The relationship between nasal airway cross-sectional area and nasal resistance

Abstract

Mouth breathing in response to an impaired nasal airway is thought to have clinical consequences. Physiologically, mouth breathing occurs whenever the body senses that nasal resistance is inappropriately high. In physical terms mouth breathing is a response that enlarges the upper airway and, by doing so, reduces airway resistance. In the past measurements of nasal resistance have been used as an index of airway impairment. Recently, we introduced a technique that estimates cross-sectional size of the airway, a variable that directly determines the magnitude of airway resistance. The purpose of the present study was to determine the precise effects of nasal airway size on nasal airway resistance so that the relationship between the two could be described in mathematic terms. There were two phases to the study—one involving a model and simulated breathing, and the other involving 100 subjects demonstrating normal and impaired nasal airways. The pressure-flow technique for estimation of nasal airway size and nasal airway resistance was used. The following equation was generated from the data: Resistance = 1.9 + 0.03 + [0.0009 ∗ flow] Area 2 The relationship between the two variables is nonlinear—that is, size of the airway has its greatest effect on resistance when the airway is <0.4 cm 2 and a much lesser effect at larger airway sizes. The study also showed that nasal airway resistance generally does not fall very much below 1.9 cm H 2O/L/S during breathing even when the airway is very large. This probably relates to the need to maintain an adequate level of airway resistance for alveolar gas exchange. Finally, the equation enables us to compare judgments of nasal airway impairment based on previous resistance studies with those based on recently introduced area measurements. The data support our contention that in adults an airway <0.4 Cm 2 is impaired.