| 1. |
- Mihaescu, Mihai, 1976-, et al.
(författare)
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Computational Fluid Dynamics Analysis of Upper Airway Reconstructed From Magnetic Resonance Imaging Data
- 2008
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Ingår i: Annals of Otology, Rhinology and Laryngology. - : SAGE Publications. - 0003-4894 .- 1943-572X. ; 117:4, s. 303-309
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: We performed flow computations on an accurate upper airway model in a patient with obstructive sleep apnea and computed the velocity, static pressure, and wall shear stress distribution in the model. Methods: Cartesian coordinates for airway boundaries were determined from cross-sectional magnetic resonance images, and a 3-dimensional computational model of the upper airway was constructed. Flow simulations were then performed within a FLUENT commercial software framework. Four different flow conditions were simulated during inspiration, assuming the steady-state condition. The results were analyzed from the perspectives of velocity, static pressure, and wall shear stress distribution. Results: We observed that the highest axial velocity was at the site of minimum cross-sectional area (retropalatal pharynx) resulting in the lowest level of wall static pressure. The highest wall shear stresses were at the same location. The pressure drop was significantly larger for higher flow rates than for lower flow rates. Conclusions: Our results indicate that the presence of airway narrowing, through change in the flow characteristics that result in increased flow velocity and reduced static pressure, can itself increase airway collapsibility. Additionally, the effects of wall shear stress on airway walls may be an important factor in the progression over time of the severity of obstructive sleep apnea.
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| 2. |
- Mihaescu, Mihai, 1976-, et al.
(författare)
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Computational Modeling of Upper Airway Before and After Adenotonsillectomy for Obstructive Sleep Apnea
- 2008
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Ingår i: The Laryngoscope. - 0023-852X .- 1531-4995. ; 118:2, s. 360-362
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Tidskriftsartikel (refereegranskat)abstract
- Adenotonsillectomy, the first-line surgical treatment for obstructive sleep apnea (OSA) in children, is successful in only 50% of obese children. Computational fluid dynamics tools, which have been applied to differentiate OSA patients from those without OSA based on the airway flow characteristics, can be potentially used to identify patients likely to benefit from surgical intervention. We present computational modeling of the upper airway before and after adenotonsillectomy in an obese female adolescent with OSA. The subject underwent upper airway imaging on a 1.5 Tesla magnetic resonance imaging (MRI) scanner, and three-dimensional airway models were constructed using airway boundary coordinates from cross-sectional MRI scans. Our results using computational simulations indicate that, in an obese child, the resolution of OSA after adenotonsillectomy is associated with changes in flow characteristics that result in decreased pressure differentials across the airway walls and thus lower compressive forces that predispose to airway collapse. Application of such findings to an obese child seeking surgical treatment for OSA can potentially lead to selection of the surgical procedure most likely to result in OSA resolution. Effective intervention for OSA in this high-risk group will result in reduction in morbidity and the public health concerns associated with OSA.
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| 3. |
- Mihaescu, Mihai, 1976-, et al.
(författare)
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Large Eddy Simulation and Reynolds-Averaged Navier–Stokes modeling of flow in a realistic pharyngeal airway model : An investigation of obstructive sleep apnea
- 2008
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Ingår i: Journal of Biomechanics. - : Elsevier BV. - 0021-9290 .- 1873-2380. ; 41:10, s. 2279-2288
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Tidskriftsartikel (refereegranskat)abstract
- Computational fluid dynamics techniques employing primarily steady Reynolds-Averaged Navier–Stokes (RANS) methodology have been recently used to characterize the transitional/turbulent flow field in human airways. The use of RANS implies that flow phenomena are averaged over time, the flow dynamics not being captured. Further, RANS uses two-equation turbulence models that are not adequate for predicting anisotropic flows, flows with high streamline curvature, or flows where separation occurs. A more accurate approach for such flow situations that occur in the human airway is Large Eddy Simulation (LES). The paper considers flow modeling in a pharyngeal airway model reconstructed from cross-sectional magnetic resonance scans of a patient with obstructive sleep apnea. The airway model is characterized by a maximum narrowing at the site of retropalatal pharynx. Two flow-modeling strategies are employed: steady RANS and the LES approach. In the RANS modeling framework both k–ε and k–ω turbulence models are used. The paper discusses the differences between the airflow characteristics obtained from the RANS and LES calculations. The largest discrepancies were found in the axial velocity distributions downstream of the minimum cross-sectional area. This region is characterized by flow separation and large radial velocity gradients across the developed shear layers. The largest difference in static pressure distributions on the airway walls was found between the LES and the k–ε data at the site of maximum narrowing in the retropalatal pharynx.
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| 4. |
- Mylavarapu, Goutham, et al.
(författare)
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Fluid-Structure Interaction Analysis in Human Upper Airways to Understand Sleep Apnea
- 2010
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Ingår i: 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition. - : AIAA. - 9781600867392 ; , s. 2010-1264-
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Konferensbidrag (refereegranskat)abstract
- Sleep apnea is characterized by partial or complete obstruction of upper airway during sleep. Existing clinical therapies are not fool-proof. Understanding human upper airway mechanisms with flow and structure dynamics is hence, essential for designing better clinical therapies. In this study, two dimensional Fluid-Structure Interaction (FSI) simulations were carried on human upper airway models using ADINA 8.4, a finite element code. Baseline model (B) is reconstructed from a mid-sagittal Magnetic Resonance Image (MRI) of an adolescent. In-house developed MATLAB code is used to de-feature various structures on the MRI scan. Vertices information from MATLAB code is then imported into Gambit 2.2, where edges are reconstructed and exported in IGES format to ADINA. Fluid and Solid surfaces are reconstructed and discretized in ADINA. Flow was assumed laminar and incompressible. Plain Strain hypothesis and small strains were assumed with solid domain. Appropriate boundary conditions with wall and fluid structure interfaces were defined in both fluid and solid domains. Pressure drop across airway is varied incrementally with a user defined time function. Airway wall displacement and flow features are obtained. The displacement of tip of soft palate and critical closing pressures required for partial or complete closure of upper airways are computed and compared across different cases. For comparative studies between a normal, narrower and corrected airway models (A, B, C), Model B is reconstructed from Model A by moving its posterior airway wall in anterior direction and Model C is reconstructed from Model B by excising the length of soft palate by 30%. It was observed that airway is more susceptible to collapse during expiration phase than inspiration. Increasing the stiffness of soft palate as in a palatal implant clinical therapy showed significant improvement in airway potency as demonstrated in Models A and B. Model B demonstrates a case of complete obstruction for significantly lower closing pressures when compared to Model A. Model A demonstrated hypo-apnea or partial obstruction of airway. Model C with excised soft palate is less susceptible to airway collapse when compared to Model B. This study is step forward to our previous studies on upper airways where airway walls were assumed static.
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| 5. |
- Mylavarapu, Goutham, et al.
(författare)
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Validation of computational fluid dynamics methodology used for human upper airway flow simulations
- 2009
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Ingår i: Journal of Biomechanics. - : Elsevier BV. - 0021-9290 .- 1873-2380. ; 42:10, s. 1553-1559
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Tidskriftsartikel (refereegranskat)abstract
- An anatomically accurate human upper airway model was constructed from multiple magnetic resonance imaging axial scans. This model was used to conduct detailed Computational Fluid Dynamics (CFD) simulations during expiration, to investigate the fluid flow in the airway regions where obstruction could occur. An identical physical model of the same airway was built using stereo lithography. Pressure and velocity measurements were conducted in the physical model. Both simulations and experiments were performed at a peak expiratory flow rate of 200L/min. Several different numerical approaches within the FLUENT commercial software framework were used in the simulations; unsteady Large Eddy Simulation (LES), steady Reynolds-Averaged Navier-Stokes (RANS) with two-equation turbulence models (i.e. k−ε, standard k−ω, and k−ω Shear Stress Transport (SST)) and with one-equation Spalart–Allmaras model. The CFD predictions of the average wall static pressures at different locations along the airway wall were favorably compared with the experimental data. Among all the approaches, standard k−ω turbulence model resulted in the best agreement with the static pressure measurements, with an average error of ∼20% over all ports. The highest positive pressures were observed in the retroglossal regions below the epiglottis, while the lowest negative pressures were recorded in the retropalatal region. The latter is a result of the airflow acceleration in the narrow retropalatal region. The largest pressure drop was observed at the tip of the soft palate. This location has the smallest cross section of the airway. The good agreement between the computations and the experimental results suggest that CFD simulations can be used to accurately compute aerodynamic flow characteristics of the upper airway.
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| 6. |
- Sawnani, Hemant, et al.
(författare)
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Influence of Gender on Pharyngeal Airway Length in Obese Adolescents
- 2010
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Ingår i: Annals of Otology, Rhinology and Laryngology. - : SAGE Publications. - 0003-4894 .- 1943-572X. ; 119, s. 842-847
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVES: Although pharyngeal airway length has been implicated in an increased male predisposition for obstructive sleep apnea (OSA) in adults, data in obese children and adolescents are lacking. Our objective was to determine the influence of gender on pharyngeal airway length in obese adolescents, and to apply computational simulations to better understand the effect of pharyngeal airway length on the airway's predisposition to collapse in this select group. METHODS: Obese subjects without OSA were recruited from our Sleep Center. Their pharyngeal airway length was measured on midline sagittal magnetic resonance images as the distance between the hard palate and the base of the epiglottis. Computational fluid dynamics analysis was used to study the effect of pharyngeal airway length on airflow characteristics. The gender groups were compared for anthropometric measurements and pharyngeal airway length by an unpaired Student's t-test. RESULTS: Our study group included 18 female and 16 male obese adolescents with a mean (+/-SD) age of 14.7 +/- 2.3 years and a mean body mass index of 38.9 +/- 6.9 kg/m2. The groups did not differ in age, body weight, or normalized pharyngeal airway length (0.44 +/- 0.08 mm/cm in girls versus 0.44 +/- 0.11 mm/cm in boys; p = 0.9). The computational fluid dynamics simulation indicated that the 3-dimensional flow field and airway wall pressures were not significantly affected by pharyngeal airway lengthening of up to 10 mm. CONCLUSIONS: Our results indicate that in obese adolescents, there is no influence of gender on pharyngeal airway length, and pharyngeal airway length alone does not significantly affect the airway's predisposition to collapse. These findings suggest that pharyngeal airway length may not explain the increased male gender predisposition for OSA in this select group.
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