Because dead space is increased in ARDS, the generous Vt approach was useful for maintaining near-normal PaCO2 and pH. Moreover, the generous Vt approach was also useful for preventing or decreasing atelectasis and reversing some of the intrapulmonary shunt caused by atelectasis or intraalveolar filling [13, 14]. However, much of the ARDS lung is not available for ventilation because of consolidation, atelectasis, and alveolar filling [15,16]. Therefore, most of the Vt is delivered to the less diseased or normal lung regions. This may cause overdistention in the aerated lung tissue. Numerous studies in experimental animal models have demonstrated that overdistention can cause increased pulmonary vascular permeability, decreased surfactant function, hemorrhage, inflammation, and hypoxemia [1,4, 7,17, 18]. Ventilation with smaller Vt may decrease mechanical stresses in the aerated lung regions and attenuate this form of acute lung injury (ALI). However, the smaller Vt approach is less effective for maintaining gas exchange than the traditional approach that used generous Vt. Some ARDS patients experience hypercapnia and acidosis while receiving ventilation with small Vt [19, 20]. Therefore, it was not clear that the smaller Vt approach would lead to improved clinical outcomes in ARDS patients.
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