Image-Based Computational Fluid Dynamics to Compare Two Repair Techniques for Mitral Valve Prolapse.

OBJECTIVE : The treatment of mitral valve prolapse involves two distinct repair techniques: chordal replacement (Neochordae technique) and leaflet resection (Resection technique). However, there is still a debate in the literature about which is the optimal one. In this context, we performed an image-based computational fluid dynamic study to evaluate blood dynamics in the two surgical techniques. METHODS : We considered a healthy subject (H) and two patients (N and R) who underwent surgery for prolapse of the posterior leaflet and were operated with the Neochordae and Resection technique, respectively. Computational Fluid Dynamics (CFD) was employed with prescribed motion of the entire left heart coming from cine-MRI images, with a Large Eddy Simulation model to describe the transition to turbulence and a resistive method for managing valve dynamics. We created three different virtual scenarios where the operated mitral valves were inserted in the same left heart geometry of the healthy subject to study the differences attributed only to the two techniques. RESULTS : We compared the three scenarios by quantitatively analyzing ventricular velocity patterns and pressures, transition to turbulence, and the ventricle ability to prevent thrombi formation. From these results, we found that the operative techniques affected the ventricular blood dynamics in different ways, with variations attributed to the reduced mobility of the Resection posterior leaflet. Specifically, the Resection technique resulted in turbulent forces, related with the risk of hemolysis formation, up to 640 Pa, while the other two scenarios exhibited a maximum of 240 Pa. Moreover, in correspondence of the ventricular apex, the Resection technique reduced the areas with low velocity to 15%, whereas the healthy case and the Neochordae case maintained these areas at 30 and 48%, respectively. Our findings suggest that the Neochordae technique developed a more physiological flow with respect to the Resection technique. CONCLUSION: Resection technique gives rise to a different direction of the mitral jet during diastole increasing the ability to washout the ventricular apex preventing from thrombi formation, but at the same time it promotes turbulence formation that is associated with ventricular effort and risk of hemolysis.

Computational fluid-structure interaction analysis of the end-to-side radio-cephalic arteriovenous fistula.

BACKGROUND AND OBJECTIVE: In the current work, we present a descriptive fluid-structure interaction computational study of the end-to-side radio-cephalic arteriovenous fistula. This allows us to account for the different thicknesses and elastic properties of the radial artery and cephalic vein. METHODS: The core of the work consists in simulating different arteriovenous fistula configurations obtained by virtually varying the anastomosis angle, i.e. the angle between the end of the cephalic vein and the side of the radial artery. Since the aim of the work is to understand the blood dynamics in the very first days after the surgical intervention, the radial artery is considered stiffer and thicker than the cephalic vein. RESULTS: Our results demonstrate that both the diameter of the cephalic vein and the anastomosis angle play a crucial role to obtain a blood dynamics without re-circulation regions that could prevent fistula failure. CONCLUSIONS: When an anastomosis angle close to the perpendicular direction with respect to the radial artery is combined with a large diameter of the cephalic vein, the recirculation regions and the low Wall Shear Stress (WSS) zones are reduced. Conversely, from a structural point of view, a low anastomosis angle with a large diameter of the cephalic vein reduces the mechanical stress acting on the vessel walls.

Fluid structure interaction analysis to stratify the behavior of different atheromatous carotid plaques.

BACKGROUND: In asymptomatic carotid stenosis (ACS), different plaque types, i.e. lipidic (LP), fibrous (FP), and calcific (CP), could have different hemodynamic and structural behaviors. METHODS: Different carotid plaques, reconstructed from medical imaging of ACS >70%, were analyzed by computing fluid structure interaction (FSI), modeling the spatial distribution of wall shear stresses (WSS), plaque displacements (D), von Mises stresses (VMS), and absorbed elastic energy (AEE) together with their maximum-in-space values at the systole (WSSsyst, Dsyst, VMSsyst and AEEsyst). RESULTS: WSS resulted significantly higher in CP, whereas D and VMS showed the highest values for LP. Regarding AEEsyst stored by the plaques, LP absorbed in average 2320 J/m3, FP 408 J/m3 (470%) and CP 99 J/m3 (2240%), (P<0.01, P<0.01, and P<0.01, respectively). CONCLUSIONS: Depending upon their nature, plaques store different deformations and inner distributions of forces, thus potentially influencing their vulnerability.

Hemodynamics and remodeling of the portal confluence in patients with malignancies of the pancreatic head: a pilot study towards planned and circumferential vein resections.

BACKGROUND: We designed a retrospective computational study to evaluate the effects of hemodynamics on portal confluence remodeling in real models of patients with malignancies of the pancreatic head. METHODS: Patient-specific models were created according to computed tomography data. Fluid dynamics was simulated by using finite-element methods. Computational results were compared to morphological findings. RESULTS: Five patients underwent total pancreatectomy, one had duodenopancreatectomy. Vein resection was performed en-bloc with the specimen. Histopathological findings showed that in patients without a vein stenosis and a normal hemodynamics, the three-layered wall of the vein was preserved. In patients with a stenosis > 70% of vein diameter and modified hemodynamics, the three-layered structure of the resected vein was replaced by a dense inflammatory infiltrate in absence of tumor infiltration. CONCLUSIONS: The portal confluence involved by malignancies of the pancreatic head undergoes a remodeling that is not mainly due to a wall infiltration by the tumor but instead to a persistent pathological hemodynamics that disrupts the balance between eutrophic remodeling and degradative process of the vein wall that can lead to the complete upheaval of the three-layered vein wall. This finding can have useful surgical application in planning resection of the vein involved by tumor growth.

Integration of activation maps of epicardial veins in computational cardiac electrophysiology.

In this work we address the issue of validating the monodomain equation used in combination with the Bueno-Orovio ionic model for the prediction of the activation times in cardiac electro-physiology of the left ventricle. To this aim, we consider four patients who suffered from Left Bundle Branch Block (LBBB). We use activation maps performed at the septum as input data for the model and maps at the epicardial veins for the validation. In particular, a first set (half) of the latter are used to estimate the conductivities of the patient and a second set (the remaining half) to compute the errors of the numerical simulations. We find an excellent agreement between measures and numerical results. Our validated computational tool could be used to accurately predict activation times at the epicardial veins with a short mapping, i.e. by using only a part (the most proximal) of the standard acquisition points, thus reducing the invasive procedure and exposure to radiation.

Image-Based Displacements Analysis and Computational Blood Dynamics after Endovascular Aneurysm Repair.

BACKGROUND: To examine intraheartbeat displacements (IHD) and geometrical changes of endografts for abdominal aortic aneurysm repair over the course of years, defined as follow-up displacements (FUD), and to correlate them with computational fluid dynamics (CFD). Despite the widespread use of endovascular aneurysm repair (EVAR), we still know little about endograft behavior after deployment. METHODS: Two cases, treated with either expanded polytetrafluoroethylene on a nitinol stent frame (PI) or with woven polyester fabric sutured to a stainless-steel Z stent skeleton (PII), were submitted to dynamic computed tomography angiography at 1, 12, and 60 months after implantation. After segmentation, IHD were computed as displacements of the reconstructed surface with respect to the diastolic instant. Similarly, FUD were studied using imaging techniques that align temporal successive segmentations. In addition, numerical simulations for blood dynamics were performed to compute viscous forces, specifically wall shear stress and time-averaged wall shear stress (TAWSS). RESULTS: IHD analysis showed slight translations without deformation for the PI endograft with respect to the stiffer stainless-steel endograft behavior of PII. FUD showed in PI motion of the metallic struts mainly focused on the distal main body of the endograft and in the zone overlapping with iliac branches. In PII, we observed a huge FUD in the middle and inferior-anterior regions of the main body. CFD analysis revealed changes of velocity patterns associated with remodeling of the iliac zone for PI and of the main body region for PII, where flow impinges the lumen wall and progressively induces deformation of the endograft wires. Measurement of TAWSS demonstrated flow disturbances in the enlarged region correlated with displacement analysis. CONCLUSIONS: Image-based displacement analysis associated with CFD enabled very subtle evaluations of endograft behavior on different temporal scales. This kind of study could be helpful both for physicians, forecasting evolution during the life span of the endograft, and manufacturers, giving them useful information about endograft implant performance and design.

Computational Analysis of Turbulent Hemodynamics in Radiocephalic Arteriovenous Fistulas to Determine the Best Anastomotic Angles.

BACKGROUND: Hemodynamics has been known to play a major role in the development of intimal hyperplasia leading to arteriovenous fistula failure. The goal of our study is to investigate the influence of different angles of side-to-end radiocephalic anastomosis on the hemodynamic parameters that promote intimal dysfunction and therefore intimal hyperplasia. METHODS: Realistic three-dimensional meshes were reconstructed using ultrasound measurements from distal side-to-end radiocephalic fistulas. The velocity at the proximal and distal radial inflows and at specific locations along the anastomosis and cephalic vein was measured through duplex ultrasound performed by a single examiner. A computational parametric study, virtually changing the inner angle of anastomosis, was performed. For this purpose, we used advanced computational models that include suitable tools to capture the pulsatile and turbulent nature of the blood flow found in arteriovenous fistulas. The results were analyzed in terms of velocity fields, wall shear stress distribution, and oscillatory shear index. RESULTS: Results show that the regions with high oscillatory shear index, which are more prone to the development of hyperplasia, are greater and progressively shift toward the anastomosis area and the proximal vein segment with the decrease of the inner angle of anastomosis. These results are specific to distal radiocephalic fistulas because they are subject to proximal and distal radial inflow. CONCLUSIONS: The results of this study show that inner anastomosis angles approaching 60-70° seem to yield the best hemodynamic conditions for maturation and long-term patency of distal radiocephalic fistulas. Inner angles greater than 90°, representing the smooth loop technique, did not show a clear hemodynamic advantage.

Prediction of Long Term Restenosis Risk After Surgery in the Carotid Bifurcation by Hemodynamic and Geometric Analysis.

This study explored the potential of hemodynamic disturbances and geometric features to predict long-term carotid restenosis after carotid endarterectomy (CEA). Thirteen CEA for carotid diameter stenosis > 70% were performed with patch graft (PG) angioplasty in nine cases, and primary closure (PC) in four cases. MRI acquisitions within one month after CEA were used for hemodynamic and geometric characterization. Personalized computational hemodynamic simulations quantified the exposure to low and oscillatory wall shear stress (WSS). Geometry was characterized in terms of flare (the expansion at the bulb) and tortuosity. At 60 months after CEA, Doppler ultrasound (DUS) was applied for restenosis detection and intima-media thickness determination. Larger flares were associated to larger exposure to low WSS (Pearson R2 values up to 0.38, P < 0.05). The two cases characterized by the highest flare and the largest low WSS exposure developed restenosis > 50% at 60 months. Linear regressions revealed associations of DUS observations of thickening with flare variables (up to R2 = 0.84, P < 0.001), and the exposure to low (but not oscillatory) WSS (R2 = 0.58, P < 0.05). Our findings suggest that arteriotomy repair should avoid a large widening of the carotid bulb, which is linked to restenosis via the generation of flow disturbances. Hemodynamics and geometry-based analyses hold potential for (1) preoperative planning, guiding the PG vs. PC clinical decision, and (2) stratifying long-term restenosis risk after CEA.

Computational fluid dynamic comparison between patch-based and primary closure techniques after carotid endarterectomy.

OBJECTIVE: The aim of the study was to provide, by means of computational fluid dynamics, a comparative analysis after carotid endarterectomy (CEA) between patch graft (PG) and primary closure (PC) techniques performed in real carotid geometries to identify disturbed flow conditions potentially involved in the development of restenosis. METHODS: Eight carotid geometries in seven asymptomatic patients who underwent CEA were analyzed. In six cases (A-F), CEA was performed using PG closure; in two cases (G and H), PC was performed. Three-dimensional carotid geometries, derived from postoperative magnetic resonance angiography, were reconstructed, and a computational fluid dynamics analysis was performed. A virtual scenario with PC closure was designed in patients in whom PG was originally inserted and vice versa. This allowed us to compare for each patient hemodynamic effects in the PG and PC scenarios in terms of oscillatory shear index (OSI) and relative residence time (RRT), considered indicators of disturbed flow. RESULTS: For the six original PG cases, the mean averaged-in-space OSI was 0.07 ± 0.01 for PG and 0.03 ± 0.02 for virtual-PC (difference, 0.04 ± 0.01; P = .0016). The mean of the percentage of area (%A) with OSI >0.2 resulted in 10.08% ± 3.38% for PG and 3.80% ± 3.22% for virtual-PC (difference, 6.28 ± 1.91; P = .008). For the same cases, the mean of the averaged-in-space RRT resulted in 5.48 ± 3.40 1/Pa for PG and 2.62 ± 1.12 1/Pa for virtual-PC (difference, 2.87 ± 1.46; P = .097). The mean of %A RRT >4.0 1/Pa resulted in 26.53% ± 12.98% for PG and 9.95% ± 6.53% for virtual-PC (difference, 16.58 ± 5.93; P = .025). For the two original PC cases, the averaged-in-space OSIs were 0.02 and 0.04 for PC and 0.03 and 0.02 for virtual-PG; the %A OSIs >0.2 were 0.9% and 7.6% for PC and 3.0% and 2.2% for virtual-PG; the averaged-in-space RRTs were 1.8 and 2.0 1/Pa for PC and 2.9 and 1.9 1/Pa for virtual-PG; the %A RRTs >4.0 1/Pa were 6.8% and 9.8% for PC and 9.4% and 6.2% for virtual-PG. These results revealed generally higher disturbed flows in the PG configurations with respect to the PC ones. CONCLUSIONS: OSI and RRT values were generally higher in PG cases with respect to PC, especially for high carotids or when the arteriotomy is mainly at the bulb region. Thus, an elective use of patch should be considered to prevent disturbed flows.

A computational fluid-structure interaction analysis of coronary Y-grafts.

Coronary artery disease is one of the leading causes of death worldwide. The stenotic coronary vessels are generally treated with coronary artery bypass grafts (CABGs), which can be either arterial (internal mammary artery, radial artery) or venous (saphenous vein). However, the different mechanical properties of the graft can influence the outcome of the procedure in terms of risk of restenosis and subsequent graft failure. In this paper, we perform a computational fluid-structure interaction (FSI) analysis of patient-specific multiple CABGs (Y-grafts) with the aim of better understanding the influence of the choice of bypass (arterial vs venous) on the risk of graft failure. Our results show that the use of a venous bypass results in a more disturbed flow field at the anastomosis and in higher stresses in the vessel wall with respect to the arterial one. This could explain the better long-term patency of the arterial bypasses experienced in the clinical practice.

Computational Fluid-Dynamic Analysis after Carotid Endarterectomy: Patch Graft versus Direct Suture Closure.

BACKGROUND: Closure technique after carotid endarterectomy (CEA) still remains an issue of debate. Routine use of patch graft (PG) has been advocated to reduce restenosis, stroke, and death, but its protective effect, particularly from late restenosis, is less evident and recent studies call into question this thesis. This study aims to compare PG and direct suture (DS) by means of computational fluid dynamics (CFD). To identify carotid regions with flow recirculation more prone to restenosis development, we analyzed time-averaged oscillatory shear index (OSI) and relative residence time (RRT), that are well-known indices correlated with plaque formation. METHODS: CFD was performed in 12 patients (13 carotids) who underwent surgery for stenosis >70%, 9 with PG, and 4 with DS. Flow conditions were modeled using patient-specific boundary conditions derived from Doppler ultrasound and geometries from magnetic resonance angiography. RESULTS: Mean value of the spatial averaged OSI resulted 0.07 for PG group and 0.03 for DS group, the percentage of area with OSI above a threshold of 0.2 resulted 10.1% and 3.7%, respectively. The mean of averaged-in-space RRT values was 4.4 1/Pa for PG group and 1.6 1/Pa for DS group, the percentage of area with RRT values above a threshold of 4 1/Pa resulted 22.5% and 6.5%, respectively. CONCLUSIONS: Both OSI and RRT values resulted higher when PG was preferred to DS and also areas with disturbed flow resulted wider. The absolute higher values computed by means of CFD were observed when PG was used indiscriminately regardless of carotid diameters. DS does not seem to create negative hemodynamic conditions with potential adverse effects on long-term outcomes, in particular when CEA is performed at the common carotid artery and/or the bulb or when ICA diameter is greater than 5.0 mm.

Computational study of the risk of restenosis in coronary bypasses.

Coronary artery disease, caused by the buildup of atherosclerotic plaques in the coronary vessel wall, is one of the leading causes of death in the world. For high-risk patients, coronary artery bypass graft is the preferred treatment. Despite overall excellent patency rates, bypasses may fail due to restenosis. In this context, the purpose of this work was to perform a parametric computational study of the fluid dynamics in patient-specific geometries with the aim of investigating a possible relationship between coronary stenosis degree and risk of graft failure. Firstly, we propose a strategy to prescribe realistic boundary conditions in the absence of measured data, based on an extension of Murray's law to provide the flow division at bifurcations in case of stenotic vessels and non-Newtonian blood rheology. Then, we carry out numerical simulations in three patients affected by severe coronary stenosis and treated with a graft, in which the stenosis degree is virtually varied in order to compare the resulting fluid dynamics in terms of hemodynamic indices potentially involved in restenosis development. Our findings suggest that low degrees of coronary stenosis produce a more disturbed fluid dynamics in the graft, resulting in hemodynamic conditions that may promote a higher risk of graft failure.

Assessment of microalgae and nitrifiers activity in a consortium in a continuous operation and the effect of oxygen depletion

Background: Industrial wastewaters with a high content of nitrogen are a relevant environmental problem. Currently, treatments to remove nitrogen are not efficient, so is necessary to develop alternative methods. The objective of this study is to investigate a consortium of microalgae - nitrifying, that due to the symbiosis between them could be an interesting alternative. Results: In this study, it was possible to obtain a consortium of nitrifying bacteria (NB) and microalgae (MA) capable of operating with low requirements of dissolved oxygen, using aerobic sludge from wastewater treatment plants. During the operation, this consortium presents removal percentages above 98% of ammonia, even at concentrations of DO of 0.5 mg O2 L-1. It is estimated that the removal was caused both by the action of nitrifying bacteria and microalgae. It was determined that approximately 60% of the ammonia feed was oxidized to nitrate by nitrifying bacteria, while the algae assimilated 40% of the nitrogen feed at steady state. A methodology for measuring the specific activities of nitrifying bacteria and microalgae by comparing the rates in the variation inorganic nitrogen compounds was established with satisfactory results. An average specific activity of 0.05 and 0.02 g NH4 + gVSS-1 d-1 for nitrifying bacteria and microalgae was determined, respectively. Conclusions: The consortium it can be obtained in a single continuous operation, and has a high capacity for nitrogen removal with low oxygen content. The consortium could prove to be a more economical method compared to traditional.

Trends in biomedical engineering: focus on Patient Specific Modeling and Life Support Systems.

Over the last twenty years major advancements have taken place in the design of medical devices and personalized therapies. They have paralleled the impressive evolution of three-dimensional, non invasive, medical imaging techniques and have been continuously fuelled by increasing computing power and the emergence of novel and sophisticated software tools. This paper aims to showcase a number of major contributions to the advancements of modeling of surgical and interventional procedures and to the design of life support systems. The selected examples will span from pediatric cardiac surgery procedures to valve and ventricle repair techniques, from stent design and endovascular procedures to life support systems and innovative ventilation techniques.

Assisted Fontan procedure: animal and in vitro models and computational fluid dynamics study.

Fontan connection with intermittent compression by wrapped latissimus dorsi (LD) was tested in vivo, in vitro and by means of computational fluid dynamics (CFD). Experimental study: LD was conditioned in four pigs for three weeks before Fontan connection by valved conduit wrapped with LD. Mock circuit: Inflatable cuff wrapped around valved conduit provided intermittent external compression, with pressure and flow measured at driving pressure of 8 or 16 mmHg. CFD study: A circuit was tested for possible increase above basal flow (4 l/min) with intermittent external compression. Experimental study: Intermittent conduit compression by LD provided mean 7% decrease of baseline PA pressure, with simultaneous flow increase of 2%. Mock circuit: By raising the driving pressure from 8 to 16 mmHg, the flow increased with baseline PVR (56%) and with elevated PVR (80%). Total pulmonary flow was reduced during intermittent external compression with both baseline and elevated PVR. CFD study: Compression with 13.0 mmHg provided 4.9% increase of total pulmonary flow with substantial increase of the peak flow (92%). In vivo and in vitro, the increased flow produced by compressing a conduit was confounded by the inevitable intermittent flow restriction. Mathematical model using lower pressure for intermittent external compression showed potential for increase in pulmonary flow.

Effect of sulphate concentration and sulphide desorption on the combined removal of organic matter and sulphate from wastewaters using expanded granular sludge bed (EGSB) reactors

During the application of anaerobic processes to high sulphate concentration wastewaters, operational problems are expected due to the occurrence of sulphate reduction. Sulphide production reduces effluent quality and may produce inhibition. The application of Expanded Granular Sludge Bed (EGSB) reactors for the combined removal of organic matter and sulphate was studied at different COD/sulphate and 3 values of pH. During the EGSB reactor operation, most of the sulphide remains in the liquid phase reducing effluent quality. The inclusion of a desorption column in the recirculation of the EGSB reactor promotes mass transfer to the gas phase, reducing the sulphide concentration in the liquid phase, significantly decreasing the chemical oxygen demand of the effluent.