Cath Lab Digest

Introduction

Catheter therapy as an alternative to surgical cardiovascular therapy has had a long and steady development process beginning in the 1960s. When Charles Dotter first described opening a superficial femoral artery with a sequence of rigid dilators, he recognized many possibilities for percutaneous approaches as alternatives to traditional surgical operations. Remarkably, he spoke of percutaneous valve replacement, even at that early juncture, at the birth of catheter therapy for cardiovascular disease.

The subsequent development of alternatives to traditional surgical approaches has been one of steady development punctuated by occasional remarkable leaps in concept and technology. The introduction of balloon angioplasty represents the most striking and critical such leap. The recent adoption of percutaneous valve replacement is probably the second leap of this magnitude. The idea took hold with early devices such as those described by Andersen in the early 1990s.^1,2 The development of the procedure and application in humans by Bonhoeffer in the pulmonic valve^3,4 and then Cribier^5,6 in the aortic position has brought the field to where it is today.

Other sections in this text characterize the details of the state of the art of percutaneous aortic valve replacement. The first two devices to have wide application in human clinical use, the Edwards-SAPIEN (Edwards Lifesciences, Irvine, California)^7–12 and the Medtronic CoreValve prosthesis (Medtronic, Inc., Minneapolis, Minnesota),^13–18 have had remarkable success. These devices are highly successful first-generation technologies. They have come to the point of much more than proof of concept.^19,20 The rapid adoption of both of these prostheses with their approval in the international market is a testimony to their utility. They are certainly more than first-generation devices, but represent the first generation in practice.

These first-generation devices, despite their rapid adoption and early success, have several limitations. The large caliber of the devices, especially the first generation of the Sapien device, have excluded a substantial number of people in the target population, the elderly with aortic valve stenosis, due to the high frequency of concomitant peripheral vascular disease. As many as three-quarters of patients are not suitable for the first-generation sheaths associated with the Sapien device. The lack of repositionability and retrievability of these devices has a tremendous impact on patient selection, the conduct of the procedure and certainly the mind-set of the operator. The Edwards valve is positioned, and the force of balloon expansion necessarily imparts a high degree of energy to the device as it opens, with resultant movement of the prosthesis as it is being deployed. The potential for malposition exists with 100% of procedures, even though the actual frequency of this complication is low. The CoreValve device is similarly challenging to position and, especially because of its length, may be placed low in the outflow tract with impingement on the mitral valve, or high in the aortic root with resultant aortic insufficiency. The inability to easily retrieve these devices poses a great challenge. The limited repositionability of the CoreValve and the complete lack of repositionability of the Edwards device are important factors in their use. The contrast with procedures such as atrial septal defect closure, where the devices are completely retrievable in most cases, highlights the importance of this design feature.

Next-generation Devices

A second generation of devices is in the early stages of human use and several devices are in the early design phase in a preclinical arena. The next-generation devices comprise a spectrum of technologies. Some are completely novel in concept and construction. Others are variations on the theme of stent-mounted devices. They share in common the use of tissue leaflets, but employ novel delivery and anchoring mechanisms. Since the human experience with all of these devices is limited, it is not realistic to report on patient implant outcomes in this review. Where some human experience has been obtained, I will try to broadly summarize the status of that experience. The development of these devices has all occurred with many iterations. The device descriptions contained here are all works in progress, and are likely to be significantly different by the time of publication. However, the broad concepts will remain constant.

Direct Flow Medical Percutaneous Aortic Valve. This novel device system (Direct Flow Medical, Inc., Santa Rosa, California) is comprised of three components: a bovine pericardial tissue valve; a sheathed delivery/recovery system; and a solidifying inflation liquid that forms the support structure.^21–23

The tissue valve is trileaflet. The leaflet tissue is attached to a Dacron fabric cuff, which conforms to the native annulus (Figures 1 and 2). The fabric cuff is inflatable and creates a seal against the native valve annulus. This minimizes the potential for paravalvular aortic insufficiency. The ventricular and aortic cuff rings are independently inflatable. These two rings encircle the native valve annulus to anchor the device and at the same time provide a large effective orifice area. The implant is initially inflated with a saline and contrast mixture. This allows fluoroscopic visualization and testing of the position and seal of the device. The saline-contrast mixture can be exchanged for an inflation medium that solidifies and hardens to form a permanent support structure. Considering that this is a fabric device, the rigidity of the hardened structure is remarkable.