Journal of Applied Physiology, Vol 60, Issue 2 398-401, Copyright © 1986 by American Physiological Society
Effect of CO2 concentrations on acoustic inferences of airway area
A. D. D'Urzo, A. S. Rebuck, V. G. Lawson and V. Hoffstein
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%.
