Abstract
INTRODUCTION: Mechanical ventilator breaths provided to deeply sedated patients have an abnormal volume distribution, encouraging alveolar collapse in dependent regions and promoting alveolar overdistention in non-dependent regions. Collapse and overdistention both start with the first breath and worsen over time, driving ventilator-induced lung injury (VILI). This is exacerbated when the lung is already injured or has increased heterogeneity. Our study investigated the impact of a single episode of lung injury on lung mechanics and the risk factors for ventilator-induced injury, compared with non-injured lungs. METHODS: Two groups of pigs were sedated and ventilated using lung-protective volume-controlled mode at 8 mL/kg, positive end-expiratory pressure (PEEP) 5 cmH(2)O, with respiratory rate and FiO2 set to maintain normal blood gas values. Animals in one group were ventilated for 50 h (50-Hour MV group, n=10). Animals in the second group had lung injury induced using oleic acid and were ventilated for 12 h post-injury (LI MV group, n=6). Both groups were compared with a never-ventilated control group (NV, n=6). Lung mechanics and injury were measured using electrical impedance tomography, esophageal pressure monitoring and tissue histology. RESULTS: End-expiratory lung-volume loss was greater in the 50-Hour MV group (P<0.05). Plateau pressure, driving pressure and lung injury score were higher in the LI MV group, (P<0.05). CONCLUSION: Risk factors for VILI developed three- to five-times faster in the group with injured lungs, demonstrating that a single lung-injury episode substantially increased the risk of VILI, compared with normal lungs, despite using a lung-protective mechanical ventilation protocol.