Vrishank Raghav

The Fluid Mechanics of Transcatheter Heart Valve Leaflet Thrombosis in the Neosinus

Background: Transcatheter heart valve (THV) thrombosis has been increasingly reported. In these studies, thrombus quantification has been based on a 2-dimensional assessment of a 3-dimensional phenomenon. Methods: Postprocedural, 4-dimensional, volume-rendered CT data of patients with CoreValve, Evolut R, and SAPIEN 3 transcatheter aortic valve replacement enrolled in the RESOLVE study (Assessment of Transcatheter and Surgical Aortic Bioprosthetic Valve Dysfunction With Multimodality Imaging and Its Treatment with Anticoagulation) were included in this analysis. Patients on anticoagulation were excluded. SAPIEN 3 and CoreValve/Evolut R patients with and without hypoattenuated leaflet thickening were included to study differences between groups. Patients were classified as having THV thrombosis if there was any evidence of hypoattenuated leaflet thickening. Anatomic and THV deployment geometries were analyzed, and thrombus volumes were computed through manual 3-dimensional reconstruction. We aimed to identify and evaluate risk factors that contribute to THV thrombosis through the combination of retrospective clinical data analysis and in vitro imaging in the space between the native and THV leaflets (neosinus). Results: SAPIEN 3 valves with leaflet thrombosis were on average 10% further expanded (by diameter) than those without (95.5±5.2% versus 85.4±3.9%; P<0.001). However, this relationship was not evident with the CoreValve/Evolut R. In CoreValve/Evolut Rs with thrombosis, the thrombus volume increased linearly with implant depth (R2=0.7, P<0.001). This finding was not seen in the SAPIEN 3. The in vitro analysis showed that a supraannular THV deployment resulted in a nearly 7-fold decrease in stagnation zone size (velocities <0.1 m/s) when compared with an intraannular deployment. In addition, the in vitro model indicated that the size of the stagnation zone increased as cardiac output decreased. Conclusions: Although transcatheter aortic valve replacement thrombosis is a multifactorial process involving foreign materials, patient-specific blood chemistry, and complex flow patterns, our study indicates that deployed THV geometry may have implications on the occurrence of thrombosis. In addition, a supraannular neosinus may reduce thrombosis risk because of reduced flow stasis. Although additional prospective studies are needed to further develop strategies for minimizing thrombus burden, these results may help identify patients at higher thrombosis risk and aid in the development of next-generation devices with reduced thrombosis risk.

How Can We Help a Patient With a Small Failing Bioprosthesis?: An In Vitro Case Study.

OBJECTIVES The aim of this study was to investigate the hemodynamic performance of a transcatheter heart valve (THV) deployed at different valve-in-valve positions in an in vitro model using a small surgical bioprosthesis. BACKGROUND Patients at high surgical risk with failing 19-mm surgical aortic bioprostheses are not candidates for valve-in-valve transcatheter aortic valve replacement, because of risk for high transvalvular pressure gradients (TVPGs) and patient-prosthesis mismatch. METHODS A 19-mm stented aortic bioprosthesis was mounted into the aortic chamber of a pulse duplicator, and a 23-mm low-profile balloon-expandable THV was deployed (valve-in-valve) in 4 positions: normal (bottom of the THV stent aligned with the bottom of the surgical bioprosthesis sewing ring) and 3, 6, and 8 mm above the normal position. Under controlled hemodynamic status, the effect of these THV positions on valve performance (mean TVPG, geometric orifice area, and effective orifice area), thrombotic potential (sinus shear stress), and migration risk (pullout force and embolization flow rate) were assessed. RESULTS Compared with normal implantation, a progressive reduction of mean TVPG was observed with each supra-annular THV position (normal: 33.10 mm Hg; 3 mm: 24.69 mm Hg; 6 mm: 19.16 mm Hg; and 8 mm: 12.98 mm Hg; p < 0.001). Simultaneously, we observed increases in geometric orifice area (normal: 0.83 cm(2); 8 mm: 1.60 cm(2); p < 0.001) and effective orifice area (normal: 0.80 cm(2); 8 mm: 1.28 cm(2); p < 0.001) and reductions in sinus shear stresses (normal: 153 dyne/cm(2); 8 mm: 40 dyne/cm(2); p < 0.001), pullout forces (normal: 1.55 N; 8 mm: 0.68 N; p < 0.05), and embolization flow rates (normal: 32.91 l/min; 8 mm: 26.06 l/min; p < 0.01). CONCLUSIONS Supra-annular implantation of a THV in a small surgical bioprosthesis reduces mean TVPG but may increase the risk for leaflet thrombosis and valve migration. A 3- to 6-mm supra-annular deployment could be an optimal position in these cases.

The Effect of Valve-in-Valve Implantation Height on Sinus Flow

Valve-in-valve transcatheter aortic valve replacement (VIV-TAVR) has proven to be a successful treatment for high risk patients with failing aortic surgical bioprostheses. However, thrombus formation on the leaflets of the valve has emerged as a major issue in such procedures, posing a risk of restenosis, thromboembolism, and reduced durability. In this work we attempted to understand the effect of deployment position of the transcatheter heart valve (THV) on the spatio-temporal flow field within the sinus in VIV-TAVR. Experiments were performed in an in vitro pulsatile left heart simulator using high-speed Particle Image Velocimetry (PIV) to measure the flow field in the sinus region. The time-resolved velocity data was used to understand the qualitative and quantitative flow patterns. In addition, a particle tracking technique was used to evaluate relative thrombosis risk via sinus washout. The velocity data demonstrate that implantation position directly affects sinus flow patterns, leading to increased flow stagnation with increasing deployment height. The particle tracking simulations showed that implantation position directly affected washout time, with the highest implantation resulting in the least washout. These results clearly demonstrate the flow pattern and flow stagnation in the sinus is sensitive to THV position. It is, therefore, important for the interventional cardiologist and cardiac surgeon to consider how deployment position could impact flow stagnation during VIV-TAVR.

On the Mechanics of Transcatheter Aortic Valve Replacement.

Transcatheter aortic valves (TAVs) represent the latest advances in prosthetic heart valve technology. TAVs are truly transformational as they bring the benefit of heart valve replacement to patients that would otherwise not be operated on. Nevertheless, like any new device technology, the high expectations are dampened with growing concerns arising from frequent complications that develop in patients, indicating that the technology is far from being mature. Some of the most common complications that plague current TAV devices include malpositioning, crimp-induced leaflet damage, paravalvular leak, thrombosis, conduction abnormalities and prosthesis-patient mismatch. In this article, we provide an in-depth review of the current state-of-the-art pertaining the mechanics of TAVs while highlighting various studies guiding clinicians, regulatory agencies, and next-generation device designers.

An Exploration of Radial Flow on a Rotating Blade in Retreating Blade Stall

The nature of radial flow during retreating blade stall on a two-bladed teetering rotor with cyclic pitch variation is investigated using laser sheet visualization and particle image velocimetry in a low-speed wind tunnel. The velocity field above the retreating blade at 270◦ azimuth shows the expected development of a radially directed jet layer close to the blade surface in the otherwise separated flow region. This jet is observed to break up into discrete structures, limiting the spanwise growth of the radial velocity in the jet layer. The discrete structures are shown to derive their vorticity from the “radial jet” layer near the surface, rather than from the freestream at the edge of the separated region. The separation line determined using velocity data shows the expected spanwise variation. The results of this study are also correlated in a limited range of extrapolation to the phenomena encountered on a full-scale horizontal axis wind turbine in yaw.

Valve mediated hemodynamics and their association with distal ascending aortic diameter in bicuspid aortic valve subjects: Valve Mediated Hemodynamics in BAV Patients

Valve mediated hemodynamics have been postulated to contribute to pathology of the ascending aorta (AAo). The objective of this study is to assess the association of aortic valve morphology and hemodynamics with downstream AAo size in subjects with bicuspid aortic valve (BAV) disease.

Thermoelectric and thermophotovoltaic micro renewable power systems for home use

Terrestrial mass-market needs for off-grid power generators offer applications and challenges for space research. Synergy between research on extraterrestrial in situ resource exploitation, and research on terrestrial mass-market needs for standalone power generators, offers avenues to advance both, with large potential impact on the quality of life. The technologies have been shown to work. However, closing the business case for such devices requires careful definition of requirements and attention to a wide array of issues. In this paper, two systems derived from space technology are discussed. The first is a thermoelectric generator for use with rudimentary wood stoves. This is designed to produce enough power for a fan to augment air flow through the stove, a DC LED floodlamp for steady lighting, and an ultraviolet LED water purifier. The power budget for this system will close well under the 10-watt level that is projected for a single semiconductor thermoelectric generator; however serious challenges remain in active thermal control. The second system is a thermophotovoltaic generator for use with trash incinerators sized for middle class families or neighborhoods. Radiation from ceramic emitters placed in the incinerator is concentrated and filtered into a set of photovoltaic cells tuned to the radiation spectrum. Power levels of several tens to hundreds of watts have already been demonstrated from such systems.

Role of Mitral Annulus Diastolic Geometry on Intraventricular Filling Dynamics

The mitral valve (MV) is a bileaflet valve positioned between the left atrium and ventricle of the heart. The annulus of the MV has been observed to undergo geometric changes during the cardiac cycle, transforming from a saddle D-shape during systole to a flat (and less eccentric) D-shape during diastole. Prosthetic MV devices, including heart valves and annuloplasty rings, are designed based on these two configurations, with the circular design of some prosthetic heart valves (PHVs) being an approximation of the less eccentric, flat D-shape. Characterizing the effects of these geometrical variations on the filling efficiency of the left ventricle (LV) is required to understand why the flat D-shaped annulus is observed in the native MV during diastole in addition to optimizing the design of prosthetic devices. We hypothesize that the D-shaped annulus reduces energy loss during ventricular filling. An experimental left heart simulator (LHS) consisting of a flexible-walled LV physical model was used to characterize the filling efficiency of the two mitral annular geometries. The strength of the dominant vortical structure formed and the energy dissipation rate (EDR) of the measured fields, during the diastolic period of the cardiac cycle, were used as metrics to quantify the filling efficiency. Our results indicated that the O-shaped annulus generates a stronger (25% relative to the D-shaped annulus) vortical structure than that of the D-shaped annulus. It was also found that the O-shaped annulus resulted in higher EDR values throughout the diastolic period of the cardiac cycle. The results support the hypothesis that a D-shaped mitral annulus reduces dissipative energy losses in ventricular filling during diastole and in turn suggests that a symmetric stent design does not provide lower filling efficiency than an equivalent asymmetric design.

Efficient Airload Determination for Bluff Body Aeromechanics

A continuous-rotation testing technique is applied to capture the variation of aerodynamic loads with attitude on objects of arbitrary shape. The technique converts the problem of measuring static air loads at various attitudes into a periodic problem. Phase-resolved ensemble-averaging is used to capture load variations with arbitrarily fine azimuthal resolution. The airload variations are obtained in closed form as discrete Fourier series. Experiments on a cylinder model of equal length and diameter were used to study the ability to capture asymmetries, and resolve support interference issues. A closed cuboid is used to correlate with prior work. A flat plate with a central cylindrical load, and a porous box are also studied. Free-swing tests using rigid tethers fixed to a pitch-yaw-roll gimbal mount are used to derive dynamic behavior in a free stream. The cylinder results showed the ability to resolve the effect of minor geometric asymmetries on airloads. The flat plate at 10 degrees pitch shows strong differences in dynamics between cases with a rounded versus squared-off edge facing the freestream. The porous box shows the differences between cases with and without one side blocked.Copyright

Study of Factors Driving Pitch, Roll, and Yaw Coupling in Bluff Body Aerodynamics

Helicopter sling loads exhibit departures from stable behavior at certain speeds posing certain safety concerns. Rolling due to pendulum motion about the sling attachment point may couple with yaw and pitch oscillations, amplifying the motion to dangerous levels at certain speeds. This is a case of coupling between the aerodynamic forces and the dynamics of the tethering system, and can a ect the handling qualities of the vehicle as well. In this work the factors driving the pitch, roll and yaw coupling in blu body aerodynamics are studied. The sensitivity of the side force and yawing moment to yaw angle is studied in a low speed wind tunnel, in steady-state load measurements simulating the e ect of velocity induced by quasi-steady pendulum-type roll motion. While the side force and yawing moment appear to be generally stable to induced yaw, at some speeds there is a reversal of the stability derivative, partially explaining observed behavior in tethered tests. The time-averaged side force and yawing moment behavior with yaw angle generally agree with predictions made using unstructured-grid Navier-Stokes computations. While the time-averaged loads at small yaw angles are small, there are substantial unsteady oscillations of the loads about these values. A hotlm constant temperature anemometer probe is used to measure spectra of velocity uctuations near the edge of the model and in the wake downstream. Spectral peaks of integrated loads observed in 2-D computations occur in the same frequency band as those seen in the measured velocity spectra.