Effect of nasal airway nonlinearities on oscillometric resistance measurements in infants

Abstract

Oscillometric measurements of respiratory system resistance (Rrs) in infants are usually made via the nasal pathways, which not only significantly contribute to overall Rrs, 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 (Rn) in upper airway casts with no respiratory flow (R0), and the effect of varying V' on Rn by simulating tidal breathing. A characteristic nonlinear relationship was found between Rn and V', exhibiting segmental linearity and a prominent breakpoint (V'bp) after log-log transformation. V'bp was linearly related to the preceeding value of end-expiratory volume acceleration (V"eE; on average r2=0.96, p<0.001). Rn depended on V', and R at end-expiration (ReE) and showed a strong dependence on V"eE in every cast (r2=0.994, p<001) with considerable inter-individual variability. The intercept of the linear regression of ReE vs. V"eE was found to be a close estimate of R0. These findings were utilised in re-analysed Rrs data acquired in vivo in a small group of infants (n=15). Using a graphical method to estimate R0 from ReE, 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 non-linearity error in ReE. This correction can be adapted to estimate R0 from a single intrabreath oscillometric measurement, which would reduce the masking effects of the upper airways on the changes in the intrathoracic resistance.

Respiratory Oscillometry in Newborn Infants: Conventional and Intra-Breath Approaches

“…These biphasic changes in Rrs, characterized by minimum values at zero V’ and local maxima at peak inspiratory and expiratory V’ (V’ maxI and V’ maxE , respectively) were a marked feature in the neonates of this study, with the non-linearity in expiration usually exceeding that of inspiration. Previous observations suggest that the non-linear, V’-dependent increase in Rrs originates from the upper airways ( 12 , 17 , 18 ), obeying the classical empirical description by Rohrer ( 19 ).…”

“…Another factor, also augmented in nasal breathing is the non-steady flow patterns that develop at fast transitions of V’ in the upper airways of irregular geometry. This leads to extra dissipation, which has been shown to depend on the rate of change in V’ (i.e., on V”) ( 12 , 17 , 18 ), and would add to the true “zero-flow” values of R eE and R eI . In the present study, the asymmetry of respiratory phase change (as characterized by the ratio V” eE /V” eI ) was shown to correlate with ΔR.…”

“…Additionally, a new tracking modality of oscillometry ( 6 ) has revealed disease-specific patterns of intra-breath changes in Zrs and has proven unique in predicting lower respiratory tract illness during infancy ( 7 ). However, a comprehensive analysis is still needed to fully characterize the intra-breath dynamics of Zrs in infants, with special regard to the substantial contribution of the upper airways ( 8 – 12 ). Confrontation of the novel intra-breath oscillometry with conventional spectral oscillometry is also lacking.…”

Ventilation Mechanics

“…However, it could be possible that nonlinearities become apparent even within a single V T maneuver, particularly at high PEEP values since the elastic properties are more nonlinear at high lung volumes. [48][49][50][51] In this case, the elastic nonlinearity is revealed by an upward curvature of the P ao at the final phase of inspiration with constant flow.…”

“…However, this hypothesis is questioned in some important ventilatory conditions in healthy subjects and particularly in patients. [48][49][50][51][52][53][54][55][56][57][58][59][60] A clear example in healthy subjects is exercise given the considerable flow values associated with high-amplitude and high-rate breathing.…”