In order to keep blood gases stable, the low and high V T groups had to be ventilated with different respiratory rates and, based on other studies [35] , we cannot completely rule out that this might have affected the outcome of our study. Our conditions were chosen to provide the same minute volume at both tidal volumes. The supplementation of 3% CO 2 to the inhaled gas mixture in the high V T groups was adequate to prevent hypocapnia, which otherwise would have occurred at a respiratory rate of 90 min 21 in the highV T RM5 group. 3% CO 2 alone did not induce hypercapnia (see Table 3 ). Thus, a protective effect of hypercapnia during MV, which is described in several studies [36, 37] , seems unlikely.