Airway area by acoustic response measurements and computerized tomography

In order to determine more precisely the accuracy with which the acoustic reflection technique (ART) can infer airway area during spontaneous breathing, we compared acoustic measurements of airway area with equivalent areas measured from computerized tomographic (CT) scans of the neck and chest in 7 patients (mean age, 54 yr; range, 33 to 69 yr) with a history of upper airway abnormalities. At the time of the study, all patients were clinically stable and had no recurrent nerve palsy. Measurements of airway area by ART and CT were performed in the supine posture while patients breathed quietly at FRC. We found that there was considerable intersubject variability in area-distance functions determined by acoustic reflections. None of the subjects had a flat tracheal plateau. Once the acoustic and CT data were aligned, we compared cross-sectional areas at various distances from the glottis. Comparison points were separated by 1 cm, and as many as 13 different CT sections were used in some subjects. Mean values for all data points (n = 83) were 2.45 +/- SD = 0.69 cm2 and 2.56 +/- SD = 0.82 cm2 for the acoustic and CT methods, respectively, Z = 0.93; p greater than 0.05. Linear regression analysis revealed a correlation coefficient (r) of 0.92; p less than 0.0001. On the basis of these findings, we conclude that the acoustic reflection technique may be used reliably for clinical and physiologic studies of the upper airways in humans.