Effects of ventilation strategy on distribution of lung inflammatory cell activity

Abstract

Introduction: Leukocyte infiltration is central to the development of acute lung injury, but it is not known how mechanical ventilation strategy alters the distribution or activation of inflammatory cells. We explored how protective (vs. injurious) ventilation alters the magnitude and distribution of lung leukocyte activation following systemic endotoxin administration.

Methods: Anesthetized sheep received intravenous endotoxin (10 ng/kg/min) followed by 2 h of either injurious or protective mechanical ventilation (n = 6 per group). We used positron emission tomography to obtain images of regional perfusion and shunting with infused ¹³N[nitrogen]-saline and images of neutrophilic inflammation with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). The Sokoloff model was used to quantify ¹⁸F-FDG uptake (Ki), as well as its components: the phosphorylation rate (k₃, a surrogate of hexokinase activity) and the distribution volume of ¹⁸F-FDG (Fe) as a fraction of lung volume (Ki = Fe × k₃). Regional gas fractions (fgas) were assessed by examining transmission scans.

Results: Before endotoxin administration, protective (vs. injurious) ventilation was associated with a higher ratio of partial pressure of oxygen in arterial blood to fraction of inspired oxygen (PaO₂/FiO₂) (351 ± 117 vs. 255 ± 74 mmHg; P < 0.01) and higher whole-lung fgas (0.71 ± 0.12 vs. 0.48 ± 0.08; P = 0.004), as well as, in dependent regions, lower shunt fractions. Following 2 h of endotoxemia, PaO₂/FiO₂ ratios decreased in both groups, but more so with injurious ventilation, which also increased the shunt fraction in dependent lung. Protective ventilation resulted in less nonaerated lung (20-fold; P < 0.01) and more normally aerated lung (14-fold; P < 0.01). Ki was lower during protective (vs. injurious) ventilation, especially in dependent lung regions (0.0075 ± 0.0043/min vs. 0.0157 ± 0.0072/min; P < 0.01). ¹⁸F-FDG phosphorylation rate (k₃) was twofold higher with injurious ventilation and accounted for most of the between-group difference in Ki. Dependent regions of the protective ventilation group exhibited lower k₃ values per neutrophil than those in the injurious ventilation group (P = 0.01). In contrast, Fe was not affected by ventilation strategy (P = 0.52). Lung neutrophil counts were not different between groups, even when regional inflation was accounted for.

Conclusions: During systemic endotoxemia, protective ventilation may reduce the magnitude and heterogeneity of pulmonary inflammatory cell metabolic activity in early lung injury and may improve gas exchange through its effects predominantly in dependent lung regions. Such effects are likely related to a reduction in the metabolic activity, but not in the number, of lung-infiltrating neutrophils.

Figure 1

Sokoloff model for ¹⁸F-fluorodeoxyglucose tracer kinetics [13]. The three compartments of the model describe the activity concentration of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) in plasma (C_p(t)), the region of interest (ROI) concentration of extravascular ¹⁸F-FDG serving as a substrate pool for hexokinase (precursor compartment, C_e(t)) and the ROI concentration of phosphorylated ¹⁸F-FDG (C_m(t)). The arrows indicate the tracer exchanges in the dynamic model and the corresponding parameters. The rate constants k₁ and k₂ account for forward and backward transport of ¹⁸F-FDG between blood and tissue. k₃ is the rate of ¹⁸F-FDG phosphorylation, reflecting hexokinase activity. The constant F_e represents the distribution volume of ¹⁸F-FDG (that is, the substrate pool for hexokinase) as a fraction of lung tissue volume.

Figure 2

Regional gas, perfusion and shunt fractions. Gas fraction (f_gas) (A), perfusion fraction (B) and shunt fraction (C) for isogravitational (dependent, middle and nondependent) regions of interest of injurious (open symbols) and protective ventilation (filled symbols) groups at baseline (triangles) and after (circles) 2 h of mechanical ventilation and endotoxemia (ETX). f_gas and perfusion fraction were stable over time in both groups. In contrast, shunt fraction dramatically increased over time in dependent regions of the injurious ventilation group, but not in those of the protective ventilation group. Horizontal lines represent median values. *P < 0.05, **P < 0.01 and ***P < 0.001. P values are derived from two-way analysis of variance with repeated measurements and Bonferroni adjustments for multiple comparisons.

Figure 3

Single-slice images of ¹⁸F-fluorodeoxyglucose uptake rate. Single-slice images of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) uptake rate (K_iP, computed voxel-by-voxel using the Patlak method [31]) in one sheep from the protective ventilation group (right panel) and in one from the injurious ventilation group (left panel). Left side in each image corresponds to the left side in the animal. ¹⁸F-FDG uptake was lower and more homogeneous in the protective ventilation experiment.

Figure 4

¹⁸F-fluorodeoxyglucose kinetics parameters. ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) kinetics parameters for isogravitational (dependent, middle and nondependent) regions of interest of injurious (open circles) and protective (closed circles) ventilation groups. (A) ¹⁸F-FDG net uptake rate (K_i). (B) ¹⁸F-FDG net uptake rate normalized for tissue density and blood fraction (K_is = K_i/(1 − f_gas − f_blood)); C) Transfer rate constant k₃ (characterizes ¹⁸F-FDG phosphorylation to ¹⁸F-FDG-6-phosphate and reflects hexokinase activity); and D) Precursor compartment for ¹⁸F-FDG phosphorylation (F_e). Note the significant regional heterogeneity in the distribution of K_is and k₃ in the injurious as compared to the protective ventilation group. Horizontal lines represent median values. *P < 0.05, **P < 0.01 and *** P < 0.001. P values are derived from two-way analysis of variance with repeated measurements, with Bonferroni adjustments for multiple comparisons.

Figure 5

Correlation between mean ¹⁸F-fluorodeoxyglucose net uptake rate at the voxel level and ¹⁸F-fluorodeoxyglucose uptake using the standard deviation. Linear regression between mean (K_iP) and standard deviation (SD(K_iP)) of voxel-level ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) net uptake rate computed using the Patlak method [31] for protective (filled circles) and injurious (open circles) ventilation groups. There was a significant correlation between mean K_iP and SD(K_iP) for both the protective ventilation group (continuous line; y = 0.48x + 0.001, r = 0.84, P = 0.034) and the injurious ventilation group (dashed line; y = 0.59x + 0.003, r = 0.98, P < 0.001). Note the offset between the two regression lines showing that, for equivalent K_iP values, protective ventilation led to lower SD(K_iP) than injurious ventilation.

Figure 6

Metabolic activity per neutrophil in dependent lung regions. Neutrophils of the injurious ventilation group exhibited higher k₃ values than those of the protective ventilation group (P = 0.012), reflecting higher hexokinase activity.