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Blood-flow downstream of stenotic and healthy aortic valves exhibits intermittent random fluctuations in the velocity field which are associated with turbulence. Such flows warrant the use of computationally demanding scale-resolving models. The aim of this work was to compute and quantify this turbulent flow in healthy and stenotic heart valves for steady and pulsatile flow conditions. Large eddy simulations (LESs) and Reynolds-averaged Navier-Stokes (RANS) simulations were used to compute the flow field at inlet Reynolds numbers of 2700 and 5400 for valves with an opening area of 70 mm2 and 175 mm2 and their projected orifice-plate type counterparts. Power spectra and turbulent kinetic energy were quantified on the centerline. Projected geometries exhibited an increased pressure-drop (>90%) and elevated turbulent kinetic energy levels (>147%). Turbulence production was an order of magnitude higher in stenotic heart valves compared to healthy valves. Pulsatile flow stabilizes flow in the acceleration phase, whereas onset of deceleration triggered (healthy valve) or amplified (stenotic valve) turbulence. Simplification of the aortic valve by projecting the orifice area should be avoided in computational fluid dynamics (CFD). RANS simulations may be used to predict the transvalvular pressure-drop, but scale-resolving models are recommended when detailed information of the flow field is required., (Copyright © 2022 by ASME.)

Objectives: The aim of this study was to perform wall stress analysis (WSA) using 4D ultrasound (US) in 40 patients with an abdominal aortic aneurysm (AAA). The geometries and wall stress results were compared with computed tomography (CT) in seven patients. Additionally, the WSA models were calibrated using 4D motion estimation, resulting in patient specific material parameters that were compared among patients., Methods: 4D-US images were acquired for 40 patients (AAA diameter 27-52 mm). Patient specific AAA geometries and wall motion were extracted from the 4D-US. WSA was performed and corresponding patient specific material properties were derived. For seven patients, CT data were available and analyzed for geometry and wall stress comparison., Results: The 4D-US based 99th percentile wall stress ranged from 198 to 390 kPa. Regression analysis showed no significant relation between wall stress and diameter of the AAA. The similarity indices between US and CT were very good and ranged between 0.90 and 0.96, and the 25th, 50th, 75th, and 95th percentile wall stresses of the US and CT data were in agreement. The characterized patient specific shear modulus had a median of 1.1 MPa (interquartile range, 0.7-1.4 MPa). Based on the maximum AAA diameter, the AAAs were divided in a small, medium, and large diameter groups. The largest AAAs revealed an increased wall stiffness compared with the smallest AAAs., Conclusions: 4D ultrasound is applicable for wall stress analysis of AAAs, and offers the opportunity to perform wall stress analysis over time, also for AAAs who do not qualify for a CT or magnetic resonance imaging. Moreover, the patient specific material properties can be determined, which could possibly improve risk assessment., (Copyright © 2016 European Society for Vascular Surgery. Published by Elsevier Ltd. All rights reserved.)

Integration of biological samples into in vitro mock loops is fundamental to simulate real device's operating conditions. We developed an in vitro platform capable of simulating the pumping function of the heart through the external pressurization of the ventricle. The system consists of a fluid-filled chamber, in which the ventricles are housed and sealed to exclude the atria from external loads. The chamber is connected to a pump that drives the motion of the ventricular walls. The aorta is connected to a systemic impedance simulator, and the left atrium to an adjustable preload. The platform reproduced physiologic hemodynamics, i.e. aortic pressures of 120/80 mmHg with 5 L/min of cardiac output, and allowed for intracardiac endoscopy. A pilot study with a left ventricular assist device (LVAD) was also performed. The LVAD was connected to the heart to investigate aortic valve functioning at different levels of support. Results were consistent with the literature, and high speed video recordings of the aortic valve allowed for the visualization of the transition between a fully opening valve and a permanently closed configuration. In conclusion, the system showed to be an effective tool for the hemodynamic assessment of devices, the simulation of surgical or transcatheter procedures and for visualization studies., (Copyright © 2015 IPEM. Published by Elsevier Ltd. All rights reserved.)

Despite the availability of multiple ultrasound approaches to left ventricular (LV) flow characterization in two dimensions, this technique remains in its childhood and further developments seem warranted. This article describes a new methodology for tracking the 2-D LV flow field based on ultrasound data. Hereto, a standard speckle tracking algorithm was modified by using a dynamic kernel embedding Navier-Stokes-based regularization in an iterative manner. The performance of the proposed approach was first quantified in synthetic ultrasound data based on a computational fluid dynamics model of LV flow. Next, an experimental flow phantom setup mimicking the normal human heart was used for experimental validation by employing simultaneous optical particle image velocimetry as a standard reference technique. Finally, the applicability of the approach was tested in a clinical setting. On the basis of the simulated data, pointwise evaluation of the estimated velocity vectors correlated well (mean r = 0.84) with the computational fluid dynamics measurement. During the filling period of the left ventricle, the properties of the main vortex obtained from the proposed method were also measured, and their correlations with the reference measurement were also calculated (radius, r = 0.96; circulation, r = 0.85; weighted center, r = 0.81). In vitro results at 60 bpm during one cardiac cycle confirmed that the algorithm properly measures typical characteristics of the vortex (radius, r = 0.60; circulation, r = 0.81; weighted center, r = 0.92). Preliminary qualitative results on clinical data revealed physiologic flow fields., (Copyright © 2015 World Federation for Ultrasound in Medicine & Biology. Published by Elsevier Inc. All rights reserved.)

The aim of this study was to develop an integrated mock circulation system that functions in a physiological manner for testing cardiovascular devices under well-controlled circumstances. In contrast to previously reported mock loops, the model includes a systemic, pulmonary, and coronary circulation, an elaborate heart contraction model, and a realistic heart rate control model. The behavior of the presented system was tested in response to changes in left ventricular contractile states, loading conditions, and heart rate. For validation purposes, generated hemodynamic parameters and responses were compared to literature. The model was implemented in a servo-motor driven mock loop, together with a relatively simple lead-lag controller. The pressure and flow signals measured closely mimicked human pressure under both physiological and pathological conditions. In addition, the system's response to changes in preload, afterload, and heart rate indicate a proper implementation of the incorporated feedback mechanisms (frequency and cardiac function control). Therefore, the presented mock circulation allows for generic in vitro testing of cardiovascular devices under well-controlled circumstances.

We consider the problem of estimating the stiffness of an artery wall using a data assimilation method applied to a 3D fluid-structure interaction (FSI) model. Recalling previous works, we briefly present the FSI model, the data assimilation procedure and the segmentation algorithm. We present then two examples of the procedure using real data. First, we estimate the stiffness distribution of a silicon rubber tube from image data. Second, we present the estimation of aortic wall stiffness from real clinical data., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Objectives: Aortic valve regurgitation, fusion and thrombosis are commonly reported clinical complications after continuous flow ventricular assist device implantations; however, the complex interaction between reduced pulsatile flow physiology and aortic valve functions has not been studied experimentally. To address this, a continuous flow left ventricular assist device was implanted in four swine ex vivo beating hearts and then operated at baseline (device off, no flow) and at device speeds ranging between 8500 and 11,500 rpm under healthy and experimentally created failing heart conditions., Methods: At baseline and after each speed increase, aortic, left ventricular, left atrial and pulse pressure signals were monitored to assess the haemodynamic status of the ex vivo heart, aortic valve opening time and the transvalvular pressure changes. Aortic root and device flows were recorded with flow probes. Left ventricular pressure-volume loops were measured with a conductance catheter. Changes in aortic leaflet motion and end-diastolic aortic root diameter were recorded with epicardial echocardiography., Results: A two-chamber healthy and failing ex vivo beating heart model was successfully created. At increasing device flows, aortic valve open time steadily decreased from 36±7% of the baseline cardiac cycle to 0% at 11,500 rpm in the healthy heart and from 18±16 to 0% in failing heart mode (P<0.05). Aortic transvalvular pressure increased from 25±5 mmHg (baseline) to 67±7 mmHg (11,500 rpm) in the healthy heart and from 10±9 mmHg (baseline) to 73±8 mmHg (11,500 rpm) in failing heart mode (P<0.05). Aortic root diameters were significantly increased at speeds exceeding 10 500 rpm in the healthy heart mode (P<0.05 vs baseline) and approached statistical significance in failing hearts., Conclusions: Increasing assist device flows resulted in pressure overload above the aortic leaflets, impaired leaflet functions, caused aortic root dilatation and altered leaflet coaptation at the central portion of the aortic valve in both modes. We conclude that the deleterious effect of the reduced pulsatile flow on the aortic valve functions and haemodynamics is immediate and such an insult may explain the structural changes of the aortic valve causing leaflet fusion and/or regurgitation in the chronic phase.

In heart valve tissue engineering, most bioreactors try to mimic physiological flow and operate with a preset transvalvular pressure applied to the tissue. The induced deformations are unknown and can vary during culturing as a consequence of changing mechanical properties of the engineered construct. Real-time measurement and control of local tissue strains are desired to systematically study the effects of mechanical loading on tissue development and, consequently, to design an optimal conditioning protocol. In this study, a method is presented to assess local tissue strains in heart valve leaflets during culturing. We hypothesize that local tissue strains can be determined from volumetric deformation. Volumetric deformation is defined as the amount of fluid displaced by the deformed heart valve leaflets in a stented configuration, and is measured, non-invasively, using a flow sensor. A numerical model is employed to relate volumetric deformation to local tissue strains in various regions of the leaflets (e.g. belly and commissures). The flow-based deformation measurement method was validated and its functionality was demonstrated in a tissue engineering experiment. Tri-leaflet, stented heart valves were cultured in vitro and during mechanical conditioning, realistic values for volumetric and local deformation were obtained.

The outcome of endovascular repair of abdominal aortic aneurysms (AAAs) is greatly compromised by the possible occurrence of endoleak. Previously, the causes and effects of endoleak on a patient-specific basis have mainly been investigated in experimental studies. In order to both reconcile and physically substantiate the various experimental findings, a lumped parameter model of an incompletely excluded AAA was developed. After experimental validation, the model was applied to study the effects on the intrasac pressure of the degree of endoleak, the degree of stent-graft compliance, and the resistance of a possible outflow tract formed by a branching vessel. It is concluded that the presence of endoleak leads to elevated intrasac pressure, the mean of which is mainly governed by the outflow tract resistance, while the pulse pressure is governed by both the endoleak resistance and the stent-graft compliance. Based on the agreement of the current results with previous findings, it is further concluded that the lumped parameter modelling method provides a useful numerical tool for validating experimental endoleak studies.