Common Anatomic Features

The HLHS encompasses a spectrum of cardiac malformations that are characterized by significant underdevelopment of the components of the left heart and the aorta, including the left ventricular cavity and mass (Figure 32.1) [1]. Aortic atresia with mitral atresia or stenosis is the most common abnormality, accounting for more than half of the patients with HLHS [43]. The ascending aorta and aortic root are markedly hypoplastic, with the ascending aorta serving only as a conduit for coronary arterial supply. In patients with aortic stenosis, the aortic valve leaflets are frequently dysplastic, and rarely may be even absent [44, 45]. At the milder end of the spectrum, some patients with aortic stenosis and mitral stenosis can overlap with patients with critical aortic stenosis that might be amenable to biventricular circulation.

The ascending aorta shows varying degrees of hypoplasia. In a large single‐center experience, the mean ascending aortic diameter was 3.3 ± 1.7 mm and 40–55% of patients had an ascending aorta of <2 mm [46, 47]. The components of the arch itself are also variably hypoplastic, and a localized coarctation of the aorta is commonly present in more than 75% of patients (Figure 32.2) [48, 49]. Ductal tissue can encircle the aortic lumen and extend from the ductal orifice proximally and distally [50], which has important surgical implications for selecting the strategy of arch repair. The mitral valve is almost always stenotic or atretic. Aortic atresia with mitral atresia is the most common anatomic subtype and occurs in 36–46% of patients undergoing stage 1 palliation, followed by aortic stenosis with mitral stenosis in 23–26%, and aortic atresia with mitral stenosis in 20–29% [46, 51, 52]. A recent pathologic review of 119 specimens mostly entered into an archive prior to surgical intervention had slightly different anatomic subgroups. The phenotypes included mitral and aortic atresia (30%), mitral and aortic stenosis (22%), and mitral stenosis with aortic atresia (48%). In this series the variability in the morphologic findings supported the notion that the lesions seen represent acquired disease occurring subsequent to closure of the embryonic interventricular communication, rather than representing abnormal embryogenesis [28].

Endocardial fibroelastosis of the left ventricle is almost always present in association with aortic atresia and open mitral valve, while it is never present in association with mitral atresia [28, 53]. Endocardial fibroelastosis is thought to develop as a result of subendocardial ischemia caused by high left ventricular intracavitary pressure throughout the cardiac cycle [53]. Coronary artery abnormalities including ventriculocoronary connections and thickening of the intima are more common in the subgroup of patients with aortic atresia and mitral stenosis [11, 54–57].

Abnormalities of the tricuspid valve are found in 35% of the HLHS patients and in up to 50% of those with mitral stenosis [58, 59]. Hypertrophy of the left ventricle, as occurs with severe outflow tract obstruction with a patent mitral valve, can result in deformation of the interventricular septum, leading to distortion of the subvalvar apparatus of the tricuspid valve and resulting in regurgitation [53]. Tricuspid valve abnormalities can also be acquired as annular dilatation secondary to volume overload of the single right ventricle, or chordal elongation with leaflet prolapse caused by subendocardial ischemia at the time of presentation or following the Norwood procedure [60]. Since the aforementioned features, such as endocardial fibroelastosis coronary artery abnormalities and the potential for tricuspid valve insufficiency, are more prevalent in patients with aortic atresia and mitral stenosis, this subgroup has been considered to be at increased risk for mortality after the Norwood procedure [54, 57, 61].

Additional Anatomic Features

Most patients with HLHS have an intact ventricular septum. If there is a ventricular septal defect, it is usually small [1]. The atrial septum will be intact or highly restrictive in up to 10% of patients with HLHS [11]. Premature closure or restriction of the patent foramen ovale, which has been postulated as a cause of HLHS, can occur as a primary event or as a secondary event to left ventricular outflow obstruction [60, 62]. An intact atrial septum can lead to an increase in left atrial pressure and left atrial hypertrophy, along with arterialization of the pulmonary veins and lymphangiectasia. Even if appropriate and timely postnatal intervention is performed, these changes in utero can persist and result in postnatal demise [11, 60]. The atrial septum can be deviated to the left on its superior aspect and attached to the roof of the left atrium, most often in those with aortic and mitral atresia [1, 63]. Nonexistence of atrial septum in the expected location can be mistaken for a large atrial septal defect [1].

Venous anomalies are uncommon in association with HLHS. Anomalous pulmonary venous connection or drainage occurs in 6% of patients [64]. Some of the patients with a highly restrictive or intact atrial septum will have a levoatriocardinal vein present that allows for decompression of the pulmonary venous return [65]. Persistent left superior caval vein occurs in about 15% of HLHS patients [60].

Apart from ventriculocoronary connections, other forms of coronary abnormalities are uncommon. Anomalous origin of coronary arteries has been reported anecdotally [66–70]. Even though the coronary arteries originate from a small ascending aorta, the diameters of the coronary arteries and ostia are usually normal [71].

A hypoplastic left ventricle that is not capable of supporting the systemic circulation can be seen in patients with interrupted aortic arch and critical left ventricular outflow obstruction with a large ventricular septal defect, unbalanced atrioventricular septal defect, or double‐outlet right ventricle [60]. These combinations can be categorized as a variant of HLHS and the treatment options are the same as for HLHS. In rare instances with the presence of a large ventricular septal defect, the development of the left ventricular chamber and mitral valve may be near normal size and allow for a complex biventricular repair.

Initial Stabilization

After birth, the medial smooth muscle fibers in the arterial duct contract, which results in wall thickening, lumen obliteration, and shortening of the duct. Functional complete closure usually occurs within 24–48 hours of birth in term neonates [78]. For this reason, prostaglandin E₁ (PGE₁) should be administered immediately to sustain ductal patency in newborns with diagnosed or suspected HLHS. Once ductal patency is confirmed by echocardiogram, the PGE₁ infusion rate can be adjusted (as low as possible) to avoid the known side effects of hypotension and respiratory depression [79, 80]. Patients with impaired ventricular function or shock will benefit from inotropic support. A decision on the use of inotropes is made by clinical features and echocardiographic findings. Since dose escalation of catecholamines may result in an undesired increase in systemic vascular resistance (SVR) that subsequently raises the pulmonary to systemic flow ratio (Qp : Qs), inodilator therapy with milrinone (a phosphodiesterase inhibitor) can be used for patients with elevated Qp : Qs and systemic hypoperfusion [60]. For the stressed neonate with increased SVR, low‐dose sedation can be used to reduce agitation and work of breathing and improve systemic oxygen balance without mechanical ventilation.

Most patients with HLHS become physiologically vulnerable due to the unrestricted pulmonary blood flow, as pulmonary vascular resistance (PVR) falls after birth. In order to avoid or minimize pulmonary overcirculation, PVR can be increased with manipulation of alveolar gas tension, frequently assisted by controlled mechanical ventilation. The use of positive end‐expiratory pressure and avoidance of hyperventilation can be helpful. In addition, the use of inspired carbon dioxide (CO₂) can be effective in attenuating the pulmonary blood flow by increasing PVR [81–83] and improving systemic oxygen delivery [84]. Although excessive supplemental oxygen, a potential pulmonary vasodilator, should generally be avoided, for preoperative patients with parenchymal lung disease the use of supplemental oxygen to prevent pulmonary vein desaturation is essential. Any of the aforementioned preoperative management strategies should be used with sufficient monitoring, as they can pose potential risks to the neonate awaiting surgery for HLHS. For the patient in whom preoperative stabilization is difficult, early surgical intervention should be considered [85, 86]. For the patient with shock and anasarca or contraindications to cardiopulmonary bypass, bilateral branch pulmonary artery banding should be considered [87, 88].

Staged Reconstruction

For the patient with HLHS, like other functional single‐ventricle lesions, a three‐step staged surgical strategy is now widely adopted. The universal aim of the neonatal stage is to create a stable parallel circulation that will permit somatic growth and lung maturation to support progression to a bidirectional cavopulmonary connection and eventual completion Fontan (or total cavopulmonary connection). For the patient with HLHS, the first stage requires relief of systemic outflow obstruction, including provision of coronary artery blood flow, an appropriately restricted amount of pulmonary blood flow, and creation of a nonrestrictive atrial septal communication. Current strategies consist of the Norwood procedure with either a Blalock–Taussig–Thomas (BTT) shunt or a right ventricle to pulmonary artery conduit (Sano modification) or hybrid palliation (Figure 32.4) [89, 90]. Usually, a superior cavopulmonary anastomosis is established as stage 2 palliation, to achieve a more efficient circulation, improve oxygenation, and unload the single ventricle [91]. Finally, the staged pathway culminates in a Fontan procedure, where the inferior caval vein (IVC) is routed to the pulmonary arteries with or without fenestration [89, 92, 93].

Stage 1 Palliation: Norwood Procedure

The goals of neonatal palliation for HLHS include relief of systemic outflow obstruction, provision of unobstructed coronary blood flow, creation of a nonrestrictive atrial septal communication to prevent pulmonary venous hypertension, and establishing reliable and appropriately limited source of pulmonary blood flow. Since Norwood and colleagues reported the first successful palliation of HLHS in 1980 [89, 94], the surgical approach has changed over time and continues to evolve. A successful Norwood palliation requires optimal surgical technique and cardiopulmonary bypass strategies to permit safe and precise reconstruction. Preoperative stabilization is essential for optimal surgical outcome. Except in the rare patient unresponsive to PGE₁ or with a highly restrictive atrial septal defect, surgery should not take place until the circulation has been stabilized and low cardiac output or acidosis has been resolved. Surgery is typically scheduled electively about 2–5 days after presentation.

After a median sternotomy, the pericardium is opened and the vascular structures are mobilized. The innominate vein is mobilized and looped with a tape to allow for atraumatic retraction and facilitate exposure of the brachiocephalic vessels. For conducting cardiopulmonary bypass, a single venous cannulation is commonly used and arterial cannulation can be established directly through the main pulmonary artery [95] or innominate artery, either directly or via a synthetic graft (Figure 32.5) [96–98]. Cardiopulmonary bypass is instituted and the patient is cooled to a bladder temperature of 18–20 °C over 30 minutes using pH‐stat perfusion strategy. With initiation of cardiopulmonary bypass, the branch pulmonary arteries are snared and a ventricular vent is placed through the right atrial free wall and directed across the tricuspid valve into the ventricle to prevent ventricular distension. The main pulmonary artery is divided just proximal to the bifurcation of the branch pulmonary arteries, and the distal pulmonary artery confluence is closed directly or with a patch. In this step, care must be taken to avoid air embolism in patients with a patent aortic valve. The arterial duct is ligated and the proximal descending aorta is mobilized including division of the first two or three sets of intercostal arteries (Figure 32.6). Circulatory arrest is established at a bladder temperature between 18 and 20 °C, cardioplegia is delivered through the arterial cannula, and atrial septectomy is accomplished.

After completing the atrial septectomy and complete mobilization of the ascending aorta, arch, brachiocephalic vessels, and proximal descending thoracic aorta, cerebral perfusion is initiated through the innominate artery in order to minimize the period of deep hypothermic circulatory arrest. The flow rate for antegrade cerebral perfusion is 40–60 mL/kg/min and is guided by right radial arterial line pressure and/or near‐infrared spectroscopy (NIRS). The aortic isthmus and the ductus arteriosus are divided, and all ductal tissue is excised from the descending thoracic aorta. An interdigitating arch reconstruction is performed in order to reduce the rate of recurrent arch obstruction [99–101]. The incision of the aortic arch begins at the aortic isthmus, extends proximally down the ascending aorta, and ends just above the adjacent “kissing” point between the ascending aorta and the pulmonary root. Great care must be taken with the diminutive ascending aorta to avoid spiraling this incision. A cutback is made in the pulmonary root at the site of implantation of the ascending aorta, extending about 2 mm below the sinotubular junction just left of the commissure that faces the ascending aorta. It is also important to end the aortic incision 2–3 mm cephalad to the level of the pulmonary root cutback. This results in a small degree of redundancy in the proximal ascending aorta and ensures a nonstenotic connection between the aortic root and the pulmonary root (Figure 32.7). A cutback is also made into the posterior left lateral descending thoracic aorta. The descending thoracic aorta has been adequately mobilized to allow it to be advanced to the arch and distal ascending aorta. The distal arch is interdigitated into the cutback in the proximal descending aorta and the posterior walls sutured together with 7‐0 polypropylene suture. A second small cutback is made in the anterior portion of the descending aorta. The connection between the adjacent sites on the ascending aorta and the cutback in the pulmonary root are accomplished with 7‐0 or 8‐0 polypropylene in the case of a very small (<2 mm) ascending aorta (Figure 32.8A). A patch is used for completion of the arch reconstruction and creation of the neo‐ascending aorta. This patch can be pulmonary homograft, pericardium, or porcine small intestinal submucosa (CorMatrix, Alpharetta, GA, USA), depending on the surgeon’s experience or preferences (Figure 32.8B). When the junction of the ascending aorta and pulmonary root is reached, the excess patch is trimmed and the proximal connection between the patch and the pulmonary root is completed (Figure 32.8C). In the case of establishing the pulmonary blood flow with a right ventricle (RV) and pulmonary artery (PA) conduit, the site for the proximal connection should be identified while the pulmonary root is open, and should be at least 1 cm below the nadir of the pulmonary valve cusp, and away from any large epicardial coronary artery [101].

Once arch reconstruction is completed, the source of pulmonary blood flow should be established. Until Sano and coworkers reintroduced the RV–PA conduit modification of the Norwood procedure in 2003 [102], pulmonary blood flow had been provided by a modified BTT shunt. A theoretical advantage of the RV–PA conduit is the elimination of diastolic runoff, and increased diastolic pressure with improved coronary perfusion pressure [103–106]. Potential disadvantages include a negative effect on RV function and ventricular arrhythmias due to the ventriculotomy, impaired growth of the PAs, and the need for an earlier stage II procedure [107–109]. The Single Ventricle Reconstruction trial, a multicenter, randomized controlled trial of 549 patients comparing the RV–PA conduit with the modified BTT shunt, showed that transplant‐free survival at 12 months was better with the RV–PA conduit, but the RV–PA conduit group had more unintended interventions [108]. In longer follow‐up of this cohort, the RV–PA conduit was no longer associated with superior transplantation‐free survival at 3 years [110], presumably related to the deleterious effects of the ventriculotomy. In contrast, a prospective observational study using propensity matching conducted by the Congenital Heart Surgeons’ Society (CHSS) showed a sustained survival benefit associated with the RV–PA conduit group [111]. This discrepancy can be potentially explained by the enrollment of more recent patients in the CHSS study with more experience with the RV–PA conduit [112, 113].

When the RV–PA conduit is selected, it can be performed under full flow during the rewarming phase. In order to prevent proximal obstruction of the RV–PA conduit, a dunk technique is performed by inserting a ringed Gore‐Tex (W.L. Gore, Newark, DE, USA) tube graft through the right ventricular free wall (Figure 32.9A,B) [101, 114]. The conduit can be positioned to the right or left of the neo‐ascending aorta, but the right side is preferred as it makes the stage 2 procedure easier. The diameter of the conduit is determined, depending on the patient’s weight and the course of the conduit, usually 5 mm if the conduit is brought to the left or 6 mm if the conduit is brought to the right. The difference in diameter is due to the slightly longer right‐sided conduit. If the modified BTT shunt is elected as a source of pulmonary blood flow, the arterial cannula is transferred from the Gore‐Tex tube graft to the neo‐ascending aorta first [100]. Distal anastomosis can be accomplished during the rewarming phase of cardiopulmonary bypass. A more centrally positioned shunt helps to promote equivalent growth of the branch pulmonary arteries (Figure 32.9C). Modified ultrafiltration is conducted routinely following successful separation from cardiopulmonary bypass.

Hybrid approaches, branch PA banding with or without ductal stenting, followed by either a conventional Norwood procedure or a Norwood procedure combined with a bidirectional Glenn shunt (comprehensive stage 2), has been reported as a rescue intervention in high‐risk neonates [87, 88, 115] or as a routine strategy [116, 117] to avoid the risk of cardiopulmonary bypass [116–120]. Important shortcomings of hybrid palliation include creation of an adequate atrial septal defect and the risk of retrograde arch obstruction, especially for the patient with aortic atresia, which can result in cerebral and coronary ischemia [121, 122]. The outcomes from two leading centers using hybrid palliation as a routine stage 1 strategy are promising and suggest that a commitment to this approach can yield excellent results, albeit at the expense of increased interstage interventional catheter‐based procedures [118, 119].

Postoperative Management

After the Norwood procedure, the output of the postischemic single RV is limited. This physiologic vulnerability peaks in the first 6–12 hours postoperatively [123]. To ensure adequate oxygen delivery to the tissues, the limited output from the single RV must be partitioned precisely between the systemic and pulmonary circulations. Assessment of cardiac output, pulmonary to systemic flow ratio, circulatory wellbeing by physical examination, and routine perioperative monitoring are notoriously inaccurate; therefore objective assessment tools are necessary [124]. Mixed venous saturation (SvO₂) can be measured to determine if oxygen delivery is adequate and provides direct information concerning the pulmonary to systemic flow ratio (Figure 32.10). NIRS takes advantage of the difference in absorption of light by oxygenated and deoxygenated hemoglobin and can determine the relative oxygen content of the tissue beneath the probe. Both SvO₂ and NIRS data have been used as primary markers for systemic oxygen delivery targeting SvO₂ >50%, cerebral rSO₂ >50%, and somatic rSO₂ >60%, to minimize organ dysfunction and the risk of secondary multisystem organ dysfunction [74, 125, 126]. Management based on SvO₂ or NIRS as a surrogate is shown in Table 32.1. Based on a large perioperative database, it appears that two‐site NIRS monitoring combined with heart rate and blood pressure is an appropriate target for goal‐directed therapy, with the added benefit of being noninvasive (Figure 32.11) [127].

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