The recent developments in imaging techniques have created new opportunities to give an accurate description of the three-dimensional morphology of vessels. Such three-dimensional reconstruction of anatomical structures from medical images has achieved importance in several applications, such as the reconstruction of human bones, spine portions, and vascular districts.

Whatever the specific anatomy, the Fontan circulation, be it created directly or subsequent to a bi-directional cavopulmonary anastomosis, transforms completely the pattern of circulation of the blood. In essence, it converts a network of circulations in parallel into one in series. The haemodynamic consequences are numerous. The most important and direct among them is, perhaps, the increase in afterload, as well as the reduction in preload, for the systemic ventricle. From the clinical point of view, the most immediate and relevant implication is the amelioration of cyanosis, this being the consequence of removing the common mixing of systemic and pulmonary venous blood within the heart.

the techniques of computational fluid dynamics are among the most powerful tools available to engineers dealing with the motion of fluids and the exchange of mass, momentum, and energy. they have recently been shown to have an increasing number of applications to the human cardiovascular system, including the fluid dynamics of surgical reconstruction of congenitally malformed parts of the cardiovascular system. in vitro models are the alternative laboratory tools with which to study fluid dynamics. the advantages of computational fluid dynamics over the in vitro models are the easy quantification of haemodynamic variables, such as rates of flow, pressure, and distribution of shear stress, and changes in geometric and fluid dynamics parameters. furthermore, using computational fluid dynamics allows the development of three-dimensional models to reproduce both the complex anatomy of the investigated region and the details of the surgical reconstruction, especially with the recent developments in magnetic resonance imaging. on the basis of the results, it is possible quantitatively to evaluate the surgical correction. this technology, which benefits greatly from the continuous improvement in hardware and software, enables cardiovascular experts and bioengineers to look at the fluid dynamics of various cardiovascular regions with increasing sophistication.

surgical repairs of many severe congenital cardiac malformations, such as the procedures used to redirect the flow of blood in the setting of absent or suboptimal perfusions, are performed either using direct vascular anastomosis, or by the insertion of interpositioned prosthetic shunts. examples of these applications can be found when considering those cardiac malformations characterized by the common physiological feature of having a single pumping ventricle, usually due to the incomplete and rudimentary form of the complementary ventricle. in this situation, since the circulation depends on the functionally single ventricle, pulmonary perfusion can be derived from the systemic circulation through a synthetic tube (gore-tex®, falstaff, az, usa), usually connected between the brachiocephalic or subclavian arteries and the right or left pulmonary arteries. this arrangement is called the modified blalock-taussig shunt (fig. 1, left). the effect is to produce parallel circulations (fig. 1, right). survival at this stage is closely dependent on the balance between systemic and pulmonary flows, and thus on the fluid-dynamics through the interposition shunt, which is often the sole source of pulmonary perfusion.

among the operations which lead to a partial rerouting of the systemic venous return into the pulmonary arteries, the bidirectional cavopulmonary anastomosis is frequently utilised in the staged surgical management of patients with functionally univentricular hearts. whereas some surgeons insist on closing any other source of pulmonary blood flow while performing a bidirectional cavopulmonary anastomosis, others maintain patency of either a stenosed native pulmonary outflow tract, or of a systemic-to-pulmonary arterial shunt. it remains controversial as to whether an additional source of pulmonary arterial blood flow can safely and usefully be left at the time of the bidirectional cavopulmonary anastomosis to increase systemic saturation, or whether it should be eliminated to reduce the volume load on the ventricle.

the bidirectional cavo-pulmonary anastomosis is a well-established palliative procedure for patients with a functionally univentricular circulation. it is usually considered one step in preparation for fontan procedure, but it may be performed as a long-term palliation for patients deemed to be at high-risk. in this subset of patients, a valuable surgical option could be to add, or maintain, an additional source of flow of blood to the lungs, either derived from a patent but banded trunk or one protected by native pulmonary stenosis, or a systemic-to-pulmonary arterial shunt. the risk and benefits of providing an additional source of pulmonary flow after construction of a bidirectional cavopulmonary anastomosis are strongly debated. in terms of benefit, the arterial saturation of oxygen is increased due to the greater ratio of pulmonary-to-systemic flow, arteriovenous fistulas are prevented and, as a consequence of the arterial pulsatile flow, the pulmonary arteries are stimulated to grow. the most significant drawbacks are volume overload of the functionally single ventricle, and higher pressures compared to an isolated bidirectional cavopulmonary anastomosis.

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.