Echocardiographic evaluation of the atria and appendages

INTRODUCTION — Imaging of the right and left atria and their appendages can provide important diagnostic and prognostic information. Left atrial (LA) imaging is especially important in patients with atrial fibrillation who are at increased risk for thromboembolic events resulting from left atrial or atrial appendage thrombus. Imaging of the LA has gained increased importance in association with new investigational procedures for atrial appendage occlusion in patients with atrial fibrillation used for stroke prophylaxis.

This topic will review the echocardiographic evaluation of the right and left atria and their appendages. The role of echocardiography in atrial fibrillation is discussed in detail elsewhere. (See "Role of echocardiography in atrial fibrillation".)

LEFT ATRIUM — The left atrium (LA) can be easily imaged in a number of views by transthoracic (surface) or transesophageal echocardiography. In addition to being a site of thrombosis in atrial fibrillation, the LA can be involved in a number of disease processes:

●It commonly dilates with age through a process that seems to be linked to "senile" left ventricular diastolic dysfunction and other diseases affecting the left ventricle such as hypertrophic, dilated, or restrictive cardiomyopathy.

●LA dilation occurs in association with chronic atrial fibrillation, Dilation can promote atrial fibrillation by perturbing propagation of orderly depolarization [1]. Conversely, permanent atrial fibrillation can promote LA dilation. (See "Mechanisms of atrial fibrillation".)

●It dilates with mitral valve disease, including both mitral stenosis and mitral regurgitation. (See "Rheumatic mitral stenosis: Clinical manifestations and diagnosis", section on 'Atrial fibrillation'.)

●It can be a site of infiltrative diseases such as amyloid cardiomyopathy.

●It occasionally becomes compressed by intrathoracic neoplasms, descending aortic aneurysms, hiatal hernias, spinal deformity, or fluid.

●It is rarely involved in myocardial infarction. (See "Supraventricular arrhythmias after myocardial infarction".)

●The LA is the most common site of origin of myxoma, the most common primary cardiac neoplasm. (See "Cardiac tumors".)

While most LA enlargement is pathological, physiologic LA enlargement may occur in association with increased stroke volume:

●Bradycardia

●Athletic heart [2]

●Pregnancy [3]

●Hyperthyroidism [4]

●Liver disease [5]

LA size and function can be estimated from the M-mode echocardiogram although two-dimensional biplane or three-dimensional quantitation is preferred. (See "Three-dimensional echocardiography", section on 'Assessment of atrial volumes'.)

M-mode echocardiogram — The standard M-mode image of the LA is taken from the long-axis parasternal view and represents the anterior to posterior dimension of that structure (image 1 and image 2). When using this methodology, it is critical to ensure that beam alignment is perpendicular to the long axis of the LA. This LA dimension is usually the smallest because of its shape and possibly its confinement between sternum and spine [6]. Thus, this dimension is the least sensitive to LA enlargement but, when increased, is a highly specific indicator of dilation [7].

LA dimension — One method for assessing LA size is to compare the LA dimension with the diameter of the aorta (image 3 and image 4). In normal subjects, these two measurements are about equal. As the LA enlarges, and in the absence of aortic dilatation, the LA diameter becomes significantly greater than that of the aorta.

Two-dimensional echocardiography — The LA can be visualized on two-dimensional transthoracic echocardiography from all of the standard imaging windows (parasternal, apical, subcostal). The LA size and volume, along with any other pathology, should be assessed. Transesophageal echocardiography can provide supplemental information about LA size and function and is of particular importance in the evaluation of LA thrombi [8,9]. (See 'Transesophageal echocardiography' below.)

LA volume — Because of the irregular shape of the LA and the single and largely unrepresentative dimension represented by the M-mode image, volume estimates are preferred for evaluating the LA [7]. These measurements require two-dimensional images, preferably two orthogonal apical planes (apical four and two chamber view) (figure 1 and image 5). The volume estimations using an area-length algorithm as well as the biplane method of disks have been validated using angiography and contrast-enhanced cardiac CT [6,10].

●In one study, the correlation between echocardiographic-derived LA volumes from the apical long-axis and four-chamber view with those obtained from contrast CT was very high if patients with very large atria (>300 mL) were included (r = 0.98) and was still quite good if these patients were excluded (r = 0.82) [10]. The echocardiographic method underestimates atrial volume by approximately 23 percent and the reproducibility of the measurement has a two standard deviation value of 10 mL.

●In another study which used gated CT angiography with automated reconstruction, the two-dimensional echocardiographic technique underestimated LA volume by 38 mL, likely due a combination of poor endocardial definition of atrial borders and, to a lesser extent, by foreshortening of the atrium [11].

The value of measuring LA volume to recognize pathology requires knowledge of normal population parameters. In males the volume is 18 to 58 mL, while in females it is 22 to 52 mL; the normal range for volume index is 21 to 28 mL/m² [12]. By measuring the atrium during both systole and diastole, one can gain a concept of normal atrial function. Mean end-systolic LA volume in a group of normal males and females is 40 mL and its smallest volume at end-diastole (after emptying both passively and actively) is 25 mL. If the normal forward stroke volume of the left ventricle is 65 mL, 15 mL comes from emptying of the atrial reservoir and 50 mL flows directly from the pulmonary veins across the mitral valve (ie, conduit volume).

Based upon mitral Doppler profiles of flow, most of reservoir left ventricular inflow (as contrasted with conduit flow) in the healthy young heart at normal heart rates enters by active left ventricular relaxation with relaxation of the atrial elastic forces (waveform 1). Atrial contraction accounts for only about 10 mL of atrial transport in the healthy young heart; this value more than doubles with aging [13,14]. Progress in developing atrial functional indices has been reported [13-17]:

●Reservoir function can be estimated by calculating the total LA emptying fraction, according to the formula:

●Conduit function can be estimated by calculating the LA passive emptying fraction and the conduit volume accordant to the following equations:

●Contractile function can be estimated by calculating the LA active emptying fraction using the equation:

The clinical utility of measuring these variables on a routine basis has not been established. Measures of LA strain and strain rate have also been applied in research studies.

Echocardiography has also been used to estimate the prevalence of LA enlargement in the general population. In a report from the Mayo Clinic, a random sample of over 2000 residents of Olmsted County (age ≥45) underwent echocardiography [18]. The prevalence of LA enlargement was 18 percent in men and 12 percent in women using LA diameter and 16 percent in both sexes using LA volume, both corrected for body surface area. LA enlargement was thought to reflect the burden of cardiovascular disease. LA dilatation is a powerful predictor of heart failure, stroke, and mortality [19-24].

The American Society of Echocardiography initially recommended a value of 28 mL/m² as the upper limit for normal LA volume index, and a value of >40 mL/m² for severe dilatation [12]. In a publication from the Normal Reference Ranges for Echocardiography (NORRE) of 734 normal volunteers, these values were reevaluated. In normal adults without cardiovascular pathology, the LA volume index could be as high as 40 mL/m² [25]. Other studies suggest an upper limit for normal of 34 mL/m². Recommendations from the American Society of Echocardiography were revised in 2015, and the new values for LA volume (indexed for body surface area in m²) are [26]:

●Normal – 16 to 34 mL/m²

●Mildly abnormal – 35 to 41 mL/m²

●Moderately abnormal – 42 to 48 mL/m²

●Severely abnormal – >48 mL/m²

LA strain — Studies suggest that speckle tracking-derived parameters of LA function may provide clinically helpful information. Speckle tracking has enabled the study of LA strain and strain rate and has eliminated the angle dependence of tissue Doppler. Since the LA is in the far field on transthoracic imaging and the atrial walls are less easily defined, measurements LA mechanics are more technically difficult than those of the left ventricle. Another issue is the considerable variability that can occur across different machine vendors [27]. When the QRS complex (ventricular end-diastole) is used as a reference for the zero baseline, the peak positive longitudinal strain (εs) corresponds to atrial reservoir function (occurring with maximal LA volume), and ε during early and late diastole (εe and εa, respectively) corresponds to conduit and atrial booster function.

A meta-analysis identified the following normal ranges, but there were high levels of heterogeneity among studies for each component [28]:

●Reservoir (40 studies) 39.4 (95% CI 38.0-40.8)

●Conduit (14 studies) 23.0 (95% CI 20.7-25.2)

●Contractile (18 studies) 17.4 (95% CI 16.0-19.0)

Because of the technical difficulties associated with the acquisition of these parameters, they have not been adopted for clinical use. However, research studies have found that global LA strain can predict maintenance of sinus rhythm after cardioversion and after pulmonary venous isolation [29]. Patients with severe organic mitral regurgitation display abnormalities in reservoir function with abnormal peak longitudinal εs, and those with greater impairments have worse prognosis after surgery [30]. Speckle tracking indices show impaired contractile function in mitral regurgitation and impaired conduit function in mitral stenosis [31]. It is likely that speckle tracking derived parameters of LA function will gain greater acceptance over time.

LA thrombus — The ability of transthoracic echocardiography to identify or exclude left atrial or atrial appendage thrombi is limited, with a reported sensitivity of 40 to 60 percent, due largely to poor visualization of the left atrial appendage, where most atrial thrombi reside [32,33]. In our clinical experience, few if any left atrial appendage thrombi can be detected in this manner. With this in mind, transesophageal echocardiography is the preferred technique for detection of LA thrombi [34]. (See 'Detection of thrombi' below.)

Transesophageal echocardiography — Although the LA and even the left atrial appendage can be imaged with transthoracic echocardiography (movie 1), transesophageal echocardiography (TEE) permits detailed examination of most of the LA, including excellent views of the left atrial appendage. (See 'Left atrial appendage' below.)

Detection of thrombi — TEE is the preferred approach for the detection of thrombi in the left atrium and appendage, given it is far more sensitive than transthoracic echocardiography (image 6 and image 7 and movie 2 and movie 3 and movie 4 and movie 5 and movie 6) [32,33]. In one study of patients undergoing mitral valve surgery, for example, TEE was 93 percent sensitive and 100 percent specific for thrombus, whereas the sensitivity of transthoracic echocardiography was only 53 percent [33].

Unless thrombi are very large and extend into the body of the LA, they are rarely identified by transthoracic echocardiography because the atrium lies in the far field of the interrogating ultrasound beam [35]. However, in one multicenter study, two atrial appendage thrombi were identified on TTE using harmonic imaging and left-sided echocardiographic contrast [36]. Although the left atrial appendage and its internal architecture (including thrombi) are readily imaged by transesophageal echocardiography, only its outline is apparent by transthoracic echocardiography.

LA thrombi are present on TEE in approximately 13 percent of nonanticoagulated patients with atrial fibrillation [37-39] and more often (33 percent in one series) in patients with rheumatic mitral stenosis [37-40]. Among patients with atrial fibrillation who have had an acute thromboembolic event, residual thrombus is rarely seen on transthoracic echocardiography but can be detected by TEE in approximately 45 percent of cases [41,42].

LA thrombi are often multiple and vary in size and, although they attach to the atrial wall, they usually demonstrate some degree of independent motion. Small thrombi must be distinguished from the normal trabeculations (pectinate muscles) of the left atrial appendage, and larger thrombi may be difficult to distinguish from tumor. (See "Contrast echocardiography: Clinical applications".)

The finding of spontaneous echo contrast, indicative of predisposing stasis, almost always accompanies thrombus and may be helpful in differentiating thrombi from tumor or normal anatomy (image 6 and image 7 and movie 2 and movie 3 and movie 6). (See "Role of echocardiography in atrial fibrillation".)

Thrombus in sinus rhythm — Atrial thrombi infrequently occur in the setting of sinus rhythm [43]. In one series of over 20,600 TEE examinations, a LA thrombus during sinus rhythm was detected in 0.1 percent of all examinations, accounting for 6 percent of all TEE examinations showing LA thrombus [44]. Atrial thrombus during sinus rhythm was usually associated with specific structural cardiac abnormalities (left ventricular dysfunction or mitral valve stenosis, either native or prosthetic) and/or a history of atrial fibrillation.

In patients with embolic stroke who manifest sinus rhythm at the time of presentation, long-term monitoring (30 days) detects a five-fold greater incidence of atrial fibrillation compared with 24 hours of monitoring [45]. With longer-term monitoring using an implantable monitoring device, the incidence was approximately 9 percent at six months, 12 percent at 12 months, and 30 percent at three years [46]. Although paroxysmal atrial fibrillation is frequently found after cryptogenic stroke associated with sinus rhythm, the absence of a temporal relationship between the events suggests that another mechanism might be present. There has been increasing evidence for an atrial cardiomyopathy which can lead to atrial thrombus before the onset of atrial fibrillation [47]. These data suggest that it may be reasonable to search for LA thrombi in those with cryptogenic stroke even when sinus rhythm is present. (See "Cryptogenic stroke and embolic stroke of undetermined source (ESUS)" and "Paroxysmal atrial fibrillation".)

Left atrial masses and tumors — In addition to thrombi, the LA may be the site of other masses and tumors. Myxoma is the most common tumor seen in the LA (movie 7 and movie 8 and image 8 and image 9 and image 10). These benign tumors most commonly arise from the inferior limb of the fossa ovale. They become manifest when they either embolize or cause obstruction to mitral inflow. (See "Cardiac tumors".)

The echocardiographic appearance of LA tumors can yield clues to their identity as myxoma; in some cases, a TEE is of more use (image 9 and image 10).

●If the tumor is encapsulated, clear spaces that represent cysts and highly reflective patches representing calcification can be appreciated.

●Careful inspection of an encapsulated tumor also demonstrates the stalk of attachment at its typical location along the interatrial septum.

●If the tumor is more amorphous, its attachment is usually broad based and may have a polypoid appearance. The reflectance or ultrasonic brightness of these masses is much less vivid.

●Since myxomas are occasionally biatrial, careful inspection of the right atrium is warranted.

LEFT ATRIAL APPENDAGE — The left atrial appendage lies within the confines of the pericardium in close relation to the free wall of the left ventricle. The left atrial appendage is sometimes visualized on transthoracic imaging in the parasternal short axis through the cardiac base and the apical two chamber view [48]. The left atrial appendage is long and thin with a narrow base of origin from the body of the left atrium. Its narrow, sharply pointed shape allows positive identification. The left atrial appendage is frequently multilobulated.

The left atrial appendage is best evaluated by TEE [48]. The widespread use of this high resolution technique has led to the realization that clinically significant thrombi are commonly sequestered in this structure; these thrombi are rarely seen with transthoracic imaging (image 6 and movie 5 and movie 6). Thrombus has a predilection to form in the dysfunctional left atrial appendage, perhaps because of its shape and the presence of trabeculations [49], and it is most common in patients with atrial fibrillation who are not anticoagulated. Similarly, in patients with mitral stenosis being considered for balloon mitral valvuloplasty, TEE is routinely performed to exclude thrombi in the left atrium and appendage so that dislodgement due to the catheter can be avoided. One study suggested that the use of left-sided contrast can enhance the evaluation of the atrial appendage and demonstrate filling defects suggestive of thrombus not visualized on the unenhanced image [50]. (See 'Detection of thrombi' above.)

The evaluation of the LA appendage morphology has become increasingly important with the development of percutaneous closure devices. The diameter of the mouth of the appendage, the number and size of the lobes, and the presence of preexisting thrombus may affect the efficacy and safety of the procedure [51,52]. TEE is used to evaluate preprocedural morphology as well as efficacy of closure [53].

Left atrial appendage function — The left atrial appendage is believed to function as a decompression chamber during left ventricular systole and other periods when LA pressure is elevated [49]. The function of the left atrial appendage on TEE has been studied in patients at risk for the development of LA thrombi [48,54,55]. A pulsed wave Doppler sample, placed within the left atrial appendage, reveals a characteristic pattern that is dependent upon the patient's underlying rhythm and atrial function (waveform 2A-B). In patients with sinus rhythm, there are well-defined filling and emptying waves (peak emptying velocity >55 cm/sec), appropriately timed with atrial contraction; there is a decrease in filling and emptying velocity of 2 and 4 cm/sec for each 10 year increase in age [56]. However, among patients with atrial fibrillation, some have well-defined wave forms while others have low velocity, poorly defined wave forms [48,54,55]. Thrombi and spontaneous echo contrast are significantly more likely to occur in the latter group with low velocity wave forms. Additionally, in one study, left atrial appendage flow velocity was related to the success of cardioversion; a flow velocity >31 cm/sec was a predictor of electrical or pharmacologic cardioversion success (odds ratio 2.8) [57]. In another study, a left atrial appendage emptying velocity >40 cm/sec, measured precardioversion was predictive of one-year maintenance of sinus rhythm [58]. (See "Cardioversion for specific arrhythmias".)

RIGHT ATRIUM

Size — M-mode techniques for measuring the right atrium (RA) have not been developed. Two-dimensional imaging can be accomplished from the four-chamber and subcostal views and volumes can be measured using an area length algorithm by a single plane tracing from the four-chamber view (figure 2). Most often, the size of the RA is judged qualitatively by comparing it with the left atrium (LA) and the right ventricle. Normally, the atria appear to be similar in size and smaller than their respective ventricle. There are data compiled on more than 2400 normal individuals to provide normal limits for RA dimensions [26] and a smaller set of data to provide ranges for normal volumes [25,26]. The reference values for normal RA volume have been recommended by the American Society of Echocardiography. Using the single plane Simpson's rule, the normal range for women is 21±6 mL/m² and for men is 25±7 mL/m² [26].

Data from a three-dimensional echocardiographic study showed that the mean RA volume among 35 normal subjects was 41±13 mL [59]. (See "Three-dimensional echocardiography", section on 'Assessment of atrial volumes'.)

Structure — Normal RA structures and their variants should be recognized to avoid confusion with pathologic lesions. The Eustachian valve is located at the junction of the inferior vena cava and the RA and it is prominent in some individuals. Rarely, it extends to insert on the interatrial septum and must be distinguished from cor triatriatum dexter, which is a rare congenital condition [60]. The Chiari network is a more fenestrated mobile structure at the junction of the vena cavae and the RA. Although these structures generally lack clinical significance, Eustachian valve endocarditis has rarely been reported [61]. In addition, the presence of a large Eustachian valve may potentiate the risk of cryptogenic stroke in conjunction with a patent foramen ovale, but this finding is inconsistent among different studies [62,63]. (See "Atrial septal abnormalities (PFO, ASD, and ASA) and risk of cerebral emboli in adults".)

In contrast to the left atrial appendage, the right atrial appendage is broad-based and less distinct in appearance. The trabeculations (pectinate muscles) extend toward the tricuspid valve and are not confined to the appendage.

Right atrial thrombus — Thrombus formation is less common in the right compared with the LA. It occurs in association with in-dwelling catheters, especially at the junction of the superior vena cava and RA.

Thrombi also can form in the right atrial appendage in patients with atrial fibrillation or prothrombotic states. However, the shallow anatomy of the right atrial appendage makes it a less likely site for thrombus formation. Among patients with AF, right atrial appendage thrombi occur in 3 to 6 percent of cases compared with approximately 13 percent with LA thrombi [37-39,64]. Other conditions that predispose to RA thrombus include tricuspid stenosis or a tricuspid valve prosthesis. The majority of patients with RA thrombi also have LA thrombi [65]. Cardioversion should be deferred even if patients have isolated right atrial thrombi.

Pulmonary emboli can rarely be imaged in transit or entrapped within the RA. On occasion, an embolus will become enmeshed in the foramen ovale as it paradoxically passes from right atrium to LA. (See "Echocardiography in detection of cardiac and aortic sources of systemic embolism", section on 'Abnormalities of the interatrial septum'.)

Right atrial tumors — Tumors of the RA are unusual, and tend to become large before the onset of symptoms (figure 3). Because surface imaging is frequently difficult, TEE appears to have incremental value in the diagnosis and characterization of these lesions [66-68]. The most primary common benign cardiac neoplasm is myxoma, approximately 15 percent of which arise in the RA. Lipomas are also common primary RA tumors. As with the LA, angiosarcoma of the RA is the most common primary malignant cardiac tumor.

A far more common cause of tumors within the RA are extensions of those that arise in the abdomen and pelvis, especially hepatoma and renal cell carcinoma, which directly invade the inferior vena cava and extend into the right heart [69-72]. Although subcostal surface imaging may define the tumor, TEE, especially the longitudinal bicaval view, can better elucidate the tumor arising from the inferior vena cava into the RA. Surgical excision may result in protracted palliation from symptoms of right-sided obstruction.

VENA CAVA — The inferior vena cava (IVC) and its junction with the right atrium (RA) can be visualized in the subcostal view by surface echocardiography, as well as in the bicaval TEE view. Inferior vena cava size and its respiratory variation can be used to estimate RA pressure [73,74].

The superior vena cava (SVC) can also be visualized in the bicaval TEE view. This view permits visualization of the course of central lines in the SVC as well as transvenous pacemaker leads in the SVC extending into the RA and right ventricle. Catheter or lead associated vegetations or thrombus may be identified, although artifact and acoustic windows may limit visualization.

SUMMARY AND RECOMMENDATIONS

●The left atrium (LA) can be easily imaged in a number of views by transthoracic echocardiography. LA size and function can be estimated on transthoracic imaging using M-mode echocardiography, although two-dimensional biplane or three-dimensional quantitation is preferred. (See 'Left atrium' above.)

●Although the LA and even the left atrial appendage can be imaged with transthoracic echocardiography, transesophageal echocardiography (TEE) permits detailed examination of most of the LA, including excellent views of the left atrial appendage. TEE is the preferred approach for the detection of thrombi in the left atrium and appendage, given it is far more sensitive than transthoracic echocardiography. (See 'Transesophageal echocardiography' above.)

●The value for normal LA size has been revised by the American Society of Echocardiography. Using the biplane Simpson's rule for the LA, the upper limit of normal is 34 mL/m² for both men and women. (See 'LA volume' above.)

●Transthoracic two-dimensional imaging of the right atrium (RA) can be accomplished from the four-chamber and subcostal views, and volumes can be measured using an area length algorithm by a single plane tracing from the four-chamber view. Normal RA structures and their variants (ie, Eustachian valve and Chiari network) should be recognized to avoid confusion with pathologic lesions. (See 'Right atrium' above.)

●The reference values for normal RA volume have been recommended by the American Society of Echocardiography. Using the single plane Simpson's rule, the normal range for women is 21±6 mL/m² and for men is 25±7 mL/m². (See 'Size' above.)

●The inferior vena cava (IVC) and its junction with the RA can be visualized in the subcostal view by transthoracic echocardiography, while the superior vena cava (SVC) is best visualized using TEE. (See 'Vena cava' above.)