How to protect the right ventricle in hypoplastic left heart syndrome

To the Editor,

The surgical management of patients with hypoplastic left heart syndrome (HLHS) is now standardized as a staged approach. It begins in the neonatal period with a first-stage palliative procedure such as a Norwood procedure with a modified Blalock-Tomas-Taussig (BTT) shunt, a Sano modification with a right ventricle (RV)–pulmonary arter (PA) shunt or with a hybrid approach. This initial stage is commonly followed by a bidirectional cavopulmonary shunt at around 4 to 12 months of age, culminating in the final procedure, the Fontan procedure, between 2 and 3 years of age. As a result of the refinement of surgical techniques and perioperative management, the early outcome of the first stage of the procedure has substantially improved, with experienced centres achieving in-hospital survival rates of more than 90%; however, the 20-year survival has dropped less than 80% even in these centres. Therefore, attention has now shifted towards how to improve the long-term outcome after Fontan completion.

In this issue of the journal, Fetch and colleagues [1] from the German Heart Center in Munich reported their experience with 335 patients with HLHS. They compared ventricular function (VF) and atrioventricular valve regurgitation (AVVR) between patients with the BTT shunt and those with the Sano shunt. They reported that progression of VF was observed in patients after they received the Sano shunt, although the degree of AVVR was similar to that among patients after the BTT and the Sano shunts. Because the Dunk technique, which causes less damage to the right ventricle, was not used as a Sano shunt until 2012, and because this study was non-randomized, it is difficult to compare the VF with the BTT shunt and the Sano shunt. When the survival of patients receiving the Sano shunt was compared to that of patients receiving the BTT shunt, survival at 6 years was 66% with the Sano shunt and 65% with the BTT shunt. A clear message from their study is that there was a significant correlation between VF and AVVR.

There have been numerous debates regarding whether the Sano shunt is better than the modified BTT shunt as a first-stage palliation of HLHS. According to the Single Ventricle Reconstruction (SVR) trial, the Sano shunt offers a superior 6-year survival, although a lasting survival benefit of the Sano shunt could not be demonstrated after 3 or after 6 years. There was no greater prevalence of RV dysfunction or tricuspid regurgitation between the Sano shunt and the BTT shunt. Similar results have been reported by the data centre of the Congenital Heart Surgeons' Society [2].

The Australia and New Zealand Fontan Registry [3] reported excellent survival into the second decade of life among patients who had the extracardiac Fontan procedure, but half of these patients suffered a late adverse event by 14 years. Patients with HLHS have a higher risk of late adverse events than other morphological groups.

Why does HLHS have poor long-term outcomes compared to other types of single ventricles? to an RV type of a single ventricle (SV)?

The adult ventricular mass is made up of a three-dimensional network of myocardial cells. This network is highly structured and arranged in layers in which the myocardial cells have a preferred orientation. In all hearts, the ventricular mass is arranged in 3 different layers: superficial (subepicardial), middle and deep (subendocardial). The superficial and deep layers are present in both the RV and the left ventricle (LV), whereas the middle layer is present only in the LV. The middle layer, unique to the LV, shows a circumferential pattern [4]. Therefore, the RV wall is thinner than the LV wall and is more susceptible to pressure and volume loads. When a volume load is applied, the RV dilates more easily than the LV, and when a pressure load is applied, the contractile force of the myocardium tends to decrease.

In patients with the Fontan and congenitally corrected transposition of the great arteries, in whom the RV is the systemic ventricle, the RV tends to dilate, and the myocardial contractility decreases more easily than that of the LV when pressure or volume load is applied. In cases of HLHS or RV, staged management of the dominant SV is required to reach the Fontan operation. Long-term results can be improved by selecting a proper procedure that does not impose volume load due to AVVR or afterload due to re-coarctation of the aorta. In addition, it is important to remove such causes as soon as possible. Minimizing damage to the myocardium will lead to improved long-term outcomes.

The most common cause of AVVR in HLHS is annular dilatation due to volume overload [5]. No matter what type of atrioventricular valve repair technique is used, it is difficult to control AVVR in the long term. Therefore, prevention of AVVR is crucial. In neonates presenting with more than moderate AVVR and RV dysfunction, we intubate and ventilate to control the Qp/Qs and wait for a few days until the AVVR decreases and the RV function improves. The Qp/Qs is controlled by putting a clip on the RV-PA graft after the Norwood-Sano procedure.

Another important cause of poor long-term outcomes of HLHS is RV dysfunction due to increased afterload. Re-coarctation of the aorta is the most common cause of RV dysfunction. Recently, it has become clear that the geometry of the neoaortic arch [6] and the prosthesis used for reconstruction of the aorta have significant influences on long-term RV function. Most surgeons prefer to use homograft or autologous pericardium or bovine pericardium to reconstruct the neoaorta. Indeed, homograft or other patch materials often comprise more than 50% of the reconstructed segment in many patients. This fact represents the big difference from other RV types of SV. However, these artificial materials have disadvantages such as lack of growth, degeneration, calcification and lack of elasticity. Artificial materials decrease the elastic properties, increase wall stiffness and decrease the distensibility of the aorta. An understanding of the elastic properties of the reconstructed aorta is vital, given that RV failure is common after the Fontan procedure and that patients with HLHS may be at particular risk. Our policy for reconstructing the neoaorta has been direct anastomosis without any patch materials. Our recent results demonstrate that more than 60% of patients had a direct anastomosis without using any patch materials [7]. This inelastic reconstructed aorta may be one of the reasons why long-term outcomes after the Fontan procedure are worse in patients with HLHS compared to those without HLHS.

Conflict of Interest: None

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