Eric Sirois

Simulated elliptical bioprosthetic valve deformation: Implications for asymmetric transcatheter valve deployment

The asymmetric, elliptical shape of a transcatheter aortic valve (TAV), after implantation into a calcified aortic root, has been clinically observed. However, the impact of elliptical TAV configuration on TAV leaflet stress and strain distribution and valve regurgitation is largely unknown. In this study, we developed computational models of elliptical TAVs based on a thin pericardial bioprosthetic valve model recently developed. Finite element and computational fluid dynamics simulations were performed to investigate TAV leaflet structural deformation and central backflow leakage, and compared with those of a nominal symmetric TAV. From the results, we found that for a distorted TAV with an elliptical eccentricity of 0.68, the peak stress increased significantly by 143% compared with the nominal circular TAV. When the eccentricity of an elliptical TAV was larger than 0.5, a central backflow leakage was likely to occur. Also, deployment of a TAV with a major calcified region perpendicular to leaflet coaptation line was likely to cause a larger valve leakage. In conclusion, the computational models of elliptical TAVs developed in this study could improve our understanding of the biomechanics involved in a TAV with an elliptical configuration and facilitate optimal design of next-generation TAV devices.

Patient-Specific Modeling of Biomechanical Interaction in Transcatheter Aortic Valve Deployment

The objective of this study was to develop a patient-specific computational model to quantify the biomechanical interaction between the transcatheter aortic valve (TAV) stent and the stenotic aortic valve during TAV intervention. Finite element models of a patient-specific stenotic aortic valve were reconstructed from multi-slice computed tomography (MSCT) scans, and TAV stent deployment into the aortic root was simulated. Three initial aortic root geometries of this patient were analyzed: (a) aortic root geometry directly reconstructed from MSCT scans, (b) aortic root geometry at the rapid right ventricle pacing phase, and (c) aortic root geometry with surrounding myocardial tissue. The simulation results demonstrated that stress, strain, and contact forces of the aortic root model directly reconstructed from MSCT scans were significantly lower than those of the model at the rapid ventricular pacing phase. Moreover, the presence of surrounding myocardium slightly increased the mechanical responses. Peak stresses and strains were observed around the calcified regions in the leaflets, suggesting the calcified leaflets helped secure the stent in position. In addition, these elevated stresses induced during TAV stent deployment indicated a possibility of tissue tearing and breakdown of calcium deposits, which might lead to an increased risk of stroke. The potential of paravalvular leak and occlusion of coronary ostia can be evaluated from simulated post-deployment aortic root geometries. The developed computational models could be a valuable tool for pre-operative planning of TAV intervention and facilitate next generation TAV device design.

Computational Evaluation of Platelet Activation Induced by a Bioprosthetic Heart Valve

It is known that bioprosthetic heart valves (BHVs) have better hemodynamics and lower thromboembolic events compared with their mechanical counterparts; however, patients implanted with BHVs still face the potential of such complications. The risk of a clinical thromboembolism is on average 0.7% per year in patients with tissue valves in sinus rhythm. In this study, we developed a computational fluid dynamic (CFD) model of a BHV implanted in an aortic root and investigated the BHV-induced platelet activation using a damage accumulation model previously applied to mechanical valves. The CFD model was validated against published experimental data, including the flow velocity profile across the valve and the transvalvular pressure drop, and close matches were obtained. Hemodynamic performance measures such as flow velocity, turbulent kinetic energy, and wall shear stress were explored. Lagrangian particle tracking was used to calculate the extent of platelet activation for central bulk flow and flow in the vicinity of the leaflets. A peak flow of 2.22 m/s was observed at 40 msec after peak systole in the vicinity of a fold at the base of the leaflets. With the platelet activation expressed as 0-100% of activation threshold levels, mean damage on one pass was 2.489 × 10(-7)% and maximum damage on one pass was 8.778 × 10(-4)%. Our results suggested that the potential for BHV-induced platelet activation was low and that the leaflets fully open geometry might play a role in the extent of blood element damage.

Hemodynamic Impact of Transcatheter Aortic Valve Deployment Configuration

Transcatheter aortic valve (TAV) replacement holds promise for a large number of patients who otherwise have limited or no treatment options. However, it also poses various challenges, due to its unique disease treatment mechanism. Successful TAV deployment and function are heavily reliant on the tissue-stent interaction [1,2]. For patients with aortic stenosis, heavy calcium deposition on the valve leaflets and the aortic root can also cause distortion of TAV geometries, resulting in a valve of an elliptical shape [3–5] instead of a nominal circular shape. In a recent study by Schultz et al. [5], the geometry and apposition of the TAV after implantation in 30 patients with aortic stenosis were evaluated using multislice computed tomography. The results indicated that none of the TAV frames reached exactly nominal designed dimensions. The difference between the orthogonal smallest and largest diameters of TAV cross section at the ventricular end was 4.4 mm, which was clearly an asymmetric elliptical shape [5]. In this study, we sought to develop computational models of valve hemodynamics under clinically relevant deployment scenarios.

Determination of radial force and coefficient of friction with a self-expanding transcatheter aortic valve stent

An experimental and analytical approach were performed to study the biomechanics of a self-expanding, nitinol transcatheter aortic valve (TAV) stent device deployed in porcine heart tissue. The radial force, pullout force, and coefficient of friction (COF) were quantified, and the interacting device tissue response was investigated. The preliminary data generated from this study may provide a useful measure for determining an adequate valve oversize in aiding against device migration.

Simulated Transcatheter Aortic Valve Flow: Implications of Elliptical Deployment and Under-Expansion at the Aortic Annulus: SIMULATED TRANSCATHETER AORTIC VALVE FLOW

Clinical use of transcatheter aortic valves (TAVs) has been associated with abnormal deployment, including oval deployment and under-expansion when placed into calcified aortic annuli. In this study, we performed an integrated computational and experimental investigation to quantify the impact of abnormal deployment at the aortic annulus on TAV hemodynamics. A size 23 mm generic TAV computational model, developed and published previously, was subjected to elliptical deployment at the annulus with eccentricity levels up to 0.68 and to under-expansion of the TAV at the annulus by up to 25%. The hemodynamic performance was quantified for each TAV deployment configuration. TAV opening geometries were fabricated using stereolithography and then subjected to steady forward flow testing in accordance with ISO-5840. Centerline pressure profiles were compared to validate the computational model. Our findings show that slight ellipticity of the TAV may not lead to degeneration of hydrodynamic performance. However, under large ellipticity, increases in transvalvular pressure gradients were observed. Under-expanded deployment has a much greater negative effect on the TAV hemodynamics compared with elliptical deployment. The maximum turbulent viscous shear stress (TVSS) values were found to be significantly larger in under-expanded TAVs. Although the maximum value of TVSS was not large enough to cause hemolysis in all cases, it may cause platelets activation, especially for under-expanded deployments.

A methodology of measuring coronary flow in a porcine aortic root using a pulsatile flow loop

The purpose of the study is to develop an ex vivo testing methodology to quantify coronary artery flow using a pulsatile left heart simulator. A dissected porcine aorta was mounted on a Vivitro left heart simulator and its coronary artery flow was measured and calibrated to match previously recorded human coronary data[1]. The method of aortic root attachment proved to fit the flow loop with no fluid leakage and the coronary flow conditions were found to match the baseline data trends with relatively good accuracy. This methodology may facilitate ex vivo measurement of coronary flow under various transcatheter valve deployment conditions.

Fluid Study of Transcatheter Aortic Valve Deployment Into Patients With Varying Coronary Ostia Position

Recently, minimally-invasive transcatheter aortic valve (TAV) replacement has emerged as a viable alternative to traditional open-chest heart valve replacement for high risk patients who otherwise have limited or no treatment options. Although significant experience with TAV procedures has been gained, various adverse effects have been observed after device implantation [1, 2]. One adverse event is the impairment of coronary artery flow. Because the TAV stent pushes the native leaflets towards the sinus of Valsalva during TAV deployment, the flow boundaries in the aortic root are consequently altered. A worst case scenario would be the occlusion of the coronary ostia. Reduced flow to the coronary arteries has also been observed for some patients following TAV intervention [3]. With IRB approval, we recently conducted a dimensional analysis of 3D aortic root geometries, reconstructed from 64-slice CT scans of 95 patients [4]. TAV-relevant dimensions were measured. The spatial distribution of the left coronary ostium was quantified (Fig. 1). In this study, we will construct a patient-specific aortic root model with varied coronary ostium locations as shown in Fig. 1, and perform a combined finite element analysis (FEA) and computational fluid dynamics (CFD) simulation to investigate hemodynamic environment changes that occur following TAV intervention.Copyright

Measuring aortic sinus pressure-inflation response using three dimensional marker tracking

In this study, we investigated the mechanical properties of porcine aortic sinus using a three-dimensional marker measuring technique called the direct linear transformation (DLT) method. By recording pressure-inflation tests of five porcine aortic roots with the use of two cameras simultaneously, we were able to obtain stress and strain responses of aortic sinus upon applied luminal pressures and compare them with previously published planar biaxial results of the same tissue. We found that the roots were stiffer in the circumferential direction for the middle region of the non-coronary sinus for all specimens. The magnitude of stress at 30% strain was higher than that observed using planar biaxial testing.

Three-Point Bending Device for flexure testing of soft tissues

This 3-Point Bending Device is intended to provide the user with a novel tool used to obtain material property information for biological tissues. Specifically, the device is intended to provide the user with the stress-strain relationship of the tested tissue in the low-strain region (≪5% strain) and provide the location of the neutral axis. The stress-strain relationship is useful because it allows the user to predict the response of tissue to an applied load. The location of the neutral axis is important because it allows the user to estimate the contributions of different layers in a multi-layered tissue specimen. No patented device or device described in the literature is capable of carrying out both of these functions.