Effect of nasal airway nonlinearities on oscillometric resistance measurements in infants

Oscillometric measurements of respiratory system resistance (R ) in infants are usually made via the nasal pathways, which not only significantly contribute to overall R but also introduce marked flow (V')-dependent changes. We employed intrabreath oscillometry in casts of the upper airways constructed from head CT images of 46 infants. We examined oscillometric nasal resistance (R ) in upper airway casts with no respiratory flow (R ) and the effect of varying V' on R by simulating tidal breathing. A characteristic nonlinear relationship was found between R and V', exhibiting segmental linearity and a prominent breakpoint (V' ) after log-log transformation. V' was linearly related to the preceding value of end-expiratory volume acceleration (V″ ; on average = 0.96, < 0.001). R depended on V', and R at end-expiration (R ) showed a strong dependence on V″ in every cast ( = 0.994, < 001) with considerable interindividual variability. The intercept of the linear regression of R versus V″ was found to be a close estimate of R . These findings were utilized in reanalyzed R data acquired in vivo in a small group of infants ( = 15). Using a graphical method to estimate R from R , we found a relative contribution of V'-dependent nonlinearity to total resistance of up to 33%. In conclusion, we propose a method for correcting the acceleration-dependent nonlinearity error in R . This correction can be adapted to estimate R from a single intrabreath oscillometric measurement, which would reduce the masking effects of the upper airways on the changes in the intrathoracic resistance. Oscillometric measurements of respiratory system resistance (R ) in infants are usually made via the nasal pathways, which not only significantly contribute to overall R but also introduce marked flow acceleration-dependent distortions. Here, we propose a method for correcting flow acceleration-dependent nonlinearity error based on in vitro measurements in 3D-printed upper airway casts of infants as well as in vivo measurements. This correction can be adapted to estimate R from a single intrabreath oscillometric measurement.