3 Echocardiography
3.1 Transthoracic Echocardiography
3.1.1 Basics
Two-dimensional (2D) echocardiography is the most important noninvasive imaging procedure in pediatric cardiology. It provides cross-sectional images of the heart and great vessels. The examination is performed using standard cross-sectional planes.
In routine diagnostics, 2D echocardiography is supplemented by examinations using M-mode, Doppler, and color Doppler imaging techniques. In some cases, three-dimensional (3D) echocardiography and tissue Doppler examinations can provide additional information.
M-mode Echocardiography
M-mode echocardiography is a one-dimensional technique (Fig. 3.1). M-mode is used to observe a small segment of the heart over a certain period. The advantage of this method is its good temporal resolution. The method is used in particular for time-dependent visualization of moving structures (e.g., cardiac valves) and for measuring size (e.g., systolic and diastolic diameter of the ventricles).
Doppler Echocardiography
Doppler echocardiography utilizes the Doppler effect. Moving an ultrasound transducer closer to or further from the receiver results in a frequency shift of the ultrasound waves. The Doppler effect can be used to measure the direction and velocity of the acoustic source. An example from everyday life is that a pedestrian can distinguish whether an ambulance with a siren is approaching (increasing pitch of the siren) or moving away (decreasing pitch). The pedestrian can also estimate whether the ambulance is approaching or moving away rapidly or slowly (extent of the increase or decrease in pitch).
Doppler echocardiography utilizes the frequency shift that occurs when ultrasound waves are reflected off moving blood cells, allowing the velocity and direction of blood flow to be determined (Table 3.1 and Table 3.2). However, measurement is reliable only if the flow direction is nearly parallel to the ultrasound beam, in other words moving directly toward or away from the ultrasound beam. The procedure therefore depends on the angle of the beam.
Anatomical structure | Measurements |
Tricuspid valve | E wave and A wave velocity, mean pressure gradient, IVRT, velocity of the regurgitation flow if there is tricuspid valve insufficiency |
Right ventricular outflow tract | Peak gradient, mean pressure gradient |
Pulmonary valve | Peak gradient, mean pressure gradient, velocity of the regurgitation flow if there is pulmonary valve insufficiency |
Pulmonary artery branches | Peak gradient, mean pressure gradient |
Mitral valve | E wave and A wave velocity, mean pressure gradient, IVRT, pressure half time (PHT) if there is a mitral valve stenosis |
Left ventricular outflow tract | Peak gradient, mean pressure gradient |
Aortic valve | Peak gradient, mean pressure gradient |
Aortic arch, aortic isthmus | Peak gradient, mean pressure gradient |
IVRT, isovolumetric relaxation time | |
Structure | Mean (m/s) | Normal range (m/s) |
Aorta | 1.5 | 1.2–1.8 |
Mitral valve inflow | 1.0 | 0.8–1.3 |
Tricuspid valve inflow | 0.6 | 0.5–0.8 |
Pulmonary artery | 0.9 | 0.7–1.1 |
The most common procedures in echocardiography are pulsed wave (PW) Doppler, continuous wave (CW) Doppler, and color Doppler.
Pulsed wave Doppler
In this method, the transducer emits and receives ultrasound waves in rapid succession. The next pulse is not sent until the previous one is received. This procedure enables flow velocities to be measured in a certain sample volume. In practice, this means that the region along the ultrasound beam where flow velocity is to be measured can be set. For example, when the ultrasound beam is directed across the right ventricular outflow tract, the pulmonary valve, and the main pulmonary artery, one can choose whether to measure blood flow velocity in the infundibulum, across the pulmonary valve, or in the main pulmonary artery.
However, the disadvantage of this method is that it cannot be used to measure high flow velocities. Blood flow moving toward the transducer is normally displayed above the zero line and blood flow moving away from the transducer is displayed below the zero line. However, if a certain maximum velocity is exceeded (Nyquist limit), aliasing occurs, meaning that the curve generated by the blood flow on the monitor is cut off and displayed on the other side of the zero line. Neither flow direction nor velocity can then be reliably assessed.
Continuous wave Doppler
In this Doppler method, continuous ultrasound waves are emitted and received. The disadvantage is that it is impossible to distinguish at which location of the beam certain Doppler signals are generated. It is thus not possible to determine precisely where along the ultrasound beam the flow velocities occur. In the above example, the maximum flow velocity in the right ventricular outflow tract through the pulmonary valve and in the pulmonary artery can be determined, but not the precise location. The advantage is that very high flow velocities can be measured using CW Doppler. Aliasing practically never occurs.
Color Doppler
Blood flow in the heart and great vessels can be displayed well using color Doppler. Blood flow directed toward the transducer is generally displayed in red; blood flow in the opposite direction appears blue. Turbulent blood flow is green or a mosaic of colors. Since the color Doppler is based on a PW Doppler technique, color reversal (aliasing) occurs when a certain velocity is exceeded.
3D echocardiography
Modern equipment makes it possible to produce a three-dimensional image. These techniques have not yet become common in routine diagnostics. They are sometimes used to obtain a detailed image of cardiac valves or to determine the volume of the heart chambers.
Tissue Doppler
Tissue Doppler imaging is a method for assessing tissue movement, which is relatively slow compared with blood flow. Special velocity and amplitude filters are required for this. Tissue movements primarily involve myocardial motion. In clinical practice, tissue Doppler imaging is most frequently used to evaluate diastolic function (see Chapter 3.1.2).
Regional myocardial deformation can be quantified using “strain” and “strain rate.” The significance of these relatively new parameters is currently being investigated in many clinical trials.
3.1.2 Standard Examination and Standard Planes
The various standard planes are presented below. In addition to 2D echocardiography, the relevant Doppler and M-mode measurements that can be made in the respective planes are explained.
Overview of the standard planes (Table 3.3):
Anatomical structure | Measurement timepoint | Suitable standard views |
Tricuspid valve annulus | Diastole | Apical four-chamber view |
Mitral valve annulus | Diastole | Apical four-chamber view, left parasternal long axis |
Diameter of the left atrium | Diastole | Left parasternal long axis |
Pulmonary valve annulus | Systole | Left parasternal long axis, tilted left parasternal long axis |
Main pulmonary artery | Systole | Left parasternal short axis, tilted left parasternal short axis |
Pulmonary artery branches | Systole | Left parasternal short axis, tilted left parasternal long axis |
Aortic valve annulus | Systole | Left parasternal long axis |
Aortic root | Systole | Left parasternal long axis |
Ascending aorta | Systole | Left parasternal long axis |
Transverse aortic arch | Systole | Suprasternal view (long axis) |
Aortic isthmus | Systole | Suprasternal view (long axis) |
Apical planes:
Four-chamber view
Five-chamber view
Two-chamber or three-chamber view
Parasternal planes:
Left parasternal longitudinal views (long axis)
Left parasternal transverse views (short axis)
Subcostal planes
Subcostal longitudinal views
Subcostal transverse views
Suprasternal planes:
Suprasternal longitudinal view
Suprasternal transverse view
Abdominal planes
The transducer positions for the various planes are depicted in Fig. 3.2.
The examination is generally performed in the supine position on neonates and infants and is best performed in the left lateral position on older children to prevent artifacts from air in the lungs.
Apical Planes
For apical planes, the transducer is placed near the cardiac apex. The transducer marking points to the patient’s left side, so the patient’s right side is displayed on the left of the monitor.
Apical Four-Chamber View
2D echocardiography
The apical four-chamber view should be used first as it enables an initial assessment of the size and function of ventricles and atria to be made. Moreover, a muscular ventricular septal defect (VSD) can be visualized very well in this plane.
The apical four-chamber view is set so that all four cardiac chambers can be seen. It is necessary to invert the image on the monitor. In this “anatomically correct” display (surgical view), the atria are displayed at the top of the monitor and the ventricles at the bottom. The ventricular septum should be as vertical as possible in the apical four-chamber view. The right ventricle can be readily recognized by the tricuspid valve that is somewhat closer to the apex than the mitral valve. In addition, there is a thick muscular band (moderator band) at the right ventricular apex that cannot be detected in the left ventricle.
Doppler
The apical four-chamber view displays the diastolic inflow through the mitral and tricuspid valves into the left and right ventricles using Doppler and color Doppler techniques.
In the PW Doppler profile, the typical M-shaped configuration of diastolic inflow is seen. The two peaks of the “M” are designated E wave and A wave. The E wave represents “early” diastolic filling and the A wave represents “atrial” contraction flow. AV valve stenosis can be quantified by determining the mean pressure gradient through the valve. This is done by tracing the contour of the M-shaped Doppler signal. Modern echocardiography equipment then calculates the mean pressure gradient using the area under the curve.
AV valve insufficiency can be seen in the color Doppler. In these cases, regurgitation through the affected valve into the atrium is seen during diastole. In the color Doppler, the regurgitation jet is blue. Using the Bernoulli equation, the maximum velocity of the regurgitation jet is used to estimate the gradient across the valve and thus the pressure in the affected chamber. If the maximum velocity of regurgitation is 3 m/s, for example, the pressure difference across the valve according to the Bernoulli equation (4 × V2) is 36 mmHg. If it is then assumed that the pressure in the right atrium, which is equivalent to the central venous pressure (CVP), is approximately 4 mmHg, the right ventricular pressure must be around 40 mmHg.
Apical Five-Chamber View
2D echocardiography
Starting with the apical four-chamber view, if the acoustic beam is aimed somewhat more parallel to the sternum, the five-chamber view can be seen. In this view, in addition to the four chambers, the left ventricular outflow tract, the aortic valve, and the ascending aorta as “fifth chamber” can be seen.
In this plane, any obstruction of the function of the aortic valve and the left ventricular outflow tract can be assessed. Ectasia of the ascending aorta can also be visualized well in this plane.
Doppler
Since the flow in the left ventricular outflow tract and across the aortic valve is almost parallel to the acoustic beam in this plane, it is particularly suitable for quantifying aortic valve stenosis and obstructions of the left ventricular outflow tract. In Doppler ultrasonography, aortic valve insufficiency is seen as regurgitation during the diastole.
Apical Four-Chamber View (posterior angulation)
2D echocardiography
Starting from the four-chamber view, tilting the transducer further backward (steeper) brings an additional structure into view, the coronary sinus (Fig. 3.3), which receives venous blood from the coronary vessels, proceeds along the lower wall of the left atrium, and empties into the right atrium. The left atrium can be clearly distinguished from the right atrium by the coronary sinus. A dilated coronary sinus is a sign of a left persistent superior vena cava, which frequently empties into the coronary sinus.
Apical Two-Chamber or Three-Chamber View
2D echocardiography
Rotating the transducer 90° clockwise from the four-chamber view position yields the two-chamber view (Fig. 3.4). This view shows the left ventricle, the left atrium, and the aorta. If the aortic valve and the ascending aorta are also included, it is called the apical three-chamber view.
Doppler
In this position, color Doppler ultrasound can be used to visualize mitral and aortic valve insufficiency. Flow velocity over the mitral valve, the left ventricular outflow tract, and the aortic valve can also be determined.
Left Parasternal Planes
Left Parasternal Long Axis
2D echocardiography
The left parasternal long axis runs parallel to the ventricular septum. The marking on the transducer points approximately to the patient’s right shoulder. The right ventricle is located immediately below the transducer, followed by the interventricular septum, the left ventricle, and the left ventricular posterior wall. In this plane, the junction of the interventricular septum and the anterior wall of the aorta and ascending aorta can also be seen. The aortic valve is readily visible and its function can be evaluated.
The left atrium is located behind the ascending aorta. The anterior leaflet of the mitral valve is contiguous with the aorta with no myocardial tissue separating them. This is called “aortomitral continuity,” a characteristic feature of the left ventricle. The anatomy and function of the mitral valve can also be readily evaluated in this position.
In addition, any movement of the septum or regional dyskinesia should be noted. Dilatation in the ascending aorta that can occur with aortic ectasia (e.g., associated with Marfan syndrome) must be excluded. Malalignment VSD with an overriding vessel (e.g., in tetralogy of Fallot or truncus arteriosus communis) can be readily visualized in the 2D view of this plane.
Doppler
Insufficiency of the aortic and mitral valves can be visualized using color Doppler. In this setting, perimembranous and sometimes muscular VSD can be visualized using color Doppler.
M-mode
The left parasternal long axis is the standard plane for many M-mode measurements. The interventricular septum and the left ventricular posterior wall appear nearly perpendicular in this plane. Positioning the beam of the M-mode through both ventricles allows the thickness of the interventricular septum, the size of the left ventricle, and the dimensions of the left ventricular posterior wall in systole and diastole to be reliably determined (see Fig. 3.1). Fractional shortening can be calculated as an indication of left ventricular function (Chapter 3.1.3).
By positioning the beam of the M-mode to include the distal ends of the mitral leaflets, the M-shaped motion of the anterior mitral leaflet and the mirror-image motion of the posterior leaflet can be seen. If there is an obstruction in the left ventricular outflow tract, the anterior mitral leaflet is “drawn into” the left ventricular outflow tract during systole. In the M-mode, this phenomenon can be seen as “systolic anterior movement” (SAM phenomenon). In a mitral valve prolapse, “sagging” of one or both mitral leaflets is observed during systole.
When the beam of the M-mode is positioned further in the cranial direction, the ascending aorta, aortic valve, and left atrium can be visualized together. The ratio of the size of the ascending aorta and the left atrium is normally about 1:1. If there is relevant patent ductus arteriosus (PDA), the size of the atrium increases disproportionately and the ratio of ascending aorta to the atrium increases to 1:1.3 to 1.5 or more. The measurements are made in the end systole when the left atrium has reached its maximum size.
The aortic valve opening can also be visualized in the left parasternal long axis. In M-mode, the opened aortic valve looks like a parallelogram. When the valve is closed, the mid-echo normally runs midway between the anterior and posterior wall of the aorta. If the aortic valve is bicuspid, the mid-echo is eccentric.
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