Even though experimental lung injury in animal experiments is the best analogy for the changes in the patients, it has to be considered that kinetics may vary between species. An important question to answer is how common PaO2-oscillations occur in patients and how injurious they might be. A limitation to the detection of PaO2-oscillations is a sensing device that detects the oscillations at very high temporal resolution. In previous studies a fiberoptic probe was used, which was measuring PaO2 based on oxygen-sensitive fluorescence quenching with a time resolution up to 10 Hz (8, 13, 14). This method is not feasible in patients. Previous studies have shown that PaO2-oscillations are translated into the peripheral hemoglobin oxygen saturation (SpO2). Given a technology that has a time resolution that is high enough (i.e. >1 Hz), measurement of SpO2-oscillations would be a valid approach to detecting and quantifying cyclical recruitment and derecruitment in a non-invasive fashion in patients on the ICU. The Masimo Rad-8 pulse-oxymeter provides such a method. In the current study it is planned to deteted SpO2-oscillations in the post-operative patients on the ICU.