Prosthetic cardiac valves have been used for many years to treat cardiac valvular disorders. The native heart valves (such as the aortic, pulmonary and mitral valves) serve critical functions in assuring the forward flow of an adequate supply of blood through the cardiovascular system. These heart valves can be rendered less effective by congenital, inflammatory or infectious conditions. Such damage to the valves can result in serious cardiovascular compromise or death. For many years the definitive treatment for such disorders was the surgical repair or replacement of the valve during open heart surgery, but such surgeries are prone to many complications. More recently a transvascular technique has been developed for introducing and implanting a prosthetic heart valve using a flexible catheter in a manner that is less invasive than open heart surgery.
In this technique, a prosthetic valve is mounted in a crimped state on the end portion of a flexible catheter and advanced through a blood vessel of the patient until the valve reaches the implantation site. The valve at the catheter tip is then expanded to its functional size at the site of the defective native valve such as by inflating a balloon on which the valve is mounted. Alternatively, the valve can have a resilient, self-expanding stent or frame that expands the valve to its functional size when it is advanced from a delivery sheath at the distal end of the catheter.
Balloon-expandable valves typically are preferred for replacing calcified native valves because the catheter balloon can apply sufficient expanding force to anchor the frame of the prosthetic valve to the surrounding calcified tissue. On the other hand, self-expanding valves typically are preferred for replacing a defective, non-stenotic (non-calcified) native valve. One drawback associated with implanting a self-expanding valve is that as the operator begins to advance the valve from the open end of the delivery sheath, the valve tends to “jump” out very quickly from the end of the sheath; in other words, the outward biasing force of the valve's frame tends to cause the valve to be ejected very quickly from the distal end of the delivery sheath, making it difficult to deliver the valve from the sheath in a precise and controlled manner and increasing the risk of trauma to the patient.
Another problem associated with implanting a percutaneous prosthetic valve in a non-stenotic native valve is that the prosthetic valve may not be able to exert sufficient force against the surrounding tissue to resist migration of the prosthetic valve. Typically, the stent of the prosthetic valve must be provided with additional anchoring or attachment devices to assist in anchoring the valve to the surrounding tissue. Moreover, such anchoring devices or portions of the stent that assist in anchoring the valve typically extend into and become fixed to non-diseased areas of the vasculature, which can result in complications if future intervention is required, for example, if the prosthetic valve needs to be removed from the patient.