One of the rarest of congenital cardiac defects is Hypoplastic Left Heart Syndrome, HLHS. This syndrome is described as a collection of anomalies affecting the left heart and connecting structures. Due to advancements in surgical intervention mortality rates in neonates with Hypoplastic Left Heart Syndrome have decreased. Management of these patients requires a symphony of multidisciplinary teams including fetal and pediatric cardiologist, cardiac surgeons, neonatologist, pulmonologist, nutritionist and family counselors. Options currently comprise of heart transplant, three-staged palliative surgery and comfort care. There may be co-morbidities present which further complicate treatment, but with surgical intervention the prognosis of this once deemed fatal condition is improved. This paper will examine the anatomy and abnormal function of the hypoplastic left heart, three staged surgical intervention and management of patient with HLHS.
Hypoplastic left heart syndrome is a congenital heart defect where the structures in the left heart are severely underdeveloped. The left ventricle, aorta and aortic arch are unable to adequately pump and deliver blood to the body due to their size. Aortic and mitral valve stenosis or atresia is present and in some cases atrial septal defect accompanies. (CDC) Normal fetal circulation involves the use of three shunts that act as liver, heart and lung bypass in order to supply the systemic circulation of oxygen rich blood. Fetal blood is oxygenated by the placenta and travels to the fetus via the umbilical vein. The ductus venous the first shunt which encountered and channels oxygenated blood pass the liver. The shunted blood is then carried via the inferior vena cava to the right atrium. Here the second shunt is encountered, the foramen ovale which is a normal atrial septal defect at this stage. The blood that does not cross the foramen ovale into the left atrium is emptied into the right ventricle which is then pumped into the pulmonary artery to be carried to the lungs. The third fetal shunt is encountered; the ductus arteriosus allows blood to be shunted from the pulmonary artery to the aorta and systemic arteries following. Blood that does go to the lungs are returned by the pulmonary veins to the left atrium and emptied into the left ventricle. (Perinatal and Pediatric Respiratory Care, Ch2 p 17-18) Shortly after birth due to changes in vascular resistance blood is redirected to the lungs for oxygenation and these shunts are no longer required leading to their closure within a few days postpartum. In HLHS, because the left heart structures are insufficient to receive and pump adequate amounts of oxygenated blood to supply the tissues the patency of the foramen ovale and the ductus arteriosus are essential to sustaining life. (PubMed – ADAM Medical Encyclopedia, 2013) Medical intervention is needed to maintain the patency of the ductus arteriosus and an atrial septal defect is created in place of the foramen ovale.
Pharmacological therapy would include; prostaglandin E1 or E2 to maintain the patency of the ductus arteriosus, diuretics, dobutamine to increase contractility of the volume loaded right ventricle. (The Lancet)
Postpartum Management of HLHS
If HLHS is not diagnosed prenatally, the baby may seem typical for a newborn but will soon exhibit symptoms of the syndrome. Symptoms include cyanosis, cold hands and feet, lethargy, weak pulse, poor sucking and feeding, pounding heart, tachypnea and shortness of breath. (A.D.AM Medical Encyclopedia) Upon auscultation a heart murmur could be heard and chest x-ray reveals enlarged spherical heart, with increase pulmonary makings. (CDC, text ch30 p527)
Once the diagnosis of Hypoplastic Left Heart is confirmed by echocardiogram treatment is aggressive to stabilize the ventilator and circulatory status. Infants are admitted into the NICU and mechanical ventilation may be instituted to reduce work of breathing, apneic episodes or other complications of prophylaxis treatment. (The Lancet, Soe04) Pharmaceutical intervention includes administration of prostaglandin infusion to secure ductal patency. Diuretics are used to reduce blood volumes and lower blood pressure. Inotropic drugs namely dobutamine is used to address increased volume load of the right heart by improving contractility. (The Lancet) Pulmonary overcirculation, systemic and coronary hypoperfusion are complications of the unbalance pulmonary vascular and systemic resistance. This is managed by increasing pulmonary vascular resistance. Which can be achieved by inducing relative hypoxemia (administering FiO2 <21% usually 17-21% with PaO2 @ 70-80%) or by hypercarbic therapy which aims to elevate PaCO2 in order to induce respiratory acidosis (target 7.20 min 7.15 max 7.30) (Peds text ch 30 p527) It is the decision of the neonates parents to decide course of surgical intervention. Options are cardiac transplant, three staged palliative surgery. When family decision or presentation of comorbidities indicate cardiac transplant, stenting of the ductus arteriosus can be performed to maintain patency until transplant. It may also be necessary to perform a balloon atrial spetostomy to maintain patency of the foramen ovale. (New England Journal of Medicine, The Lancet)
Goals of surgical intervention is delivery of oxygenated blood to the body via the expanded aorta, creation of a new path for deoxygenated blood to reach the lungs reduce the mixing of oxygenated and deoxygenated blood mixing. The first stage is the Norwood procedure which is usually performed with in the first week of life. The Norwood procedure consists of removing the wall between the atria and expanding the aorta by using the pulmonary valve and artery. The newly constructed aorta and coronary arteries are connected to the right ventricle. A modified Blalock-Taussig shunt is placed between the right subclavian artery and the right pulmonary artery. (peds text, pg528) This stage reduces over circulation to the lungs and improves the circulation of higher oxygenated blood to the body thereby, matching pulmonary and systemic circulation by means of a univentricular pump. (The Lancet) Research suggests that the success of survival after the Norwood procedure can be increased when Extracorporeal Membrane Oxygenation (ECMO) support is implemented immediate post-operative or during the procedure. (CHEST, 2003) “ECMO can enable myocardial recovery and improve oxygen delivery which may in turn improve neurological outcomes” (the Lancet)
The second stage cavo-pulmonary shunt also called the Bi-directional Glenn Procedure is preformed when the infant is four to six months. This procedure aims to reduce the workload of the right ventricle by separating the pulmonary artery from systemic circulation and returning oxygen poor blood directly to the lungs. The superior vena cava is detached from the right atrium and attached directly to the pulmonary artery. The B-T shut placed in stage one is removed. (The Lancet) Fontan Procedure is the final stage of the three stage palliative surgery. It is generally done between 18months and three years of age. The goal of this stage is to eliminate the mixing of blood in the right heart; where all systemic venous return flows directly to the lungs, with completely saturated blood returning to the right heart. By redirecting the inferior vena cava to the pulmonary artery, oxygen –poor blood from the lower part of the body will circulate directly into the lungs.
Blood flow in a infant that has survived this staged reconstruction is as follows; oxygen-poor blood returns directly to the lungs via the new connection between the pulmonary arteries and the superior and inferior vena cava. Gas exchange occurs in the lungs which then return oxygen-rich blood via the pulmonary arteries
Superior and inferior vena cava carry deoxygenated blood directly to the lungs by dumping into the pulmonary artery. Blood is then returned to the heart by the pulmonary veins to the left and right atria which are now connected then to the right ventricle which functions as the main pumping chamber. Blood exits the heart via the reconstructed aorta which has been attached to the pulmonary artery trunk.
The circulatory pathway is not restored to normal but redirected to ensure adequate gas exchange in the lungs as well as creating a sufficient pump in the right ventricle to support adequate systemic circulation.