Role of echocardiography in atrial fibrillation

INTRODUCTION — Atrial fibrillation (AF) is the most common treated arrhythmia. Echocardiography plays a key role in evaluation and management of patients with AF.

The topic will review the use of echocardiography in evaluating patients with AF. An overview of AF is presented separately. (See "Atrial fibrillation: Overview and management of new-onset atrial fibrillation".)

OBJECTIVES — The role of echocardiographic imaging among patients with AF can be divided into two main categories:

●Assessment of cardiac chamber sizes and function, the atrial contribution to left ventricular filling, the pericardium, and valvular function. This information may be helpful in determining the conditions associated with AF, the risk for recurrent AF following cardioversion, and the hemodynamic benefit of maintaining sinus rhythm. This information is generally obtained from transthoracic echocardiography (TTE), with moderately invasive transesophageal echocardiography (TEE) generally reserved for assessment of the left atrial appendage for thrombus prior to cardioversion. (See "Epidemiology, risk factors, and prevention of atrial fibrillation" and "Antiarrhythmic drugs to maintain sinus rhythm in patients with atrial fibrillation: Recommendations".)

●Identification of patients at increased risk for thromboembolic complications of AF before cardioversion and in patients with chronic AF. (See "Prevention of embolization prior to and after restoration of sinus rhythm in atrial fibrillation" and "Atrial fibrillation in adults: Selection of candidates for anticoagulation".)

INDICATIONS — Nearly all patients presenting with their first episode of AF will benefit from transthoracic (surface) echocardiographic (TTE) evaluation of left atrial size, left ventricular cavity size and regional/global systolic function, and mitral valve morphology and function. Examination of prior TTE data (if available) may allow for assessment of the atrial contribution to left ventricular filling (transmitral Doppler peak A wave velocity) when the patient is in sinus rhythm so as to have an assessment of the atrial contribution to ventricular filling. (See 'Transthoracic echocardiography' below.)

A more selected subgroup may benefit from the additional information obtained from transesophageal echocardiographic (TEE) evaluation for left atrial thrombi to allow for early cardioversion if no thrombi are identified. (See 'Transesophageal echocardiography' below and "Prevention of embolization prior to and after restoration of sinus rhythm in atrial fibrillation".)

TRANSTHORACIC ECHOCARDIOGRAPHY — TTE provides detailed information about cardiac anatomy and function. In comparison to TEE, TTE is less useful for the detection of atrial thrombus, especially thrombus in the right atrial appendage or left atrial appendage (LAA), which is better detected by TEE. (See 'Left atrial thrombi' below.)

Left atrial size — TTE is particularly helpful in assessing the size of the body of the left atrium. The normal left atrial dimension in adults is less than 4.0 cm (or less than 2.0 cm/m² body surface area) or biplane derived left atrial volume index of less than 34 mL/m² body surface area. Left atrial enlargement is common in AF, particularly in patients with mitral valve disease (both stenosis and regurgitation), left ventricular cavity dilation, annular calcification, or hypertension [1]. In addition, sustained AF itself can lead to a further increase in left atrial size [2], an effect that is reversible after cardioversion and maintenance of sinus rhythm [3]. Pulsed Doppler studies have shown that the time to recovery of atrial mechanical function is directly related to the duration of AF (eg, within 24 hours in patients with AF for less than 2 weeks; up to a week for patients who have been in AF for two to six weeks; and up to a month for patients with sustained AF for more than six weeks) [4]. However, the routine serial assessment of atrial mechanical function recovery is not recommended and TTE Doppler assessment of atrial recovery does not predict long term maintenance of sinus rhythm. (See "Echocardiographic evaluation of the atria and appendages".)

Regardless of the mechanism, left atrial enlargement is important prognostically. It decreases the probability that long-term maintenance of sinus rhythm will be successful [5-7]. Patients with chronic (more than one year) AF, rheumatic mitral valve disease, or severe left atrial enlargement (dimension greater than 6.0 cm or left atrial volume greater than 48 mL/m²) are at greatest risk for recurrent AF [7]. If, however, the duration of AF is brief, an attempt at cardioversion is reasonable for most patients regardless of absolute left atrial size. (See "Atrial fibrillation: Cardioversion".)

Although TTE can provide anatomic imaging of the body of the left atrium, TEE is preferred when looking for left atrial thrombi and assessing LAA and right atrial appendage anatomy and function (abnormalities of which predispose to the thrombus formation), as these areas are not well seen on TTE. Few data are known regarding the impact of AF on atrial appendage anatomy, though sustained AF does lead to progressively more impaired LAA ejection velocity. (See 'Transesophageal echocardiography' below.)

Mitral valve function — TTE is quite useful in the assessment of mitral valve anatomy and function, which can influence the risk of thrombus formation. As an example, occult mitral stenosis in the adult may initially present with AF, often in the setting of acute thromboembolism. In this setting, long-term oral anticoagulation is indicated even if cardioversion to sinus rhythm is successful and independent of CHA₂DS₂-VASc score (these clinical thromboembolism scores were derived from non-valvular AF populations). Long-term maintenance of sinus rhythm is unlikely unless the mitral stenosis (by surgery or percutaneous balloon mitral valvuloplasty) or severe mitral regurgitation (surgical repair or replacement) is corrected. (See "Surgical and investigational approaches to management of mitral stenosis" and "Percutaneous mitral balloon commissurotomy in adults".)

Mitral regurgitation is commonly found among patients with AF. More than moderate mitral regurgitation appears to protect against clinical thromboembolism in chronic AF, presumably by minimized stasis in the left atrium and LAA [8-11] (see "Mechanisms of thrombogenesis in atrial fibrillation"). However, mitral regurgitation does not appear to protect from the formation of LAA thrombus as identified on TEE.

As mentioned, examination of a prior TTE when the patient is in sinus rhythm (if available) will provide useful information on the relative atrial contribution to total left ventricular filling by examining the transmitral peak A wave velocity or peak E to peak A wave ratio. For those patients with a relatively high/large transmitral A wave, the contribution of left atrial systole to left ventricular filling is greater. Thus, these patients may derive greater hemodynamic benefit from rhythm control.

Left ventricular function — All patients with newly discovered AF should have a TTE to assess for LV size and function and other structural cardiac conditions that may impact treatment. TTE assessment of left ventricular systolic function helps to guide the choice of pharmacologic therapy for ventricular rate control in chronic AF. (See "Control of ventricular rate in patients with atrial fibrillation who do not have heart failure: Pharmacologic therapy".)

TTE can also detect left ventricular hypertrophy, focal wall motion abnormalities suggestive of myocardial infarction, and conditions less frequently associated with AF, including pericarditis (pericardial effusion), pulmonary embolus (dilated and poorly functioning right ventricle), and aortic stenosis (AF is generally poorly tolerated in this disorder and does not occur until very late in the disease). (See "Medical management of asymptomatic aortic stenosis in adults", section on 'Atrial fibrillation'.)

Left ventricular dysfunction, as determined from the TTE, independently predicts an increased risk of a stroke in patients with AF. Analysis of 1066 patients entered into three prospective clinical trials evaluating the role of anticoagulation in nonvalvular AF (BAATAF, SPINAF, and SPAF) found that, among patients in the placebo or control groups, the incidence of a stroke was 9.3 percent per year in patients with moderate to severe left ventricular dysfunction compared with 4.4 percent per year in those with normal or mildly abnormal left ventricular systolic function (figure 1) [12]. The predictive value of left ventricular dysfunction for thromboembolic risk has been confirmed in many other studies. (See "Atrial fibrillation in adults: Selection of candidates for anticoagulation", section on 'CHA2DS2-VASc score'.)

While TTE is recommended for all patients presenting with their first episode of AF, repeated TTE is not indicated when the patient has recurrent episodes unless there is a concern that the clinical situation has changed (eg, new heart failure).

TRANSESOPHAGEAL ECHOCARDIOGRAPHY — The preceding observations provide the rationale for the performance of TTE in all patients presenting with their first episode of AF. On the other hand, TEE should be reserved for patients in whom the diagnostic information will lead to alterations in therapy. It is a moderately invasive imaging technique that provides superior visualization of posterior structures, such as the left atrium and left atrial appendage (LAA).

TEE has particular value in estimating thromboembolic risk in different clinical settings:

●It can detect LAA and right atrial appendage thrombi (movie 1 and movie 2) for patients being considered for early cardioversion. In this setting, there is little additional benefit from TTE prior to TEE, as most of the necessary information can be derived from the TEE with its superior assessment of both appendages. (See "Prevention of embolization prior to and after restoration of sinus rhythm in atrial fibrillation".)

While data suggest a role of TEE for patients who have not received a full month of therapeutic anticoagulation prior to cardioversion, there does not appear to be a role for routine TEE prior to cardioversion in patients who have been adequately anticoagulated with warfarin or direct oral anticoagulant (DOAC) for at least four weeks prior to cardioversion. However, TEE immediately prior to elective cardioversion should be considered for those patients at increased risk for left atrial thrombi (eg, rheumatic mitral valve disease, recent/prior thromboembolism, severe left ventricular systolic dysfunction) or those with a transiently subtherapeutic international normalized ratio (INR) or who have missed doses of DOAC in the month prior to elective cardioversion. (See "Prevention of embolization prior to and after restoration of sinus rhythm in atrial fibrillation".)

●Among patients in paroxysmal or chronic AF, abnormalities in the LAA (thrombus, dense spontaneous echo contrast, or flow velocity ≤20 cm/s) or the presence of a complex aortic plaque increases the risk of a thromboembolic event and are more likely in patients with clinical risk factors for thromboembolism (figure 2) [13].

●Another role of TEE is for assessment of the adequacy of complete exclusion of the LAA for those undergoing surgical or percutaneous (eg, Watchman device) LAA occlusion. This assessment is helpful in determining the duration of anticoagulation following LAA occlusion. (See "Atrial fibrillation: Left atrial appendage occlusion".)

Left atrial thrombi — A main advantage of TEE is that it provides superior visualization of posterior structures, such as the left atrium and LAA as well as the anterior right atrial appendage. This is particularly important for the detection of thrombi, spontaneous echocontrast (a precursor to thrombus), and depressed atrial appendage ejection velocities as these metrics are not assessable with TTE.

The ability of TTE to identify or exclude left atrial or atrial appendage thrombi (as well as right atrial appendage thrombi) is quite limited, with a reported sensitivity of 39 to 63 percent, due largely to poor visualization of the LAA [14,15]. By contrast, TEE permits detection of thrombus in both the left atrium (movie 3 and movie 4) and the LAA (movie 1 and movie 2).

TEE evidence of thrombus in the body of the left atrium is very uncommon. The vast majority of thrombi are seen in the LAA with thrombi seen in approximately 13 percent of patients presenting with nonrheumatic AF of more than three days duration [16-18]. The prevalence is increased in high-risk patients with mitral stenosis (33 percent in one series) [19], left ventricular systolic dysfunction, enlargement of the left atrium or LAA, spontaneous echo contrast, a recent thromboembolic event (43 percent in one report) [20], and CHADS2 score [21]. Recurrent embolization in the last setting may be due to migration of the residual thrombus. On the other hand, the apparent lack of atrial thrombi in 57 percent of these patients probably reflects migration of the entire thrombus during the embolic event, a thrombus not visualized by TEE due to its small size, or another source for the embolus. Thrombus in the right atrial appendage is far less common.

The sensitivity and specificity of TEE for left atrial thrombi (in patients in whom the left atrium was directly examined at surgery) are 93 to 100 percent and 99 to 100 percent, respectively [14,15]. In a review of 231 patients in whom only 5.2 percent had a left atrial thrombus, TEE has a positive and negative predictive value of 86 and 100 percent, respectively [14]. For patients who are not candidates for TEE due to esophageal stricture or another contraindication, intracardiac echocardiography with the catheter in the main pulmonary artery has been shown to be at least as efficacious as TEE for identifying atrial appendage thrombi [22].

One potential limitation of these studies is that they were performed by experienced operators and the accuracy may not be replicable at all institutions. Additionally, the complementary role of three-dimension real-time TEE for atrial appendage thrombus is unknown. The use of an endocardial border definition echocontrast agent may help define small thrombi in the atrial appendage [23]. (See "Prevention of embolization prior to and after restoration of sinus rhythm in atrial fibrillation".)

Studies from the Stroke Prevention in Atrial Fibrillation (SPAF) investigators confirmed the usefulness of TEE for predicting thromboembolism [13,24]. This study involved 786 patients with nonrheumatic AF, 382 of whom were at high clinical risk for a thromboembolism (eg, women >75 years of age and patients with systolic blood pressure >160 mmHg or a history of previous thromboembolism, impaired left ventricular function, or recent congestive heart failure). The rate of stroke was increased over threefold when TEE evidence of dense spontaneous echo contrast was present, increased by threefold for reduced (<20 cm/second) LAA peak ejection velocity and for LAA thrombus, and increased by fourfold by complex aortic plaque.

Spontaneous echo contrast — Spontaneous echo contrast (SEC or "smoke") refers to the presence of dynamic, smoke-like echoes seen during TEE in the left atrium or atrial appendage (movie 3 and movie 2). Although most widely studied in the left atrium, SEC also occurs in the right atrium [16,25]. (See 'Right atrial thrombi' below.)

SEC is thought to reflect increased erythrocyte aggregation caused by low shear rate due to altered atrial flow dynamics and uncoordinated atrial systole [26,27]. Erythrocyte aggregation is mediated by plasma proteins, especially fibrinogen, which promotes red cell rouleaux formation by moderating the normal electrostatic forces (due to negatively charged membranes) which keep erythrocytes from aggregating [28].

SEC is a strong risk factor for and may be the preceding stage to thrombus formation and thromboembolic events [13,24,29,30]. The following clinical characteristics of SEC have been identified:

●SEC is present in over 50 percent of all patients with AF and in over 80 percent of those with LAA thrombi or a recent thromboembolic event [12,13,20,24,25,29,30]. Furthermore, serial TEE studies have shown that SEC subsequently develops in many patients with chronic AF (44 percent in one report) who do not have SEC on their initial TEE [29].

●The LAA peak outflow velocity can be estimated by TEE and SEC semiquantitatively graded as marked or dense if present throughout the entire cardiac cycle, or faint when intermittent [24,29]. The risk of thromboembolism increases as these parameters worsen [24].

●SEC is associated with clinical risk factors for thromboembolism, including a prior thromboembolic event, left ventricular systolic dysfunction, and hypertension (figure 2) [24,31] as well as CHADS2 risk score. On the other hand, it is less common in patients with mitral regurgitation [29] which, as noted above, appears to protect against clinical thromboembolism in chronic AF, presumably by minimized stasis in the left atrium and atrial appendage [8-11].

●Warfarin, which leads to thrombus resolution and a lower incidence of thromboembolism, does not affect the presence of SEC, since it does not change the underlying hemodynamic abnormality [28,30,32]. (See "Atrial fibrillation in adults: Use of oral anticoagulants".)

Blood flow velocity — The ability to estimate blood flow velocity in the left or right atrium and left and right atrial appendage permits a more quantifiable measure of stasis. A low LAA blood flow velocity (less than 20 cm/second) is associated with the presence of appendage thrombus [25,29,33] and with denser SEC [24,29]. The risk of stroke increases sharply with marked reductions in blood flow velocity (<15 cm/sec), particularly in the LAA or posterior left atrium [34].

It has been suggested that left atrial blood flow velocity also may be a predictor of the likelihood of maintaining sinus rhythm after cardioversion. In one report, a high peak LAA blood flow velocity (>40 cm/sec) identified patients with an increased likelihood of remaining in sinus rhythm one year after cardioversion [35]. In comparison, low blood velocity was of limited predictive value. Other reports have been conflicting on the predictive value of low LAA blood flow velocity for the maintenance of sinus rhythm [36,37].

An explanation for ongoing thromboembolism in patients with paroxysmal AF and apparently maintained sinus rhythm may be related to a mechanical discordance between the body of the left atrium and the LAA (ie, an AF LAA pulse wave Doppler phenotype with sinus rhythm electrocardiogram and body of the left atrium motion) [38]. The reproducibility and consistency of this finding are unknown, but retrospective data suggest a discordance in up to 25 percent of patients with paroxysmal AF.

Right atrial thrombi — Few data are available comparing the sensitivity, specificity, and accuracy of TTE and TEE for right atrial and right atrial appendage thrombi, but the right atrial appendage is rarely seen by TTE. By contrast, right atrial or atrial appendage thrombi are also easily seen by TEE (image 1). They are much less common than left atrial thrombi in patients in AF, occurring in 3 to 6 percent of cases (versus 15 to 20 percent for left atrial thrombi) [16,25]. The majority of patients with right atrial thrombi also have markedly depressed right ventricular systolic function, rheumatic tricuspid stenosis or prosthetic valve, or left atrial thrombi [12]. Cardioversion should be deferred even if patients have isolated right atrial thrombi.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Atrial fibrillation".)

SUMMARY AND RECOMMENDATIONS

●Routine performance of transthoracic echocardiography is suggested for all patients presenting with their first episode of atrial fibrillation (AF) to obtain information regarding atrial size, ventricular function, valvular function, and possible pericardial effusion. Repeated transthoracic echocardiographic examinations for recurrent presentations of AF are not necessary unless the clinical presentation has changed. If available, data from prior transthoracic echocardiography (TTE) when the patient was in sinus rhythm is useful to determine the patient's relative dependence on sinus rhythm/atrial contribution to total left ventricular filling. (See 'Indications' above and 'Transthoracic echocardiography' above.)

●The main advantage of moderately invasive transesophageal echocardiography (TEE) is its ability to detect left and right atrial appendage thrombi and patients at risk for thrombi because of the presence of spontaneous echo contrast or reduced left atrial appendage (LAA) blood flow velocity as well as aortic plaque. The main clinical use of TEE for AF is in the management of early cardioversion in patients with AF of more than 48 hours or high-risk patients with AF of shorter duration who are candidates for cardioversion. (See 'Indications' above and 'Transesophageal echocardiography' above and "Prevention of embolization prior to and after restoration of sinus rhythm in atrial fibrillation".)

●There does not appear to be a role for routine TEE prior to cardioversion in patients who have been adequately anticoagulated with warfarin or direct oral anticoagulant (DOAC) for at least four weeks prior to cardioversion. However, TEE immediately prior to elective cardioversion should be considered for those patients at increased risk for left atrial thrombi (eg, rheumatic mitral valve disease, recent/prior thromboembolism, severe left ventricular systolic dysfunction) or those with a transiently subtherapeutic international normalized ratio (INR) or who have missed doses of DOAC in the month prior to elective cardioversion. (See 'Indications' above and 'Transesophageal echocardiography' above and "Prevention of embolization prior to and after restoration of sinus rhythm in atrial fibrillation", section on 'Transesophageal echocardiography-based approach'.)