the norwood procedure involves three separate stages of operative corrections. the first stage involves re-fashioning the pulmonary trunk into a neo-aorta so that it is possible to establish an unrestricted systemic circulation. an interpositional, or systemic-to-pulmonary arterial, shunt is then created between the neo-aorta and the pulmonary arteries to allow pulmonary perfusion and gas exchange. two of the available options for the systemic-to-pulmonary shunt are the central shunt and the right modified blalock-taussig shunt. in the setting of a central shunt, pulmonary perfusion is derived from a conduit placed between the pulmonary arterial bed and the neo-aorta whereas, in the modified blalock-taussig shunt, the conduit is interposed between one of the pulmonary arteries and the brachiocephalic artery. in subsequent stages, pulmonary perfusion is provided directly by deoxygenated blood. this is achieved by connecting, first, the superior caval vein, and then the inferior caval vein, to the pulmonary arteries. it is usually during the second stage that the systemic-to-pulmonary shunt is removed.