1. Field of the Invention
The present invention relates generally to the construction and use of vascular catheters. More particularly, the invention relates to intravascular atherectomy catheters having a distal housing for severing atheroma and collecting the severed atheroma in a forward portion of said housing.
Arteriosclerosis, also known as atherosclerosis, is a common human ailment arising from the deposition of fatty-like substances, referred to as atheroma or plaque, on the walls of blood vessels. Such deposits occur both in peripheral blood vessels that feed the limbs of the body and in coronary blood vessels which feed the heart. When deposits accumulate in localized regions of a blood vessel, blood flow is restricted and the person's health is at serious risk.
Numerous approaches for reducing and removing such vascular deposits have been proposed, including balloon angioplasty where a balloon-tipped catheter is used to dilatate a region of atheroma, atherectomy where a blade or other cutting element is used to sever and remove the atheroma, and laser angioplasty where laser energy is used to ablate at least a portion of the atheroma.
Of particular interest to the present invention are atherectomy catheters and methods where a cutting blade is advanced past an opening in a housing at the distal end of a vascular catheter. By positioning the housing so that at least a portion of the atheroma passes through the opening, the atheroma can be severed and translated forwardly by advancing the cutting blade. Typically, such cutting blades are circular, and are rotated (or oscillated) and advanced simultaneously to effect the desired cutting.
Although atherectomy catheters have enjoyed widespread success in both peripheral and coronary applications, certain design limitations persist, particularly with regard to the small diameter catheters used in coronary applications. Such small diameter coronary atherectomy catheters must be able to be advanced through very tight bends which are found in the coronary arteries. Atherectomy catheters, which have a rigid or semi-flexible housing with a hollow nose cone at their distal end are designed to track over conventional movable guide wires when they are being introduced to the site of a lesion. Ideally, the atherectomy catheter will follow the entry path without substantial displacement of the guide wire. The rigid nature of the housing, however, prevents precise tracking of the guide wire which can cause lateral displacement, which is exacerbated by housings which are elongated to provide collection capacity for severed atheroma at their forward ends.
The cutter housing on atherectomy catheters, by virtue of their rigid or semi-flexible structure, can also cause trauma to the blood vessel wall while the catheter is being positioned, particularly around tight bends in the coronary arteries. While such trauma could be decreased by providing a soft distal tip on the housing, a soft tip that does not resist kinking and twisting can exacerbate problems with binding on the movable guide wire (the soft tip can collapse and bind the guide wire) and will generally not be suitable for adding atheroma collection capacity to the catheter. The inherent collapsibility of the soft tip thus limits its ability to receive and hold severed atheroma as well as interfering with the catheter's ability to track over a movable guide wire.
For these reasons, it would be desirable to provide improved atherectomy catheter designs. In particular, it would be desirable to provide atherectomy catheters having improved distal ends, where the distal end is both sufficiently soft to avoid damaging a blood vessel wall and sufficiently strong to avoid collapsing during use. The soft distal tip should facilitate placement of the atherectomy catheter over conventional movable guide wires, even in the coronary arteries where very tight bends must be crossed. The soft distal tip should further provide for increased atheroma collection capacity and should be suitable for receiving severed atheroma which is advanced forward from the associated cutter housing. Preferably, the catheter nose cone should have a graduated flexibility, with a very high flexibility near its distal tip with decreasing flexibility in the proximal direction. Such a stiffness profile will improve the ability to position the catheter, particularly in the coronary arteries.
2. Description of the Background Art
Atherectomy catheters having axially translatable cup-shaped blades are described in U.S. Pat. Nos. 4,979,951 and Reissue 33,569. The latter patent illustrates a relatively inflexible nose cone which has been partially hollowed to receive severed atheroma. An atherectomy catheter having a coil-tip fixed guide wire which receives a conventional movable guide wire is disclosed in PCT published application WO 91/01156, corresponding to copending application U.S. Ser. No. 07/697,913, assigned to the assignee of the present application. An atherectomy catheter system which includes a cylindrical component for providing atheroma receiving capacity is illustrated in PCT published application WO 91/12773, corresponding to copending application U.S. Ser. No. 07/482,421, assigned to the assignee of the present invention. Balloon angioplasty catheters having flexible distal tips which receive a movable guide wire are disclosed in U.S. Pat. Nos. 4,921,483; 4,597,755; 4,582,181; and 4,571,240. The disclosures of each of these references are incorporated herein by reference.