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.

At first sight, the venous and arterial poles of the heart seem to belong to a single structure. This feature is demonstrated by the continuity seen in the fibrous skeleton between the leaflets of the aortic valve with those of both the mitral and tricuspid valves. Moreover, the orifice of the aortic valve is deeply wedged in between the fibrous annuluses supporting the two atrioventricular valves.

Patients with a functionally univentricular circulation represent a challenge for both surgeons and cardiologists. In this introductory paper, we aim to describe the variability in cardiac phenotype amongst these patients, and to review how function might relate to anatomy.

The concept that the adult heart is a terminally differentiated organ has been generally accepted for many years. As a post-mitotic organ, the heart has been considered to be characterized by a predetermined number of parenchymal cells. This number is held to be defined at birth, and is preserved until death of the organ and/or the organism. According to this concept, the increase of myocardial mass, or in other words, myocardial hypertrophy, is interpreted on the basis of growth of single terminally differentiated myocytes, able to produce contractile elements and organelles, but not capable of division. Ventricular remodelling has been envisaged, therefore, as a balance between the phenomenon leading to either increase in cardiac mass, in other words hypertrophy, or loss of myocytes, as occurring in apoptosis or necrosis.

Many complex cardiac defects produce a functionally single ventricle, in which there is mixing of the systemic and pulmonary circulations. The output from the functionally single ventricle is divided between the two circulations: the proportion going to the systemic and pulmonary vascular beds being determined by the relative resistance to flows within the respective circulations.

Cardiac and pulmonary physiology in patients with functionally univentricular hearts is markedly different from that found in the setting of the normal circulation. The physiology of both the heart and the lungs remains abnormal after palliative procedures, such as creation of systemic-to-pulmonary shunts, banding of the pulmonary trunk, or creation of the cavo-pulmonary connections. In this brief review, I will concentrate on two main aspects of physiology, namely oxygenation and haemodynamics. I will make comparisons between the normal circulation and the functionally univentricular circulation, the latter both before and after creation of a cavo-pulmonary connection. To finish, I will discuss the physiology of the cavo-pulmonary connection itself, and consider some clinical implications.

The ventricle of a functionally univentricular heart is dilated, hypertrophic, and hypocontractile, certainly after the completion of a Fontan circulation (reviewed in Reference). It is difficult, however, to determine the reasons for these abnormal ventricular conditions. The abnormal size of the ventricle, along with the less than adequate function, can be due to the congenital malformation itself, the previous surgical conditions, or the very abnormal working conditions of the ventricle during the various stages of palliation. Moreover, because of the varying morphology of the pumping ventricle in patients with a functionally univentricular arrangement, it is difficult to assess size, shape, hypertrophy, stress, strain, contraction and relaxation, both in cross sectional and serial follow-up.

During the past decades, different variants of the Fontan circulation have become the primary therapeutic option for physiological correction of congenital cardiac malformations in which restoration to biventricular circulations is impossible. Subsequent to creation of the Fontan circulation, the pulmonary and systemic circulations are in series, with only one pumping chamber. Thus, the functionally single ventricle must provide energy for flow of blood to the lungs, as well as to the body. Generally, patients who have undergone these procedures have a good prognosis, although they have subnormal cardiac output at rest, while their central venous pressure is significantly elevated. Despite an adequate haemodynamic situation at rest, exercise performance is usually reduced. Thus far, this reduction has been attributed to cardiac factors, such as the absence of the morphologically right ventricle, or impaired function of the morphologically left ventricle. Due to the mechanical coupling between the heart and the peripheral circulation, however, the inadequate response to exercise in terms of cardiac output might also be dependant on insufficient peripheral vascular adjustments. In this review, we assess the existing experimental and clinical studies which provide detailed analysis of ventriculo-arterial mechanics in the setting of the Fontan circulation.

The Fontan circuit is characterized by a pulmonary circulation dependent on central venous pressure in the absence of a subpulmonary ventricle. Patients with this circulation usually exhibit a low cardiac output. In addition to their abnormal cardiac state, recent studies have demonstrated impaired cardiac autonomic nervous activity, elevated neurohormonal factors, and impaired endothelial function, findings also encountered in adults with chronic heart failure. One major component of these impairments is the abnormal cardiorespiratory response during exercise. Despite some pathophysiologic similarities to adults with cardiac disease, those with the Fontan circulation usually have a congenitally malformed heart underscoring abnormal haemodynamics, with further compromise from multiple surgical procedures. It differs in many aspects from the situation in the adults with impaired systolic ventricular function in the setting of chronic cardiac failure. In this review, I will focus on the cardiorespiratory response during exercise, seeking to clarify the characteristics in these particular patients with the Fontan circulation.

Those patients with functionally univentricular hearts are the ones in which surgical repair so as to produce biventricular circulations is deemed impossible, despite the fact that in most instances there are two ventricles within the ventricular mass. Before the era of the Fontan procedure, the typical course of patients with such functionally univentricular arrangements consisted of cardiac failure and death within the first year of life. Despite the advent of effective palliative therapy, ventricular dysfunction remains a significant clinical problem for these patients.

Magnetic resonance imaging is a powerful diagnostic tool, able to give us accurate anatomical and functional insights concerning the structure of the heart, and capable of overcoming some of the drawbacks and limitations of echocardiography and catheterization. The aim of our current review is to highlight the advantages and applications of the technique in the evaluation of patients with functionally univentricular hearts before and after conversion to the Fontan circulation. Most of these patients will have undergone surgery several times in their life, with a certain body size, and with less than optimal echocardiographic windows because of the surgical scars. Moreover, vascular access is sometimes unavailable due to previous catheterizations. Furthermore, resonance imaging is non-invasive, and can be performed without sedation in patients older than 8 years. Since many of the current techniques, some of them still experimental, will be used extensively in future clinical practice, physicians should be aware of the full spectrum of capabilities of resonance imaging. We will highlight all these applications in the sections which follow.

The identification of the important relationship between shape and function of ventricular chambers represents a milestone of modern cardiology. Application of the law of Laplace for an ideal sphere furnishes intuitive insights on the progression of heart failure. A dilated heart, by virtue of its large size, must generate greater stress in the myocardial wall to achieve sufficient pressure so as to eject the required amount of blood. The mural hypertrophy represents a compensatory mechanism, guaranteeing a lower stress. When the ratio between the radius of the chamber and the thickness of its wall increases abnormally, the heart fails.

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.

Since the pioneering work of Fontan and Baudet, who suggested that a dysfunctional right ventricle could be bypassed by connecting the pulmonary arteries directly to the right atrium in the so-called atriopulmonary anastomosis, much experience has been gained in the field of the functionally univentricular circulation. In view of the continuing need to optimize the fluid dynamics of the connection, research on this topic remains very active. In particular, it is relevant to consider the power dissipated during flow across a connection of this type, due to the low level of the pressure head available for perfusion. The flow to the lungs in this setting is driven only by the low pressure in the caval veins, thus making it essential to minimize the losses of energy in the connection between the terminal part of the venous system and the pulmonary arteries.

When Fontan and Baudet devised what has become known as the Fontan operation, they understandably assumed that inflow and outflow valves should be included to achieve a pump-like action of the subpulmonary right atrial cavity. Over the following years, however, it became apparent that valves did not function satisfactorily in this situation. Worse, the implanted valves had a tendency to become obstructive, which often lead to critical elevation of the already raised systemic venous pressure. Surgeons gradually realised that the outcomes of surgery designed to create the Fontan circulation were likely to be better without inclusion of valves in the subpulmonary right atrium, and they stopped putting them in.