BACKGROUND: Pressure-controlled ventilation (PCV) is frequently used recently as the initial mode of mechanical ventilation in the patients with respiratory failure. Theoretically, because of its high initial inspiratory flow, pressure-controlled ventilation has lower peak inspiratory pressure and improved gas exchange than volume-controlled ventilation (VCV). But the data from previous studies showed controversial results about the gas exchange. Moreover, the comparison study between PCV and VCV with various inspiration:expiration time ratios (I:E ratios) is rare. So this study was performed to compare the respiratory mechanics and gas exchange between PCV and VCV with various I:E raitos. METHODS: Nine patients receiving mechanical ventilation for respiratory failure were enrolled. They were ventilated by both PCV and VCV with various I:E ratios (1:2, 1:1.3 and 1.7:1). FiO2, tidal volume, respiratory rate and external positive end-expiratory pressure (PEEP) were kept constant throughout the study. After 20 minutes of each ventilation mode, arterial blood gas, airway pressures, expired CO2 were measured. RESULTS: In both PCV and VCV, as the I:E ratio increased, the mean airway pressure was increased, and PaCO2 and physiologic dead space fraction were decreased. But P(A-a)O2 was not changed. In all three different I:E ratios, peak inspiratory pressure was lower during PCV, and mean airway pressure was higher during PCV. But PaCO2 level, physiologic dead space fraction and P(A-a)O2 were not different between PCV and VCV with three different I:E ratios. CONCLUSION: There was no difference in gas exchange between PCV and VCV under the same tidal voulme, frequency and I:E ratio.