Andersons pediatric cardiology 1805

morphogenesis of the lesions making up the group of hearts with univentricular atrioventricular connection Keywords double-inlet ventricle, absent atrioventricular connection, Hypoplastic left heart syndrome; pulmonary atresia with intact ventricular septum morphogenesis Introduction For the anatomist and pathologist, the terms “univentricular” and “single” have proven to be among the most controversial words used to describe a congenitally malformed heart Since the mid-1990s, light has begun to emerge at the end of the tunnel for those seeking a logical framework for the use of these words In no small way, this reflects the recognition by clinicians that it is frequently not possible surgically to create a biventricular circulation in a patient having two well-formed ventricles In these circumstances, conversion to the Fontan circulation permits both the systemic and pulmonary circulations to be supported by only one of the ventricles In other circumstances, as when there is a doubleoutlet ventricle with remote interventricular communication, it is the combined ventricular mass that ejects to the systemic circulation These facts underscore the grouping of the markedly heterogeneous anatomic entities discussed in this chapter The arrangement in the setting of double-outlet ventricles is discussed in Chapter 39 The anatomy involved when both circulations are supported by only one ventricle becomes easier to understand in consideration of the extensive controversies in the earlier literature, which has offered numerous illogical definitions of “single ventricles” or “univentricular hearts.” These definitions reflected the interpretation of ventricular morphology in patients having doubleinlet ventricles as opposed to atrioventricular valvar atresia.1,2 It was, of course, patients having tricuspid atresia who were first converted to the Fontan circulation The majority of patients now treated in this fashion, however, have hypoplastic left heart syndrome Such patients have unequivocally biventricular hearts, as do many of those with pulmonary atresia and an intact ventricular septum, who are treated in comparable fashion Here we address the basic morphology of all these lesions, along with those found in other patients with obviously biventricular morphologic arrangements We begin, however, by explaining why the topic has proved so controversial for both morphologists and clinicians, while accepting that such considerations are now largely of historical interest Philosophical Considerations Relative to Nomenclature It is very unusual to find hearts with a solitary chamber within the ventricular mass When found, they exhibit a double inlet to a ventricle that has particularly coarse apical trabeculations and in which the only septal structure present is the muscular outlet chamber (Fig 69.1) Such hearts have not only a univentricular arrangement of the ventricular mass but also a univentricular arrangement of the atrioventricular junctions This is because, in the setting of all variants of doubleinlet ventricle, the atrial chambers connect to only one ventricle irrespective of whether or not the ventricular mass is itself univentricular This fact is significant because, subsequent to a study that appeared in 1964,1 it came to be accepted that the criterion for the definition of a “single ventricle” was the presence of a double-inlet atrioventricular connection This was accepted despite the fact that those promoting this terminology recognized that, in the majority of the patients having double-inlet ventricle, a second chamber was present within the ventricular mass (Fig 69.2).2 At the time, such hearts were considered to be “univentricular” because the second chamber was interpreted as representing an “infundibular outlet chamber,” and hence not a “true” ventricle There was a problem with this approach, since at that time the hearts with atrioventricular valvar atresia were deemed to be biventricular, with the small chamber in patients having tricuspid atresia accepted as being a right ventricle When comparisons are made between the morphology of the small chamber in hearts with tricuspid atresia and those with double-inlet left ventricle—provided that the comparison is made in the setting of hearts having the same ventriculoarterial connections—the chambers are virtually indistinguishable (Fig 69.3).3 Such similarity is no more than should be expected, since in both tricuspid atresia and double-inlet left ventricle the larger ventricle is of left ventricular morphology The smaller chamber is an incomplete right ventricle lacking its inlet component In double-inlet left ventricle, this occurs because both atrioventricular junctions are supported by the dominant ventricle (see Fig 69.2) In classic tricuspid atresia, in contrast, the right ventricle lacks its inlet because there is congenital absence of the right atrioventricular connection (Fig 69.4) In both settings, as in the hearts with an indeterminate ventricle, it is the atrioventricular connections that are univentricular With one small exception, therefore, it follows that all