Partitioning of ventilation between nose and mouth: the role of nasal resistance.

We have examined the relationship between nasal resistance (Rna) and the distribution of ventilation between the nose and mouth in 10 normal breathing children and 15 children who met clinical criteria of mouth breathing. We studied Rna by posterior rhinometry. We used a face mask divided into separate oral and nasal chambers to measure oral and nasal components of ventilation. Each chamber of the mask was connected to a separate pneumotachograph. We measured oral and nasal tidal volumes (VTna) by integration of the oral and nasal flow, and calculated the total tidal volume (VTtot) by summing the oral and nasal components. The nasal fraction of ventilation (F-VTna) was calculated by dividing VTna by VTtot. We found a weak inverse correlation between Rna and F-VTna, but eight of 25 children did not breathe as one might predict on the basis of Rna, and eight of 15 children who appeared to be mouth breathers actually breathed through the nose. We administered a vasoconstricting nasal spray and a placebo nasal spray to the children and, although Rna changed significantly, we observed no change in the distribution of flow between the nose and mouth. In summary we found that clinical criteria of mouth breathing do not accurately identify children who actually breathe mainly through the mouth. Moreover Rna is only a weak predictor of the pattern of breathing; hence other factors may be important determinants of the distribution of flow between the nose and mouth.(ABSTRACT TRUNCATED AT 250 WORDS)