by Douglas H. King, M.D.
One of the fastest growing areas within the discipline of pediatric cardiology is the study of fetal echocardiography. This particular discipline has evolved with the advent of improved imaging techniques, especially that of computer enhancement which now allows us to evaluate the cardiac anatomy and fetuses as young as 16-18 weeks. The purpose of this discussion is to review the indications for fetal echocardiography and to explore the use of fetal echocardiography in both the evaluation and treatment of congenital heart disease and fetal dysrhythmias.
Indications for fetal scanning include those groups of patients who would be considered to be at high risk for having a child with a cardiac malformation. These groups include those with:
1. Positive family history.
2. Maternal conditions such as diabetes mellitus, which does increase the likelihood of having a congenital heart defect.
3. Those fetuses who are the product of in vitro fertilization, which does increase the incidence of congenital heart disease.
4. Exposure to potentially teratogenic drugs such as lithium or Dilantin.
Recurrent risks for congenital heart disease in families with a congenital heart defect vary according to various studies. In a family who has given birth to a previous child with congenital heart disease, the recurrence risk ranges between 2 and 3%. A particularly high-risk group would include the fetus of a parent who has congenital heart disease. The recurrence risk for these fetuses is approximately 4%.
Fetuses who are the product of in vitro fertilization have a recurrence risk as high as 7-8% and should be screened for congenital heart disease. Parents with genetic syndromes, such as velocardiofacial syndrome, may have a recurrence risk as high as 50% for having a child with a congenital heart defect. Consequently, the fetuses born to a parent with a deletion of the 22nd chromosome should have a fetal echocardiogram.
Fetal echocardiography should be considered for conditions which put the fetus at risk for structural or functional cardiac malformations. Mothers with diabetes have a 13% chance of giving birth to a child with a major congenital anomaly and there is approximately a 6% chance of giving birth to an infant with congenital heart disease.
The most common heart defects associated with maternal Type I diabetes include transposition of the great arteries, VSD, coarctation of the aorta, and abnormal septal hypertrophy. Mothers with connective tissue disease are at risk for giving birth to a child with complete heart block, as well as other congenital abnormalities. Mothers with isoimmunization secondary to RH incompatibility are at risk for having children with congestive heart failure secondary to severe anemia. Finally, mothers of advanced maternal age are at risk for giving birth to infants with chromosomal abnormalities. These mothers of course should be given the option of both amniocentesis and fetal ultrasound scanning.
Among the many teratogenic agents that have been associated with congenital heart disease is phenytoin. This medication has been associated with septal defects, coarctation of the aorta, patent ductus arteriosus, as well as pulmonary and aortic stenosis. Mothers taking lithium during the first trimester of pregnancy are at risk for giving birth to a child with Ebstein’s anomaly of the tricuspid valve. Those mothers who have ingested large amounts of alcohol put their fetuses at risk for both atrial and ventricular septal defects. Isotretinoin, used for acne, has been associated with congenital heart defects causing both abnormalities of the great vessels as well as atrioseptal defects. Mothers taking oral contraceptives during their first trimester of pregnancy have given birth to infants with septal defects as well as children with tetralogy of Fallot. In those mothers in whom a fetal arrhythmia is noted, a fetal echocardiogram should be seriously considered. Fetuses with either sustained tachycardia, i.e., heart rates equal to or greater than 180 or those with sustained bradycardia, i.e., heart rates less than 90-100, should be thoroughly evaluated.
If a congenital heart defect is found in a fetus, there should be serious consideration towards amniocentesis, especially if there are any other associated congenital anomalies. The risk of genetic defects may be as high as 40% for congenital cardiac malformations detected at 18-20 weeks. This figure of 40% is much higher than is seen in liveborn infants with congenital heart defects where chromosomal or other syndromes are noted in approximately 10% of children with congenital heart defects. The risk for higher association in the 18-20 week-old fetus stems from the fact that many fetuses with congenital malformations may be spontaneously aborted during the second and third trimester.
The echocardiographic evaluation of fetal dysrhythmias is one area in which considerable strides have been made, in both evaluation and management. Many fetuses with persistent dysrhythmias will present with fetal hydrops and the treatment following delivery carries with it a high risk of morbidity and mortality. Fetal echocardiography and fetal Doppler permits an accurate in utero diagnosis of fetal dysrhythmias and is helpful in monitoring treatment with maternally administered medications.
Supraventricular tachycardia is the most frequent dysrhythmia noted in utero. The decision to administered medications to the mother depends on the age of the fetus and the overall clinical situation. If the supraventricular tachycardia is not sustained and there is no evidence of fetal hydrops, then the infant can be followed expectantly, observing the serial ultrasounds for signs of hydrops or persistent tachycardia. If hydrops is present and the tachycardia is sustained, then the decision for early delivery depends on fetal lung maturity and the overall age of the fetus. In infants less than 34 weeks with an immature lung status, serious consideration should be given towards administration of medications to the mother.
The most frequently used medication for fetal tachycardia is digoxin. A maternal digoxin level of 2 ng/mL is usually required for conversion and this typically requires a maintenance dose in the mother of at least 0.75 mg per day. A second-line drug used for treatment of fetal supraventricular tachycardia is flecainide. The success rate with a combination of digoxin and flecainide is as high as 80% for conversion of fetal tachycardia.
The in utero treatment of fetal dysrhythmias highlights the importance of a team approach and treatment of critically ill fetuses. Generally a pediatric cardiologist works closely with a perinatologist in making decisions with regards to optimum treatment of the fetal dysrhythmia. If fetal treatment is not successful, the perinatologist and neonatologist will work closely together in terms of determining the optimal time for delivery of the premature fetus.
Prenatal screening for fetuses with congenital heart disease provides both the physician and parents with valuable information. If a congenital heart defect is present, the parents can make informed choices as to whether to continue the pregnancy or whether to terminate the pregnancy prior to 24 weeks. If a pregnancy is carried to term, the parents can be prepared for the outcome of the pregnancy and the infant can be delivered appropriately in a center equipped to deal with infants with cardiac problems. Many studies have highlighted improved morbidity and decreased mortality in those children with complex congenital heart disease who were identified in utero and were delivered at a tertiary care center. In the future, prenatal therapy of selected congenital heart defects will become more commonplace, including pulmonary and aortic valvotomies in fetuses with severe right and left ventricular outflow tract obstruction.