Possible Subclinical Leaflet Thrombosis in Bioprosthetic Aortic Valves

To the Editor: Makkar et al. (Nov. 19 issue)(1) report possible subclinical leaflet thrombosis in up to 40% of patients involved in a clinical trial of transcatheter aortic-valve replacement (TAVR). In contrast, we found a relatively low incidence (7%) of possible subclinical valve leaflet thrombosis among patients in our series in which 255 patients underwent TAVR with the use of a CoreValve prosthesis. A retrospective review of our series (unpublished data) showed that in 104 patients, cardiac computed tomography (CT) at a median of 7 days after implantation (range, 3 to 87) (in 51 patients), transesophageal echocardiography at a median . . 

In silico study of the ageing effect upon aortic valves

A fluid–structure interaction (FSI) numerical model of the aortic valve was used to simulate and compare young and physiological aged operating conditions. The effect of normal ageing was considered by introducing alterations typically associated with senility: namely mild stiffening of the tissues and progressive dilation of the aortic chamber. The aim of this study is to provide a haemodynamic baseline which allows to assess the typical physiological variations associated with advancing age. Results were analysed in terms of leaflets kinematics, flow dynamics, pressure and valve performance parameters. The study indicates that the normal changes occurring with ageing, such as stiffening and progressive aortic root dilation, can result in substantial alterations in the haemodynamics and mechanical efficiency of the aortic valve. In particular, mild tissue stiffening and aortic root dilation reduce the valve efficiency over the cardiac cycle. The concomitant presence of both phenomena can lead to some mitigation of the impairment. The observed changes, which can be associated with normal and healthy ageing, need to be taken into consideration when evaluating the real pathological contribution of aortic valve diseases occurring in aged patients

Standard mechanical testing is inadequate for the mechanical characterisation of shape-memory alloys: Source of errors and a new corrective approach

Thanks to its unique behaviour characterised by a superelastic response, Nitinol has now become the material of preference in a number of critical applications, especially in the area of medical implants. However, the reversible phase transformation producing its exceptional comportment is also responsible for a number of phenomena that make its mechanical characterisation particularly complex, by hindering the assumptions at the very basis of common uniaxial tensile testing. This necessarily reduces the level of safety and design optimization of current applications, which rely on incorrect mechanical parameters. In this study, the spurious effects introduced by the unconventional material behaviour during uniaxial tensile testing are analysed by means of digital image correlation (DIC), identifying the onset of undesirable material inhomogeneities and bending moments that are dependent on the test setup and strongly limit the reliability of standard characterisation. Hence, a more accurate and systematic testing approach, exploiting the ability of DIC to analyse the local mechanical response at specific regions of the test specimen, is presented and discussed

Computational Analysis of Balloon Catheter Behaviour at Variable Inflation Levels

Aortic valvuloplasty is a minimally invasive procedure for the dilatation of stenotic aortic valves. Rapid ventricular pacing is an established technique for balloon stabilization during this procedure. However, low cardiac output due to the pacing is one of the inherent risks, which is also associated with several potential complications. This paper proposes a numerical modelling approach to understand the effect of different inflation levels of a valvuloplasty balloon catheter on the positional instability caused by a pulsating blood flow. An unstretched balloon catheter model was crimped into a tri-folded configuration and inflated to several levels. Ten different inflation levels were then tested, and a Fluid-Structure Interaction model was built to solve interactions between the balloon and the blood flow modelled in an idealised aortic arch. Our computational results show that the maximum displacement of the balloon catheter increases with the inflation level, with a small step at around 50% inflation and a sharp increase after reaching 85% inflation. This work represents a substantial progress towards the use of simulations to solve the interactions between a balloon catheter and pulsating blood flow

Biological Equivalence of GGTA-1 Glycosyltransferase Knockout and Standard Porcine Pericardial Tissue Using 90-Day Mitral Valve Implantation in Adolescent Sheep

Objective There is growing interest in the application of genetically engineered reduced antigenicity animal tissue for manufacture of bioprosthetic heart valves (BHVs) to reduce antibody induced tissue calcification and accelerated structural valve degeneration (SVD). This study tested biological equivalence of valves made from Gal-knockout (GalKO) and standard porcine pericardium after 90-day mitral valve implantation in sheep. Methods GalKO (n = 5) and standard (n = 5) porcine pericardial BHVs were implanted in a randomized and blind fashion into sheep for 90-days. Valve haemodynamic function was measured at 30-day intervals. After explantation, valves were examined for pannus, vegetation, inflammation, thrombus, and tissue calcification. Results Nine of 10 recipients completed the study. There was no difference between study groups for haemodynamic performance and no adverse valve-related events. Explanted BHVs showed mild pannus integration and minimal thrombus, with no difference between the groups. Limited focal mineral deposits were detected by x-ray. Atomic spectroscopy analysis detected tissue calcium levels of 1.0 µg/mg ± 0.2 for GalKO BHVs and 1.9 µg/mg ± 0.9 for standard tissue BHVs (p = 0.4), considered to be both low and equivalent. Conclusions This is the first demonstration of biological equivalence between GalKO and standard pig pericardium. The GalKO mutation causes neither intrinsic detrimental biological nor functional impact on BHV performance. Commercial adaptation of GalKO tissue for surgical or transcatheter BHVs would remove the clinical disparity between patients producing anti-Gal antibody and BHVs containing the Gal antigen. GalKO BHVs may reduce accelerated tissue calcification and SVD, enhancing patient choices, especially for younger patients

Investigation of the Thermomechanical Response of Cyclically Loaded NiTi Alloys by Means of Temperature Frequency Domain Analyses

Nickel–Titanium (NiTi) shape memory alloys subjected to cyclic loading exhibit reversible temperature changes whose modulation is correlated with the applied load. This reveals the presence of reversible thermomechanical heat sources activated by the applied stresses. One such source is the elastocaloric effect, accounting for the latent heat of Austenite–Martensite phase transformation. It is, however, observed that when the amplitude of cyclic loads is not sufficient to activate or further propagate this phase transformation, the material still exhibits a strong cyclic temperature modulation. The present work investigates the thermomechanical behaviour of NiTi under such low-amplitude cyclic loading. This is carried out by analysing the frequency domain content of temperature sampled over a time window. The amplitude and phase of the most significant harmonics are obtained and compared with the theoretical predictions from the first and second-order theories of the Thermoelastic Effect, this being the typical reversible thermomechanical coupling prevailing under elastic straining. A thin strip of NiTi, exhibiting a fully superelastic behaviour at room temperature, was investigated under low-stress amplitude tensile fatigue cycling. Full-field strain and temperature distributions were obtained by means of Digital Image Correlation and IR Thermography. The work shows that the full field maps of amplitude and phase of the first three significant temperature harmonics carry out many qualitative information about the stress and structural state of the material. It is, though, found that the second-order theory of the Thermoelastic Effect is not fully capable of justifying some of the features of the harmonic response, and further work on the specific nature of thermomechanical heat sources is required for a more quantitative interpretation

Transcatheter aortic valves produce unphysiological flows which may contribute to thromboembolic events: An in-vitro study.

PURPOSE: Transcatheter aortic valve implantation (TAVI) has been associated with large incidence of ischemic events, whose sources are still unclear. In fact, sub-acute complications cannot be directly related to the severity of the calcification in the host tissues, nor with catheter manipulation during the implant. A potential cause could be local flow perturbations introduced by the implantation approach, resulting in thrombo-embolic consequences. In particular, contrary to the surgical approach, TAVI preserves the presence of the native leaflets, which are expanded in the paravalvular space inside the Valsalva sinuses. The purpose of this study is to verify if this configuration can determine hemodynamic variations which may promote blood cell aggregation and thrombus formation. METHODS: The study was performed in vitro, on idealized models of the patient anatomy before and after TAVI, reproducing a range of physiological operating conditions on a pulse duplicator. The fluid dynamics in the Valsalva sinuses was analyzed and characterized using phase resolved Particle Image Velocimetry. RESULTS: Comparison of the flow downstream the valve clearly indicated major alterations in the fluid mechanics after TAVI, characterized by unphysiological conditions associated with extended stagnation zones at the base of the sinuses. CONCLUSION: The prolonged stasis observed in the Valsalva sinuses for the configuration modelling the presence of transcatheter aortic valves provides a fluid dynamic environment favourable for red blood cell aggregation and thrombus formation, which may justify some of the recently reported thromboembolic and ischemic events. This suggests the adoption of anticoagulation therapies following TAVI, and some caution in the patients׳ selection

Investigation of the thermomechanical response of cyclically loaded niti alloys by means of temperature frequency domain analyses

Nickel\u2013Titanium (NiTi) shape memory alloys subjected to cyclic loading exhibit reversible temperature changes whose modulation is correlated with the applied load. This reveals the pres-ence of reversible thermomechanical heat sources activated by the applied stresses. One such source is the elastocaloric effect, accounting for the latent heat of Austenite\u2013Martensite phase transfor-mation. It is, however, observed that when the amplitude of cyclic loads is not sufficient to activate or further propagate this phase transformation, the material still exhibits a strong cyclic temperature modulation. The present work investigates the thermomechanical behaviour of NiTi under such low-amplitude cyclic loading. This is carried out by analysing the frequency domain content of temperature sampled over a time window. The amplitude and phase of the most significant harmonics are obtained and compared with the theoretical predictions from the first and second-order theories of the Thermoelastic Effect, this being the typical reversible thermomechanical coupling prevailing under elastic straining. A thin strip of NiTi, exhibiting a fully superelastic behaviour at room temperature, was investigated under low-stress amplitude tensile fatigue cycling. Full-field strain and temperature distributions were obtained by means of Digital Image Correlation and IR Thermography. The work shows that the full field maps of amplitude and phase of the first three significant temperature harmonics carry out many qualitative information about the stress and structural state of the material. It is, though, found that the second-order theory of the Thermoelastic Effect is not fully capable of justifying some of the features of the harmonic response, and further work on the specific nature of thermomechanical heat sources is required for a more quantitative interpretation

In vitro haemodynamic testing of Amplatzer plugs for paravalvular leak occlusion after Transcather Aortic Valve Implantation

Objective: We aimed to in-vitro test Amplatzer devices (Amplatzer Vascular Plug II and Amplatzer Vascular Plug III, SJM St. Paul, MN) in closing PVL generated by transcatheter balloon expandable aortic valve prosthesis in order to quantify the effective treatment of PVL. Background: Transcatheter aortic valve replacement (TAVI) procedures represent the treatment of choice for high risk patients. Despite evolving technologies paravalvular leak (PVL) is still a major unaddressed issue. This severe complication significantly impair long-term survival. Percutaneous treatment of this complication is usually performed with the implantation of not specifically designed and not approved vascular devices. Methods: A 23mm Sapien XT (Edwards Lifesciences, Irvine, CA) was implanted in a rubber aortic root and a semi-elliptical shape PVL was created. The vascular occluder devices were implanted in the PVL and hemodynamic performance was tested in a pulse duplicator according to international standard ISO 5840-3:2013. Different type of comparison tests together with high speed camera recording allowed us to define the global efficiency of the occluders and their interaction with the transcatheter prosthesis. Results: The results revealed that the use of vascular plugs was not per se sufficient to produce an effective or substantial reduction of PVL with a maximum efficiency inferior of 50%. Recorded video showed clearly that the vascular plug always interfered with the leaflet of the prosthetic valve. Conclusions: Current used devices do not guarantee effective treatment of PVL and may otherwise compromise the structural integrity of the prosthetic valve implanted. Specific designed devices are required