Semiautomated method for noise reduction and background phase error correction in MR phase velocity data

Background phase distortion and random noise can adversely affect the quality of magnetic resonance (MR) phase velocity measurements. A semiautomated method has been developed that substantially reduces both effects. To remove the background phase distortion, the following steps were taken: The time standard deviations of the phase velocity images over a cardiac cycle were calculated. Static regions were identified as those in which the standard deviation was low. A flat surface representing an approximation to the background distortion was fitted to the static regions and subtracted from the phase velocity images to give corrected phase images. Random noise was removed by setting to zero those regions in which the standard deviation was high. The technique is demonstrated with a sample set of data in which the in-plane velocities have been measured in an imaging section showing the left ventricular outflow tract of a human left ventricle. The results are presented in vector and contour form, superimposed on the conventional MR angiographic images.

Heart valve function: a biomechanical perspective

Heart valves (HVs) are cardiac structures whose physiological function is to ensure directed blood flow through the heart over the cardiac cycle. While primarily passive structures that are driven by forces exerted by the surrounding blood and heart, this description does not adequately describe their elegant and complex biomechanical function. Moreover, they must replicate their cyclic function over an entire lifetime, with an estimated total functional demand of least 3x10(9) cycles. As in many physiological systems, one can approach HV biomechanics from a multi-length-scale approach, since mechanical stimuli occur and have biological impact at the organ, tissue and cellular scales. The present review focuses on the functional biomechanics of HVs. Specifically, we refer to the unique aspects of valvular function, and how the mechanical and mechanobiological behaviours of the constituent biological materials (e.g. extracellular matrix proteins and cells) achieve this remarkable feat. While we focus on the work from the authors' respective laboratories, the works of most investigators known to the authors have been included whenever appropriate. We conclude with a summary and underscore important future trends.

Standardized Definition of Structural Valve Degeneration for Surgical and Transcatheter Bioprosthetic Aortic Valves

Bioprostheses are prone to structural valve degeneration, resulting in limited long-term durability. A significant challenge when comparing the durability of different types of bioprostheses is the lack of a standardized terminology for the definition of a degenerated valve. This issue becomes especially important when we try to compare the degeneration rate of surgically inserted and transcatheter bioprosthetic valves. This document, by the VIVID (Valve-in-Valve International Data), proposes practical and standardized definitions of valve degeneration and provides recommendations for the timing of clinical and imaging follow-up assessments accordingly. Its goal is to improve the quality of research and clinical care for patients with deteriorated bioprostheses by providing objective and strict criteria that can be utilized in future clinical trials. We hope that the adoption of these criteria by both the cardiological and surgical communities will lead to improved comparability and interpretation of durability analyses.

Review of hydrodynamic principles for the cardiologist: applications to the study of blood flow and jets by imaging techniques

An understanding of the basic concepts of the physics of blood flow is of vital importance to the cardiologist as he or she attempts to utilize new blood flow imaging modalities, such as Doppler ultrasound and nuclear magnetic resonance imaging. Concepts such as the Bernoulli equation and its limitations, the continuity equation and volume flow calculations and the theory of free and confined jets have applications in cardiac blood flow-related problems. For example, mitral regurgitant flow may be treated with the free jet theory. Aortic stenosis results in confined jet flow. It is important that the cardiologist understand the basic principles behind these hydrodynamic concepts so that he or she can use them in appropriate applications. The limitations of the simplification of complex hydrodynamic relations that are used clinically need to be clearly understood so that these simplified principles are not used improperly or used to draw oversimplified conclusions.

Integrated mechanism for functional mitral regurgitation: leaflet restriction versus coapting force: in vitro studies

BACKGROUND

Functional mitral regurgitation in patients with ischemic or dilated ventricles has been related to competing factors: altered tension on the leaflets due to displacement of their papillary muscle and annular attachments, which restricts leaflet closure, versus global ventricular dysfunction with reduced transmitral pressure to close the leaflets. In vivo, however, geometric changes accompany dysfunction, making it difficult to study these factors independently. Functional mitral regurgitation also paradoxically decreases in midsystole, despite peak transmitral driving pressure, suggesting a change in the force balance acting to create a regurgitant orifice, with rising transmitral pressure counteracting forces that restrict leaflet closure. In vivo, this mechanism cannot be tested independently of annular contraction that could also reduce midsystolic regurgitation.

METHODS AND RESULTS

An in vitro model was developed that allows independent variation of papillary muscle position, annular size, and transmitral pressure, with direct regurgitant flow rate measurement, to test the hypothesis that functional mitral regurgitation reflects an altered balance of forces acting on the leaflets. Hemodynamic and echocardiographic measurements of excised porcine valves were made under physiological pressures and flows. Apical and posterolateral papillary muscle displacement caused decreased leaflet mobility and apical leaflet tethering or tenting with regurgitation, as seen clinically. It reproduced the clinically observed midsystolic decrease in regurgitant flow and orifice area as transmitral pressure increased. Tethering delayed valve closure, increased the early systolic regurgitant volume before complete coaptation, and decreased the duration of coaptation. Annular dilatation increased regurgitation for any papillary muscle position, creating clinically important regurgitation; conversely, increased transmitral pressure decreased regurgitant orifice area for any geometric configuration.

CONCLUSIONS

The clinically observed tented-leaflet configuration and dynamic regurgitant orifice area variation can be reproduced in vitro by altering the three-dimensional relationship of the annular and papillary muscle attachments of the valve so as to increase leaflet tension. Increased transmitral pressure acting to close the leaflets decreases the regurgitant orifice area. These results are consistent with a mechanism in which an altered balance of tethering versus coapting forces acting on the leaflets creates the regurgitant orifice.

A new method for quantification of regurgitant flow rate using color Doppler flow imaging of the flow convergence region proximal to a discrete orifice. An in vitro study

While color Doppler flow mapping has yielded a quick and relatively sensitive method for visualizing the turbulent jets generated in valvular insufficiency, quantification of the degree of valvular insufficiency has been limited by the dependence of visualization of turbulent jets on hemodynamic as well as instrument-related factors. Color Doppler flow imaging, however, does have the capability of reliably showing the spatial relations of laminar flows. An area where flow accelerates proximal to a regurgitant orifice is commonly visualized on the left ventricular side of a mitral regurgitant orifice, especially when imaging is performed with high gain and a low pulse repetition frequency. This area of flow convergence, where the flow stream narrows symmetrically, can be quantified because velocity and the flow cross-sectional area change in inverse proportion along streamlines centered at the orifice. In this study, a gravity-driven constant-flow system with five sharp-edged diaphragm orifices (ranging from 2.9 to 12 mm in diameter) was imaged both parallel and perpendicular to the direction of flow through the orifice. Color Doppler flow images were produced by zero shifting so that the abrupt change in display color occurred at different velocities. This "aliasing boundary" with a known velocity and a measurable radial distance from the center of the orifice was used to determine an isovelocity hemisphere such that flow rate through the orifice was calculated as 2 pi r2 x Vr, where r is the radial distance from the center of the orifice to the color change and Vr is the velocity at which the color change was noted. Using Vr values from 54 to 14 cm/sec obtained with a 3.75-MHz transducer and from 75 to 18 cm/sec obtained with a 2.5-MHz transducer, we calculated flow rates and found them to correlate with measured flow rates (r = 0.94-0.99). The slope of the regression line was closest to unity when the lowest Vr and the correspondingly largest r were used in the calculation. The flow rates estimated from color Doppler flow imaging could also be used in conjunction with continuous-wave Doppler measurements of the maximal velocity of flow through the orifice to calculate orifice areas (r = 0.75-0.96 correlation with measured areas).(ABSTRACT TRUNCATED AT 250 WORDS)

Chordal cutting: a new therapeutic approach for ischemic mitral regurgitation

BACKGROUND

Mitral regurgitation (MR) conveys adverse prognosis in ischemic heart disease. Because such MR is related to increased leaflet tethering by displaced attachments to the papillary muscles (PMs), it is incompletely treated by annular reduction. We therefore addressed the hypothesis that such MR can be reduced by cutting a limited number of critically positioned chordae to the leaflet base that most restrict closure but are not required to prevent prolapse. This was tested in 8 mitral valves: a porcine in vitro pilot with PM displacement and 7 sheep with acute inferobasal infarcts studied in vivo with three-dimensional (3D) echo to quantify MR in relation to 3D valve geometry.

METHODS AND RESULTS

In all 8 valves, PM displacement restricted leaflet closure, with anterior leaflet angulation at the basal chord insertion, and mild-to-moderate MR. Cutting the 2 central basal chordae reversed this without prolapse. In vivo, MR increased from 0.8+/-0.2 to 7.1+/-0.5 mL/beat after infarction and then decreased to 0.9+/-0.1 mL/beat with chordal cutting (P<0.0001); this paralleled changes in the 3D leaflet area required to cover the orifice as dictated by chordal tethering (r(2)=0.76).

CONCLUSIONS

Cutting a minimum number of basal chordae can improve coaptation and reduce ischemic MR. Such an approach also suggests the potential for future minimally invasive implementation.

Fluid Mechanics of Heart Valves

Valvular heart disease is a life-threatening disease that afflicts millions of people worldwide and leads to approximately 250,000 valve repairs and/or replacements each year. Malfunction of a native valve impairs its efficient fluid mechanic/hemodynamic performance. Artificial heart valves have been used since 1960 to replace diseased native valves and have saved millions of lives. Unfortunately, despite four decades of use, these devices are less than ideal and lead to many complications. Many of these complications/problems are directly related to the fluid mechanics associated with the various mechanical and bioprosthetic valve designs. This review focuses on the state-of-the-art experimental and computational fluid mechanics of native and prosthetic heart valves in current clinical use. The fluid dynamic performance characteristics of caged-ball, tilting-disc, bileaflet mechanical valves and porcine and pericardial stented and nonstented bioprostheic valves are reviewed. Other issues related to heart valve performance, such as biomaterials, solid mechanics, tissue mechanics, and durability, are not addressed in this review.

Altered shear stress stimulates upregulation of endothelial VCAM-1 and ICAM-1 in a BMP-4- and TGF-beta1-dependent pathway

OBJECTIVE

Hemodynamics has been associated with aortic valve (AV) inflammation, but the underlying mechanisms are not well understood. Here we tested the hypothesis that altered shear stress conditions stimulate the expression of cytokines and adhesion molecules in AV leaflets via a bone morphogenic protein (BMP)- and transforming growth fact (TGF)-beta1-dependent pathway.

METHODS AND RESULTS

The ventricularis or aortic surface of porcine AV leaflets were exposed for 48 hours to unidirectional pulsatile and bidirectional oscillatory shear stresses ex vivo. Immunohistochemistry was performed to detect expressions of the 4 inflammatory markers VCAM-1, ICAM-1, BMP-4, and TGF-beta1. Exposure of the aortic surface to pulsatile shear stress (altered hemodynamics), but not oscillatory shear stress, increased expression of the inflammatory markers. In contrast, neither pulsatile nor oscillatory shear stress affected expression of the inflammatory markers on the ventricularis surface. The shear stress-dependent expression of VCAM-1, ICAM-1, and BMP-4, but not TGF-beta1, was significantly reduced by the BMP inhibitor noggin, whereas the TGF-beta1 inhibitor SB431542 blocked BMP-4 expression on the aortic surface exposed to pulsatile shear stress.

CONCLUSIONS

The results demonstrate that altered hemodynamics stimulates the expression of AV leaflet endothelial adhesion molecules in a TGF-beta1- and BMP-4-dependent manner, providing some potential directions for future drug-based therapies for AV diseases.

Left ventricular blood flow patterns in normal subjects: a quantitative analysis by three-dimensional magnetic resonance velocity mapping

OBJECTIVES

Magnetic resonance velocity mapping was used to investigate the hypothesis of a vortex motion within the left ventricle interacting with mitral valve motion and inflow velocity.

BACKGROUND

In vitro flow visualization studies have suggested the presence of a large anterior vortex inside the left ventricle during mitral inflow. However, to our knowledge the occurrence of this phenomenon has not been demonstrated in the human left ventricle.

METHODS

Magnetic resonance velocity mapping was performed in 26 healthy volunteers using a flow-adjusted gradient sequence for three-dimensional flow velocity acquisition in the long-axis plane of the left ventricle. By computer processing, the flow vectors in the left ventricle were visualized and animated dynamically.

RESULTS

The early diastolic mitral inflow was apically directed, and a large counterclockwise anterior vortex was created within the left ventricle shortly after the onset of the mid-diastolic semiclosure of the anterior mitral leaflet. During mid-diastolic diastasis, mitral inflow ceased until the flow accelerated again at atrial systole. The final closure of the mitral valve was preceded by a smaller vortex seen at the tips of the mitral leaflets. At systolic ejection, all flow vectors were directed toward the left ventricular outflow tract. The anterior vortex had a radius of 1.62 +/- 0.24 cm (mean +/- SD), and the average angular velocity (i.e., the rotation of an element about the center of the vortex within the central core) was 30.08 +/- 9.98 radians/s. The maximal kinetic energy of the anterior vortex was 4.3 x 10(-4) +/- 7.1 x 10(-5) J.

CONCLUSIONS

The hypothesis of a diastolic vortex formation in the human left ventricle was confirmed, and its close temporal relation to the motion of the anterior mitral leaflet was demonstrated.

Papillary muscle displacement causes systolic anterior motion of the mitral valve. Experimental validation and insights into the mechanism of subaortic obstruction

BACKGROUND

Systolic anterior motion (SAM) of the mitral valve in hypertrophic cardiomyopathy (HCM) has generally been explained by a Venturi effect related to septal hypertrophy, causing outflow tract narrowing and high velocities. Patients with HCM, however, also have primary abnormalities of the mitral apparatus, including anterior and inward or central displacement of the papillary muscles, and leaflet elongation. These findings have led to the hypothesis that changes in the mitral apparatus can be a primary cause of SAM by altering the forces acting on the mitral valve and its ability to move in response to them. Despite suggestive observations, however, it has never been prospectively demonstrated that such changes can actually cause SAM.

METHODS AND RESULTS

To test this hypothesis in vivo, anterior papillary muscle displacement was created in 7 dogs studied by echocardiography, with controlled cardiac output and heart rate. In all 7 dogs, papillary muscle displacement caused SAM, with an outflow tract gradient (33 +/- 19 mm Hg) and mitral regurgitation in 6. As in patients with HCM, the mitral valve was displaced anteriorly and the coaptation point shifted toward the insertion of the leaflets, creating longer distal residual leaflets that moved anteriorly.

CONCLUSIONS

Primary changes in the mitral apparatus can cause SAM without septal hypertrophy. In this model, SAM appears to be determined by the ability of the leaflets to move anteriorly (papillary muscle displacement causing slack and increased residual leaflet length) and their interposition into the outflow stream by anterior displacement, determining the direction of this motion. Geometric factors observed in HCM and in patients with SAM without HCM can therefore play a primary role in causing SAM.

Fluid mechanics of artificial heart valves

1. Artificial heart valves have been in use for over five decades to replace diseased heart valves. Since the first heart valve replacement performed with a caged-ball valve, more than 50 valve designs have been developed, differing principally in valve geometry, number of leaflets and material. To date, all artificial heart valves are plagued with complications associated with haemolysis, coagulation for mechanical heart valves and leaflet tearing for tissue-based valve prosthesis. For mechanical heart valves, these complications are believed to be associated with non-physiological blood flow patterns. 2. In the present review, we provide a bird's-eye view of fluid mechanics for the major artificial heart valve types and highlight how the engineering approach has shaped this rapidly diversifying area of research. 3. Mechanical heart valve designs have evolved significantly, with the most recent designs providing relatively superior haemodynamics with very low aerodynamic resistance. However, high shearing of blood cells and platelets still pose significant design challenges and patients must undergo life-long anticoagulation therapy. Bioprosthetic or tissue valves do not require anticoagulants due to their distinct similarity to the native valve geometry and haemodynamics, but many of these valves fail structurally within the first 10-15 years of implantation. 4. These shortcomings have directed present and future research in three main directions in attempts to design superior artificial valves: (i) engineering living tissue heart valves; (ii) development of advanced computational tools; and (iii) blood experiments to establish the link between flow and blood damage.

The Fluid Mechanics of Transcatheter Heart Valve Leaflet Thrombosis in the Neosinus

BACKGROUND

Transcatheter heart valve (THV) thrombosis has been increasingly reported. In these studies, thrombus quantification has been based on a 2-dimensional assessment of a 3-dimensional phenomenon.

METHODS

Postprocedural, 4-dimensional, volume-rendered CT data of patients with CoreValve, Evolut R, and SAPIEN 3 transcatheter aortic valve replacement enrolled in the RESOLVE study (Assessment of Transcatheter and Surgical Aortic Bioprosthetic Valve Dysfunction With Multimodality Imaging and Its Treatment with Anticoagulation) were included in this analysis. Patients on anticoagulation were excluded. SAPIEN 3 and CoreValve/Evolut R patients with and without hypoattenuated leaflet thickening were included to study differences between groups. Patients were classified as having THV thrombosis if there was any evidence of hypoattenuated leaflet thickening. Anatomic and THV deployment geometries were analyzed, and thrombus volumes were computed through manual 3-dimensional reconstruction. We aimed to identify and evaluate risk factors that contribute to THV thrombosis through the combination of retrospective clinical data analysis and in vitro imaging in the space between the native and THV leaflets (neosinus).

RESULTS

SAPIEN 3 valves with leaflet thrombosis were on average 10% further expanded (by diameter) than those without (95.5±5.2% versus 85.4±3.9%; P<0.001). However, this relationship was not evident with the CoreValve/Evolut R. In CoreValve/Evolut Rs with thrombosis, the thrombus volume increased linearly with implant depth (R²=0.7, P<0.001). This finding was not seen in the SAPIEN 3. The in vitro analysis showed that a supraannular THV deployment resulted in a nearly 7-fold decrease in stagnation zone size (velocities <0.1 m/s) when compared with an intraannular deployment. In addition, the in vitro model indicated that the size of the stagnation zone increased as cardiac output decreased.

CONCLUSIONS

Although transcatheter aortic valve replacement thrombosis is a multifactorial process involving foreign materials, patient-specific blood chemistry, and complex flow patterns, our study indicates that deployed THV geometry may have implications on the occurrence of thrombosis. In addition, a supraannular neosinus may reduce thrombosis risk because of reduced flow stasis. Although additional prospective studies are needed to further develop strategies for minimizing thrombus burden, these results may help identify patients at higher thrombosis risk and aid in the development of next-generation devices with reduced thrombosis risk.

Elevated cyclic stretch alters matrix remodeling in aortic valve cusps: implications for degenerative aortic valve disease

Matrix metalloproteinases (MMPs) and cathepsins are proteolytic enzymes that are upregulated in diseased aortic valve cusps. The objective of this study was to investigate whether elevated cyclic stretch causes an increased expression and activity of these proteolytic enzymes in the valve cusp. Circumferentially oriented fresh porcine aortic valve cusp sections were stretched to 10% (physiological), 15% (pathological), and 20% (hyperpathological) in a tensile stretch bioreactor for 24 and 48 h. The expression and activity of MMP-1, MMP-2, MMP-9, tissue inhibitor of MMP-1, and cathepsin L, S, and K were quantified and compared with fresh controls. Cell proliferation and apoptosis were also analyzed. As a result, at 10% physiological stretch, the expression and activity of remodeling enzymes were comparable with fresh controls. At 15% stretch, the expression of MMP-1, -2, -9 and cathepsin S and K were upregulated, whereas the expression of cathepsin L was downregulated compared with controls. A similar trend was observed at 20% stretch, but the magnitudes of upregulation and downregulation of the expression were less than those observed at 15%. In addition, there were significantly higher cell proliferation and apoptosis at 20% stretch compared with those of other treatment groups. In conclusion, elevated mechanical stretch on aortic valve cusps may detrimentally alter the proteolytic enzyme expression and activity in valve cells. This may trigger a cascade of events leading to an accelerated valve degeneration and disease progression.

The Björk-Shiley aortic prosthesis: flow characteristics, thrombus formation and tissue overgrowth

Thrombus formation and tissue overgrowth were observed in nine Björk-Shiley aortic prostheses recovered six months or longer after implantation. These pathologic findings may be attributed to the flow characteristics of the prosthesis. The open disc of the valve separates the flow into two unequal regions. Varying degrees of thrombus formation were observed in the minor outflow region, including the depression in the aortic face of the disc and the metal strut bridging this area. Tissue overgrowth was noted along the perimeter of the prosthesis adjacent to the minor outflow region. That overgrowth further reduced the available cross section for flow in this already constrained area. In vitro velocity measurements with a laser-Doppler anemometer identified a zone of stagnation about 20 mm wide near the aortic face of the disc. The average velocities in the major and minor outflow regions were around 100 and 25 cm/sec, respectively, and the corresponding peak-shear stresses were approximately 700 and 150 dynes/cm2. There is reason, then, to attribute the thrombus formation and tissue overgrowth to the stagnation zone and the low shear in the minor outflow region.

Biaixal stress-stretch behavior of the mitral valve anterior leaflet at physiologic strain rates

Characterization of the mechanical properties of the native mitral valve leaflets at physiological strain rates is a critical step in improving our understanding of MV function and providing experimental data for dynamic constitutive models. We explored, for the first time, the effects of strain rate (from quasi-static to physiologic) on the biaxial mechanical properties of the native mitral valve anterior leaflet (MVAL). A novel high-speed biaxial testing device was developed, capable of achieving in vitro strain rates reported for the MVAL (Sacks et al., Ann. Biomed. Eng. 30(10):1280-1290, 2002). Porcine MVAL specimens were loaded to physiological load levels with cycle periods of 15, 1, 0.5, 0.1, and 0.05 s. The resulting loading stress-strain responses were found to be remarkably independent of strain rate. The hysteresis, defined as the fraction of the membrane strain energy between the loading and unloading curves tension-areal stretch curves, was low (approximately 12%) and did not vary with strain rate. The results of the present work indicated that MVAL tissues exhibit complete strain rate insensitivity at and below physiological strain rates under physiological loading conditions. These novel results suggest that experimental tests utilizing quasi-static strain rates are appropriate for constitutive model development for mitral valve tissues. The mechanisms underlying this quasi-elastic behavior are as yet unknown, but are likely an important functional aspect of native mitral valve tissues and clearly warrant further study.

Surface strains in the anterior leaflet of the functioning mitral valve

Abstract-The mitral valve (MV) is a complex anatomical structure whose function involves a delicate force balance and synchronized function of each of its components. Elucidation of the role of each component and their interactions is critical to improving our understanding of MV function, and to form the basis for rational surgical repair. In the present study, we present the first known detailed study of the surface strains in the anterior leaflet in the functioning MV. The three-dimensional spatial positions of markers placed in the central region of the MV anterior leaflet in a left ventricle-simulating flow loop over the cardiac cycle were determined. The resulting two-dimensional in-surface strain tensor was computed from the marker positions using a C0 Lagrangian quadratic finite element. Results demonstrated that during valve closure the anterior leaflet experienced large, anisotropic strains with peak stretch rates of 500%-1,000%/s. This rapid stretching was followed by a plateau phase characterized by relatively constant strain state. We hypothesized that the presence of this plateau phase was a result of full straightening of the leaflet collagen fibers upon valve closure. This hypothesis suggests that the MV collagen fibers are designed to allow leaflet coaptation followed by a dramatic increase in stiffness to prevent further leaflet deformation, which would lead to valvular regurgitation. These studies represent a first step in improving our understanding of normal MV function and to help establish the principles for repair and replacement.

Adjacent solid boundaries alter the size of regurgitant jets on Doppler color flow maps

Recent studies have attempted to predict the severity of regurgitant lesions from jet size on Doppler flow maps. Jet size is a function of both regurgitant volume and fluid entrained from the receiving chamber and, for a free jet, is a function of its momentum at the orifice. However, regurgitant jets often approach or attach to cardiac walls, potentially altering their momentum and ability to expand by entrainment. Therefore, this study addressed the hypothesis that adjacent walls influence regurgitant jet size as seen on Doppler flow maps. Steady flow was driven through circular orifices (0.02 to 0.05 cm2) at physiologic velocities of 2 to 5 m/s. At a constant flow rate and orifice velocity, orifice position was varied to produce three jet geometries: free jets, jets adjacent to a horizontal chamber wall lying 1 cm below the orifice and wall jets with the orifice at the level of the wall. Doppler color flow imaging was performed at identical instrument settings for all jets. Two long-axis views of the jet were obtained: a vertical view perpendicular to the wall, resembling that most commonly used in patients to image the length of the jet, and a horizontal view parallel to the chamber wall. Velocities along the jet were also measured by Doppler mapping.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonlinear power loss during exercise in single-ventricle patients after the Fontan: insights from computational fluid dynamics

BACKGROUND

We previously demonstrated that power loss (PL) through the total cavopulmonary connection (TCPC) in single-ventricle patients undergoing Fontan can be calculated by computational fluid dynamic analysis using 3-dimensional MRI anatomic reconstructions. PL through the TCPC may play a role in single-ventricle physiology and is a function of cardiac output. We hypothesized that PL through the TCPC increases significantly under exercise flow conditions.

METHODS AND RESULTS

MRI data of 10 patients with a TCPC were analyzed to obtain 3-dimensional geometry and flow rates through the superior vena cava, inferior vena cava, left pulmonary artery, and right pulmonary artery. Steady computational fluid dynamic simulations were performed at baseline conditions using MRI-derived flows. Simulated exercise conditions of twice (2x) and three times (3x) baseline flow were performed by increasing inferior vena cava flow. PL, head loss, and effective resistance through the TCPC were calculated for each condition. Each condition was repeated at left pulmonary artery/right pulmonary artery ratios of 30/70 and 70/30 to determine the effects of pulmonary flow splits on exercise PL. For each patient, PL increases dramatically in a nonlinear fashion with increasing cardiac output, even when normalized to calculate head loss or resistance. Flow splits had a significant effect on PL at exercise, with most geometries favoring right pulmonary artery flow.

CONCLUSIONS

The relationship between cardiac output and PL is nonlinear and highly dependent on TCPC geometry and pulmonary flow splits. This study demonstrates the importance of studying the TCPC under exercise conditions, because baseline conditions may not adequately characterize TCPC efficiency.

Pressure recovery distal to a stenosis: potential cause of gradient "overestimation" by Doppler echocardiography

Doppler ultrasound is currently being widely applied to measure intracardiac pressure gradients noninvasively. In comparative invasive studies, it is generally assumed that pressure is effectively uniform distal to the stenosis. As the poststenotic jet expands, however, its velocity decreases, and pressure is recovered to the extent permitted by turbulence, so that the measured gradient will be lower if the distal catheter is positioned downstream from the vena contracta. This can lead to apparent Doppler "overestimation" of the pressure gradient because of this phenomenon of pressure recovery. This study demonstrates that pressure recovery can be important in a variety of clinical settings studied by in vitro models. Although most prominent in streamlined tunnels modeled after the obstruction in patients with hypertrophic cardiomyopathy, these effects are important even for central stenoses at physiologic flow rates. Because precise catheter position is not always known or controlled, these findings suggest an important advantage for Doppler gradient estimation, because it provides the maximal gradient at the vena contracta, which determines the load on the proximal chamber.

Flow in prosthetic heart valves: state-of-the-art and future directions

Since the first successful implantation of a prosthetic heart valve four decades ago, over 50 different designs have been developed including both mechanical and bioprosthetic valves. Today, the most widely implanted design is the mechanical bileaflet, with over 170,000 implants worldwide each year. Several different mechanical valves are currently available and many of them have good bulk forward flow hemodynamics, with lower transvalvular pressure drops, larger effective orifice areas, and fewer regions of forward flow stasis than their earlier-generation counterparts such as the ball-and-cage and tilting-disc valves. However, mechanical valve implants suffer from complications resulting from thrombus deposition and patients implanted with these valves need to be under long-term anti-coagulant therapy. In general, blood thinners are not needed with bioprosthetic implants, but tissue valves suffer from structural failure with, an average life-time of 10-12 years, before replacement is needed. Flow-induced stresses on the formed elements in blood have been implicated in thrombus initiation within the mechanical valve prostheses. Regions of stress concentration on the leaflets during the complex motion of the leaflets have been implicated with structural failure of the leaflets with bioprosthetic valves. In vivo and in vitro experimental studies have yielded valuable information on the relationship between hemodynamic stresses and the problems associated with the implants. More recently, Computational Fluid Dynamics (CFD) has emerged as a promising tool, which, alongside experimentation, can yield insights of unprecedented detail into the hemodynamics of prosthetic heart valves. For CFD to realize its full potential, however, it must rely on numerical techniques that can handle the enormous geometrical complexities of prosthetic devices with spatial and temporal resolution sufficiently high to accurately capture all hemodynamically relevant scales of motion. Such algorithms do not exist today and their development should be a major research priority. For CFD to further gain the confidence of valve designers and medical practitioners it must also undergo comprehensive validation with experimental data. Such validation requires the use of high-resolution flow measuring tools and techniques and the integration of experimental studies with CFD modeling.

Hemodynamics and mechanobiology of aortic valve inflammation and calcification

Cardiac valves function in a mechanically complex environment, opening and closing close to a billion times during the average human lifetime, experiencing transvalvular pressures and pulsatile and oscillatory shear stresses, as well as bending and axial stress. Although valves were originally thought to be passive pieces of tissue, recent evidence points to an intimate interplay between the hemodynamic environment and biological response of the valve. Several decades of study have been devoted to understanding these varied mechanical stimuli and how they might induce valve pathology. Here, we review efforts taken in understanding the valvular response to its mechanical milieu and key insights gained from in vitro and ex vivo whole-tissue studies in the mechanobiology of aortic valve remodeling, inflammation, and calcification.

In vitro flow experiments for determination of optimal geometry of total cavopulmonary connection for surgical repair of children with functional single ventricle

OBJECTIVES

This study sought to evaluate the effect of offsetting cavopulmonary connections at varying pulmonary flow ratios to determine the optimal geometry of the connection.

BACKGROUND

Previous investigators have demonstrated energy conservation within the streamlined contours of the total cavopulmonary connection compared with that of the atriopulmonary connection. However, their surgical design of connecting the two cavae directly opposite each other may result in high energy losses. Others have introduced a unidirectional connection with some advantages but with concerns about the formation of arteriovenous malformation in the lung excluded from hepatic venous return. Thus, an optimal surgical design has not been determined.

METHODS

In the present models, the caval connections were offset through a range of 0.0 to 2.0 diameters by 0.5 superior cava diameter increments. Flow ratios were fixed for superior and inferior cavae and varied for right and left pulmonary arteries as 70:30, 60:40, 50:50, 40:60 and 30:70 to stimulate varying lung resistance. Pressure measurements and flow visualization were done at steady flows of 2, 4 and 6 liters/min to stimulate rest and exercise.

RESULTS

Our data show that the energy losses at the 0.0-diameter offset were double the losses of the 1.0 and 1.5 diameters, which had minimal energy losses. This result was attributable to chaotic patterns seen on flow visualization in the 0.0-diameters offset. Energy savings were more evident at the 50:50 right/left pulmonary artery ratio. Energy losses increased with increased total flow rates.

CONCLUSIONS

The results strongly suggest the incorporation of caval offsets in future total cavopulmonary connections.

Elevated cyclic stretch induces aortic valve calcification in a bone morphogenic protein-dependent manner

Calcified aortic valve (AV) cusps have increased expression of bone morphogenic proteins (BMPs) and transforming growth factor-beta1 (TGF-beta1). Elevated stretch loading on the AV is known to increase expression of matrix remodeling enzymes and pro-inflammatory proteins. Little, however, is known about the mechanism by which elevated stretch might induce AV calcification. We investigated the hypothesis that elevated stretch may cause valve calcification via a BMP-dependent mechanism. Porcine AV cusps were cultured in a stretch bioreactor, at 10% (physiological) or 15% (pathological) stretch and 70 beats per minute for 3, 7, and 14 days, in osteogenic media supplemented with or without high phosphate (3.8 mmol/L), TGF-beta1 (1 ng/ml), as well as the BMP inhibitor noggin (1, 10, and 100 ng/ml). Fresh cusps served as controls. Alizarin red and von Kossa staining demonstrated that 15% stretch elicited a stronger calcification response compared with 10% stretch in a fully osteogenic medium containing high phosphate and TGF-beta1. BMP-2, -4, and Runx2 expression was observed after 3 days on the fibrosa surface of the valve cusp and was stretch magnitude-dependent. Cellular apoptosis was highest at 15% stretch. Tissue calcium content and alkaline phosphatase activity were similarly stretch-dependent and were significantly reduced by noggin in a dose dependent manner. These results underline the potential role of BMPs in valve calcification due to altered stretch.