In patients undergoing cardiac surgery under cardiopulmonary bypass, some organs like brain and heart are preserved while others (skin, gut and skeletal muscle) are being underperfused. This phenomenon is related to silent peripheral vasoconstriction that is not clinically available but threatens end-organ perfusion and carries the risk of multi-organ failure. By measuring non-invasively the somatic-to-cerebral oxygen saturation gradient, the present study aims at detecting silent peroperative hypoperfusion episodes. The investigators hypothesize that gradient, measured during the surgical procedure, will predict the occurrence of anaerobic metabolism, ascertained by an elevation of blood lactate concentration, measured in intensive care unit.