[Role of the nose in inspiration in mouth breathing: quantitative determination by analysis of expiratory gases].

It is widely accepted that increased nasal resistance plays a major role in habitual mouth breathing. We investigated oronasal air flow distribution during voluntary mouth breathing in subjects without nasal obstruction. To determine whether nasal air flow contributes to total inspiratory flow, we administered 100% O2 by a nasal mask while the lips were kept apart by a mouth piece. Expired O2 concentrations measured at the mouth were a sensitive indicator of nasal admixture during inspiration. Theoretical considerations predict that mixed expired pO2 from two consecutive steady state periods should allow calculation of nasal admixture. Measurements made on 22 healthy volunteers revealed a very variable degree of nasal contribution to inspiratory air flow (mean +/- SD, 25 +/- 15%, range 3-70%). There was no correlation between this proportion and anthropometric data, smoking habits, nasal resistance, or presence of rhinitis. We suggest that changes in the position of the soft palate, tongue, and/or pharyngeal wall associated with respiration are mainly responsible for the within and between subject variation observed in this study. This explanation is consistent with recent experimental findings on the pharyngeal dilating muscles.