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BOOK REVIEW |
| Missoula, Montana, USA |
This beautifully produced book is a timely and welcome text that records the papers presented at the Second International Symposium on Stentless Bioprosthesis held in Noordveg, The Netherlands. It is a natural follow-up to the first edition published in 1995, which recorded the papers presented at the first symposium held in Paris.
An initial perusal of the table of contents clearly shows that at the present time, work on stentless valves is practically centered on the aortic valve. The first two sections report isolated and still rather esoteric papers on monocusps for right ventricular reconstruction, Ross procedure on a previous arterial switch, and mitral replacement with xenogeneic mitral and pericardial stentless valves. The bulk of the information corresponds to what most readers would expect from the title, that is the porcine aortic stentless bioprosthesis. The new so-called "stentless bioprosthesis" is the latest develop-ment in the already long history of tissue valve replace-ment. This has erupted into the market with force and has been received with enthusiasm due to several factors. In the first place, it is because of the recent return of interest in tissue valves among the surgical community. This is due to the shift towards a progressively older patient population with greater problems of permanent anti-coagulation and the excellent durability of the standard bioprosthesis in these elderly patients. Secondly, better cardiopulmonary and myocardial protection techniques have reduced the anxiety attached to longer and more complex procedures. Increased familiarity with aortic root replacement and larger experience with stentless homografts have resulted in more standardized implan-tation techniques. Thirdly, the realization that residual gradients, obviously more significant in small aortic annuli, negatively affect the long-term outcome of valve replacements. This problem which might demand root enlargement procedures, offsets the technical simplicity of a standard prosthetic replacement. Finally, although not yet proven, it is expected that the absence of a stent should reduce mechanical stress and, therefore, increase the durability of the glutaraldehyde-treated porcine aortic valve. It has been pointed out that the original porcine valves placed in the 1960s, were all stentless and that the advent of the stented bioprosthesis was universally welcomed as a positive development. However, it has to be remembered that these early porcine valves were not glutaraldehyde-treated and, therefore, had handling characteristics similar to homografts. Present-day stentless valves are much stiffer because they are cross-linked and reinforced with Dacron or pericardium, maintaining their shape during implantation.
As could be expected, the largest section of the book is dedicated to the outcome with aortic stentless biopros-theses, used mostly in Europe because of less regulatory restrictions than in the US. Most reported series are small (around 100 patients), started between 1990 and 1993, and applied to older patients, often with a small aortic annulus. Because of this lack of information on the long-term behavior of the stentless valve, selection by the surgeon of the specific type to use is logically centered on the handling characteristics and ease of implantation of the bioprosthesis. The majority of models are porcine (some composite) although one (Sorin) is made of pericardium. They are implanted with a double-suture-line technique similar to a homograft implantation. A departure from this well-proven technique is the single supra-annular composite Cryolife-O'Brien porcine prosthesis and the intermediate solution of the "BioSud" prosthesis reported by Navia and colleagues from Argentina, which has a familiar-looking but flexible stent that is secured to the patient with a single suture line at the aortic annulus and a single stitch at each commissure. The most popular models are the Medtronic Freestyle and St. Jude Toronto SVP, although the contributions from Brazil (Biocord and Labcor) are significant. The versatility of the Freestyle valve that can be used as a root replacement, inclusion cylinder, or subcoronary, is an attractive solution since it can be tailored to the patient's specific needs and surgeon's preferences. In terms of comparisons between models, there is a paper from Lille, France, which concludes that at one-year follow-up, no differences could be found between the Freestyle and the Toronto SVP valves.
It is surprising to find the excellent papers from the groups of Dotty, Westaby, and Huysmans, which deal mostly with surgical techniques, within the outcomes section and not in the implantation techniques section that otherwise is rather poor. The exception is the paper by Goldman and colleagues from Toronto, which reports 6 patients who developed significant proximal coronary stenosis after implantation of the Toronto SVP stentless bioprosthesis. A useful technical reminder.
Probably, the strongest part of the book is the section on hemodynamic results following aortic valve replacement with the Toronto SVP and Medtronic Freestyle valves. They all show lower gradients than standard prostheses, reduction in gradients with time, and better left ventricular mass reduction. These are the strong points of stentless valves and the main argument for their increasing popularity. The issue of their expected durability remains speculative. Two major factors must play an essential role in this respect: geometric design and tissue treatment. A significant reduction of mechanical stress because of being stentless must be accepted. The value of the different anticalcification treatments applied to these valves remains largely speculative. There is enough evidence to suggest that the behavior of the valve leaflets and aortic wall is different in terms of calcification after the different preservation treatments. Although in my opinion, the paper from Leiden is misplaced in the first section of the book rather than in the last section, it reports interesting data. It is the only experimental study in large animals (piglets) where the behavior of the Freestyle valve is evaluated in the pulmonary position. All specimens showed degeneration of cusps and wall, with the presence of inflammatory nodules at microscopy. The authors point out previous work from their laboratory with the same valves in the aortic position and better results. Although proprietary and disclosure constraints unfortunately limit their value considerably, the ongoing work and future perspectives are well represented in the last section of this book.
The publishers must be congratulated for an outstanding publication with excellent and single-style illustrations and the editors must be encouraged to continue their efforts to maintain the interest in this field which points towards the tissue valves we will be using in the next century.
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