Hidden Microatelectases Increase Vulnerability to Ventilation-Induced Lung Injury

Area of Science:

• Pulmonary Medicine
• Critical Care Medicine
• Respiratory Physiology

Background:

• Mechanical ventilation of lungs with microatelectases may precipitate acute lung injury (ALI).
• Bleomycin-induced lung injury causes surfactant dysfunction and microatelectases, but normal elastance and oxygenation initially.
• Computational models suggest increased alveolar strain during low positive end-expiratory pressure (PEEP) ventilation in the presence of microatelectases.

Purpose of the Study:

• To test the hypothesis that mechanical ventilation, in the presence of microatelectases, aggravates and unmasks ALI.
• To investigate the effect of different PEEP levels on lung mechanics and injury in a bleomycin-induced ALI model.

Main Methods:

• Rats were challenged with bleomycin or saline.
• Animals were ventilated with either PEEP = 1 cmH2O (PEEP1) or PEEP = 5 cmH2O (PEEP5) for 3 hours.
• Lung tissue elastance, bronchoalveolar lavage (BAL) fluid, and lung histology (light and electron microscopy) were analyzed.

Main Results:

• Ventilation with PEEP1 significantly increased tissue elastance in bleomycin-injured lungs, indicating distal airspace instability.
• BAL analysis revealed increased neutrophils, protein, and albumin in the PEEP1 group.
• Histological examination showed increased septal wall thickness, interstitial edema, epithelial injury, and endoplasmic reticulum stress markers in the PEEP1 group.

Conclusions:

• Hidden microatelectases increase pulmonary vulnerability to mechanical ventilation.
• Low PEEP ventilation (1 cmH2O) aggravates lung injury, characterized by increased epithelial damage and inflammation, in the presence of microatelectases.
• Mechanical ventilation strategies should consider the presence of microatelectases to mitigate ventilator-induced lung injury.