Airspace collapse is a hallmark of parenchymal lung injury. Strategies to reopen and maintain patency of these regions offer three advantages: improved gas exchange, less ventilator-induced lung injury, and improved lung compliance. Elevations in intrathoracic pressure to achieve these goals, however, may overdistend healthier lung regions and compromise cardiac function. Positive end-expiratory pressure is a widely used technique to maintain alveolar patency, but its beneficial effects must be balanced against its harmful effects. Mechanical approaches to achieve this balance are clinically difficult to do. Thus gas exchange algorithms with modest PaO2 goals are commonly used today. Recruitment maneuvers and long inspiratory time strategies may be useful adjuncts.