Cerebral oxygenation can be monitored using near infrared spectroscopy. Different devices are available to assess cerebral oxygenation noninvasively. Beach chair position is assumed to influence cerebral oxygenation. Litte is known about the influence of prone position and head down position on cerebral oxygenation. The investigators hypothesize that these positions influence cerebral oxygenation. Further the investigators presume that the two cerebral oximeters INVOS and ForeSight provide comparable oxygenation data.

Inclusion Criteria:

age >18 and <80 years written informed consent

Exclusion Criteria:

cerebrovascular disease coronary heart disease severe organ failure hemoglobin < 10mg/dl injury of forehead pregnancy

Patients scheduled for surgical intervention in beach chair position, prone position and head down position