This is a prospective randomized clinical trial investigating the effect of sevoflurane vs. desflurane on tissue microcirculation in patients undergoing off-pump coronary artery bypass surgery. Eligible patients are going to be randomized to receive sevoflurane or desflurane, as a maintenance anesthetic, during the surgery. Tissue oxygen saturation and microcirculatory changes, such as an occlusion and recovery slope, during vascular occlusion test will be recorded during the surgery. The primary end point of the study is the difference in tissue oxygen saturation between the groups.