Julien Cisonni

Effect of the velopharynx on intraluminal pressures in reconstructed pharynges derived from individuals with and without sleep apnea

The most collapsible part of the upper airway in the majority of individuals is the velopharynx which is the segment positioned behind the soft palate. As such it is an important morphological region for consideration in elucidating the pathogenesis of obstructive sleep apnea (OSA). This study compared steady flow properties during inspiration in the pharynges of nine male subjects with OSA and nine body-mass index (BMI)- and age-matched control male subjects without OSA. The k-ωSST turbulence model was used to simulate the flow field in subject-specific pharyngeal geometric models reconstructed from anatomical optical coherence tomography (aOCT) data. While analysis of the geometry of reconstructed pharynges revealed narrowing at velopharyngeal level in subjects with OSA, it was not possible to clearly distinguish them from subjects without OSA on the basis of pharyngeal size and shape alone. By contrast, flow simulations demonstrated that pressure fields within the narrowed airway segments were sensitive to small differences in geometry and could lead to significantly different intraluminal pressure characteristics between subjects. The ratio between velopharyngeal and total pharyngeal pressure drops emerged as a relevant flow-based criterion by which subjects with OSA could be differentiated from those without.

Validation of theoretical models of phonation threshold pressure with data from a vocal fold mechanical replica (L)

This paper analyzes the capability of a mucosal wave model of the vocal fold to predict values of phonation threshold lung pressure. Equations derived from the model are fitted to pressure data collected from a mechanical replica of the vocal folds. The results show that a recent extension of the model to include an arbitrary delay of the mucosal wave in its travel along the glottal channel provides a better approximation to the data than the original version of the model, which assumed a small delay. They also show that modeling the vocal tract as a simple inertive load, as has been proposed in recent analytical studies of phonation, fails to capture the effect of the vocal tract on the phonation threshold pressure with reasonable accuracy.

Numerical simulation of pharyngeal airflow applied to obstructive sleep apnea: effect of the nasal cavity in anatomically accurate airway models

Repetitive brief episodes of soft-tissue collapse within the upper airway during sleep characterize obstructive sleep apnea (OSA), an extremely common and disabling disorder. Failure to maintain the patency of the upper airway is caused by the combination of sleep-related loss of compensatory dilator muscle activity and aerodynamic forces promoting closure. The prediction of soft-tissue movement in patient-specific airway 3D mechanical models is emerging as a useful contribution to clinical understanding and decision making. Such modeling requires reliable estimations of the pharyngeal wall pressure forces. While nasal obstruction has been recognized as a risk factor for OSA, the need to include the nasal cavity in upper-airway models for OSA studies requires consideration, as it is most often omitted because of its complex shape. A quantitative analysis of the flow conditions generated by the nasal cavity and the sinuses during inspiration upstream of the pharynx is presented. Results show that adequate velocity boundary conditions and simple artificial extensions of the flow domain can reproduce the essential effects of the nasal cavity on the pharyngeal flow field. Therefore, the overall complexity and computational cost of accurate flow predictions can be reduced.

The Influence of Geometrical and Mechanical Input Parameters on Theoretical Models of Phonation

Summary The influence of initial aperture and mechanical properties on the onset pressure thresholds and oscillation frequencies is experimentally assessed on a deformable vocal fold replica in case of strong and weak acoustical coupling. The mechanical replica enables to vary the initial aperture while mechanical properties are maintained and therefore to mimic abduction and adduction gestures of human phonation. Depending on initial conditions (geometrical, mechanical and acoustical) one or two oscillation regions are experimentally found for which important di erences are observed for both oscillation onset pressure thresholds and oscillation frequencies. Measured onset pressure thresholds are used to validate the outcome of a theoretical model of phonation using a reduced mechanical model. The applied coupling sti ness in the theoretical model is estimated from the measured frequency response instead of imposed by an ‘ad-hoc’ criterion. The variations in coupling sti ness result in a qualitative agreement between predicted and measured values for all assessed experimental conditions. In addition, the Young’s modulus of the replica is qualitatively estimated to be within the range observed ‘in-vivo’.

Experimental validation of quasi-one-dimensional and two-dimensional steady glottal flow models

Physical modelling of phonation requires a mechanical description of the vocal fold coupled to a description of the flow within the glottis. In this study, an in-vitro set-up, allowing to reproduce flow conditions comparable to those of human glottal flow is used to systematically verify and discuss the relevance of the pressure and flow-rate predictions of several laminar flow models. The obtained results show that all the considered flow models underestimate the measured flow-rates and that flow-rates predicted with the one-dimensional model are most accurate. On the contrary, flow models based on boundary-layer theory and on the two-dimensional numerical resolution of Navier–Stokes equations yield most accurate pressure predictions. The influence of flow separation on the predictions is discussed since these two models can estimate relevant flow separation positions whereas this phenomenon is treated in a simplified ad-hoc way in the one-dimensional flow modelling. Laminar flow models appear to be unsuitable to describe the flow downstream of the glottal constriction. Therefore, the use of flow models taking into account three-dimensional effects as well as turbulence is motivated.

Increasing the complexity of glottal flow models: in‐vitro validation for steady flow conditions

Quasi one‐dimensional glottal flow descriptions predict vocal folds oscillations characteristics which are qualitatively relevant to in‐vitro and in‐vivo experimental data. The current paper considers the resolution of the 2D Navier‐Stokes equations in order to obtain a refined description of the flow phenomena adapted to more realistic glottal geometry. The pressure and flow rate predictions obtained from quasi one‐dimensional flow models and the resolution of the 2D Navier‐Stokes equations are examined for steady flows within a rigid glottis. The models predictions are validated against in‐vitro measurements performed on rigid constriction replicas comparable to the geometrical conditions of the glottis and mounted in a suitable set‐up. The confrontation between the experimental and computed data tends to show that the accuracy of the estimated pressures increases with the complexity of the flow model whereas the inverse tendency can be observed for the estimated flow rates. A focus is made on the flow ...

Theoretical and experimental results of phonation threshold pressure vs. oscillation frequency of the vocal folds

The dynamical principles of the vocal fold oscillation at phonation were set forth by Titze (I. R. Titze, J. Acoust. Soc. Am. 83, 1536‐1552 , 1988), by representing motion of the tissues as a surface wave propagating in the direction of the airflow. An important result of his work was an equation for the phonation threshold value of lung pressure, defined as the minimum value required to initiate the vocal fold oscillation. Titzes model assumed a small time delay for the mucosal wave to travel along the vocal folds, with the consequence that the phonation threshold pressure results independent of the oscillation frequency. Here, we consider an extension of his model for an arbitrary time delay. Our results show that the threshold pressure increases with oscillation frequency following a x/sin(x) law. We investigate the validity of the theoretical equation by comparing it with pressure measures from a mechanical replica of the vocal folds, under various configurations. In general, the equation shows good ...

Blood flow velocity prediction in aorto-iliac stent grafts using computational fluid dynamics and Taguchi method

Covered Endovascular Reconstruction of Aortic Bifurcation (CERAB) is a new technique to treat extensive aortoiliac occlusive disease with covered expandable stent grafts to rebuild the aortoiliac bifurcation. Post stenting Doppler ultrasound (DUS) measurement of maximum peak systolic velocity (PSVmax) in the stented segment is widely used to determine patency and for follow up surveillance due to the portability, affordability and ease of use. Anecdotally, changes in hemodynamics created by CERAB can lead to falsely high PSVmax requiring CT angiography (CTA) for further assessment. Therefore, the importance of DUS would be enhanced with a proposed PSVmax prediction tool to ascertain whether PSVmax falls within the acceptable range of prediction. We have developed a prediction tool based on idealized models of aortoiliac bifurcations with various infra-renal PSV (PSVin), iliac to aortic area ratios (R) and aortoiliac bifurcation angles (α). Taguchi method with orthogonal arrays (OA) was utilized to minimize the number of Computational Fluid Dynamics (CFD) simulations performed under physiologically realistic conditions. Analysis of Variance (ANOVA) and Multiple Linear Regression (MLR) analyses were performed to assess Goodness of fit and to predict PSVmax. PSVin and R were found to contribute 94.06% and 3.36% respectively to PSVmax. The Goodness of fit based on adjusted R2 improved from 99.1% to 99.9% based on linear and exponential functions. The PSVmax predictor based on the exponential model was evaluated with sixteen patient specific cases with a mean prediction error of 9.9% and standard deviation of 6.4%. Eleven out of sixteen cases (69%) in our current retrospective studies would have avoided CTA if the proposed predictor was used to screen out DUS measured PSVmax with prediction error greater than 15%. The predictor therefore has the potential to be used as a clinical tool to detect PSVmax more accurately post aortoiliac stenting and might reduce diagnostic errors and avoid unnecessary expense and risk from CTA follow-up imaging.

Stability of a Cantilevered Flexible Plate with Non-uniform Thickness in Viscous Channel Flow

Most studies analysing the instability of a cantilevered flexible plate in an axial flow are based on models assuming an inviscid flow and uniform properties for the plate. However, for some applications, such as biomechanical fluid-structure interaction (FSI) systems, these simplifications may not be valid due the scale of the problems and the non-uniform geometric and mechanical properties of the soft tissue. In this study, a parametric investigation is conducted to determine the conditions leading to flutter instability of a cantilevered flexible plate with a non-uniform thickness immersed in a two-dimensional viscous channel flow. It is shown that, depending on the mass ratio, the thinning and thickening of the plate free-end can stabilise or destabilise the FSI system and change the critical mode at instability onset.

Numerical Investigation of the Post-stall Flow Patterns over a NACA 0021 Hydrofoil with Sinusoidal Leading Edge

As passive flow-control devices disrupting flow separation, leading-edge protuberances can provide superior hydrodynamic performance to hydrofoils at high angles of attack. Most experimental and numerical investigations carried out for low Reynolds number conditions have related the relative improvements observed post-stall to “bi-periodic” flow structures, developing over tubercles pairs. In this study, a numerical approach is employed to show the emergence of higher-order patterns in the flow over a stalling NACA 0021 hydrofoil with sinusoidal leading edge. The effect of the number of sinusoidal tubercles defining the leading edge of the hydrofoil model on the prediction of “bi-periodic” or “tri-periodic” flow structures is particularly analyzed to interpret the uncertainty found on the resulting hydrodynamic performance.