Culture of 3D Bioprinted Bone Constructs Requires an Increased Fluid Dynamic Stimulation

In vitro flow-induced mechanical stimulation of developing bone tissue constructs has been shown to favor mineral deposition in scaffolds seeded with cells directly exposed to the fluid flow. However, the effect of fluid dynamic parameters, such as shear stress (SS), within 3D bioprinted constructs is still unclear. Thus, this study aimed at correlating the SS levels and the mineral deposition in 3D bioprinted constructs, evaluating the possible dampening effect of the hydrogel. Human mesenchymal stem cells (hMSCs) were embedded in 3D bioprinted porous structures made of alginate and gelatin. 3D bioprinted constructs were cultured in an osteogenic medium assessing the influence of different flow rates (0, 0.7 and 7 ml/min) on calcium and collagen deposition through histology, and bone volume (BV) through micro-computed tomography. Uniform distribution of calcium and collagen was observed in all groups. Nevertheless, BV significantly increased in perfused groups as compared to static control, ranging from 0.35±0.28 mm³, 11.90±8.74 mm³ and 25.81±5.02 mm³ at week 3 to 2.28±0.78 mm³, 22.55±2.45 mm³ and 46.05±5.95 mm³ at week 6 in static, 0.7 and 7 ml/min groups, respectively. SS values on construct fibers in the range 10-100 mPa in 7 ml/min samples were twice as high as those in 0.7 ml/min samples showing the same trend of BV. The obtained results suggest that it is necessary to enhance the flow-induced mechanical stimulation of cell-embedding hydrogels to increase the amount of mineral deposited by hMSCs, compared to what is generally reported for the development of in vitro bone constructs. STATEMENT OF SIGNIFICANCE: : Culture of 3D Bioprinted Bone Constructs Requires an Increased Fluid Dynamic Stimulation, In this study, we evaluated for the first time how the hydrogel structure dampens the effect of flow-induced mechanical stimulation during the culture of 3D bioprinted bone tissue constructs. By combining computational and experimental techniques we demonstrated that those shear stress thresholds generally considered for culturing cells seeded on scaffold surface, are no longer applicable when cells are embedded in 3D bioprinted constructs. Significantly, more bone volume was formed in constructs exposed to shear stress values generally considered as detrimental than in constructs exposed shear stress values generally considered as beneficial after 3 weeks and 6 weeks of dynamic culture using a perfusion bioreactor.

2D µ-Particle Image Velocimetry and Computational Fluid Dynamics Study Within a 3D Porous Scaffold

Transport properties of 3D scaffolds under fluid flow are critical for tissue development. Computational fluid dynamics (CFD) models can resolve 3D flows and nutrient concentrations in bioreactors at the scaffold-pore scale with high resolution. However, CFD models can be formulated based on assumptions and simplifications. μ-Particle image velocimetry (PIV) measurements should be performed to improve the reliability and predictive power of such models. Nevertheless, measuring fluid flow velocities within 3D scaffolds is challenging. The aim of this study was to develop a μPIV approach to allow the extraction of velocity fields from a 3D additive manufacturing scaffold using a conventional 2D μPIV system. The μ-computed tomography scaffold geometry was included in a CFD model where perfusion conditions were simulated. Good agreement was found between velocity profiles from measurements and computational results. Maximum velocities were found at the centre of the pore using both techniques with a difference of 12% which was expected according to the accuracy of the μPIV system. However, significant differences in terms of velocity magnitude were found near scaffold substrate due to scaffold brightness which affected the μPIV measurements. As a result, the limitations of the μPIV system only permits a partial validation of the CFD model. Nevertheless, the combination of both techniques allowed a detailed description of velocity maps within a 3D scaffold which is crucial to determine the optimal cell and nutrient transport properties.

Simulation of oxygen transfer in stented arteries and correlation with in-stent restenosis

Computational models are used to study the combined effect of biomechanical and biochemical factors on coronary in-stent restenosis, which is a postoperative remodeling and regrowth pathology of the stented arteries. More precisely, we address numerical simulations, on the basis of Navier-Stokes and mass transport equations, to study the role of perturbed wall shear stresses and reduced oxygen concentration in a geometrical model reconstructed from a real porcine artery treated with a stent. Joining in vivo and in silico tools of investigation has multiple benefits in this case. On one hand, the geometry of the arterial wall and of the stent closely correspond to a real implanted configuration. On the other hand, the inspection of histological tissue samples informs us on the location and intensity of in-stent restenosis. As a result, we are able to correlate geometrical factors, such as the axial variation of the artery diameter and its curvature; the numerical quantification of biochemical stimuli, such as wall shear stresses; and the availability of oxygen to the inner layers of the artery, with the appearance of in-stent restenosis. This study shows that the perturbation of the vessel curvature could induce hemodynamic conditions that stimulate undesired arterial remodeling.

A novel flow chamber for biodegradable alloy assessment in physiologically realistic environments

In order to better understand the in vivo corrosion of biodegradable alloys, it is necessary to replicate the physiological environment as closely as possible. In this study, a novel flow chamber system is developed that allows the investigation of biodegradable alloy corrosion in a simulated physiological environment. The system is designed to reproduce flow conditions encountered in coronary arteries using a parallel plate setup and to allow the culturing of cells. Computational fluid dynamics and analytical methods are used as part of the design process to ensure that suitable flow conditions are maintained in the test region. The system is used to investigate the corrosion behavior of AZ31 alloy foils of different thickness, in test media with and without proteins and in static and dynamic solutions. It is observed that pulsatile flows, similar to those in the coronary arteries, significantly increase corrosion rates and lead to a different corrosion surface morphologies relative to static immersion tests.

Virtual surgeries in patients with congenital heart disease: a multi-scale modelling test case

The objective of this work is to perform a virtual planning of surgical repairs in patients with congenital heart diseases--to test the predictive capability of a closed-loop multi-scale model. As a first step, we reproduced the pre-operative state of a specific patient with a univentricular circulation and a bidirectional cavopulmonary anastomosis (BCPA), starting from the patient's clinical data. Namely, by adopting a closed-loop multi-scale approach, the boundary conditions at the inlet and outlet sections of the three-dimensional model were automatically calculated by a lumped parameter network. Successively, we simulated three alternative surgical designs of the total cavopulmonary connection (TCPC). In particular, a T-junction of the venae cavae to the pulmonary arteries (T-TCPC), a design with an offset between the venae cavae (O-TCPC) and a Y-graft design (Y-TCPC) were compared. A multi-scale closed-loop model consisting of a lumped parameter network representing the whole circulation and a patient-specific three-dimensional finite volume model of the BCPA with detailed pulmonary anatomy was built. The three TCPC alternatives were investigated in terms of energetics and haemodynamics. Effects of exercise were also investigated. Results showed that the pre-operative caval flows should not be used as boundary conditions in post-operative simulations owing to changes in the flow waveforms post-operatively. The multi-scale approach is a possible solution to overcome this incongruence. Power losses of the Y-TCPC were lower than all other TCPC models both at rest and under exercise conditions and it distributed the inferior vena cava flow evenly to both lungs. Further work is needed to correlate results from these simulations with clinical outcomes.

Computational models to predict stenosis growth in carotid arteries: which is the role of boundary conditions?

This work addresses the problem of prescribing proper boundary conditions at the artificial boundaries that separate the vascular district from the remaining part of the circulatory system. A multiscale (MS) approach is used where the Navier-Stokes equations for the district of interest are coupled to a non-linear system of ordinary differential equations which describe the circulatory system. This technique is applied to three 3D models of a carotid bifurcation with increasing stenosis resembling three phases of a plaque growth. The results of the MS simulations are compared to those obtained by two stand-alone models. The MS shows a great flexibility in numerically predicting the haemodynamic changes due to the presence of a stenosis. Nonetheless, the results are not significantly different from a stand-alone approach where flows derived by the MS without stenosis are imposed. This is a consequence of the dominant role played by the outside districts with respect to the stenosis resistance.

Chondrocyte response to high regimens of cyclic hydrostatic pressure in 3-dimensional engineered constructs

PURPOSE

Despite widespread use of 3-dimensional (3D) micro-porous scaffolds to promote their potential application in cartilage tissue engineering, only a few studies have examined the response to hydrostatic pressure of engineered constructs. A high cyclic pressurization, currently believed to be the predominant mechanical signal perceived by cells in articular cartilage, was used here to stimulate bovine articular chondrocytes cultured in a synthetic 3D porous scaffold (DegraPol).

METHODS

Construct cultivation lasted 3 days with applied pressurization cycles of amplitude 10 MPa, frequency 0.33 Hz, and stimulation sessions of 4 hours/day.

RESULTS

At 3 days of culture, with respect to pre-culture conditions, the viability of the pressurized constructs did not vary, whereas it underwent a 16% drop in the unpressurized controls. Synthesis of alfa-actin was 34% lower in all cultured constructs. Synthesis of collagen II/collagen I did not vary in pressurized constructs, was 76% lower in unpressurized controls, and was around 230% higher in pressurized constructs with respect to unpressurized controls. Chondrocytes showed a phenotypic spherical morphology at time zero and at 3 days of pressurized culture.

CONCLUSIONS

Although the passage from 2D expansion to 3D geometry was effective to guide cell differentiation, only mechanical conditioning enabled the maintenance and further cell differentiation toward a mature chondrocytic phenotype.

Expansion and drug elution model of a coronary stent

The present study illustrates a possible methodology to investigate drug elution from an expanded coronary stent. Models based on finite element method have been built including the presence of the atherosclerotic plaque, the artery and the coronary stent. These models take into account the mechanical effects of the stent expansion as well as the effect of drug transport from the expanded stent into the arterial wall. Results allow to quantify the stress field in the vascular wall, the tissue prolapse within the stent struts, as well as the drug concentration at any location and time inside the arterial wall, together with several related quantities as the drug dose and the drug residence times.

A new bioreactor for the controlled application of complex mechanical stimuli for cartilage tissue engineering

Mechanical stimuli have been shown to enhance chondrogenesis on both animal and human chondrocytes cultured in vitro. Different mechanical stimuli act simultaneously in vivo in cartilage tissue and their effects have been extensively studied in vitro, although often in a separated manner. A new bioreactor is described where different mechanical stimuli, i.e. shear stress and hydrostatic pressure, can be combined in different ways to study the mechanobiology of tissue engineered cartilage. Shear stress is imposed on cells by forcing the culture medium through the scaffolds, whereas a high hydrostatic pressure up to 15 MPa is generated by pressurizing the culture medium. Fluid-dynamic experimental tests have been performed and successful validation of the bioreactor has been carried out by dynamic culture of tissue-engineered cartilage constructs. The bioreactor system allows the investigation of the combined effects of different mechanical stimuli on the development of engineered cartilage, as well as other possible three-dimensional tissue-engineered constructs.

Toward optimal hemodynamics: computer modeling of the Fontan circuit

The construction of efficient designs with minimal energy losses is especially important for cavopulmonary connections. The science of computational fluid dynamics has been increasingly used to study the hemodynamic performance of surgical operations. Three-dimensional computer models can be accurately constructed of typical cavopulmonary connections used in clinical practice based on anatomic data derived from magnetic resonance scans, angiocardiograms, and echocardiograms. Using these methods, the hydraulic performance of the hemi-Fontan, bidirectional Glenn, and a variety of types of completion Fontan operations can be evaluated and compared. This methodology has resulted in improved understanding and design of these surgical operations.

Use of rapid prototyping models in the planning of percutaneous pulmonary valved stent implantation

Percutaneous replacement of the pulmonary valve is a recently developed inter-ventional technique which involves the implantation of a valved stent in the pulmonary trunk. It relies upon careful consideration of patient anatomy for both stent design and detailed procedure planning. Medical imaging data in the form of two-dimensional scans and three-dimensional interactive graphics offer only limited support for these tasks. The paper reports the results of an experimental investigation on the use of arterial models built by rapid prototyping techniques. An analysis of clinical needs has helped to specify proper requirements for such model properties as cost, strength, accuracy, elastic compliance, and optical transparency. Two different process chains, based on the fused deposition modelling technique and on the vacuum casting of thermoset resins in rubber moulds, have been tested for prototype fabrication. The use of anatomical models has allowed the cardiologist's confidence in patient selection, prosthesis fabrication, and final implantation to be significantly improved.

MRI-based multiscale models for the hemodynamic and structural evaluation of surgically reconstructed aortic arches

The surgical reconstruction of the aortic arch is necessary in pediatric patients suffering from different types of congenital heart malformations, in particular, coarctation of the aorta. Among the reconstruction techniques used in surgical practice end-to-end anastomosis (E/E), Gore-tex graft interposition (GGI) and Gore-tex patch graft aortoplasty (GPGA) are compared in this study with a control model, employing a computational fluid-structure-interaction scheme. This study analyzes the impact of introducing synthetic materials on aortic hemodynamics and wall mechanics. Three-dimensional (3D) geometries of a porcine aortic arch were derived from magnetic resonance imaging (MRI) images. Inlet conditions were derived from MRI velocimetry. A multiscale approach was used for the imposition of outlet conditions, wherein a lumped parameter net provided an active afterload. Evidence was found that ring-like repairs increased blood velocity, whereas GPGA limited it. Vortex presence was greater and longer lasting in GGI. The highest power losses corresponded to GPGA. GGI had an intermediate effect, while E/E dissipated only slightly more than the control case. Wall stresses peak in a longitudinal strip on the subject's left side of the vessel, particularly in the frontal area. There was a concentration of stress at the suture lines. All surgical techniques performed equally well in restoring physiological pressures.

Multiscale modelling as a tool to prescribe realistic boundary conditions for the study of surgical procedures

This work was motivated by the problems of analysing detailed 3D models of vascular districts with complex anatomy. It suggests an approach to prescribing realistic boundary conditions to use in order to obtain information on local as well as global haemodynamics. A method was developed which simultaneously solves Navier-Stokes equations for local information and a non-linear system of ordinary differential equations for global information. This is based on the principle that an anatomically detailed 3D model of a cardiovascular district can be achieved by using the finite element method. In turn the finite element method requires a specific boundary condition set. The approach outlined in this work is to include the system of ordinary differential equations in the boundary condition set. Such a multiscale approach was first applied to two controls: (i) a 3D model of a straight tube in a simple hydraulic network and (ii) a 3D model of a straight coronary vessel in a lumped-parameter model of the cardiovascular system. The results obtained are very close to the solutions available for the pipe geometry. This paper also presents preliminary results from the application of the methodology to a particular haemodynamic problem: namely the fluid dynamics of a systemic-to-pulmonary shunt in paediatric cardiac surgery.

Effects of pulmonary afterload on the hemodynamics after the hemi-Fontan procedure

A computational fluid dynamics study based on the application of the finite volume method has been performed to investigate the effects of the pulmonary afterload on the hemodynamics after the hemi-Fontan procedure. This operation is generally used as part of a series of staged procedures to treat complex congenital malformations of the heart. It consists of re-directing the superior vena caval flow from the right atrium into the pulmonary arteries, by-passing the right ventricle while excluding the inferior caval flow from the lungs. To reproduce correctly the pulmonary afterload conditions, a simplified lumped-parameter mechanical model of the pulmonary circulation has been developed and linked to the finite volume solver. In addition, the effect of a stenosis in the left pulmonary artery was also examined. In this paper the adopted methodology is presented, together with some of the preliminary results. The model has been used to simulate the local fluid dynamics for different values of the pulmonary arteriolar resistance and lung resistances, allowing a quantitative evaluation of the dissipated energy and the flow distribution into the lungs. The results show that both flow distribution into the lungs and energy dissipation after the hemi-Fontan procedure are only minimally affected by the pulmonary arteriolar resistance.

Modeling of the Norwood circulation: effects of shunt size, vascular resistances, and heart rate

Hypoplastic left heart syndrome is the most common lethal cardiac malformation of the newborn. Its treatment, apart from heart transplantation, is the Norwood operation. The initial procedure for this staged repair consists of reconstructing a circulation where a single outlet from the heart provides systemic perfusion and an interpositioning shunt contributes blood flow to the lungs. To better understand this unique physiology, a computational model of the Norwood circulation was constructed on the basis of compartmental analysis. Influences of shunt diameter, systemic and pulmonary vascular resistance, and heart rate on the cardiovascular dynamics and oxygenation were studied. Simulations showed that 1) larger shunts diverted an increased proportion of cardiac output to the lungs, away from systemic perfusion, resulting in poorer O2 delivery, 2) systemic vascular resistance exerted more effect on hemodynamics than pulmonary vascular resistance, 3) systemic arterial oxygenation was minimally influenced by heart rate changes, 4) there was a better correlation between venous O2 saturation and O2 delivery than between arterial O2 saturation and O2 delivery, and 5) a pulmonary-to-systemic blood flow ratio of 1 resulted in optimal O2 delivery in all physiological states and shunt sizes.

In vitro steady-flow analysis of systemic-to-pulmonary shunt haemodynamics

A modified Blalock-Taussig shunt is a connection created between the systemic and pulmonary arterial circulations to improve pulmonary perfusion in children with congenital heart diseases. Survival of these patients is critically dependent on blood flow distribution between the pulmonary and systemic circulations which in turn depends upon the flow resistance of the shunt. Previously, we investigated the pressure-flow relationship in rigid shunts with a computational approach. to estimate the pulmonary blood flow rate on the basis of the in vivo measured pressure drop. The present study aims at evaluating, in vitro how the anastomotic distensibility and restrictions due to suture presence affect the shunt pressure-flow relationship. Two actual Gore-Tex shunts (3 and 4 mm diameters) were sutured to compliant conduits by a surgeon and tested at different steady flow rates (0.25-11 min(-1)) and pulmonary pressures (3-34 mmHg). Corresponding computational models were also created to investigate the role of the anastomotic restrictions due to sutures. In vitro experiments showed that pulmonary artery pressure affects the pressure-flow relationship of the anastomoses. particularly at the distal site. However, this occurrence scarcely influences the total shunt pressure drop. Comparisons between in vitro and computational models without anastomotic restrictions show that the latter underestimates the in vitro pressure drops at any flow rate. The addition of the anastomotic restrictions (31 and 47% of the original area of 3 and 4 mm shunts, respectively) to the computational models reduces the gap, especially at high shunt flow rate and high pulmonary pressure.

Computational model of the fluid dynamics in systemic-to-pulmonary shunts

A systemic-to-pulmonary shunt is a connection created between the systemic and pulmonary arterial circulations in order to improve pulmonary perfusion in children with congenital heart diseases. Knowledge of the relationship between pressure and flow in this new, surgically created, cardiovascular district may be helpful in the clinical management of these patients, whose survival is critically dependent on the blood flow distribution between the pulmonary and systemic circulations. In this study a group of three-dimensional computational models of the shunt have been investigated under steady-state and pulsatile conditions by means of a finite element analysis. The model is used to quantify the effects of shunt diameter (D), curvature, angle, and pulsatility on the pressure-flow (DeltaP-Q) relationship of the shunt. Size of the shunt is the main regulator of pressure-flow relationship. Innominate arterial diameter and angles of insertion have less influence. Curvature of the shunt results in lower pressure drops. Inertial effects can be neglected. The following simplified formulae are derived: DeltaP=(0. 097Q+0.521Q(2))/D(4) and DeltaP=(0.096Q+0.393Q(2))/D(4) for the different shunt geometries investigated (straight and curved shunts, respectively).

Use of mathematical model to predict hemodynamics in cavopulmonary anastomosis with persistent forward flow

BACKGROUND

The bidirectional cavopulmonary anastomosis with additional pulmonary blood flow is used as a staged procedure or a definitive palliation of univentricular hearts. In this paper the flow competition occurring between the caval and the pulmonary flows is investigated. The hemodynamics in the superior vena cava and the blood flow distribution into the lungs, as well as the systemic arterial oxygen availability, are correlated with the severity of the right ventricle outflow tract obstruction and the pulmonary arteriolar resistance.

MATERIALS AND METHODS

Computer models of the pre- and postoperative hemodynamics of univentricular hearts were developed. The effects of increasing severity of the right ventricle outflow tract obstruction, with a pulmonary arteriolar resistance ranging from 0.8 to 7.9 nonindexed Woods units, were simulated.

RESULTS

The study indicates that the presence of an additional pulmonary blood flow from the native pulmonary artery may be beneficial. Since an excessive additional blood flow may cause central venous hypertension, its optimal value should be chosen according to the value of pulmonary arteriolar resistance. The model was utilized to simulate four clinical cases.

CONCLUSIONS

The simulations show that the model can predict the postoperative hemodynamics and could therefore be usefully applied to predict quantitatively the effect of the native pulmonary blood flow following bidirectional cavopulmonary anastomosis.

Calculating blood flow from Doppler measurements in the systemic-to-pulmonary artery shunt after the Norwood operation: a method based on computational fluid dynamics

Hypoplastic left heart syndrome is currently the most lethal cardiac malformation of the newborn infant. Survival following a Norwood operation depends on the balance between systemic and pulmonary blood flow, which is highly dependent on the fluid dynamics through the interposition shunt between the two circulations. We used computational fluid dynamic (CFD) models to determine the velocity profile in a systemic-to-pulmonary artery shunt and suggested a simplified method of calculating the blood flow in the shunt based on Doppler measurements. CFD models of systemic-to-pulmonary shunts based on the finite element method were studied. The size of the shunt has been varied from 3 to 5 mm. Velocity profiles at proximal and distal positions were evaluated and correlations between maximum and mean spatial velocity were found. Twenty-one Doppler measurements in the proximal and distal part of the shunt were obtained from six patients with hypoplastic left heart syndrome. Combining Doppler velocities and CFD velocity profiles, blood flow rate in the shunt was calculated. Flow rate evaluated from aortic Doppler and oxygen saturation measurements were performed for comparison. Results showed that proximal shunt Doppler velocities were always greater than the correspondent distal ones (ratio equal to 1.15 +/- 0.11). CFD models showed a similar behaviour (ratio equal to 1.21 +/- 0.03). CFD models gave a V(mean)/V(max) ratio of 0. 480 at the proximal junction and of 0.579 at the distal one. The agreement between the flow evaluated in the proximal and distal areas of the shunt was good (0.576 +/- 0.150 vs. 0.610 +/- 0.166 l/min). Comparison of these data with saturation data and aortic Doppler measurements correlate less well (0.593 +/- 0.156 vs. 1.023 +/- 0.493 l/min). A formula easily to quantify shunt flow rate is proposed. This could be used to evaluate the effects of different therapeutic and pharmacological manoeuvres in this unique circulation.

Humidity measurements in passive heat and moisture exchangers applications: a critical issue

A reliable, quantitative assessment of humidification performances of passive heat and moisture exchangers in mechanically-ventilated patients is still to be achieved, although relevant efforts have been made to date. One of the major problems to tackle consists in the difficulty of humidity measurements, both in vivo (during either anaesthesia or intensive care unit treatments) and in vitro set-ups. In this paper a review of the basic operation principles of humidity sensors as well as an analysis of their usage within in vivo and in vitro tests are presented. Particular attention is devoted to the limitations arising from the specific measurement set-up, as they may affect the results notably.

Computational fluid dynamics in paediatric cardiac surgery

Computational fluid dynamics techniques have been applied to study both the local and the global haemodynamics created by different surgical reconstructions, currently used to treat complex congenital heart defects. These operations are characterised by competition of flows which can lead to postoperative failure of the surgical treatment. Different techniques have been used in order to improve knowledge of the global haemodynamics in patients submitted to such operations, and to devise possible optimal hydraulic designs of the connections. The adopted approach has combined highly-detailed, three-dimensional models of the connections with simplified zero-dimensional, lumped-parameter network models of the overall circulation of the patient. Three-dimensional models of the connections have been developed by means of the finite element method. Local fluid dynamics features have been analysed and then 'incorporated' in mathematical models able to predict some clinically relevant postoperative haemodynamic data. Results emphasise the impact of local geometry on global haemodynamics.

Computational fluid dynamic simulations of cavopulmonary connections with an extracardiac lateral conduit

Complex congenital heart defects due to the absence of a ventricular chamber can often be treated by the Fontan surgical procedure. The objective of this work was to quantify the haemodynamics in the Fontan operation (cavopulmonary connection) with extracardiac lateral conduit. Four different models based on the finite element method were constructed with different lengths of inferior anastomosis (range 18-25 mm) and inclinations of the conduit (33 and 47.5 degrees). Mass conservation and Navier-Stokes equations were solved by means of the FIDAP code, based on the finite element method. The left-to-right pulmonary flow ratio and percentage inferior caval blood to the left lung were the highest with the smallest anastomosis and highest inclination: 1.35 and 83.26%, respectively. Dissipated power percentage was higher with the largest anastomosis than with the smallest (19.4 vs 15.8%). It was concluded that, when performing a total cavopulmonary connection, an extracardiac lateral conduit: (i) diverts more flow to the left lung, and (ii) shows higher energy losses when compared with a connection with intra-atrial tunnel. This study could be useful to evaluate the incidence of pulmonary arteriovenous malformations.

Computational fluid dynamic and magnetic resonance analyses of flow distribution between the lungs after total cavopulmonary connection

Total cavopulmonary connection is a surgical procedure adopted to treat complex congenital malformations of the right heart. It consists basically in a connection of both venae cavae directly to the right pulmonary artery. In this paper a three-dimensional model of this connection is presented, which is based on in vivo measurements performed by means of magnetic resonance. The model was developed by means of computational fluid dynamics techniques, namely the finite element method. The aim of this study was to verify the capability of such a model to predict the distribution of the blood flow into the pulmonary arteries, by comparison with in vivo velocity measurements. Different simulations were performed on a single clinical case to test the sensitivity of the model to different boundary conditions, in terms of inlet velocity profiles as well as outlet pressure levels. Results showed that the flow distribution between the lungs is slightly affected by the shape of inlet velocity profiles, whereas it is influenced by different pressure levels to a greater extent.

Computational transient simulations with varying degree and shape of pulmonic stenosis in models of the bidirectional cavopulmonary anastomosis

The bidirectional cavopulmonary anastomosis is a surgical technique utilized to treat severe congenital malformations of the right part of the heart. It is obtained by anastomosing the superior vena cava to the superior aspect of the undivided right pulmonary artery. Transient simulations with a three-dimensional model of the bidirectional cavopulmonary anastomosis were carried out to evaluate the haemodynamics of different types of pulmonic stenosis (shape and severity of the obstruction). Models with a tunnel-like (supravalvar) or discrete (valvar) pulmonic stenosis with different values of reduction of cross-sectional area (60 and 75%) were investigated and compared to a model without stenosis. Calculations were based on a finite element method analysis. The results showed that a tighter stenosis can lead to a blood volume flow to the left lung reaching 70% of the total pulmonary flow. Moreover, the flow fields are highly influenced by the presence and shape of the pulmonic stenosis; the most intense jets in the left pulmonary artery occur for a discrete pulmonic stenosis of 75%. The flow in the right pulmonary artery is nearly steady because it is damped down by the steady caval flow.

A mathematical model of circulation in the presence of the bidirectional cavopulmonary anastomosis in children with a univentricular heart

The bidirectional cavopulmonary anastomosis is used as a staged procedure or a definitive palliation of univentricular hearts. It is often performed in the presence of an additional blood flow arising from the native pulmonary outflow tract. In this paper, the effects of the severity of the pulmonary outflow obstruction and the pulmonary arteriolar resistance are analysed with regard to the haemodynamics in the superior vena cava and the blood distribution into the lungs. A computer model has been developed, which can represent both the preoperative and the postoperative (systemic and pulmonary) circulations in a patient with a double-outlet univentricular heart. It is particularly detailed in the region of the large vessels and includes components that account for local three-dimensional effects due to the actual shape of the anastomosis. Results have indicated that the mean pressure in the superior vena cava increases from 8.2 to 19.2 mmHg with pulmonary arteriolar resistance ranging from 0.8 to 7.9 Woods units and pulmonary outflow obstruction ranging from 50 to 100%. The percentage flow distribution to the right lung has turned out to be heavily affected by the flow competition and has ranged from 43 to 50% of the total flow to the lungs in the systolic phase, and from 51 to 62% in the diastolic phase. The model allows routinely used clinical indices to be computed, as well as the evaluation of new indices, which is potentially helpful in the clinical assessment of postoperative haemodynamics (e.g. the right-to-left lung flow ratio and the superior vena cava-to-pulmonary flow ratio).

Mesh updating in fluid-structure interactions in biomechanics: an iterative method based on an uncoupled approach

In this study, a computational uncoupled approach to fluid-structure interaction problems in biofluid mechanics is presented. It is based on the finite element method and is applied to study the local fluid dynamics in two specific situations: the left ventricular ejection phase and the motion of an isolated red blood cell along a small artery. Particularly, the focus is on the algorithms developed to deal with mesh updating, because both examined districts are characterized by geometrical deformations of the fluid domain edges. This is currently a challenging issue in the application of computational fluid dynamics techniques to living systems, especially to the cardiovascular system. Although the chosen approach uses a commercial computational fluid dynamics package for the solution of the fluid domain, original algorithms have been developed to perform the boundary displacement calculations correctly, as well as the corresponding mesh updating. Results are reported and compared with available data in the literature pertinent to the two studied problems.

A computational pulsatile model of the bidirectional cavopulmonary anastomosis: the influence of pulmonary forward flow

The bidirectional cavopulmonary anastomosis (BCPA or bidirectional Glenn) is an operation to treat congenital heart diseases of the right heart by diverting the systemic venous return from the superior vena cava to both lungs. The main goal is to provide the correct perfusion to both lungs avoiding an excessive increase in systemic venous pressure. One of the factors which can affect the clinical outcome of the surgically reconstructed circulation is the amount of pulsatile blood flow coming from the main pulmonary artery. The purpose of this work is to analyse the influence of this factor on the BCPA hemodynamics. A 3-D finite element model of the BCPA has been developed to reproduce the flow of the surgically reconstructed district. Geometry and hemodynamic data have been taken from angiocardiogram and catheterization reports, respectively. On the basis of the developed 3-D model, four simulations have been performed with increasing pulsatile blood flow rate from the main pulmonary artery. The results show that hemodynamics in the pulmonary arteries are greatly influenced by the amount of flow through the native main pulmonary artery and that the flow from the superior vena cava allows to have a similar distribution of the blood to both lungs, with a little predilection for the left side, in agreement with clinical postoperative data.

A new pulsatile blood pump for adult cardiopulmonary bypass: design criteria and preliminary fluid dynamic evaluation

A new pulsatile pumping device for adult cardiopulmonary bypass has been designed. Its main characteristic consists in having a fully disposable pumping head, since polymeric materials have been adopted for the housing as well as for the built-in inlet and outlet valves. Furthermore, the valves show an innovative design, as they are ring-shaped and accomplish their task by virtue of their elastic deformability. The design phase of the pumping head and the first fluid dynamic evaluations have been performed by numerical methods. Particularly, a three-dimensional CAD model of the pumping head (in the current configuration) is presented in this paper. On the basis of this model, computational fluid dynamic analysis of the hydraulic behaviour has been performed for some components. The obtained results show complex velocity patterns in the pumping chamber during the filling phase as well as limited pressure gradients across the inlet valve.

Use of computational fluid dynamics in the design of surgical procedures: application to the study of competitive flows in cavo-pulmonary connections

Computational fluid dynamic methods based on a finite-element technique were applied to the study of (1) competition of flows in the inferior and superior venae cavae in total cavopulmonary connection, and (2) competition between flow in the superior vena cava and forward flow from a stenosed pulmonary artery in bidirectional cavopulmonary anastomosis. Models corresponding to various degrees of offsetting and shape of the inferior vena caval anastomosis were simulated to evaluate energy dissipation and flow distribution between the two lungs. A minimal energy loss with optimal flow distribution between the two lungs was obtained by enlarging the inferior vena caval anastomosis toward the right pulmonary artery. This modified technique of total cavopulmonary connection is described. A computational model of the operation was developed in an attempt to understand the mechanisms of postoperative failure. In tight pulmonary artery stenosis (75%), the pulsatile forward flow is primarily directed to the left pulmonary artery, with little influence on superior vena caval pressure and the right pulmonary artery. Pulsatile forward flows corresponding to 15%, 30%, 45%, and 60% of the systemic artery output increased the mean pulmonary artery and superior vena caval pressures by 1, 1.7, 2.4, and 3.6 mm Hg, respectively. Although the modeling studies were not able to determine the cause of postoperative failure, they emphasize the impact of local geometry on flow dynamics. More simulations are required for further investigation of the problem.

A numerical fluid mechanical study of repaired congenital heart defects. Application to the total cavopulmonary connection

A computational fluid dynamics study based on the application of the finite element method has been performed to investigate the local hemodynamics of the total cavopulmonary connection. This operation is used to treat congenital malformations of the right heart and consists of a by-pass of the right ventricle. In this paper the adopted methodology is presented, together with some of the preliminary results. A three-dimensional parametric model of the connection and a lumped-parameter mechanical model of the pulmonary circulation have been developed. The three-dimensional model has been used to simulate the local fluid dynamics for different designs of the connection, allowing a quantitative evaluation of the dissipated energy in each of the examined configurations. The pulmonary afterload of the three-dimensional model has been reproduced by coupling it with the pulmonary mechanical model. The results show that, from a comparative point of view, the energetic losses can be greatly reduced if a proper hydraulic design of the connection is adopted, which also allows control of the blood flow distribution into the lungs.

Fluid-structure interaction problems in bio-fluid mechanics: a numerical study of the motion of an isolated particle freely suspended in channel flow

In this paper a problem belonging to the moving boundary class is tackled with a 2-D application of computational fluid dynamics techniques. The motion of an isolated rigid particle freely suspended in an incompressible Newtonian fluid in a narrow channel is studied numerically at a low Reynolds number, yet different from zero. The actual problem consists of two coupled problems: the motion of the viscous fluid and that of the rigid particle suspended and convected with the fluid. The full Navier-Stokes equations (i.e. both transient and convective terms are included) are solved in the fluid domain by means of the finite element method, while the motion of the particle is determined on the basis of a rigid act of motion. Results from simulations corresponding to differential initial positions of the particle are shown in this paper: they allow one to study the rotational motions of the particle as well as its displacements. The goal of the paper is to analyse the lateral displacement behaviour of the particle, already observed in experimental studies in microcirculation. In particular, lateral migrations are supposed to be due to inertial forces acting in the fluid around the moving particle combined with the proximity of the resting wall (wall effect). Preliminary results are in fairly good agreement with those available in the literature.

Computational fluid dynamics of artificial heart valves

A large number of in vitro studies during the last thirty years have assessed the fluid dynamic behavior of different artificial heart valves. The present study illustrates the utility of the Finite Element Method for fluid dynamic evaluation of prosthetic heart valves. The valves investigated were the Bjork-Shiley Convex-Concave (curved disc), the Medtronic-Hall (flat disc) and the Carbomedics (bileaflet). These three types were chosen in order to clarify the role of different occluder geometries on global and local fluid dynamics. The Finite Element Method was used to calculate pressure and velocity fields in the fluid domain around each valve. There were significant differences, mainly in local fluid dynamics, between the three valves. The Reynolds number also plays an important role.