Results from experimental and clinical studies have shown that mechanical ventilation or/and hyperoxia may aggravate a pre-existing lung injury or even cause lung injury in healthy lungs by affecting the expression of MMP-9, but the MMP-9 effects are controversial. How are MMP-9 regulated when multicausative factors of injury such as different FiO 2 , PIP, and respiratory time (RT) impose simultaneously on lungs? Methods: Newborn New Zealand white rabbits were randomly allocated to an

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... ed air control group or to one of the 2 × 3 × 3 ventilation strategies by using a factorial design, with different FiO 2 , PIP, and RT. Then, lung wet-to-dry ratio (W/D), lung histopathology scores, transmission electron microscope, and cells in BALF were analyzed in these different groups. MMP-9 levels were studied by immunohistochemistry and ELISA. Results: MMP-9 levels were significantly different among 3 PIP ventilation regimes (F = 7.215) and MPIP group was the highest among 3 PIP groups. The lung histopathology score in 100% oxygen was significantly higher than in 45% oxygen group (F = 9.037) and MPIP group was the lowest among 3 PIP groups (F = 57.515) and RT 6 h was more serious than RT 1 h. MMP-9 positively correlated with monocytes, but negatively correlated with neutrophils and lung injury histopathology scores. Conclusions: Different PIP and FiO 2 exert simultaneously on newborn lung in newborn rabbits ventilation, only mechanical stretch stimulation affects MMP-9 synthesis. Advisable mechanical stretch can promote MMP-9 expression and has protective role in lung in VILI. HPIP causes barotraumas and LPIP induces atelectrauma.