Airway compliance measurements in mouse models of respiratory diseases

Abstract

The quantification of airway compliance (C_aw) is essential to the study of airway alterations in disease models. However, the required measurements of airway pressure and volume are difficult to acquire in mice. We hypothesized that the inflation limb of full-range pressure-volume (PV) curves could be used to quantify C_aw, as it contains a segment where only the airway tree is distended. The study objective was to assess the feasibility of the approach by analysis of full-range PV curves previously collected in three mouse models: an elastase model of emphysema, a genetic model spontaneously developing emphysema (leukotriene C4 synthase knockout; LTC4S-KO), and a bleomycin model of lung fibrosis. Attempts to validate results included C_aw change relative to respiratory system compliance (ΔC_aw/ΔC), the minute work of breathing (mWOB), and the elastance at 20.5 Hz (E_rs_20.5) from prior respiratory mechanics measurements in the same subjects. C_aw was estimated at 3% of total compliance in healthy mice or 2.3 ± 1 μL/cmH₂O (n = 17). The technique detected changes in models of respiratory obstructive and restrictive diseases relative to control mice as well as differences in the two emphysema models studied. The changes in C_aw were consistent with those seen in ΔC_aw/ΔC, mWOB, or E_rs_20.5, with some variations according to the model, as well as with results reported in the literature in humans and mice. Direct C_aw measurements in subjects as small as mice could prove useful to further characterize other respiratory disease models associated with airway remodeling or to assess treatment effects.

Keywords: LTC4 synthase; airway compliance; emphysema; fibrosis; pressure-volume curves.

Figure 1.

Study design used for the determination of airway compliance in three mouse models of respiratory diseases. A: diagram representation of the study design including the number of animals (n) per experimental group. B: diagram summarizing of the experimental procedures used in each study included in the analysis.

Figure 2.

Full-range pressure-volume curves and lung opening pressures in three models of respiratory diseases. A–C: average (±SD) full-range pressure-volume (PV) curves. A: control (n = 5) and elastase-treated mice (n = 5). B: wild-type (WT, n = 4) and leukotriene C4 synthase knockout (KO, n = 6) mice. C: control (n = 4) and bleomycin-treated mice (n = 6). In the elastase-induced model of emphysema (A), only the full-range PV curves constructed using the automated method (flexiVent FX) were included (3). D–F: lung opening pressure (P_op) in the three mouse models studied and their respective controls. Individual results (n = 4–9/group) and group means (±SD) are presented for each group. G and H: correlation between the lung opening pressure (P_op) and the compliance of the respiratory system in the elastase (G) and the genetic (H) models of emphysema. The compliance of the respiratory system (C) was calculated as the slope of the PV deflation limb between 3 and 7 cmH₂O. The dashed lines represent the 95% confidence interval of the best-fit line (solid line) and R² is the coefficient of determination assessing the quality of the fit. *P < 0.05, t test.

Figure 3.

Determination of airway compliance. A–C: average (±SD) pressure-volume (PV) inflation curve segments for the determination of airway compliance. A: control (n = 5) and elastase-treated mice (n = 5). B: wild-type (WT, n = 4) and leukotriene C4 synthase knockout (KO, n = 6) mice. C: control (n = 4) and bleomycin-treated (n = 6) mice. In the elastase-induced model of emphysema (A), only the curves from the animals subjected to the automated method (flexiVent FX) were included (3). D–F: airway compliance (C_aw) calculated by linear regression as the slope of the PV inflation segment. Individual results (n = 4–9/group) and group means (±SD) are presented for each group. *P < 0.05, t test.

Figure 4.

Inspiratory work of breathing and respiratory elastance at 20.5 Hz. A–C: normalized inspiratory work of breathing per units of time (mWOB). A: control (n = 9) and elastase-treated (n = 8) mice. B: wild-type (WT, n = 4) and leukotriene C4 synthase knockout (KO, n = 6) mice. C: control (n = 4) and bleomycin-treated (n = 6) mice. D–F: respiratory elastance at 20.5 Hz (E_rs_20.5) in the three mouse models studied and their respective controls. Individual results (n = 4–9/group) and group means (±SD) are presented for each group. G–I: correlation between airway compliance (C_aw) and mWOB in the genetic model of emphysema (H) or E_rs_20.5 in the bleomycin model of lung fibrosis (I). Resistance at the same frequency (R_rs_20.5) was also extracted to complement the evaluation. A statistical correlation (Pearson’s r = 0.5731, P = 0.0162) was found between R_rs_20.5 and C_aw in the elastase-induced model of emphysema (G). However, the low coefficient of determination (R² = 0.3284) for the best-fit curve (solid line) and the wide 95% confidence interval (dash lines) suggest that the correlation is driven by a single point slightly outside of the group. *P < 0.05, t test.