Comparison of feline nasal cavity dimensions measured by acoustic rhinometry and nasal casts.

BACKGROUND: The goal of this study was to evaluate the relationship between feline nasal cavity geometry determined in vivo by acoustic rhinometry (AR(in vivo)) and by nasal cavity casts. Cast cross-sectional areas were measured by acoustic rhinometry (AR(cast)), a fluid-displacement method (FDM), and slicing. A volume comparison between AR(in vivo) and AR(cast) was studied in cats with varying degrees of nasal obstruction after application of phenylpropanolamine, saline, or compound 48/80. METHOD: After measurements of AR(in vivo), impression material was injected into the nasal cavity to produce casts. Subsequently, the cross-sectional areas of the nasal impressions were measured by AR(cast) and FDM using ethanol. All casts were weighed to determine exact volume. Six casts also were sliced into segments of equal thickness for determination of cross-sectional area. RESULTS: Cast volume determined by AR(cast) was consistent with results obtained using FDM and weight. Volumes of the first 3 cm determined by AR(in vivo) ranged between 78 +/- 9% of cast volumes determined by AR(cast) for decongested cavities and 16 +/- 15% for congested cavities. CONCLUSION: AR(in vivo) does not reflect cast geometry, probably because of (1) underestimation by AR because of methodological problems caused by the cavity geometry, (2) deformation of compliant structures within the nasal passageways resulting from the casting procedure, and/or (3) the casting material reaches parts of the nasal cavity not accessible to sound, e.g., sinuses or recesses. Nevertheless, this study does not preclude the use of AR as a sensitive method suited to evaluate relative changes in nasal volume caused by experimental challenges of the nasal mucosa. Compared with AR(in vivo), casts still may be of use but it is less sensitive to measure relative changes after experimental challenge.