Korean J Orthod.  2010 Apr;40(2):66-76. 10.4041/kjod.2010.40.2.66.

Morphological characteristics of the upper airway and pressure drop analysis using 3D CFD in OSA patients

• Mo SS
• Ahn HT
• Lee JS
• Chung YS
• Moon YS
• Pae EK
• Sung SJ

• KMID: 2273214
• DOI: http://doi.org/10.4041/kjod.2010.40.2.66

Abstract

OBJECTIVE
Obstructive sleep apnea (OSA) is a common disorder which is characterized by a recurrence of entire or partial collapse of the pharyngeal airway during sleep. A given tidal volume must traverse the soft tissue tube structure of the upper airway, so the tendency for airway obstruction is influenced by the geometries of the duct and characteristics of the airflow in respect to fluid dynamics.
METHODS
Individualized 3D FEA models were reconstructed from pretreatment computerized tomogram images of three patients with obstructive sleep apnea. 3D computational fluid dynamics analysis was used to observe the effect of airway geometry on the flow velocity, negative pressure and pressure drop in the upper airway at an inspiration flow rate of 170, 200, and 230 ml/s per nostril.
RESULTS
In all 3 models, large airflow velocity and negative pressure were observed around the section of minimum area (SMA), the region which narrows around the velopharynx and oropharynx. The bigger the Out-A (outlet area)/ SMA-A (SMA area) ratio, the greater was the change in airflow velocity and negative pressure.
CONCLUSIONS
Pressure drop meaning the difference between highest pressure at nostril and lowest pressure at SMA, is a good indicator for upper airway resistance which increased more as the airflow volume was increased.

Keyword

OSA; Upper airway; Computational fluid dynamics; Pressure drop

MeSH Terms

Airway Obstruction
Airway Resistance
Humans
Hydrodynamics
Oropharynx
Recurrence
Sleep Apnea, Obstructive
Tidal Volume

Figure

• 

  Fig. 1 Construction of 3D upper airway. A, Segmentation of the air region. The black color in the dotted box shows the nasal cavity; B, 3D surface model. 3D surface model of the upper airway constructed from the segmented images using Bionix body builder software. Smoothing was performed two times with the built-in options of Laplacian and boundary-edge smoothing algorithms for the most optimal model; C, AWL (airway length) measurements were done on the lateral cephalogram. Palatal plane (PP), tip of the uvular (TU), tip of the epiglottis (TE), upper esophageal sphincter (UES), upper esophageal sphincter plane (UESP); D, 3D model generation. The inside of the 3D surface model was meshed into the tetrahedron element; E, the inlet (nostril) and outlet (on the UESP) regions were assigned respectively.

• 

  Fig. 2 Morphological characteristics of upper airway models for 3 subjects. Section of minimum area (SMA) is indicated by the dotted line in the right side of the model. A, Model A; B, model B; C, model C. The shapes of the SMA were compared by a 5 mm unit mesh diagram. The left (L), right (R), ventral (V), dorsal (D), inferior and superior views were compared. SMA, The area of the section of minimum area; SMA-W, the lateral width of the SMA; SMA-T, the anterior-posterior thickness of the SMA; In-A, the cross-sectional area of left and right nostrils; Out-A, the area of hypopharynx in the model.

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  Fig. 3 Comparison of the area measurements of upper airway models. In-A, Cross-sectional area of the left and right nostrils; SMA-A, the area of the section of minimum area; Out-A, the area of hypopharynx in the model.

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  Fig. 4 Comparison of the linear measurements of upper airway models. SMA-W, The lateral width of the SMA (section of minimum area); SMA-T, the anterior-posterior thickness of the SMA; AWL, the length of upper airway.

• 

  Fig. 5 Comparison of the changes of the airflow velocity at the flow rate of 200 ml/s. A, Maximum velocity was observed at the SMA (section of minimum area) region. More airjet regions were observed; B, maximum velocity was observed near the tip of the epiglottis, but the maximum velocity magnitude was lower than those in model A; C, maximum airflow velocity was observed at the narrowest portions at the SMA.

• 

  Fig. 6 Axes and negative pressure patterns of model A, B, and C at the flow rate of 200 ml/s. A, Negative pressure increases at the collapsed regions. The maximum negative pressure occurs at the narrowest region below the SMA (section of minimum area) region; B, the maximum negative pressure occurs at the epiglottis region; C, the maximum negative pressure were observed at the SMA which is narrowest region and the epiglottis tip region.

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  Fig. 7 Comparison of the cross sectional area ratio and pressure drop of upper airway models. SMA-A, The area of the section of minimum area; In-A, the cross-sectional area of left and right nostrils; Out-A, the area of hypopharynx in the model.

• 

  Fig. 8 Changes of the maximum pressure drop by the air flow rate. In all models, maximum pressure drop (differences between the highest pressure in the inlet region and the lowest pressure in the section of minimum area region) increased as the air flow rate increased, and the largest pressure drop occurred in model A.

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