The effects of long-term conventional mechanical ventilation on the lungs of adult rats

Background., Ventilation-induced lung injury is often studied in animal models by using ventilation strategies with high-tidal volumes and high-oxygen concentration over a relatively short period of time. The injury induced by these ventilation strategies includes alterations to the surfactant system and up-regulation of inflammatory markers. Whether these responses to ventilation occur with more clinically relevant ventilation strategies is not known. Objective: To assess how healthy adult rats respond to 24 hrs of conventional mechanical ventilation with respect to lung physiology, markers of inflammation, and alterations to pulmonary surfactant, and how this is affected by the oxygen concentration. Interventions. Adult rats were mechanically ventilated for 24 hrs with a tidal volume of 8 mL/kg, 5 cm H2O positive end-expiratory pressure, at 60 breaths/min with either 21% or 100% oxygen. Animals were monitored for blood oxygenation and other physiologic parameters. After ventilation, lungs were lavaged and analyzed for inflammatory markers and pulmonary surfactant. These outcomes were compared with measurements obtained from spontaneously breathing rats exposed to either 21% or 100% oxygen for 24 hrs. Main Results: Twenty-four hours of ventilation did not result in significant changes in blood oxygenation. Inflammatory markers, such as interleukin-6 concentration and the number of neutrophils in the lavage, were increased in ventilated animals compared with the nonventilated controls, regardless of the level of inspired oxygen. The amount of active surfactant was increased after ventilation; however, the surface activity of this material was impaired as compared with controls. Conclusion: Prolonged mechanical ventilation of health lungs with a physiologically benign strategy can contribute to the inflammatory response and cause alterations to pulmonary surfactant.