The phrase “HLHS” was first suggested by Noonan and Nadas [2] to characterize a very small left ventricle with poorly developed aortic and mitral valves. The left ventricle (LV) is usually a slit-like cavity (Figure 1) with thick muscle, especially when mitral atresia co-exists. The aortic valve is atretic or markedly narrowed with annular hypoplasia. Similarly, the mitral valve is markedly stenotic, hypoplastic or atretic (Figure 1). The LV is very small when the mitral valve is open. Endocardial fibroelastosis is often present. Hypoplasia of the ascending aorta is present, and its diameter is usually 2–3 mm (Figure 2). However, it is adequate to supply ample coronary blood flow retrogradely. The left atrium is very small (Figure 1). The interatrial septum is usually thickened with a small patent foramen ovale (PFO) and rarely the atrial septum is intact. A patent ductus arteriosus (PDA) is classically present and is necessary for the baby to survive [3,4,5].

Tricuspid atresia (TA) is a cyanotic CHD and is characterized as a congenital absence or agenesis of the morphologic tricuspid valve [6,7]. The right atrium is dilated and the tricuspid valve is atretic (Figure 3). In the most common muscular variety, atretic tricuspid valve is seen as a localized fibrous thickening or a dimple in the floor of the right atrium at the anticipated site of the tricuspid valve [8,9,10,11,12]. An atrial septal defect is necessary for survival and is typically a stretched PFO. The mitral valve is usually bicuspid and morphologically a mitral valve. The LV is evidently a morphological LV, but it is enlarged and hypertrophied [8,9,10,11,12]. Usually, a ventricular septal defect (VSD) is present. The VSD is most commonly in the muscular ventricular septum [13,14]. The right ventricle (RV) is hypoplastic and is not sufficiently large in size to support pulmonary circulation. The origin of great arteries is variable and on the basis of which a classification of this disease entity was developed [6]: Type I, normally related great arteries; Type II, d-transposition of the great arteries; Type III, malpositions of the great arteries other than d-transposition; and Type IV, truncus arteriosus [6]. Some of the echocardiographic features are demonstrated in Figure 3.

In DILV, the ventricle most frequently has LV morphology, although RV, mixed, indeterminate, or undifferentiated morphologies have been reported [1,15]. The main ventricle is largely a morphological LV with an outlet chamber connected to it which has a RV morphology. The AV valves may be normal (Figure 4), or one of them may be hypoplastic, stenotic, or even atretic. The great arteries are most frequently transposed and the aorta arises from the hypoplastic RV, and the pulmonary artery comes off the main LV chamber. L-transposition happens more often than d-transposition. The great vessels are normally related in 30% of cases. Double-outlet RV may be seen, in which both great vessels arise from the rudimentary RV. Pulmonary stenosis is present in two thirds of patients and such stenosis is seen irrespective of the great artery relationship. The stenosis may be at the valvar or subvalvar level, or the pulmonary valve/artery may be atretic. Subaortic obstruction may be present in patients with transposition of the great arteries and is due to the stenosis of the VSD or, more accurately, bulbo-ventricular foramen. Patients with subaortic obstruction often have associated aortic coarctation [1,15].

Defects in the atrial and ventricular septae, together with defects in one or both AV valves, described in the past as common AV canal or complete endocardial cushion defect [16], are now generally called AV septal defects (AVSDs). In the complete form, there is usually a large inlet VSD contiguous with a primum atrial septal defect (ASD), and a common AV valve with a single valve annulus. A variant of this, called the intermediate form, has the same features but with distinct and separate right and left AV valves. Transitional and partial forms are also described, and are similar to or the same as ostium primum ASDs. Complete AVSDs are also categorized based on the ventricular sizes, namely balanced and unbalanced (Figure 5) [17]. Unbalanced AVSDs comprise 10–15% of all complete AVSDs. The unbalanced types may be LV-dominant with a large LV and small RV, or RV-dominant with a large RV and small LV (Figure 5). RV-dominant AVSDs are more frequently seen [17].

Mitral atresia may be seen with aortic atresia or a normal aortic root; the former is generally characterized as HLHS which was discussed in a preceding section. In this segment, mitral atresia with normal aortic root will be reviewed. While this division is arbitrary, it is considered appropriate [18] because of the differences in therapy for these two defects, in that HLHS babies almost always require the Norwood procedure [19,20] in the neonatal period, while babies with mitral atresia with normal aortic root need different types of intervention at different ages based on the associated cardiac defects [18]. Mitral atresia with normal aortic root is an uncommon complex CHD and is usually associated with several other defects. The atretic mitral valve may be an absent AV connection or an imperforate valve membrane. The left atrium (LA) is hypoplastic or small (Figure 6) and the right atrium is enlarged and hypertrophied. A PFO or ASD is generally seen; a PFO in 2/3rds of patients and an ASD in 1/3rd. A restrictive atrial septal defect may be present and, rarely, atrial septum is intact; in such cases the levoatriocardinal vein may help to decompress the LA. A single ventricle is most commonly associated ventricular anatomy, although in a few cases there are two ventricles. If two ventricles are present, the LV is small and communicates with the RV via a VSD which is usually small (Figure 6). The RV is always enlarged and hypertrophied. Transposition of the great vessels is frequently seen. Double-outlet right ventricle is present in some babies. In most patients the pulmonary valve is normal and not stenotic. However, in 25–30% of cases valvar and/or subvalvar stenosis or atresia is seen. The aortic valve and aortic root are near normal in size by definition. Coarctation of the aorta or interrupted aortic arch is seen in nearly 30% of cases. Such aortic obstruction is seen only in subjects with a normal pulmonary valve. PDA is seen in nearly 80% of patients [15,18].

There are other cardiac defects that may have functionally one ventricle. These will be briefed.

The management at initial presentation, typically in the neonatal period, depends upon the pathophysiology of the defect itself plus the associated cardiac abnormalities, namely the status of PBF, the presence of obstruction to SBF, and other cardiac defects.

Independent of the nature of palliative intervention during the neonatal period, bidirectional Glenn operation [76] by anastomosis of the superior vena cava (SVC) to the right PA, end-to-side (Figure 14) is undertaken at an approximate age of six months. If a prior BT or Sano shunt is present, it is ligated at the time of bidirectional Glenn. While performing the procedure at six months is commonly accepted, the Glenn can be performed as early as three months subject to demonstrating normal PA pressures and anatomy.

In babies who have persistent left SVC, bilateral bidirectional Glenn (Figure 15) is performed particularly in patients with a small or absent left innominate vein. A bidirectional Glenn procedure may also be undertaken for patients with infrahepatic interruption of the IVC with azygos or hemiazygos continuation, and such a procedure may be called the Kawashima procedure.

Prior to undertaking the bidirectional Glenn procedure, it must be ensured that the PA pressures are normal and the branch PAs are adequate in size. This is most often accomplished by cardiac catheterization and cineangiography. However, some institutions employ echocardiogrms or other imaging studies such as magnetic resonance imaging (MRI) or computed tomography (CT) to accomplish this goal. If stenosis of the PAs is present, it may be relieved with balloon angioplasty or stent placement, as appropriate, or it may be repaired while performing the bidirectional Glenn procedure. If atrioventricular valve regurgitation, aortic coarctation, subaortic obstruction, and other abnormalities are present, they should also be repaired at the time of bidirectional Glenn surgery.

Although the author prefers the term “Fontan/Kruetzer Procedure” because of the simultaneous description of the procedure by both groups [35,36], it is more commonly referred to as “Fontan Procedure” in the literature and therefore, it will be so used in the rest of the presentation. During the final stage, the IVC blood flow is rerouted into the PA. At the same time a fenestration between the conduit and the atrial mass is created. Arbitrary division of these procedures into Stage IIIA (diversion of IVC into the PA) and Stage IIIB (closure of the fenestration) [33,34] may be undertaken.