CONGENITAL heart disease is

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1 601 This is the second in a series of technical bulletins to be published by the American Institute of Ultrasound in Medicine (). The is a multidisciplinary organization dedicated to promoting the safe and effective use of of ultrasonography in clinical medicine. The purpose of a technical bulletin is to keep the ultrasound community informed about practical scientific developments relating to the clinical practice of diagnostic ultrasonography. Topics are generally selected for their broad application to healthcare professionals who use diagnostic ultrasound technology in their practice of medicine. Performance of the Basic Fetal Cardiac Ultrasound Examination CONGENITAL heart disease is a leading cause of infant mortality with an estimated incidence of about 4 8 per 1,000 live births. 1,2 Nonetheless, structural cardiac defects are among the most frequently missed abnormalities by prenatal ultrasound. 3,4 This technical bulletin describes a basic ultrasound evaluation of the fetal heart. These guidelines are appropriate for lowrisk fetuses that are already being scanned for standard prenatal ultrasound indications. 5 Suspected problems may require more comprehensive echocardiography. Fetal cardiac screening is designed to maximize the detection of structural anomalies within the constraints of an indicated prenatal ultrasound study. The procedure identifies a high-risk group for karyotyping and provides useful information about obstetrical management related to delivery site or multidisciplinary care. Detection rates can be diminished by maternal obesity, lower transducer frequency, abdominal scars, early gestational age, oligohydramnios, and prone fetal position. 6 The level of operator experience is also an important factor that should be considered for fetal cardiac screening. 7 BASIC CARDIAC EXAMINATION Abasic cardiac examination can be performed optimally between 18 and 22 weeks of gestation. Some anatomy can be difficult to evaluate during early pregnancy and a later scan may be hindered by acoustic shadowing. Many heart structures, however, can be well visualized beyond 22 weeks of gestation, especially if the fetus is not prone. Cardiac structures are often well visualized when the left ventricular apex is directed anteriorly toward the maternal abdominal wall (Figures 1B, 1C, 1D). Chamber disproportion can sometimes be better appreciated when the interventricular and interatrial septae are perpendicular to the ultrasound beam (Figures 1A, 1E). Higher frequency transducers will improve the likelihood of detecting subtle defects but at the expense of reduced penetration. General Considerations Normal cardiac rate and regular rhythm should be confirmed. Heart rate typically ranges from 120 to 160 beats per minute. Bradycardia (< 120 beats per minute) can be transiently observed in normal second-trimester fetuses. By contrast, third-trimester bradycardia may be clinically significant and warrants further investigation. Careful attention should be given to cardiac axis and position. The heart is normally deviated about 45 ± 20 (2 SD) degrees toward the left side of the fetus 8,9 (Figure 2). Abnormal axis increases the risk of a cardiac malformation, which may also be associated with a chromosomal 1998 by the American Institute of Ultrasound in Medicine J Ultrasound Med 17: , 1998

2 602 A B C D E Figure 1 Satisfactory visualization of the fetal heart depends on orientation of cardiac structures relative to the maternal abdomen. anomaly. The heart may be abnormally displaced from its usual position in the anterior left central chest. This can be caused by a diaphragmatic hernia or space-occupying lesion, such as cystic adenomatoid malformation. Some sonographic views may reveal a small hypoechoic rim around the fetal heart that can be mistaken for a pericardial effusion. An isolated finding of this type often represents a normal variation. 10,11 The basic fetal cardiac screening examination entails a critical analysis of the four-chamber view, which should not be mistaken for a simple chamber count (Table 1) This view is obtained from an axial plane across the fetal thorax (Figures 3, 4). A similiar view of an adult heart would be obtained in a much different manner by directing the probe from the cardiac apex toward the patient s right shoulder. This distinction occurs since the heart lies horizontally as a result of cephalad displacement by a prominent fetal liver. EXTENDED BASIC CARDIAC EXAMINATION IF technically feasible, optional views of the outflow tracts can be obtained as part of an extended cardiac screening examination. Evaluation of outflow tracts can increase the detection rates for major cardiac malformations above that achievable by the fourchamber view alone This exam minimally requires that normal great vessels are approximately equal in size and that they cross each other at right angles from their origins as they exit from their respective ventricular chambers. Failure to confirm these criteria in a well-visualized study should alert the examiner that a more detailed evaluation may be warranted.

3 603 L 45 LV RA LA Spine R L 45 LV RA LA P Spine R P-point Location 96.3% 3.7% Figure 2 The cardiac axis can be measured from a four-chamber view of the fetal heart. A line through the interventricular axis is extended to the posterior border of the heart to produce point P, the location of which can be used to define fetal cardiac position. Adapted with permission. 8 The outflow tracts are typically obtained by angling the transducer toward the fetal head from a four-chamber view when the interventricular septum is tangential to the ultrasound beam (Figures 1B, 1C, 1D). A supplementary technique for evaluating the outflow tracts has also been described for the fetus when the interventricular septum is perpendicular to the ultrasound beam (Figures 1A, 1D). 19 Left Ventricular Outflow Tract The left ventricular outflow tract (LVOT) view may detect small ventricular septal defects and conotruncal abnormalities that are not seen during the basic cardiac exam alone. This view confirms the presence of a great vessel originating from the left ventricle (Figure 5). Continuity should be documented between the anterior aortic wall and ventricular septum. The aortic valve should be freely moving and not thickened. When the LVOT is truly the aorta, it may even be possible to trace the vessel into its arch from which three arteries originate into the neck. However, identification of these aortic arch vessels should not be considered as a routine part of the extended basic cardiac examination. TABLE 1 BASIC FETAL CARDIAC SCREENING EXAMINATION General Atria Ventricles AV Valves Four cardiac chambers are present Majority of heart located in left chest Heart occupies about one-third of the thoracic area Normal cardiac situs, axis, and position Pericardial effusion not seen Atria appear approximately equal in size Foramen ovale flap lies in the left atrium Lower rim of atrial septum (septum primum) is present Ventricles approximately equal in size Foramen ovale flap lies in the left atrium Ventricular septum appears intact from apex to crux Both atrioventricular valves open and move freely Tricuspid valve septal insertion more apical than mitral valve

4 604 Figure 3 The four-chamber view of the heart is obtained from an axial scanning plane across the fetal thorax. Corresponding views of the left (LVOT) and right (OT) ventricular outflow tracts are found by angling the transducer toward the fetal head. Right Ventricular Outflow Tract This scanning plane documents the presence of a great vessel from the morphologic right ventricle with a moderator band (Figure 6). The pulmonary artery is always slightly larger than the aortic root during fetal life. The right ventricular outflow tract (OT) normally arises from the right ventricle and courses toward the left of the more posteriorly positioned ascending aorta. It crosses the ascending aorta at about a 90-degree angle just above its origin. The pulmonary outflow valve should be freely moving and not thickened. A pulmonary artery bifurcation cannot always be seen due to fetal position. The OT can be confirmed as a pulmonary artery only if its distal end appears bifurcated. It divides toward the left side into the ductus arteriosus, which is contiguous with the descending aorta. The right side branches into the right pulmonary artery. Additional studies are required to demonstrate the clinical utility of the OT view when routinely applied to a low-risk fetal population. Figure 4 Key components of a normal four-chamber view include an intact interventricular septum and atrial septum primum. There is no disproportion between the left (LV) and right () ventricles. A moderator band helps to identify the morphologic right ventricle. Note how the offset atrioventricular septal valve leaflets insert into the crux. Ventricular Septum Moderator Band LV Crux Pulmonary Veins Atrial Septum

5 605 LV Aorta Figure 5 A left ventricular outflow tract (LVOT) view emphasizes that a great vessel can be seen exiting the left vessel. The aortic valve leaflets should be freely moving and not thickened. FETAL ECHOCARDIOGRAM Afetal echocardiogram should be performed if recognized risk factors raise the likelihood of congenital heart disease beyond what would be expected for a low-risk screening population. 20 Unfortunately, a high proportion of prenatally detectable cases of congenital heart disease occur in patients without any risk factors or extracardiac anomalies. 21 Specific details of this specialized procedure are not within the scope of this document. Clinicians, however, should be familiar with some of the reasons why patients could be referred for this comprehensive evaluation (Table 2). This detailed examination should be performed by those familiar with the prenatal diagnosis of congenital heart disease. In addition to information provided by the basic screening exam, a detailed analysis of cardiac structure and function may further characterize visceroatrial situs, systemic and pulmonary venous connections, foramen ovale mechanism, atrioventricular connections, ventriculoarterial connections, great vessel relationships, and sagittal views of the aortic and ductal arches. Figure 6 Cardiac position and axis are identical to Figure 5. A right ventricular outflow tract (OT) view emphasizes that a great vessel can be seen exiting the morphologic right ventricle (). Note that the OT exits the ventricle at about 90 degrees to the aortic outflow tract. The pulmonary valve leaflets should be freely moving and not thickened. Pulmonary Artery Aorta Pulmonary Artery Bifurcation

6 606 Additional sonographic techniques can be used to study the heart. For example, Doppler ultrasonography can measure blood flow velocity or identify abnormal flow patterns across valves and within heart chambers. M-mode echocardiography also offers an important method for analyzing cardiac dysrhythmias, suspected ventricular dysfunction, and abnormal wall thickness. TABLE 2 COMMON INDICATIONS FOR FETAL ECHOCARDIOGRAPHY First degree relative of proband has congenital heart disease (mother, father, sibling) First degree relative of proband has a genetic disorder associated with cardiac disease Maternal diabetes exists prior to pregnancy Maternal diabetes diagnosis is made during early pregnancy Cardiac teratogen exposure (e.g., alcohol, lithium, valproic acid, vitamin A) Major structural fetal anomaly Fetal hydrops ACKNOWLEDGMENTS This technical bulletin was developed under the auspices of the Education Committee of the American Institute of Ultrasound in Medicine. The Education Committee wishes to express special recognition to Wesley Lee, MD, from William Beaumont Hospital in Royal Oak, Michigan, who was instrumental in the development of this document. Appreciation is also extended to other individuals who provided additional input: Lindsey Allan, MD Richard Barr, MD, PhD Beryl Benacerraf, MD Patricia Cantu, RT(R), RDMS, T, RDCS Harris Cohen, MD Christine Comstock, MD Joshua Copel, MD Greggory DeVore, MD Peter Doubilet, MD Julia Drose, BA, RT, RDMS, T Sturla Eik-Nes, MD Harris Finberg, MD Carol Ann Gorman, RNC, RDMS Gerald Holzman, MD James Huhta, MD Charles Kleinman, MD Janet Kirk, MD Michael Manco-Johnson, MD Jon Meilstrup, MD Christopher Merritt, MD Richard Meyer, MD Erik Paulson, MD Kathryn Reed, MD Norman Silverman, MD Ramada Smith, MD Mark Wax, MD Wesley Lee, MD Chair, Education Committee American Institute of Ultrasound in Medicine

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