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Numerical Studies on Obstructive Apnea Syndrome Due to a Narrowed Airway
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*  Numerical Studies on Obstructive Apnea Syndrome
      Due to a Narrowed Airway

“Quality of sleep” has attracted a great deal of attention in the healthcare community recently. Obstructive apnea syndrome is one of the most common reasons for sleeplessness and it is the focus of this research. Obstructive apnea syndrome significantly reduces the airflow in and out of the airway in that it causes a thickening of the soft tissue in the patient’s larynx. Especially during the sleep, the area around the patient’s larynx becomes extremely narrow. As a result, the patient experiences difficulty in breathing and becomes restless. Obstructive apnea syndrome causes those who suffer from it to not be able to concentrate at work. This, in part, can have a negative effect on the patient’s overall quality of life.

The following section describes the processes involved in this research including importing the medical image reconstruction of the surface and geometric models, generating the mesh of the upper airway, and the flow analysis. Simulated velocity and pressure fields are useful in diagnosing respiratory diseases. We learn from this study that by analyzing the flow field in the upper airways and understanding the characteristics of the realistic flow field, more complete information is obtained that can significantly help with diagnosis.

*  Image Reconstruction and Numerical Models

In this research, the 3D geometry of the human airway was reconstructed using patient X-ray data. The 3D geometry was generated using the commercial software package Amira-TGS (Fig. 4). There were a total of 455 images scanned from the X-rays. Each image consisted of 512 x 512 voxels. The gap between the two scanned neighboring images was 0.8mm. After obtaining the reconstructed 3D model, we used the commercial software package ICEMTM to generate a computational fluid dynamics-based (CFD) 3D tetrahedral unstructured mesh. The simulation of the airflow in the trachea was rendered in 3D. In this study, the gravitational effect, heat source, heat transfer, phase change, and chemical reactions were all ignored. The airflow in the trachea was assumed to be laminar and incompressible. The fluid inhaled from the opening was considered to be normal air. The parameters at the inlet were preset as 1,038 for Re(ˇ×U D / ν), and 1.589ˇD10-5 m-2s-1 for kinetic viscosity ν. A no-slip condition was imposed on the surface of the trachea. The initial velocity and pressure in the trachea were zero.

After constructing the mesh, we performed the Finite Volume Method-based simulation using the commercial software package CFDRCTM. The computer used to execute the simulation was a PC Cluster provided by the National Center for High-Performance Computing (NCHC). The computational time required to reach the preset convergence condition was, on average, one day for each case studied.

Fig. 4. The pre and post surgery changes in the airway of a patient with obstructive apnea syndrome
ˇ¶ Fig. 4. The pre and post surgery changes in the airway of a patient with obstructive apnea syndrome

*  Results and Discussion

This research numerically analyzes the flow field of the narrowed upper airway resulting from obstructive apnea syndrome. By comparing the simulation results (constructed using Magnetic Resonance Imaging (MRI)) obtained from the airway geometry of a patient before and after surgery, we found that, before surgery, it was more difficult for the patient to breathe due to an abrupt drop in air pressure in the pharynx and throat. The condition greatly improved post surgery. 

The purpose of this study was to analyze the flow field of a patient suffering from obstructive apnea syndrome, pre and post surgery, by means of numerical simulations, and to provide a pre-surgery evaluation of the patient’s condition. By observing the pressure difference resulting from the changed vessels of the pharynx and larynx, the patient can better understand his condition and the physician can diagnose the disease more easily (Fig. 5). 

Fig. 5. The pre and post surgery drop in air pressure during inhalation of a patient with obstructive apnea syndrome
ˇ¶ Fig. 5. The pre and post surgery drop in air pressure during inhalation of a patient with obstructive apnea syndrome

*  References

[1] Granta, O., Bailie, N., Watterson, J., Cole, J., Gallagherb, G., Hannab, B., 2004, Numerical Model of a Nasal Septal Perforation, MEDINFO, 1352-1356.
[2] Castro Ruiz, P., Castro Ruiz, F., Costas López, A., Cenjor Español, C., 2005, Computational fluid dynamics simulations of the airflow in the human nasal cavity., Acta Otorrinolaringol Esp 56, 403-410.
[3] Reimersdahl, Th., Horschler, I., Gerndt, A., Kuhlen, T., Meinke, M., Schlondorff, G., Schroder, W., Bischof, C.H., 2001, Airflow simulation inside a model of the human nasal cavity in a virtual reality based rhinological operation planning system, International Congress Series 1230, 87–92.
[4] Lindemann, J., Brambs, H.J., Keck, T., Wiesmiller, K.M., Rettinger, G., Pless, D., 2005, Numerical simulation of intranasal airflow after radical sinus surgery, American Journal of Otolaryngology–Head and Neck Medicine and Surgery 26, 175 – 180.
[5] Xu, C., Sin S., McDonough, J.M., Udupa, J.K., Guez, A., Arens, A., Wootton, D.M., 2006, Computational fluid dynamics modeling of the upper airway of children with obstructive sleep apnea syndrome in steady flow, Journal of Biomechanics 39, 2043-2054
[6] Vos, W., Backer, J.D., Devolder, A., Vanderveken, O., Verhulst, S., Salgado, R., 2007, Correlation between severity of sleep apnea and upper airway morphology based on advanced anatomical and functional imaging, Journal of Biomechanics 40, 2207-2213.
[7] Jeong, S.J., Kim, W.S., Sung, S.J., 2007, Numerical investigation on the flow characteristics and aerodynamic force of the upper airway of patient with obstructive sleep apnea using computational fluid dynamics, Medical Engineering & Physics 29, 637-651.



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