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

Abstract

Background: Oscillometry has been employed widely as a non-invasive and standardized measurement of respiratory function in children and adults; however, limited information is available on infants.

Aims: To establish the within-session variability of respiratory impedance (Zrs), to characterize the degree and profile of intra-breath changes in Zrs and to assess their impact on conventional oscillometry in newborns.

Methods: 109 healthy newborns were enrolled in the study conducted in the first 5 postpartum days during natural sleep. A custom-made wave-tube oscillometry setup was used, with an 8-48 Hz pseudorandom and a 16 Hz sinusoidal signal used for spectral and intra-breath oscillometry, respectively. A resistance-compliance-inertance (R-C-L) model was fitted to average Zrs spectra obtained from successive 30-s recordings. Intra-breath measures, such as resistance (Rrs) and reactance (Xrs) at the end-expiratory, end-inspiratory and maximum-flow points were estimated from three 90-s recordings. All natural and artifact-free breaths were included in the analysis.

Results: Within-session changes in the mean R, C and L values, respectively, were large (mean coefficients of variation: 10.3, 20.3, and 26.6%); the fluctuations of the intra-breath measures were of similar degree (20-24%). Intra-breath analysis also revealed large swings in Rrs and Xrs within the breathing cycle: the peak-to-peak changes amounted to 93% (range: 32-218%) and 41% (9-212%), respectively, of the zero-flow Zrs magnitude.

Discussion: Intra-breath tracking of Zrs provides new insight into the determinants of the dynamics of respiratory system, and highlights the biasing effects of mechanical non-linearities on the average Zrs data obtained from the conventional spectral oscillometry.

Keywords: infant oscillometry; intra-breath method; nasal resistance; respiratory compliance; respiratory reactance; respiratory resistance.

FIGURE 1

Definition of specific intra-breath measures of resistance (Rrs) and reactance (Xrs). Shown are the time points of end inspiration (eI)- and end expiration (eE) (indicated by dotted lines), their differences (Δ) and peak-to-peak changes in inspiration (ppI) and expiration (ppE).

FIGURE 2

Examples of short term changes in impedance (Zrs) and breathing pattern. Each graph represents a 12-s period. (A) Slightly irregular tidal flow but stable and low Zrs. (B) Slow downward drift in reactance (Xrs) during regular breathing. (C) Increasing flow dependence of Zrs during steady-state breathing; this probably reflects spontaneous development of nasal obstruction. (D) Transient decrease of expiratory flow limitation after a spontaneous sigh (arrow).

FIGURE 3

Typical patterns of intrabreath changes in respiratory impedance (Zrs). (A) Insignificant changes in reactance (Xrs) and mild flow non-linearity in resistance (Rrs); (B) marked fall in Xrs and increase in Rrs during expiration; (C) marked fall in Xrs in inspiration; (D) increases in Xrs and Rrs with both inspiratory and expiratory flow.

FIGURE 4

Clusters established in the relationships between inspiratory and expiratory flow dependences of reactance. X_V’maxI and X_V’maxE are reactance values at peak inspiratory and expiratory flows, X₀ is the average zero-flow value of reactance. For definitions of Patterns A-D, see text or the legend to Table 1.

FIGURE 5

Correlogram for selected measures of tidal breathing, spectral oscillometry and intra-breath oscillometry. For definition of variables, see legend to Table 4.

FIGURE 6

Effect of asymmetry in volume acceleration (V”) ratio on T_PTEF/T_E. eE: end expiration; eI: end inspiration; T_PTEF: time to peak tidal expiratory flow; T_E: expiratory time. For definitions of Patterns A–D, see text or the legend to Table 1.