The aim of this study is to examine the effects of patients' clinical and demographic datas on cerebral oxygen saturation values. Brain oxygenation is measured using near infrared spectroscopy (NIRS). The primary outcome measure of the study is the regional oxygen saturation (rSO2) values measured on the monitor at twelve time points. The other primary outcome measures consist of baseline arterial and venous blood gas analysis and baseline activated clotting time measurements.