patients with congenitally malformed hearts can be placed into one of two groups.4,5 The first group is made up of those with biventricular atrioventricular connections, specifically concordant, discordant, and mixed atrioventricular connections (see Chapter 2) In the second group, the atrioventricular junctions are supported by only one ventricle, either because of double-inlet ventricle or because one of the junctions is completely absent There is a very small third group in which there is absence of one of the atrioventricular connections but with straddling and overriding of the solitary atrioventricular valve This combination produces a uniatrial but biventricular connection (Fig 69.5).6 In all the patients having univentricular atrioventricular connections, apart from the very small minority having solitary and indeterminate ventricles, a second ventricle is to be found within the ventricular mass (Fig 69.6) In the presence of this second chamber, it follows that the hearts themselves are not “univentricular.” When found, the second ventricle is incomplete, lacking at the least its inlet and often also lacking any direct outlet other than the ventricular septal defect On rare occasions, it may be possible surgically to septate the dominant ventricle in the setting of double inlet and hence to restore biventricular circulations Patients with univentricular atrioventricular connections currently are almost always treated by conversion to the Fontan circulation, as are many patients with biventricular atrioventricular connections In such circumstances, all the hearts will become functionally univentricular subsequent to creation of the Fontan arrangement The addition of the word “functionally,” therefore, provides the logic for the grouping of the various entities to be described in this chapter.7 FIG 69.1 Anatomically solitary ventricle The heart has been opened in clam-like fashion, showing a double inlet to and a double outlet from a ventricle with exceedingly coarse apical trabeculations The only septum present is the muscular outlet septum FIG 69.2 Heart with double-inlet left ventricle (LV) sectioned to replicate the parasternal echocardiographic projection There is a dominant LV with an incomplete right ventricle positioned anterosuperiorly The channel between the chambers is a ventricular septal defect, being positioned between the apical and outlet septal components LAVV and RAVV, Left and right atrioventricular valves FIG 69.3 Comparison of the small chamber in classic tricuspid atresia (right) with the second ventricular chamber seen in the “Holmes heart,” which has a double-inlet left ventricle, but with concordant ventriculoarterial connections (left) In terms of their component makeup, with both chambers lacking an anatomic inlet other than the ventricular septal defect (VSD), the chambers are identical Conventional wisdom, however, had dictated that tricuspid atresia should be interpreted in terms of a “biventricular heart,” whereas a double-inlet left ventricle represented a “single ventricle.” RV, Right ventricle