Deterioration of Regional Lung Strain and Inflammation during Early Lung Injury

Abstract

Rationale: The contribution of aeration heterogeneity to lung injury during early mechanical ventilation of uninjured lungs is unknown.

Objectives: To test the hypotheses that a strategy consistent with clinical practice does not protect from worsening in lung strains during the first 24 hours of ventilation of initially normal lungs exposed to mild systemic endotoxemia in supine versus prone position, and that local neutrophilic inflammation is associated with local strain and blood volume at global strains below a proposed injurious threshold.

Methods: Voxel-level aeration and tidal strain were assessed by computed tomography in sheep ventilated with low Vt and positive end-expiratory pressure while receiving intravenous endotoxin. Regional inflammation and blood volume were estimated from 2-deoxy-2-[(18)F]fluoro-d-glucose (¹⁸F-FDG) positron emission tomography.

Measurements and main results: Spatial heterogeneity of aeration and strain increased only in supine lungs (P < 0.001), with higher strains and atelectasis than prone at 24 hours. Absolute strains were lower than those considered globally injurious. Strains redistributed to higher aeration areas as lung injury progressed in supine lungs. At 24 hours, tissue-normalized ¹⁸F-FDG uptake increased more in atelectatic and moderately high-aeration regions (>70%) than in normally aerated regions (P < 0.01), with differential mechanistically relevant regional gene expression. ¹⁸F-FDG phosphorylation rate was associated with strain and blood volume. Imaging findings were confirmed in ventilated patients with sepsis.

Conclusions: Mechanical ventilation consistent with clinical practice did not generate excessive regional strain in heterogeneously aerated supine lungs. However, it allowed worsening of spatial strain distribution in these lungs, associated with increased inflammation. Our results support the implementation of early aeration homogenization in normal lungs.

Keywords: acute respiratory distress syndrome; endotoxemia; mechanical ventilation; positron emission tomography computed tomography; ventilator-induced lung injury.

Figure 1.

(A and B) Ratios of Pa_O2/Fi_O2 (A) and peripheral blood neutrophil counts (B) at baseline and after 6 and 24 hours of low Vt mechanical ventilation and mild endotoxemia. (C) Driving pressure (Ers · Vt, solid line) and delta transpulmonary pressure (El · Vt, dashed line). The equal driving pressure with different transpulmonary pressure in supine and prone positions was explained by the difference in El (D) and Ecw (E). ^prone and ^vsupine versus baseline. ^#prone and *supine versus 6 hours. One symbol indiates P < 0.05; two symbols indicate P < 0.01; three symbols indicate P < 0.001. Open gray square = prone; solid black circle = supine. Histologic findings showed regional differences in lung injury. (F) Box plot of lung injury score evaluated in three regions from ventral to dorsal in supine (black) and prone (gray) animals. ^#P < 0.05 versus ventral region. (G) Examples of hematoxylin and eosin staining in high-power fields for each one of the regions in one supine and one prone animal. Scale bars, 10 μm. Ecw = chest wall elastance; El = lung elastance; Ers = respiratory system elastance.

Figure 2.

(A) Lung aeration decreased faster in supine than prone conditions when a mechanical ventilation strategy compatible with clinical practice using low positive end-expiratory pressure and Vt was applied for 24 hours to mild endotoxemic animals. (B and C) In the supine lung, on average 48% of regions are either nonaerated or poorly aerated at the end of 24 hours of mechanical ventilation and endotoxemia (B), whereas in prone animals 27% of the lung is poorly aerated and none nonaerated (C). Nonaerated, fraction of gas (Fgas) < 0.1; poorly aerated, 0.1 ≤ Fgas < 0.5; normally aerated, 0.5 ≤ Fgas < 0.9; and hyperaerated, 0.9 ≤ Fgas. (D) Aeration heterogeneity, measured as the variance normalized by the squared mean along time. There is marked contrast between the progressions of aeration heterogeneity in supine versus prone animals, with increase significantly only in supine. Values were computed at end-expiration. Group effect is indicated in the figure. ^##P < 0.01 and ^###P < 0.001, comparison among time points. Prone animals had no difference between time points. Open gray squares = prone; solid black circles = supine. All data refer to the whole lung.

Figure 3.

Normalized strain computed from measurements at the voxel level in supine and prone animals at baseline and after 6 and 24 hours of low-Vt mechanical ventilation and mild endotoxemia. (A) Transverse slice at approximately two-thirds of the cephalocaudal axis is presented along time showing voxel-level strain in a cold-to-hot color scale (dark blue = compression/no strain; red = higher strain value within the image) superimposed on the computed tomography scan. Note the heterogeneous spatial distribution of strains in supine animals, in contrast to the more homogeneous distribution in prone sheep, also shown in the normalized strain distribution presented for each animal (different colors). (B and C) Heterogeneity (variance normalized by squared mean strain) increased in the supine position (B), leading to an increase in the ratio of maximum (95th percentile) to mean strain in this group (C). ^###P < 0.001.

Figure 4.

Normalized strain in one supine (top) and one prone (bottom) animal at baseline and after 24 hours of low-Vt mechanical ventilation and mild endotoxemia. Strains are color coded from low and slight compression (−1/L; blue) to expansion (+3/L; yellow). Values at the ends of the scale were highlighted making the center (1/L) transparent with a gradual increase in opacity for both sides, as shown in the color bars associated with the strain histograms on the right. In both groups, there was heterogeneity in isogravitational levels. Note the larger heterogeneity in the spatial distribution of strains in the supine position, with more extreme values seen in subdiaphragmatic (dark blue) and nondependent (yellow) regions, whereas a more homogeneous pattern is observed in prone conditions.

Figure 5.

Voxel-level normalized strain versus end-inspiratory aeration (fraction of gas [Fgas]) at baseline and after 6 and 24 hours of mild endotoxemia and low-Vt mechanical ventilation. Data refer to all animals in supine (top) and prone (bottom). The boxes represent median and interquartile range of strains for voxels in the aeration intervals: <0.1, 0.1–0.3, 0.3–0.5, 0.5–0.7, 0.7–0.9, and >0.9, centered in the mean aeration within each aeration interval. Voxels between the 5th and 95th strain percentiles are depicted in a two-dimensional histogram, with the gray scale indicating the fraction of total lung volume represented by a pair of strain and aeration (black is highest). Gray scale is the same within groups. In the supine animals (top), strain–aeration relationships showed an inverted U-shaped pattern. Strain increased with aeration up to an Fgas approximately 0.6 followed by a decrease with aeration for Fgas above that value. This pattern was consistent across time points, with progressive decrease in median strain at low aerations and increase at high aerations when compared with median strain at baseline (dashed line). The decrease in gray scale for prone (bottom) at 24 hours indicates a slight spread of the distribution.

Figure 6.

(A) After 24 hours of low-Vt mechanical ventilation and mild endotoxemia, 2-deoxy-2-[(18)F]fluoro-d-glucose uptake rate, a marker of inflammation, was increased relative to baseline both in animals in supine and prone positions. (B and C) This increase was caused more by an increase in the phosphorylation rate (B) than by volume of distribution (C), indicating predominance of cellular metabolic activation. At baseline, 2-deoxy-2-[(18)F]fluoro-d-glucose uptake rate spatial distribution was mostly homogeneous in both groups. (D) After 24 hours, it remained homogeneous in prone but showed a vertical gradient in supine. (E) Not only did the tissue density increase, but also regions that became atelectatic (<0.1) had a higher increase in tissue-normalized uptake when compared with regions of constant normal (0.5–0.7) aeration. (F) Within aerated regions, first (red) and third (blue) tertile of strain had no difference in normalized uptake increase. **P < 0.01 and ***P < 0.001 versus normally aerated region. Fgas = fraction of gas.

Figure 7.

Gene expression in three regions with different aeration, blood volume, and strain conditions after 24 hours of low-Vt mechanical ventilation and mild endotoxemia. In prone animals, sampled regions were selected to match the three regions along the gravitational axis sampled in the supine animals. Note that mid regions have more samples than dorsal and ventral because tissue was sampled at mid and caudal zones, which showed no difference in a paired Wilcoxon test and were treated as one region. Points are a dot plot representation of fold change relative to β-actin and a control noninjured animal measured with RT-qPCR, and gray lines indicate median (horizontal) and first and third quartiles. Comparison between regions in the same group: *P < 0.05, **P < 0.01, and ***P < 0.001.

Figure 8.

Two patients with sepsis at the first 96 hours of mechanical ventilation had relationships between voxel-level normalized strain and end-inspiratory aeration (fraction of gas) consistent with the relationships observed in the animal experiments. (A) Strain increased from lowest aeration toward normal aeration but did not decrease in more aerated regions. The boxes represent median and interquartile range of strains for voxels in the aeration intervals: <0.1, 0.1–0.3, 0.3–0.5, 0.5–0.7, 0.7–0.9, and >0.9, centered in mean aeration within ranges. Voxels between the 5th and 95th percentiles of strain are depicted in a two-dimensional histogram, with the gray scale representing the fraction of total lung volume in each bin (black is highest). End-inspiratory aeration was chosen to emphasize hyperaerated regions. Inspiratory images were transformed to the expiratory computed tomography scan references using the same transformation estimated by elastic image registration to calculate the voxel-level strain. (B and C) Normalized 2-deoxy-2-[(18)F]fluoro-d-glucose uptake (B) and blood volume per gram of tissue (C) in atelectatic (<0.1), normal (0.5–0.7), and high-aeration (>0.7) regions. In both patients, regions of high aeration and potential hyperinflation have higher normalized uptake and blood volume than normally aerated regions. Fgas = fraction of gas.