Computational fluid dynamics study of the inspiratory upper airway and clinical severity of obstructive sleep apnea.

The apnea-hypopnea index (AHI) is a widely accepted measure for the severity of obstructive sleep apnea (OSA). Current methods to determine AHI fail to provide anatomic information for treatment decisions. In this report, we studied three-dimensional models of upper airways acquired by computed tomographic scanning with geometric measurements and computational fluid dynamics (CFD) analysis and evaluated the correlations with AHI.Participants had CT scans of their upper airways after standard polysomnography studies. Three-dimensional surface models of upper airways were generated for cross-sectional area measurements of the choanae (ACH) and the smallest cross-sectional area (Amin). Computational fluid dynamic analysis was then performed by using this three-dimensional model. Pressure differences required to set tidal volume during inspiration (ΔPmin-INSP) and expiration (ΔPmax-EXP) and minimum negative pressure produced in the level of ACH (Pmin-INSP at ACH) and Amin (Pmin-INSP at Amin) were calculated. Correlations of these parameters and the body mass index with AHI were analyzed. Statistical differences between groups of different AHI ranges were also compared.The pressure distribution simulated by CFD demonstrated abrupt pressure drops in Amin level, and this phenomenon was more significant in severe OSA. All parameters except ACH and Pmin-INSP at Amin significantly correlated with the AHI, and there were significant statistical differences between the OSA groups and the normal group. The results indicate that, in our study group, the geometry of pharyngeal airway and its CFD simulation correlate well with AHI. This model may be further applied for clinical evaluation.

Hemodynamic analysis of vascular stenting treatment outcome: computational fluid dynamics method vs optical flow method.

BACKGROUND AND PURPOSE: Computational fluid dynamics method (CFDM) and optical flow method (OFM) effectively provide the hemodynamic information based on the digital subtraction angiogram (DSA). However, the quantitative analysis in comparison of CFDM and OFM is still absent. The goal of this study is to apply CFDM and OFM in quantitative analysis of stenting treatment. MATERIAL AND METHOD: A left carotid stenosis patient underwent stenting of percutaneous transluminal angioplasty was analyzed as an example. CFDM and OFM for hemodynamic analysis on digital subtraction angiography before and after stenting treatment were presented. RESULTS: Improvement gains of blood flow velocities on left internal carotid artery after stenting treatment for different initial conditions on the common carotid artery were 1.91 ∼ 2.13, 1.62 ∼ 2.09, and 0.69 by CFDM with Newtonian and non-Newtonian fluids and OFM, respectively. With the CFDM analysis, the flow mapping by OFM using time resolved DSA data on the fly to estimate hemodynamic significance of a cervical carotid stenosis was explained. CONCLUSION: Quantificative blood flow estimations by CFDM and OFM to evaluate the treatment outcomes to patient with carotid stenosis are practical. Both methods are able to provide quantitative information of blood flow for stenting treatment. It is advantagious to use both methods in treatment evaluation.

Hemodynamic analysis of capillary in finger nail-fold using computational fluid dynamics and image estimation.

Red blood cell (RBC) dynamics in capillaries is a useful diagnostic tool for many diseases. Previous study showed that optical flow estimation (OFE) is capable of accurately calculating RBC velocities using image registration technique. The computational fluid dynamics (CFD) method is explored in this study to calculate the RBC velocity in capillaries of finger nail-fold for six cases. The two-dimensional capillary images were reconstructed to three-dimensional, assuming circular cross sections. The no-slip boundary conditions were applied on the vessel walls. The initial velocity of the RBC going into each capillary was calculated by OFE. The velocities of multiple points along each capillary calculated by CFD, V(CFD), were compared with OFE calculations, V(OFE). The calculated RBC velocity was in the range of 56-685µm/s. The average difference (V(CFD) - V(OFE)) with one standard deviation is -2.66±18.61µm/s for all the 48 calculation points, and 0.03±0.12µm/s for all except one points (47 points), indicating that CFD can provide a reasonable accuracy in RBC velocity calculation in finger nail-fold capillaries.

Computational fluid dynamic study on obstructive sleep apnea syndrome treated with maxillomandibular advancement.

Maxillomandibular advancement is one of the treatments available for obstructive sleep apnea. The influence of this surgery on the upper airway and its mechanism are not fully understood. The present research simulates the flow fields of narrowed upper airways of 2 patients with obstructive sleep apnea treated with maxillomandibular advancement. The geometry of the upper airway was reconstructed from computed tomographic images taken before and after surgery. The consequent three-dimensional surface model was rendered for measurement and computational fluid dynamics simulation. Patients showed clinical improvement 6 months after surgery. The cross-sectional area of the narrowest part of the upper airway was increased in all dimensions. The simulated results showed a less constricted upper airway, with less velocity change and a decreased pressure gradient across the whole conduit during passage of air. Less breathing effort is therefore expected to achieve equivalent ventilation with the postoperative airway. This study demonstrates the possibility of computational fluid dynamics in providing information for understanding the pathogenesis of OSA and the effects of its treatment.