Increasing the inspiratory time and I:E ratio during MV increased histopathologic signs of lung injury. Mice were mechanically ventilated for 4 hours with either low tidal volume (LVT 9 ml/kg) or high tidal volume (HVT 34 ml/kg) and an inspiratory/expiratory ratio of 1:2 or 1:1, respectively. An alternative end point was defined as decreasing of mean arterial blood pressure below 40 mm Hg, which predicts death with certainty in this model. Controls (ctr) were subjected to LVT 1:2 ventilation only during operation and were killed before the 4-hour ventilation protocol started. Histopathology of lungs from ctr, HVT 1:2, and HVT 1:1 groups, stained with the periodic acid-Schiff (PAS) reaction, are shown: In contrast to the ctr group, both ventilated groups had damage of the alveolar walls with septal thickening, necrosis, and desquamation of alveolar epithelial cells type I, formation of hyaline membranes (red arrows), and increased numbers of intraalveolar cells (predominantly neutrophils and macrophages, black arrows). PAS reaction highlighted the more severe and more continuous as well as thicker hyaline membranes along the alveolar surfaces of lungs from the HVT 1:1 group. Top panel: magnification × 200; Bottom panel: magnification × 400. Representative images from each group (n = 4 each) are shown.