One hour before the surgery, patients are premedicated with lorazepam (per os, 2.5 mg). Before induction of anesthesia, the NIRS sensors (INVOS 3100, Somanetics, MI, USA) arepositioned on both sides of the forehead. Sensors to detect depth of anesthesia are also mounted on the forehead to monitor EMG and EEG activities. These signals are used to calculate response (RE) and state entropy (SE), respectively (GE Healthcare, Chicago, USA). Induction of anesthesia is achieved by iv midazolam (30 μg/kg), sufentanil (0.4-0.5 μg/kg), and propofol (0.3-0.5 mg/kg), and iv propofol (50 mg/kg/min) is administered to maintain anesthesia. Intravenous boluses of rocuronium (0.6 mg/kg for induction and 0.2 mg/kg every 30 minutes for maintenance) is administered iv to ensure neuromuscular blockade. A cuffed tracheal tube (internal diameter of 7, 8, or 9 mm) is used for tracheal intubation, and patients are mechanically ventilated (Dräger Zeus, Lübeck, Germany) in volume-controlled mode with decelerating flow. A tidal volume of 7 ml/kg and a positive end-expiratory pressure of 4 cmH2O are applied, and the ventilation frequency is adjusted to 12-14 breaths/min to maintain end-tidal CO2 partial pressure of 36 38 mmHg. Mechanical ventilation is performed with a fraction of inspired oxygen of 0.5 during the entire OPCAB procedure and before CPB, and it is increased to 0.8 after CPB. As a standard part of the cardiac anesthesia procedure, esophageal and rectal temperature probes are introduced, and a central venous line is inserted into the right jugular vein. The left radial artery is also cannulated to monitor systolic, diastolic and mean arterial (MAP) blood pressures and arterial blood gas samples.

The membrane oxygenator is primed with 1,500 ml lactated Ringer's solution prior to CPB. Intravenous heparin (150 or 300 U/kg for OPCAB and CPB procedures, respectively) is injected into the patient, and an activated clotting time of 300 s is achieved during OPCAB and of 400 s during CPB procedures. During CPB, mild hypothermia is allowed, the mechanical ventilation is stopped, and the ventilator is disconnected without applying positive airway pressure. Before restoring ventilation, the lungs are inflated 3-5 times to a peak airway pressure of 30 cmH2O to facilitate lung recruitment. Normothermia is maintained in the OPCAB patients.

After securing arterial and peripheral venous lines and placement of NIRS and entropy sensors, data collection is initiated immediately before anesthesia induction in all groups of patients. Since catheterization of the jugular vein is scheduled after anesthesia induction, ScvO2 and gSO2 data are not available at the first protocol stage. After induction and before surgical incision, all measurements are repeated. For the patients undergoing CPB procedures, the whole data set is registered at the beginning of CPB after clamping the aorta and 5 min before the end of CPB. For the patients undergoing OPCAB procedures, collection of the full set of data is performed during performance of the first proximal anastomosis between the aorta and saphenous vein graft. The final stage of the protocol is allocated to the end of the operation after sternal closure. All invasive (i.e. arterial and venous blood gas) and non-invasive (i.e. NIRS) data are registered simultaneously at each protocol stage.

Sample sizes are estimated to enable the detection of a 10% difference in the primary outcome parameter gSO2 that we considered clinically significant. Accordingly, sample-size estimation based on an ANOVA test with four groups of patients indicated that 24 patients were required in each group to detect a significant difference between the protocol groups (the assumed variability of 10%, power of 80% and the significance level of 5%).

Two-way repeated measures ANOVA with the inclusion of an interaction term is used for all measured variables with the protocol stage as within-subject factor (protocol stages) and group allocation as between-subject factor to establish the effects of T2DM and the surgical procedure on the oxygen saturation indices.