In the mid-1970s, linear- array scanners that could produce 2-D images of the beating heart were developed

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Chapter 9 / Echocardiography 139

139

From: Essential Cardiology: Principles and Practice, 2nd Ed.

Edited by: C. Rosendorff © Humana Press Inc., Totowa, NJ

9 Echocardiography

Daniel G. Blanchard, MD

and Anthony N. DeMaria, ^MD

INTRODUCTION

Echocardiography is the evaluation of cardiac structures and function utilizing images produced by ultrasound (US) energy. Echocardiography started as a crude one-dimensional technique but has evolved into one that images in two and three dimensions (2-D, 3-D) and that can be performed from the chest wall, from the esophagus, and from within vascular structures. Clinically useful M- mode recordings became available in the late 1960s and early 1970s. In the mid-1970s, linear- array scanners that could produce 2-D images of the beating heart were developed. Eventually, these evolved into the phased-array instruments currently in use. In addition to 2-D imaging, the Doppler examination has become an essential component of the complete echocardiographic evaluation. Doppler US technology blossomed in the early 1980s with the development of pulsed-wave (PW), continuous-wave (CW), and 2-D color-flow imaging. The field of cardiac US continues to grow rapidly: recent clinical additions include 3-D imaging, harmonic imaging, and contrast echocardiography.

PHYSICS AND PRINCIPLES

US is sonic energy with a frequency higher than the audible range (greater than 20,000 Hz).

US is created by a transducer that consists of electrodes and a piezoelectric crystal that deforms when exposed to an electric current. This crystal creates US energy and then generates an elec- trical signal when struck by reflected US waves. US is useful for diagnostic imaging because, like light, it can be focused into a beam that obeys the laws of reflection and refraction. A US beam travels in a straight line through a medium of homogeneous density but if the beam meets an interface of different acoustic impedance, part of the energy is reflected. This reflected energy can then be evaluated and used to construct an image of the heart (1).

Because the velocity of sound in soft tissue is relatively constant (approx 1540 m/s), the distance from the transducer to an object that reflects US can be calculated using the time a sound wave takes to make the round trip from the transducer to the reflector and back again. Sophisticated computers can examine reflections from multiple structures simultaneously and display them on a screen as 1-D images. If the US beam is then electronically swept very rapidly across a sector, a 2-D image can be generated.

Several characteristics of US are important in obtaining high-quality images. High-frequency US energy yields excellent resolution, and such beams tend to diverge less over distance than low- frequency signals. High-frequency beams, however, tend to reflect and scatter more as they pass through tissue and are thus subject to greater attenuation than low-frequency signals. Therefore, echocardiographic examinations should utilize the highest frequency that is capable of obtaining signals from the targets in the US field of interest (1).

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140 Blanchard and DeMaria

TWO-DIMENSIONAL ECHOCARDIOGRAPHY:

STANDARD EXAMINATION

A US beam can image the heart from multiple areas on the chest wall. Several years ago, M- mode imaging (which detects motion along a single beam of US) was the primary tool of clinical echocardiography. M-mode has been largely supplanted by 2-D imaging. To help standardize the 2-D examination, the American Society of Echocardiography recognizes three orthogonal imaging planes: the long-axis, the short-axis, and the four-chamber planes (Fig. 1) (2). It is important to remember that the long and short axes are those of the heart, not of the entire body. These three planes can be imaged in four basic transducer positions: parasternal, apical, subcostal, and suprasternal (Fig. 2). From these parasternal positions, the transducer angle can be modified to obtain views of the mitral valve, the base of the heart, the tricuspid valve, and the right ventricular outflow tract (Fig. 3A,B). From the apical and subcostal transducer positions, both ventricles and all cardiac valves can be examined (Fig. 3C–E). The transducer can also be placed in the suprasternal position to image the thoracic aorta and great vessels.

A complete examination utilizing these imaging planes and transducer positions visualizes the cardiac valves, chamber sizes, and ventricular function in the great majority of cases. Echocardio- graphy is an accepted method for evaluating cardiac systolic function, and assessments of ejection fraction and regional ventricular dysfunction correlate well with those made with angiographic and radionuclide methods. In occasional patients, however, examination is limited owing to US artifacts, marked obesity, severe lung disease (with lung tissue interposed between chest wall and heart), or chest wall deformities.

An additional advance that has improved imaging quality is harmonic imaging. Until recently, all US transducers transmitted and received signals at the same frequency. With harmonic imag-

Fig. 1. The three basic tomographic imaging planes used in echocardiography: long-axis, short-axis, and four- chamber. LV, left ventricle; LA, left atrium; RV, right ventricle; RA, right atrium; PA, pulmonary artery; AO, aorta. (From ref. 1, with permission.)

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ing, the transducer transmits at a given (fundamental) frequency but receives at the higher harmonic frequency (for example, transmission at a frequency of 2.5 MHz and reception at 5 MHz). This technology helps to limit artifacts and often improves visualization of regional ventricular function and cardiac anatomy (3).

DOPPLER ECHOCARDIOGRAPHY

Two-dimensional imaging provides abundant information about cardiac structure but no direct data on blood flow. This important area of cardiac imaging is addressed by Doppler echocardiography. When a sound signal strikes a moving object, the frequency of the reflected signal is altered in a way that is proportional to the velocity at which the object is moving and to its direction. The velocity of the moving object can be calculated by the Doppler equation:

v = f_d. c/2f_o (cos q),

where v is the velocity of red blood cells under examination, f_d is the Doppler frequency shift recorded, f₀the transmitted frequency, and c the velocity of sound (4). The angle q is the angle between the US beam and the direction of red blood cell flow (i.e., if the US beam is directed parallel to blood flow, the angle is 0 degrees). The importance of this angle cannot be overstated, as echocardiography computer systems assume it to be zero degrees. If the angle q is greater than 20 degrees, significant errors in velocity calculation occur (5).

Thus, the echocardiography system evaluates the change in frequency (the Doppler shift) of US reflected by red blood cells and translates this into velocity of blood flow. By convention,

Fig. 2. Visualization of the heart’s basic tomographic imaging planes by various transducer positions. The long- axis plane (A) can be imaged in the parasternal, suprasternal, and apical positions; the short-axis plane (B) in the parasternal and subcostal positions; and the four-chamber plane (C) in the apical and subcostal positions.

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Fig. 3. (Opposite page) (A) Two-dimensional image of the heart in the parasternal long-axis view. The cardiac chambers correlate with the diagram in Fig. 2A. (B) Short-axis plane through the heart at the level of the papillary muscle. (C) Two-dimensional image of the apical four-chamber plane. (D) Two-dimensional image of the apical three-chamber plane. (E) Two-dimensional image of the subcostal four-chamber plane. RA, right atrium;

RV, right ventrical; LV, left ventrical; LA, left atrium; AO, aorta. (From ref. 1, with permission.)

spectral Doppler tracings (1) plot velocity with respect to time and (2) display blood flow toward the transducer above an arbitrary “zero” line and flow away from the transducer below this line.

As an example, Fig. 4 shows a normal Doppler tracing of blood flow through the mitral valve and the typical early filling (E) and late filling from atrial contraction (A). In this example, the transducer is in the apical position.

There are three main forms of Doppler imaging: PW, CW, and color-flow Doppler. Through a technique called range gating, PW Doppler can examine flow in discrete, specific areas in the heart and vasculature. This capability is extremely useful in assessing local flow disturbances, but because of the phenomenon of “aliasing,” high velocities cannot be accurately recorded (for a more complete discussion of this phenomenon, the reader is referred to refs. 5 and 6). The normal velocity of flow through the tricuspid valve is 0.3 to 0.7 m/s and through the pulmonary artery 0.6 to 0.9 m/s. Normal flow velocity through the mitral valve is 0.6 to 1.3 m/s and 1.0 to 1.7 m/s through the LV outflow tract.

Unlike PW Doppler, CW Doppler records all blood flow velocities encountered along the Doppler US beam. Therefore, there is ambiguity of flow location, but CW Doppler is free from

“aliasing” and can successfully record very high flow velocities. Color-flow imaging, a major advance in echocardiography, is an extension of PW Doppler. This technique assesses the velocity of flow in multiple sample volumes along multiple beam paths and then assigns a color to each velocity. This color “map” is then superimposed on the 2-D image to obtain a real-time, moving description of blood flow. By convention, flow moving toward the transducer is color coded in

Fig. 4. Normal pulsed-wave Doppler tracing from the left ventricular inflow tract displays the early rapid filling (E) and atrial contraction (A) phases of diastolic flow. The transducer is in the apical position, and the sample volume is at the mitral leaflet tips. (From ref. 1, with permission.)

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shades of red, flow moving away from the transducer in blue (Fig. 5). Very high-velocity flow is assigned a speckled or green color. Color-flow Doppler is an essential part of the complete echocardiographic examination and is an excellent tool for both screening and semiquantitation of valvular regurgitation and stenosis.

Recently, there has been much interest in using mitral inflow velocity patterns to evaluate left ventricular (LV) diastolic function (7). Normally, the E wave is larger than the A wave (see Fig. 4).

In cases of LV relaxation impairment, the early diastolic transmitral pressure gradient is blunted, causing a decrease in the peak E wave velocity and the rate of flow deceleration. Accompanying this, the peak A wave velocity increases (Fig. 6A). In patients with advanced diastolic dysfunction and markedly increased left atrial pressure and LV stiffness, the E/A ratio becomes abnormally high and the E wave develops a very rapid deceleration of flow velocity (i.e., a short deceleration time). This is the so-called “restrictive” filling pattern (Fig. 6B). In general, the former “relaxation” abnormality (small E, large A) represents mild diastolic dysfunction, whereas the “restrictive” pattern indicates severe diastolic dysfunction and significantly elevated left atrial pressure.

This restrictive pattern can occur in restrictive cardiomyopathy, advanced LV systolic dysfunction, pericardial disease, and severe valvular disease (e.g., severe mitral or aortic regurgitation). The restrictive pattern also has been associated with increased risk of death in patients with advanced heart failure.

Despite the utility of transmitral flow patterns in assessing diastolic properties, these should not be interpreted as pathognomonic findings of diastolic dysfunction but rather as a component of a complete clinical and echocardiographic evaluation. In this regard, pulmonary vein flow patterns and tissue Doppler imaging of the mitral annulus are also quite useful and may help detect elevated left atrial pressure when mitral inflow patterns are equivocal or (falsely) appear normal (Fig. 6C) (7). The reader is referred to ref. 7 for a complete discussion of the use of US in diastolic function.

Bernoulli and Continuity Equations

The modified Bernoulli equation states that the gradient across a discrete stenosis in the heart or vasculature can be estimated thus:

Pressure gradient = 4 ([Stenotic orifice velocity]²- [Proximal velocity]²).

Fig. 5. Apical four-chamber images with color-flow Doppler during diastole and systole. Red flow indicates movement toward the transducer (diastolic filling); blue flow indicates movement away from the transducer (systolic ejection). RA, right atrium; RV, right ventricle; LV, left ventricle. (See Color Plate 1, following p. 268.

From ref. 1, with permission.)

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Fig. 6. (A) Pulsed-wave Doppler tracing of diastolic relaxation abnormality. The transducer is in the apical position with the sample volume at the mitral leaflet tips. (B) Pulsed-wave Doppler tracing of diastolic restrictive abnormality. (From ref. 1, with permission.)

If the blood velocity proximal to the stenosis is less than 1.5 m/s, this proximal velocity term can be ignored. The resulting equation states that the pressure gradient across a discrete stenosis is four times the square of the peak velocity through the orifice. This equation can be used to calculate pressure gradients across any flow-limiting orifice (8). In addition, if valvular regurgitation is present, the Bernoulli equation can be used to calculate pressure gradients across the tricuspid and mitral valves. This is quite helpful in measuring pulmonary artery pressure, as the peak right ventricular (RV) and pulmonary artery pressures equal 4 (peak TR velocity)²plus the right atrial pressure (which can be estimated on physical examination).

The continuity equation states that the product of cross-sectional area and velocity is constant in a closed system of flow:

A₁V₁ = A₁V₂

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The most common use of the continuity equation is calculating aortic valve area, where the product of the cross-sectional area and flow velocity of the LV outflow tract (LVOT) equals the product of the cross-sectional area and velocity of the aortic valve orifice (9). LVOT area is defined as p(d/2)². This area is multiplied by the LVOT peak systolic velocity (measured by PW Doppler) and then divided by the peak velocity through the stenotic orifice (measured by CW Doppler) to obtain the aortic valve area.

TRANSESOPHAGEAL AND HANDHELD ECHOCARDIOGRAPHY Occasionally transthoracic echocardiography (TTE) does not provide adequately detailed information regarding cardiac anatomy. This is most often true in the evaluation of posterior cardiac structures (e.g., the left atrium and mitral valve), prosthetic cardiac valves, small vegetations or thrombi, and the thoracic aorta. Transesophageal echocardiography (TEE) is well-suited for these situations, as the esophagus is, for much of its course, immediately adjacent to the left atrium and the thoracic aorta (10).

TEE images can be recorded from a variety of positions, but most authorities recommend three basic positions: (1) posterior to the base of the heart, (2) posterior to the left atrium, and (3) inferior to the heart (Fig. 7A,B). There are several specific instances in which TEE is recommended. These include assessment and evaluation of (1) cardiac anatomy when TTE is inadequate, (2) valvular vegetations and infective intracardiac abscesses (Fig. 8A), (3) prosthetic valve function, (4) cardiac embolic sources, including atrial appendage thrombi (Fig. 8B), patent foramen ovale, and interatrial septal aneurysm, and (5) aortic dissection and atherosclerosis (11).

Fig. 6. (Continued) (C) Doppler assessment of progressive diastolic dysfunction utilizing transmitral pulsed- wave Doppler, pulmonary venous Doppler, and mitral annular tissue Doppler imaging. IVRT, isovolumic relaxation time; Dec. Time, E wave deceleration time; E, early LV filling velocity; A, atrial component of LV filling; PVs, systolic pulmonary vein velocity; PVd, diastolic pulmonary vein velocity; Pva, pulmonary vein velocity resulting from atrial contraction; S_m, systolic myocardial velocity; E_m, early diastolic myocardial velocity;

A_m, myocardial velocity during LV filling produced by atrial contraction. (From ref. 7, with permission.)

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Fig. 7. (A) Standard TEE imaging planes in transverse and longitudinal axes. (B) Transverse four-chamber TEE plane; SVC, IVC, superior and inferior vena cava; LAA, left atrial appendage; RUPV, LUPV, right and left upper pulmonary vein; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. (From ref. 1, with permission.)

Recent technologic advances have led to production of small, lightweight (5–6 lb) echocardiographic units. These handheld devices are very portable, and facilitate point-of-care echo evaluation by the physician. The quality of images from these scanners, however, still does not equal that of state-of-the-art standard ultrasound instruments. In addition, current handheld scanners have marginal spectral and color Doppler capabilities. The appropriate use of these scanners is currently controversial, and recommendations will evolve over time. Several studies have shown benefits from handheld scanning in detection of cardiac and aortic pathology, while other reports have shown a relative lack of utility, especially in critically ill patients.

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This area is definitely in flux, but at this time it may be best to view handheld and limited echo examinations as extensions of the stethoscope. Performed by a competent individual, the diagnostic capability of handheld scanning is at least the equal of auscultation, and probably significantly superior (12).

CONTRAST ECHOCARDIOGRAPHY

Contrast echocardiography has grown explosively in the last few years. For many years, the main agent used for echocardiographic “contrast” injection was agitated saline, which contains numerous air microbubbles that are strong reflectors of US energy. When injected intravenously, agitated saline produces dense opacification of the right heart structures and is an excellent method

Fig. 8. (A) Short-axis TEE image through the cardiac base. A large septated abscess cavity (A) is present between the aortic root (AO) and the left atrium (LA). (B) TEE image of a thrombus in the left atrial appendage (arrow). RVOT, right ventricular outflow tract. (From ref. 1, with permission.)

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Fig. 9. Contrast microbubble injection demonstrating a shunt (arrow) from the right atrium (RA) to the left atrium (LA). RV, right ventricle; LV, left ventricle. (From ref. 1, with permission.)

for detecting intracardiac shunts. As the air microbubbles dissolve rapidly into the bloodstream, they do not pass through the pulmonary circulation. Therefore, any air microbubbles entering the left side of the heart must arrive there through a shunt (Fig. 9).

Direct injection of agitated saline into the aorta or left ventricle produces US opacification of the myocardium and LV cavity, respectively (13). LV opacification markedly enhances US images and endocardial border definition, but intraarterial contrast injection is clearly impractical for rou- tine use. Extensive research in the past few years has resulted in the creation of several echocardiographic contrast agents (including Optison^™, Definity^®, and Imagent^™) that survive transit through the pulmonary circulation and reach the left side of the heart after intravenous injection. The current generation of these agents have microbubbles filled with various perfluorocarbon gases instead of air. Because these gases are dense and much less soluble in blood than air, they can persist in the circulation, producing consistent and dense opacification of the LV cavity. These microbubbles also flow along with blood through the coronary vessels, and new technology now permits quantitation of myocardial bloodflow by measuring contrast transit characteristics through the myocardium. In addition, harmonic imaging enhances the US backscatter from contrast microbubbles (which resonate in an US field) while it decreases the signal returning from the myocardium (which does not resonate) (14). Echocontrast agents are especially useful in stress echocardiography, as the enhanced LV endocardial border definition improves detection of regional dysfunction. Recent studies have also shown that regional abnormalities in myocardial perfusion can be assessed during stress testing.

VALVULAR HEART DISEASE Aortic Valve

AORTIC STENOSIS

The thin leaflets of the aortic valve are usually well-visualized by echocardiography. Aortic valve disease is often best imaged from the parasternal views. In cases of acquired (calcific) aortic stenosis (AS), the valve leaflets are markedly thickened and calcified, and their motion severely

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Fig. 10. Parasternal long-axis view demonstrates a thickened, stenotic aortic valve (AV). AO, aorta; LV, left ventricle; LA, left atrium. (From ref. 1, with permission.)

restricted (Fig. 10). In congenital AS, systolic “doming” of the leaflets is seen, often along with congenital anomalies of the valve leaflets (e.g., bicuspid, unicuspid). Attempts at valve area planimetry by transthoracic echocardiography have generally been unsuccessful, although planimetry with TEE has yielded better results. Thus, standard 2-D imaging accurately detects AS, but not its severity.

The cornerstone of quantification is the Doppler examination. CW Doppler can record the peak velocity of blood flow through the aortic valve, which then can be used to calculate the peak instan- taneous systolic gradient with the modified Bernoulli equation. As mentioned above, the aortic valve orifice area is then calculated via the continuity equation in the following manner:

First, the area of the LVOT just proximal to the aortic valve is calculated using this equation: p r², where r is half of the diameter of the LVOT measured in the parasternal long axis view; next, the velocity of flow in the LVOT is measured using PW Doppler; finally, the area of the valve orifice is calculated by multiplying the LVOT area by the LVOT velocity and dividing the result by the peak flow velocity through the stenotic orifice (9,10). These calculations correlate quite well with catheterization-derived values and are valid as long as the LVOT flow velocity is less than 1.5 in/s.

AORTIC INSUFFICIENCY

Two-dimensional imaging may show a normal aortic valve in cases of aortic insufficiency (Al), but it can also demonstrate leaflet abnormalities, aortic root enlargement, LV dilation, and diastolic “flutter” of the anterior mitral valve leaflet. In acute severe Al, M-mode imaging can reveal early diastolic closure of the mitral valve (an uncommon but extremely important finding). Although 2-D imaging provides clues to the presence of Al, the Doppler examination is much more useful and easily detects the abnormal flow. Indeed, color-flow Doppler is a very rapid screening tool that detects Al with nearly 100% sensitivity. Quantitation of Al, however, is considerably more difficult.

There are several approaches for semiquantitation of Al by echocardiography. The first utilizes color-flow imaging. In the parasternal views, severity can be estimated by the diameter (or cross- sectional area) of the color jet in the LVOT. Mild Al generally has a jet diameter smaller than 25%

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Fig. 11. (A) Parasternal long-axis image showing a multicolored jet (indicating turbulent flow) of aortic regurgitation in the left ventricular outflow tract. The jet is narrow in width, suggesting mild regurgitation. (See Color Plate 2, following p. 268.) (B) Pulsed-wave Doppler tracing (from the suprasternal transducer position) in a case of severe aortic regurgitation. The sample volume is in the descending thoracic aorta, and holodiastolic flow reversal (arrow) is present. AO, aorta; LA, left atrium; LV, left ventricle. (From ref. 1, with permission.)

of the outflow tract diameter (Fig. 11A), whereas a severe Al color jet often occupies more than 75% of the outflow tract during diastole. Findings with moderate AT fall between these.

A second method uses CW Doppler to calculate the Al “pressure half-time” (see “Mitral Steno- sis” section). This parameter is a function of the gradient between the aorta and left ventricle during diastole. In severe Al, this gradient decreases very quickly (producing a short pressure half-time) but with mild Al it decreases much more slowly (producing a long pressure half-time). In general, a pressure half-time of 200 to 250 ms or less strongly suggests severe aortic insufficiency.

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152 Blanchard and DeMaria In the third method, PW Doppler is utilized to detect diastolic reversal of flow in the descending aorta. Holodiastolic flow reversal suggests severe Al (Fig. 11B). Several other techniques for evaluating severity of Al (e.g., calculation of regurgitant flow volume and orifice area using flow convergence measurements) are beyond the scope of this chapter (1).

Although echocardiographic assessment of aortic stenosis is quantitative and generally accurate, assessment of Al is semiquantitative at best. Therefore, clinical examination and correlation are essential. Despite this, echocardiography is quite useful with aortic valve disease and can help to determine proper timing of valve surgery.

Mitral Valve MITRAL STENOSIS

Detection of mitral stenosis (MS) was one of the earliest clinical applications of cardiac US.

Rheumatic MS is characterized by tethering and fibrosis of the mitral leaflets, principally at the distal tips. The leaflets are sometimes calcified and usually are thickened and display characteristic “doming” during diastole (Fig. 12A). The posterior leaflet of the valve may be pulled ante- riorly during diastole secondary to commissural fusion with the longer anterior leaflet. The left atrium is almost always enlarged. In the parasternal short-axis view, the commissural fusion is appar- ent and produces a “fish-mouth” appearance of the orifice (Fig. 12B) (15). Doppler examination reveals abnormally high diastolic flow velocity through the mitral valve and often detects coexistent mitral regurgitation.

Echocardiographic quantitation of MS severity is done in two ways. First, the mitral orifice area can be measured directly via planimetry in the parasternal short-axis view. Gain artifacts must be avoided, and care must be taken to find the smallest orifice area at the distal end of the leaflets.

Properly done, this technique is accurate and correlates well with catheterization data. The second commonly used technique is the “pressure half-time” method. The pressure half-time is the inter- val required for transmitral flow velocity to decrease from its maximum to the velocity that represents half of the pressure equivalent. As the severity of MS increases, the rate of flow deceleration decreases (i.e., the pressure gradient between left atrium and LV remains high during diastole), prolonging the pressure half-time. The pressure half-time method also correlates well with planimetry measurements, but is not accurate immediately after mitral valvuloplasty.

In addition to valve area quantitation, echocardiography is useful in predicting success of percu- taneous mitral valvuloplasty. A score based on four variables (mitral valvular thickening, calcification, mobility, and subvalvular involvement) has been devised and tested. Each variable is rated on a scale of 1 to 4 (where 4 is most severe) and the individual components are summed. A score of 8 to 12 or greater predicts a poor response to valvuloplasty and an increased risk of complications.

MITRAL REGURGITATION

As it is for Al, echocardiography is extremely accurate for detecting mitral regurgitation (MR), but quantitation is more difficult. Two-dimensional imaging in MR may reveal thickened, abnormal mitral valve leaflets (for example, in cases of rheumatic disease, myxomatous degeneration, mitral valve prolapse, or ruptured mitral chordae tendineae). With severe MR, the left atrium and ventricle are often enlarged. Doppler echocardiography is the primary method of semiquantitation of MR.

Color-flow imaging shows a jet of aliased flow in the left atrium during systole, and the size of this color jet correlates roughly with angiographic MR severity (Fig. 13; see Color Plate 3, following p. 268) (17). Eccentrically directed MR, however, may produce a color jet of misleadingly small cross-sectional area on US imaging, even when left ventriculography demonstrates severe MR.

Volumetric analysis with PW Doppler can be used to calculate regurgitant volumes, but its accuracy is limited. PW Doppler interrogation of the pulmonary veins (by TTE or TEE) is helpful in quantifying MR, as systolic flow reversal within the vein is quite specific for severe regurgitation.

Recent work has shown that flow convergence is a useful marker in cases of valvular regurgitation (18). With significant MR, there is often a large zone of high-velocity (aliased) color flow proxi-

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Fig. 12. (A) Parasternal long-axis view of mitral stenosis. The left atrium (LA) is enlarged, mitral opening is limited, and “doming” of the anterior mitral leaflet is present. (B) Parasternal short-axis plane in mitral stenosis. RV, right ventricle; LV, left ventricle; AO, aorta. (From ref. 1, with permission.)

mal to the mitral valve leaflets. This finding (even with a relatively small color jet in the left atrium) often indicates MR of at least moderate severity.

MITRAL VALVE PROLAPSE

Echocardiography is the diagnostic procedure of choice for mitral valve prolapse. This condition is defined by the bulging back of the mitral valve leaflets into the left atrium, with a portion of the leaflets passing the level of the mitral valve annulus on the parasternal long-axis view (Fig. 14).

M-mode imaging also can detect mitral valve prolapse, but it is less sensitive than 2-D imaging.

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Fig. 14. Parasternal long-axis image through the mitral valve in late systole. The plane of the annulus (A) is drawn in a dotted line. The posterior leaflet prolapses past the level of the annulus. LA, left atrium; AO, aorta;

LV, left ventricle. (From ref. 1, with permission.)

Fig. 13. Parasternal long-axis view in a case of severe mitral regurgitation. The color Doppler jet is directed posteriorly and is eccentric (black arrows). The jet “hugs” the wall of the left atrium (LA) and wraps around all the way to the aortic root (white arrows). LV, left ventricle. (See Color Plate 3, following p. 268. From ref. 1, with permission.)

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Chapter 9 / Echocardiography 155 Rupture of a chordae tendineae is well-visualized by US. Imaging usually reveals the involved chord and leaflet as well as the severity of MR. TEE is especially beneficial for assessing the feasi- bility of mitral valve repair.

PROSTHETIC CARDIAC VALVES

Echocardiography can assess the anatomy and function of bioprosthetic and mechanical heart valves. In general, however, evaluation is considerably more limited than that of native valves.

Because of acoustic shadowing, the areas distal to prosthetic (especially mechanical) valves are obscured, limiting detection of valvular regurgitation, thrombi, and vegetations. Because of this, TEE has become indispensable in the evaluation of prosthetic valve dysfunction and associated abnormalities.

Right-Sided Valvular Disease and Pulmonary Hypertension

Two-dimensional echocardiography can detect rheumatic involvement of the tricuspid and pulmonic valves and congenital pulmonic stenosis. Color-flow imaging detects and helps to semiquan- tify tricuspid and pulmonic regurgitation, similar to insufficiency of the mitral and aortic valves.

Measurement of the peak tricuspid regurgitation velocity by CW Doppler is helpful for estimating peak systolic pulmonary artery and right ventricular pressures (via the modified Bernoulli equation) (9).

The 2-D findings associated with right ventricular overload and pulmonary hypertension include enlargement of the right ventricle and right atrium, dilation of the pulmonary artery and inferior vena cava, flattening of the interventricular septum (with loss of the normal curvature toward the right), and hypertrophy of the right ventricular free wall. Doppler examination often shows moderate to severe tricuspid regurgitation in these cases.

DISEASES OF THE AORTA Aortic Dissection

In the last several years, echocardiography has fundamentally changed the diagnostic approach to suspected aortic dissection. TTE is a reasonably accurate screening tool for ascending aortic dissection (type A) but is not sensitive for detecting descending aortic dissection (type B). Diag- nostic findings include a dilated aorta with a thin, linear mobile signal in the lumen representing the dissected intimal flap. Color Doppler imaging may reveal normal or high-velocity flow in the true lumen and slow (stagnant) flow in the false channel. Occasionally, the entrance into the false channel is defined. Although TTE is sometimes helpful, TEE has become a diagnostic procedure of choice for aortic dissection (12). Its sensitivity and specificity rival those of magnetic resonance imaging, and TEE has the advantage of being portable and rapid. In addition, LV and valvular function can be defined during the examination. TEE can also help detect thrombosis of the false lumen, traumatic transection of the aorta, and intramural aortic hematoma (an increasingly recog- nized disorder with a prognosis similar to that of dissection) (19).

Aortic Aneurysm and Atherosclerosis

Aneurysms of the aorta may appear saccular or fusiform, and on echocardiography are seen as focal or diffuse areas of aortic enlargement. TTE is useful for detecting ascending aortic dilation and can sometimes visualize descending thoracic and abdominal aortic aneurysms (Fig. 15). Sinus of Valsalva aneurysms (asymmetric dilations of the aortic root) are also well-visualized, and the aortic insufficiency or shunts often associated with these aneurysms are well-defined. Echocardiography has been used extensively to aid decision making on the timing of aortic valve and root replace- ment in patients with Marfan’s syndrome (12).

TEE has played a major role in the detection of aortic atherosclerosis. This disease has been underappreciated in the past but appears to be a powerful risk factor for stroke and peripheral

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Fig. 15. Parasternal long-axis image demonstrates severe aortic root (AO) enlargement. LA, left atrium; LV, left ventricle. (From ref. 1, with permission.)

emboli. TEE is currently the procedure of choice for detecting aortic atheromas, which character- istically appear as asymmetric, calcified plaques that protrude into the aortic lumen (12).

INFECTIVE ENDOCARDITIS

Echocardiography is an integral part of the diagnosis and management of infective endocarditis. Clearly, the diagnosis remains a clinical one, but echocardiographic detection of vegetations is now included in most modern diagnostic algorithms and strategies. The hallmark of endocarditis is an infective valvular vegetation (Fig. 16), and TTE detects these with reasonable sensitivity (although as many as 20% of patients with proven native valve endocarditis may have unremarkable TTE findings).

TEE is considerably more accurate than TTE for visualizing vegetations and is significantly better in detecting valvular abscesses and prosthetic valve endocarditis (see Fig. 8A) (20). Echo- cardiography also helps visualize associated abnormalities such as valvular regurgitation, purulent pericarditis, and intracardiac fistulae. Accurate visualization of these abnormalities helps guide management and is useful in assessing the need for cardiac surgery. A common clinical dilemma concerns the appropriate use of TEE in persons with endocarditis. It seems reasonable to use TTE as the first screening test for most patients with suspected endocarditis. If the study is technically limited or findings are equivocal or diagnostic of vegetations in patients at high risk for perivalvular complications, TEE should be performed. If TTE findings are unremarkable or vegetations are detected in patients at low risk for complications, TEE is probably unnecessary. Patients at high risk (e.g., those with prosthetic cardiac valves, congenital heart disease, or infection with virulent organisms) should undergo TEE if endocarditis is strongly suspected, even if TTE results are unremarkable (20).

Despite all technologic advances, infective endocarditis remains a clinical diagnosis, and the utility of echocardiography should not be overestimated. Myxomatous valvular degeneration can masquerade as vegetations, and an old, healed vegetation can be mistaken for an active lesion.

Therefore, echocardiographic results should be integrated with all available clinical data.

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ISCHEMIC HEART DISEASE

Echocardiography is an important technique for detecting and analyzing myocardial ischemia and infarction (MI). LV ischemia quickly produces dysfunction and hypokinesis of the involved ventricular segment. If coronary flow is not restored, permanent damage occurs with resulting akinesis and thinning of the affected myocardial segment. If the region of dysfunctional myocardium is identified, the infarct-related coronary artery often can be inferred (21). Echocardiography detects these abnormalities, along with the LV dilation and depression of ejection fraction that accompany severe ischemic heart disease. The LV myocardium can be divided into 16 wall segments according to a format adopted by the American Society of Echocardiography (Fig. 17) (21).

By grading the contraction of each of these segments, a semiquantitative wall motion score can be calculated. This parameter has been used to assess prognosis for both acute MI and chronic ischemic heart disease.

Although echocardiography can help estimate the extent of damage in acute MI, the technique is also valuable for detecting post-MI complications. Easily visualized findings include pericardial effusion (from pericarditis or LV free wall rupture), ventricular septal rupture, mitral regurgitation (from LV enlargement or papillary ischemia), LV pseudoaneurysm, and RV dysfunction associated with inferior wall Ml. Long-term LV remodeling and aneurysm formation can also be assessed (Fig. 18) (22).

Stress Echocardiography

Echocardiography can been combined with stress testing to increase the accuracy of ischemia detection (23). In this technique, side-by-side cine loops of 2-D images made before and after (or during) stress are displayed on a computer monitor. Normally, the LV myocardium becomes hyper- contractile with exercise, and end-diastolic LV cavity size decreases. Stress-induced segmental hypo-kinesis is abnormal, and the affected coronary artery can be predicted from which particular area(s) exhibit inducible ventricular dysfunction. Multiple wall segment abnormalities and LV dilation with stress are ominous findings that suggest severe stenoses in multiple coronary arteries and widespread ischemia (23).

Fig. 16. Parasternal long-axis view demonstrates a vegetation (arrow) on the anterior mitral valve leaflet. AO, aorta; LV, left ventricle; LA, left atrium. (From ref. 1, with permission.)

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Fig. 17. Sixteen-segment format for identification of left ventricular wall segments. Coronary arterial territories are also included. LAX, parasternal long-axis; SAX PM, short-axis at papillary muscle level; 4C, apical four- chamber; 2C, apical two-chamber; ANT, anterior; SEPT, septal; POST, posterior; LAT, lateral; INF, inferior.

(From ref. 21.)

Stress echocardiography can be performed with either exercise or a graded infusion of dobutamine. In general, both types of stress are safe and are tolerated well, and accuracy rates are com- parable with those of nuclear stress imaging. Stress echocardiography tends to be slightly less sensitive than nuclear stress imaging but slightly more specific. Dobutamine echocardiography has assumed an important role in the detection of myocardial viability and the phenomenon of

“hibernation” (24). Technical innovations such as harmonic imaging and contrast echocardiography have increased the accuracy and applicability of stress echocardiography. In addition, US contrast agents help facilitate the direct quantitation of myocardial perfusion during stress.

CARDIOMYOPATHIES

Cardiomyopathies are generally separated into three categories: dilated (DCM), hypertrophic (HCM), and restrictive (RCM). Echocardiography plays an important role in the clinical evaluation, providing information on cavity size, ventricular wall thickness, valvular lesions, and systolic function. In cases of classic HCM, echocardiography alone may be diagnostic. In cases of dilated, restrictive, and nonclassic HCM, however, additional clinical information may be needed to arrive at a firm diagnosis. These diseases are discussed in further detail in Chapter 33. In this section, we focus primarily on their US features.

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Hypertrophic Cardiomyopathy

HCM is a primary abnormality of the myocardium that exhibits unprovoked hypertrophy and often affects the septum disproportionately. The first and fundamental echocardiographic abnormality is LV hypertrophy, which is often severe. Classically, the septum is involved more extensively than other areas (Fig. 19A), but the hypertrophy may also be concentric or apical. Asymme- tric septal hypertrophy leads to the second classic US feature of HCM: dynamic LVOT obstruction.

This is associated with systolic anterior motion (SAM) of the mitral valve (see arrow, Fig. 19A).

Systolic encroachment of the abnormally thickened septum into the LVOT creates a pressure drop via the Venturi effect, which then draws the mitral leaflets toward the septum, causing dynamic obstruction. Like severe LVH, SAM is not pathognomonic for HCM and can occur in other conditions such as hypovolemia and hyperdynamic states (1).

The third manifestation of classic HCM is mid-systolic partial closure of the aortic valve. This occurs only in obstructive HCM cases and is probably a manifestation of the sudden late systole pressure drop caused by SAM. Therefore, when this sign is present, significant LVOT obstruction is likely (25). The fourth sign of HCM is seen on CW Doppler imaging through the LVOT. Nor- mally, flow veIocity in this area peaks early during systole and has a maximum of 1.7 m/s. In HCM with outflow tract obstruction, the peak systolic flow velocity is abnormally high. As opposed to valvular aortic stenosis, however, the CW spectral tracing of obstructive HCM peaks late in systole, creating a characteristic “sabertooth” pattern (Fig. 21B). Catheterization data would pre- dict this type of tracing, as the outflow tract gradient is not severely elevated in early systole but rises in mid- and late systole because of dynamic obstruction. The peak CW velocity can be used to calculate the systolic gradient via the modified Bernoulli equation, although recent studies have suggested that this calculation may not be consistently accurate in HCM.

Dilated Cardiomyopathy

Echocardiographic findings in DCM include four-chamber cardiac dilation and marked LV enlargement. Systolic function is depressed, often severely. In addition, the LV walls are often thin, with concomitant left atrial enlargement, limited mitral and aortic valve opening (due to low stroke

Fig. 18. Apical four-chamber images of a large apical infarction with aneurysm. Diastole (D) is displayed on the left, systole (S) on the right. During systole, the base on the ventricle contracts but the apex is dyskinetic (arrows). RA, right atrium; RV, right ventricle; LV, left ventricle; LA, left atrium. (From ref. 1, with permission.)

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volume), and mitral annular dilation (with secondary mitral regurgitation) (26). Unfortunately, these findings are not specific for DCM and can be caused by severe ischemic heart disease, viral myocarditis, cardiac toxins, and nutritional deficiencies. Ischemic heart disease can often be predicted by the presence of regional LV dysfunction, but, again, this finding is not always reliable.

Diastolic dysfunction is common in DCM, and Doppler interrogation of mitral inflow may show an abnormal relaxation, restrictive, or “pseudonormal” pattern, depending on left atrial pressure and loading conditions. A restrictive pattern of inflow is associated with poor prognosis for DCM.

Fig. 19. (A) Parasternal long-axis view (during systole) of hypertrophic cardiomyopathy (HCM). Asymmetric septal hypertrophy is present, as well as systolic anterior motion of the anterior mitral leaflet (arrow). (B) Con- tinuous-wave Doppler tracing through the left ventricular outflow tract (from the apical transducer position) in hypertrophic obstructive cardiomyopathy. In comparison to valvular aortic stenosis, the rise in velocity is delayed (reflecting dynamic rather than fixed outflow obstruction). LV, left ventricle; LA, left atrium; RV, right ventricle; AO, aorta. (From ref. 1, with permission.)

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Restrictive Cardiomyopathy

RCM is a fairly rare condition that is characterized on US by (1) a diffuse increase in LV wall thickness in the absence of severe cavity dilation and (2) marked biatrial enlargement (27).

Systolic function may be normal or modestly decreased. Doppler examination may show a mitral inflow relaxation abnormality early in the course of RCM, but this tends to evolve into a restrictive pattern as the disease progresses. RCM may be idiopathic or secondary to infiltrative diseases such as hemochromatosis and hypereosinophilic endocardial disease. The most common canse of RCM, however, is amyloidosis, which causes biventricular hypertrophy and diffuse thickening of the interatrial septum and cardiac valves (Fig. 20). A “ground-glass” or speckled appearance of the myocardium has been described with amyloid, but this sign has minimal clinical usefulness.

As with the other cardiomyopathies, the echocardiographic findings in RCM are often helpful but ultimately nonspecific.

CARDIAC MASSES

Echocardiography has become the procedure of choice for the detection of intracardiac thrombi, vegetations, and tumors. It also visualizes a number of “pseudo-masses” or benign anatomic variants (e.g., prominent eustachian valve, Chiari network, prominent right ventricular moderator band, and LV false chordae tendineae). US can also detect intracardiac foreign bodies, including pacemaker leads, intracardiac catheters, and endomyocardial bioptomes.

Intracardiac Thrombi

Thrombi can develop in any chamber of the heart and may cause embolic events (28). The major predisposing factors for intracardiac thrombus formation include low cardiac output, localized stasis

Fig. 20. Apical four-chamber view of cardiac amyloid. RV, right ventricle; RA, right atrium; LA, left atrium;

LV, left ventricle. (From ref. 1, with permission.)

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162 Blanchard and DeMaria of flow, and myocardial injury. The echocardiographic appearance of thrombi is quite variable:

thrombi can be freely mobile or attached to the endocardium, and they may be laminar and homogeneous in density or heterogeneous with areas of central liquefaction or calcification. Thrombi typically have identifiable borders on US and should be visible in multiple imaging planes (28).

Thrombi within the right heart are often laminar but can be quite mobile (especially venous thromboemboli that have migrated to the right side of the heart), and they increase the risk of pulmonary embolism. Left atrial thrombi occur most often in the setting of LV systolic dysfunction, mitral stenosis, atrial fibrillation, and severe left atrial enlargement. TEE is clearly superior to TTE for detecting these thrombi, especially those within the left atrial appendage (see Fig. 8B).

Because approx 50% of left atrial thrombi are limited to the appendage, TEE is the procedure of choice for detecting them. Left atrial thrombi are often accompanied by spontaneous US contrast (or “smoke”) in the left atrium, which indicates stagnant flow and increased likelihood of embolic events.

LV thrombi usually occur in settings of systolic dysfunction (28), including DCM, acute MI, and chronic LV aneurysm. Most LV thrombi are located in the apex and thus are best visualized in the apical views. LV thrombi may be laminar and fixed, protruding or mobile, and homogeneous or heterogeneous in US density. Artifacts can sometime mimic apical thrombi. A true LV thrombus has a density that is distinct from that of the myocardium, moves concordantly with the underlying tissue, and is visible in multiple imaging planes. Finally, an LV thrombus rarely occurs in areas of normally functioning myocardium.

Cardiac Tumors

Cardiac tumors can be benign or malignant; malignancies may be primary, metastatic, or the result of direct extension from adjacent tumors. Although primary cardiac malignancies are exceed- ingly rare, metastatic spread to the heart from lung cancer, breast cancer, lymphoma, or melanoma is fairly common, especially in the later stages of disease. Such tumors may be seen within the cardiac chambers, but pericardial or epicardial involvement is more common.

Myxomas are by far the most common primary cardiac tumors, and about 75% are found in the left atrium (29). On 2-D imaging, these tumors generally appear gelatinous, speckled, and sometimes globular. Tissue heterogeneity is frequently seen, but calcifications are rare. Although they can originate from any portion of the atrial wall, myxomas are usually attached by a pedicle to the interatrial septum. Large myxomas are almost always mobile and may move back and forth into the mitral annulus. Doppler examination may demonstrate valvular regurgitation, obstruction, or both. TTE accurately detects most large myxomas (see Fig. 21), but TEE is superior for delineating small tumors (29). Less common benign primary cardiac tumors include rhabdomyomas (associated with tuberous sclerosis), fibromas (which tend to grow within the LV myocardial wall), and papillary fibroelastomas (which grow on valves and tend to embolize systemically).

CONGENITAL HEART DISEASE

In this section we focus primarily on the echocardiographic recognition of the more common congenital lesions seen in adults.

Atrial Septal Defect

Most ostium secundum and ostium primum atrial septal defects (ASD) are easily seen with TTE (30). Sinus venosus defects, however, can be difficult to detect without TEE. As the normal interatrial septum is thin and parallel to the US beam from the apical position, artifactual “dropout” in the area of the fossa ovalis can be mistaken for ASD. Therefore, subcostal imaging is usually superior. Ostium secundum defects (the most common form of ASD) are distinguished by localized absence of tissue in the middie portion of the interatrial septum (Fig. 22A; see Color Plate 3, following p. 268). The absence of any septal tissue interposed between the defect and the base of the interventricular septum together with the loss of normal apical displacement of the

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Fig. 21. Apical four-chamber view of a large left atrial myxoma (arrows) that is attached to the lateral wall of the left atrium (LA). RA, right atrium; RV, right ventricle; LV, left ventricle; PE, pericardial effusion; PL, pleural. (From ref. 1, with permission.)

tricuspid annulus suggests an ostium primum defect. Cleft anterior mitral valve leaflet, mitral regurgitation, and inlet ventricular septal defect often occur with ostium primum ASDs. Sinus venosus ASDs are seen in the superior and posterior portions of the interatrial septum and are usually associated with anomalous drainage of one or more pulmonary veins into the right atrium (1).

Additional 2-D findings seen in ASD include right atrial and RV enlargement, flattening of the interventricular septum, and paradoxical septal motion. Doppler interrogation often demonstrates blood flow though the defect, but atrial inflow from the vena cava and pulmonary veins sometimes mimics ASD. To prevent misdiagnosis of ASD, intravenous injection of agitated saline is recommended (see “Contrast Echocardiography”). Finally, Doppler and 2-D imaging can be used to estimate roughly the pulmonary-to-systemic blood flow ratio in patients with ASD or other intracardiac shunts.

Ventricular Septal Defect

The majority of VSDs in adults are perimembranous. Inlet (AV canal), trabecular, and outlet (supracristal) defects are much rarer. Although large VSDs are often visible on 2-D imaging alone (Fig. 22B), color-flow imaging is essential for detection of small defects (31). CW Doppler measurement of peak systolic flow velocity through a VSD can be used to estimate the pressure gradient between the two ventricles via the modified Bernoulli equation (the estimated RV systolic pressure is the systolic arterial pressure minus the calculated Bernoulli gradient) (9). Associated 2-D and Doppler findings include cardiac enlargement (possibly with RV pressure overload), mitral and tricuspid valvular abnormalities and regurgitation, coexistent ASD (most often with inlet VSD), ventricular septal aneurysms, and aortic insufficiency (especially with supracristal VSD). During intravenous injection of agitated saline, “negative” contrast jets are sometimes seen at the right ventricular aspect of the VSD.

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Fig. 22. (A) Apical four-chamber view of an ostium secundum atrial septal defect. On the left, a defect in the mid-atrial septum is present (arrows). On the right, there is color flow through the shunt. RA, right atrium;

RV, right ventricle; LA, left atrium; LV, left ventricle. (See Color Plate 4, following p. 268.) (B) Apical four- chamber image of an inlet ventricular septal defect. RV, right ventricle; RA, right atrium; LA, left atrium; LV, left ventricle. (From ref. 1, with permission.)

Patent Ductus Arteriosus

Patent ductus arteriosus (PDA) is a connection between the distal portion of the aortic arch and the pulmonary artery (usually just to the left of its bifurcation). Two-dimensional imaging occasionally detects a PDA, but color Doppler interrogation is considerably more likely to demonstrate the

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characteristic high-velocity diastolic flow in the proximal pulmonary artery (32). Additional 2-D findings include LV enlargement and volume overload. If “Eisenmenger” physiology supervenes, the right side of the heart enlarges and LV dilation may reverse to some degree. Therefore, absence of LV or RV enlargement suggests a small shunt.

Conotruncal and Aortic Abnormalities

The most common congenital cardiac anomaly in adults is a bicuspid aortic valve (prevalence of 1 to 2% in men and somewhat less in women). This anomaly is often associated with aortic insufficiency or stenosis, as well as coarctation of the aorta. Tetralogy of Fallot is one of the more frequent conotruncal abnormalities. The classic echo features include a large perimembranous VSD, pulmonic stenosis, RV enlargement and hypertrophy, and anterior displacement of the aortic valve.

Coarctation of the aorta, which is often associated with a bicuspid aortic valve, is best visualized from the supersternal position. Two-dimensional imaging sometimes detects the coarctation, but acoustic shadowing and dropout may limit evaluation of the descending aorta. Doppler examination is more reliable and shows abnormally high flow velocity in the descending aorta. A classic finding in coarctation is holodiastolic antegrade flow in the descending aorta, indicating a pressure gradient throughout diastole and, therefore, severe coarctation. Another congenital abnormality seen occasionally in adult patients is Ebstein’s anomaly. Two-dimensional imaging in classic cases reveals a deformed tricuspid valve, including an elongated anterior leaflet and an apically displaced septal leaflet. Associated findings include enlargement of the right side of the heart and tricuspid regurgitation. ASDs are present in a significant minority of cases.

PERICARDIAL DISEASE

Echocardiography is an accurate, reliable tool for the detection of pericardial effusion, intra- pericardial masses, and cardiac tamponade. Pericardial fluid is identified as a “dark” or echo-free space immediately adjacent to the epicardium (Fig. 23). Pericardial effusions may be concentric or

Fig. 23. Parasternal long-axis image of a large pericardial effusion (PE). LV, left ventricle; LA, left atrium;

AO, aorta. (From ref. 1, with permission.)