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The efficacy of inverse ratio ventilation in ARDS is not clear. Furthermore, the mechanism responsible has not been determined. We designed an animal study to determine if inverse ratio ventilation improves gas exchange and by what mechanism. ⋯ We conclude that increasing mean airway pressure by prolongation of the inspiratory time improves gas exchange in our model of ARDS, but when mean airway pressure is increased further, allowing the development of intrinsic PEEP, the beneficial effects on gas exchange are less. Increasing mean airway pressure with intrinsic PEEP is not equivalent to other methods of increasing mean airway pressure.