Evaluating Ecogeographic Variation in Human Nasal Passages Using In-Silico Decongestion of the Nasal Cycle
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To protect the lungs from desiccation and thermal damage, inspired air must be heated to core body temperature (37°C) and 100% saturated with water vapor upon reaching pulmonary tissues. The majority of air conditioning occurs in the nasal passages, where heat and moisture are transferred to inspired air from nasal mucosa via concurrent convection and evaporation. Given that physiological demand for air conditioning is largely dependent on the external environment, many studies have noted strong associations between climate and ecogeographic patterning of human nasal morphology. Specifically, these studies have shown that individuals indigenous to cold-dry environments exhibit relatively longer, taller, and narrower nasal passages than individuals from hot-humid climates. These apparent climate-mediated morphologies are assumed to reflect functional differences, with longer, taller, and narrower nasal passages in cold-dry climates enhancing respiratory heat and moisture exchange via the increased relative mucosal surface area. However, due to the nasal cycle, direct associations between nasal passage dimensions and mucosal surface area to airway volume (SA/V) have been challenging to quantitatively establish. The nasal cycle refers to the alternating congestion and decongestion of the venous sinuses lining the nasal turbinates and part of the septum. In this study, we tested associations between nasal passage height, breadth, and length dimensions and passage SA/V ratio via 3D morphometric assessments of computed tomography (CT) cranial scans in 8 individuals - four of European ancestry (EA) and four of African ancestry (AA). 3D models of the nasal passages were created using 3D Slicer software. Airway models were artificially dilated in-silico to simulate fully decongested nasal passages. Morphometric measurements of passage height, breadth, and length dimensions; mucosal surface area; and airway volume were collected from both congested and decongested airway models. Consistent with expectations, there appears to be no noticeable difference in median congested SA/V ratios (p=0.8) between EA subjects (SA/V=0.99) and AA subjects (SA/V=0.99). There was a trend for EA subjects to have greater median decongested SA/V ratios (SA/V=0.51) than AA subjects (SA/V=0.49), however, this difference did not reach statistical significance with the number of subjects included in our study (p=0.2). Further analysis reveals EA individuals' nasal length to be significantly longer than AA individuals (p=0.028) while all other measurements (nasal height and nasal breadth) demonstrate expected trends but are not significant (all p≥0.8). Our results provide evidence to support that the congestion level of subjects on CT imaging could significantly impact morphometric analysis of the nasal cavity and computational fluid dynamics (CFD) analysis of nasal airflow - and thus is important to consider when creating models for CFD analysis. Future research employing CFD analyses may provide insight into how morphological differences impact intranasal heat and moisture exchange thus providing further insight into how ecogeographic variation in human nasal morphology may reflect climatically adaptive differences in nasal function.