Background and Goal of Study: The cerebral tissue oxygen-saturation (ScO2) is normally about 10% lower than the central venous oxygen saturation (SvO2) due to the notably high oxygen extraction of the brain. The micro and macroangiopathies in diabetes mellitus may increase the physiological saturation gap between SvO2 and ScO2 (gSO2). Our aim was to evaluate how diabetes mellitus affects ScO2 and gSO2 during the different phases of a cardiac surgery. Materials and Methods: In a prospective consecutive descriptive manner, two groups of patients less than 80 years of age with no history of smoking were involved. Patients with type 2 diabetes mellitus (n=52) and control patients without T2DM (n=107) were studied with or without applying cardiopulmonary bypass (CPB). ScO2 was measured by using near-infrared spectroscopy and SvO2 was determined by analysing of central venous blood gas samples. ScO2 was registered before and after the anaesthesia induction, at the beginning and at the end of the CPB procedures, or during the OPCAB procedure, and following sternal closure. SvO2 data were also obtained simultaneously under each condition following the anaesthesia induction. Normocapnia was maintained during the whole procedure by ventilation and by using pH-stat management during CPB. Results and Discussion: SvO2 did not differ between the T2DM and control patients at any stage of the surgery. Conversely, the ScO2 values were lower in patients with T2DM before anaesthesia induction than those obtained in the controls (60.4±8.1%[SD] vs. 67.2±7.9%, p<0.05) and this difference endured in all stages of the surgery. After anaesthesia induction, the gSO2 was significantly higher in patients with T2DM than in the control population (23.42±1.5% vs. 15.79±1%, p<0.05). The gSO2 increased at the beginning of the CPB both in the T2DM and control patients. However, no significant change was observed in gSO2 during the OPCAB surgery, with uniformly lower values maintained in the 
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Group T2DM. The lower ScO2 in patients with T2DM indicates impaired microcirculation consequently increasing gSO2. Although the SvO2 is a gold standard parameter to assess global oxygen balance, this increased gap precludes the estimation the regional oxygen demand and thereby diminishing the diagnostic value of SvO2 in T2DM patients. Conclusion: SvO2 is not suitable to detect cerebral tissue oxygen balance in the presence of T2DM, particularly when cardiac output is high or oxygen consumption is low. 

11 / Differences between mixed venous oxygen saturation and regional cerebral oxygenation in diabetes mellitus