The widespread use of veno-arterial extracorporeal membrane oxygenation (VA-ECMO) in the last decade has encouraged the concept of various return cannulas aimed at ensuring bidirectional perfusion and avoiding limb ischemia. Yet, the local hemodynamics modification owing to bidirectional flow cannulas is poorly understood. Computational fluid dynamics simulations of two different bidirectional flow cannulas inserted within an idealized artery were performed. The cannula designs differ in elbow configuration. One design has a single hole (single-hole, SH cannula), while the other has four holes (multi-hole, MH cannula) at the elbow region. Simulations were run at two different perfusion flow rates, namely, 1.4 and 3.5 L/min, and compared with the hemodynamics of a standard one-directional cannula. Both cannulas achieved improved distal perfusion compared to the standard configuration, delivering between 14% and 23%, and 22.5% and 37% of the total ECMO flow through the SH and MH configurations, respectively. However, secondary circulations arose from the distal opening, resulting in a large stagnation region along the MH cannula body. Such a condition, which is recognized as a risk factor for thrombus formation, was less pronounced in the SH solution. The analysis shows that distal openings provide adequate limb perfusion, potentially reducing ischemia risk. The multi-opening solution results in more effective outcomes to some degree but may present stagnation if not properly sized, thus potentially compromising local hemodynamics.

BACKGROUND AND AIMS:The mature left ventricular myocardium is arranged in a complex three-dimensional network of fibers that form a counterclockwise helix in the endocardial layer and a clockwise helix in the epicardial layer. There are no data in the literature on the development of left ventricular myocardium during the fetal life. The aims of this paper were to study the physiological maturation steps of the LV myocardium in fetuses from 17 to 40 gestational weeks, by means of speckle tracking applied to the endocardial and epicardial aspect of the left ventricle, and, to confirm our finds, through the histologic study of the myocardium of demised fetuses. METHODS AND RESULTS:We studied longitudinal endocardial and epicardial strain by echocardiography in 105 fetuses. Twenty non-diseased fetal hearts from autopsies were selected to assess the layer thickness and cardiac fiber orientation in relation to gestational age. Echocardiography showed a progressive increasing of epicardial/endocardial longitudinal strain ratio with gestational age (r=0.51; p<0.0001). The strain rate E/A ratio increased over time (r=0.27; p=0.018). Histological data revealed that during the same gestational period, the proportion of the epicardial layer increased fourfold, the mesocardiac layer decreased and the endocardial layer remained stable. We found an excellent correlation between the epicardial to endocardial strain ratio and epicardial to endocardial wall thickness (r=0.950, p<0.001). CONCLUSIONS:Left ventricular myocardium maturation begins early during fetal life. As the fetus develops, both the relative tissue volume and peak systolic strain rates shift together from the endocardium towards the epicardium. It is a slow process, completed late in fetal life.

Topology optimization provides a rigorous framework for determining optimal flow-path designs in fluid mechanics. A popular technique is the density-based approach, which treats fluid–solid interfaces by considering the solid phase as a porous medium with low permeability. Mathematically, such a problem is governed by the Navier–Stokes equations combined with the Brinkman penalization. To consistently solve the optimization problem, this study examines the theoretical foundations of the method, focusing on dimensionless parameters such as the Reynolds number and a specific Darcy number, Da*. Through dimensional analysis, we derive scaling guidelines for the Brinkman penalization in relation to fluid properties and domain geometry. Numerical simulations show that incorrect scaling can yield nonphysical results, including excessive fluid penetration into solid regions, undermining the optimization. Our findings demonstrate that the invariance of the solution can be maintained by appropriately scaling Da* with the relative velocity in the porous region, ensuring accurate and reliable results in different scenarios. This work provides a systematic framework for parameter selection in fluid topology optimization, addressing key modeling and computational challenges. By emphasizing the importance of dimensional analysis, it contributes to a broader understanding of topology optimization, paving the way for its more robust and consistent application in fluid mechanics.

Mountain streams are hot spots for the exchange of gases such as oxygen or carbon dioxide with the atmosphere. Air-water gas exchange is accelerated by air bubbles entrained in turbulent flow and depends on bubble concentration and size. Yet, our understanding of gas exchange mechanisms and our ability to upscale gas fluxes is hampered by a severe lack of data on bubble size distributions in streams. Here, we measured bubble size distributions in 16 step-pool systems across six stream reaches in two mountain ranges (Dolomites, Italy; Vosges, France), combining Acoustic Bubble Spectrometry and ambient underwater sound recording. Bubble size distributions in our study streams were similar to those reported previously for ocean breaking waves: bubble concentrations decreased with bubble size following a power-law scaling, with a power exponent generally ranging from -2/3 to -10/3, increasing with bubble size, and varying both within and among step-pool systems. Total bubble concentrations exhibited a bilinear power law relationship with turbulent kinetic energy dissipation rates estimated from Acoustic Doppler Velocimetry, suggesting distinct bubble formation regimes under low and high turbulence. Bubble concentrations generally decreased with distance from steps, but invisible microbubbles still exhibited significant concentrations several meters downstream from steps where no visible macrobubbles were recorded. Our findings provide novel scaling laws for bubble size distributions, offering insights into the different gas exchange regimes observed in mountain streams. Additionally, they underscore the potential of bubble-mediated gas exchange even under absence of visible bubbles, highlighting the complexities involved in upscaling gas exchange mechanisms in such environments.

OBJECTIVE:Posterior systolic curling (PSC) is a morphological and functional abnormality of the posterior mitral valve annulus, known in the literature as a significant risk factor for the onset of malignant ventricular arrhythmias and sudden cardiac death. This study proposes a semi-automatic algorithm to acquire the Mitral valve annulus to Infero-basal wall Rotation Angle (MIRA) and an innovative semi-automatic echocardiographic parameter, called Mitral Annulus to Infero-Basal wall Angle (MAIBA) for diagnosing PSC. METHODS:Both algorithms leverage cardiac tissue tracking and biomedical image-processing techniques. They were tested on 100 patients (44 female, median age 58 years) and classified into PSC-positive and PSC-negative groups based on an initial eyeball estimation. All subjects underwent echocardiographic exams using parasternal long-axis view transthoracic echocardiography. RESULTS:The MIRA and MAIBA algorithms achieved Pearson correlation coefficients of 0.70 and 0.80, respectively, with respective percentage accuracies of 89% and 91%, respectively, supporting a strong correlation with manual measurements. Bland-Altman analyses confirmed agreement between semi-automatic and manual methods, and inter-observer reliability was excellent, with intra-class correlation coefficient values of 0.98 for MIRA and 0.97 for MAIBA. The optimal cutoff value for the MAIBA angle was set equal to 69° through receiver operator characteristic curve analysis and Youden's index. The diagnostic accuracy of PSC identification was 76% for MIRA and 85% for MAIBA, demonstrating robust potential for clinical application. CONCLUSION:In conclusion, statistical analyses underscore MIRA and MAIBA angles as promising diagnostic tools, potentially enhancing clinicians' ability to identify PSC with high reliability and accuracy, paving the way for improved diagnostic support in cases of arrhythmogenic risk.

The role of hemodynamics has often been overlooked in mathematical modeling aimed at replicating the restenosis process in stented arteries. This study seeks to address this gap by proposing a simplified model of tissue growth driven by the distribution of mean shear stress acting on the vessel wall. Using an iterative sequence of three-dimensional Computational Fluid Dynamics simulations applied to idealized coronary and femoral arteries, combined with a semi-empirical parametrization of endothelium growth, we demonstrated that the progression of restenosis can be effectively modeled and differentiated according to the intensity of time-varying flow velocities. Notably, restenosis develops faster in the femoral artery (approximately 17 days) compared to the coronary artery (approximately 25 days). The progress of tissue accretion is well defined by the evolution of time-averaged wall shear stress. After an initial decrease (triggering phase), significant increases in wall shear stress are observed during the main accretion phase until the shear stress eventually recovers a sufficient level to arrest the process (stabilization phase). This process, attributed to varying hemodynamic conditions within the stent, highlights the significant influence of local flow dynamics and emphasizes the necessity of accurately modeling both the anatomical structure and the corresponding hemodynamics of arteries when predicting in-stent restenosis.

Headwater streams are important sources of greenhouse gases to the atmosphere. The magnitude of gas emissions originating from such streams, however, is modulated by the characteristic microtopography of the riverbed, which might promote the spatial heterogeneity of turbulence and air entrainment. In particular, recent studies have revealed that step-and-pool configurations, usually found in close sequences along mountain streams, are important hotspots of gas evasion. Yet, the mechanisms that drive gas transfer at the water-air interface in a step-and-pool configuration are not fully understood. Here, we numerically simulated the hydrodynamics of an artificial step-and-pool configuration to evaluate the contribution of turbulence and air entrainment to the total gas evasion induced by the falling jet. The simulation was validated using observed hydraulic features (stage, velocity) and was then utilized to determine the patterns of energy dissipation, turbulence-induced gas exchange and bubble-mediated transport. The results show that gas evasion is led by bubble entrainment and is mostly concentrated in a small and irregular region of a few square decimeters near the cascade, where the local gas transfer velocity (k) peaks at 500 md-1. The enhanced spatial heterogeneity of k in the pool does not allow one to define a priori the region of the domain where the outgassing takes place and makes the value of the spatial mean of k inevitably scale-dependent. Accordingly, we propose that the average mass transfer velocity should be used with caution to describe the outgassing in spatially heterogeneous flow fields, such as those encountered in step-and-pool rivers.

The work proposes and discusses a theoretical approach to predict the behavior of an open-channel supercritical flow that overpasses a step, either forward or backward facing, non-orthogonal to the flow direction. In this case, a sequence of oblique shock waves and expansion fans is generated close to and downstream of the step. The proposed model is verified by comparing the theoretical predictions with the results provided by a two-dimensional, depth averaged numerical model. Applications include the combined use of oblique steps and abrupt wall deflections to suppress wave fronts that characterize supercritical flow in channel bends. Special attention is devoted to the supercritical to subcritical transition (and vice versa) in overpassing a forward-facing step; this is found to be a rather intriguing problem characterized by complex solutions and by hysteresis. Besides the classic smooth (everywhere supercritical) and choked (with a hydraulic jump and a subcritical flow upstream of the step) solutions, an additional intermediated flow configuration can occur for particular characteristics of the supercritical current and step height. The domain of existence of the different solutions, as well as the hysteresis domain, are obtained based on the theoretical and the numerical models.

Diffuser/nozzle pipes produce a directional flow resistance that is often exploited in microcirculation to generate a pumping action. This work presents an approximate time-dependent theoretical solution based on the mechanical energy conservation equation to predict the laminar flow rate through an ideal diffuser/nozzle pump. The theoretical solution is then used to characterize the dimensionless parameters that control the dynamics of the valveless pump in the pulsatile flow regime. A suitable numerical model is also implemented to solve the flow in a parametrized two-dimensional axial-symmetric domain subjected to an oscillating pressure, and its results are used to assess the theoretical solution. The pump dynamics and the main model parameters, such as the energy-loss coefficients, result in the following dependence on the ratios between the viscous force, the advective inertia, and the temporal inertia, i.e., the Reynolds (Red), Womersley (Wod), and Strouhal (St) numbers referred to throat diameter. In particular, The Womersley number plays an essential role in controlling the global energy loss when Red < 100. The flow transition is also investigated and found when Red exceeds a critical value, which increases with Wod. Finally, the pump efficiency is found to reach its maximum when the convective and temporal inertia become comparable, i.e., St=O(1), consistent with the observed range of St in real-world diffuser/nozzle pumps. This optimum range of functioning of the pump is also observed for cerebrospinal pulsatile flow in the Sylvius aqueduct, suggesting that the modeled mechanism is used to promote or enhance cerebrospinal fluid circulation.

Steps are dominant morphologic traits of high-energy streams, where climatically- and biogeochemically-relevant gases are processed, transported to downstream ecosystems or released into the atmosphere. Yet, capturing the imprint of the small-scale morphological complexity of channel forms on large-scale river outgassing represents a fundamental unresolved challenge. Here, we combine theoretical and experimental approaches to assess the contribution of localized steps to the gas evasion from river networks. The framework was applied to a representative, 1 km-long mountain reach in Italy, where carbon dioxide concentration drops across several steps and a reference segment without steps were measured under different hydrologic conditions. Our results indicate that local steps lead the reach-scale outgassing, especially for high and low discharges. These findings suggest that steps are key missing components of existing scaling laws used for the assessment of gas fluxes across water-air interfaces. Therefore, global evasion from rivers may differ substantially from previously reported estimates.

This work presents and discusses a series of experiments focusing on the transport of floating particles, mimicking seeds and propagules, within an array of randomly arranged cylinders mimicking emergent vegetation stems. The focus is on the temporary capture process by which particles colliding with a cylinder are trapped by surface tension for finite but relatively long retention times, thus promoting a large mechanical dispersion. Video analysis of the particle paths within the array shows that the probability of particles being captured, either temporarily or permanently, as well as the mean retention time, vary with flow velocity while being weakly affected by stem density. On the contrary, stem density plays a significant role in determining the frequency of the temporary captures; in particular, the probability of having temporary, rather than permanent, captures increases with vegetation density. We also propose some relationships to predict the probability of having temporary capture events and their mean duration based on experimental results.

NeoChord-DS1000-System (NC) and The Harpoon-Mitral-Repair-System (H-MRS) are two trans-apical chordal implantation devices developed for the treatment of degenerative mitral valve (MV) regurgitation (DMR) either if as Fibroelastic-Deficiency (FED), Forma-Frusta (FF), or Barlow (B) presentation. The aim of this study is to evaluate some of the advantages and disadvantages of these two different devices by performing numerical simulation analyses focused on different transventricular access sites in all subsets of DMR presentations. By applying a novel approach for the development of patient-specific MV domains we worked out a set of numerical simulations of the artificial chordae implantation. Different leaflet insertions and ventricle access sites were investigated, and resulting contact-area (CA), tensioning-forces (F) and leaflet's stress (LS) were calculated. The analyses showed that: i) NC-approach maintains low LS when performed with a posterior access site and optimizes the overlap between the leaflets at the systolic peak; ii) H-MRS-system presents better results in case of a more anterior ventricular entry site; however, for FED prolapse large variation of F and LS with respect to NC-approach are found; iii) an accidental contact between artificial sutures and the anterior leaflet may occur when valve function is restored through an excessive anterior access site. Present findings set light on specific technical aspects of transapical off-pump chords implantation, either performed with NC and H-MRS systems and highlight the advantages and disadvantages proper to the two devices. Our study also paves the basis for a systematic application of computational methodology, in order to plan a patient-specific mini-invasive approach thus maximizing the outcomes.

Headwater streams represent a key component of the global carbon cycle, as they are hotspots for the evasion of carbon dioxide from surface waters. Gas emissions from rivers and streams are modulated by the gas transfer velocity at the water-air interface, k, which is physically related to the energy dissipated by the flow field, ε. Here we study how local relations between gas transfer rate and energy dissipation can be spatially averaged in presence of heterogeneous flow fields, e.g. as induced by changes in the local slope. Furthermore, we develop mathematical tools for the quantification of the fraction of gas emission that is related to localized energy losses (e.g. sudden drops and step-pool formations). The study complements numerical simulations and direct measures of stream CO2 outgassing in an Italian headwater catchment. Our theoretical results indicate that reach-scale relations between k and ε in general differ from the corresponding local scaling laws. In particular, we show that high energy heterogeneous streams are characterized by a gas transfer velocity significantly higher than that of an equivalent homogeneous stream. The empirical data suggest that the outgassing is highly heterogeneous along a river network, with the outgassing generated by localized gas emissions in correspondence of hydraulic discontinuities that might be a dominant contribution to the total gas evasion in many settings. These results offer a clue for the interpretation of empirical data about stream outgassing in heterogeneous reaches and complex river networks.

The correct estimation of the distensibility of deformable aorta replicas is a challenging issue, in particular when its local characterization is necessary. We propose a combined in-vitro and in-silico approach to face this problem. First, we tested an aortic silicone arch in a pulse-duplicator analyzing its dynamics under physiological working conditions. The aortic flow rate and pressure were measured by a flow meter at the inlet and two probes placed along the arch, respectively. Video imaging analysis allowed us to estimate the outer diameter of the aorta in some sections in time. Second, we replicated the in-vitro experiment through a Fluid-Structure Interaction simulation. Observed and computed values of pressures and variations in aorta diameters, during the cardiac cycle, were compared. Results were considered satisfactory enough to suggest that the estimation of local distensibility from in-silico tests is reliable, thus overcoming intrinsic experimental limitations. The aortic distensibility (AD) is found to vary significantly along the phantom by ranging from 3.0 × 10−3 mmHg−1 in the ascending and descending tracts to 4.2 × 10−3 mmHg−1 in the middle of the aortic arch. Interestingly, the above values underestimate the AD obtained in preliminary tests carried out on straight cylindrical samples made with the same material of the present phantom. Hence, the current results suggest that AD should be directly evaluated on the replica rather than on the samples of the adopted material. Moreover, tests should be suitably designed to estimate the local rather than only the global distensibility.

Emergent vegetation has a significant impact on floating particle transport and diffusion in open channel flow. The random walk model of a Lagrangian approach has proven more than suitable to describe the rather unpredictable moving trajectory of particles within emergent vegetation. However, compared to the large computational costs of the Lagrangian model, which also requires more input data, a simplified model based on the Eulerian approach can be by far preferable and cost‐effective for rapid first‐order prediction of particle transport and diffusion within vegetated areas. In this study, we developed a one‐dimensional advection‐diffusion model to simulate particle transport processes within vegetated areas, and to explore the impacts of vegetation on particles transport, diffusion and removal. The model parameters, including the probability of a particle colliding with a stem, and the probability of it being temporarily trapped or permanently captured by a stem, as well as the mean retention time that govern the random walk model, are introduced to estimate the mean velocity, diffusion coefficient, removal rate of particles of Eulerian model, distinguishing from the standard advection and diffusion parameters. The validity of the parameters is verified through the stochastic model with a large number of realizations. The comparison between the dispersal kernel as well as the spatio‐temporal distribution of floating particles predicted by Eulerian model and stochastic model is quite satisfactory and suggests that the Eulerian model we proposed is properly described.

(1) Background: The realization of appropriate aortic replicas for in vitro experiments requires a suitable choice of both the material and geometry. The matching between the grade of details of the geometry and the mechanical response of the materials is an open issue that deserves attention. (2) Methods: To explore this issue, we performed a series of Fluid–Structure Interaction simulations, which compared the dynamics of three aortic models. Specifically, we reproduced a patient-specific geometry with a wall of biological tissue or silicone, and a parametric geometry based on in vivo data made in silicone. The biological tissue and the silicone were modeled with a fiber-oriented anisotropic and isotropic hyperelastic model, respectively. (3) Results: Clearly, both the aorta’s geometry and its constitutive material contribute to the determination of the aortic arch deformation; specifically, the parametric aorta exhibits a strain field similar to the patient-specific model with biological tissue. On the contrary, the local geometry affects the flow velocity distribution quite a lot, although it plays a minor role in the helicity along the arch. (4) Conclusions: The use of a patient-specific prototype in silicone does not a priori ensure a satisfactory reproducibility of the real aorta dynamics. Furthermore, the present simulations suggest that the realization of a simplified replica with the same compliance of the real aorta is able to mimic the overall behavior of the vessel.

Data used in the associated study entitled Retention of floating particles captured by emergent vegetation through capillarity.

The gas transfer velocity, k , modulates gas fluxes across air-water interfaces in rivers. While the theory postulates a local scaling law between k and the turbulent kinetic energy dissipation rate ε , empirical studies usually interpret this relation at the reach-scale. Here, we investigate how local k ( ε ) laws can be integrated along heterogeneous reaches exploiting a simple hydrodynamic model, which links stage and velocity to the local slope. The model is used to quantify the relative difference between the gas transfer velocity of a heterogeneous stream and that of an equivalent homogeneous system. We show that this aggregation bias depends on the exponent of the local scaling law, b , and internal slope variations. In high-energy streams, where b > 1 , spatial heterogeneity of ε significantly enhances reach-scale values of k as compared to homogeneous settings. We conclude that small-scale hydro-morphological traits bear a profound impact on gas evasion from inland waters.