title: Effect of CO2 concentrations on acoustic inferences of airway area.
abstract: To determine the effect of gas composition on the accuracy of measurements of airway area and distance using an acoustic reflection technique, we employed glass-tube models to simulate pharyngeal (Phar-model), laryngeal (Lar-model), and tracheal (Trach-model) regions of upper and central airways. We made repeated measurements of area-distance functions using gas mixtures containing 0, 2, 4, 6, 8, and 10% CO2, 80% He, and balance O2. The actual area of the model was calculated from the roentgenographic data and compared favorably with an area measured by acoustic reflections using a gas mixture containing 0% CO2. With the different gas mixtures, calculated area was overestimated only at the highest levels of CO2, with Phar-model area increasing from (mean +/- SD) 4.66 +/- 0.03 cm2 measured with 0% CO2 to 4.93 +/- 0.05 cm2 (P less than 0.05) measured with CO2 concentration of 10%. To assess the effect of CO2 concentration on measurements of distance, we isolated two discrete points located in the Phar-model and Lar-model regions. When measurements were performed using 10% CO2 mixture, Phar-model point was shifted by 1.02 +/- 0.03 cm and Lar-model point was shifted by 2.16 +/- 0.09 cm away from the microphone compared with their axial position determined, using 0% CO2 mixture (P less than 0.05). Differences in area-distance calculations at the higher levels of CO2 did not exceed the within-run variability of the technique (10 +/- 4%). We conclude that CO2 absorbers are not required during measurements of airway area by acoustic reflections, provided CO2 concentration does not exceed 10%.
authors.forename: A D
authors.initials: AD
authors.lastname: D'Urzo
authors.forename: A S
authors.initials: AS
authors.lastname: Rebuck
authors.forename: V G
authors.initials: VG
authors.lastname: Lawson
authors.forename: V
authors.initials: V
authors.lastname: Hoffstein
mesh_terms: D000162:Acoustics; D000818:Animals; D002245:Carbon Dioxide; D006801:Humans; D007830:Larynx / Q000033:anatomy & histology; D008953:Models, Anatomic; D010614:Pharynx / Q000033:anatomy & histology; D012137:Respiratory System / Q000033:anatomy & histology*; D014132:Trachea / Q000033:anatomy & histology
chemical_list: D002245:Carbon Dioxide