Novel approaches to cardiac valve repair: from structure to function: Part II

Treatment options for isolated aortic valve insufficiency: a review

Aortic insufficiency (AI) is a valvular disease with increasing prevalence in older patients. The modern era provides numerous options for the management of AI which is explored here. Traditional interventions included aortic valve replacement with either mechanical or bioprosthetic aortic valves. While the former is known for its durability, it has grown out of favor due to the potential complications of anticoagulation. The preference for bioprosthetic valves is thus on the rise, especially with the advancements of transcatheter technology and the use of valve-in-valve therapy. Surgical options are also no longer limited to replacement but include complex techniques such as those required for aortic valve repair, Ozaki neocuspidization, Ross procedure and valve-sparring aortic root repair. Transcatheter options for the management of AI are not commercially available currently; however, preliminary data from ad-hoc trials, showed promising results and potential use of transcatheter technology in a variety of patients with pure AI.

Profiling Genome-Wide DNA Methylation Patterns in Human Aortic and Mitral Valves

Cardiac valves exhibit highly complex structures and specialized functions that include dynamic interactions between cells, extracellular matrix (ECM) and their hemodynamic environment. Valvular gene expression is tightly regulated by a variety of mechanisms including epigenetic factors such as histone modifications, RNA-based mechanisms and DNA methylation. To date, methylation fingerprints of non-diseased human aortic and mitral valves have not been studied. In this work we analyzed the differential methylation profiles of 12 non-diseased aortic and mitral valve tissue samples (in matched pairs). Analysis of methylation data [reduced representation bisulfite sequencing (RRBS)] of 16,101 promoters genome-wide revealed 584 differentially methylated (DM) promoters, of which 13 were reported in endothelial mesenchymal trans-differentiation (EMT), 37 in aortic and mitral valve disease and 7 in ECM remodeling. Both functional classification as well as network analysis showed that the genes associated with the DM promoters were enriched for WNT-, Cadherin-, Endothelin-, PDGF-, HIF-1 and VEGF- signaling implicated in valvular physiology and pathophysiology. Additional enrichment was detected for TGFB-, NOTCH- and Integrin- signaling involved in EMT as well as ECM remodeling. This data provides the first insight into differential regulation of human aortic and mitral valve tissue and identifies candidate genes linked to DM promoters. Our work will improve the understanding of valve biology, valve tissue engineering approaches and contributes to the identification of relevant drug targets.

Starting an aortic valve repair program: is it worthwhile? Aortic valve repair compared to replacement

OBJECTIVES

Increasing evidence suggests that aortic valve (AV) repair may have better long-term outcomes than AV replacement for aortic insufficiency. However, most literature is limited to centres of excellence and has not sufficiently been replicated in nascent programs. This single-centre experience of a newly established AV repair program aims to compare short-term outcomes of AV sparing versus replacement surgery.

METHODS

A retrospective cohort study of patients who underwent elective surgery for aortic regurgitation or aortic root dilatation at the Leuven University Hospital between 2013 and 2018 was performed. Patients with a critically ill presentation, endocarditis, aortic stenosis or requiring redo surgery were excluded. Patients were assigned to repair versus replacement based on preoperative intention to preserve the AV. Nearest neighbour propensity score matching was performed to compare both groups. Safety (mortality, morbidity), efficiency (cross-clamp and bypass times) and efficacy end points (repair rate, postoperative echocardiography) were compared.

RESULTS

One hundred and seven patients underwent AV surgery (48 repair, 59 replacement), from which 2 groups of 23 matched patients were created. There were 1 death and 2 reoperations after repair and no death and 1 reoperation after replacement. Extracorporeal circulation and aortic cross-clamp time were significantly longer while ventilation and total hospital stay were significantly shorter after AV repair. Echocardiographic follow-up showed comparable aortic regurgitation but lower transvalvular gradients after repair. Freedom from major complications was comparable in both cohorts.

CONCLUSIONS

Early results suggest the feasibility of replicating experienced centres' perioperative and short-term outcomes in nascent programs.

Microstructure of the juvenile sheep aortic valve hinge region and the trilamellar sliding hypothesis

Background: The aortic valve mechanism performs extremely sophisticated functions which depend on the microstructure of its component parts. The hinge mechanism of the aortic leaflets plays a crucial part in the overall function. However, the detailed microstructure and its relation to function has not been adequately studied.

Methods: The aortic roots of juvenile sheep were fixed under physiologic pressure. Sections through all three sinuses were then performed to illustrate the microstructure of the hinge mechanism in different regions of the aortic root.

Results: The hinge region in the different sinuses showed unique microstructure with a trilamellar topology with a dominant core consisting of glycosaminoglycans. The exact arrangement of the trilamellar structures varies around the aortic sinuses, which could have functional implications. These features allow the hinge to perform its complex functions through what we have described as “the trilamellar sliding hypothesis”.

Conclusion: The microstructure of the hinge mechanism is unique and enables it to perform it sophisticated functions.

Heart Valve Biomechanics and Underlying Mechanobiology

Heart valves control unidirectional blood flow within the heart during the cardiac cycle. They have a remarkable ability to withstand the demanding mechanical environment of the heart, achieving lifetime durability by processes involving the ongoing remodeling of the extracellular matrix. The focus of this review is on heart valve functional physiology, with insights into the link between disease-induced alterations in valve geometry, tissue stress, and the subsequent cell mechanobiological responses and tissue remodeling. We begin with an overview of the fundamentals of heart valve physiology and the characteristics and functions of valve interstitial cells (VICs). We then provide an overview of current experimental and computational approaches that connect VIC mechanobiological response to organ- and tissue-level deformations and improve our understanding of the underlying functional physiology of heart valves. We conclude with a summary of future trends and offer an outlook for the future of heart valve mechanobiology, specifically, multiscale modeling approaches, and the potential directions and possible challenges of research development. © 2016 American Physiological Society. Compr Physiol 6:1743-1780, 2016.

Biocompatibility and Systemic Safety of a Novel Implantable Annuloplasty Ring for the Treatment of Mitral Regurgitation in a Minipig Model

Prosthetic annuloplasty rings are a common treatment modality for mitral regurgitation, and recently, percutaneous implantation techniques have gained popularity due to their favorable safety profile. Although in common use, biocompatibility of annuloplasty rings has been reported only sparsely in the literature, and none of these reports used the percutaneous technique of implantation. We report on the biocompatibility and the systemic safety of a novel transcatheter mitral valve annuloplasty ring (AMEND™) in 6 minipigs. This device is composed of a nitinol tube surrounded by a braided polyethylene terephthalate fabric tube. The device produced no adverse inflammatory response, showing gradual integration between the metal ring and the fabric by normal host fibrocellular response, leading to complete neoendocardium coverage. There was no evidence for adverse reactions, rejection, or intolerance in the valvular structure. In 2 animals, hemopericardium resulted from the implantation procedure, leading to right-sided cardiac insufficiency with pulmonary edema and liver congestion. The findings reported herein can serve as a case study for the expected healing pathology reactions after implantation of transcatheter mitral valve annuloplasty rings.

Coupled Simulation of Heart Valves: Applications to Clinical Practice

The last few decades have seen great advances in the understanding of heart valves, and consequently, in the development of novel treatment modalities and surgical procedures for valves afflicted by disease. This is due in part to the profound advancements in computing technology and noninvasive medical imaging techniques that have made it possible to numerically model the complex heart valve systems characterized by distinct features at different length scales and various interacting processes. In this article, we highlight the importance of explicitly coupling these multiple scales and diverse processes to accurately simulate the true behavior of the heart valves, in health and disease. We examine some of the computational modeling studies that have a direct consequence on clinical practice.

TexMi: development of tissue-engineered textile-reinforced mitral valve prosthesis

Mitral valve regurgitation together with aortic stenosis is the most common valvular heart disease in Europe and North America. Mechanical and biological prostheses available for mitral valve replacement have significant limitations such as the need of a long-term anticoagulation therapy and failure by calcifications. Both types are unable to remodel, self-repair, and adapt to the changing hemodynamic conditions. Moreover, they are mostly designed for the aortic position and do not reproduce the native annular-ventricular continuity, resulting in suboptimal hemodynamics, limited durability, and gradually decreasing ventricular pumping efficiency. A tissue-engineered heart valve specifically designed for the mitral position has the potential to overcome the limitations of the commercially available substitutes. For this purpose, we developed the TexMi, a living textile-reinforced mitral valve, which recapitulates the key elements of the native one: annulus, asymmetric leaflets (anterior and posterior), and chordae tendineae to maintain the native annular-ventricular continuity. The tissue-engineered valve is based on a composite scaffold consisting of the fibrin gel as a cell carrier and a textile tubular structure with the twofold task of defining the gross three-dimensional (3D) geometry of the valve and conferring mechanical stability. The TexMi valves were molded with ovine umbilical vein cells and stimulated under dynamic conditions for 21 days in a custom-made bioreactor. Histological and immunohistological stainings showed remarkable tissue development with abundant aligned collagen fibers and elastin deposition. No cell-mediated tissue contraction occurred. This study presents the proof-of-principle for the realization of a tissue-engineered mitral valve with a simple and reliable injection molding process readily adaptable to the patient's anatomy and pathological situation by producing a patient-specific rapid prototyped mold.

Time-dependent mechanical properties of aortic valve cusps: effect of glycosaminoglycan depletion

Aortic valve (AV) performance is closely linked to its structural components. Glycosaminoglycans (GAGs) are thought to influence the time-dependent properties of living tissues. This study investigates the effect of GAGs on the viscoelastic behaviour of the AV. Fresh porcine AV cusps were either treated enzymatically to remove GAGs or left untreated (control). The specimens were tested for stress relaxation and tensile properties under equibiaxial load conditions. The stress relaxation curves were fitted using a double exponential decay equation and the early relaxation constant (τ(1)) and late relaxation constant (τ(2)) calculated for each specimen. Immunohistochemistry confirmed the successful depletion of both sulphated and non-sulphated GAGs from the AV cusps. A statistical increase in τ(1) was found for both the radial and circumferential directions between the control and -GAGs group (radial, control 17.37s vs. -GAGs 25.65 s; circumferential, control 21.47s vs. -GAGs 27.37 s). It was also found that τ(1) differed between the two directions for the control group but not after GAG depletion (control, radial 17.37s vs. circumferential 21.47 s; -GAGs, radial 25.65 s vs. circumferential 27.37s). No effect on stiffness was found. The results showed that the presence of GAGs influences the mechanical viscoelastic properties of the AV but has no effect on the stiffness. The natural anisotropy, which reflects the relaxation kinematics, is lost after GAG depletion.

Functional mitral regurgitation: modern concepts for ventricular geometry reshaping

Functional mitral valve regurgitation (MR), a condition affecting millions of primarily elderly patients worldwide, is associated with poor clinical outcomes. Functional MR has traditionally been considered a disorder of regional or global left ventricular (LV) remodeling secondary to myocardial disease, in which anatomically normal leaflets fail to coapt adequately. The primary mechanisms of MR are mitral annular dilatation and leaflet restriction secondary to LV remodeling. Although annuloplasty is commonly used to correct valve incompetence, the effects of altered ventricular mechanics on MR need to be specifically addressed. This review focuses on current concepts of geometric reconfiguration of the LV and mitral-ventricular apparatus to reduce MR.

Autonomic dysregulation as a novel underlying cause of mitral valve prolapse: a hypothesis

Mitral valve prolapse is a common valvular abnormality that is caused by myxomatous degeneration, characterized macroscopically by leaflet thickening and redundancy accompanied with histologically marked proliferation of the spongiosa and mucopolysaccharide acid replacement of leaflet collagen in the prolapse leaflets. Nevertheless, the discrepant natural history and various concomitant syndromes cannot be explained completely by the current genetic autosomal dominant inheritance theory. In addition, autonomic dysregulation has been commonly reported in mitral valve prolapse, but has never been indicated as a major underlying cause. This article attempts to interpret the occurrence of primary pathology and progression in mitral valve prolapse on a common basis of improper autonomic tone. The imbalanced background of autonomic nervous firing leads to disharmonized synthetic/catabolism balance in the extracellular matrix, disrupted transition in the interstitial cellular component and invalided anti-inflammatory pathway in the endothelium, which trigger and accelerate the progression of this condition. Such a hypothesis not only unifies the seemingly disparate syndromes and valvular disorder, but also has implications for future biopharmaceutical and mechanical treatment.

Management-oriented classification of mitral valve regurgitation

Mitral regurgitation (MR) has previously been classified into rheumatic, primary, and secondary MR according to the underlying disease process. Carpentier's/Duran functional classifications are apt in describing the mechanism(s) of MR. Modern management of MR, however, depends primarily on the severity of MR, status of the left ventricular function, and the presence or absence of symptoms, hence the need for a management-oriented classification of MR. In this paper we describe a classification of MR into 4 phases according to LV function: phase I = MR with normal left ventricle, phase II = MR with normal ejection fraction (EF) and indirect signs of LV dysfunction such as pulmonary hypertension and/or recent onset atrial fibrillation, phase III = EF ≥ 30%-< 50% and/or mild to moderate LV dilatation (ESID 40-54 mm), and phase IV = EF < 30% and/or severe LV dilatation (ESDID ≥ 55 mm). Each phase is further subdivided into three stages: stage "A" with an effective regurgitant orifice (ERO) < 20 mm, stage "B" with an ERO = 20-39 mm, and stage "C" with an ERO ≥ 40 mm. Evidence-based indications and outcome of intervention for MR will also be discussed.

Early and long-term results of reoperative total aortic root replacement with reimplantation of the coronary arteries

BACKGROUND

Total root replacement with biologic valves and reimplantation of the coronaries gives good early and midterm results. There is continuing concern, however, regarding the risks and long-term results for reoperation after total replacement of the aortic root with reimplantation of the coronaries.

METHODS

Between June 1981 and July 2010, a total of 84 patients underwent reoperative aortic root replacement with reimplantation of the coronaries (60 male, mean age 38 ± 15 years). All patients had undergone first-time total aortic root replacement with homografts (82 patients) or autografts (2 patients). Indication for reoperation was structural valve deterioration in 72 patients (85%) and infective endocarditis in 12 patients (15%). Mean interval between first operation and reoperation was 11.1 ± 4.7 years (range, 1 month-24.7 years). Median length of follow-up was 9.7 ± 5.6 years (range, 1 month-24.4 years).

RESULTS

Thirty-day mortality was 2.4% (n = 2 patients). Both patients died postoperatively of low-output syndromes with multiorgan failure. At reoperation, 74 patients received homografts (87%), 7 patients underwent a Ross procedure (9%), and 3 received stentless porcine roots (4%). One patient required pacemaker implantation (1%). Actuarial survivals were 89% ± 4% and 81% ± 5% at 5 and 10 years, respectively. Nine patients underwent a successful third root replacement during follow-up. Freedom from third-time aortic root operation was 97% ± 3% at 10 years.

CONCLUSIONS

Reoperative aortic root replacement can be performed safely with good short-term and midterm outcomes in a young patient cohort.

On the multiscale modeling of heart valve biomechanics in health and disease

Theoretical models of the human heart valves are useful tools for understanding and characterizing the dynamics of healthy and diseased valves. Enabled by advances in numerical modeling and in a range of disciplines within experimental biomechanics, recent models of the heart valves have become increasingly comprehensive and accurate. In this paper, we first review the fundamentals of native heart valve physiology, composition and mechanics in health and disease. We will then furnish an overview of the development of theoretical and experimental methods in modeling heart valve biomechanics over the past three decades. Next, we will emphasize the necessity of using multiscale modeling approaches in order to provide a comprehensive description of heart valve biomechanics able to capture general heart valve behavior. Finally, we will offer an outlook for the future of valve multiscale modeling, the potential directions for further developments and the challenges involved.

An integrated pericardial valved stent special for percutaneous tricuspid implantation: an animal feasibility study

BACKGROUND

Various percutaneous valve replacement approaches have been reported in animals to replace the aortic and pulmonary valve. To broaden the indications of percutaneous approach to atrioventricular valves replacement, we developed a novel valved stent and evaluated the feasibility and safety of percutaneous implantation of the device in the tricuspid position.

MATERIALS AND METHODS

A unidirectional semilunar valve of porcine pericardium was sutured to a valvular ring. Then the ring with pericardial valve was mounted on a double-edge nitinol stent to construct the tricuspid valved stent. Transcatheter tricuspid valved stent implantation was performed on 10 healthy sheep. These sheep were followed up shortly after procedure with echocardiography evaluation and 64-slice CT imaging examination during the periodical follow-up at 1 mo and at 6 mo post-implantation. Additionally, two sheep were sacrificed after the procedure for anatomic and histological evaluation one at 1 h and the other at 1 mo, respectively.

RESULTS

Percutaneous valve implantation was successful in eight of 10 sheep. Two sheep died during the procedure due to migration of stent and fatal arrhythmia. The pressure of right heart did not significantly change after the procedure. Further echocardiography and imaging confirmed the stents were in desired position during the follow-up. The remaining six sheep with normal valvular and cardiac functionality survived for 6 mo after implantation.

CONCLUSIONS

The tricuspid stent with a valvular ring and pericardial valve can be implanted in tricuspid annulus percutaneous. The double-edge stent could substitute the native tricuspid valve chronically.

Management of patients with aortic dissection

INTRODUCTION

The incidence of acute diseases of the aorta will continue to rise as our population ages.

METHODS

Selective literature review.

RESULTS/DISCUSSION

At centers specializing in maximum care, it is important that a specific strategy should be established in cases of clinically suspected, acute aortic diseases, and non-invasive diagnostic measures should be taken without delay. If an acute aortic dissection is diagnosed, the necessary treatment must be provided immediately. As this disease is life-threatening, a cooperating network of referring physicians and institutions should be set up in order to ensure optimal treatment for these desperately ill patients. Acute type A aortic dissections generally require surgery, while the primary treatment of uncomplicated type B dissections is conservative and complicated type B dissections can be treated primarily with stent-graft implantation. The referring medical specialists and institutions should follow the patients closely after treatment so that any problems that may develop can be recognized and treated in timely fashion.

Catheter-based modification of heart valve diseases: from experimental to clinical application

Efforts to modify cardiac valve defects using catheter-based techniques are increasing at the present time. We present observations on cardiac valve morphology and disease and review the progress being made to address valve defects with these innovative methods. Some new procedures developed through animal experimentation have already been put to use in clinical practice, but the newness of these techniques and the small number of cases in which they have been applied to date precludes an evaluation of their long-term durability. Although at the present time cardiac surgery remains the standard for treating most cases of valve disease, in certain situations a catheter-based treatment might provide a reasonable alternative, even if only temporary, especially for individuals with serious disease who are not suitable candidates for surgery.

Heart valve function: a biomechanical perspective

Heart valves (HVs) are cardiac structures whose physiological function is to ensure directed blood flow through the heart over the cardiac cycle. While primarily passive structures that are driven by forces exerted by the surrounding blood and heart, this description does not adequately describe their elegant and complex biomechanical function. Moreover, they must replicate their cyclic function over an entire lifetime, with an estimated total functional demand of least 3x10(9) cycles. As in many physiological systems, one can approach HV biomechanics from a multi-length-scale approach, since mechanical stimuli occur and have biological impact at the organ, tissue and cellular scales. The present review focuses on the functional biomechanics of HVs. Specifically, we refer to the unique aspects of valvular function, and how the mechanical and mechanobiological behaviours of the constituent biological materials (e.g. extracellular matrix proteins and cells) achieve this remarkable feat. While we focus on the work from the authors' respective laboratories, the works of most investigators known to the authors have been included whenever appropriate. We conclude with a summary and underscore important future trends.

Assessment of Mitral Annulus Size and Function by Real-time 3-Dimensional Echocardiography in Cardiomyopathy: Comparison with Magnetic Resonance Imaging

OBJECTIVE

We sought to assess mitral annular (MA) size and function in hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) using real-time 3-dimensional (3D) echocardiography (RT3DE).

METHODS

The study included 30 patients with HCM, 20 patients with DCM, and 30 control subjects. RT3DE measurements included end-systolic and end-diastolic MA area (MAA) (MAA(3D)), MA diameter(3D), MA fractional area change (MAFAC), and MA fractional shortening. In subgroup of 50 patients, magnetic resonance imaging (MRI) was used for MAA(MRI) and MA diameter(MRI) measurement.

RESULTS

End-diastolic MAA(3D) was larger in HCM than in control group (P < .0001). Higher MAFAC and MA fractional shortening were present in HCM than in control group (P = .001 and P = .006, respectively). End-systolic and end-diastolic MAA(3D) in DCM were higher than in HCM and control groups (P < .0001). Lower MAFAC and MA fractional shortening were present in DCM than in HCM and control groups (P < .0001). MAFAC correlated well with left ventricular function in control subjects (r = 0.94, P < .0001), whereas correlation was less in DCM (r = 0.53, P = .02) and HCM (r = 0.42, P < .01). RT3DE and MRI measurements were comparable.

CONCLUSION

RT3DE assessment of MA size and function in control subjects and patients with cardiomyopathy is accurate and well correlated with MRI.

Mitral valve annuloplasty with a semirigid annuloplasty band in ischemic mitral regurgitation: early results

OBJECTIVE

A trigone-to-trigone semirigid annuloplasty band (C-G Future Band, Medtronic, Inc., Minneapolis, Minnesota, USA) was introduced in 2001 for mitral valve repair. We report our early clinical and echocardiographic results with this new device to correct ischemic mitral regurgitation.

METHODS

Between January 2002 and December 2004, among 216 patients operated on for mitral regurgitation, 107 patients had a C-G Future Band annuloplasty and 85 consecutive patients (72.6% male; mean age 66.9 +/- 8.6 years) received this annuloplasty band to correct ischemic mitral regurgitation. Mean follow-up was 14.3 +/- 9.8 months (range 0.2-37 months). Clinical and echocardiographic assessment was accomplished preoperatively, postoperatively, at 6 and 12 months, and at two years.

RESULTS

Perioperative mortality was 3.7% (three in-hospital deaths), whereas overall survival at two years was 88.7 +/- 4.2%. Immediately after repair, echocardiographic mitral regurgitation was dramatically reduced (2.5 +/- 0.6 vs. 0.9 +/- 0.6; P < 0.0001); ejection fraction increased from 43.8 +/- 11% preoperatively to 44.8 +/- 12% postoperatively (P = 0.007). At the time of follow-up, New York Heart Association (NYHA) functional class was significantly improved (mean preoperative NYHA class 2.04 +/- 0.9 vs. mean postoperative NYHA class 1.25 +/- 0.6; P < 0.0001). No patient experienced thromboembolic events and no late mitral valve reoperation occurred.

CONCLUSIONS

Early and mid-term mitral valve function is satisfactory with trigone-to-trigone semirigid band annuloplasty, with excellent repair durability immediately after the operation and at two years. Moreover, after annuloplasty repair, an improvement in clinical functional status is obtained. A wider use of this semirigid annuloplasty band can be recommended.

Value of assessment of tricuspid annulus: real-time three-dimensional echocardiography and magnetic resonance imaging

Aim

To detect the accuracy of real-time three-dimensional echocardiography (RT3DE) and two-dimensional echocardiography (2DE) for tricuspid annulus (TA) assessment compared with magnetic resonance imaging (MRI).

Methods

Thirty patients (mean age 34 ± 13 years, 60% males) in sinus rhythm were examined by MRI, RT3DE, and 2DE for TA assessment. End-diastolic and end-systolic TA diameter (TAD) and TA fractional shortening (TAFS) were measured by RT3DE, 2DE, and MRI. End-diastolic and end-systolic TA area (TAA) and TA fractional area changes (TAFAC) were measured by RT3DE and MRI. End-diastolic and end-systolic right ventricular (RV) volumes and ejection fraction (RV-EF) were measured by MRI.

Results

The TA was clearly delineated in all patients and visualized as an oval-shaped by RT3DE and MRI. There was a good correlation between TAD_MRI and TAD_3D (r = 0.75, P = 0.001), while TAD_2D was fairly correlated with TAD_3D and TAD_MRI (r = 0.5, P = 0.01 for both). There were no significant differences between RT3DE and MRI in TAD, TAA, TAFS, and TAFAC measurements, while TAD_2D and TAFS_2D were significantly underestimated (P < 0.001). TAFS_2D was not correlated with RV-EF, while TAFS_3D and TAFAC_3D were fairly correlated with RV-EF (r = 0.49, P = 0.01, and r = 0.47, P = 0.02 respectively).

Conclusion

RT3DE helps in accurate assessment of TA comparable to MRI and may have an important implication in the TV surgical decision-making processes. RT3DE analysis of TA function could be used as a marker of RV function.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

An ex vivo study of the biological properties of porcine aortic valves in response to circumferential cyclic stretch

Normal physiological mechanical forces cause constant tissue renewal in aortic valve leaflets (AVL) while altered mechanical forces incite changes in their structural and biological properties. The current study aims at characterizing the remodeling properties of AVL subjected to cyclic circumferential stretch in a sterile ex vivo bioreactor. The leaflets cultured were stretched at a maximum rate of 300%s⁻¹ corresponding to a 15% strain for 48 h. Collagen, sulfated glycosaminoglycan (sGAG), and elastin contents of the stretched, fresh, and statically incubated leaflets were measured. Cusp morphology and cell phenotype were also examined. AVLs exposed to cyclic stretch showed a significant increase in collagen content (p < 0.05) when compared to fresh and statically incubated AVLs. sGAG content was significantly reduced in the stretched AVLs (p < 0.05) when compared to the fresh leaflets and was comparable between stretched and statically incubated AVLs. There was no statistically significant change in elastin content in all the three groups of AVLs (p > 0.05). Native aortic valve morphology was well preserved in stretched leaflets. Immunohistochemistry and immunoblotting studies showed an increased expression of α-smooth muscle actin (α-SMA) in stretched leaflets while α-SMA expression was reduced in statically incubated AVLs when compared to the fresh leaflets. To conclude, circumferential cyclic stretch altered the extracellular matrix remodeling activity of valvular cells, and consequently the extracellular matrix composition of the AVLs. Most interestingly, the contractile and fibrotic phenotypic expression of valve interstitial cells was enhanced. These results show that circumferential cyclic stretch is a possible mediator for AVL remodeling activity.

Pathology of infectious and inflammatory diseases in prosthetic heart valves

Prosthetic heart valves, both mechanical and biological (xenograft valves, stented or unstented), show an inflammatory reaction (infective endocarditis), associated predominantly with bacterial/fungal infection. Somewhat surprisingly, no immune-mediated reaction has been reported thus far. This may, among other reasons, be related to the fact that the tissues are "fixed" with aldehydes and are virtually isolated from host circulation, separated by synthetic material (the valve stent and the fabric covering it). Stentless valves (especially these without fabric covering them), however, have no such "isolation" from the host circulation. While the Toronto-Stentless Porcine Valve has a covering of fabric, the Medtronic Freestyle valve has no such covering. It is perhaps not so surprising therefore that at the intermediate time point of 5 to 6 years, some valves are beginning to show such an immune reaction.

Current Perspective on Aortic Valve Repair and Valve-Sparing Aortic Root Replacement

Aortic valve repair and valve-sparing aortic root replacement are attractive concepts because they offer the possibility of valve competence without structural deterioration due to nonviability and they preclude the need for anticoagulation. Enthusiasm for aortic valve repair has waxed and waned over the past 45 years due in part to the inherent technical difficulties and poor mid-term results. Renewed interest in the concept of aortic valve repair has paralleled the development of valve-sparing aortic root replacement over the last 20 years. A current perspective on aortic valve repair and valve-sparing aortic root replacement is presented in the following review. Historical background, indications for repair, technical considerations, and outcomes data are discussed.

Ventricular septal defect closure in right coronary cusp prolapse and aortic regurgitation complicating VSD in the outlet septum: which treatment is most appropriate?

BACKGROUND

There is currently not a standardized technique for the sizing and shaping of surgical closure of the ventricular septal defect (VSD) patch in patients with right coronary aortic cusp prolapse and aortic regurgitation (AR) complicating VSD in the outlet septum.

METHODS

Forty-six VSD patients who had aortic valve prolapse were divided into groups DC (direct closure, n=19), and SPC (small patch closure, n=27). Preoperative and postoperative echocardiography with Doppler color flow interrogation was performed on all patients.

RESULTS

In the DC group, among seven patients who had aortic valve prolapse but no AR preoperative, one patient developed AR during postoperative follow-up period. In the remaining 12 patients who had mild AR associated with aortic valve prolapse prior to the procedure, AR was diminished in four and unchanged in six patients. However, AR was aggravated in two patients who required further operations for AV repair or replacement. In the SPC group, among the eight patients who had no preoperative AR, AR progressed in one patient postoperatively. In the remaining 19 patients who had mild AR, AR was diminished in 15 and unchanged in four. The outcome from the operative procedure was significantly better in the SPC group than DC group with mild preoperative AR (chi(2)=7.82; P<0.05).

CONCLUSIONS

Small patch closure for this type of VSD is safer and more reliable in improving mild AR than that of direct closure, especially in patients with mild AR.

Mitral valve repair or replacement in elderly people

PURPOSE OF REVIEW

Much has been published so far to describe and praise the benefits of mitral valve repair, and to compare it with valve replacement. Now, with mitral valve surgery in elderly people gaining greater acceptance worldwide, repair or replacement remains a controversial issue. This is especially true in the ageing population, in whom many of the complications associated with a mechanical valve can be avoided by using a bioprosthesis. This review will try to assess the latest views in the field and come up with possible answers to this ongoing question.

RECENT FINDINGS

The causes of mitral regurgitation in this age group are separately reviewed and discussed in the light of our better understanding of the pathophysiology of the disease. Mitral surgery is recommended when the effective regurgitant orifice reaches 40 mm. Repair in degenerative disease seems to be feasible, with good long-term results. In chronic ischaemic regurgitation, the concept of a tethered 'normal valve' is changing. The 'poor' ventricle may be able to withstand surgery as long as the subvalvular apparatus is preserved; on the other hand, repair and replacement seem to have the same survival advantage in high-risk patients.

SUMMARY

Mitral valve surgery is well tolerated in elderly people. Early intervention leads inevitably to better outcome. The majority of valvular disorders in this age group are amenable to repair, with good reproducible results. Replacement with a bioprosthesis remains a viable option for complex regurgitant jets.

Biaixal stress-stretch behavior of the mitral valve anterior leaflet at physiologic strain rates

Characterization of the mechanical properties of the native mitral valve leaflets at physiological strain rates is a critical step in improving our understanding of MV function and providing experimental data for dynamic constitutive models. We explored, for the first time, the effects of strain rate (from quasi-static to physiologic) on the biaxial mechanical properties of the native mitral valve anterior leaflet (MVAL). A novel high-speed biaxial testing device was developed, capable of achieving in vitro strain rates reported for the MVAL (Sacks et al., Ann. Biomed. Eng. 30(10):1280-1290, 2002). Porcine MVAL specimens were loaded to physiological load levels with cycle periods of 15, 1, 0.5, 0.1, and 0.05 s. The resulting loading stress-strain responses were found to be remarkably independent of strain rate. The hysteresis, defined as the fraction of the membrane strain energy between the loading and unloading curves tension-areal stretch curves, was low (approximately 12%) and did not vary with strain rate. The results of the present work indicated that MVAL tissues exhibit complete strain rate insensitivity at and below physiological strain rates under physiological loading conditions. These novel results suggest that experimental tests utilizing quasi-static strain rates are appropriate for constitutive model development for mitral valve tissues. The mechanisms underlying this quasi-elastic behavior are as yet unknown, but are likely an important functional aspect of native mitral valve tissues and clearly warrant further study.

Direct-Access Valve Replacement

OBJECTIVES

This study validates the off-pump antegrade transventricular route for ultrasound-guided direct-access aortic valved stent implantation.

BACKGROUND

Direct-access aortic valved stent implantation offers numerous advantages over the remote-access percutaneous approach and may one day provide an alternative to surgical aortic valve replacement.

METHODS

Valved stents were implanted off-pump in 12 pigs (68.5.0 +/- 7.3 kg) via direct-access transapical approach using a left-sided mini-thoracotomy and continuous ultrasonic and fluoroscopic guidance. Acute valved stent function was studied with intravascular and intracardiac ultrasound. All valved stents were tested in vitro before insertion. Macroscopic analysis was performed at necropsy.

RESULTS

In 8 of 12 pigs, valved stents were delivered to the target site over the native aortic valve leaflets without interference of coronary blood flow and with good acute valve function. Two valved stents were deployed and supra-annularly occluded the coronary orifice, leading to fatal outcome. Two valved stents dislodged into the left ventricle, one because of size mismatch and one that failed to unfold correctly.

CONCLUSIONS

Twelve pigs underwent deployment of a valved stent in the aortic position. Six valves observed for an average 4.5-h period showed satisfactory postimplantation valve function.

Percutaneous valve procedures: what is the future?

PURPOSE OF REVIEW

Since its introduction 27 years ago by Andreas Gruntzig, interventional cardiology has expanded its scope from coronary disease to peripheral, congenital, and also valve diseases.Percutaneous mitral commissurotomy and aortic valvuloplasty have been performed since the mid 1980s. Balloon commissurotomy has been used in tens of thousands of patients worldwide and provides good short- and long-term results in a wide range of patients. It has virtually replaced surgical commissurotomy in the management of mitral stenosis. On the other hand, percutaneous aortic valvuloplasty is almost abandoned worldwide due to its lack of efficacy and the risks involved.

RECENT FINDINGS

The new techniques of percutaneous valve intervention are aortic valve replacement, and mitral valve repair. Both are at an early stage since the first in-man applications only started in 2002. Preliminary series show that these techniques are feasible; however, they must be further evaluated in comparison with contemporary treatment to accurately assess efficacy and risks. Today potential applications concern high-risk patients, however, after thorough evaluation this may be extended to others in the future.

SUMMARY

Therefore, percutaneous interventions already play an important role in management of valvular heart disease, which is likely to grow in the future.