Computational Fluid Dynamics Study of Obstructive Sleep Apnea Syndrome using Static and Quasi-Dynamic Methods in Obese Adolescents

Obstructive Sleep Apnea Syndrome (OSAS) is a disorder characterized by partial or complete narrowing of the pharynx during sleep, resulting in periods of airflow cessation, oxygen desaturation, and sleep disruption. Both anatomic and physiologic factors affecting upper airway size, shape, and function may play a role in the causation of OSAS. Computational fluid dynamic (CFD) analysis was used to model the effect of airway geometry on internal pressure and velocity in the upper airway of obese adolescents with obstructive sleep apnea syndrome (OSAS). Two separate studies were conducted, both based upon magnetic resonance images (MRI) and flow data: (1) MR images acquired during quiet tidal breathing provided a patient-specific static model of the upper airway in twelve OSAS and twelve control subjects and were solved at the patient-specific maximum inspiratory flow rate using a low-Reynolds number k-o turbulence model; (2) volume-gated MR images acquired during normal tidal breathing at ten volume increments of the respiratory cycle provided a quasi-dynamic view of one OSAS and one control subject and were solved using a low-Reynolds number k-o turbulence model, driven by flow data averaged over 12 consecutive breathing cycles. In addition, rhinomanometry and anatomical volume data was collected to compare against CFD calculations.;In the static study, restricted airways created increased resistance. Pharyngeal and nasopharynx resistance and minimum surface pressure calculations were significantly different between OSAS and control subject groups. Bernoulli's principle was used to calculate nasopharynx resistance given inspiratory flow rate and relevant cross-sectional areas. There was a strong correlation between CFD and Bernoulli's results, validating Bernoulli's principle as accurate candidate nasopharynx resistance calculation method.;In the quasi-dynamic study, collapse in the OSAS initiated in the proximal nasopharynx and continued downstream into the oropharynx. A high velocity turbulent jet, lasting throughout inspiration, was located at the point of initial collapse in the nasopharynx. A second, intensified, high velocity jet formed in the oropharynx during later inspiration. Pharyngeal flow resistance at 10% inspiration was 0.220 kPa/I/s and increased continuously up to 1.725 kPa/I/s at 90% inspiration. Expiratory resistance averaged 0.091 kPa/I/s. Tube laws (pressure vs. cross-section area), derived for different locations along the airway, indicated that the oropharynx was more compliant than the nasopharynx (1.028 mm2/Pa vs. 0.449 mm2/Pa), and had a lower theoretical limiting flow rate, confirming the oropharynx as the flow-limiting segment of the airway in this subject. This new method may help to differentiate anatomical and functional factors in airway collapse.