Cannulation : Typically blood is drained by gravity through two cannulae inserted into the superior and inferior vena cavae. During bypass, if the SVC and IVC are snared, the entire venous return to the heart is diverted to the pump (total cardio pulmonary bypass). Whenever the right side of the heart has to be opened, the patient has to be on total bypass. Otherwise, when a chamber is opened, air will enter the tubing, causing air lock and cessation of venous retu1-n. For an aortic valve replacement or coronary artery bypass, the right side need not be opened. So the operation can be done using a single, two stage venous cannulae passing through right atrium into the inferior vena cava (atrio-venous cannula). The oxygenated blood is returned through an arterial cannula inserted into the ascending aorta or femoral artery.