Total Cavopulmonary Connection Flow With Functional Left Pulmonary Artery Stenosis

Biomechanical Analysis of Age-Dependent Changes in Fontan Power Loss

The hemodynamics in Fontan patients with single ventricles rely on favorable flow and energetics, especially in the absence of a subpulmonary ventricle. Age-related changes in energetics for extracardiac and lateral tunnel Fontan procedures are not well understood. Vorticity (VOR) and viscous dissipation rate (VDR) are two descriptors that can provide insights into flow dynamics and dissipative areas in Fontan pathways, potentially contributing to power loss. This study examined power loss and its correlation with spatio-temporal flow descriptors (vorticity and VDR). Data from 414 Fontan patients were used to establish a relationship between the superior vena cava (SVC) to inferior vena cava (IVC) flow ratio and age. Computational flow modeling was conducted for both extracardiac conduits (ECC, n = 16) and lateral tunnels (LT, n = 25) at different caval inflow ratios of 2, 1, and 0.5 that corresponded with ages 3, 8, and 15+. In both cohorts, vorticity and VDR correlated well with PL, but ECC cohort exhibited a slightly stronger correlation for PL-VOR (>0.83) and PL-VDR (>0.89) than that for LT cohort (>0.76 and > 0.77, respectively) at all ages. Our data also suggested that absolute and indexed PL increase (p < 0.02) non-linearly as caval inflow changes with age and are highly patient-specific. Comparison of indexed power loss between our ECC and LT cohort showed that while ECC had a slightly higher median PL for all 3 caval inflow ratio examined (3.3, 8.3, 15.3) as opposed to (2.7, 7.6, 14.8), these differences were statistically non-significant. Lastly, there was a consistent rise in pressure gradient across the TCPC with age-related increase in IVC flows for both ECC and LT Fontan patient cohort. Our study provided hemodynamic insights into Fontan energetics and how they are impacted by age-dependent change in caval inflow. This workflow may help assess the long-term sustainability of the Fontan circulation and inform the design of more efficient Fontan conduits.

Evaluation of the total hydrodynamic energy loss using 4D flow MRI in a case with Fontan failure

Fontan Failure (FF) is a common problem for single-ventricle patients as they reach adulthood. Although several mechanisms may cause FF, an optimized blood flow stream through the surgical conduits is essential to avoid excessive energy loss (EL). Recent clinical studies showed EL is related to the quality of life, exercise capacity, and hepatic function since the single-ventricle feeds pulmonary and systemic circulation serially. 4D flow MRI effectively estimates EL in Fontan circulation and allows clinicians to compare the effectiveness of the treatment strategy concerning pre-intervention. Here, we present 26-year-old women with FF who had normal cardiac catheterization findings and were treated according to high EL definitions that are measured through 4D flow MRI.

Switching the Left and the Right Hearts: A Novel Bi-ventricle Mechanical Support Strategy with Spared Native Single-Ventricle

End-stage Fontan patients with single-ventricle (SV) circulation are often bridged-to-heart transplantation via mechanical circulatory support (MCS). Donor shortage and complexity of the SV physiology demand innovative MCS. In this paper, an out-of-the-box circulation concept, in which the left and right ventricles are switched with each other is introduced as a novel bi-ventricle MCS configuration for the "failing" Fontan patients. In the proposed configuration, the systemic circulation is maintained through a conventional mechanical ventricle assist device (VAD) while the venous circulation is delegated to the native SV. This approach spares the SV and puts it to a new use at the right-side providing the most-needed venous flow pulsatility to the failed Fontan circulation. To analyze its feasibility and performance, eight SV failure modes have been studied via an established multi-compartmental lumped parameter cardiovascular model (LPM). Here the LPM model is experimentally validated against the corresponding pulsatile mock-up flow loop measurements of a representative 15-year-old Fontan patient employing a clinically-approved VAD (Medtronic-HeartWare). The proposed surgical configuration maintained the healthy cardiac index (3-3.5 l/min/m2) and the normal mean systemic arterial pressure levels. For a failed SV with low ejection fraction (EF = 26%), representing a typical systemic Fontan failure, the proposed configuration enabled a ~ 28 mmHg amplitude in the venous/pulmonary waveforms and a 2 mmHg decrease in the central venous pressure (CVP) together with acceptable mean pulmonary artery pressures (17.5 mmHg). The pulmonary vascular resistance (PVR)-SV failure case provided a ~ 5 mmHg drop in the CVP, with venous/pulmonary pulsatility reaching to ~ 22 mmHg. For the high PVR failure case with a healthy SV (EF = 44%) pulmonary hypertension is likely to occur as expected. While this condition is routinely encountered during the heart transplantation and managed through pulmonary vasodilators a need for precise functional assessment of the spared failed-ventricle is recommended if utilized in the PVR failure mode. Comprehensive in vitro and in silico results encourage this novel concept as a low-cost, more physiological alternative to the conventional bi-ventricle MCS pending animal experiments.

Hemodynamic analysis of unilateral and bilateral renal artery stenosis based on fluid-structure interaction

Renal artery stenosis (RAS) hypertension is a common type of secondary hypertension. This paper aimed to explore how unilateral renal artery stenosis (Uni-RAS) and bilateral renal artery stenosis (Bi-RAS) caused renovascular hypertension with the fluid-structure interaction (FSI) method. Based on a real RAS model, 20 ideal models with different stenosis degrees were established by modifying the stenosis segment. The hemodynamic parameters at different degrees of stenosis, mass flow rate (MFR), pressure drop (PD), fractional flow reserve (FFR), oscillatory shear index (OSI), and relative residence time (RRT), were numerically calculated by the computational fluid dynamics (CFD) method. The numerical results showed that RAS caused the decrease of MFR, and the increase of PD and the proportion of high OSI and high RRT. In the case of RAS, it could not be regarded as a reference indicator for causing renovascular hypertension that the value of FFR was greater than 0.9. In addition, the results of the statistical analysis indicated that Uni-RAS and Bi-RAS were statistically different for MFR, PD and the proportion of high RRT.

Numerical simulation of the cavopulmonary connection flow with conduit stenoses of varying configurations

The circulation in the total cavopulmonary connection (TCPC) is a low-energy system which operation and efficiency are subjected to multiple factors. Some retrospective studies report that the abnormal narrowing of vessels in the system, i.e. stenosis, is one of the most dangerous geometric factors which can result in heart failure. In the present study, the effect of varying extracardiac conduit (ECC) stenosis on the hemodynamics in a surrogate TCPC model is investigated using high-fidelity numerical simulations. The efficiency of the surrogate TCPC model was quantified according to the power loss, relative perfusion in lungs and the percentage of conduit surface area with abnormally low and high wall shear stress for venous flow. Additionally, the impact of respiration and asymmetry in the stenosis geometry to the system was examined. The results show that the flow in the TCPC model exhibits pronounced unsteadiness even under the steady initial boundary conditions, while the uneven pulmonary flow distribution and the presence of the ECC stenosis amplify the chaotic nature of the flow. Energy efficiency of the system is shown to strongly correlate with amount of vortical structures in the model and their range of scales. Finally, the study demonstrates that the presence of respiration in the model adds to perturbations in the flow which causes increase in the power loss. Results obtained in the study provide valuable insights on how the ECC stenosis effect the flow in the surrogate TCPC model under different flow conditions.

Hemodynamic Parameters for Cardiovascular System in 4D Flow MRI: Mathematical Definition and Clinical Applications

Blood flow imaging becomes an emerging trend in cardiology with the recent progress in computer technology. It not only visualizes colorful flow velocity streamlines but also quantifies the mechanical stress on cardiovascular structures; thus, it can provide the detailed inspections of the pathophysiology of diseases and predict the prognosis of cardiovascular functions. Clinical applications include the comprehensive assessment of hemodynamics and cardiac functions in echocardiography vector flow mapping (VFM), 4D flow MRI, and surgical planning as a simulation medicine in computational fluid dynamics (CFD).For evaluation of the hemodynamics, novel mathematically derived parameters obtained using measured velocity distributions are essential. Among them, the traditional and typical parameters are wall shear stress (WSS) and its related parameters. These parameters indicate the mechanical damages to endothelial cells, resulting in degenerative intimal change in vascular diseases. Apart from WSS, there are abundant parameters that describe the strength of the vortical and/or helical flow patterns. For instance, vorticity, enstrophy, and circulation indicate the rotating flow strength or power of 2D vortical flows. In addition, helicity, which is defined as the cross-linking number of the vortex filaments, indicates the 3D helical flow strength and adequately describes the turbulent flow in the aortic root in cases with complicated anatomies. For the description of turbulence caused by the diseased flow, there exist two types of parameters based on completely different concepts, namely: energy loss (EL) and turbulent kinetic energy (TKE). EL is the dissipated energy with blood viscosity and evaluates the cardiac workload related to the prognosis of heart failure. TKE describes the fluctuation in kinetic energy during turbulence, which describes the severity of the diseases that cause jet flow. These parameters are based on intuitive and clear physiological concepts, and are suitable for in vivo flow measurements using inner velocity profiles.

A Tribute to Ajit Yoganathan's Cardiovascular Fluid Mechanics Lab: A Survey of Its Contributions to Our Understanding of the Physiology and Management of Single-Ventricle Patients

INTRODUCTION

Among patients with congenital heart disease, those born with only a single working ventricle represent a particularly complex sub-population, typically requiring multiple surgeries and suffering from high levels of mortality and morbidity. Their cardiac care is complex and has evolved considerably since surgical palliation was first introduced more than 50 years ago. Improvements in treatment have been driven both by growing clinical experience and by knowledge gained through experimental and computational studies of blood flow in these patients. The Cardiovascular Fluid Mechanics Lab at the Georgia Institute of Technology, founded 30 years ago by Dr. Ajit Yoganathan, has pioneered work in this field.

METHODS

In this review, key contributions of Dr. Yoganathan's Cardiovascular Fluid Dynamics Lab are surveyed, including experimental flow loop studies as well as computational fluid dynamics analyses that address many of the critical challenges that cardiologists and surgeons face in treating these patients, including how to reconstruct cardiovascular anatomy to minimize power loss, balance blood flow distribution at key bifurcation points, and avoid other unfavorable hemodynamic conditions.

CONCLUSIONS

Among many contributions in this field, work from the Cardiovascular Fluid Mechanics Lab has led to novel medical devices and patient-specific computational modeling workflows and software tools. These key contributions from this group have enhanced our understanding of the physiology and management of single-ventricle patients.

An in-vitro evaluation of the flow haemodynamic performance of Gore-Tex extracardiac conduits for univentricular circulation

OBJECTIVE(S)

The Fontan procedure is a common palliative intervention for sufferers of single ventricle congenital heart defects that results in an anastomosis of the venous return to the pulmonary arteries called the total cavopulmonary connection (TCPC). In patients with palliated single ventricular heart defects, the Fontan circulation passively directs systemic venous return to the pulmonary circulation in the absence of a functional sub-pulmonary ventricle. Therefore, the Fontan circulation is highly dependent on favourable flow and energetics, and minimal energy loss is of great importance. The majority of in vitro studies, to date, employ a rigid TCPC model. Recently, few studies have incorporated flexible TCPC models, without the inclusion of commercially available conduits used in these surgical scenarios.

METHOD

The methodology set out in this study successfully utilizes patient-specific phantoms along with the corresponding flowrate waveforms to characterise the flow haemodynamic performance of extracardiac Gore-Tex conduits. This was achieved by comparing a rigid and flexible TCPC models against a flexible model with an integrated Gore-Tex conduit.

RESULTS

The flexible model with the integrated Gore-Tex graft exhibited greater levels of energy losses when compared to the rigid walled model. With this, the flow fields showed greater levels of turbulence in the complaint and Gore-Tex models compared to the rigid model under ultrasound analysis.

CONCLUSION

This study shows that vessel compliance along with the incorporation of Gore-Tex extracardiac conduits have significant impact on the flow haemodynamics in a patient-specific surgical scenario.

Methodology for Hemodynamic Assessment of a Three-Dimensional Printed Patient-Specific Vascular Test Device

Assessing hemodynamics in vasculature is important for the development of cardiovascular diagnostic parameters and evaluation of medical devices. Benchtop experiments are a safe and comprehensive preclinical method for testing new diagnostic endpoints and devices within a controlled environment. Recent advances in three-dimensional (3D) printing have enhanced benchtop tests by allowing generation of patient-specific and pathophysiologic conditions. We used 3D printing, coupled with image processing and computer-aided design (CAD), to develop a patient-specific vascular test device from clinical data. The proximal pulmonary artery (PA) tree including the main, left, and right pulmonary arteries, with a stenosis within the left PA was selected as a representative anatomy for developing the vascular test device. Three test devices representing clinically relevant stenosis severities, 90%, 80%, and 70% area stenosis, were evaluated at different cardiac outputs (COs). A mock circulatory loop (MCL) generating pathophysiologic pulmonary pressure and flow was used to evaluate the hemodynamics within the devices. The dimensionless pressure drop–velocity ratio characteristic curves for the three stenosis severities were obtained. At a fixed CO, the dimensionless pressure drop increased nonlinearly with an increase in (a) the velocity ratio for a fixed stenosis severity and (b) the stenosis severity at a specific velocity ratio. The dimensionless pressure drop observed in vivo was similar (within 1%) to that measured in moderate area stenosis of 70% because both flows were viscous dominated. The hemodynamics of the 3D printed test device can be used for evaluating diagnostic endpoints and medical devices in a preclinical setting under realistic conditions.

Evaluation of pulmonary artery stenosis in congenital heart disease patients using functional diagnostic parameters: An in vitro study

Congenital pulmonary artery (PA) stenosis is often associated with abnormal PA hemodynamics including increased pressure drop (Δp) and reduced asymmetric flow (Q), which may result in right ventricular dysfunction. We propose functional diagnostic parameters, pressure drop coefficient (CDP), energy loss (E_loss), and normalized energy loss (E¯_loss) to characterize pulmonary hemodynamics, and evaluate their efficacy in delineating stenosis severity using in vitro experiments. Subject-specific test sections including the main PA (MPA) bifurcating into left and right PAs (LPA, RPA) with a discrete LPA stenosis were manufactured from cross-sectional imaging and 3D printing. Three clinically-relevant stenosis severities, 90% area stenosis (AS), 80% AS, and 70% AS, were evaluated at different cardiac outputs (COs). A benchtop flow loop simulating pulmonary hemodynamics was used to measure Q and Δp within the test sections. The experimental Δp-Q characteristics along with clinical data were used to obtain pathophysiologic conditions and compute the diagnostic parameters. The pathophysiologic Q_LPA decreased as the stenosis severity increased at a fixed CO. CDP_LPA, E_loss,LPA (absolute), and E¯_loss,LPA (absolute) increased with an increase in LPA stenosis severity at a fixed CO. Importantly, CDP_LPA and E¯_loss,LPA had reduced variability with CO, and distinct values for each LPA stenosis severity. Under variable CO, a) CDP_LPA values were 14.5-21.0 (70% AS), 60.7- 2.2 (80% AS), ≥ 261.6 (90% AS), and b) E¯_loss,LPA values (in mJ per Q_LPA) were -501.9 to -1023.8 (70% AS), -1247.6 to -1773.0 (80% AS), -1934.5 (90% AS). Hence, CDP_LPA and E¯_loss,LPA are expected to assess the true functional severity of PA stenosis.

Giessen Procedure as Comprehensive Stage II Palliation With Aortic Arch Reconstruction After Hybrid Bilateral Pulmonary Artery Banding and Ductal Stenting for Hypoplastic Left Heart Syndrome

This article reviews our experience using hybrid stage I palliation in the neonatal period and subsequently with comprehensive stage II palliation for hypoplastic left heart syndrome. Between June 1998 and April 2017, 154 patients with the diagnosis of hypoplastic left heart syndrome and variants underwent a hybrid stage I palliation (bilateral pulmonary artery banding and ductal stenting). One hundred thirty-nine patients could be further univentricularly palliated. One hundred twenty-one patients underwent a comprehensive stage II operation with an operative mortality of 6.6%. The hybrid procedure provides reproducible results with reduced in-hospital, interstage, and long-term mortality and lower rates of aortic arch reinterventions.

New imaging tools in cardiovascular medicine: computational fluid dynamics and 4D flow MRI

Blood flow imaging is a novel technology in cardiovascular medicine and surgery. Today, two types of blood flow imaging tools are available: measurement-based flow visualization including 4D flow MRI (or 3D cine phase-contrast magnetic resonance imaging), or echocardiography flow visualization software, and computer flow simulation modeling based on computational fluid dynamics (CFD). MRI and echocardiography flow visualization provide measured blood flow but have limitations in temporal and spatial resolution, whereas CFD flow calculates the flow according to assumptions instead of flow measurement, and it has sufficiently fine resolution up to the computer memory limit, and it enables even virtual surgery when combined with computer graphics. Blood flow imaging provides profound insight into the pathophysiology of cardiovascular diseases, because it quantifies and visualizes mechanical stress on the vessel walls or heart ventricle. Wall shear stress (WSS) is a stress on the endothelial wall caused by the near wall blood flow, and it is thought to be a predictor of atherosclerosis progression in coronary or aortic diseases. Flow energy loss (EL) is the loss of blood flow energy caused by viscous friction of turbulent diseased flow, and it is expected to be a predictor of ventricular workload on various heart diseases including heart valve disease, cardiomyopathy, and congenital heart diseases. Blood flow imaging can provide useful information for developing predictive medicine in cardiovascular diseases, and may lead to breakthroughs in cardiovascular surgery, especially in the decision-making process.

Increased Energy Loss Due to Twist and Offset Buckling of the Total Cavopulmonary Connection

The hemodynamic energy loss through the surgically implanted conduits determines the postoperative cardiac output and exercise capacity following the palliative repair of single-ventricle congenital heart defects. In this study, the hemodynamics of severely deformed surgical pathways due to torsional deformation and anastomosis offset are investigated. We designed a mock-up total cavopulmonary connection (TCPC) circuit to replicate the mechanically failed inferior vena cava (IVC) anastomosis morphologies under physiological venous pressure (9, 12, 15 mmHg), in vitro, employing the commonly used conduit materials: Polytetrafluoroethylene (PTFE), Dacron, and porcine pericardium. The sensitivity of hemodynamic performance to torsional deformation for three different twist angles (0 deg, 30 deg, and 60 deg) and three different caval offsets (0 diameter (D), 0.5D, and 1D) are digitized in three dimensions and employed in computational fluid dynamic (CFD) simulations to determine the corresponding hydrodynamic efficiency levels. A total of 81 deformed conduit configurations are analyzed; the pressure drop values increased from 80 to 1070% with respect to the ideal uniform diameter IVC conduit flow. The investigated surgical materials resulted in significant variations in terms of flow separation and energy loss. For example, the porcine pericardium resulted in a pressure drop that was eight times greater than the Dacron conduit. Likewise, PTFE conduit resulted in a pressure drop that was three times greater than the Dacron conduit under the same venous pressure loading. If anastomosis twist and/or caval offset cannot be avoided intraoperatively due to the anatomy of the patient, alternative conduit materials with high structural stiffness and less influence on hemodynamics can be considered.

Hemodynamic effects of left pulmonary artery stenosis after superior cavopulmonary connection: a patient-specific multiscale modeling study

OBJECTIVE

Currently, no quantitative guidelines have been established for treatment of left pulmonary artery (LPA) stenosis. This study aims to quantify the effects of LPA stenosis on postoperative hemodynamics for single-ventricle patients undergoing stage II superior cavopulmonary connection (SCPC) surgery, using a multiscale computational approach.

METHODS

Image data from 6 patients were segmented to produce 3-dimensional models of the pulmonary arteries before stage II surgery. Pressure and flow measurements were used to tune a 0-dimensional model of the entire circulation. Postoperative geometries were generated through stage II virtual surgery; varying degrees of LPA stenosis were applied using mesh morphing and hemodynamics assessed through coupled 0-3-dimensional simulations. To relate metrics of stenosis to clinical classifications, pediatric cardiologists and surgeons ranked the degrees of stenosis in the models. The effects of LPA stenosis were assessed based on left-to-right pulmonary artery flow split ratios, mean pressure drop across the stenosis, cardiac pressure-volume loops, and other clinically relevant parameters.

RESULTS

Stenosis of >65% of the vessel diameter was required to produce a right pulmonary artery:LPA flow split <30%, and/or a mean pressure drop of >3.0 mm Hg, defined as clinically significant changes.

CONCLUSIONS

The effects of <65% stenosis on SCPC hemodynamics and physiology were minor and may not justify the increased complexity of adding LPA arterioplasty to the SCPC operation. However, in the longer term, pulmonary augmentation may affect outcomes of the Fontan completion surgery, as pulmonary artery distortion is a risk factor that may influence stage III physiology.

NUMERICAL STUDY OF BIDIRECTIONAL GLENN WITH UNILATERAL PULMONARY ARTERY STENOSIS

Purpose: Hypoplastic left heart syndrome (HLHS) is a congenital heart disease and is usually associated with pulmonary artery stenosis. The superior vena cava-to-pulmonary artery (bidirectional Glenn) shunt is used primarily as a staging procedure to the total cava-to-pulmonary connection for single-ventricle complex. When HLHS coexists with pulmonary artery stenosis, the surgeons then face a multiple problem. This leads to high demand of optimized structure of Glenn surgery. The objective of this article is to investigate the influence of various anastomotic structures and the direction of superior vena cava (SVC) in Glenn on hemodynamics under pulse inflow conditions and try to find an optimal structure of SVC in Glenn surgery with unilateral pulmonary artery stenosis.

Method: First, 3D patient-specific models were constructed from medical images of a HLHS patient before any surgery by using the commercial software Mimics, and another software Free-form was used to deform the reconstructed models in the computer. Four 3D patient-specific Glenn models were constructed: model-1 (normal Glenn), model-2 (lean the SVC back to the stenotic pulmonary artery), model-3 (lean the SVC towards the stenotic pulmonary artery), model-4 (add patch at junction of the SVC toward stenosis at pulmonary artery). Second, a lumped parameter model (LPM) was established to predict boundary conditions for computational fluid dynamics (CFD). In addition, numerical simulations were conducted using CFD through the finite volume method. Finally, hemodynamic parameters were obtained and evaluated.

Results: It was showed that model-4 have relatively balanced vena cava blood perfusion into the left pulmonary artery (LPA) and right pulmonary artery (RPA), this may be due to less helical flow and the patch at junction of the SVC. Near stenosis of pulmonary artery, model-4 performed with the higher wall shear stress (WSS), which would benefit endothelial cell function and gene expression. In addition, results showed that model-4 performed with the lower oscillatory shear index (OSI) and wall shear stress gradient (WSSG), which would decrease the opportunity of vascular intimal hyperplasia.

Conclusion: It is benefited that surgeons adds patch at junction of the SVC towards stenosis at pulmonary artery. These results can impact the surgical design and planning of the Glenn surgery with unilateral pulmonary artery stenosis.

Computer modeling to tailor therapy for congenital heart disease

The inherent structural and physiological complexity of congenital heart disease lends itself strongly to simulation. Complex hemodynamic and structural problems unique to congenital heart disease may be difficult to understand and the response to therapy or intervention uncertain. Methodologies borrowed from engineering, computing and mathematical sciences can be applied to such problems and used to inform clinical decisions. Therapy thus informed by modeling experiments has the potential to contribute significantly to improved clinical outcomes. This field remains in its infancy, and will only become used routinely if validation of current methods is carried out in the clinical setting.

Quantitative evaluation of hemodynamics in the Fontan circulation: a cross-sectional study measuring energy loss in vivo

Flow energy loss (EL) at the Fontan anastomosis has been thought to reflect flow efficiencies and to influence on hemodynamics in the Fontan circulation and has been often discussed in numerical studies. However, in vivo EL measurements have to date not been reported. We directly measured EL in the Fontan circulation and examined the relationship between the structural configuration and EL, as well as the influence of EL, on the hemodynamics in the Fontan circulation. We performed a catheterization study measuring simultaneous pressure and flow velocity to calculate EL in nine patients (mean age 2.3 ± 0.3 years) 1 year after the Fontan procedure. The measured EL was 9.66 ± 8.50 mW. One patient with left pulmonary artery stenosis recorded the highest EL (17.78 mW), and one patient with bilateral superior vena cava and left phrenic nerve palsy recorded the second highest EL (14.62 mW). EL significantly correlated with time constant tau and weakly with max-dp/dt during the isovolumic diastolic phase (r = 0.795 and -0.574, respectively). EL also correlated with max-dp/dt during the isovolumic systolic phase (r = 0.842) but not with ejection fraction or systemic blood flow (r = 0.384 and -0.034, respectively). In conclusion, inefficient structural configuration and phrenic nerve palsy seem to be related with increased in EL at the Fontan anastomosis. Although these preliminary findings also suggest that EL is associated with an impaired relaxation of the ventricle, a long-term study with a large population is warranted to reach such a definitive conclusion.

Fontan hemodynamics from 100 patient-specific cardiac magnetic resonance studies: a computational fluid dynamics analysis

OBJECTIVES

This study sought to quantify average hemodynamic metrics of the Fontan connection as reference for future investigations, compare connection types (intra-atrial vs extracardiac), and identify functional correlates using computational fluid dynamics in a large patient-specific cohort. Fontan hemodynamics, particularly power losses, are hypothesized to vary considerably among patients with a single ventricle and adversely affect systemic hemodynamics and ventricular function if suboptimal.

METHODS

Fontan connection models were created from cardiac magnetic resonance scans for 100 patients. Phase velocity cardiac magnetic resonance in the aorta, vena cavae, and pulmonary arteries was used to prescribe patient-specific time-averaged flow boundary conditions for computational fluid dynamics with a customized, validated solver. Comparison with 4-dimensional cardiac magnetic resonance velocity data from selected patients was used to provide additional verification of simulations. Indexed Fontan power loss, connection resistance, and hepatic flow distribution were quantified and correlated with systemic patient characteristics.

RESULTS

Indexed power loss varied by 2 orders of magnitude, whereas, on average, Fontan resistance was 15% to 20% of published values of pulmonary vascular resistance in single ventricles. A significant inverse relationship was observed between indexed power loss and both systemic venous flow and cardiac index. Comparison by connection type showed no differences between intra-atrial and extracardiac connections. Instead, the least efficient connections revealed adverse consequences from localized Fontan pathway stenosis.

CONCLUSIONS

Fontan power loss varies from patient to patient, and elevated levels are correlated with lower systemic flow and cardiac index. Fontan connection type does not influence hemodynamic efficiency, but an undersized or stenosed Fontan pathway or pulmonary arteries can be highly dissipative.

Mock circulatory system of the Fontan circulation to study respiration effects on venous flow behavior

We describe an in vitro model of the Fontan circulation with respiration to study subdiaphragmatic venous flow behavior. The venous and arterial connections of a total cavopulmonary connection (TCPC) test section were coupled with a physical lumped parameter (LP) model of the circulation. Intrathoracic and subdiaphragmatic pressure changes associated with normal breathing were applied. This system was tuned for two patients (5 years, 0.67 m2; 10 years, 1.2 m2) to physiological values. System function was verified by comparison to the analytical model on which it was based and by consistency with published clinical measurements. Overall, subdiaphragmatic venous flow was influenced by respiration. Flow within the arteries and veins increased during inspiration but decreased during expiration, with retrograde flow in the inferior venous territories. System pressures and flows showed close agreement with the analytical LP model (p < 0.05). The ratio of the flow rates occurring during inspiration to expiration were within the clinical range of values reported elsewhere. The approach used to set up and control the model was effective and provided reasonable comparisons with clinical data.

Fontan conversion templates: patient-specific hemodynamic performance of the lateral tunnel versus the intraatrial conduit with fenestration

Intraatrial-conduit Fontan is considered a modification of both extracardiac and lateral-tunnel Fontan. In this study, the patient-specific hemodynamic performance of intraatrial-conduit and lateral-tunnel Fontan with fenestration, considered as conversion templates, was investigated based on the authors' patient cohort. Pulsatile computational fluid dynamics simulations were performed using patient-specific models of intraatrial-conduit and lateral-tunnel Fontan patients. Real-time "simultaneous" inferior and superior vena cava, pulmonary artery, and fenestration flow waveforms were acquired from ultrasound. Multiple hemodynamic performance indices were investigated, with particular focus on evaluation of the pulsatile flow performance. Power loss inside the lateral-tunnel Fontan appeared to be significantly higher than with the intraatrial-conduit Fontan for patient-specific cardiac output and normalized connection size. Inclusion of the 4-mm fenestration at a 0.24 L/min mean flow resulted in a lower cavopulmonary pressure gradient and less time-averaged power loss for both Fontan connections. Flow structures within the intraatrial conduit were notability more uniform than within the lateral tunnel. Hepatic flow majorly favored the left lung in both surgical connections: conversion from lateral-tunnel to intraatrial-conduit Fontan resulted in better hemodynamics with less power loss, a lower pressure gradient, and fewer stagnant flow zones along the conduit. This patient-specific computational case study demonstrated superior hemodynamics of intraatrial-conduit Fontan over those of lateral-tunnel Fontan with or without fenestration and improved performance after conversion of the lateral tunnel to the intraatrial conduit. The geometry-specific effect of the nonuniform hepatic flow distribution may motivate new rationales for the surgical design.

Total cavopulmonary connection in patients with apicocaval juxtaposition: optimal conduit route using preoperative angiogram and flow simulation

OBJECTIVES

Single ventricle with apicocaval juxtaposition (ACJ) is a rare, complex anomaly, in which the optimal position of the conduit for completion of total cavopulmonary connection (TCPC) is still controversial. The purpose of this study was to identify a preoperative method for optimal conduit position using the IVC anatomy and computational fluid dynamics (CFD).

METHODS

Twenty-four patients with ACJ (5.3 ± 5.7 years) who underwent TCPC were enrolled. A conduit was placed ipsilateral to the cardiac apex in each of 11 patients, of which 9 were intra-atrial and 2 extracardiac (group A) and, in a further 13 patients, extracardiac on the contralateral side (group B). As control, 10 patients with tricuspid atresia were also enrolled (group C). The location of the IVC in relation to the spine was evaluated from the frontal view of preoperative angiogram, using the following index: IVC-index = IVC width overlapping the vertebra/width of the vertebra × 100%. Energy loss was calculated by CFD simulation.

RESULTS

IVC-index of group B was larger than groups A and C (45 ± 26 vs. 20 ± 21 and 28 ± 19%, P = 0.03). Postoperative catheterizations showed that, due to its curvature, conduit length in group B was significantly longer than the others (65 ± 12 vs. 36 ± 14 and 44 ± 10 mm, P < 0.001), although there was no statistical difference in central venous pressure or cardiac output. CFD studies revealed less energy loss in group A conduits compared with group B (1.6 ± 0.3 vs. 3.6 ± 0.6 mW, P = 0.05), although this did not appear to be clinically significant. Moreover, CFD simulation showed significant energy loss within the Fontan circulation when the conduit was either compressed or kinked: 4.9 and 18.2 mW respectively.

CONCLUSIONS

In patients with ACJ, placement of a straighter and shorter conduit on the ventricular apical side provides better laminar blood flow with less energy loss. However, conduit compression and kinking are far more detrimental to the Fontan circulation. A preoperative IVC-index is pivotal for avoiding these factors and deciding the optimal conduit route.

Treatment planning for a TCPC test case: a numerical investigation under rigid and moving wall assumptions

The hemodynamics in patients with total cavopulmonary connections (TCPC) is generally very complex and characterized by patient-to-patient variability. To better understand its effect on patients' outcome, CFD models are widely used, also to test and optimize surgical options before their implementation. These models often assume rigid geometries, despite the motion experienced by thoracic vessels that could influence the hemodynamics predictions. By improving their accuracy and expanding the range of simulated interventions, the benefit of treatment planning for patients is expected to increase. We simulate three types of intervention on a patient-specific 3D model, and compare their predicted outcome with baseline condition: a decrease in pulmonary vascular resistance obtainable with medications; a surgical revision of the connection design; the introduction of a fenestration in the TCPC wall. The simulations are performed both with rigid wall assumption and including patient-specific TCPC wall motion, reconstructed from a 4DMRI dataset. The results show the effect of each option on clinically important metrics and highlight the impact of patient-specific wall motion. The largest differences between rigid and moving wall models are observed in measures of energetic efficiency of TCPC as well as in hepatic flow distribution and transit time of seeded particles through the connection.

Numerical study for blood flow in pulmonary arteries after repair of tetralogy of Fallot

Pulmonary regurgitation (PR) is a common phenomenon in pulmonary arteries in patients after repair of tetralogy of Fallot (TOF). The regurgitation fraction of left pulmonary artery (LPA) is usually greater than right pulmonary artery (RPA) according to clinic data. It may be related to blood flow in pulmonary arteries. Therefore, understanding hemodynamics in pulmonary arteries helps to comprehend the reason. The aim of this study is to use 3D reconstructed pulmonary artery models from magnetic resonance imaging (MRI) and to use numerical approaches for simulation of flow variations in pulmonary arteries after repair of TOF. From the numerical results, the blood flow is influenced by the bifurcation angles and geometry of pulmonary artery. The regurgitation happens first in LPA after repair of TOF due to the small angle between LPA and main pulmonary artery (MPA). The recirculation region which obstructs forward blood flow to the left lung is found in LPA during acceleration of systole. We also analyze the pressure distribution; the extreme pressure variations are in dilation area of MPA. Numerical data including regurgitation in MPA, LPA, and RPA are compared with phase contrast MR measured data. Good agreements are found between numerical results and measured data.

Presurgical evaluation of Fontan connection options for patients with apicocaval juxtaposition using computational fluid dynamics

Apicocaval juxtaposition (ACJ) is a rare congenital heart defect associated with single ventricle physiology where optimal positioning of the Fontan conduit for completion of total cavopulmonary connection (TCPC) is still controversial. In ACJ, the cardiac apex is ipsilateral with the inferior vena cava (IVC), risking kinking and collapse of the Fontan conduit at the apex of the heart. The purpose of this study is to evaluate two viable routes for Fontan conduit connection in patients with ACJ, using computational fluid dynamics. Internal energy loss evaluations were used to determine contribution of conduit curvature to the energy efficiency of each cavopulmonary anastomosis configuration. This percentage of energy loss contribution was found to be greater in the case of a curved extracardiac conduit connection (44%, 4.1 mW) traveling behind the ventricular apex, connecting the IVC to the left pulmonary artery, than the straighter lateral tunnel conduit (6%, 1.4 mW) installed through the ventricular apex. In contrast, net energy loss across the anastomosis was significantly lower with extracardiac TCPC (9.3 mW) in comparison with lateral tunnel TCPC (23.2 mW), highlighting that a curved Fontan conduit is favorable provided that it is traded off for a superior cavopulmonary connection efficiency. Therefore, a relatively longer and curved Fontan conduit has been demonstrated to be a suitable connection option independent of anatomical situations.

Pulsatile venous waveform quality affects the conduit performance in functional and "failing" Fontan circulations

OBJECTIVE

To investigate the effect of pulsatility of venous flow waveform in the inferior and superior caval vessels on the performance of functional and "failing" Fontan patients based on two primary performance measures - the conduit power loss and the distribution of inferior caval flow (hepatic factors) to the lungs.

METHODS

Doppler angiography flows were acquired from two typical extra-cardiac conduit "failing" Fontan patients, aged 13 and 25 years, with ventricle dysfunction. Using computational fluid dynamics, haemodynamic efficiencies of "failing", functional, and in vitro-generated mechanically assisted venous flow waveforms were evaluated inside an idealised total cavopulmonary connection with a caval offset. To investigate the effect of venous pulsatility alone, cardiac output was normalised to 3 litres per minute in all cases. To quantify the pulsatile behaviour of venous flows, two new performance indices were suggested.

RESULTS

Variations in the pulsatile content of venous waveforms altered the conduit efficiency notably. High-frequency and low-amplitude oscillations lowered the pulsatile component of the power losses in "failing" Fontan flow waveforms. Owing to the offset geometry, hepatic flow distribution depended strongly on the ratio of time-dependent caval flows and the pulsatility content rather than mixing at the junction. "Failing" Fontan flow waveforms exhibited less balanced hepatic flow distribution to lungs.

CONCLUSIONS

The haemodynamic efficiency of single-ventricle circulation depends strongly on the pulsatility of venous flow waveforms. The proposed performance indices can be calculated easily in the clinical setting in efforts to better quantify the energy efficiency of Fontan venous waveforms in pulsatile settings.

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.

Risk analysis of unruptured aneurysms using computational fluid dynamics technology: preliminary results

BACKGROUND AND PURPOSE

The decision as to the treatment of incidental IAs is complex. There are no certain quantitative methods that can be used to evaluate the risk of rupture in IAs. In recent years, CFD technology has been recognized as a potential risk-analysis tool. The aim of this article was to propose a hemodynamic parameter, EL, to determine the effects of stable unruptured aneurysms and of those that ruptured during the subsequent observation period.

MATERIALS AND METHODS

Four incidentally found ICA-PcomA aneurysms ruptured during the period of observation (ruptured-IAs). Another 26 unruptured aneurysms (stable-IAs) with similar location, size, and morphology were compared for the differences in hemodynamic factors, such as EL and WSS.

RESULTS

The EL calculated at the ruptured-IAs was nearly 5 times higher on average than that at the stable-IAs (ruptured, 0.00374 ± 0.0011; stable, 0.000745 ± 0.0001 mW/mm(3), P < .001). However, there was no difference between the ruptured and stable groups according to the results of time-averaged WSS (P = .8) for ruptured- and stable-IAs. According to flow visualization, though the mean average inflow speed of ruptured-IAs was 2 times higher than that of the stable-IAs, the flow inside ruptured-IAs appeared to undergo longer resident tracks, with stronger impact on the aneurysm wall. On the contrary, the flow inside stable-IAs passed smoothly through the aneurysms.

CONCLUSIONS

These preliminary results indicated that EL may be a useful parameter for the quantitative estimation of the risks of rupture for IAs.

Computer-Aided Patient-Specific Coronary Artery Graft Design Improvements Using CFD Coupled Shape Optimizer

This study aims to (i) demonstrate the efficacy of a new surgical planning framework for complex cardiovascular reconstructions, (ii) develop a computational fluid dynamics (CFD) coupled multi-dimensional shape optimization method to aid patient-specific coronary artery by-pass graft (CABG) design and, (iii) compare the hemodynamic efficiency of the sequential CABG, i.e., raising a daughter parallel branch from the parent CABG in patient-specific 3D settings. Hemodynamic efficiency of patient-specific complete revascularization scenarios for right coronary artery (RCA), left anterior descending artery (LAD), and left circumflex artery (LCX) bypasses were investigated in comparison to the stenosis condition. Multivariate 2D constraint optimization was applied on the left internal mammary artery (LIMA) graft, which was parameterized based on actual surgical settings extracted from 2D CT slices. The objective function was set to minimize the local variation of wall shear stress (WSS) and other hemodynamic indices (energy dissipation, flow deviation angle, average WSS, and vorticity) that correlate with performance of the graft and risk of re-stenosis at the anastomosis zone. Once the optimized 2D graft shape was obtained, it was translated to 3D using an in-house "sketch-based" interactive anatomical editing tool. The final graft design was evaluated using an experimentally validated second-order non-Newtonian CFD solver incorporating resistance based outlet boundary conditions. 3D patient-specific simulations for the healthy coronary anatomy produced realistic coronary flows. All revascularization techniques restored coronary perfusions to the healthy baseline. Multi-scale evaluation of the optimized LIMA graft enabled significant wall shear stress gradient (WSSG) relief (~34%). In comparison to original LIMA graft, sequential graft also lowered the WSSG by 15% proximal to LAD and diagonal bifurcation. The proposed sketch-based surgical planning paradigm evaluated the selected coronary bypass surgery procedures based on acute hemodynamic readjustments of aorta-CA flow. This methodology may provide a rational to aid surgical decision making in time-critical, patient-specific CA bypass operations before in vivo execution.

Larger aortic reconstruction corresponds to diminished left pulmonary artery size in patients with single-ventricle physiology

BACKGROUND

Pulmonary artery size is a crucial determinant of hemodynamic energy loss in total cavopulmonary connections. We investigated the effect of aortic arch reconstruction on left pulmonary artery size based on their anatomic proximity.

METHODS

Thirty-two patients undergoing the Fontan operation, 16 with hypoplastic left heart syndrome and 16 with non-hypoplastic left heart syndrome, were selected from the multicenter Fontan magnetic resonance imaging database at the Georgia Institute of Technology. The 16 datasets were consecutive with full anatomic reconstructions of the total cavopulmonary connection and aortic arch with no artifacts. The size of the aorta along the transverse arch and left pulmonary artery size in the region below the aortic arch was quantified by using a previously validated skeletonization technique.

RESULTS

The transverse aortic and left pulmonary artery measurements (median, maximum, and minimum, respectively) for non-hypoplastic left heart syndrome were 2.2, 3.1, and 1.5 cm/m and 1.2, 1.6, and 0.2 cm/m, respectively, compared with 2.5, 4.1, and 2.0 cm/m and 0.9, 1.5, and 0.4 cm/m for patients with hypoplastic left heart syndrome. Thus the transverse aortic diameter of patients with hypoplastic left heart syndrome was, on average, 24% greater than that for patients with non-hypoplastic left heart syndrome (P < .05), whereas the left pulmonary artery diameter of patients with hypoplastic left heart syndrome was smaller than that of patients with non-hypoplastic left heart syndrome (P < .05). Regression analysis showed a significant negative correlation (P < .05) between aortic and left pulmonary artery diameters in both the hypoplastic left heart syndrome and non-hypoplastic left heart syndrome groups. However, when the study population was regrouped into reconstructed aorta and nonreconstructed aorta groups, the negative correlation was only significant for patients with reconstructed aortas, regardless of ventricular pathology (P < .02).

CONCLUSIONS

Stage 1 aortic reconstruction procedures that result in a large aorta limit left pulmonary artery size in patients undergoing the Fontan operation.

Modelling in congenital heart disease. Art or science?

Despite the advances in imaging modalities and surgical techniques, the management of adults with congenital heart disease (ACHD) over the years has remained largely empirical rather than evidence-based. Animal models have been difficult to develop and very costly, while clinical trials are difficult to design and perform in ACHD, leaving gaps in our understanding of the pathophysiology and treatment of congenital heart disease. Disease modelling, both hypothetical and patient-specific, provides an alternative solution to many of these problems. Advances in cardiovascular imaging and diagnostics have led to the easy acquisition of large quantities of structural and functional information, which cannot be handled "intuitively". Computational modelling introduces mathematical rigour in the analysis and utilisation of these data by quantitative simulation and testing of clinically relevant hypotheses through experimentally validated models. Close multidisciplinary collaboration between bioengineers and clinicians is essential for transforming data and images derived from models of disease into clinically useful information.

Hemodynamic energy dissipation in the cardiovascular system: generalized theoretical analysis on disease states

BACKGROUND

We present a fundamental theoretical framework for analysis of energy dissipation in any component of the circulatory system and formulate the full energy budget for both venous and arterial circulations. New indices allowing disease-specific subject-to-subject comparisons and disease-to-disease hemodynamic evaluation (quantifying the hemodynamic severity of one vascular disease type to the other) are presented based on this formalism.

METHODS AND RESULTS

Dimensional analysis of energy dissipation rate with respect to the human circulation shows that the rate of energy dissipation is inversely proportional to the square of the patient body surface area and directly proportional to the cube of cardiac output. This result verified the established formulae for energy loss in aortic stenosis that was solely derived through empirical clinical experience. Three new indices are introduced to evaluate more complex disease states: (1) circulation energy dissipation index (CEDI), (2) aortic valve energy dissipation index (AV-EDI), and (3) total cavopulmonary connection energy dissipation index (TCPC-EDI). CEDI is based on the full energy budget of the circulation and is the proper measure of the work performed by the ventricle relative to the net energy spent in overcoming frictional forces. It is shown to be 4.01+/-0.16 for healthy individuals and above 7.0 for patients with severe aortic stenosis. Application of CEDI index on single-ventricle venous physiology reveals that the surgically created Fontan circulation, which is indeed palliative, progressively degrades in hemodynamic efficiency with growth (p<0.001), with the net dissipation in a typical Fontan patient (Body surface area=1.0 m(2)) being equivalent to that of an average case of severe aortic stenosis. AV-EDI is shown to be the proper index to gauge the hemodynamic severity of stenosed aortic valves as it accurately reflects energy loss. It is about 0.28+/-0.12 for healthy human valves. Moderate aortic stenosis has an AV-EDI one order of magnitude higher while clinically severe aortic stenosis cases always had magnitudes above 3.0. TCPC-EDI represents the efficiency of the TCPC connection and is shown to be negatively correlated to the size of a typical "bottle-neck" region (pulmonary artery) in the surgical TCPC pathway (p<0.05).

CONCLUSIONS

Energy dissipation in the human circulation has been analyzed theoretically to derive the proper scaling (indexing) factor. CEDI, AV-EDI, and TCPC-EDI are proper measures of the dissipative characteristics of the circulatory system, aortic valve, and the Fontan connection, respectively.

Patient-specific modeling of cardiovascular mechanics

Advances in numerical methods and three-dimensional imaging techniques have enabled the quantification of cardiovascular mechanics in subject-specific anatomic and physiologic models. Patient-specific models are being used to guide cell culture and animal experiments and test hypotheses related to the role of biomechanical factors in vascular diseases. Furthermore, biomechanical models based on noninvasive medical imaging could provide invaluable data on the in vivo service environment where cardiovascular devices are employed and on the effect of the devices on physiologic function. Finally, patient-specific modeling has enabled an entirely new application of cardiovascular mechanics, namely predicting outcomes of alternate therapeutic interventions for individual patients. We review methods to create anatomic and physiologic models, obtain properties, assign boundary conditions, and solve the equations governing blood flow and vessel wall dynamics. Applications of patient-specific models of cardiovascular mechanics are presented, followed by a discussion of the challenges and opportunities that lie ahead.

Modified control grid interpolation for the volumetric reconstruction of fluid flows

Complex applications in fluid dynamics research often require more highly resolved velocity data than direct measurements or simulations provide. The advent of stereo PIV and PCMR techniques has advanced the state-of-the-art in flow velocity measurement, but 3D spatial resolution remains limited. Here a new technique is proposed for velocity data interpolation to address this problem. The new method performs with higher quality than competing solutions from the literature in terms of accurately interpolating velocities, maintaining fluid structure and domain boundaries, and preserving coherent structures.

Hemodynamic performance of stage-2 univentricular reconstruction: Glenn vs. hemi-Fontan templates

Flow structures, hemodynamics and the hydrodynamic surgical pathway resistances of the final stage functional single ventricle reconstruction, namely the total cavopulmonary connection (TCPC) anatomy, have been investigated extensively. However, the second stage surgical anatomy (i.e., bi-directional Glenn or hemi-Fontan template) has received little attention. We thus initiated a multi-faceted study, involving magnetic resonance imaging (MRI), phase contrast MRI, computational and experimental fluid dynamics methodologies, focused on the second stage of the procedure. Twenty three-dimensional computer and rapid prototype models of 2nd stage TCPC anatomies were created, including idealized parametric geometries (n = 6), patient-specific anatomies (n = 7), and their virtual surgery variant (n = 7). Results in patient-specific and idealized models showed that the Glenn connection template is hemodynamically more efficient with (83% p = 0.08 in patient-specific models and 66% in idealized models) lower power losses compared to hemi-Fontan template, respectively, due to its direct end-to-side anastomosis. Among the several secondary surgical geometrical features, stenosis at the SVC anastomosis or in pulmonary branches was found to be the most critical parameter in increasing the power loss. The pouch size and flare shape were found to be less significant. Compared to the third stage surgery the hydrodynamic resistance of the 2nd stage is considerably lower (both in idealized models and in anatomical models at MRI resting conditions) for both hemi- and Glenn templates. These results can impact the surgical design and planning of the staged TCPC reconstruction.

The total cavopulmonary connection resistance: a significant impact on single ventricle hemodynamics at rest and exercise

Little is known about the impact of the total cavopulmonary connection (TCPC) on resting and exercise hemodynamics in a single ventricle (SV) circulation. The aim of this study was to elucidate this mechanism using a lumped parameter model of the SV circulation. Pulmonary vascular resistance (1.96+/-0.80 WU) and systemic vascular resistances (18.4+/-7.2 WU) were obtained from catheterization data on 40 patients with a TCPC. TCPC resistances (0.39+/-0.26 WU) were established using computational fluid dynamic simulations conducted on anatomically accurate three-dimensional models reconstructed from MRI (n=16). These parameters were used in a lumped parameter model of the SV circulation to investigate the impact of TCPC resistance on SV hemodynamics under resting and exercise conditions. A biventricular model was used for comparison. For a biventricular circulation, the cardiac output (CO) dependence on TCPC resistance was negligible (sensitivity=-0.064 l.min(-1).WU(-1)) but not for the SV circulation (sensitivity=-0.88 l.min(-1).WU(-1)). The capacity to increase CO with heart rate was also severely reduced for the SV. At a simulated heart rate of 150 beats/min, the SV patient with the highest resistance (1.08 WU) had a significantly lower increase in CO (20.5%) compared with the SV patient with the lowest resistance (50%) and normal circulation (119%). This was due to the increased afterload (+35%) and decreased preload (-12%) associated with the SV circulation. In conclusion, TCPC resistance has a significant impact on resting hemodynamics and the exercise capacity of patients with a SV physiology.

Neonatal Aortic Arch Hemodynamics and Perfusion During Cardiopulmonary Bypass

The objective of this study is to quantify the detailed three-dimensional (3D) pulsatile hemodynamics, mechanical loading, and perfusion characteristics of a patient-specific neonatal aortic arch during cardiopulmonary bypass (CPB). The 3D cardiac magnetic resonance imaging (MRI) reconstruction of a pediatric patient with a normal aortic arch is modified based on clinical literature to represent the neonatal morphology and flow conditions. The anatomical dimensions are verified from several literature sources. The CPB is created virtually in the computer by clamping the ascending aorta and inserting the computer-aided design model of the 10 Fr tapered generic cannula. Pulsatile (130 bpm) 3D blood flow velocities and pressures are computed using the commercial computational fluid dynamics (CFD) software. Second order accurate CFD settings are validated against particle image velocimetry experiments in an earlier study with a complex cardiovascular unsteady benchmark. CFD results in this manuscript are further compared with the in vivo physiological CPB pressure waveforms and demonstrated excellent agreement. Cannula inlet flow waveforms are measured from in vivo PC-MRI and 3 kg piglet neonatal animal model physiological experiments, distributed equally between the head-neck vessels and the descending aorta. Neonatal 3D aortic hemodynamics is also compared with that of the pediatric and fetal aortic stages. Detailed 3D flow fields, blood damage, wall shear stress (WSS), pressure drop, perfusion, and hemodynamic parameters describing the pulsatile energetics are calculated for both the physiological neonatal aorta and for the CPB aorta assembly. The primary flow structure is the high-speed canulla jet flow (∼3.0 m/s at peak flow), which eventually stagnates at the anterior aortic arch wall and low velocity flow in the cross-clamp pouch. These structures contributed to the reduced flow pulsatility (85%), increased WSS (50%), power loss (28%), and blood damage (288%), compared with normal neonatal aortic physiology. These drastic hemodynamic differences and associated intense biophysical loading of the pathological CPB configuration necessitate urgent bioengineering improvements—in hardware design, perfusion flow waveform, and configuration. This study serves to document the baseline condition, while the methodology presented can be utilized in preliminary CPB cannula design and in optimization studies reducing animal experiments. Coupled to a lumped-parameter model the 3D hemodynamic characteristics will aid the surgical decision making process of the perfusion strategies in complex congenital heart surgeries.

Numerical investigation of regurgitation phenomena in pulmonary arteries of Tetralogy of Fallot patients after repair

Pulmonary regurgitation is a very common phenomenon in pulmonary arteries after repair of patients of Tetralogy of Fallot (TOF) which is the most common complex congenital heart diseases. The aim of this study is to use numerical approaches to simulate flow variations in pulmonary artery after repair of patients of TOF. We analyze the flow patterns in an in-vitro bifurcation pulmonary artery and consider effects of various regurgitation fractions (RF or b/f) in left pulmonary artery (LPA) and right pulmonary artery (RPA). We not only observe the variation of flow patterns, but also analyze the results of b/f and net volumetric flow rates in LPA and RPA. In general, the b/f of LPA is higher than RPA in the measured data provided by phase-contrast magnetic resonance imaging (PC-MRI). We validate the result using numerical approaches to analyze the flow patterns in pulmonary artery in this study. The results will be useful for medical doctors when they perform operations for TOF patients.