Hypoplastic Left Heart Syndrome (HLH)

This is the 13th most common congenital heart disease. It occurs in 0.05-0.25/1,000 live births (1.5% of all congenital heart disease at all ages).

HLH is most probably due to reduced blood flow from the ductus venosus to the left heart, possibly due to small atrial communication.

The left ventricle is small with or without mitral atresia and with or without aortic valve atresia.

The ascending aorta may be as small as 2 mm but enough to supply coronary arteries in a retrograde fashion. The PDA is large shunting right-to-left. Babies are not born with coarctation but it frequently develops. A patent foramen ovale could be small or even closed.

The right ventricle provides blood to two pulmonary arteries and to aorta through the PDA. Pulmonary blood flow depends upon pulmonary vascular resistance which in turn depends upon patent foramen ovale. When patent foramen ovale is restrictive left atrial pressure will be elevated causing increased pulmonary vascular resistance and less pulmonary blood flow. The more the pulmonary blood flow the better is the systemic oxygen saturation. Although hypoplastic left heart is the most lethal congenital heart disease, the cardiac anatomy is adequate for intrauterine life. The atrial septum is thick and this may be due to increased left atrial pressure in utero leading to LA hypertrophy or this might be the primary pathology causing decreased right-to-left shunting in utero at the PFO level leading to hypoplasia of the left heart.

Coronary and head blood flow may be decreased after birth due to coarctation of the aorta secondary to ductal tissue in the aortic arch.

At birth babies do well due to increased pulmonary vascular resistance therefore adequate systemic flow once pulmonary vascular resistance decreases will cause increase in pulmonary blood flow and decrease in systemic blood flow leading to shock. After birth as the pulmonary blood flow increased this will increased the load on the right ventricle. In addition, the coronary blood flow will decrease as the PA is restricted causing RV ischemia. Prostaglandin by reopening the patent ductus arteriosus will decrease LV pressure and improve coronary circulation, however, pulmonary blood flow will increase because of PGE’s pulmonary vasodilatory effect.

Poor cardiac output after birth will lead to poor renal perfusion and increase in intravenous volume as well as in hyperkalemia. These child will develop a poor appetite and this will lead to hypoglycemia. The increase in intravascular volume, hyperkalemia and hypoglycemia coupled with decreased coronary blood flow due to the restriction of the PDA will lead to myocardial injury.

Due to hypoplasia of the LV, there will be reduced LV forces, resulting in right axis deviation and RVH pattern.  The R wave progression in the chest leads is also abnormal, with prominent S wave in left chest leads.

LV hypoplasia, mitral and aortic atresia are all well seen by echocardiography.  There may be no blood flow by color and Doppler across the mitral and aortic valves. The flow in the ascending aorta may be retrograde, where blood from the PDA flows towards the aortic valve to supply blood to the coronary arterial circulation.
It is important to assess the extent of the atrial communication.  An optimal ASD size is that which allows left to right shunting of blood returning from the pulmonary venous circulation with mild stenosis.  Large ASDs will allow the LA pressure to drop resulting in drop in the pulmonary vascular resistance leading to excessive pulmonary blood flow.  Therefore, large ASDs may be detrimental, particularly that these patients will be destined to Fontan circulation or cardiac transplantation.

Rashkind or blade septostomy may be needed to optimize atrial communication.  Otherwise not necessarily needed prior to stage I Norwood procedure.  Angiography is frequently needed prior to Glenn or Fontan procedures.

These babies typically present with acidosis and shock. They should be intubated and mechanically ventilated with correction of acidosis. Prostaglandin is started to increase the pulmonary blood flow.

As the ductus arteriosus is opened by Prostaglandin the pulmonary blood flow will increase significantly particularly that Prostaglandin is a vasodilation therefore this should be controlled by increasing the pulmonary vascular resistance. This could be achieved by hypercarbia, controlled acidosis and mild hypoxia. Hypercarbia and respiratory incidence could be achieved by providing subambient oxygen through mixing carbon dioxide or nitrogen with room air.

Surgical management includes the Norwood procedure or cardiac transplantation.

The Norwood procedure includes an initial palliative procedure followed by the Fontan procedure. Palliation includes using of the pulmonary valve and proximal MPA as neo-aorta and neo-aorta by ligating the distal main pulmonary artery and connecting the proximal main pulmonary artery to the aortic arch. This will improve the systemic blood flow and then the pulmonary arteries are fed by a 3-1/2 to 4 mm systemic to pulmonary arterial shunt. In addition, the atrial septum is removed surgically and the PDA is ligated. The pulmonary veins may develop progressive stenosis, the etiology is unclear. Tricuspid regurgitation my develop and become progressive which is of significance since the right ventricle is the systemic ventricle. Coarctation of the aorta is common in those who survive the first step of palliation because of residual ductal tissue. Therefore, at the time of palliation the area of coarctation could be bypassed by a homograft. The first stage of Norwood procedure is performed at 1-2 weeks of age and the second stage at about 2 years of age. Alternatively, three stages could be done. The first a Norwood and systemic to pulmonary arterial shunt, the second at about 6 months a bi-directional Glenn and the third stage would take down the systemic to pulmonary arterial shunt at about 18-24 months where the Fontan is completed by connecting the inferior vena cava to the pulmonary arterial circulation.