The objective of this research is to compare the effect of a low dose Vasopressin infusion or placebo during cardiopulmonary bypass on vasopressor requirements, hemodynamics and tissue oxygen saturation and its recovery using tNIRS and vascular occlusion test (VOT) in elective on pump cardiac surgery patients using a non-invasive near infrared spectrometry (NIRS) machine. The clinical implications are that NIRS may permit earlier, non-invasive detection of significant physiologic derangements and allow more accurate and timely titration of medications, fluids and surgical intervention. The use of a low dose vasopressin seems to be preventive for the incidence of observed post-cardiotomy vasodilatory shock and may improve the function of microcirculation that will be assessed using VOT and tNIRS. Finally, it may decrease both catecholamine dose and duration of their administration, it is considered as a useful agent for decreasing all heir side-effects