Analysis of regional mechanics in canine lung injury using forced oscillations and 3D image registration

Abstract

Acute lung injury is characterized by heterogeneity of regional mechanical properties, which is thought to be correlated with disease severity. The feasibility of using respiratory input impedance (Z(rs)) and computed tomographic (CT) image registration for assessing parenchymal mechanical heterogeneity was evaluated. In six dogs, measurements of Z(rs) before and after oleic acid injury at various distending pressures were obtained, followed by whole lung CT scans. Each Z(rs) spectrum was fit with a model incorporating variable distributions of regional compliances. CT image pairs at different inflation pressures were matched using an image registration algorithm, from which distributions of regional compliances from the resulting anatomic deformation fields were computed. Under baseline conditions, average model compliance decreased with increasing inflation pressure, reflecting parenchymal stiffening. After lung injury, these average compliances decreased at each pressure, indicating derecruitment, alveolar flooding, or alterations in intrinsic tissue elastance. However, average compliance did not change as inflation pressure increased, consistent with simultaneous recruitment and strain stiffening. Image registration revealed peaked distributions of regional compliances, and that small portions of the lung might undergo relative compression during inflation. The authors conclude that assessments of lung function using Z(rs) combined with the structural alterations inferred from image registration provide unique but complementary information on the mechanical derangements associated with lung injury.

Figure 1

Comparison of total respiratory compliance computed from the Jacobian approach of Eq. (6) () to the segmented image approach () for all six dogs at baseline (white) and after lung injury (black) for pressure changes between 5–10 (circles), 10–15 (squares), and 15–20 (triangles) cm H₂O. Linear regression yielded a significant correlation (solid line, r ² = 0.78, p < 0.01) between to two approaches which was indistinguishable from the line of identity (dashed line)

Figure 2

Best-fit tissue compliance distributions obtained for the six dogs studied at baseline and after lung injury at four different mean airway pressures. See text for details

Figure 3

Summary of distributed tissues model parameters: (a) R, (b) I, (c) η, (d) , (e) σ_C, and (f) coefficient of tissue variation vs. mean airway pressure for all six dogs. Data are presented as means ± standard errors at baseline (white) and after lung injury (black), assuming the P(C) for each dog yielding the best fit to the Z _rs data. *Significantly different from baseline data at same mean airway pressure using two-tailed paired t test. **Significantly different from corresponding data at 5 and 10 cm H₂O under same condition using ANOVA and Tukey HSD criterion (p < 0.05)

Figure 4

(a) Total segmented lung volume vs. mean airway pressure at baseline (white) and after lung injury (black). *Significantly higher from baseline data at same mean airway pressure using two-tailed paired t test (p < 0.05). (b) Total segmented compliance () vs. inflation pressure at baseline (white) and after lung injury (black)

Figure 5

Image registration in (a) transverse and (b) coronal sections for a representative dog at baseline and after ALI for three different inflation pressures pairs. Volume changes are color coded such that yellow, orange, and red correspond to expansion, blue and purple to contraction, and green to no change in volume

Figure 6

Percent of total lung volume undergoing expansion and compression at the three different inflation pressures for baseline (white) and lung injury (black) conditions as predicted from image registration. Data are presented as means ± standard errors. Approximately 85–90% of total lung volume expands as airway pressure increases

Figure 7

Distributions of regional compliances throughout the lung for a representative dog at three inflation pressure pairs at baseline (gray) and after lung injury (black). Dashed line denotes zero compliance

Figure 8

Summary of the (a) averages and (b) standard deviations of regional compliances for all the six dogs as computed from the registered images according to Eq. (5), along with (c) the total respiratory compliance as determined from Eq. (6). Data are presented as means ± standard errors