Biophysical determinants of alveolar epithelial plasma membrane wounding associated with mechanical ventilation

Abstract

Mechanical ventilation may cause harm by straining lungs at a time they are particularly prone to injury from deforming stress. The objective of this study was to define the relative contributions of alveolar overdistension and cyclic recruitment and "collapse" of unstable lung units to membrane wounding of alveolar epithelial cells. We measured the interactive effects of tidal volume (VT), transpulmonary pressure (PTP), and of airspace liquid on the number of alveolar epithelial cells with plasma membrane wounds in ex vivo mechanically ventilated rat lungs. Plasma membrane integrity was assessed by propidium iodide (PI) exclusion in confocal images of subpleural alveoli. Cyclic inflations of normal lungs from zero end-expiratory pressure to 40 cmH2O produced VT values of 56.9 ± 3.1 ml/kg and were associated with 0.12 ± 0.12 PI-positive cells/alveolus. A preceding tracheal instillation of normal saline (3 ml) reduced VT to 49.1 ± 6 ml/kg but was associated with a significantly greater number of wounded alveolar epithelial cells (0.52 ± 0.16 cells/alveolus; P < 0.01). Mechanical ventilation of completely saline-filled lungs with saline (VT = 52 ml/kg) to pressures between 10 and 15 cmH2O was associated with the least number of wounded epithelial cells (0.02 ± 0.02 cells/alveolus; P < 0.01). In mechanically ventilated, partially saline-filled lungs, the number of wounded cells increased substantially with VT, but, once VT was accounted for, wounding was independent of maximal PTP. We found that interfacial stress associated with the generation and destruction of liquid bridges in airspaces is the primary biophysical cell injury mechanism in mechanically ventilated lungs.

Keywords: epithelial wounding; injury; lung mechanics.

Fig. 1.

Representative optical section of an injured rat lung labeled by airspace instillation of a solution containing propidium iodide (PI) and green fluorescent dextran. The blue autofluorescence arises from the connective tissue of the lung parenchyma. The nuclei of alveolar epithelial cells with plasma membrane defects can be identified by their red nuclear fluorescence.

Fig. 2.

A comparison of hyperventilation-induced epithelial injury estimates between “dry” lungs (DLV), “wet” lungs, i.e., partially liquid-flooded lungs (WLV), and completely saline-filled lungs subjected to total liquid ventilation (TLV) is shown. The two DLV groups (open symbols) differ with respect to positive end-expiratory pressure and tidal volume settings. Open circles represent data from lungs ventilated between zero end-expiratory pressure and 40 cmH₂O pressure. The open squares represent data from lungs ventilated between 3 and 40 cmH₂O pressure. The two WLV groups differ with respect to the biophysical properties of the alveolar liquid. Closed circles represent data from partially saline-filled lungs. Closed squares represent data from partially perfluorocarbon-filled lungs. The two TLV groups differ with respect to the duration of mechanical ventilation. Crosses represent data from lungs that were ventilated for 20 min. Bars represent data from lungs that were ventilated for 60 min. Cell injury index (CII) refers to the number of wounded epithelial cells per alveolus in post hoc-labeled lungs. Individual and group mean values and their SDs are shown. Dashed and solid lines identify the groups to which statistical comparisons of data means apply. NS stands for not significant at a P value <0.05.

Fig. 3.

The effect of tidal volume on epithelial injury estimates in 4 groups of 8 wet lungs, i.e., saline-flooded lungs (WLV), is shown. The numbers associated with each group refer to the airway pressure limits (equal to apparent transpulmonary pressure) between which the lungs were “cycled.” CII refers to the number of wounded epithelial cells/alveolus in lungs in which the initial saline instillate contained PI.