lesions, the excimer laser need only be capable of delivering laser energy at lower energy levels, i.e., 60 fluence or less at 40 Hertz or less. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     For a better understanding of the present invention, reference is made to the following description and accompanying drawings, while the scope of the invention is set forth in the appended claims. 
     FIG. 1 is a schematic representation of a catheter tip segment in accordance with the invention that is in position to ablate atherosclerotic lesions. 
     FIG. 2 is a schematic representation of a laser support catheter in accordance with the invention. 
     FIG. 3 is a cross-section taken from lines  3 - 3  of FIG. 2 in accordance with an embodiment using a single row of optical fibers. 
     FIG. 4 is a distal end view of FIG. 2 for the embodiment of FIG.  3 . 
     FIG. 5 is a cross-section taken from lines  5 — 5  of FIG. 2 in accordance with an embodiment using a double row of optical fibers. 
     FIG. 6 is a distal end view of FIG. 3 for the embodiment of FIG.  5 . 
    
    
     DESCRIPTION OF THE INVENTION 
     Turning to FIG. 1, a coronary vessel  6  is shown having atherosclerotic blockage  8 . A catheter contains optical fibers  12  and has a tip  16  that is inserted through the vessel  6  to reach the atherosclerotic blockage  8 . An excimer laser that is connected to the catheter emits ultraviolet laser radiation energy through optical fibers  12 . The laser radiation energy emerges from the catheter tip  16  to strike atherosclerotic blockage  8  to ablate the same. 
     The catheter tip  16  has a length of at least 1 centimeter and a diameter of less than 1 millimeter. Also, a guide wire  22  extends through a centerline of the catheter and is used in a conventional manner for guiding the catheter. Preferably, the tip length is at least 1 centimeter and up to 30 centimeters, although the optimal maximum length need only be as long as required for the catheter to come within laser striking distance of a lesion within the artery and traverse the lesion. 
     Presently available laser catheters that have tip diameters greater than 1 millimeter are unable to effectively ablate the calcium that is commonly present in atherosclerotic lesions. Experimental laser catheters have been developed that enable delivery of elevated laser energy levels that risk obtaining unwanted dissections. Dissections arise when the vessel wall stretches too much under stress from expanding gas bubbles that form at the tip of the laser catheter as the ablation of the atherosclerotic lesion transforms the lesion into water and carbon dioxide. Further, existing laser catheters are relatively stiff, which prevents them from negotiating acute bends common in the arterial systems. 
     By reducing the diameter of the catheter tip  16  to less than 1 millimeter, several advantages are realized. The smaller tip will cause less tissue to become vaporized per laser radiation emission. This results in a smaller gas bubble being formed than would otherwise be the case. The higher laser energy required to ablate calcium can be employed without increasing the likelihood of major dissections. In addition, the smaller tip is more effective for crossing heavily calcified lesions than are catheters having wider tips. Catheters having a tip diameter less than one millimeter are capable of crossing certain lesions that resist their crossing by wider diameter tip catheters. Furthermore, long lesions, which are not completely resistant to wider catheters, may be crossed with greater speed. That is, the use of the smaller catheter tip is more effective in crossing heavily calcified lesions than the use of the wider catheter tip. 
     By providing for a length of the catheter tip  16  to be at least one centimeter, additional advantages arise. One advantage resides in the tip  16  being more flexible because the diameter is smaller than for the wider conventional catheter tips. This greater flexibility enhances the ability of the laser catheter to negotiate bends within the arterial system, which often are present before reaching the lesion. That is, it enhances the likelihood of the laser catheter reaching the intended site of treatment. Another advantage is that the thinner tip enhances the likelihood of the laser catheter traversing severely calcified lesions that wider tips can not traverse. 
     A further advantage is that the catheter shaft is wider than the catheter tip  16  and thus readily pushable. Very thin catheters tend to store longitudinal energy when they are pushed forward over a guide wire  22 . This reduces the ability to advance the tip  16 . The thicker shaft of the present catheter is stiffer than the tip  16 , which improves longitudinal energy transfer to the thin tip segment of the catheter. Thus, the present invention combines optimal pushability, because of the thicker shaft, with optimal flexibility, because of the thinner tip  16 . 
     The use of a high energy miniaturized excimer laser may achieve the same luminal diameter as traditional lower energy laser catheters, because the relatively larger energy output of the miniaturized laser catheter should result in the formation of relatively larger gas bubbles. Gas bubble formation of a small high energy laser catheter may be comparable to that of a larger traditional energy laser catheter. Thus, the extent of vessel wall stretching may be comparable so the use of the miniaturized laser catheter is suited for calcified vessels of average and large diameter, in addition to being suited for small vessels. 
     In any size vessel over 2 millimeters in diameter that requires endovascular therapy, therefore, in which an excimer laser catheter is used to ablate calcified vessels, preferably a catheter tip segment of about at least 1 centimeter in length should be provided to have a diameter that is less than 1 millimeter. The catheter shaft, however, has a more conventional cross- sectional dimension that is larger than one millimeter. The use of high laser energy of more than 60 fluence and more than 40 Hertz has the specific goal of effectively treating heavily calcified lesions while the small tip dimension allows the high laser energy to be delivered without excessive gas bubble formation. Lower laser energy levels may be used such as 60 fluence at 40 Hertz to ablate holes through non-calcified tissue. A pilot hole was drilled through calcified tissue rapidly by emitting 2660 pulses of laser energy at 100 fluence/80 Hertz so the fastest and best results for ablating calcified tissue was for at least 80 fluence/80 Hertz excimer laser parameter settings. 
     The smaller tip segment (at least 1 cm in length) is flexible and enhances the ability to negotiate bends within the arterial system, affording better and easier access to the site of the lesion. Since the remainder of the catheter  14 , i.e., the shaft, may be thicker and thus more rigid, the resulting device provides enhanced “push” and torque characteristics. 
     FIGS. 3-4 show a further embodiment of a single row laser support catheter  70  and FIGS. 5-6 show an additional embodiment of a double row laser support catheter  72 . Both share the following conventional components depicted in FIG.  2 : capillary tube  74 , proximal coupler  76 , tail tubing  78 , bifurcate cover  80 , proximal shaft  82 , optical fibers  84  and an inner lumen  86 . The proximal shaft  82  is preferably made of hytrel material. Also shown is a distal jacket  88  preferably made of hytrel material, a distal tip  90  preferably made of platinum and an excimer laser  92  (shown in box form). The main difference between the embodiments of FIGS. 3-4 and FIGS. 5-6 is that the embodiment of FIGS. 3-4 has a single row of optical fibers  84  concentrically arranged, while the embodiment of FIGS. 5-6 has two rows of optical fibers  84  concentrically arranged separated from each other radially. 
     With respect to the embodiment of FIGS. 3-4, a suitable optical fiber arrangement has 30 fibers each being a 45 to 50 micron fiber. With respect to the embodiment of FIGS. 5-6, a suitable optical fiber arrangement has 68 fibers each being a 45 to 50 micron fiber. The guide wire (not shown) has an internal diameter preferably of 0.014 to 0.016 inches. There are dimensional constraints on how small in diameter the catheter may be made. One constraint is the diameter of the optical fibers and the number of rows of optical fibers. While two rows are preferred for better consistent performance of laser energy delivery over that for just one single row, three rows may be used instead of just two rows to provide even better laser energy delivery performance. Of course, using three rows of optical fibers will make the catheter more densely packed and thus less flexible and more stiff than using two rows. Another constraint is the diameter of the guide wire and the wall thickness of the catheter, which must be taken into account for the dimension of the catheter. Taking all these dimensional constraints into consideration, the catheter preferably has a dimension of about 0.8 to 0.9 millimeters. 
     While the drawings show catheters that have a generally circular cross-section, the cross-section may instead be polygonal. 
     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood that various changes and modifications may be made without departing from the spirit and scope of the present invention.