Blood gas analysis. Blood gas analyses revealed that infusion of saline and application of 3% CO 2 in the high V T groups were adequate to keep the acid-base status stable and that ventilation resulted in normocapnia in the lowV T RM5 and highV T RM5 group, with pCO 2 levels ranging from 35 to 40 mmHg ( Table 3) . The pO 2 /FiO 2 ratio was between 530 and 620 mmHg in both RM5 groups and in the highV T noRM group (Figure 3 ). Most mice ventilated with highV T noRM had physiological pCO 2 levels after six hours. The pO 2 /FiO 2 ratio was decreased significantly to around 200 mmHg in the lowV T noRM group, indicating acute respiratory failure. In this group, impaired gas exchange was further indicated by an increase in pCO 2 levels to about 60 mmHg, leading to respiratory acidosis with a mean pH of 7.22. Mice in the lowV T RM60 group showed very heterogeneous pO 2 /FiO 2 ratios between 260 and 570 mmHg and markedly increased pCO 2 levels as well as initiating hypercapnic acidosis. Bicarbonate (HCO 3 2 ) was only slightly reduced, resulting in negative values for standard base excess, but did not indicate metabolic imbalance.