cardiac surgery is a discipline aimed at restoring the best cardiac function of diseased hearts. it is often necessary to implant devices, and surgical tools are of great help in achieving faster and safer procedures. paediatric cardiac surgery is a more recent specialization of cardiac surgery. in paediatric cardiac surgery, the problem is usually related to congenital malformations, consisting in the main of anatomical abnormalities which produce, among other problems, poor oxygenation of the blood, high cardiac afterload, cardiac failure, hemodynamic problems during growth, and so on. the paediatric cardiac surgeon uses his skill and experience to determine the optimal correction in each individual case. in many cases, however, it is not easy to predict the effects of the given surgical procedure that, for success, depends on many different parameters.

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patients with congenitally malformed hearts characterised by a functionally single ventricle are currently treated using several procedures that bypass the right heart, such as the fontan and hemi-fontan operations, the bidirectional cavopulmonary anastomosis, and the total cavopulmonary connection. all these options are based on the procedures introduced for palliative correction of tricuspid atresia by fontan and baudet in 1971. introduced with the purpose of reducing the pre-operative volume overload, the surgical task mainly consists of separating the pulmonary from the systemic circulation. irrespective of the specific operation performed, we can call the resulting circulation the fontan circulation.

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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.

clinical experience in the treatment of patients with functionally univentricular hearts has shown the fontan operation to be much less than a perfect therapy. even in ideal patients, acquired elevation of the venous pressures in the systemic circulation, especially in the regions drained by the inferior caval vein, is only a matter of time. for this reason the treatment of this complex form of congenital heart disease is still the object of active scientific debate, as well as considerable ongoing medical and basic research.

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 bidirectional cavopulmonary anastomosis is commonly used in the palliation of patients with a functionally univentricular physiology. the management of alternative sources of flow of blood to the lungs, as well as the magnitude of acceptable accessory blood flow at the time of surgery, nonetheless, remains controversial. these issues are particularly significant when a cavopulmonary anastomosis is performed in infants who may become candidates for a fontan procedure. indeed, a long-standing volume overload, which is invariably associated with the maintenance of accessory sources of pulmonary blood flow, may result in systemic ventricular dysfunction. these observations prompted us to review the influence of antegrade pulmonary blood flow in the management of infants undergoing a cavopulmonary anastomosis.

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.

more than 30 years ago, fontan and baudet proposed bypass of a dysfunctional right ventricle by connecting the pulmonary arteries directly to the right atrium, the so-called atriopulmonary anastomosis. since then, much experience has been accrued in the field of the functionally univentricular circulation. the proposed connections have been subjected to several modifications, aiming towards minimizing the losses of energy in the cavopulmonary system, and thereby improving the clinical outcomes. a remarkable improvement was achieved with the introduction of the concept of the total cavopulmonary connection, specifically the combination of a bi-directional glenn anastomosis with a tubular intracardiac extension from the inferior caval venous to the pulmonary arteries. this design was shown to avoid the dissipation of energy associated with the swirling patterns seen in the traditional atrio-pulmonary anastomosis.

hypoplasia, with or without dysplasia, of the subpulmonary ventricle is found in association with a wide spectrum of complex congenital cardiac anomalies. in these cardiac defects, the systemic ventricle is usually normal, while the subpulmonary one, usually of right morphology, is incapable of supporting the entire flow of blood to the lungs. there is now an alternative to the more classical functionally univentricular type of repair, which can be achieved by means of the total cavopulmonary connection, namely the partial biventricular, or the so called “one and a half ventricle” repair. in selected cases, by reducing the preload on the subpulmonary ventricle by construction of a bi-directional cavo-pulmonary shunt, it is possible to achieve complete separation of the pulmonary and systemic circulations, whilst still maintaining pulsatile flow of blood to the lungs. in this review, we describe our experience with the “one-and-a-half ” ventricular option, analysing the role of preoperative evaluation of the subpulmonary ventricle, and describing our results over the short and intermediate term.

most paediatric cardiologists think that the optimal treatment for patients with functionally single ventricle is the relatively early construction, at from 2 to 7 years of age, of the total cavo-pulmonary circulation. this approach is based on the desire to produce an acyanotic child, such an outcome fostering the belief that the patient is cured. when using this therapeutic approach, the total connection is usually preceded by earlier creation of a partial connection, achieved by creating a bi-directional anastomosis between the superior caval vein and the right pulmonary artery at the age of from 3 to 9 months, without providing additional flow of blood to the lungs. this leads to severe cyanosis by the age of 3 years, when the flow increases in the inferior caval vein, and is the signal for early completion of the total connection. in this policy, the absence of additional flow to the lungs is justified by the fear of altering the ventricular function essential for the success of the total connection, because of the volume, and eventually the pressure, overload of the additional flow on the functionally single ventricle,which may impair its compliance and/or contractility, or be responsible for atrioventricular valvar regurgitation. these alterations in myocardial functions would then contraindicate, or increase, the mortality and morbidity of the total connection, producing deterioration in its long-term results.

based on the revolutionary concept of the dispensable right ventricle, the fontan operation stands as the extreme example of physiologic repair in pediatric cardiac surgery, serving as the common surgical denominator for the entire spectrum of functionally univentricular conditions. also called ortho-terminal repair, it makes allowance for a circulatory arrangement that is in-series by effectively driving the pumpless systemic venous return directly through the pulmonary circulation. if this arrangement is to work, then two conditions must be fulfilled. first, there must not be any anatomical or functional hindrance along the whole fontan pathway, from the caval venous inflows to the aortic outflow. second, there must be a well-suited systemic pump, essentially recruited to the dual function of drawing blood from and, simultaneously, driving blood through, the lungs. this latter function, driving the blood through the lungs, takes place during the venous phase of the systemic output.

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.

in this review, i will first consider the general physiology induced by hypoplasià of the left ventricle, examining in particular its practical consequences in the fetus and the neonate. i will then explore the particularity of the physiology after the first stage of the norwood sequence of operations, comparing the two approaches of an aorta-pulmonary shunt versus a conduit placed from the right ventricle to the pulmonary arteries.

the norwood procedure and its modifications have become the most commonly used surgical procedures for patients with hypoplastic left heart syndrome. many efforts have been made to understand the fluid dynamics of the norwood circulation, with the major impetus coming from the lumped-parameter model described by migliavacca et al. in this model, a large number of variables have been analysed to understand their effect on the norwood circulation. in the clinical setting, some of these parameters can be influenced by the surgeon during the operation. the shunt has been demonstrated to be the critical element in regulating the pulmonary and systemic flows, with the goal of the surgeon being to obtain balanced flows. in order to do this, it is possible to influence the flow through the shunt in three ways:

1. by augmenting the pressure of perfusion of the shunt, with a suboptimal aortic reconstruction that can leave some residual pressure gradient. this can work like an increment of systemic vascular resistance;

2. by varying the size of the shunt;

3. by varying the type of architecture of the shunt, since flow through the shunt is dependent on the site of anastomosis, the angle with the vessels, and so on.

in order to validate or verify the results obtained from the lumped-parameter model, we have reviewed our experience with the norwood procedure since 1992.

efforts to treat babies with hypoplastic left heart syndrome began over 30 years ago, when surgeons first reported limited success with palliative surgery. cayler et al. performed an anastomosis between the ascending aorta and the right pulmonary artery, bilateral banding of the pulmonary arteries, and an atrial septectomy. their follow-up extended for only 7 months, but the authors predicted that the rapid advances being made in cardiac surgery at the time heralded a new era where hypoplastic left heart syndrome would be cured. unfortunately, that prediction remains far from true today, although dramatic advances have been made in the therapy of this otherwise lethal condition.

the hypoplastic left heart syndrome includes a spectrum of underdevelopment of the left-sided cardiac structures, characterized by aortic valvar atresia or severe stenosis with a small ascending aorta and hypoplasia of the left ventricle. without surgical treatment, practically all infants with this complex cardiac anomaly die within the first month of life. since norwood et al. reported successful surgical palliation in 1981, multi-staged reconstructive surgery, involving the norwood procedure as the first stage, followed by a bi-directional glenn anastomosis and subsequent completion of the fontan circulation, has become the established surgical treatment.