Hypoplastic Left Heart Syndrome
(See also Overview of Congenital Cardiovascular Anomalies.)
Hypoplastic left heart syndrome consists of hypoplasia of the left ventricle and ascending aorta, maldevelopment and hypoplasia of the aortic and mitral valves (frequently aortic atresia is present), an atrial septal defect, and a patent ductus arteriosus. Unless normal closure of the patent ductus arteriosus is prevented with prostaglandin infusion, cardiogenic shock and death ensue. A loud, single 2nd heart sound (S2) and nonspecific systolic murmur are common. Diagnosis is by emergency echocardiography. Definitive treatment is staged surgical correction or heart transplantation.
Hypoplastic left heart syndrome (HLHS) accounts for 2% of congenital heart anomalies. Because the mitral valve, left ventricle, and aortic valve are hypoplastic (often with aortic atresia), oxygenated blood coming into the left atrium from the lungs is diverted across the atrial communication into the right heart, where it mixes with desaturated systemic venous return (see Figure: Hypoplastic left heart.). This relatively desaturated blood exits the right ventricle through the pulmonary artery to the lungs and through the ductus arteriosus to the systemic circulation. Systemic blood flow is maintained only through the right-to-left ductal shunt; thus immediate survival depends on patency of the ductus arteriosus.
Hypoplastic left heart.
Symptoms appear when the ductus arteriosus begins to close during the first 24 to 48 h of life. Subsequently, the clinical picture of cardiogenic shock (eg, tachypnea, dyspnea, weak pulse, pallor, cyanosis, hypothermia, metabolic acidosis, lethargy, oliguria or anuria) rapidly develops. When systemic circulation is compromised, coronary and cerebral perfusion may be reduced, leading to symptoms of myocardial or cerebral ischemia. Perfusion of the kidneys, liver, and mesentery is also inadequate and oliguria or anuria are common. If the ductus arteriosus is not reopened, death rapidly ensues.
The patient often presents with a history of poor feeding, increased work of breathing, pale or gray coloration, and lethargy. Physical examination shows a very active precordium with a marked parasternal lift associated with very poor peripheral perfusion, cool extremities, bluish gray skin color, and absent or barely palpable pulses. The 2nd heart sound (S2) is loud and single. A soft, nonspecific systolic murmur is often present, as is hepatomegaly. Severe metabolic acidosis is typical, often worsening if supplemental oxygen is administered, which is characteristic of hypoplastic left heart syndrome.
Diagnosis of hypoplastic left heart syndrome is suspected clinically, particularly in neonates with metabolic acidosis that worsens after receiving oxygen; oxygen lowers pulmonary vascular resistance and thus increases the relative proportion of the right ventricle output that flows to the lungs rather than through the patent ductus arteriosus to the body. Diagnosis is confirmed by emergency echocardiography.
Cardiac catheterization is rarely required.
Chest x-ray shows cardiomegaly and pulmonary venous congestion or pulmonary edema. ECG shows right ventricular hypertrophy and diminished left ventricular forces, though it may be within normal limits for a neonate.
Fortunately, most patients with hypoplastic left heart syndrome are now diagnosed with prenatal ultrasonography or fetal echocardiography, allowing initiation of prostaglandin E1 and other appropriate therapies immediately after birth and before organ hypoperfusion can occur.
All infants with hypoplastic left heart syndrome should be stabilized immediately in a neonatal ICU or pediatric cardiac ICU. Vascular access should be established rapidly via an umbilical venous catheter and/or peripheral IV, whichever is quicker. PGE1 (beginning at 0.01 to 0.1 mcg/kg/min IV) is infused to prevent closure of the ductus arteriosus or to reopen a constricted ductus. Neonates, particularly those that are critically ill at presentation, usually require tracheal intubation and mechanical ventilation. Metabolic acidosis is corrected via infusion of sodium bicarbonate. Severely ill neonates with cardiogenic shock may require inotropic drugs (eg, milrinone) and diuretics to improve cardiac function and control volume status.
It is critical to keep pulmonary vascular resistance relatively high and systemic vascular resistance low to prevent marked pulmonary overcirculation at the expense of systemic perfusion. These resistance ranges are maintained by avoiding hyperoxia, alkalosis, and hypocarbia, all of which may lead to pulmonary vasodilation. Because oxygen is one of the most potent pulmonary vasodilators, infants are ventilated with room air or even hypoxic gas mixtures to aim for systemic saturations of 70 to 80%. If the infant requires mechanical ventilation, Pco2 can be controlled in the high normal or mildly elevated range. Systemic vascular resistance is managed by avoiding, or minimizing, the use of vasoconstricting drugs (eg, epinephrine or high-dose dopamine). Milrinone may be beneficial because it can cause systemic vasodilation.
Pearls & Pitfalls
Survival ultimately requires staged surgical procedures that enable the right ventricle to function as the systemic ventricle and control pulmonary blood flow.
Stage 1, done during the first week of life, is the Norwood procedure. The main pulmonary artery is divided, the distal stump is closed with a patch, and the hypoplastic aorta and proximal pulmonary artery are combined into a neoaorta. The ductus arteriosus is ligated. Pulmonary blood flow is reestablished by inserting a right-sided modified Blalock-Taussig shunt or a right ventricular-pulmonary artery conduit (Sano modification). Finally, the atrial septal communication is enlarged. An alternative hybrid procedure, often a joint effort of heart surgeons and interventional cardiologists, involves inserting a stent into the ductus arteriosus (to maintain systemic blood flow) and placing bilateral branch pulmonary artery bands (to limit pulmonary blood flow). In some centers, the hybrid procedure is reserved for higher risk patients (eg, premature infants, those with multisystem organ dysfunction).
Stage 2, done at 3 to 6 mo of age, consists of a bidirectional Glenn or hemi-Fontan procedure (connection of the superior vena cava to the right pulmonary artery). The 3rd stage, done at 18 to 36 mo, is a modified Fontan procedure.
Survival rate is 75% for stage 1, 95% for stage 2, and 90% for stage 3. Overall survival rate is about 70% at 5 yr after surgical correction. As with other children with complex congenital heart disease, survivors may have some degree of neurodevelopmental disability, which may be due to preexisting developmental abnormalities of the CNS or to overt or occult CNS hypoperfusion or thromboemboli occurring during the multistage procedures.
In some infants, heart transplantation is considered the procedure of choice for hypoplastic left heart syndrome; however, PGE1 infusion must be continued along with careful management of pulmonary and systemic vascular resistance until a donor heart is available. Because availability of donor hearts is very limited, about 20% of infants die while awaiting transplant. The 5-yr survival rates after transplantation and after multistage surgery are similar. After heart transplantation, immunosuppressants are required. These drugs make patients more susceptible to infections and cause pathologic changes in the coronary arteries of the transplanted heart in a significant percentage of patients over a 5-yr period. The only known treatment for allograft coronary artery disease is retransplantation.
Endocarditis prophylaxis is recommended for at least 6 mo after each surgical intervention and subsequently for as long as the patient remains cyanotic or has a residual defect adjacent to a surgical patch or prosthetic material.
In hypoplastic left heart syndrome, there is hypoplasia of the left ventricle and ascending aorta and maldevelopment and hypoplasia of the aortic and mitral valves; an atrial septal defect and a patent ductus arteriosus are necessary for systemic blood flow (and thus immediate survival).
Symptoms of cardiogenic shock (eg, tachypnea, dyspnea, weak pulse, pallor, cyanosis, hypothermia, metabolic acidosis, lethargy, oliguria or anuria) appear when the ductus arteriosus begins to close during the first 24 to 48 h of life, becoming more pronounced if supplemental oxygen is given.
Initially, give PGE1 to keep the ductus arteriosus open, give as little oxygen as possible (to avoid increasing pulmonary flow at the expense of systemic flow), and avoid vasoconstrictors; give sodium bicarbonate as needed.
Definitive treatment requires staged operations.