Preterm infants often need peripheral artery catheters for invasive blood pressure recording and to facilitate blood sampling. Near infrared spectroscopy is a method to evaluate cerebral oxygenation and as well as cerebral blood flow. Sampling procedures with identical sampling volumes are performed at a short (40 seconds) and a long (70 seconds) time intervall while changes of cerebral oxygenation are measured. The investigators hypothesise that slower sampling decrease changes in cerebral blood flow.