The role of echocardiography in atrial fibrillation and cardioversion.(Imaging Techniques)

Heart 2003;89:1447-1454

Atrial fibrillation (AF) is the most commonly encountered arrhythmia in clinical practice. (1) Recent advances in technology and in the understanding of the pathophysiology of AF have led to more definitive and potentially curative therapeutic approaches. (1) In this setting, echocardiography has a unique and important role in the assessment of cardiac structure and function, risk stratification, and increasingly in guiding the management of AF. Because of its recognised value, echocardiography has become established in guidelines for management of AF (2) and utilisation of echocardiography has increased, particularly of transoesophageal echocardiography to guide direct current cardioversion or detect cardiac sources of embolism. Even more recently the development of intracardiac echocardiography has led to real-time guidance of percutaneous interventions, including radiofrequency ablation and left atrial appendage closure procedures for patients with AF.

In this review, we highlight the echocardiographic modalities that are available and their role in the evaluation and management of AF.

ATRIAL FIBRILLATION

AF affects approximately 0.4% of the general population and its prevalence is increasing. (3 4) AF frequently accompanies common conditions such as hypertension, chronic heart failure, and valvar or ischaemic heart disease, and is an important sequela of cardiothoracic surgery. (5) Importantly, AF is associated with significant mortality and morbidity, particularly from thromboembolic stroke. (3 6-9) The risk of stroke is greater in the elderly and with concomitant valvar (particularly rheumatic) heart disease; however, non-valvar AF is responsible for 75 000 strokes and hospitalisation costs of $1 billion dollars annually in the USA. (10) In addition, AF may be associated with reduced functional capacity and impaired cardiac performance, particularly when ventricular rates are not adequately controlled.

Despite a mounting disease burden, there have been significant advances in the management of AF because of a greater mechanistic understanding of pathophysiology. (1) The aetiology of AF is complex and may be modulated by factors such as autonomic tone, atrial wall stress, inflammation, and ischaemia. The arrhythmia is characterised by disorganised electrical activity with multiple re-entrant circuits in the atria that can lead to electrical and structural remodelling of the atria, which in turn reinforces the establishment of AF. Once changes in atrial structure and mechanical function become gross, they can be detected by echocardiography.

Perhaps most significant is the discovery that a proportion of AF cases caused by triggering from foci within the pulmonary veins can be successfully ablated with radiofrequency treatment. Additionally, the left atrial appendage (LAA) has become a focus of attention with surgical and percutaneous closure techniques becoming available. Echocardiography has an increasingly important therapeutic role in guiding these ablation and LAA closure procedures.

ECHOCARDIOGRAPHIC MODALITIES

Transthoracic echocardiography

Transthoracic echocardiography (TTE), including two dimensional (2D) imaging and complete Doppler assessment of valves, is recommended for all subjects with AF. (2) TTE allows rapid, safe, relatively comprehensive assessment of cardiac structure and function that can help to define the underlying aetiology of AF and the risk of complications. Recent advances such as harmonic imaging, alone or with micro-bubble contrast agents, allows enhanced endocardial border definition for assessment of left ventricular volumes and function. New modalities such as colour M mode (CMM) and tissue Doppler imaging (TDI) allow more accurate assessment of diastolic function and estimated filling pressures. Assessment of systolic and diastolic left ventricular function in AF may, however, be complicated by irregular RR interval and rapid ventricular rate. Transthoracic imaging usually provides suboptimal visualisation of the atrial appendages and has inadequate sensitivity and specificity for diagnosing LAA thrombus. (11) However, efforts continue to overcome this deficiency, including the use of microbubble contrast agents for better visualisation.

Transoesophageal echocardiography

Transoesophageal echocardiography (TOE), performed with 3-7 MHz multiplane transducers mounted on steerable modified fibroscopy probes, allows high resolution interrogation of posterior cardiac structures, including the atria, interatrial septum, and pulmonary veins. Most importantly, the atrial appendages can be visualised for thrombus. More accurate evaluation of valvar lesions, especially prosthetic dysfunction, is possible by TOE and alternative thromboembolic sources can be readily identified, including complex atheroma of the ascending thoracic aorta and arch. Although TOE is very safe, with major complications occurring in < 0.02% of procedures, it is not indicated for …