Power calculation: The outcome is unclear because no similar study has been done before. If tcPCO2 becomes in medium (SD) 5 (1) kPa in a group and 5.5 (1) kPa in the second group, 63 patients are required before obtaining a significance of p <0.05 and a power of 0.80. As the outcome is uncertain, the investigators intend to include 65 patients.