The Role of Echocardiography in AF

Atrial fibrillation is the most common persistent arrhythmia, with 0.4% of the U.S. population affected. The prevalence of AF increases with age, affecting 4% of people older than 60 and 8% of those older than 80.¹

The complications of AF are well recognized. The lack of effective atrial contraction results in changes in ventricular preload, and hence heart failure, low cardiac output and decreased vital organ perfusion. Rapid ventricular rate, which is frequently associated with AF, may result in myocardial ischemia or even tachycardia-mediated cardiomyopathy. Blood stagnation in the fibrillating atria may result in thrombus formation and potential embolization. These emboli are frequently the etiology of transient ischemic attacks, strokes and other vital organ damage. An estimated 20% of all strokes are due to AF.² Patients with AF are frequently symptomatic, with decreased quality of life. In developed countries, AF is a leading reason for medical care and hospitalization.³

Anatomical Assessment

Echocardiography is an important tool in the evaluation of AF. Transthoracic echocardiography (TTE) should be used to define atrial sizes, ventricular dimensions, wall thickness, global and segmental wall motion, valve function and pericardial abnormalities. Attention should also be paid to hemodynamic parameters such as left atrial (LA) pressure. LA pressure can be estimated from the ratio between peak early diastolic transmitral flow velocity (E wave) and the early diastolic mitral annular tissue velocity (E’). An E/E’ ratio of 15 or more is usually associated with elevated LA pressure (> 14 mm Hg).⁴

Assessment of right-sided structures and function is also important. Occasionally the etiology of AF may be related to pulmonary hypertension with associated right atrial enlargement and right ventricular dysfunction. Echocardiographic evaluation should include estimation of the right atrial pressure (from the echocardiographic assessment of the inferior vena cava diameter and respiratory variations) and the pulmonary artery pressure (from the Doppler assessment of systolic tricuspid regurgitation velocity and the pulmonic regurgitation end-diastolic velocity).

Risk Stratification

TTE findings can help to stratify risks for embolic stroke and the need for anticoagulation. For example, a patient with AF and significant left ventricular wall motion abnormality depicted by echocardiography will usually need anticoagulation. On the other hand, a young patient with normal clinical findings and normal echocardiogram (lone AF) may not need it. Occasionally, intra-atrial clots can be seen by TTE. However, sensitivity of TTE is low, and absence of clot on TTE does not rule out its presence (especially for clots located in the LA appendage).

In patients with nonvalvular AF, 90% of the LA clots are located in the LA appendage. Transesophageal echocardiography (TEE) is the technique of choice for evaluation of the LA appendage. TEE is much more sensitive than TTE in diagnosis of LA appendage clot.⁵ In addition, TEE can demonstrate echocardiographic markers of blood stagnation, including LA and LA appendage spontaneous contrast (“smoke”) and low LA appendage blood flow velocity.

TEE may therefore be indicated in patients with AF, especially if establishing the presence or absence of LA appendage clot is needed. TEE can also help diagnose other conditions that may cause stroke in patients with AF. One such condition is aortic arch atherosclerosis with protruding or mobile plaques, which is responsible for 12% of strokes in patients with or without AF.⁶

Cardioversion

TEE is especially important in the strategy and risk stratification of cardioversion in AF. Cardioversion without prior anticoagulation is associated with a 7% risk of stroke, and in the past, this association was believed to be due to the resumed sinus rhythm that resulted in mechanical activity of the atrium leading to the dislodgement of an LA clot. Anticoagulation decreased this risk to 1%.⁷ Subsequently, LA contractility was shown to diminish immediately after cardioversion, despite normal sinus rhythm. This atrial stunning is the result of the precardioversion AF.⁸

The stunning of the left atrium may result in blood stagnation while in normal sinus rhythm (even worse than during AF), with new clot formation and new risk of stroke. LA stunning may occur in electrical, chemical or spontaneous cardioversion. Therefore, AHA/ACC guidelines⁹ recommend that patients undergoing cardioversion (chemical or electrical) be anticoagulated for 3 to 4 weeks before the procedure to allow organization of possible atrial clot and to prevent the presence of a fresh clot at the time of cardioversion. These patients should also receive at least another 3 or 4 weeks of postcardioversion anticoagulation to prevent clot formation caused by postcardioversion LA stunning. This approach to cardioversion has been referred to as the conventional strategy.

A more recent TEE-guided strategy calls for early anticoagulation with intravenous heparin followed by TEE. If no atrial clot or dense LA spontaneous contrast (“sludge”) is present, the patient undergoes immediate cardioversion. The postcardioversion anticoagulation continues as in the conventional strategy. The more rapid, TEE-guided strategy is safe and therefore an acceptable alternative to the conventional strategy.¹⁰

Newer echocardiographic technologies will further improve our ability to assess and treat AF and its complications. Strain imaging is a useful tool in evaluation of atrial muscle mechanics and the stratification of risk of future AF.¹¹ Real-time three-dimensional TEE can identify atrial anatomical details and guide percutaneous, catheter-based ablation¹² and atrial appendage closure procedures.¹³ Echocardiography is a technique of unique value in the evaluation of many aspects of AF, including anatomy, hemodynamics, treatment strategies, complications and risk stratification. n

References:

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