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.