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Ann Thorac Surg 1998;65:235
© 1998 The Society of Thoracic Surgeons

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Special Report

Stentless Aortic Bioprostheses: Compelling Data From the Second International Symposium

Oxford Heart Centre, John Radcliffe Hospital, Oxford, United Kingdom;
Leiden University Hospital, Leiden, the Netherlands;
Division of Cardiovascular Surgery, The Toronto Hospital, University of Toronto, Toronto, Ontario, Canada

Accepted for publication October 30, 1997.

Mr Westaby, Oxford Heart Center, John Radcliffe Hospital, Oxford OX2 7AE, United Kingdom.

Abstract

Background. Stentless aortic xenografts are an important addition to the range of prosthetic valves. So far their use has been restricted to a limited number of study centers. This report summarizes the principal findings from the Second International Symposium on Stentless Bioprostheses. Attention is focused on the Toronto SPV and Freestyle valves recently approved by the United States Food and Drug Administration.

Methods. Stentless xenografts are used predominantly in elderly patients with aortic stenosis. Implant techniques are more complex than for stented valves, as reflected by longer ischemic and cardiopulmonary bypass times. The valves have been subjected to detailed serial echocardiographic assessment and clinical follow-up.

Results. The hemodynamic characteristics resemble those of the aortic homograft. There is a progressive increase in effective orifice area and decrease in transvalvular pressure gradients with time. Left ventricular mass index and wall thickness normalize between 6 and 12 months postoperatively. Left ventricular remodeling is accompanied by improved symptomatic status and a low incidence of valve-related complications. Limited comparative studies suggest important benefits over stented xenografts. Improved hemodynamics may translate into better bioprosthetic durability.

Conclusions. Reproducible and reliable implant methods should be taught carefully, but the hemodynamic advantages are substantial. Stentless xenografts are ideal for the elderly patient with aortic stenosis.

Valve prostheses continue to improve towards the goal of optimum hemodynamic function, indefinite durability, and a minimal risk of thromboembolism. Stented porcine xenografts or pericardial valves are hybrids of biological and mechanical structures, hence the term bioprosthesis. Most bioprostheses do not need anticoagulation in the aortic position, but the valve stent is both obstructive and a focus for stress on the tissue components. Stentless biological valves such as the aortic homograft and pulmonary autograft have excellent hemodynamic features, and there has been a resurgence of interest in these valves for aortic valve replacement in young patients. An understanding that these benefits need not be restricted to the young has prompted further evaluation of stentless porcine xenografts, first used by Binet and associates [1] in the early 1960s.

In 1994 the proponents and manufacturers of stentless bioprostheses came together in Paris to discuss the role and relative merits of these devices. Three years later, the meeting was reconvened in Noordwijk, Holland, with a scientific content that reflected the increased awareness and benefits of stentless valve technology. Because most mitral stentless valves are in an early stage of evaluation, this report concentrates on those aortic devices that are currently available for use or evaluation in Europe and the United States.

Why Stentless Valves?

Accelerated fatigue tests show the valve stent to be both obstructive and the major factor governing stress on the biological component [2]. Stent mounting and tissue preservation methods lead to suboptimal valve geometry, so that the durability of stented xenografts is less favorable than that of freehand-sewn aortic homografts. Degeneration, calcification, and cusp rupture occur earlier, especially in children or young adults [3]. A similar time course of valve failure (9 years) occurred when aortic homografts were stent-mounted [4]. Despite freedom from reoperation exceeding this time, bioprosthetic valve degeneration causes hemodynamic deterioration long before explantation. For the last few years of its lifespan, the stented xenograft is deformed, calcified, and poorly compliant, mimicking native valve pathology. Although the durability of stented xenografts used to be sufficient to avoid reoperation in most patients older than 70 years, longevity is increasing and freedom from structural failure is far less satisfactory than for mechanical valves [5].

The type of valve prosthesis has an important bearing on postoperative ventricular performance and outcome, particularly in smaller sizes [6][7]. A residual pressure drop across some mechanical valves and stented bioprostheses, together with their nonphysiologic flow profile, conveys an unfavorable influence on long-term outcome. During normal activity (let alone physical exercise), Doppler-measured mean and peak pressure gradients increase from about 25 and 45 mm Hg at rest to 40 and 70 mm Hg, respectively (symptom-limited master two-step test) [8]. Prosthesis-related left ventricular pressure increase is now emerging as the principal determinant of incomplete regression of left ventricular hypertrophy and interstitial fibrosis postoperatively [9]. Persistent valve gradients also result in impaired left ventricular diastolic function [10]. This has an important effect on late onset of fatal congestive heart failure irrespective of ejection fraction. Suboptimal left ventricular performance impairs quality of life and increases mortality should aortic reoperation be required.

The improved hemodynamic function of stentless valves is of great importance. For a specified external diameter, effective orifice areas are larger (and the pressure drop lower) than for stented valves [11][12][13]. The ideal stentless aortic valve is the patient’s own pulmonary valve. When this is used for root replacement, the flow characteristics are perfect and the durability seemingly indefinite. However, the Ross procedure exchanges a single-valve operation for a double root replacement with an inherent increase in risk [14]. Aortic homografts convey excellent hemodynamic function, but limited homograft availability ensures that they are not used routinely.

With modern myocardial preservation techniques, the extended ischemic time required for more complex implant methods has little effect on clinical outcome. Reproducible and reliable operative techniques allow widespread application of stentless xenografts for degenerative aortic disease in the elderly [15].

Stentless Valve Hemodynamics

In Noordwijk the superlative hemodynamic performance of stentless bioprostheses was confirmed by all groups with a large experience. Both in vitro and in vivo, the mean systolic gradient and effective orifice area of a size 23 stentless valve are equivalent to those of a 31-mm stented xenograft [11][16]. Detailed echocardiographic studies with the Toronto SPV valve (St. Jude Medical, St. Paul, MN) (Fig 1) and Freestyle valve (Medtronic, Minneapolis, MN) (Fig 2) valves show valve gradients, mechanical energy loss at the valve, peak systolic pressure, and left ventricular wall stress to be directly equivalent to those of an aortic homograft [17][18]. The data clearly demonstrate a significantly lower resistance to ventricular ejection when a stentless valve is used.

View larger version (1K): [in this window] [in a new window]   The Toronto SPV valve. (Reprinted courtesy of St. Jude Medical, St. Paul, MN. All rights reserved.)

View larger version (1K): [in this window] [in a new window]   The Freestyle valve. (Reprinted by permission of Medtronic, Minneapolis, MN.)

David and associates [20] have case-matched Toronto SPV valve patients with those receiving the Hancock II valve. When the duration of follow-up exceeds 5 years, the actuarial survival of patients with the stentless valve is 93% versus 86% for the stented valve. Proportional hazard analysis shows valve-related complications to be three times more common in the patients with a stented bioprosthesis. This suggests that improved hemodynamic function and left ventricular mechanics translate directly into better event-free survival.

Another characteristic finding for stentless bioprostheses has been the progressive and reproducible improvement in valve function with time [21]. Detailed echocardiographic follow-up studies of both the Toronto SPV and Freestyle valves show a consistent fall in mean systolic valve gradients and increase in effective orifice area progressively from the time of implantation to 18 months postoperatively [20][21]. By this time, valve gradients are reproducibly less than 10 mm Hg, even in the small aortic root, and on exercise or during dobutamine stress echocardiography these gradients remain less than 20 mm Hg. When the Freestyle valve is implanted by the aortic root replacement method, the postoperative valve gradients are even lower because a larger valve is employed in the supraannular position. In this case, the gradients do not fall significantly with time [15].

Kon and colleagues [15] presented a comparative study between the Freestyle valve and cryopreserved aortic homografts as free-standing total root replacements. Initial mean systolic gradient at hospital discharge was 7.73 ± 4.84 mm Hg for the Freestyle valve versus 6.66 ± 3.20 mm Hg for the cryopreserved homograft. At 3 years, there was no difference in mean systolic gradient, but whereas 100% of the Freestyle valves were free from aortic regurgitation, only 29% of homograft patients had no aortic regurgitation. Huysmans and associates documented the changes in hemodynamic function for the Freestyle valve implanted by the root inclusion technique, demonstrating a mean transvalvular pressure gradient of 10.5 mm Hg at discharge, which fell to 4.9, 4.2, and 2.0 mm Hg at 6 months, 1 year, and 2 years postoperatively.

Left Ventricular Remodeling

Residual left ventricular hypertrophy has a major effect on ventricular function and late outcome after aortic valve replacement [22]. In a 22-year follow-up, Lund [23] showed that the completeness of regression of left ventricular hypertrophy was a dominant factor, determining late outcome. Impaired left ventricular diastolic function (related to residual hypertrophy) was the sole predictor of fatal congestive heart failure, irrespective of ejection fraction (usually normal). Ventricular hypertrophy may also provoke ventricular dysrrhythmias after aortic valve replacement and account for late sudden death. Suboptimal ventricular function late after aortic valve replacement also increases operative mortality if reoperation becomes necessary.

There is great interest in the relationship between superior hemodynamics, valve durability, and improvement in left ventricular dynamics. Del Rizzo and colleagues [12] presented serial echocardiograms of 115 Toronto SPV patients at 3, 6, 12, and 24 months postoperatively. There were significant changes in mean valve gradient (-3.0 ± 4.0 mm Hg; p < 0.001), effective orifice area (+0.15 ± 0.42 cm²; p < 0.0001), and both left ventricular mass (-37.8 ± 57.9 g; p < 0.0001) and left ventricular mass index (-21.1 ± 30.5 g/m²; p < 0.0001) from 0 to between 3 and 6 months of follow-up. The thickness of the intraventricular septum and left ventricular posterior wall decreased by 1.2 ± 1.7 mm (p < 0.0001) and 1.4 ± 1.9 mm (p < 0.0001), respectively. Repeated-measures analysis (multivariate analysis of variance) showed a statistically discernable relationship between mean pressure gradient and left ventricular mass over time (p < 0.0001). Univariate analysis of variance demonstrated that reduction in left ventricular mass was greatest between operation and 6 months of follow-up, whereas multivariate analysis demonstrated a relationship between regression of hypertrophy and velocity in the left ventricular outflow tract, velocity across the aortic valve, and effective orifice area. Regression analysis showed that mean valve gradient varied with velocity across the aortic valve and effective orifice area but was independent of velocity in the left ventricular outflow tract or left ventricular outflow tract dimension.

Walther and colleages [17] presented 133 patients randomly assigned to receive a stentless xenograft, a conventional bioprosthesis, or a mechanical valve. At operation, there were no differences between the groups in sex, annular dimensions, or left ventricular hypertrophy (although larger diameter stentless valves were implanted). Although all patients showed a decrease in left ventricular hypertrophy after aortic valve replacement, the decrease was significantly greater in those with stentless valves. Jin and colleagues [18] studied 176 consecutive unselected aortic bioprosthesis patients with the Freestyle valve, with the aim of identifying those physiologic mechanisms that underlie left ventricular remodeling. All implantations were performed by the modified subcoronary technique [15]. The patients were prospectively investigated by Doppler echocardiography at 2 weeks and at 3 to 6, 12, and 24 months after operation (Food and Drug Administration protocol). By 2 years after the operation, left ventricular mass index had fallen from 162 ± 64 to 109 ± 36 g/m², and T/R ratio from 0.61 ± 0.25 to 0.43 ± 0.10 (virtually normal) (Table 1). Left ventricular stroke volume index increased from 29.4 ± 10 to 42 ± 17 mL/m². Myocardial stroke work progressed from 3.1 ± 1.6 to 5.2 ± 2.2 mJ/cm³ (all p < 0.01 by analysis of variance), whereas left ventricular outflow tract diameter remained unchanged. In the meantime, stentless valve effective orifice area increased from 1.59 ± 0.75 to 2.2 ± 0.72 cm². The transvalvular increase in mean flow velocity fell significantly from 82 ± 31 to 49 ± 24 cm/second, whereas mean pressure drop fell from 9.7 ± 5.0 to 5.2 ± 3.7 mm Hg (all p < 0.001 by analysis of variance). Correspondingly the deceleration time of aortic flow velocity increased from 153.6 ± 64.1 to 202.7 ± 37.6 milliseconds (p < 0.001 by analysis of variance). These findings show that, as left ventricular hypertrophy regresses, the left ventricular systolic function improves. Left ventricular stroke volume increased as the heart rate fell, and cardiac output remained constant. The stroke volume increase was due solely to the changes in flow velocity time integral at outflow tract level. Despite an apparent increase in effective orifice area of the valve and the fall in left ventricular mass and septal thickness, the diameter of the left ventricular outflow tract remained unchanged. In contrast, at the aortic valve level, the increased stroke volume was not accompanied by any change in flow velocity time integral, so that effective cross-sectional area of the jet at this level must have increased correspondingly. The changes occurred progressively, so that by 2 years the effective orifice area had increased by 38% and the mean pressure drop across the Freestyle valve had fallen by 45%.

These reports illustrate the important benefits of stentless bioprostheses on left ventricular mechanics. At the outflow tract level, the left ventricular dimensions remain constant, and an increase in stroke volume is produced by an increase in blood velocity time integral. At the valve level, the velocity time integral remains unchanged although stroke volume increases. This represents a change in flow pattern, with a wider flow jet and a more rectangular flow profile. At the same time, the increased stroke volume is also mediated by prolongation of the deceleration period of ejection, when left ventricular pressure is below that in the aorta. At the ventricular level, hypertrophy regresses and relative wall thickness falls. There is also a striking increase (>70%) in external ventricular work per cubic centimeter of myocardium. All of these changes have the effect of lowering energy expenditure during ejection by reducing blood flow velocity and acceleration, and thus transvalvular pressure gradients. In contrast, the stent-mounted xenograft provides a fixed resistance to flow, which does not improve with time, and systolic pressure gradients that increase dramatically with exercise. The argument for stentless bioprostheses in the aortic position is now compelling, particularly for the elderly, in whom left ventricular structure and function may normalize within 6 to 12 months of operation [18]. Improved valve hemodynamics with removal of the adverse stress relationship between tissue and stent may also translate into improved valve durability.

Outcome After Stentless Valve Replacement

Perhaps the most encouraging information from Noordwijk was David’s report of prolonged event-free survival for elderly aortic stenosis patients compared with patients with stented bioprostheses. Survival curves for this age group are not dissimilar to those of younger patients after homograft implantation. Anticoagulation with warfarin is not required for patients in sinus rhythm, so bleeding complications are eliminated.

For the Cryolife-O’Brien valve (Fig 3), Hvass and O’Brien reported a very low incidence of late complications, with a linearized rate of 0.69 ± 0.3 for embolic events, 0.35 ± 0.2 for endocarditis, and 1.74 ± 0.5 for total late deaths. Structural valve deterioration did not occur in any patient over the 5-year follow-up period. The Toronto SPV valve has been followed up carefully up to 8 years with a similar very low incidence of valve-related complications. Butany, from the Department of Pathology at the Toronto Hospital, studied eight valve explants from a total of 171 patients. Three were removed for infective endocarditis (2 early, 1 late), 4 at autopsy, and 1 at heart transplantation. Some pannus overgrowth was seen at the suture lines, and in 1 case this extended onto the cusps. The cloth covering was intact with mild fibrous tissue ingrowth. Minor mineralization was noted in two valves and cusp tears in two. Up to 5 years there was no significant structural degeneration.

View larger version (1K): [in this window] [in a new window]   The CryoLife-O’Brien stentless valve. (Reprinted by permission of Cryolife International, Inc, Kennesaw, GA.)

Surgery for aortic stenosis is one of medicine’s great success stories, but there is an increasing realization that the type of valve prosthesis has an important bearing on outcome. Stentless porcine xenografts are now freely available and user friendly, more so than aortic homografts. The Food and Drug Administration suggests that reproducible and reliable implant techniques should be taught carefully. There are unequivocal differences in rehabilitation of the left ventricle after use of a stentless xenograft versus a stented bioprosthesis, and although duration of follow-up is relatively short, it becomes progressively more difficult to justify the use of first-generation technology.

Acknowledgments

The Programme Directors thank Mr Martin Fijneman for his excellent organization of the Symposium and Miss Katherine L Ely for editorial assistance.

Footnotes

Toronto SPV is a registered trademark of St. Jude Medical, Inc.

References

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