Spatial and temporal heterogeneity of ventilator-associated lung injury after surfactant depletion

Abstract

Volutrauma and atelectrauma have been proposed as mechanisms of ventilator-associated lung injury, but few studies have compared their relative importance in mediating lung injury. The objective of our study was to compare the injury produced by stretch (volutrauma) vs. cyclical recruitment (atelectrauma) after surfactant depletion. In saline-lavaged rabbits, we used high tidal volume, low respiratory rate, and low positive end-expiratory pressure to produce stretch injury in nondependent lung regions and cyclical recruitment in dependent lung regions. Tidal changes in shunt fraction were assessed by measuring arterial Po(2) oscillations. After ventilating for times ranging from 0 to 6 h, lungs were excised, sectioned gravitationally, and assessed for regional injury by evaluation of edema formation, chemokine expression, upregulation of inflammatory enzyme activity, and alveolar neutrophil accumulation. Edema formation, lung tissue interleukin-8 expression, and alveolar neutrophil accumulation progressed more rapidly in dependent lung regions, whereas macrophage chemotactic protein-1 expression progressed more rapidly in nondependent lung regions. Temporal and regional heterogeneity of lung injury were substantial. In this surfactant depletion model of acute lung injury, cyclical recruitment produced more injury than stretch.

Fig. 1

Lung chemokines. Monocyte chemoattractant protein-1 (MCP-1; A), interleukin-8 (IL-8; B), and growth-related oncogene (GRO; C) production in lung tissue supernatant from regional lung sections as detected by ELISA. All results were normalized to the weight of solids in the individual lung slice. Lung slice 1 is the most nondependent, and lung slice 9 is the most dependent. Control rabbits (●) were killed immediately after vascular access was established. The 0-h rabbits (○) were killed immediately after lavage. The remaining rabbits were ventilated for 1.5 h (■), 3 h (□), or 6 h (◆) with high tidal volume, low respiratory rate, and low positive end-expiratory pressure to produce nondependent stretch and dependent cycling atelectasis. The error bars are 2 times the square root of residual mean squares from a standard variance table and represent the residual variability in the population after accounting for variations due to time and slice number.

Fig. 2

Lung tissue enzyme activity. Inducible nitric oxide synthase (iNOS) activity in lung tissue supernatant (A) and myeloperoxidase (MPO) activity in lung tissue homogenates (B) from regional lung sections as detected by radioactive citrulline assay and colorimetric assay, respectively. Lung slice 1 is the most nondependent, and lung slice 9 is the most dependent. ● Control rabbits; ○, rabbits ventilated after lavage for 0 h; ■, rabbits ventilated after lavage for 1.5 h; □, rabbits ventilated after lavage for 3 h; ◆, rabbits ventilated after lavage for 6 h. The error bars are 2 times the square root of residual mean squares from a standard variance table. OD, optical density; prot, protein.

Fig. 3

Wet-to-dry equivalent. Wet-to-dry equivalent from regional lung slices based on the ratio of the original lung slice weight to the weight of the sample solids. Lung slice 1 is the most nondependent, and lung slice 9 is the most dependent. ● Control rabbits; ○, rabbits ventilated after lavage for 0 h; ■, rabbits ventilated after lavage for 1.5 h; □, rabbits ventilated after lavage for 3 h; ◆, rabbits ventilated after lavage for 6 h. The error bar is 2 times the square root of residual mean squares from a standard variance table.

Fig. 4

Regional neutrophils. Average intra-alveolar neutrophils counted from 10 hematoxylin and eosin-stained 150 × 200 μm histopathological sections from each regional lung slice. Lung slice 1 is the most nondependent, and lung slice 9 is the most dependent. ● Control rabbits; ○, rabbits ventilated after lavage for 0 h; ■, rabbits ventilated after lavage for 1.5 h; □, rabbits ventilated after lavage for 3 h; ◆, rabbits ventilated after lavage for 6 h. The error bar is 2 times the square root of residual mean squares from a standard variance table.

Fig. 5

Photomicrographs of regional lung slices. Images (150 × 200 μm) of hematoxylin and eosin-stained lung slices from the most nondependent regions and dependent regions, respectively, of control rabbits, and those ventilated for 0 h and 6 h after lavage.