The oxygenation index (OI) was calculated as follows: (FiO 2 × 100 × MAP in cmH 2 O)/PaO 2 in mmHg. The ventilation index (VI) was calculated as follows: (PaCO 2 in mmHg × respiratory rate × (PIP -PEEP in cmH 2 O))/1000. VI is used as determinant for CO 2 elimination because the respiratory rate, PIP, and PEEP were kept constant throughout the study period [17] . Dead space (V D ) was calculated according to the Bohr-Enghoff equation: V D = V Te × (1 -(P ET-CO2 /PaCO 2 )) [18] .