Blood gas analysis. One aim of the second series of experiments was to investigate whether a higher PEEP can prevent the worsening of gas exchange in mice ventilated at a PEEP = 2 cmH 2 O and RM60 or noRM. The pO 2 /FiO 2 ratio was around 500 mmHg in the PEEP6_RM60 group and around 400 mmHg in the PEEP6_noRM group (Fig. 10) . Both groups differed significantly from the control group PEEP6_RM5, which showed unimpaired gas exchange with a pO 2 /FiO 2 ratio of about 600 mmHg This further demonstrates that regular RMs are necessary even with a PEEP of 6 cmH 2 O. In accordance, pCO 2 levels were significantly elevated in the groups PEEP6_RM60 and PEEP6_noRM, compared to the control group (data not shown).