Assessment of variables affecting late left ventricular flow propagation velocity.

UNLABELLED: Early colour M-mode flow propagation velocity (Vpe) in the left ventricle is a well-known non-invasive index for assessing left ventricular relaxation. However, the utility and determinants of late colour M-mode flow propagation (Vpa) have received little attention to date. Vpa as a representation of the left ventricular vortex travelling velocity during late filling could have a distinct role in differentiating potential subgroups in diastolic failure. The aim of the present study was to establish the normal values of late flow propagation in a healthy population of various ages (18-79 years), and to examine the general and echocardiographic variables that affect Vpa. METHODS: We studied 75 apparently healthy subjects (age range, 18-79 years; 38 women, 37 men) as part of an outpatient clinic check-up screening. General parameters were recorded, including age, gender, height, weight, blood pressure, and heart rate. In addition, conventional grey-scale M-mode, 2D, as well as colour M-mode, 2D, and pulsed wave (tissue) Doppler echocardiographic parameters were obtained in a single centre and using a single operator setting. Backward linear regression analysis (dependent variable: Vpa) was performed to find the optimal model, taking into account multicollinearity and maximum coefficient of determination (R2). Due to the heteroscedasticity of the collected data, a logarithmic transformation was used. In addition, separate linear backward regression analysis was performed for the male and female subgroups. RESULTS: Vpa values were 26-179 cm/s. The optimal regression model after elimination included the following variables: age (beta = 0.684, P < 0.001), height (beta = 0.521, P < 0.001), gender (beta = 0.343, P < 0.05), left ventricular Vpe (beta = 0.299, P < 0.01), left ventricular posterior systolic (M-mode) wall thickness (beta = 0.288, P < 0.01), interventricular septum thickness diastole (beta = 0.346, P < 0.005), transmitral Doppler E-wave deceleration time apical 4-chamber (beta = -0.297, P < 0.05), and tissue Doppler peak E-wave mitral annulus (beta = 0.459, P < 0.005). The total coefficient of determination (R2) for this model was 0.540 (P < 0.001); 0.673 (P < 0.001) for men and 0.645 (P < 0.001) for women. CONCLUSION: Vpa, representing left ventricular vortex travelling velocity during late filling, shows a large range of values in normal healthy subjects. It is mainly depending on age, gender and left ventricular mass. Moreover, substantially different determinants are found between men and women. Further study is required to explore these findings.

A fast strong coupling algorithm for the partitioned fluid-structure interaction simulation of BMHVs.

The numerical simulation of Bileaflet Mechanical Heart Valves (BMHVs) has gained strong interest in the last years, as a design and optimisation tool. In this paper, a strong coupling algorithm for the partitioned fluid-structure interaction simulation of a BMHV is presented. The convergence of the coupling iterations between the flow solver and the leaflet motion solver is accelerated by using the Jacobian with the derivatives of the pressure and viscous moments acting on the leaflets with respect to the leaflet accelerations. This Jacobian is numerically calculated from the coupling iterations. An error analysis is done to derive a criterion for the selection of useable coupling iterations. The algorithm is successfully tested for two 3D cases of a BMHV and a comparison is made with existing coupling schemes. It is observed that the developed coupling scheme outperforms these existing schemes in needed coupling iterations per time step and CPU time.

Anatomical and functional changes in the upper airways of sleep apnea patients due to mandibular repositioning: a large scale study.

The obstructive sleep apnea-hypopnea syndrome (OSAHS) is a sleep related breathing disorder. A popular treatment is the use of a mandibular repositioning appliance (MRA) which advances the mandibula during the sleep and decreases the collapsibility of the upper airway. The success rate of such a device is, however, limited and very variable within a population of patients. Previous studies using computational fluid dynamics have shown that there is a decrease in upper airway resistance in patients who improve clinically due to an MRA. In this article, correlations between patient-specific anatomical and functional parameters are studied to examine how MRA induced biomechanical changes will have an impact on the upper airway resistance. Low-dose computed tomography (CT) scans are made from 143 patients suffering from OSAHS. A baseline scan and a scan after mandibular repositioning (MR) are performed in order to study variations in parameters. It is found that MR using a simulation bite is able to induce resistance changes by changing the pharyngeal lumen. The change in minimal cross-sectional area is the best parameter to predict the change in upper airway resistance. Looking at baseline values, the ideal patients for MR induced resistance decrease seem to be women with short airways, high initial resistance and no baseline occlusion.

Design of an artificial left ventricular muscle: an innovative way to actuate blood pumps?

Blood pumps assist or take over the pump function of a failing heart. They are essentially activated by a pusher plate, a pneumatic compression of collapsible sacs, or they are driven by centrifugal pumps. Blood pumps relying upon one of these actuator mechanisms do not account for realistic wall deformation. In this study, we propose an innovative design of a blood pump actuator device which should be able to mimic fairly well global left ventricular (LV) wall deformation patterns in terms of circumferential and longitudinal contraction, as well as torsion. In order to reproduce these basic wall deformation patterns in our actuator device, we designed a novel kind of artificial LV "muscle" composed of multiple actively contracting cells. Its contraction is based on a mechanism by which pressurized air, inside such a cell, causes contraction in one direction and expansion perpendicular to this direction. The organization and geometry of the contractile cells within one artificial LV muscle, the applied pressure in the cells, and the governing LV loading conditions (preload and afterload) together determine the global deformation of the LV wall. Starting from a simple plastic bag, an experimental model based on the above mentioned principle was built and connected to a lumped hydraulic model of the vascular system (including compliance and resistance). The wall deformation pattern of this device was validated visually and its pump performance was studied in terms of LV volume and pressure and heart rate. Our experimental results revealed (i) a global LV motion resembling a real LV, and (ii) a close correlation between our model and a real LV in terms of end-systolic volume and pressure, end-diastolic volume and pressure, stroke volume, ejection fraction and pressure-volume relationship. Our proposed model appears promising and it can be considered as a step forward when compared to currently applied actuator mechanisms, as it will likely result in more physiological intracavity blood flow patterns.

Modeling fluid flow through irregular scaffolds for perfusion bioreactors.

Direct perfusion of 3D tissue engineered constructs is known to enhance osteogenesis, which can be partly attributed to enhanced nutrient and waste transport. In addition flow mediated shear stresses are known to upregulate osteogenic differentiation and mineralization. A quantification of the hydrodynamic environment is therefore crucial to interpret and compare results of in vitro bioreactor experiments. This study aims to deal with the pitfalls of numerical model preparation of highly complex 3D bone scaffold structures and aims to provide more accurate wall shear stress (WSS) estimates. microCT imaging techniques were used to reconstruct the geometry of both a titanium (Ti) and a hydroxyapatite scaffold, starting from 430 images with a resolution of 8 microm. To tackle the tradeoff between model size and mesh resolution we selected two concentric regions of interest (cubes with a volume of 1 and 3.375 mm(3), respectively) for both scaffolds. A flow guidance in front of the real inlet surface of the scaffold was designed to mimic realistic inlet conditions. With a flow rate of 0.04 mL/min perfused through a 5 mm diameter scaffold at an inlet velocity of 33.95 microm/s we obtained average WSSs of 1.10 and 1.46 mPa for the 1 mm(3) and the 3.375 mm(3) model of the hydroxyapatite scaffold compared to 1.40 and 1.95 mPa for the 1 mm(3) model and the 3.375 mm(3) model of the Ti scaffold, showing the important influence of the scaffold micro-architecture heterogeneity and the proximity of boundaries. To assess that influence we selected cubic portions, of which the WSS data were analyzed, with the same size and the same location within both 1 and 3.375 mm(3) cubic models. Varying the size of the inner portions simultaneously in both model selections gives a quantification of the sensitivity to boundary neighborhood. This methodology allows to get more insight in the complex concept of tissue engineering and will likely help to understand and eventually improve the fluid-mechanical aspects.