# In-Vitro Experimental Modeling of Oscillatory Respiratory Flow in a CT-Scanned OSAHS Tract
## 1 Introduction  
The human airway is a key organ for the body to exchange air with the external environment. However, this passageway is susceptible to inhaled particulate matters, extreme atmospheric conditions, bad individual habits, and natural aging, resulting in a wide spectrum of respiratory diseases ranging from the simple common cold, snoring, nasal airway blockage and obstructive sleep apnea-hypopnea syndrome (OSAHS) [1]. In the case of such disorders, OSAHS is highly prevalent and poses a high risk of damaging human health and life [2; 3; 4; 5]. Its severity is highly correlated to the flow characteristics of the upper airway [6]. Thus, the knowledge of OSAHS respiratory flow in the upper airway is of scientific significance for the establishment and development of OSAHS physiology and pathology, and of clinical medicine references for the diagnosis and therapy of OSAHS diseases. Compared with the in-vivo measurement of respiratory flow in a realistic human airway, the in-vitro measurement of respiratory flow can effectively provide more detailed flow fields, allowing the respiratory physiologist to deepen the understanding of OSAHS pathology and the respiratory physician to improve diagnosis and therapy for OSAHS diseases. Nonetheless, OSAHS physiological studies of airway anatomy has earned much less attention than healthy airways. Only a small number of studies have focused on respiratory flow in OSAHS-like tract, although much experimental research on the respiratory flow has been carried out in simplified smooth multiple-generation airwaymodels, which were briefly reviewed in [PERSON] et al. [7]. Following a series of numerical works on steady inspiratory or expiratory flow in smooth symmetrical and asymmetrical multiple-generation bifurcating airway models [8; 9; 10], [PERSON] and his collaborators extended their numerical methods to unsteady respiratory flow in a human upper airway (HUA) model before and after OSAHS surgery [11]. Their numerical results confirmed that after surgery, the HUA flow resistance decreases with even more streamlined flow patterns, demonstrating the potential application of OSAHS respiratory flow in OSAHS surgery. Later, they numerically simulated the steady and unsteady flows in a HUA model with OSAHS resulting from nasopharyngeal obstruction [12]. Their numerical results indicated that the respiratory flow with OSAHS features strong flow injection caused by narrowing of the pharynx at both inspiration and expiration stages. Recently, [PERSON] et al. [13] numerically simulated steady inspiration flows in a nasopharyngeal obstruction model with OSAHS. Their model is quite similar to that used in [PERSON] et al. [12]. The numerical results found that inspiratory flow velocity reached a maximum at the obstructed area with a strong negative pressure gradient. Very recently, [PERSON] et al. [14] used the large eddy simulation (LES) method to investigate oscillatory respiration flows in an idealized extra-thoracic (ETA) airway. The simulated results indicated that the unsteady airflow structures are highly impacted by inspiration and expiration phases, and the techniques to model the breathing process. As for the experimental measurements of respiratory flow, [PERSON] and [PERSON] [15] used tomographic particle image velocimetry (PIV) techniques to measure the airflow in disordered and corrected nasal cavity models. Later, [PERSON] and his collaborators evaluated the feasibility of 3D steady tomographic PIV measurements in a normal nasal cavity [16]. More recently, [PERSON] et al. [17] implemented a smoke-wire visualization of steady flow at the mid-sagittal plane of a HUA model with OSAHS by using a high-speed camera. Their experimental visualization, together with numerical simulation, suggested that for OSAHS patients, HUA narrowing is a symptom of OSAHS development and deterioration. All of the above in-vitro experimental models for OSAHS respiratory flow were limited to steady flow cases.