Purpose—Mitral regurgitation (MR) is the second most common valve disease in industrialized countries. Despite its high prevalence, little is known about its impact on the flow dynamics in the left ventricle (LV). Because of the interdependence between valvular function and hemodynamics in the heart chambers, an exploration of the dynamics in the LV could lead to a diagnosis of MR. This in vitro study aimed to develop an advanced left heart simulator capable of reproducing several conditions of MR and to evaluate their impact on the LV flow dynamics in terms of flow structures and viscous energy dissipation (VED). Methods—A simulator, previously developed to test mechanical and biological valves, was upgraded with an original anatomically-shaped mitral valve made from a hydrogel. The valve can be used in healthy or pathological configurations. The nature and severity of the disease was controlled by applying specific strain to the chordae. In this study, in addition to a healthy condition, two different severities of MR were investigated: moderate MR and severe MR. Planar time-resolved particle image velocimetry measurements were performed in order to evaluate the velocity field in the LV and the VED induced by each condition. Results—Our results showed that MR led to flow disturbances in the LV that were characterized by an increase in mitral inflow velocity and by elevated values of VED. Interestingly VED increased in proportion to the severity of MR and with a dissipation predominating during systole. Conclusion—Considering these results, the introduction of new parameters based on LV VED could provide crucial information regarding the coupling between the mitral valve and the LV and allow for better stratification of patients with MR.

A high-power Y-junction circulator is proposed for radar applications in the Ku-band (12–18 GHz). The design consists of two identical circulators combined using power splitters at all ports. The overall circulator network achieves 20-dB isolation level and return loss over the Ku-band with a peak power level of 14 kW. An internally matched divider is used at one port of the overall system to eliminate the cavity resonances produced by the practical asymmetries between the two circulators. The asymmetry problem is analyzed in detail to understand the consequences of these practical variations fully. Experimental validation is done by a fabricated prototype and shows a good agreement between the simulated and the measured results with differences in the range of ±2 dB.

Despite the availability of in vivo, instantaneous, and three-dimensional intracardiac flow data, their clinical analysis from a Lagrangian perspective remains limited due to their high computational cost. As an example, identifying Lagrangian coherent structures (LCS) in cardiac flows is not routinely performed in clinical settings despite their ability to identify mixing and stagnation regions along with locations of elevated shear stresses. Here, we explore a recently developed approach, “Lagrangian descriptors”, which quantifies the finite time Euclidean arc-length of Lagrangian trajectories released from a grid of initial positions. Through the evaluated arc-lengths of a set of trajectories, signatures of the LCS (computed from the same initial condition) are captured. Notably, the Lagrangian descriptor approach extracts the LCS within the flow at least five times faster than the common geometrical approach (i.e., using finite-time Lyapunov exponents). In this work, we apply, for the first time, the Lagrangian descriptors approach to in vivo 4D-MRI velocity fields inside left ventricles. The results show the ability of this approach to rapidly reveal the LCS within the left ventricle and how their organization can be altered under healthy and pathological conditions.

In cardiovascular flows, blood transport has been revealed using the Lagrangian coherent structures (LCS). Through a geometric approach, the extracted LCS highlight mixing, stagnation, and elevated shear stress regions. In geophysical flows, graph-theoretic approaches are used to represent fluid transport as a complex flow network. By using classical graph measures, one can extract the LCS along with direct measures of local dispersion and mixing. Also, from the adjacency matrix of the flow network, we can identify the coherent sets in the flow where the fluid within each set is minimally mixed with that in other sets. Notably, the geometric approach can only detect the borders of such regions. This study shows the first application of complex networks analysis to instantaneous planar velocity fields (acquired via PIV) downstream different arrangements of healthy and repaired mitral valves. Using the in- and out-degrees of the transport matrix, the instantaneous local mixing and dispersion are highlighted. Moreover, the LCS are revealed by computing the discrete finite time entropy of the network. Finally, the flow transport matrix is partitioned using a fuzzy c-means clustering algorithm to reveal the flow coherent sets which can better reveal fluid transport mechanisms.    

Abstract

Introduction

Study aims were to compare hemodynamics and viscous energy dissipation (VED) in 3D printed mitral valves–one replicating a normal valve and the other a valve with severe mitral annular calcification (MAC). Patients with severe MAC develop transmitral gradients, without the commissural fusion typifying rheumatic mitral stenosis (MS), and may have symptoms similar to classical MS. A proposed mechanism relates to VED due to disturbed blood flow through the diseased valve into the ventricle.

Methods

A silicone model of a normal mitral valve (MV) was created using a transesophageal echocardiography dataset. 3D printed calcium phantoms were incorporated into a second valve model to replicate severe MAC. The synthetic MVs were tested in a left heart duplicator under rest and exercise conditions. Fine particles were suspended in a water/glycerol blood analogue for particle image velocimetry calculation of VED.

Results

Catheter mean transmitral gradients were slightly higher in the MAC valve compared to the normal MV, both at rest (3.2 vs. 1.3 mm Hg) and with exercise (5.9 vs. 5.0 mm Hg); Doppler gradients were 2.7 vs. 2.1 mm Hg at rest and 9.9 vs 8.2 mm Hg with exercise. VED was similar between the two valves at rest. During exercise, VED increased to a greater extent for the MAC valve (240%) versus the normal valve (127%).

Conclusion

MAC MS is associated with slightly increased transmitral gradients but markedly increased VED during exercise. These energy losses may contribute to the exercise intolerance and exertional dyspnea present in MAC patients.

Abstract

Purpose

Aortic valve replacement remains the only viable solution for symptomatic patients with severe aortic valve stenosis. Despite their improved design and long history of successful operation, bileaflet mechanical heart valves are still associated with post-operative complications leading to valve dysfunction. Thus, the flow dynamics can be highly disturbed downstream of the dysfunctional valve.

Methods

In this in vitro study, the flow dynamics downstream of healthy and dysfunctional bileaflet mechanical heart valves have been investigated using particle image velocimetry measurements. Proper orthogonal decomposition of the velocity field has been performed in order to explore the coherent flow features in the ascending aorta in the presence of a dysfunctional bileaflet mechanical heart valve.

Results

The ability of proper orthogonal decomposition derived metrics to differentiate between heathy and dysfunctional cases is reported. Moreover, reduced-order modeling using proper orthogonal decomposition is thoroughly investigated not only for the velocity field but also for higher order flow characteristics such as time average wall shear stress, oscillatory shear index and viscous energy dissipation.

Conclusion

Considering these results, proper orthogonal decomposition can provide a rapid binary classifier to evaluate if the bileaflet mechanical valve deviates from its normal operating conditions. Moreover, the study shows that the size of the reduced-order model depends on which flow parameter is required to be reconstructed.

In cardiovascular flows, Lagrangian coherent structures have been used to explore the skeleton of blood transport. Revealing these transport barriers is instrumental to quantify the mixing and stagnation of blood as well as to highlight locations of elevated strain rate on blood elements. Nevertheless, the clinical use of Lagrangian coherent structures in cardiovascular flows is rarely reported due largely to its nonintuitive nature and computational expense. Here, we explore a recently developed approach called “Lagrangian descriptors,” which quantifies the finite time Euclidean arc length of Lagrangian trajectories released from a grid of initial positions. Moreover, the finite time arc lengths of a set of trajectories capture signatures of Lagrangian coherent structures computed from the same initial condition. Remarkably, the Lagrangian descriptors approach has the most rapid computational performance among all its Lagrangian counterparts. In this work, we explore the application of Lagrangian descriptors for the first time in cardiovascular flows. For this purpose, we consider two in vitro flow models studied previously by our group: flow in an abdominal aortic aneurysm and that in a healthy left ventricle. In particular, we will demonstrate the ability of the Lagrangian descriptors approach to reveal Lagrangian coherent structures computed via the classical geometrical approach, though at a significantly reduced computational cost.

Abstract

Purpose

Identification of coherent structures in cardiovascular flows is crucial to describe the transport and mixing of blood. Coherent structures can highlight locations where minimal blood mixing takes place, thus, potential thrombus formation can be expected thither. Graph-based approaches have recently been introduced in order to describe fluid transport and mixing between multiple Lagrangian trajectories, where each trajectory serves as a node that can be connected to another trajectory based on their relative distance during the course of time.

Methods

In this study, we compute the Lagrangian trajectories from in vitro planar instantaneous velocity fields in two models of abdominal aortic aneurysms, (AAA) namely single bulge and bi-lobed. Then, we construct unweighted and undirected graphs based on the pairwise distance of Lagrangian trajectories. We report local measures of the graph namely the degree and the clustering coefficient. We also perform spectral clustering of the graph Laplacian to extract the flow coherent sets.

Results

Local graph measures reveal fluid regions of high mixing such as vortex boundaries. Through spectral clustering, the fluid is partitioned into a reduced number of coherent sets where within each set, inner mixing of fluid is maximized while the fluid mixing between different coherent sets is minimized. The approach reveals multiple coherent sets adjacent to the AAA bulge that have sustained this adjacency to the wall through their coherent motion during one cardiac cycle.

Conclusion

Identifying coherent sets enables tracking their transport during the cardiac cycle and identify their role in the flow dynamics. Moreover, the size and the transport of the long-residing coherent sets inside the AAA bulges can be deduced which may aid in predicting thrombus formation at such a location.

Voltage source Multilevel Inverters (MLIs) are vital components for medium voltage and high-power applications due to their advantages like modularity and better power quality. However, the number of components used is significant. In this paper, an improved asymmetrical multilevel inverter topology is proposed producing 17-levels output voltage utilizing two dc sources. The circuit is developed to reduce the number of isolated dc-sources used without reducing output levels. The circuit utilizes six two-quadrant switches, three four-quadrant switches and four capacitors. The capacitors are self-balancing and do not require extra attention, i.e. the control system is simple for the proposed MLI. Detailed analysis of the topology under linear and non-linear loading conditions is carried out. Comparison with other similar topologies shows that the proposed topology is superior in device count, power quality, Total Standing Voltage (TSV), and cost factor. The performance of the topology is validated for different load conditions through MATLAB/Simulink environment and the prototype developed in the laboratory. Furthermore, thermal analysis of the circuit is done, and the losses are calculated via PLECS software. The topology offers a total harmonic distortion (THD) of 4.79% in the output voltage, with all the lower order harmonics being less than 5% complying with the IEEE standards.

In this work, we propose a high-quality control solution for islanded microgrids with multiparallel power converters; it uses a full state-variable direct model predictive control (FSV-DMPC) and has a simple structure. Unlike the conventional cascaded control loops, the proposed FSV-DMPC solution tracks the optimal reference generated by a robust droop loop using a unified cost function. This proposal enables the FSV-DMPC to be inserted into the entire control framework with plug-and-play capability; it is robust to parameter variations while also guaranteeing dynamics and stability. We conduct a deep analysis of the proposed approach, taking into account both the characteristics of the solution and the bounded stability of the system. Through comprehensive comparative studies with a classical double-loop linear controller, we validate that our solution achieves superior output voltage regulation during the load transients in terms of voltage error and settling time. Meanwhile, similar steady-state performances are accomplished for both methods. Finally, we verify our approach experimentally in different scenarios through a lab-constructed microgrid test bench. Experimental data confirm that the proposed approach achieves excellent steady-state and transient performances and obtains accurate load sharing.