The relationship between nasal resistance to airflow and the airspace minimal cross-sectional area

Abstract The relationship between nasal resistance (R) and airspace minimal cross-sectional area (mCSA) remains unclear. After the introduction of acoustic rhinometry, many otolaryngologists believed that mCSA measurements would correlate with subjective perception of nasal airway obstruction (NAO), and thus could provide an objective measure of nasal patency to guide therapy. However, multiple studies reported a low correlation between mCSA and subjective nasal patency, and between mCSA and R. This apparent lack of correlation between nasal form and function has been a long-standing enigma in the field of rhinology. Here we propose that nasal resistance is described by the Bernoulli Obstruction Theory. This theory predicts two flow regimes. For mCSA > Acrit , the constriction is not too severe and there is not a tight coupling between R and mCSA. In contrast, when mCSA < Acrit , nasal resistance is dominated by the severe constriction and it is predicted to be inversely proportional to the minimal cross-sectional area (R∝mCSA−1 ). To test this hypothesis, computational fluid dynamics (CFD) simulations were run in 3-dimensional models based on computed tomography scans of 15 NAO patients pre- and post-surgery (i.e., 60 unilateral nasal cavities). Airspace cross-sectional areas were quantified perpendicular to airflow streamlines. Our computational results are consistent with the theory. Given that in most people mCSA > Acrit (estimated to be 0.37 cm2 ), this theory suggests that airway constrictions are rarely an exclusive contributor to nasal resistance, which may explain the weak correlation between mCSA and subjective nasal patency.