Fluid mechanics of blood flow in human fetal left ventricles based on patient-specific 4D ultrasound scans

Assessment of fetal intraventricular diastolic fluid dynamics using ultrasound vector flow mapping

OBJECTIVE

The purpose of this study was to investigate the feasibility of visualizing and quantifying the normal pattern of vortex formation in the left ventricle (LV) and right ventricle (RV) of the fetal heart during diastole using vector flow mapping (VFM).

METHODS

A total of 36 healthy fetuses in the second trimester (mean gestational age: 23 weeks, 2 days; range: 22-24 weeks) were enrolled in the study. Color Doppler signals were recorded in the four-chamber view to observe the phase of the diastolic vortices in the LV and RV. The vortex area and circulation were measured, and parameters such as intraventricular pressure difference (IVPD), intraventricular pressure gradient (IVPG), and average energy loss (EL_AVG) were evaluated at different diastolic phases, including isovolumic relaxation (D1), early diastole (D2), and late diastole (D3).

RESULTS

Healthy second-trimester fetal vortex formations were observed in both the LV and RV at the end of diastole, with the vortices rotating in a clockwise direction towards the outflow tract. There were no significant differences in vortex area and circulation between the two ventricles (p > 0.05). However, significant differences were found in IVPD, IVPG, and EL_AVG among the diastolic phases (D1, D2, and D3) (p < 0.05). Trends in IVPD, IVPG, and EL_AVG during diastole (D1-D2-D3) revealed increasing IVPD and EL_AVG values, as well as decreasing IVPG values. Furthermore, during D3, the RV exhibited significantly higher IVPD, IVPG, and EL_AVG compared to the LV (p > 0.05).

CONCLUSION

VFM is a valuable technique for analyzing the formation of vortices in the left and right ventricles during fetal diastole. The application of VFM technology has the potential to enhance the assessment of fetal cardiac parameters.

Recasting Current Knowledge of Human Fetal Circulation: The Importance of Computational Models

Computational hemodynamic simulations are becoming increasingly important for cardiovascular research and clinical practice, yet incorporating numerical simulations of human fetal circulation is relatively underutilized and underdeveloped. The fetus possesses unique vascular shunts to appropriately distribute oxygen and nutrients acquired from the placenta, adding complexity and adaptability to blood flow patterns within the fetal vascular network. Perturbations to fetal circulation compromise fetal growth and trigger the abnormal cardiovascular remodeling that underlies congenital heart defects. Computational modeling can be used to elucidate complex blood flow patterns in the fetal circulatory system for normal versus abnormal development. We present an overview of fetal cardiovascular physiology and its evolution from being investigated with invasive experiments and primitive imaging techniques to advanced imaging (4D MRI and ultrasound) and computational modeling. We introduce the theoretical backgrounds of both lumped-parameter networks and three-dimensional computational fluid dynamic simulations of the cardiovascular system. We subsequently summarize existing modeling studies of human fetal circulation along with their limitations and challenges. Finally, we highlight opportunities for improved fetal circulation models.

Fluid Mechanical Effects of Fetal Aortic Valvuloplasty for Cases of Critical Aortic Stenosis with Evolving Hypoplastic Left Heart Syndrome

Fetuses with critical aortic stenosis (FAS) are at high risk of progression to HLHS by the time of birth (and are thus termed "evolving HLHS"). An in-utero catheter-based intervention, fetal aortic valvuloplasty (FAV), has shown promise as an intervention strategy to circumvent the progression, but its impact on the heart's biomechanics is not well understood. We performed patient-specific computational fluid dynamic (CFD) simulations based on 4D fetal echocardiography to assess the changes in the fluid mechanical environment in the FAS left ventricle (LV) directly before and 2 days after FAV. Echocardiograms of five FAS cases with technically successful FAV were retrospectively analysed. FAS compromised LV stroke volume and ejection fraction, but FAV rescued it significantly. Calculations to match simulations to clinical measurements showed that FAV approximately doubled aortic valve orifice area, but it remained much smaller than in healthy hearts. Diseased LVs had mildly stenotic mitral valves, which generated fast and narrow diastolic mitral inflow jet and vortex rings that remained unresolved directly after FAV. FAV further caused aortic valve damage and high-velocity regurgitation. The high-velocity aortic regurgitation jet and vortex ring caused a chaotic flow field upon impinging the apex, which drastically exacerbated the already high energy losses and poor flow energy efficiency of FAS LVs. Two days after the procedure, FAV did not alter wall shear stress (WSS) spatial patterns of diseased LV but elevated WSS magnitudes, and the poor blood turnover in pre-FAV LVs did not significantly improve directly after FAV. FAV improved FAS LV's flow function, but it also led to highly chaotic flow patterns and excessively high energy losses due to the introduction of aortic regurgitation directly after the intervention. Further studies analysing the effects several weeks after FAV are needed to understand the effects of such biomechanics on morphological development.

Fluid Mechanics of Fetal Left Ventricle During Aortic Stenosis with Evolving Hypoplastic Left Heart Syndrome

In cases of fetal aortic stenosis and evolving Hypoplastic Left Heart Syndrome (feHLHS), aortic stenosis is associated with specific abnormalities such as retrograde or bidirectional systolic transverse arch flow. Many cases progressed to hypoplastic left heart syndrome (HLHS) malformation at birth, but fetal aortic valvuloplasty can prevent the progression in many cases. Since both disease and intervention involve drastic changes to the biomechanical environment, in-vivo biomechanics likely play a role in inducing and preventing disease progression. However, the fluid mechanics of feHLHS is not well-characterized. Here, we conduct patient-specific echocardiography-based flow simulations of normal and feHLHS left ventricles (LV), to understand the essential fluid dynamics distinction between the two cohorts. We found high variability across feHLHS cases, but also the following unifying features. Firstly, feHLHS diastole mitral inflow was in the form of a narrowed and fast jet that impinged onto the apical region, rather than a wide and gentle inflow in normal LVs. This was likely due to a malformed mitral valve with impaired opening dynamics. This altered inflow caused elevated vorticity dynamics and wall shear stresses (WSS) and reduced oscillatory shear index at the apical zone rather than mid-ventricle. Secondly, feHLHS LV also featured elevated systolic and diastolic energy losses, intraventricular pressure gradients, and vortex formation numbers, suggesting energy inefficiency of flow and additional burden on the LV. Thirdly, feHLHS LV had poor blood turnover, suggesting a hypoxic environment, which could be associated with endocardial fibroelastosis that is often observed in these patients.

Hemodynamic and Structural Comparison of Human Fetal Heart Development Between Normally Growing and Hypoplastic Left Heart Syndrome-Diagnosed Hearts

Congenital heart defects (CHDs) affect a wide range of societies with an incidence rate of 1.0–1.2%. These defects initiate at the early developmental stage and result in critical health disorders. Although genetic factors play a role in the formation of CHDs, the occurrence of cases in families with no history of CHDs suggests that mechanobiological forces may also play a role in the initiation and progression of CHDs. Hypoplastic left heart syndrome (HLHS) is a critical CHD, which is responsible for 25–40% of all prenatal cardiac deaths. The comparison of healthy and HLHS hearts helps in understanding the main hemodynamic differences related to HLHS. Echocardiography is the most common imaging modality utilized for fetal cardiac assessment. In this study, we utilized echocardiographic images to compare healthy and HLHS human fetal hearts for determining the differences in terms of heart chamber dimensions, valvular flow rates, and hemodynamics. The cross-sectional areas of chamber dimensions are determined from 2D b-mode ultrasound images. Valvular flow rates are measured via Doppler echocardiography, and hemodynamic quantifications are performed with the use of computational fluid dynamics (CFD) simulations. The obtained results indicate that cross-sectional areas of the left and right sides of the heart are similar for healthy fetuses during gestational development. The left side of HLHS heart is underdeveloped, and as a result, the hemodynamic parameters such as flow velocity, pressure, and wall shear stress (WSS) are significantly altered compared to those of healthy hearts.

Numerical Investigation of the Fetal Left Heart Hemodynamics During Gestational Stages

Flow-driven hemodynamic forces on the cardiac tissues have critical importance, and have a significant role in the proper development of the heart. These mechanobiological mechanisms govern the cellular responses for the growth and remodeling of the heart, where the altered hemodynamic environment is believed to be a major factor that is leading to congenital heart defects (CHDs). In order to investigate the mechanobiological development of the normal and diseased hearts, identification of the blood flow patterns and wall shear stresses (WSS) on these tissues are required for an accurate hemodynamic assessment. In this study, we focus on the left heart hemodynamics of the human fetuses throughout the gestational stages. Computational fetal left heart models are created for the healthy fetuses using the ultrasound images at various gestational weeks. Realistic inflow boundary conditions are implemented in the models using the Doppler ultrasound measurements for resolving the specific blood flow waveforms in the mitral valve. Obtained results indicate that WSS and vorticity levels in the fetal left heart decrease with the development of the fetus. The maximum WSS around the mitral valve is determined around 36 Pa at the gestational week of 16. This maximum WSS decreases to 11 Pa at the gestational week of 27, indicating nearly three-times reduction in the peak shear stress. These findings reveal the highly dynamic nature of the left heart hemodynamics throughout the development of the human fetus and shed light into the relevance of hemodynamic environment and development of CHDs.

Mending a broken heart: In vitro, in vivo and in silico models of congenital heart disease

Birth defects contribute to ∼0.3% of global infant mortality in the first month of life, and congenital heart disease (CHD) is the most common birth defect among newborns worldwide. Despite the significant impact on human health, most treatments available for this heterogenous group of disorders are palliative at best. For this reason, the complex process of cardiogenesis, governed by multiple interlinked and dose-dependent pathways, is well investigated. Tissue, animal and, more recently, computerized models of the developing heart have facilitated important discoveries that are helping us to understand the genetic, epigenetic and mechanobiological contributors to CHD aetiology. In this Review, we discuss the strengths and limitations of different models of normal and abnormal cardiogenesis, ranging from single-cell systems and 3D cardiac organoids, to small and large animals and organ-level computational models. These investigative tools have revealed a diversity of pathogenic mechanisms that contribute to CHD, including genetic pathways, epigenetic regulators and shear wall stresses, paving the way for new strategies for screening and non-surgical treatment of CHD. As we discuss in this Review, one of the most-valuable advances in recent years has been the creation of highly personalized platforms with which to study individual diseases in clinically relevant settings.

A transition point for the blood flow wall shear stress environment in the human fetal left ventricle during early gestation

Development of the fetal heart is a fascinating process that involves a tremendous amount of growth. Here, we performed image-based flow simulations of 3 human fetal left ventricles (LV), and investigated the hypothetical scenario where the sizes of the hearts are scaled down, leading to reduced Reynolds number, to emulate earlier fetal stages. The shape and motion of the LV were retained over the scaling to isolate and understand the effects of length scaling on its fluid dynamics. We observed an interesting cut-off point in Reynolds number (Re), across which the dependency of LV wall shear stress (WSS) on Re changed. This was in line with classical fluid mechanic theory where skin friction coefficient exhibited first a decreasing trend and then a plateauing trend with increasing Re. Below this cut-off point, viscous effects dominated, stifling the formation of LV diastolic vorticity structures, and WSS was roughly independent of Reynolds number. However, above this cut-off, inertial effects dominated to cause diastolic vortex ring formation and detachment, and to cause WSS to scale linearly with Reynolds number. Results suggested that this transition point is found at approximately 11 weeks of gestation. Since WSS is thought to be a biomechanical stimuli for growth, this may have implications on normal fetal heart growth and malformation diseases like Hypoplastic Left Heart Syndrome.

Computational Modeling of Blood Flow Hemodynamics for Biomechanical Investigation of Cardiac Development and Disease

The heart is the first functional organ in a developing embryo. Cardiac development continues throughout developmental stages while the heart goes through a serious of drastic morphological changes. Previous animal experiments as well as clinical observations showed that disturbed hemodynamics interfere with the development of the heart and leads to the formation of a variety of defects in heart valves, heart chambers, and blood vessels, suggesting that hemodynamics is a governing factor for cardiogenesis, and disturbed hemodynamics is an important source of congenital heart defects. Therefore, there is an interest to image and quantify the flowing blood through a developing heart. Flow measurement in embryonic fetal heart can be performed using advanced techniques such as magnetic resonance imaging (MRI) or echocardiography. Computational fluid dynamics (CFD) modeling is another approach especially useful when the other imaging modalities are not available and in-depth flow assessment is needed. The approach is based on numerically solving relevant physical equations to approximate the flow hemodynamics and tissue behavior. This approach is becoming widely adapted to simulate cardiac flows during the embryonic development. While there are few studies for human fetal cardiac flows, many groups used zebrafish and chicken embryos as useful models for elucidating normal and diseased cardiogenesis. In this paper, we explain the major steps to generate CFD models for simulating cardiac hemodynamics in vivo and summarize the latest findings on chicken and zebrafish embryos as well as human fetal hearts.

A pilot study of bladder voiding with real-time MRI and computational fluid dynamics

Lower urinary track symptoms (LUTS) affect many older adults. Multi-channel urodynamic studies provide information about bladder pressure and urinary flow but offer little insight into changes in bladder anatomy and detrusor muscle function. Here we present a novel method for real time MRI during bladder voiding. This was performed in a small cohort of healthy men and men with benign prostatic hyperplasia and lower urinary tract symptoms (BPH/LUTS) to demonstrate proof of principle; The MRI urodynamic protocol was successfully implemented, and bladder wall displacement and urine flow dynamics were calculated. Displacement analysis on healthy controls showed the greatest bladder wall displacement in the dome of the bladder while men with BPH/LUTS exhibited decreased and asymmetric bladder wall motion. Computational fluid dynamics of voiding showed men with BPH/LUTS had larger recirculation regions in the bladder. This study demonstrates the feasibility of performing MRI voiding studies and their potential to provide new insight into lower urinary tract function in health and disease.

Right ventricle in hypoplastic left heart syndrome exhibits altered hemodynamics in the human fetus

Hypoplastic left heart syndrome (HLHS) represents approximately 9% of all congenital heart defects and is one of the most complex, with the left side of the heart being generally underdeveloped. Numerous studies demonstrate that intracardiac fluid flow patterns in the embryonic and fetal circulation can impact cardiac structural formation and remodeling. This highlights the importance of quantifying the altered hemodynamic environment in congenital heart defects, like HLHS, relative to a normal heart as it relates to cardiac development. Therefore, to study human cardiovascular fetal flow, computational fluid dynamic simulations were performed using 4D patient-specific ultrasound scans in normal and HLHS hearts. In these simulations, we find that the HLHS right ventricle exhibits a greater cardiac output than normal; yet, hemodynamics are relatively similar between normal and HLHS right ventricles. Overall, this study provides detailed quantitative flow patterns for HLHS, which has the potential to guide future prevention and therapeutic interventions, while more immediately providing additional functional detail to cardiologists to aid in decision making.

Impact of Aortoseptal Angle Abnormalities and Discrete Subaortic Stenosis on Left-Ventricular Outflow Tract Hemodynamics: Preliminary Computational Assessment

Discrete subaortic stenosis (DSS) is an obstruction of the left ventricular outflow tract (LVOT) due to the formation of a fibromuscular membrane upstream of the aortic valve. DSS is a major risk factor for aortic regurgitation (AR), which often persists after surgical resection of the membrane. While the etiology of DSS and secondary AR is largely unknown, the frequent association between DSS and aortoseptal angle (AoSA) abnormalities has supported the emergence of a mechanobiological pathway by which hemodynamic stress alterations on the septal wall could trigger a biological cascade leading to fibrosis and membrane formation. The resulting LVOT flow disturbances could activate the valve endothelium and contribute to AR. In an effort to assess this hypothetical mechano-etiology, this study aimed at isolating computationally the effects of AoSA abnormalities on septal wall shear stress (WSS), and the impact of DSS on LVOT hemodynamics. Two-dimensional computational fluid dynamics models featuring a normal AoSA (N-LV), a steep AoSA (S-LV), and a steep AoSA with a DSS lesion (DSS-LV) were designed to compute the flow in patient-specific left ventricles (LVs). Boundary conditions consisted of transient velocity profiles at the mitral inlet and LVOT outlet, and patient-specific LV wall motion. The deformation of the DSS lesion was computed using a two-way fluid-structure interaction modeling strategy. Turbulence was accounted for via implementation of the k-ω turbulence model. While the N-LV and S-LV models generated similar LVOT flow characteristics, the DSS-LV model resulted in an asymmetric LVOT jet-like structure, subaortic stenotic conditions (up to 2.4-fold increase in peak velocity, 45% reduction in effective jet diameter vs. N-LV/S-LV), increased vorticity (2.8-fold increase) and turbulence (5- and 3-order-of-magnitude increase in turbulent kinetic energy and Reynolds shear stress, respectively). The steep AoSA subjected the septal wall to a 23% and 69% overload in temporal shear magnitude and gradient, respectively, without any substantial change in oscillatory shear index. This study reveals the existence of WSS overloads on septal wall regions prone to DSS lesion formation in steep LVOTs, and the development of highly turbulent, stenotic and asymmetric flow in DSS LVOTs, which support a possible mechano etiology for DSS and secondary AR.

Torsional motion of the left ventricle does not affect ventricular fluid dynamics of both foetal and adult hearts

Left ventricular torsion is caused by shortening and relaxation of the helical fibres in the myocardium, and is thought to be an optimal configuration for minimizing myocardial tissue strains. Characteristics of torsional motion has also been proposed to be markers for cardiac dysfunction. However, its effects on fluid and energy dynamics in the left ventricle have not been comprehensively investigated. To investigate this, we performed image-based flow simulations on five healthy adult porcine and two healthy human foetal left ventricles (representing two different length scales) at different degrees of torsional motions. In the adult porcine ventricles, cardiac features such as papillary muscles and mitral valves, and cardiac conditions such as myocardial infarctions, were also included to investigate the effect of twist. The results showed that, for all conditions investigated, ventricular torsional motion caused minimal changes to flow patterns, and consistently accounted for less than 2% of the energy losses, wall shear stresses, and ejection momentum energy. In contrast, physiological characteristics such as chamber size, stroke volume and heart rate had a much greater influence on flow patterns and energy dynamics. The results thus suggested that it might not be necessary to model the torsional motion to study the flow and energy dynamics in left ventricles.

Fluid dynamics and forces in the HH25 avian embryonic outflow tract

The embryonic outflow tract (OFT) eventually undergoes aorticopulmonary septation to form the aorta and pulmonary artery, and it is hypothesized that blood flow mechanical forces guide this process. We performed detailed studies of the geometry, wall motions, and fluid dynamics of the HH25 chick embryonic OFT just before septation, using noninvasive 4D high-frequency ultrasound and computational flow simulations. The OFT exhibited expansion and contraction waves propagating from proximal to distal end, with periods of luminal collapse at locations of the two endocardial cushions. This, combined with periods of reversed flow, resulted in the OFT cushions experiencing wall shear stresses (WSS or flow drag forces) with elevated oscillatory characteristics, which could be important to signal for further development of cushions into valves and septum. Furthermore, the OFT exhibits interesting double-helical flow during systole, where a pair of helical flow structures twisted about each other from the proximal to distal end. This coincided with the location of the future aorticopulmonary septum, which also twisted from the proximal to distal end, suggesting that this flow pattern may be guiding OFT septation.

Quantifying blood flow dynamics during cardiac development: demystifying computational methods

Blood flow conditions (haemodynamics) are crucial for proper cardiovascular development. Indeed, blood flow induces biomechanical adaptations and mechanotransduction signalling that influence cardiovascular growth and development during embryonic stages and beyond. Altered blood flow conditions are a hallmark of congenital heart disease, and disrupted blood flow at early embryonic stages is known to lead to congenital heart malformations. In spite of this, many of the mechanisms by which blood flow mechanics affect cardiovascular development remain unknown. This is due in part to the challenges involved in quantifying blood flow dynamics and the forces exerted by blood flow on developing cardiovascular tissues. Recent technologies, however, have allowed precise measurement of blood flow parameters and cardiovascular geometry even at early embryonic stages. Combined with computational fluid dynamics techniques, it is possible to quantify haemodynamic parameters and their changes over development, which is a crucial step in the quest for understanding the role of mechanical cues on heart and vascular formation. This study summarizes some fundamental aspects of modelling blood flow dynamics, with a focus on three-dimensional modelling techniques, and discusses relevant studies that are revealing the details of blood flow and their influence on cardiovascular development.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

Human fetal hearts with tetralogy of Fallot have altered fluid dynamics and forces

Studies have suggested the effect of blood flow forces in pathogenesis and progression of some congenital heart malformations. It is therefore of interest to study the fluid mechanic environment of the malformed prenatal heart, such as the tetralogy of Fallot (TOF), especially when little is known about fetal TOF. In this study, we performed patient-specific ultrasound-based flow simulations of three TOF and seven normal human fetal hearts. TOF right ventricles (RVs) had smaller end-diastolic volumes (EDVs) but similar stroke volumes (SVs), whereas TOF left ventricles (LVs) had similar EDVs but slightly increased SVs compared with normal ventricles. Simulations showed that TOF ventricles had elevated systolic intraventricular pressure gradient (IVPG) and required additional energy for ejection but IVPG elevations were considered to be mild relative to arterial pressure. TOF RVs and LVs had similar pressures because of equalization via ventricular septal defect (VSD). Furthermore, relative to normal, TOF RVs had increased diastolic wall shear stresses (WSS) but TOF LVs were not. This was caused by high tricuspid inflow that exceeded RV SV, leading to right-to-left shunting and chaotic flow with enhanced vorticity interaction with the wall to elevate WSS. Two of the three TOF RVs but none of the LVs had increased thickness. As pressure elevations were mild, we hypothesized that pressure and WSS elevation could play a role in the RV thickening, among other causative factors. Finally, the endocardium surrounding the VSD consistently experienced high WSS because of RV-to-LV flow shunt and high flow rate through the over-riding aorta. NEW & NOTEWORTHY Blood flow forces are thought to cause congenital heart malformations and influence disease progression. We performed novel investigations of intracardiac fluid mechanics of tetralogy of Fallot (TOF) human fetal hearts and found essential differences from normal hearts. The TOF right ventricle (RV) and left ventricle had similar and elevated pressure but only the TOF RV had elevated wall shear stress because of elevated tricuspid inflow, and this may contribute to the observed RV thickening. TOF hearts also expended more energy for ejection.

Characterization of the hemodynamic wall shear stresses in human umbilical vessels from normal and intrauterine growth restricted pregnancies

Significant reductions in blood flow and umbilical diameters were reported in pregnancies affected by intrauterine growth restriction (IUGR) from placental insufficiency. However, it is not known if IUGR umbilical blood vessels experience different hemodynamic wall shear stresses (WSS) compared to normal umbilical vessels. As WSS is known to influence vasoactivity and vascular growth and remodeling, which can regulate flow rates, it is important to study this parameter. In this study, we aim to characterize umbilical vascular WSS environment in normal and IUGR pregnancies, and evaluate correlation between WSS and vascular diameter, and gestational age. Twenty-two normal and 21 IUGR pregnancies were assessed via ultrasound between the 27th and 39th gestational week. IUGR was defined as estimated fetal weight and/or abdominal circumference below the 10th centile, with no improvement during the remainder of the pregnancy. Vascular diameter was determined by 3D ultrasound scans and image segmentation. Umbilical artery (UA) WSS was computed via computational flow simulations, while umbilical vein (UV) WSS was computed via the Poiseuille equation. Univariate multiple regression analysis was used to test for the differences between normal and IUGR cohort. UV volumetric flow rate, UA and UV diameters were significantly lower in IUGR fetuses, but flow velocities and WSS trends in UA and UV were very similar between normal and IUGR groups. In both groups, UV WSS showed a significant negative correlation with diameter, but UA WSS had no correlation with diameter, suggesting a constancy of WSS environment and the existence of WSS homeostasis in UA, but not in UV. Despite having reduced flow rate and vascular sizes, IUGR UAs had hemodynamic mechanical stress environments and trends that were similar to those in normal pregnancies. This suggested that endothelial dysfunction or abnormal mechanosensing was unlikely to be the cause of small vessels in IUGR umbilical cords.

Fetal regional myocardial strain rate in the membranous ventricular septum: changes with gestational age and the left ventricular mass and predictive value for a complete membranous ventricular septum (without defect)

To describe the fetal regional myocardial strain rate in the membranous ventricular septum across gestation and to determine their predictive value for a complete membranous ventricular septum (without defect) after delivery. In 1150 fetuses, the peak systolic strain rate (SRs), peak early diastolic strain rate (SRe) and peak late diastolic strain rate (SRa) in the membranous ventricular septum were measured at four time points across gestation (18-20, 24-26, 30-32 and 36-38 weeks). The integrity of the interventricular septum was examined at 12 weeks' postnatal age. The correlations between myocardial strain rates and gestational age as well as fetal left ventricular mass were analyzed, and the performance of myocardial strain rates in predicting a complete membranous ventricular septum was deducted. Strain rate absolute values in the membranous ventricular septum all increased across gestation. They all significantly correlated with gestational age and left ventricular mass. At 24 weeks during pregnancy, the areas under the receiver operating characteristics curve (AUC) for SRe and SRa were all > 0.72 (p < 0.05) in predicting a complete membranous ventricular septum, while the AUC for SRs was only 0.55. The sensitivity, specificity and accuracy of the cut off value (> 1.53 s^-1) for SRe was 62.5, 85.7 and 73.3%, respectively, and the sensitivity, specificity and accuracy of the cut off value (> 1.51 s^-1) for SRa was 75.2, 71.9 and 73.8%, respectively. The changes of myocardial strain rates in the membranous ventricular septum across gestation maybe can be used to predict a complete membranous ventricular septum after delivery.

Feasibility of Ultrasound-Based Computational Fluid Dynamics as a Mitral Valve Regurgitation Quantification Technique: Comparison with 2-D and 3-D Proximal Isovelocity Surface Area-Based Methods

Current Doppler echocardiography quantification of mitral regurgitation (MR) severity has shortcomings. Proximal isovelocity surface area (PISA)-based methods, for example, are unable to account for the fact that ultrasound Doppler can measure only one velocity component: toward or away from the transducer. In the present study, we used ultrasound-based computational fluid dynamics (Ub-CFD) to quantify mitral regurgitation and study its advantages and disadvantages compared with 2-D and 3-D PISA methods. For Ub-CFD, patient-specific mitral valve geometry and velocity data were obtained from clinical ultrasound followed by 3-D CFD simulations at an assumed flow rate. We then obtained the average ratio of the ultrasound Doppler velocities to CFD velocities in the flow convergence region, and scaled CFD flow rate with this ratio as the final measured flow rate. We evaluated Ub-CFD, 2-D PISA and 3-D PISA with an in vitro flow loop, which featured regurgitation flow through (i) a simplified flat plate with round orifice and (ii) a 3-D printed realistic mitral valve and regurgitation orifice. The Ub-CFD and 3-D PISA methods had higher precision than the 2-D PISA method. Ub-CFD had consistent accuracy under all conditions tested, whereas 2-D PISA had the lowest overall accuracy. In vitro investigations indicated that the accuracy of 2-D and 3-D PISA depended significantly on the choice of aliasing velocity. Evaluation of these techniques was also performed for two clinical cases, and the dependency of PISA on aliasing velocity was similarly observed. Ub-CFD was robustly accurate and precise and has promise for future translation to clinical practice.

Flow dynamics and energy efficiency of flow in the left ventricle during myocardial infarction

Cardiovascular disease is a leading cause of death worldwide, where myocardial infarction (MI) is a major category. After infarction, the heart has difficulty providing sufficient energy for circulation, and thus, understanding the heart's energy efficiency is important. We induced MI in a porcine animal model via circumflex ligation and acquired multiple-slice cine magnetic resonance (MR) images in a longitudinal manner-before infarction, and 1 week (acute) and 4 weeks (chronic) after infarction. Computational fluid dynamic simulations were performed based on MR images to obtain detailed fluid dynamics and energy dynamics of the left ventricles. Results showed that energy efficiency flow through the heart decreased at the acute time point. Since the heart was observed to experience changes in heart rate, stroke volume and chamber size over the two post-infarction time points, simulations were performed to test the effect of each of the three parameters. Increasing heart rate and stroke volume were found to significantly decrease flow energy efficiency, but the effect of chamber size was inconsistent. Strong complex interplay was observed between the three parameters, necessitating the use of non-dimensional parameterization to characterize flow energy efficiency. The ratio of Reynolds to Strouhal number, which is a form of Womersley number, was found to be the most effective non-dimensional parameter to represent energy efficiency of flow in the heart. We believe that this non-dimensional number can be computed for clinical cases via ultrasound and hypothesize that it can serve as a biomarker for clinical evaluations.

Fluid mechanics of human fetal right ventricles from image-based computational fluid dynamics using 4D clinical ultrasound scans

There are 0.6–1.9% of US children who were born with congenital heart malformations. Clinical and animal studies suggest that abnormal blood flow forces might play a role in causing these malformation, highlighting the importance of understanding the fetal cardiovascular fluid mechanics. We performed computational fluid dynamics simulations of the right ventricles, based on four-dimensional ultrasound scans of three 20-wk-old normal human fetuses, to characterize their flow and energy dynamics. Peak intraventricular pressure gradients were found to be 0.2–0.9 mmHg during systole, and 0.1–0.2 mmHg during diastole. Diastolic wall shear stresses were found to be around 1 Pa, which could elevate to 2–4 Pa during systole in the outflow tract. Fetal right ventricles have complex flow patterns featuring two interacting diastolic vortex rings, formed during diastolic E wave and A wave. These rings persisted through the end of systole and elevated wall shear stresses in their proximity. They were observed to conserve ∼25.0% of peak diastolic kinetic energy to be carried over into the subsequent systole. However, this carried-over kinetic energy did not significantly alter the work done by the heart for ejection. Thus, while diastolic vortexes played a significant role in determining spatial patterns and magnitudes of diastolic wall shear stresses, they did not have significant influence on systolic ejection. Our results can serve as a baseline for future comparison with diseased hearts.

Patient-specific CFD models for intraventricular flow analysis from 3D ultrasound imaging: Comparison of three clinical cases

BACKGROUND

As the intracardiac flow field is affected by changes in shape and motility of the heart, intraventricular flow features can provide diagnostic indications. Ventricular flow patterns differ depending on the cardiac condition and the exploration of different clinical cases can provide insights into how flow fields alter in different pathologies.

METHODS

In this study, we applied a patient-specific computational fluid dynamics model of the left ventricle and mitral valve, with prescribed moving boundaries based on transesophageal ultrasound images for three cardiac pathologies, to verify the abnormal flow patterns in impaired hearts. One case (P1) had normal ejection fraction but low stroke volume and cardiac output, P2 showed low stroke volume and reduced ejection fraction, P3 had a dilated ventricle and reduced ejection fraction.

RESULTS

The shape of the ventricle and mitral valve, together with the pathology influence the flow field in the left ventricle, leading to distinct flow features. Of particular interest is the pattern of the vortex formation and evolution, influenced by the valvular orifice and the ventricular shape. The base-to-apex pressure difference of maximum 2mmHg is consistent with reported data.

CONCLUSION

We used a CFD model with prescribed boundary motion to describe the intraventricular flow field in three patients with impaired diastolic function. The calculated intraventricular flow dynamics are consistent with the diagnostic patient records and highlight the differences between the different cases. The integration of clinical images and computational techniques, therefore, allows for a deeper investigation intraventricular hemodynamics in patho-physiology.

Patient-specific CFD simulation of intraventricular haemodynamics based on 3D ultrasound imaging

BACKGROUND

The goal of this paper is to present a computational fluid dynamic (CFD) model with moving boundaries to study the intraventricular flows in a patient-specific framework. Starting from the segmentation of real-time transesophageal echocardiographic images, a CFD model including the complete left ventricle and the moving 3D mitral valve was realized. Their motion, known as a function of time from the segmented ultrasound images, was imposed as a boundary condition in an Arbitrary Lagrangian-Eulerian framework.

RESULTS

The model allowed for a realistic description of the displacement of the structures of interest and for an effective analysis of the intraventricular flows throughout the cardiac cycle. The model provides detailed intraventricular flow features, and highlights the importance of the 3D valve apparatus for the vortex dynamics and apical flow.

CONCLUSIONS

The proposed method could describe the haemodynamics of the left ventricle during the cardiac cycle. The methodology might therefore be of particular importance in patient treatment planning to assess the impact of mitral valve treatment on intraventricular flow dynamics.