In our double blind randomized study 40 patients undergoing cardiac surgery will be randomly divided in two groups. The group A who will be infused with 2 IU/hr. vasopressin and the group B who will be infused with normal saline (placebo) intraoperatively starting at the time we give I.V. Heparin, throughout CPB and infusion will be ended when we give the Protamine after coming off bypass. VOTs will be measured at the following time points: Pre-induction of anesthesia, post-induction of anesthesia and at 30 minute intervals during the cardiopulmonary bypass. And we will select cases that are expected to have a longer CPB time such as Redo CABG, CABG + valve replacement, double valve replacement, Bentall procedure and Mitral valve repair with mini thoracotomy.