Hypoplastic left heart syndrome

Hypoplastic left heart syndrome (HLHS) refers to the grouping of certain congenital cardiac defects. The main characteristics are the marked hypoplasia (underdevelopment) or even absence of the left ventricle and severe hypoplasia of the aorta. Often a localized pinching (coarctation) of the aorta is also present. The main pulmonary artery is enlarged, and gives rise to a large ductus arteriosus. This allows blood to flow from the right ventricle into the aorta and out to the body. Other characteristics of HLHS often include a combination of aortic and mitral stenosis or aortic and mitral atresia.

Diagram 2.19 Hypoplastic left heart syndrome (HLHS) 1 - patent foramen ovale 2 - pinched aorta 3 - patent ductus arteriosus 4 - narrowed aorta 5 - hypoplastic left ventricle 6 - aortic atresia The lack of a developed left ventricle, plus the aortic coarctation leads to reversed blood flow through the aorta. Partially oxygenated blood reaches the aorta after travelling through the patent foramen ovale, up the pulmonary trunk and through the patent ductus arteriosus. The major blood flow to the systemic circulation is through the PDA.

HLHS is also used to describe cases in which the right ventricle is hypoplastic and the child also suffers from transposition of the great arteries. In such cases the hypoplastic right heart pumps into the same artery that the left ventricle normally pumps blood through, and therefore the circulation is the same as in HLHS. This is sometimes referred to as functional HLHS.

Diagram 2.20 Functional hypoplastic left heart syndrome 1 - patent foramen ovale or atrial septal defect secundum 2 - tricuspid valve atresia 3 - hypoplastic aortic arch 4 - patent ductus arteriosus 5 - transposition of the great arteries 6 - restrictive muscular ventricular septal defect 7 - hypoplastic right ventricle De-oxygenated blood flows from the right atrium through an atrial septal defect into the left atrium. There it mixes with oxygenated blood returning from the lungs. A portion of this blood flows from the left ventricle up the pulmonary trunk to the lungs and through the patent ductus arteriosus to the body. Another portion of this blood flows through a ventricular septal defect, and is pumped by the hypoplastic right ventricle up the aorta.

HLHS is a severe form of congenital heart disease. Without surgical intervention, HLHS is fatal. Infants are often diagnosed within 24 to 48 hours of birth. Symptoms appear when the ductus arteriosus begins to close.

There are three options for treating these children: supportive care until death occurs, staged reconstruction of the heart, or a heart transplant.

Supportive care

Before surgical treatment was developed, at least 90 per cent of infants with this condition died by the age of one month. Even today, some infants are not candidates for surgical therapy. For these infants, supportive care is the only option. Exactly what this care entails will depend on the condition of the infant, but would likely include assistance with breathing, the provision of fluids and management of pain.

Reconstruction of the heart

Staged reconstruction is the treatment of choice for hypoplastic left heart syndrome. Reconstruction takes place in three stages.

The first stage, known as a Norwood operation, is undertaken as soon as possible after birth. This stage calls for a homograft¹ to be used to enlarge the rudimentary aortic arch and then join it to the pulmonary trunk. This is known as an anastomosis. The pulmonary trunk will have been disconnected from the left and right pulmonary arteries. The newly created blood vessel functions as the patient's aorta. The right ventricle then becomes a common ventricle, pumping blood through the aorta to the rest of the body.

Blood flow to the lungs is provided by means of a modified Blalock-Taussig shunt joining the innominate artery to the pulmonary artery. This increases the flow of oxygen-enriched blood that will be sent to the body. The Norwood operation also involves making a hole in the wall between the left and right atria. In medical terms this is known as an atrial septectomy.

Diagram 2.21 Stage 1 of hypoplastic left heart syndrome reconstruction (Norwood) 1 - Blalock-Taussig shunt (temporary) 2 - atrial septum removed 3 - patch where pulmonary trunk is disconnected from left and right pulmonary artery 4 - aorta and pulmonary trunk anastomosed together and the aorta made larger Blood flows through the anastomosed aorta and pulmonary trunk to the aortic arch. A shunt connects the aorta to the pulmonary arteries, providing the lungs with blood to oxygenate.

After the Norwood operation, children require medical management with various drugs. Some drugs, such as digoxin, are needed to improve the strength of the heart's contractions, and other drugs, such as diuretics, are required to remove excess fluid from the body.

The second stage of reconstruction is undertaken between three and ten months of age. Stage two involves performing a bidirectional Glenn shunt or a hemi-Fontan. (This is also known as a partial Fontan or a Bidirectional Cavo-Pulmonary Shunt.)

A bidirectional Glenn shunt is performed by connecting the superior vena cava (SVC) to the right branch of the pulmonary artery. As a result, venous blood from the head and upper limbs will pass directly to the lungs, bypassing the right ventricle. However, the venous blood from the lower half of the body will continue to return to the heart.

While avoiding the risk of failure of a complete Fontan operation, the partial Fontan helps to relieve symptoms. The procedure decreases the volume of blood delivered to the single ventricle, thus reducing the amount of work that the ventricle must perform. Post-operatively, oxygen saturation is improved.

In addition, because the bi-directional Glenn is a low-pressure shunt, it does not carry the risk of causing thickening and hardening of the blood vessels of the lungs. (This is a normal response of the lung's blood vessels to high pressures.) This operation creates a more favorable setting in which to complete a Fontan reconstruction at one and a half to two years of age.

Diagram 2.22 Stage 2 of hypoplastic left heart syndrome reconstruction 1 - superior vena cava connected to pulmonary artery 2 - takedown of Blalock-Taussig shunt De-oxygenated blood from the superior vena cava is routed directly to the lungs. A mix of oxygenated and de-oxygenated blood is sent to the body through the anastomosed aorta and pulmonary trunk.

At 18 to 24 months of age, if the patient does well after the Glenn shunt and pulmonary resistance remains low, the third stage of reconstruction is possible. This involves the completion of the Fontan procedure (as described in the earlier discussion of tricuspid atresia).

With the Fontan operation, the venous blood from the lower part of the body is also diverted to the lungs, thus creating the 'complete' Fontan circulation. However, if the child is not well or the pulmonary resistance is considered too high, then no further surgical treatment is possible.

Diagram 2.23 Stage 3 of hypoplastic left heart syndrome reconstruction (Fenestrated Fontan) 1 - conduit joining inferior vena cava and superior vena cava to right pulmonary artery 2 - artificial wall containing small hole (to separate blue blood from red blood in atria and to channel blue blood from lower body to pulmonary artery. De-oxygenated blood from the superior vena cava and inferior vena cava is routed directly to the lungs. Oxygenated blood is sent to the body through the anastomosed aorta and pulmonary trunk.

Transplantation

Heart transplantation is carried out using the same basic techniques as for other transplants. In the case of pediatric heart transplants, the greatest problem is that infant donor hearts are in short supply. Thus transplantation is a treatment for only a limited number of babies with hypoplastic left heart syndrome. In addition, recipients can survive only with the assistance of heavy doses of expensive medication to prevent rejection of the transplanted tissue. These drugs also have significant side-effects, such as the risk of infection and lymphatic cancer, and must be taken by recipients for the rest of their lives. For those reasons, heart transplants are not considered the standard treatment for children suffering from HLHS.