Hearts which, at first sight, seem to have a solitary chamber within their ventricular mass have long been the subject of controversy. As difficult as it is to manage these cardiac malformations medically and surgically, it has been at least as challenging, to date, merely to describe and classify them. Even the most commonly used terms, “single ventricle” and “univentricular heart”, spark heated debate. In distant times, when congenitally malformed hearts were pathological curiosities, these entities were described as “cor triloculare biatriale”. Therein lies the beginning of the problem, since when hearts of this type were examined by more enlightened pathologists, such as the great Maude Abbott,1 it became plain that the apparently solitary ventricular mass in reality possessed a second, albeit much smaller, chamber. Abbott described this second structure as the “outlet chamber”. This convention of describing a “single ventricle”, albeit with a co-existing “outlet chamber”, that presumably lacked ventricular status, continued throughout the first half of the twentieth century, although it had been recognised by then that hearts could rarely be found with truly solitary ventricles, and these were typically deemed to be common structures. Van Praagh et al.2 neatly summarised the problem with this approach when they pointed out that the so-called “single ventricle” possessed two ventricular chambers, whilst the “common ventricle” described the truly solitary arrangement. In their seminal investigation of 1964, Van Praagh et al.2 analysed only those hearts unified because of double inlet atrioventricular connection, or alignment. They excluded arbitrarily from their investigation all hearts with atrioventricular valvar atresia, despite the similarity in morphology between many of these latter lesions and the hearts with double inlet.3

The recent significant reduction in operative mortality associated with surgical completion of the Fontan circulation is clearly multi-factorial. Better understanding of the fundamental physiological and anatomical issues have led to refinements in operative technique, early neutralization of potential risk factors, widespread use of a staged approach, and selective application of the concept of fenestration or incomplete partitioning. While further reduction of operative mortality is important, and may be anticipated in the future, major emphasis now must be placed on optimizing functional outcome, and understanding and managing the late complications associated with the unique physiology produced by the Fontan circulation.

The nomenclature and classification of patients with a functionally univentricular heart has been debated for decades. We review here the approach taken for dealing with this group of patients by the International Working Group for Mapping and Coding of Nomenclatures for Paediatric and Congenital Cardiac Disease. We discuss the approach of this Nomenclature Working Group in the context of other historical and contemporary ideas about this topic.

Many patients with a functional univentricular heart were treated in the 1970s and 1980s, using an atriopulmonary connection to create the Fontan circulation.1–3 Although this procedure, in many patients, was initially successful, and provided arterial saturations of oxygen close to normal, as these patients were followed over the years, in some cases they developed significant complications. One complication of the atriopulmonary connection is progressive right atrial dilation, which leads to atrial arrhythmias, such as atrial flutter or fibrillation.4,5 The combination of these two problems leads to low cardiac output, diminished quality of life, and poor categorization within the classification of the New York Heart Association. This, and other issues, has led most centres to abandon the atriopulmonary connection as a means of creating the Fontan circulation in favour of the lateral tunnel with cavopulmonary connections,6 or the extracardiac conduit.7

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.