Saturday, August 16, 2008
STRUCTURAL HEART DISEASE
1. CYANOTIC PRESENTATIONS
1. CYANOTIC PRESENTATIONS
- Essentials of Diagnosis & Typical Features
- Cyanosis, initially without associated respiratory distress.
- Failure to increase PaO2 with supplemental oxygen.
- Chest x-ray with decreased lung markings suggests right heart obstruction.
General ConsiderationThe causes of cyanotic heart disease that occurs in the newborn period are transposition of the great vessels (TOGV), total anomalous pulmonary venous return (TAPVR), truncus arteriosus (some types), tricuspid atresia, and pulmonary atresia or critical pulmonary stenosis.
Infants with these disorders present with early cyanosis. The hallmark of many of these lesions is cyanosis in an infant without associated respiratory distress. In most of these infants, tachypnea develops over time either because of increased pulmonary blood flow or secondary to metabolic acidemia from progressive hypoxemia. Diagnostic aids include comparing the blood gas or oxygen saturation in room air to that in 100% FIO2. Failure of PaO2 or SaO2 to increase suggests cyanotic heart disease. Other useful aids are chest x-ray, electrocardiography, and echocardiography.
TOGV is the most common form of cyanotic heart disease presenting in the newborn period. Examination generally reveals a systolic murmur and single S2. Chest x-ray shows a generous heart size and a narrow mediastinum with normal or increased lung markings. There is little change in PaO2 or SaO2 with supplemental oxygen. Total anomalous pulmonary venous return, in which venous return is obstructed, presents early with severe cyanosis and tachypnea because the pulmonary venous return is obstructed, resulting in pulmonary edema. The chest x-ray typically shows a small to normal heart size with marked pulmonary edema. Infants with right heart obstruction (pulmonary and tricuspid atresia, critical pulmonary stenosis, and some forms of truncus arteriosus) have decreased lung markings on chest x-ray and, depending on the severity of hypoxia, may develop metabolic acidemia. Those forms with an underdeveloped right heart will have left-sided predominance on electrocardiography. Although tetralogy of Fallot is the most common form of cyanotic heart disease, the obstruction at the pulmonary valve is often not severe enough to result in cyanosis in the newborn. In all cases, diagnosis can be confirmed by echocardiography.
2. ACYANOTIC PRESENTATIONS
Essentials of Diagnosis & Typical Features
- Most newborns who present with acyanotic heart disease have left-sided outflow obstruction.
- Differentially diminished pulses (coarctation) or decreased pulses throughout (aortic atresia).
- Metabolic acidemia.
- Chest x-ray showing large heart and pulmonary edema.
Newborn infants who present with serious acyanotic heart disease usually have congestive heart failure secondary to left-sided outflow tract obstruction. Infants with left-to-right shunt lesions (eg, ventricular septal defect) may have murmurs in the newborn period, but clinical symptoms do not occur until pulmonary vascular resistance drops enough to cause significant shunting and subsequent congestive heart failure (usually at 3–4 weeks of age).
Infants with left-sided outflow obstruction generally do well the first day or so until the ductus arteriosus—the source of all or some of the systemic flow—narrows. Tachypnea, tachycardia, congestive heart failure, and metabolic acidosis develop. On examination, all of these infants have abnormalities of the pulses. In aortic atresia and stenosis, pulses are all diminished, whereas in coarctation syndromes, differential pulses (diminished or absent in the lower extremities) are evident. Chest x-ray films in these infants show a large heart and pulmonary edema. Diagnosis is confirmed with echocardiography.
3. TREATMENT OF CYANOTIC & ACYANOTIC LESIONS
Early stabilization includes supportive therapy as needed (eg, intravenous glucose, oxygen, ventilation for respiratory failure, pressor support). Specific therapy includes infusions of prostaglandin E1, 0.025–0.1 µg/kg/min, to maintain ductal patency. In some cyanotic lesions (eg, pulmonary atresia, tricuspid atresia, critical pulmonary stenosis) in which lung blood flow is ductus dependent, this improves pulmonary blood flow and PaO2 by allowing shunting through the ductus to the pulmonary artery. In left-sided outflow tract obstruction, systemic blood flow is ductus dependent; prostaglandins improve systemic perfusion and resolve the acidosis. Further specific management—including palliative surgical and cardiac catheterization procedures—is discussed in Chapter 18.
PERSISTENT PULMONARY HYPERTENSION IN THE NEWBORN INFANT
- Essentials of Diagnosis & Typical Features
- Onset of symptoms on day 1 of life.
- Hypoxia with poor response to high concentrations of inspired oxygen.
- Right to left shunts through the foramen ovale, ductus arteriosus, or both.
- Most often associated with parenchymal lung disease.
Persistent pulmonary hypertension results when the normal decrease in pulmonary vascular resistance after birth does not occur. Most infants so affected are full term or postterm, and many have experienced perinatal asphyxia. Other clinical associations include hypothermia, meconium aspiration syndrome, hyaline membrane disease, polycythemia, neonatal sepsis, chronic intrauterine hypoxia, and pulmonary hypoplasia.
There are three causes of persistent pulmonary hypertension: (1) acute vasoconstriction resulting from perinatal hypoxia, (2) prenatal increase in pulmonary vascular smooth muscle development, and (3) decreased cross-sectional area of the pulmonary vascular bed because of inadequate vessel number. In the first, an acute perinatal event leads to hypoxia and failure of the pulmonary vascular resistance to drop. In the second, abnormal muscularization of the pulmonary resistance vessels results in persistent hypertension after birth. The third category includes infants with pulmonary hypoplasia (eg, diaphragmatic hernia).
Clinically, the syndrome is characterized by onset on the first day of life, usually from birth. Respiratory distress is prominent, and PaO2 is usually poorly responsive to high concentrations of inspired oxygen. Many of the infants have associated myocardial depression with resulting systemic hypotension. Echocardiography reveals right-to-left shunting at the level of the ductus arteriosus or foramen ovale (or both). Chest x-ray usually shows lung infiltrates related to associated pulmonary pathology (eg, meconium aspiration, hyaline membrane disease). If the majority of right-to-left shunt is at the ductal level, pre- and postductal differences in PaO2 and SaO2 will be observed.
Therapy for persistent pulmonary hypertension involves supportive therapy for other postasphyxia problems (eg, anticonvulsants for seizures, careful fluid and electrolyte management for renal failure). Intravenous glucose should be provided to maintain normal blood sugar, and antibiotics should be administered for possible infection. Specific therapy is aimed at both increasing systemic arterial pressure and decreasing pulmonary arterial pressure to reverse the right-to-left shunting through fetal pathways. First-line therapy includes oxygen and ventilation (to reduce pulmonary vascular resistance) and colloid or crystalloid infusions (10 mL/kg, up to 30 mL/kg) to improve systemic pressure. Ideally, systolic pressure should be greater than 60 mm Hg. With compromised cardiac function, systemic pressors can be used as second-line therapy (eg, dopamine, 5–20 µg/ kg/min; dobutamine, 5–20 µg/kg/min; or both). Metabolic acidemia should be corrected with sodium bicarbonate because acidemia exacerbates pulmonary vasoconstriction. In some cases, a mild respiratory alkalosis may improve oxygenation. Recent studies using the inhaled gas nitric oxide, which is identical or very similar to endogenous endothelium-derived relaxing factor, at doses of 5–20 ppm, have shown it to be a very promising and specific pulmonary vasodilator. In addition, use of high-frequency oscillatory ventilation has proved effective in many of these infants, particularly those with severe associated lung disease.
Infants for whom conventional therapy is failing (poor oxygenation despite maximum support) may require ECMO. The infants are placed on bypass, with blood exiting the baby from the right atrium and returning to the aortic arch after passing through a membrane oxygenator. The lungs are essentially at rest during the procedure, and with resolution of the pulmonary hypertension the infants are weaned from ECMO back to conventional ventilator therapy. This therapy can save infants who might otherwise die but has major side effects that must be considered prior to its institution. The use of ECMO to treat this condition has declined with the advent of high-frequency ventilation and inhaled NO as therapies. Neurodevelopmental outcome among survivors of ECMO is similar to that of infants with persistent pulmonary hypertension managed without the procedure. Approximately 10–15% of survivors have significant neurologic sequelae, with cerebral palsy or cognitive delays. Other sequelae such as chronic lung disease, sensorineural hearing loss, and feeding problems have also been described in this population.
Irregularly irregular heart rates, commonly associated with premature atrial contractions and less commonly with premature ventricular contractions, are noted often in the first days of life of well newborns. These arrhythmias are benign and of no consequence. Clinically significant bradyarrhythmias are seen in association with congenital heart block. Heart block can be seen in an otherwise structurally normal heart (associated with maternal lupus) or with structural cardiac abnormalities.
Treatment is with isoproterenol (starting dose: 0.1 µg/kg/min) to improve cardiac output or with temporary pacing. On electrocardiography, tachyarrhythmias can be either wide complex (eg, ventricular tachycardia) or narrow complex (eg, supraventricular tachycardia). Supraventricular tachycardia is the most common tachyarrhythmia in the neonate and may be a sign of structural heart disease, myocarditis, left atrial enlargement, and aberrant conduction pathways. Acute treatment is ice to the face and, if unsuccessful, adenosine given intravenously at a dose of 75–100 µg/kg. If there is no response, the dose can be increased up to 300 µg/kg. Long-term therapy is with digoxin or propranolol. Digoxin should not be used in cases with Wolff-Parkinson-White syndrome. Cardioversion is rarely needed for supraventricular tachycardia but is needed acutely for hemodynamically unstable ventricular tachycardia.