Background

Infective endocarditis (IE), a noncontagious infection of the endocardium and heart valves, has an incidence ranging from 2.6 to 11.6 per million^1,2 and a high in-hospital mortality rate of approximately 20% despite medical and surgical advances. The contemporary epidemiology of IE has changed significantly compared to earlier eras, and these changes have an important influence on the evaluation and outcome of this condition. In the large, prospective, multinational International Collaboration on Endocarditis (ICE) registry, the mean age of diagnosis of definite IE is the sixth decade, with approximately 2 : 1 male predominance and frequently occurring in the presence of comorbid conditions such as kidney disease and diabetes mellitus.³ The major predisposition to IE is underlying degenerative valvular disease (particularly mitral valve disease), present in approximately 30% of cases, but a very low prevalence of underlying rheumatic heart disease.³ The microbiology of the condition has shifted toward more virulent organisms, particularly Staphylococcus aureus as the most common cause, reflecting the growing influence of health care–associated infection as a cause of bacteremia.

The diagnosis and management of this condition has greatly improved with the advent of echocardiography. The first use of echocardiography in the diagnosis of IE was described in 1973 by Dillon and colleagues,⁴ who applied M-mode echo to visualize evidence of vegetations. The high-frequency oscillations of vegetative lesions and/or destruction of aortic or mitral leaflet integrity could be documented using this technique (Fig. 22-1). M-mode, however, did not allow estimation of lesion size, morphology, and localization of specific leaflet involvement, which was provided by real-time, two-dimensional (2D) echocardiography first described by Gilbert and colleagues.⁵ Over time, with the widespread use of 2D echocardiography and the improved spatial resolution offered by transesophageal echocardiographic (TEE) approaches,⁶ echocardiography has become the diagnostic modality of choice in all cases of suspected IE. Echocardiography not only provides noninvasive evidence of endocardial infection but also offers important hemodynamic information regarding the presence and severity of valvular regurgitation and other structural complications, findings key to the management decision-making process in these patients. For these reasons, echocardiography is considered essential in all cases suspected of IE.⁷

Figure 22-1 M-mode echocardiogram recorded in a patient with aortic valve endocarditis.

Note mass of shaggy echoes (vegetation) in diastole.

Basic Principles and Echocardiographic Approach

Clinical characteristics may identify the host at risk for IE and guide the use of echocardiography for diagnostic confirmation. IE is associated with fever in more than 90% of cases, and a similar percentage will have positive blood cultures for a microorganism associated with this condition.³ Most patients present with symptom duration of less than 1 month.³ Approximately half of patients will have evidence of a new or changing heart murmur reflecting valvular regurgitation or other endocardial damage, but a very small percentage will have peripheral manifestations of classical, subacute IE (Osler nodes, Janeway lesions, splinter hemorrhages, Roth spots).³

Historically, the association between infective lesions on the endocardium and the clinical manifestations of IE has been recognized for over a century. However, the clinical manifestations of the disease remained nonspecific and challenged accurate clinical diagnosis of the disease antemortem.⁸ In 1977, Pelletier and Petersdorf⁹ proposed case definitions for IE that relied mainly on clinical characteristics and, specifically, on the demonstration of continuous bacteremia. Although a first step in standardization of the diagnosis of IE, these criteria were highly specific but lacked sensitivity. In 1981, Von Reyn and colleagues¹⁰ expanded on these clinical criteria and offered levels of diagnostic certainty (rejected, possible, probable, and definite) for suspected IE. These modifications improved sensitivity and specificity, but did not incorporate evidence of endocardial involvement by the relatively novel application of echocardiography at the time. Subsequently, in 1994, the Duke Criteria proposed by Durack and colleagues¹¹ incorporated visualization of endocardial involvement by echocardiography with microbiologic and clinical criteria for the first time. Indeed, evidence of endocardial involvement, including new valvular regurgitation, intracardiac vegetation, paraannular abscess, or new dehiscence of prosthetic valve, became one of the two major diagnostic criteria for IE by the Duke case definition, along with microbiologic criteria.

Several comparative studies of the Von Reyn and Duke Criteria in various cohorts have subsequently established the superior sensitivity of the Duke criteria compared to earlier case definitions.^12–19 The negative predictive value of these criteria was more than 92% with a high sensitivity (100%) and specificity (88%).^20,21 Recent modifications of the Duke criteria (Box 22-1) have added greater specificity to the schema.²²

Technical Details, Quantitation, Data Analysis

Echocardiographic features of IE include vegetations, abscess, aneurysm, fistula, leaflet perforation, and valvular dehiscence (Table 22-1).²³ Vegetations occur at a region of endocardial denudation, often resulting from preexisting valvular disease (Figs. 22-2, 22-3, and 22-4). Disruption of the endocardial surface results in platelet adhesion and fibrin deposition, to which microorganisms adhere during transient bacteremia to form infected vegetation. Vegetations are visualized in nearly 90% of patients with definite IE.³ By echocardiography, vegetations appear as irregularly shaped, discrete, oscillating, echogenic masses that are adherent to valves, chordate, or other endocardial surfaces in the path of turbulent jets passing through regurgitant valves or septal defects. They are typically located on the low-pressure side of high-velocity jets; hence, in cases of regurgitation, they are located on the atrial aspect of the mitral and tricuspid valves and on the ventricular aspect of the aortic and pulmonic valves. They often display the same echogenicity as midmyocardial structures and may be heterogenous with echodense or echolucent areas. Vegetations may also be found on noncardiac structures such as intracardiac devices. In the presence of vegetation, perforation of a valve leaflet may be visualized as a defect in the body of a valve leaflet with evidence of flow through the defect (Fig. 22-5).

TABLE 22-1 Pathologic Features of Infective Endocarditis: Echocardiographic Appearance and Clinical Significance

Figure 22-2 2D TTE parasternal long-axis view recorded in a patient with aortic valve endocarditis.

Arrow points to a heterogeneous irregularly shaped mass attached to aortic valve leaflet prolapsing into the LV outflow tract. Ao, Aorta.

Figure 22-3 TEE views of an aortic valve vegetation. The 2D images show a heterogeneous mass, in the long-axis (left) and short-axis (right) views. Attachment of vegetation to the right coronary cusp is evident (arrows). Ao, Aorta.

Figure 22-4 3D TEE full-volume data set cropped to reveal aortic valve vegetation.

Anatomic relationships are displayed precisely.

Figure 22-5 Leaflet perforation and aortic regurgitation owing to endocarditis. A, 2D TEE long-axis view of the aortic valve demonstrate loss of structural integrity and leaflet perforation. B, 2D TEE long-axis view of aortic valve with color Doppler demonstrating severe aortic regurgitation. Ao, Aorta.

Machine settings such as frame rate, sector arc size, gray scale, and focal zones must be optimized, and a careful examination including nonstandard views should be conducted to exclude the presence of vegetations. Increased gain settings and improper focal zones can make vegetations appear larger than actual size. On the other hand, color flow imaging may change pulse repetition frequency and impair visualization of the vegetation. The size of the vegetations should be measured within the resolution characteristics of the transducer so that machine settings can be duplicated for future comparative studies, if clinically indicated.

Vegetations may be present in patients with negative blood cultures, which is termed culture-negative endocarditis and represents approximately 10% of definite IE. Culture-negative endocarditis results from prior antibiotic therapy, fastidious organisms such as HACEK species, or unusual organisms such as Coxiella burnetii (Q fever) or Bartonella quintana. Vegetations also need to be differentiated from other masses with similar appearance on echocardiography such as Libman-Sacks (noninfective) endocarditis, degenerative change, Lambl’s excrescences, thrombus, or tumors. Because there are no echocardiographic features that can reliably differentiate infective from noninfective endocardial lesions, the key to a proper diagnosis lies in integrating imaging data and clinical information. Visualization of distinct vegetation may be impaired by severe, underlying valvular degeneration, particularly prominent calcification of valve leaflets.

Tissue destruction in IE may result in other structural complications that can be identified on echocardiography. Paravalvular abscess complicated 30% to 40% of cases of IE in earlier reports⁶ but were found in 14% of patients with definite IE in a larger, contemporary study³ (Fig. 22-6). An abscess is a result of invasive infection that spreads generally along contiguous tissue planes, particularly with aortic valve infection. Development of a new atrioventricular conduction abnormality, worsening clinical picture, persistent bacteremia/fever in the setting of aortic valve endocarditis, injection drug use, infection with an invasive pathogen (staphylococcal), or prosthetic valves should prompt a search for an aortic root abscess.²⁴

Figure 22-6 Aortic root abscess owing to endocarditis. 2D TEE orthogonal views of the aortic valve demonstrating thickening and heterogeneous echodensity of the aortic annulus (arrows). This is most prominent at the junction of the anterior mitral valve leaflet and aortic valve.

TEE is the diagnostic test of choice when an abscess is suspected clinically. An abscess is diagnosed by TEE as the visualization of a “thickened area” or “mass with a heterogeneous echogenic or echolucent appearance” within the myocardium or annular region.⁶ On color Doppler imaging, flow within the area is supportive of the diagnosis. Abscesses complicating native valve IE most commonly involve the aortic valve annulus at the junction of aortic root and anterior mitral valve leaflet. Hence, they may extend into the adjacent interventricular septum, the right ventricular outflow tract, the interatrial septum, and the anterior mitral valve leaflet. Location of the abscess, particularly in the posterior annulus of the mitral valve when calcification is present, may limit its visualization by echocardiography.²⁵ In the ICE cohort, 22% of cases of definite aortic valve IE were complicated by a paravalvular abscess.²⁴ These patients were more likely to have prosthetic valves and coagulase-negative staphylococcal infection. Surgical experience suggests an even higher prevalence of abscess formation not visualized by TEE.²⁵ New paravalvular regurgitation around a prosthetic valve may also suggest the presence of an abscess or infected sewing ring of the prosthesis.

If an abscess is identified, a careful examination is required to rule out extension into these areas, because this additional information has considerable implications for the type of reparative surgery to be undertaken. Loss of structural integrity of the valve, presence of valvular regurgitation and vegetations, and thickening of adjacent tissues help differentiate a native aortic valve abscess from fluid in the transverse pericardial sinus. These associated findings are also helpful in evaluating the presence of an abscess in patients with prosthetic valves in the aortic position. Postsurgical changes, including an echolucent space between a prosthetic valve sewing ring and the aortic root with or without paravalvular regurgitation, may pose a diagnostic dilemma in a patient with prosthetic valve and fever. In such cases, comparison with an intraoperative TEE, if available, may be very helpful to document the chronicity of the abnormality.

In rare cases, exposure of abscesses to high intravascular pressures and progressive burrowing infection may lead to pseudoaneurysm formation (color flow imaging demonstrating flow in echolucent space that is contiguous with the bloodstream). Because of further tissue invasion, these paravalvular cavities or pseudoaneurysms can form fistulous connections (aortoatrial or aortoventricular), leaflet perforation and even myocardial perforation. Fistula formation complicated 1.6% of cases of native valve IE and 3.5% of cases of prosthetic valve endocarditis in a cohort of 4681 cases of definite IE²⁶ (Fig. 22-7). These occurred with similar frequency in the three sinuses of Valsalva. Fistulas from the right or noncoronary sinus generally track or exit into the right ventricle, whereas fistulas from the left sinus exit to the left atrium.²⁷ TEE is the modality of choice to investigate these structural complications. Color flow Doppler imaging demonstrates flow turbulence, abnormal flow in echolucent spaces, and shunting of blood flow in cases of fistulous connections between cardiac chambers.

Figure 22-7 Aortic to right ventricular fistula owing to endocarditis. A, 2D and color Doppler TEE short-axis views of the aortic valve demonstrating a left-to-right shunt between the aorta and right ventricle. Also note large vegetations on the tricuspid valve and the bright echoes of an intracardiac device seen in the right atrium. Ao, Aorta. B, Continuous-wave Doppler recordings showing the difference in flow characteristics between an aorta-to-right-ventricle fistula showing continuous, low-velocity flow (left) and a perimembranous ventricular septal defect showing systolic, high-velocity flow (right).

Progressive tissue destruction due to infection can also result in mitral chordal rupture in native valve IE and valvular dehiscence in prosthetic valve endocarditis. Valvular dehiscence is an uncommon, serious complication and portends poor outcome for the patient. On echocardiography, valvular dehiscence is defined as rocking motion of the valve with excursion of at least 15° in at least one direction (Fig. 22-8, A). This structural deterioration is often accompanied by severe, paravalvular regurgitation (Fig. 22-8, B).

Figure 22-8 Mechanical aortic valve prosthesis with paravalvular abcess and valve dehiscence. A, 2D TEE long-axis view of a mechanical aortic valve recorded in systole. Note large, echolucent space (arrow) posterior to the valve. B, 2D color Doppler TEE image of the mechanical aortic valve demonstrating severe paravalvular regurgitation (arrow) due to valve dehiscence. Ao, Aorta.

In addition to these complications of IE, it is important to recognize that new valvular regurgitation may represent endocardial infection and is a major diagnostic Duke criterion for IE. Determining the mechanism of regurgitation as well as quantifying its severity is important: Severe regurgitation is poorly tolerated clinically because of acute onset without time for ventricular compensation and is an indication for surgical intervention.

In patients with mechanical prosthetic valves, a complete examination requires using a combination of transthoracic and transesophageal approaches. Transthoracic echocardiography (TTE) allows visualization of the ventricular aspect of the valve but is limited for examining the atrial aspect because of beam attenuation and shadowing. For the same reason, assessment of valvular regurgitation in case of mitral and triscuspid valve prosthesis is problematic with a transthoracic approach. Doppler echocardiography may be used to interrogate prosthetic valves for the presence of regurgitation using the velocity-time integral. TEE allows for much better assessment of valvular regurgitation and evaluation for vegetations on the atrial aspect of valve prostheses.

In patients with cardiac implantable electronic devices, there is substantial artifact generation, and TEE is superior to TTE in suspected infection in such devices.^28–30 Visualization of the lead in the proximal superior vena cava from TEE views may identify vegetations attached to cardiac implantable electronic device leads that are difficult to visualize by other modalities. During the examination, it is important to visualize the entire course of the prosthetic device throughout the vasculature and cardiac structures. A careful evaluation of the cardiac valves is also important because of the high rate of concomitant valve infection, particularly tricuspid valve (Figs. 22-9 and 22-10).

Figure 22-9 2D TTE image of an infected cardiac implanted electronic device.

Note the presence of heterogeneous, echodense masses (arrow) on the anterior and septal tricuspid valve leaflets.

Figure 22-10 2D TEE transgastric view of an infected cardiac implanted electronic device.

A heterogeneous, irregular mass (arrow) is seen attached to the intracardiac lead. Note the absence of involvement of the tricuspid valve.

Clinical Utility and Outcome Data

Utility in Diagnosis

General Considerations

The diagnostic yield of echocardiography is dependent on several variables including echocardiographic modality, vegetation size, vegetation location, native or prosthetic valve, presence of valve calcification (increases the sensitivity, but reduces the specificity of identifying vegetations), and preexisting valvular degeneration. With TTE, in approximately 15% of cases, variable sound transmission due to patient-related factors such as obesity, presence of lung disease, and chest wall deformities may limit diagnostic capability. Although vegetations as small as 2 to 5 mm can be detected, the sensitivity of TTE is low (approximately 40%) with such small lesions.³¹ The sensitivity of TTE is highest for tricuspid valve infection because of the proximity to the chest wall. In a study of injection drug users with suspected right-sided IE,³² TTE detected the presence of vegetations at the same rate as TEE; however, TEE was more effective at characterizing the vegetations. The diagnostic utility of TTE for suspected IE is highest in patients with intermediate to high likelihood of this disease³³ (e.g., a patient with a new or changed heart murmur and bacteremia). The limited sensitivity of TTE for the visualization of an intracardiac vegetation or abscess precludes “ruling out” the diagnosis of IE based on a negative study. Despite its poor sensitivity, TTE is often performed as the initial diagnostic study to assess for the presence and severity of valvular lesions and determine ventricular function. In a patient with a low pretest probability for IE and a normal TTE examination, a diagnosis other than IE should be pursued rather than proceeding to TEE examination (Fig. 22-11). TTE is also recommended for the reassessment of high-risk patients with definite IE (those with virulent organisms, clinical deterioration, persistent or recurrent fever, new murmur, or persistent bacteremia).

Figure 22-11 Clinical applications of echocardiography in the evaluation and management of infective endocarditis (IE). CIED, Cardiac implantable electronic device.

Although TTE and TEE have concordant results in approximately half of patients with suspected IE, TEE results in additional diagnostic information in a high percentage of patients.³⁴ TEE has several major advantages over TTE for the assessment of IE. These include better spatial resolution due to a higher-frequency transducer, lack of acoustic interference from adjacent structures such as the lungs and chest wall, and proximity to posterior structures such as the mitral valve and left atrium. TEE can detect vegetations as small as 1 to 2 mm and can detect vegetations 2 to 5 mm with a sensitivity of nearly 100%.³¹ TEE should be performed in patients with a high clinical probability of IE and a negative TTE.³⁵

Specific subsets of patients in whom TEE should be performed, even as the primary imaging modality (without preceding TTE) for diagnosis of IE, include (1) patients with prosthetic heart valves and suspected IE; (2) patients with persistent staphylococcal bacteremia without known source, or nosocomial staphylococcal bacteremia; and (3) patients with suspected infection in a cardiac implantable electronic device. Evidence suggests that TEE is also better than TTE for diagnosing native pulmonic valve IE,³⁶ although this is the least commonly infected valve and typically is associated with congenital heart disease as a predisposing condition. In patients with suspected culture-negative IE (approximately 10% of all cases of definite IE), TEE has higher sensitivity than TTE for visualizing diagnostic findings of endocardial involvement.³⁷

Cost-effective analyses also support a diagnostic approach using echocardiography, particularly TEE. Heidenreich and co-workers³⁸ have shown that in suspected IE, a diagnostic strategy that focuses on TEE as the initial imaging modality is more cost effective than a staged procedure with TTE and is a superior strategy compared with empiric antibiotic therapy alone. In this study, TEE was optimal for patients who had a prior probability of IE that is observed commonly in clinical practice (4% to 60%) with a modestly reduced cost compared with the use of TTE as initial study.

It is important to note that the utility of a diagnostic tool such as echocardiography is optimal in the appropriate clinical context or pretest probability of disease (greater than 2% to 3%). Although there are few empiric data to quantify the pretest probability of disease, there is general consensus that certain characteristics increase the likelihood of disease (Table 22-2). Recent literature suggests that imaging technologies such as echocardiography can be overused in certain clinical scenarios, such as the evaluation of suspected IE. Kurupuu and colleagues^38a have shown that 53% of echocardiograms could be avoided without loss of diagnostic accuracy by using a simple algorithm in patients with a low pretest probability of disease. Similarly, Greaves and associates³⁹ have shown that the absence of five simple clinical criteria was associated with a zero probability of a TTE demonstrating evidence of IE: (1) vasculitic/embolic phenomena, (2) central venous access, (3) a recent history of injection drug use, (4) prosthetic heart valve, and (5) positive blood cultures. Collectively, these studies have shown that in patients with very low pretest probability of disease, echocardiography may be avoided without the loss of diagnostic accuracy. Recommendations for the appropriate use of TTE and TEE in cases of suspected or known IE are provided in Tables 22-3 and 22-4. The evaluation of possible IE in specific clinical scenarios is discussed next.

TABLE 22-2 Pretest Probability Estimates for Infective Endocarditis According to Patient Characteristics

Clinical Feature	Estimate of Pretest Probability
Viridans group streptococcal bacteremia	14% (95% CI: 6-22%)
Unexplained bacteremia	5-40%
Bacteremia and recent injection drug use	31% (95% CI: 19-44%)
Admission with fever and recent injection drug use	13% (95% CI: 7-19%)
Persistently positive blood cultures and predisposing heart disease	>50%
Persistently positive blood cultures and a new regurgitant murmur	>90%
Collective absence of vasculitic/embolic phenomena, central venous access, recent history of injection drug use, a prosthetic valve, and positive blood cultures	0%
Firm alternate diagnosis or resolution of endocarditis syndrome within 4 days	<2%
Gram-negative bacteremia with clear noncardiac source of infection	<2%

Adapted from Heidenreich PA, Masoudi FA, Maini B, et al: Echocardiography in patients with suspected endocarditis: a cost-effectiveness analysis. Am J Med 107:198-208, 1999; and Greaves K, Mou D, Patel A, Celermajer DS: Clinical criteria and the appropriate use of transthoracic echocardiography for the exclusion of infective endocarditis. Heart 89:273-275, 2003.

TABLE 22-3 ACC/AHA Practice Guidelines and the Appropriateness Criteria for the Use of TTE in Suspected or Known IE

Indications	Level of Evidence
Class I
1. TTE to detect valvular vegetations with or without positive blood cultures is recommended for the diagnosis of infective endocarditis	B
2. TTE is recommended to characterize the hemodynamic severity of valvular lesions in known infective endocarditis	B
3. TTE is recommended for assessment of complications of infective endocarditis (e.g., abscesses, perforation, and shunts).	B
4. TTE is recommended for reassessment of high-risk patients (e.g., those with virulent organisms, clinical deterioration, persistent or recurrent fever, new murmur, or persistent bacteremia)	C
Class IIa
TTE is reasonable to diagnose infective endocarditis of a prosthetic valve in the presence of persistent fever without bacteremia or a new murmur	C
Class IIb
TTE may be considered for the reevaluation of prosthetic valve endocarditis during antibiotic therapy in the absence of clinical deterioration	C
Class III
TTE is not indicated to reevaluate uncomplicated (including no regurgitation on baseline echocardiogram) native valve endocarditis during antibiotic treatment in the absence of clinical deterioration, new physical findings, or persistent fever	C
Appropriateness Criteria	Appropriateness Score (1-9)
1. Initial evaluation of suspected infective endocarditis with positive blood cultures or a new murmur	A (9)
2. Transient fever without evidence of bacteremia or a new murmur	I (2)
3. Transient bacteremia with a pathogen not typically associated with infective endocarditis and/or a documented nonendovascular source of infection	I (3)
4. Re-evaluation of infective endocarditis at high risk for progression or complication or with a change in clinical status or cardiac exam	A (9)
5. Routine surveillance of uncomplicated infective endocarditis when no change in management is contemplated	I (2)

Adapted from Bonow RO, Carabello BA, Kanu C, et al: ACC/AHA 2006 guidelines for the management of patients with valvular heart disease. Circulation 114:e84-e231, 2006; and Douglas PS, Khandheria B, Stainback RF, et al: ACCF/ASE/ACEP/ASNC/SCAI/SCCT/SCMR 2007 appropriateness criteria for transthoracic and transesophageal echocardiography. J Am Soc Echocardiogr 20:787-805, 2007.

TABLE 22-4 ACC/AHA Practice Guidelines and the Appropriateness Criteria for the Use of TEE in Suspected or Known IE

Indications	Level of Evidence
Class I
1. TEE is recommended to assess the severity of valvular lesions in symptomatic patients with infective endocarditis	C
2. TEE is recommended to diagnose IE in patients with valvular heart disease and positive blood cultures if TTE is nondiagnostic	C
3. TEE is recommended to diagnose complications of IE with potential impact on prognosis and management (e.g., abscess, perforation, and shunts)	C
4. TEE is recommended as first-line diagnostic study to diagnose prosthetic valve endocarditis and assess for complications	C
5. TEE is recommended for preoperative evaluation in patients with known IE unless the need for surgery is evident on TTE and unless preoperative imaging will delay surgery in urgent cases	C
6. Intraoperative TEE is recommended for patients undergoing valve surgery for infective endocarditis	C
Class IIa
TEE is reasonable to diagnose possible infective endocarditis in patients with persistent staphylococcal bacteremia without known source	C
Class IIb
TEE might be considered to detect IE in patients with nosocomial staphylococcal bacteremia	C
Appropriateness Criteria	Appropriateness Score (1-9)
1. Evaluation of valvular structure and function to assess suitability for, and assist in planning of, an intervention	A (9)
2. To diagnose infective endocarditis with a low pretest probability (e.g., transient fever, known alternative source of infection, or negative blood cultures/atypical pathogen for endocarditis)	I (3)
3. To diagnose infective endocarditis with a moderate or high pretest probability (e.g., staphylococcal bacteremia, fungemia, prosthetic heart valve, or intracardiac device)	A (9)

Levels of evidence are defined as:

A, Data derived from multiple randomized clinical trials or metaanalysis.

B, Data derived from a single randomized study or from nonrandomized studies.

C, Consensus opinion of experts, cases studies or standards of care.

For the appropriateness score, A indicates the test is considered appropriate and I indicates it is inappropriate.

Adapted from Bonow RO, Carabello BA, Kanu C, et al: ACC/AHA 2006 guidelines for the management of patients with valvular heart disease. Circulation 114:e84-e231, 2006; and Douglas PS, Khandheria B, Stainback RF, et al: ACCF/ASE/ACEP/ASNC/SCAI/SCCT/SCMR 2007 appropriateness criteria for transthoracic and transesophageal echocardiography. J Am Soc Echocardiogr 20:787-805, 2007.

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