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.

Inclusion Criteria:

• Preterm infant (Gestational Age <37+0)
• Peripheral arterial catheter (ulnary or radial artery)

Exclusion Criteria:

• Complex organ malformation

Preterm infants (Gestational Age <37+0)