Echocardiographic Evaluation of Congenital Heart Disease
Meryl S. Cohen
Jack Rychik
Methodologies using ultrasonic imaging have been developed over the last three decades that allow for accurate and timely assessment of the form and the function of the heart. Echocardiography is noninvasive and portable; thus, it has become the imaging modality of choice in the initial diagnostic evaluation of the patient suspected of having congenital or acquired heart disease. Structural abnormalities as well as alterations in flow and hemodynamics may be quickly and easily identified with minimal disturbance to the patient. Unlike angiography, in which images of opacified blood are radiographically exhibited and in which structural form is assumed from the visualization of nonopacified regions, echocardiography allows for direct, real-time imaging of cardiac structures based on their ultrasonic reflective properties. Multiple advantages are inherent in this diagnostic tool. Echocardiography is safe, portable, and radiation free. Views and sweeps of the heart from different angles and positions may be performed freely to conceptually reconstruct a three-dimensional (3D) image. Presently, 3D echocardiography can also be performed to better define valve abnormalities as well as define all of the rims of atrial septal defect (ASD) and ventricular septal defect (VSD). Serial assessments can be performed at different points in time or used continuously as a monitoring tool during procedures or interventions. Familiarity with the principles used to generate the echocardiographic image and its limitations will aid in the appropriate interpretation and clinical application of ultrasound in the surgical management of infants and children with congenital heart disease.
PRINCIPLES OF ULTRASOUND PHYSICS AND APPLICATIONS
Ultrasonic Frequencies
Ultrasonic energy is generated by the delivery of electrical impulses to piezoelectric crystals, which resonate at a set frequency. The range of ultrasound frequencies used for conventional cardiac imaging is 2.0 to 10 MHz. The choice of a particular frequency for imaging is based on its tissue penetration and resolution characteristics. High-frequency ultrasound is dissipated quickly in tissue and can be propagated only for short distances but allows for greater resolution of structure, whereas low-frequency ultrasound penetrates greater distances before attenuation but resolution is sacrificed. This concept is illustrated by the formula: wavelength = velocity/frequency, where the velocity of ultrasound in biologic tissue is constant at 1,540 m/sec. To ultrasonically resolve two points in space, they must be the distance of at least one wavelength from each other; hence, the higher the frequency, the smaller is the wavelength and the greater is the ability to resolve points that are in close proximity. For example, using 2.0-MHz ultrasound, one can resolve two points that are a minimum of 0.78-mm apart. If the objects are any closer, they will not be resolved and will appear as one. At 7.5 MHz, two points can be resolved at a minimum of 0.21-mm apart, hence resolution is greater. In practice, high-frequency transducers are chosen for use in newborns and small children to maximize resolution, whereas in older children and adolescents lower frequencies are used to maximize penetration. Higher resolution can also be used in transesophageal probes because penetration is not a major obstacle.
The Doppler Principle
In 1842, Christian Johann Doppler described the change in frequency of energy emission of an object in motion in relation to its velocity of motion toward or away from a stationary observer as follows:
Fd = 2VFo(cos Y)/c
where Fd is the frequency shift, Fo is the emitting frequency, Y is the angle of incidence between the direction of motion of the object and the emitted frequency, V is the velocity of motion of the object, and c is the velocity of the energy in the medium (a constant). In the setting of reflected ultrasound, this principle can be used to assay for the velocity of blood flow moving through the chambers of the heart. Rearranging the equation, we obtain
V = Fd Fo(cos Y)c/2
Hence, the velocity of blood flow can be derived from the Doppler frequency shift of reflected ultrasound from moving blood if the emitting frequency and the angle of incidence between the direction of motion of blood and the interrogating ultrasound beam are known. In the clinical setting, it is cumbersome to measure the angle of incidence between flow direction and the ultrasound beam. Every effort is therefore made to align the ultrasound beam parallel to the direction of blood flow; angle Y is thereby assumed to be 0 (cosine of Y = 1). Doppler velocity assessments based on this assumption continue to be valid for angles of incidence of ≤20 degrees because the cosine of 20 remains close to unity; however, at angles >20 degrees this assumption is not valid, and Doppler-derived velocities may be underestimated.
Information relating to velocity and direction of blood flow can be obtained by several methods including pulsed-wave, continuous-wave, or color Doppler techniques. Pulsed-wave and continuous-wave Doppler quantify the blood flow velocity. In the pulsed-wave Doppler technique, ultrasound crystals fire pulses of energy and then stop to “listen” for reflected sound. This technique permits the spatial determination of velocity by allowing for interrogation of flow within a selected region of interest. Distance is calculated from the time it takes for reflected ultrasound to return to the transducer during the listening phase. Pulsed-wave Doppler is limited by its inability to assess peak velocities when there are significant disturbances of flow and velocities are high. Once a region of disturbed flow is identified, continuous-wave
Doppler may be applied to determine the peak velocity in the region. Continuous-wave Doppler emits and listens simultaneously. Thus, continuous-wave Doppler will assess all velocities within a line of interrogation—velocities within as well as proximal to and distal to the site of interest. Combining pulsed-wave and continuous-wave Doppler when assessing disturbed flow patterns is ideal to give the location of the turbulence and the maximum velocity.
Doppler may be applied to determine the peak velocity in the region. Continuous-wave Doppler emits and listens simultaneously. Thus, continuous-wave Doppler will assess all velocities within a line of interrogation—velocities within as well as proximal to and distal to the site of interest. Combining pulsed-wave and continuous-wave Doppler when assessing disturbed flow patterns is ideal to give the location of the turbulence and the maximum velocity.
Color Doppler is a pulsed-wave method in which flow within a region is assigned a color based on velocity and direction and is displayed as an overlay onto the two-dimensional image. By convention, flow toward the transducer is designated as varying shades of red, whereas flow away from the transducer is blue. Color flow is a pictorial indicator of blood flow direction and velocity but does not quantify the maximum velocity of a flow jet.
Modified Bernoulli Theorem
Based on the concept of exchange of potential energy into kinetic energy, the velocity of flow between cardiac structures can be used to calculate the pressure difference, thereby providing hemodynamic information. Bernoulli showed that the difference in potential energy, or pressure, between two sites is equal to the kinetic energy loss in addition to the energy losses caused by inertial and frictional forces. If the loss of energy because of inertia or friction is assumed to be minimal, as can be done when assessing flow across a discrete, short segmental narrowing (such as a valve or VSD), then these contributing variables may be ignored and the modified Bernoulli formula can be applied to calculate the pressure difference between upstream point 1 (proximal point) and downstream point 2 (distal point):
P1−P2 = 4(V22−V22)
If the proximal blood flow velocity is ≤1.0 m/sec, as is the case within most of the structures of the normal heart, the formula can be further simplified to
Pressure difference = 4V22.
Hence, the pressure difference across an area of discrete stenosis can be calculated based on the peak velocity across the region of narrowing obtained by Doppler echocardiography (Fig. 72.1A, and 72.1B). Systolic pressure gradients derived from peak velocity data reflect the peak instantaneous pressure gradient, which usually occurs during the upstroke of systole and not at peak systole. This is why Doppler-derived gradients may be higher than cardiac catheterization gradients in which peak-to-peak pressure gradients are measured. Lesions in which the time for reaching peak pressure is delayed in one chamber relative to the other will add to a further exaggeration of differences between the catheterderived and Doppler-derived gradients. For example, in aortic stenosis, peak ascending aorta pressure is reached much later than peak left ventricular pressure. Hence, whereas the peak-to-peak (catheter) gradient may be 40 to 50 mmHg when comparing the two time-delayed peaks, the peak instantaneous (Doppler echocardiography) gradient, which will likely occur during early systole when aortic pressure is low and left ventricular pressure is rapidly on the rise, may be up to 30 to 40 mmHg higher, providing an echo-derived gradient of 70 to 80 mmHg. The mean gradient, which can be calculated from the echocardiogram by integrating the sum of the peak instantaneous gradients within the systolic cycle, is most likely the best reflection of the afterload work imposed on the ventricle; however, clinical correlates were developed in the era before echocardiography, and the standard for grading and treatment of valvular stenosis of a congenital cause today is still based on the peak-to-peak catheter-based gradient assessment.
 Fig. 72.1. (A) The Bernoulli principle defines the relationship between kinetic energy (velocity) and potential energy (pressure drop) across a discrete stenosis such that inertial and frictional forces can be ignored. (B) Practical example of the use of the Bernoulli principle. Continuous-wave Doppler measurement across a stenotic right ventricular to pulmonary artery conduit in a patient with L-transposition of the great arteries after atrial switch operation and Rastelli procedure. The peak velocity is 4 m/sec generating a peak instantaneous gradient across the conduit of 65 mmHg. |
APPROACH TO IMAGING CHILDREN WITH UNREPAIRED CONGENITAL HEART DISEASE
Echocardiographic evaluation of a child with potential heart disease should be performed in a standardized, systematic manner with identification of all segments of the anatomy from multiple planes. In addition to the identification of all structural defects, physiologic information via the Doppler methodologies is also obtained. To facilitate the study, patients ranging from 3 months to 3 years of age should generally be sedated. A complete echocardiographic study consists of two-dimensional views and sweeps (incremental views obtained while rotating the transducer through a plane or on an axis), starting with subcostal imaging in which the liver is used as an acoustic window to the heart and ending with the suprasternal windows (Table 72.1). These standardized views and tomographic sweeps are acquired in a sequential manner during the pediatric echocardiogram, and synthesized into a comprehensive picture of cardiac anatomy, function, and blood flow (Fig. 72.2A-72.2G).
Table 72.1 Views and Sweeps for Echocardiographic Imaging of the Child with Suspected Congenital Heart Disease | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Abdomen and Situs
The echocardiographic study commences with a determination of situs. A transverse or coronal view of the abdomen just below the sternum at the level of the diaphragms displays the position of the liver, stomach, inferior vena cava, and descending aorta. In situs solitus, the following should be noted: (1) the liver and the inferior vena cava traversing throughout the body of the liver are to the right of the spine; (2) the stomach is to the left of the spine; and (3) the aorta is retroperitoneal and lies just anterior and slightly to the left of the spine (Fig. 72.3). In situs inversus, the mirror image is found, whereas in heterotaxy syndrome (asplenia or polysplenia), the liver is often in the midline, and the inferior vena cava and aorta may be juxtaposed on the same side of the spine either to the right or the left. In heterotaxy syndrome, systemic venous anatomy is clinically important as the majority of associated cardiac defects require single-ventricle palliation. In polysplenia, the inferior vena cava may be interrupted at the infrarenal level, with no inferior vena cava seen at the level of the diaphragms. In this case, a dilated azygos vein should be sought in the retroperitoneal space adjacent to the spine, which carries inferior vena caval blood to the superior vena cava (Fig. 72.4). A dilated superior vena cava helps confirm this finding. In addition to interruption of the inferior vena cava, the hepatic venous drainage into the atria should be identified. Occasionally, the hepatic veins drain to both atria particularly when there is a common atrium. The size of the hepatic veins should also be noted because inordinate dilation may be a sign of (1) right atrioventricular valve insufficiency, stenosis, or atresia or (2) infradiaphragmatic total
anomalous pulmonary venous connection (TAPVC). A left superior vena cava has no hemodynamic significance in the structurally normal heart. Typically, this vessel crosses anterior to the left pulmonary artery and enters the coronary sinus posteriorly. However, bilateral superior venae cavae are common in heterotaxy syndrome. Identification of this anomaly becomes important particularly if single-ventricle palliation is planned (Fig. 72.5). Without a bridging vein between the superior venae cavae, a bilateral bidirectional superior cavopulmonary anastomosis is necessary in single-ventricle palliation; when a large enough bridging vein is present, the left superior vena cava can be ligated rather than connected to the pulmonary artery. In heterotaxy syndrome, the left superior vena cava may empty directly into the left atrium or by way of an unroofed coronary sinus resulting in a right-to-left shunt.
anomalous pulmonary venous connection (TAPVC). A left superior vena cava has no hemodynamic significance in the structurally normal heart. Typically, this vessel crosses anterior to the left pulmonary artery and enters the coronary sinus posteriorly. However, bilateral superior venae cavae are common in heterotaxy syndrome. Identification of this anomaly becomes important particularly if single-ventricle palliation is planned (Fig. 72.5). Without a bridging vein between the superior venae cavae, a bilateral bidirectional superior cavopulmonary anastomosis is necessary in single-ventricle palliation; when a large enough bridging vein is present, the left superior vena cava can be ligated rather than connected to the pulmonary artery. In heterotaxy syndrome, the left superior vena cava may empty directly into the left atrium or by way of an unroofed coronary sinus resulting in a right-to-left shunt.
 Fig. 72.2. (A) Subcostal frontal sweep. Coronal plane with sweep from anterior (position 1) to posterior (position 3). At position 1, structures visualized include the right ventricular outflow tract and the apex of the left ventricle. At position 2, the left ventricular outflow tract is seen. At position 3, the right and left atria and atrial septum are highlighted. Ao, aorta; LA, left atrium; LV, left ventricle; MPA, main pulmonary artery; RA, right atrium; RV, right ventricle; SVC, superior vena cava. (All tomographic sweep and view figures are reproduced with permission from Lai WW et al. Guidelines and standards for performance of a pediatric echocardiogram: a report from the Task Force of the Pediatric Council of the American Society of Echocardiography. J Am Soc Echocardiogr 2006;19:1413-1430.) (B) Subcostal sagittal sweep. Sagittal plane sweep from right to left. At position 1, the entry of the superior vena cava is seen. At position 2, the right ventricle inflow and outflow is visualized. At position 3, the short axis of the left ventricle and the right ventricle cavity wrapping around anteriorly are seen, and at position 4, the apex of the left ventricle. Ao, aorta; LV, left ventricle; RA, right atrium; RPA, right pulmonary artery; RV, right ventricle; SVC, superior vena cava. (C) Apical four-chamber view demonstrates the atria, ventricles, and atrioventricular valves. Posterior angulation visualizes the coronary sinus; anterior angulation visualizes the left ventricular outflow tract and proximal aorta. Ao, aorta; CS, coronary sinus; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. (D) Parasternal long axis views with sweep from rightward/inferior (position 1), which focuses on the tricuspid valve and right ventricle inflow to leftward/superior (position 2) with visualization of the right ventricle outflow and pulmonary valve. At position 2, the mitral valve, left ventricle inflow, left ventricle outflow tract, and aortic valve in long axis are well seen. Ao, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle. (E) Parasternal short-axis sweep from superior (position 1) to inferior (position 3). Structures viewed in sequence in this sweep include the aortic valve in cross section with the identification of valve leaflets, course of the coronary arteries, conal septum, pulmonary valve, ventricular septum, mitral valve, papillary muscles, muscular septum, right ventricle cavity, and left ventricle cavity. LA, left atrium; LV, left ventricle; MV, mitral valve; PMs, papillary muscles; RA, right atrium; RV, right ventricle; TV, tricuspid valve. (F) Suprasternal frontal view. Structures well seen include the innominate vein, superior vena cava, pulmonary arteries, and pulmonary vein entry into the left atrium. Ao, aorta; Innom V, innominate vein; PVs, pulmonary veins; RPA, right pulmonary artery. (G) Suprasternal arch view, also known as the “candy cane view” as the ascending and descending aorta are displayed with the origin of cephalic vessels. Ao, aorta; Innom V, innominate vein; RPA, right pulmonary artery. |
 Fig. 72.2. (Continued) |
 Fig. 72.2. (Continued) |
 Fig. 72.3. Coronal view used to determine situs. Situs solitus or normal situs is seen with the aorta to the left of the spine and the inferior vena cava (IVC) running through the liver on the right. |
 Fig. 72.4. Coronal view in a patient with polysplenia type of heterotaxy syndrome. The stomach is left-sided, the aorta runs to the right along the spine, and there is interruption of the inferior vena cava such that the azygos runs adjacent to the aorta and inserts into the left superior vena cava (not seen). |
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