Clinical Applications Of Cardiac Ultrasound

4.1. Transvaginal and Transabdominal Fetal Echocardiography

The human fetal heart is fully developed and functional by 11 wk after conception. Using transvaginal ultrasound, the structure and functional characteristics of the fetal heart can be observed as early as 9 wk of gestational age (8). This technique remains the most useful type of fetal cardiac imaging until approx 16 wk of gestation. At that time, transabdominal imaging becomes the preferred method (Fig. 1). Fetal imaging is routinely performed at 16-20 wk of gestational age, and image quality improves until about 24-28 wk of gestational age (8). The quality of fetal images can be reduced by loss of amniotic fluid, maternal body habitus, fetal bone density, or the fetal position.

M-mode, 2D, and Doppler ultrasound techniques are useful for an analysis of the anatomy and function of the fetal heart, as well as for the diagnoses and monitoring of fetal arrhythmias. In general, fetal echocardiography has contributed to: (1) improved understanding of the natural history of many forms of congenital heart disease; (2) improved monitoring and obstetric care of fetuses with structural heart diseases and arrhythmias; and (3) attempts at in utero correction of valvar abnormalities (9,10).

4.2. Transesophageal Echocardiography

Transesophageal echocardiography allows imaging of the heart from the esophagus or stomach, which improves image resolution by eliminating much of the acoustic interference from the lungs and chest wall and at the same time allowing for a reduced distance of the ultrasound source from the heart. Transesophageal imaging is performed using either a biplane probe (two single-plane arrays set at perpendicular planes) or a rotating single-array probe that provides multiple planes of view (an omniplane probe).

Fig. 1. Fetal echocardiogram at approx 24 wk of gestation. Four-chamber views are shown with pulse wave Doppler analysis of mitral inflow and aortic outflow. Mitral inflow is characterized by two waves, an E wave representing passive filling of the left ventricle in diastole and an A wave representing active filling of the ventricle with atrial systole. Doppler flows are less than 1 m/s, indicating unobstructed blood flow, and the interval between aortic outflow signals is approx 0.5 s, indicating a fetal heart rate of 120 beats/min.

Fig. 1. Fetal echocardiogram at approx 24 wk of gestation. Four-chamber views are shown with pulse wave Doppler analysis of mitral inflow and aortic outflow. Mitral inflow is characterized by two waves, an E wave representing passive filling of the left ventricle in diastole and an A wave representing active filling of the ventricle with atrial systole. Doppler flows are less than 1 m/s, indicating unobstructed blood flow, and the interval between aortic outflow signals is approx 0.5 s, indicating a fetal heart rate of 120 beats/min.

requires sedation or anesthesia and thus adequate patient monitoring . It is significantly more invasive than standard transthoracic echocardiography and can be complicated by airway compromise or dysphagia (3,12,13).

4.3. Transthoracic Echocardiography

Transthoracic echocardiography is the most common method for cardiac imaging. It is noninvasive, can be performed in any cooperative patient, and only rarely requires sedation or anesthesia. Yet, images obtained are limited by patient size and can be complicated by interference from soft tissues, bone, or lung. The transthoracic echocardiogram is performed from standard windows on the chest (Fig. 2) and requires the use of multiple transducers at varied ultrasound frequencies to maximize the 2D image resolution and Doppler ultrasound information obtained. Most commonly, images are obtained by trained and licensed cardiac sonographers and then interpreted by a cardiologist.

4.4. Standard Transthoracic Examination

The standard transthoracic cardiac echo includes images from parasternal, apical, suprasternal notch, and subcostal imaging windows (Fig. 2). Two-dimensional sectors are imaged in each window to provide anatomical details and functional analyses. The highest frequency transducer set at the lowest depth possible is used to maximize image resolution while scanning for anatomical detail. Two-dimensional images are then used to guide Doppler ultrasound interrogations, often with a lower frequency transducer that will optimize Doppler information. Two-dimensional images are also used to guide M-mode measurements of chamber sizes and functions, and Doppler gradients are calculated across valves and shunts to maximize the hemodynamic information obtained.

Subcostal

Fig. 2. Diagram of the chest showing transducer position for standard transthoracic echocardiographic windows. A typical examination in a cooperative patient is performed in a standard order: parasternal, apical, subcostal, and then suprasternal notch views. Perpendicular imaging planes can be obtained from each position by rotating the transducer 90°.

Subcostal

Fig. 2. Diagram of the chest showing transducer position for standard transthoracic echocardiographic windows. A typical examination in a cooperative patient is performed in a standard order: parasternal, apical, subcostal, and then suprasternal notch views. Perpendicular imaging planes can be obtained from each position by rotating the transducer 90°.

Today, transesophageal probes come in sizes appropriate for use in adults, children, and infants. Transesophageal echocardiography is used when improved resolution is required or when transthoracic windows are unavailable, as is typical in the operating room or cardiac catheterization laboratory (3). It also has become a routine form of intraoperative monitoring for open heart surgery and is useful to detect incomplete repairs prior to separation from cardiopulmonary bypass (11). Further, it is a useful adjunct to interventional cardiac catheterization procedures. Transesophageal echocardiography typically

Fig. 3. Transthoracic parasternal long-axis views in a newborn infant. These views demonstrate the long axis of the left side of the heart. Frame 1 is in diastole with an open mitral valve to accommodate left ventricular filling and a closed aortic valve (arrow). Frame 2 is in systole with an open aortic valve (arrow) and closed mitral valve (*). White dots on the sector border represent centimeter marks. Ao, aorta; LA, left atrium; LV, left ventricle; MV, = mitral valve; RV, right ventricle.

Fig. 3. Transthoracic parasternal long-axis views in a newborn infant. These views demonstrate the long axis of the left side of the heart. Frame 1 is in diastole with an open mitral valve to accommodate left ventricular filling and a closed aortic valve (arrow). Frame 2 is in systole with an open aortic valve (arrow) and closed mitral valve (*). White dots on the sector border represent centimeter marks. Ao, aorta; LA, left atrium; LV, left ventricle; MV, = mitral valve; RV, right ventricle.

The standardized transthoracic echocardiograms are obtained by scanning at four regions on the chest wall: the parasternal window, the apical window, the subcostal region, and the suprasternal notch (Fig. 2). Parasternal "long-axis" views are used to obtain long-axis images of the left side of the heart, including the left atrium, left ventricle, and aorta (Fig. 3). A subtle tilt of the transducer inferiorly from this position gives views of the right atrium, tricuspid valve, and right ventricle, and tilting leftward brings the pulmonary valve and main pulmonary artery into view.

Turning the transducer and scan plane by 90° results in short-axis views of the heart in planes from the base of the heart (region of the aorta and tricuspid and pulmonary valves) to the apex (Fig. 4A-D). M-mode measurements of the left-sided chambers are obtained from parasternal short-axis windows and can be used to assess chamber sizes and functions (Fig. 4E,F). Apical windows reveal standard four-chamber views of the left atrium, mitral valve, left ventricle, right atrium, tricuspid valve, and right ventricle (Fig. 5A,B). This view sends the ultrasound beam parallel to the septal structures, so it is not adequate to assess the integrity of the atrial or ventricular septa. Tilting the transducer anteriorly results in a five-chamber view that allows excellent visualization of the left ventricular outflow tract and aorta. Doppler gradients across the mitral, tricuspid, and aortic valves can be obtained from this view (Fig. 5C), and the velocity of tricuspid valve regurgitation can be used to estimate right ventricular and pulmonary artery systolic pressures.

Subcostal views are particularly useful in patients with lung disease or in those who have had recent open heart surgery. From subcostal images, the orientation of the heart in the chest and the major vascular connections can be established. Subcostal views also provide excellent visualization of the intraatrial septum (Fig. 6A) and four-chamber views in patients with poor apical windows. Suprasternal notch views are most useful for visualization of the aortic arch, its branching vessels, and the descending thoracic aorta (Fig. 6C), as well as for determining the Doppler shifts across the aortic valve. This view is also important to exclude vascular abnormalities, including coarcta-tion of the aorta.

Essentials of Human Physiology

Essentials of Human Physiology

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