Uses and Abuses of Fetal Echocardiography: A Pediatric Cardiologist's View*

Transcription

1 lacc VoL 8, No REVIEWS Uses and Abuses of Fetal Echocardiography: A Pediatric Cardiologist's View* JAMES C. HUHTA, MD, FACC Houston, Texas The most valuable and widely applied technique for evaluation of the human fetus is transabdominal ultrasonography, which may be useful from the first few weeks of gestation until the time immediately before birth. Proliferation of high quality equipment (1), including imaging systems that utilize dynamic focusing, plus the availability of a large body of data on the subject (2) have led to sophistication and accuracy in the recognition of cardiac abnormalities on the part of noncardiologist providers of prenatal imaging services. There are three ultrasound techniques commonly used to evaluate the cardiovascular system: M-mode, two-dimensional and Doppler echocardiography. Most equipment for use by obstetricians is not designed for performing M-mode or Doppler portions of the examination, but there is every indication that Doppler techniques will become commonplace in evaluating the high risk fetus in the next few years. Fetal echocardiography combines many such new developments in technology for assessing the cardiovascular anatomy and function of the fetus. Applications of these include 1) the diagnosis of fetal cardiac and extracardiac anatomy. 2) fetal arrhythmia diagnosis and monitoring during transplacental therapy, and 3) cardiovascularfunction assessment including measurement of ventricular ejection and the state of the fetal myocardium and placenta using new Doppler tools for the measurement of blood flow velocities. Interventions for the treatment of congenital defects of the kidneys and brain have already been performed and it appears that the practice of cardiac surgery will be extended to the fetus in the near future (3), thereby focusing more attention on the accuracy of complete cardiac diagnosis by ultrasonography. Indications and Safety Routine screening of fetuses has not been shown to significantly alter outcome and, in the absence of specific in- *Editorials published in Journal ofthe American College ofcardiology reflect the views of the authors and do not necessarily represent the views of lacc or the American College of Cardiology. From The Lillie Frank Abercrombie Section of Cardiology. Department of Pediatrics, Texas Children's Hospital, Houston, Texas. Address for reprints: James C. Huhta, MD, The Lillie Frank Abercrombie Section of Cardiology, Department of Pediatrics, Texas Children's Hospital, 6621 Fannin, Houston, Texas dications in the prenatal period, it should not be standard practice to order an ultrasound examination. When noninvasive evaluation of the fetus is indicated, brief examination of the heart is important and, if an abnormality is suspected, the pediatric cardiologist enters the picture to provide expertise in the noninvasive evaluation of cardiac anatomy and function. Other indications for referral include evidence of fetal hydrops or polyhydramnios, the presence of a noncardiac congenital defect, an abnormality of fetal rhythm and a positive family history of congenital heart disease. The extremely rapid proliferation of the use of ultrasound in the early assessment of the fetus has led to concern on the part of many physicians and consumers and the government that prolonged, high intensity exposure to ultrasound energy may be harmful to the developing fetus in the first trimester (4). Evaluation of the fetal heart by Doppler echocardiography requires higher intensity exposure than is required by standard B-mode or real-time two-dimensional scanning and may exceed the 100 mw/cm 2 spatial peak temporal average federal guidelines, especially with continuous wave Doppler technology. It is suspected that the intensities of a brief diagnostic ultrasound examination are unlikely to be dangerous in later gestation fetuses, but the fetus in need of frequent examinations may experience significant exposure. Therefore, because the risk-benefit ratio of repetitive diagnostic examinations lasting up to 30 minutes each is unclear, we have adopted the policy that repeated pulsed and continuous wave Doppler examinations should be performed only when a potentially life-threatening abnormality is present in the fetus (5,6). Anatomic Diagnosis of Congenital Heart Disease Two-dimensional echocardiography can be used to perform a complete segmental anatomic assessment of the heart and great arteries in the newborn infant (7-9) and there is evidence that such assessment may be possible for much of the cardiac anatomy of the fetus after 18 to 20 weeks' gestation (l0-12). Experience in the diagnosis of abnormalities of atrial situs, atrioventricular (AV) connection, ventriculoarterial connection and the atrial and ventricular septa have been reported (13-25). However, early experi-

2 452 HUHTA lacc Vol. 8, No.2 ence indicates that the same areas that cause difficulty in the child will cause difficulty in the fetus: namely, ventricular septal defect, semilunar valve stenosis and extracardiac anatomy. Only a limited number of workers in the field have reported results with state of the art equipment performing a detailed cardiac examination before 18 weeks' gestation. Although one can recognize that the fetal heart has four chambers, an examination of the anatomy comparable with what can be obtained in a newborn infant is not yet routinely possible. If prenatal diagnosis of congenital heart disease is to achieve a standing comparable with diagnosis after birth by echocardiography plus angiography, one should not have a double standard of accuracy in diagnosis of cardiac anatomy: one for the newborn child and one for the fetus. Methodology for complete diagnosis of intracardiac and extracardiac congenital defects is the first step in both situations. One difficulty that arises is the definition of the primary diagnosis. Congenital heart disease often consists of a combination of defects rather than an isolated defect and one defect may be palliated by another. For example, a 23 week gestation fetus with trisomy 21 was found to have an AV canal defect (Fig. 1). At the second examination, which included Doppler echocardiography, infundibular and valvular pulmonary stenosis (tetralogy of Fallot) were also present. There was dilation of the aorta compared with the right pulmonary artery from right to left shunting in utero as well as a left superior vena cava to the coronary sinus. The outlook for that child after birth was one of gradually increasing cyanosis rather than the more usual congestive heart failure in the first few months. Therefore, the prognosis for any given fetus with cardiac disease must be individualized while keeping the limitations of the test in mind. An unintentional abuse offetal echocardiography results from poor communication between the perinatologist, the pediatric cardiologistand the parents. The identification of a cardiac abnormality in a third trimester fetus sometimes leads to hasty decisions about early delivery or caesarean section, which can result in the additional problem of respiratory distress syndrome in the newborn child. Often there is little evidence that the cardiac problem has any effect on the viability of the fetus by this late date or on its ability to tolerate a normal vaginal delivery. A team approach combining the skills of many subspecialties is useful in such situations. Conversely, when the fetus appears moribund with nonimmune hydrops fetalis, referral to such a specialty team is indicated to establish the cause or to provide assistance in the delivery. Diagnosis offetal Arrhythmias The diagnosis of arrhythmias in the fetus is one area in which prenatal cardiac evaluation has clearly shown practical benefit. Arrhythmias in the fetus may be benign, symptomatic only late in gestation or life-threatening from the moment of appearance. Because direct recording of the fetal electrocardiogram is difficult and unpredictable, M-mode echocardiography has been used with success to evaluate fetal rhythm and has found widespread application (26-33). In addition, fetal Doppler techniques (34), which evaluate the blood flow velocities instead of cardiac chamber wall motion, are equally useful in diagnosis and may be superior to the M-mode technique in the uncommon situation where severe dilation of the atria has reduced their movement. Premature atrial contractions (Fig. 2) occur frequently and have little importance except as an occasional precursor of supraventricular tachycardia or when they may be mistaken for bradycardia. Supraventricular tachycardia is now a well recognized cause of fetal hydrops and cardiac failure in utero. Because transplacental therapy is available to treat supraventricular tachycardia, a method to diagnose the cardiac rhythm and to evaluate the effects of treatment is very important. In animal experiments, atrial pacing of the fetus at rates greater than 300 beats/min causes hydrops and premature labor within I to 2 days (Gest A, Hansen T, personal communication, January 1986). Failure to convert with drug therapy aimed at supraventricular tachycardia should suggest another diagnosis. Ventricular tachycardia in the fetus is rare and there is not yet sufficient experience with diagnosis of this entity to comment. Bradycardia is due either to some form of AV block or to slowing of the sinus node. In a fetus near term, episodes of bradycardia at a rate less than 80 beats/min indicate fetal distress that could be secondary to a cardiac defect. Complete heart block in the fetus may be due to collagen vascular disease in the mother or to congenital heart disease, such as corrected transposition of the great arteries or left atrial isomerism and AV canal defect. The diagnosis is based on detection of independent atrial and ventricular contractions with a faster atrial rate. Robert Carpenter at Baylor College of Medicine recently performed in utero transabdominal transthoracic pacing of a moribund severely hydropic 27 week gestation fetus with congenital complete heart block and a normal heart. Such a capability and the well established efficacy of transplacental therapy are just two examples of the feasibility of treating arrhythmias that may otherwise lead to a stillbirth. The key to success is accurate diagnosis and a knowledge of the natural history. Cardiovascular Function Assessment Ventricular dysfunction. The heart is one of the few organ systems whose function in the fetus can be evaluated by noninvasive means. Cardiac function in the fetus can be estimated using measurements of ventricular chamber sizes and shortening fraction by M-mode techniques (35-38), systolic time intervals (39) and acceleration time of the great

3 Figure 1 (right). Imaging of cardiac anatomy in a 23 week gestation fetus. Upper panel, the atrioventricular canal defect with its anterior bridging leaflet (AL) and normal pulmonary venous connections (white arrow). Middle panel, The aortic arch (AAo) is dilated compared with the right pulmonary artery (RPA). Lower panel, There is a left superior vena cava connecting to the coronary sinus (C8) anterior to the spine (8). A = anterior; DAo = descending aorta; I = inferior; LeA = left carotid artery; LV = left ventricle; P = posterior; RA = right atrium; RV = right ventricle; 8 = superior. Figure 2 (below). Fetal Doppler echocardiogram of the mitral valve (MV) in a 22 week gestation fetus with an irregular heart rate. During normal beats the normal rapid filling velocities (E) are followed by the contribution of atrial systole (A) occurring immediately before the fetal ventricular ejection (open arrows). Premature atrial contractions cause an early or absent ventricular response depending on the degree of prematurity. Numbers indicate the A-A interval in milliseconds. LA left atrium; LV = left ventricle. E 500 E 500 I E i 420 E+A J I E 1 1

4 Figure 3 (left). Two-dimensional (upper panel) and M-mode (lower panel) echocardiograms from a 32 week gestation fetus with cardiomyopathy showing a dilated and poorlyfunctioning left ventricle(lv). The location of the measurement is shown in the upper panel (arrow). Somewhat preserved right ventricular function suggests primary left ventricular myocardial disease. RV = right ventricle. LV 1.7cm Figure 4 (below). Doppler echocardiogram showing mild triscuspid valve insufficiency (II) in a normal 20 week gestation fetus. The mild nature of the insufficiency is suggested by the relatively low peak velocity of the jet. This finding was not detectable I year after birth. A = atrial systole; E = rapid filling phase of diastole; RA = right atrium; RV = right ventricle. RV t I - - 1s RA

5 lacc Vol. 8, No.2 HUHTA 455 Figure 5 (right). Doppler ultrasound sampling in the umbilical artery (UmArt) show a significant difference between systolic and diastolic velocities (systolic/diastolic ratio, 0.31/0.08 = 3.9). UmArt Figure 6 (below). Doppler echocardiogram of the fetal ductus and main pulmonary artery(mpa). The increased velocityof blood flow in the normal fetal ductus (0.8 mls) on pulsed Doppler echocardiography compared with the main pulmonary artery indicates mild physiologic constriction at this level. Note the diastolic runoff in the ductus due to a high pulmonary/systemic resistance ratio ks Ductus MPA artery Doppler flow velocity (40). Ventricular dysfunction may result from end-stage fetal distress, as in hydrops fetalis, or may be due to primary cardiomyopathy. The shortening fraction of the left ventricle is a useful measure of ventricular function in combination with the ventricular size (41). Growth in ventricular sizes studied longitudinally has been shown to be useful in detecting intrauterine growth retardation. However, few data are available concerning the feasibility of treating ventricular dysfunction in utero. In one 32 week gestation fetus with primary cardiomyopathy (endocardial fibroelastosis), transplacental treatment with digoxin was attempted. This baby presented with a dilated, poorly functioning left ventricle (Fig. 3) and no evidence of outflow tract obstruction as is seen in critical aortic stenosis. There was no improvement in the left ventricular shortening fraction with this treatment and the baby died shortly after an uncomplicated delivery at term. Valvular insufficiency. An associated finding with ventricular dysfunction is insufficiency ofone or both AV valves (5,6,42). Pulsed Doppler echocardiography can be used to sample at the valve or in the atrium to detect this abnormality. Although not a feature of the normal fetal Doppler examination, mild tricuspid valve insufficiency can occur in a normal fetus (Fig. 4), but is encountered most com-

6 456 HUHTA JACC Vol. 8, No.2 monly in distressed babies with hydrops fetalis, and in those with some types of arrhythmia. In association with congenital heart disease and hydrops, AV valve insufficiency is a poor prognostic sign (12). Pulsed Doppler echocardiography is limited in the quantitationof the severity of valve insufficiency, but color Doppler techniques may be useful in this regard in the future. Systolic and diastolic flow velocities. Many workers are now using Doppler techniques for monitoring umbilical and uterine blood flow velocities (43-47). The same limitations mentioned earlier apply to the quantitation of blood flow but the patterns of systolic and diastolic flow velocities in the fetal aorta and the umbilical cord give clues to the hemodynamic situation of the fetus (Fig. 5). The placental circulation is a high flow, low resistance circuit and one would expect significant diastolic flow when the placental resistance is normal. One finds that the ratio of systolic to diastolic umbilical artery flow velocity is normally less than 3 at term and less than 4 before 30 weeks' gestation (46). In situations where there is growth retardation related to placental insufficiency, the resistance of the placental circuit increases and the ratio of systolic to diastolic velocity in the umbilical artery markedly increases above normal values. Figure 7. ColorDopplerflow map from a 26 weekgestation fetus. Upper panel, Normal systolic flow toward the transducer in the ascending (AAo, red) and descending (DAo, blue) aortas. Middle panel, The normal pattern of flow in the right ventricular outflow tract appearsblue as flowawayfrom the transducer. Lower panel, The increased ductus systolic velocity in Figure 6 is confirmed by color Dopplerechocardiography which shows increasing velocity (change from blueto green) from the mainpulmonary artery (MPA) to the ductus (open arrow) in a normal fetus. Ao = aorta; LV = left ventricle; RV = right ventricle. Future Directions In the future, technical advances in transumbilical vascular access to the fetus will allow cardiac catheterization to be performed in utero in selected cases. With the current explosion of interventional techniques for the treatment of congenital heart defects without surgery there is a high probability that these skills could be applied to the fetus. But decisions about such treatment require research into more readily applicable methods of fetal assessment such as: 1) quantitation of ventricular stroke volume by pulsed Doppler echocardiography; 2) noninvasive measurement of ventricular pressures by quantitation of the velocity of the AV valve insufficiency jet; 3) evaluation of the pulmonary to systemic resistance ratio by Doppler technique, perhaps by sampling the velocity pattern in the ductus arteriosus (Fig. 6); 4) improved neurologic examination, perhaps by ultrasound evaluation of fetal eye movements; and 5) color Doppler echocardiography, which may be very important in the future as a screening tool for the evaluation of fetal hemodynamics (Fig. 7). Ethical Considerations Given what has been accomplished thus far in fetal diagnosis and what is just over the horizon, it is timely to ask, "So what?" Without well established in utero methods of treating the defective or malfunctioning heart, is it useful or even advisable to work in this area, further refining our diagnostic acumen? Is this an area that will experience rapid

7 JACC Vol. 8. No.2 HUHTA 457 and sustained growth and deserve substantial research funding and trainee involvement, or is this field ahead of its time or impractical in the face of limited resources and abortion on demand? No more than 30 years ago, the postpartum diagnosis of a "blue baby" meant an early death. The introduction of cardiac surgery and high quality cineangiographic techniques dramatically altered the outcome for such infants. It is possible that a similar evolution could occur in interventional fetal/perinatal medicine today. The widespread availability of ultrasound equipment for fetal diagnosis should lead to interventions to affect fetal cardiovascular development that have yet to be imagined (48). Earlier intervention for defects, that appear hopeless later in gestation may significantly alter the natural history. The element that is uncertain for such achievements in fetal care is the conviction on the part of the medical community that progress in treating the fetus with congenital heart disease before delivery will benefit mankind both now and in the future. Although the natural history of some defects results in spontaneous abortion due to severe congestive heart failure before birth, parents and physicians are faced with the choice of treatment or termination long before this information is available. In such a situation, termination of a pregnancy after the diagnosis of a defect represents a value judgment about fetal life and its future quality, and the practice of termination fundamentally changes the approach of physicians to this group of patients. We are living in a genuinely ambivalent society that praises the possibility that physicians will someday eliminate patients with congenital defects rather then treating them. Never before has our profession been so challenged by the presupposition that there is some life that is not worthy to be lived. The acceptance of "wrongful life" is only one example of this attitude (49). Surely the knowledge and skills accumulated in such a noble endeavor as improving the welfare of the fetus in trouble with congenital heart disease are worthy of our best efforts. Knowledge gained in this way will have immediate application to monitoring of the fetus in complicated pregnancies where the heart is anatomically normal. References I. Sahn DJ. Resolution and display requirements for ultrasound Doppler/evaluation of the heart in children, infants and the unborn human fetus. J Am Coli Cardiol 1985;5(suppl 1): DeVore GR. Fetal echocardiography-a new frontier. Clin Obstet Gynecol 1984;27: Turley K, Vlahakes GJ, Harrison MR, et al. Intrauterine cardiothoracic surgery: the fetal lamb model. Ann Thorac Surg 1982;34: Maulik D, Nanda NC. Biological effects of ultrasound. 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