The goal of this study is to determine if off-pump CABG surgery is associated with better cerebral tissue oxygenation when compared with on-pump CABG. In addition, the investigators would like to compare the results of two different monitors of brain tissue oxygenation with each other, and assess the correlation between brain tissue oxygenation and oxygenation measurements in renal and muscular tissue, as well as with conventional hemodynamic and metabolic variables, such as central venous oxygen saturation (ScvO2). Finally, the investigators aim to determine if duration and extent of cerebral oxygen desaturation predict outcome variables such as postoperative cognitive performance.