Vasopressin is a hormone secreted by the posterior pituitary gland. Its main functions are constriction of blood vessels and reabsorption of water and thereby regulating water balance and hemodynamics of the body. Copeptin is a cleavage product in the synthesis of vasopressin and is released in equimolar amounts. Its stability makes it easier to determine the amount of vasopressin. It is known, that the use of a heart-lung machine in cardiac procedures has effects on the water balance of the body and may affect the vasopressin secretion.