TSRA Content:
Authors: Stephanie N. Nguyen, MD, and Alexis Schiazza, BS
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Carlos Mery, MD.
Introduction
The single ventricle pathway is a palliation strategy used for patients with a wide variety of congenital cardiac malformations that do not allow a biventricular circulation due to anatomic or functional abnormalities of the ventricles and/or valves. The pathway involves a series of three major procedures that culminate in total cavopulmonary connection, or Fontan circulation. In the final stage of the single-ventricle pathway, all systemic venous return flows passively into the lungs via the pulmonary artery, thus bypassing the right heart completely, and all pulmonary venous return enters the univentricular heart for systemic circulation.
Single Ventricle Lesions
Single ventricle palliation is most commonly performed for the following congenital heart defects:
- Hypoplastic left heart syndrome (HLHS) - characterized by an underdeveloped left ventricle associated with varying degrees of mitral and aortic stenosis or atresia. Systemic flow is dependent on a patent ductus arteriosus (PDA) and mixing occurs at the atrial level through an atrial septal defect (ASD).
- Tricuspid atresia - the tricuspid valve is undeveloped and blood flows from the right atrium across an atrial septal defect (ASD) to the left atrium. Blood then reaches the pulmonary circulation via a ventricular septal defect (VSD) or a patent ductus arteriosus (PDA). The right ventricle is typically hypoplastic. Commonly, the great arteries are transposed, and the pulmonary artery is either stenotic or atretic.
- Unbalanced atrioventricular septal defect (AVSD) - a malformation where the crux of the heart is absent, resulting in a primum ASD, an inlet-type VSD, and a common atrioventricular (AV) valve. In an unbalanced AVSD, the common AV valve does not align in a balanced fashion but is dominant to either the left or right ventricle, while the other is hypoplastic.
- Double inlet left ventricle (DILV) - the left ventricle receives the inflow of both AV valves, and the right ventricle is underdeveloped. The aorta and pulmonary artery (PA) tend to be transposed.
Clinical Presentation
The clinical presentation of these patients depends on anatomy and, in particular, on the balance between the pulmonary and systemic circulations. Patients without pulmonary obstruction will present with progressive symptoms of pulmonary overcirculation and congestive heart failure as the pulmonary vascular resistance drops in the first days of life (resulting in more flow into the pulmonary circulation rather than the systemic circulation). On the other hand, significant pulmonary obstruction (valvular/subvalvular pulmonary stenosis or pulmonary atresia) will present with severe cyanosis when the ductus arteriosus closes (since pulmonary blood flow was dependent on left-to-right shunting through the ductus).
Patients with HLHS or systemic obstruction may present with circulatory collapse once the ductus arteriosus closes (since the systemic circulation was dependent on right-to-left shunting through the ductus). In such cases of ductal-dependent circulation, prostaglandin E infusion helps maintain ductal patency and allows stabilization of these patients prior to surgical intervention.
Staged Single Ventricle Reconstruction/Palliation
Stage 1:
The management strategy at initial presentation (typically in the neonatal period) is largely dependent on the status of pulmonary and systemic blood flow. Therefore, the goals of the first stage are to:
- Provide unobstructed systemic flow
- Provide an adequate source of pulmonary blood flow
- Assure adequate pulmonary and systemic venous drainage into the functional ventricle
- Allow adequate mixing of blood from the systemic and pulmonary veins at the atrial level
The type of procedure used at the first stage of palliation is determined by the anatomy of the defect and are as follows:
- Shunts – used in cases of inadequate pulmonary blood flow, such as tricuspid atresia or other cases of right ventricular outflow tract obstruction. The most common shunt is the modified Blalock-Thomas-Taussig (BT) shunt, in which a Gore-Tex graft is sutured to the subclavian artery and to one of the branch PAs to increase pulmonary blood flow and promote PA development.
- PA banding – used in cases of excessive pulmonary blood flow. The band is usually tightened to achieve a systemic oxygen saturation of 80-85%.
- Norwood procedure - used for HLHS. The main PA is divided, and the proximal PA is amalgamated to the hypoplastic ascending aorta. The ascending, transverse, and proximal descending aorta are then reconstructed with a patch to form a neoaorta (Damus-Kaye-Stansel procedure). The atrial septum is widely opened to allow adequate mixing. A source of pulmonary blood flow is provided in the form of an RV-PA conduit (Sano shunt) or a BT shunt).
- Hybrid procedures – a more recently developed approach that is typically reserved for HLHS patients who are considered too high-risk for the Norwood procedure. Hybrid stage I palliation consists of an atrial septostomy, ductal stent, and PA banding. This allows for adequate systemic blood flow via the ductus while protecting the pulmonary vascular bed. The aortic reconstruction is then performed as part of the second stage of palliation.
Postoperative management of patients after stage I palliation is challenging since the amount of blood flow into the systemic and pulmonary circulations depends on the relative difference in their vascular resistance. Thus, changes in pulmonary vascular resistance change systemic flow and vice versa. Interstage mortality is highest between the first and second stages; therefore, close follow-up and home monitoring are essential.
Stage II:
The second stage of palliation involves the creation of a communication between the superior vena cava (SVC) and the PA (superior cavopulmonary anastomosis) to offload the single functional ventricle. The most common form of second stage procedure is the bidirectional Glenn anastomosis in which the SVC is divided, and the distal end is anastomosed to the right PA in an end-to-side fashion. The alternative superior cavopulmonary connection is the hemi Fontan where continuity with the right atrium is maintained but patched closed in preparation for a lateral tunnel Fontan. In both cases, venous return from the head and upper body bypasses the ventricle and flows passively into the pulmonary circulation. Therefore, pulmonary hypertension must be avoided in early infancy. More than moderate pulmonary hypertension is a contraindication to stage II palliation. The second stage is usually performed between 2 and 6 months of age.
Stage III:
The third stage, or Fontan procedure, is the last planned procedure in the univentricular pathway and is usually performed between 18 months and 4 years of age. It involves connecting the inferior vena cava (IVC) to the PA, therefore diverting all systemic venous return passively into the pulmonary system. This is usually accomplished by either placing an extracardiac conduit connecting the IVC to the right PA or by connecting the atrium to the PA and baffling the IVC flow into the atrial-PA anastomosis (i.e. lateral tunnel Fontan). A communication or fenestration may be left between the Fontan conduit or baffle and the systemic atrium to allow decompression of the circuit. An adequate Fontan circulation requires no obstruction or distortion of the PA, low pulmonary vascular resistance, unobstructed pulmonary venous drainage, minimal AV valve regurgitation, adequate ventricular function, and no systemic obstruction. Nonetheless, Fontan circulation is associated with multisystem failure over time, largely as a result of high systemic venous pressures. These include protein-losing enteropathy, Fontan-associated liver disease, plastic bronchitis, and overall poor exercise tolerance. Management of failing Fontan circulation poses a significant challenge and oftentimes includes workup for heart +/- liver transplantation.