Left atrial appendage: structure, function, and role in thromboembolism

Abstract

The left atrial appendage (LAA) is derived from the left wall of the primary atrium, which forms during the fourth week of embryonic development. It has developmental, ultrastructural, and physiological characteristics distinct from the left atrium proper. The LAA lies within the confines of the pericardium in close relation to the free wall of the left ventricle and thus its emptying and filling may be significantly affected by left ventricular function. The physiological properties and anatomical relations of the LAA render it ideally suited to function as a decompression chamber during left ventricular systole and during other periods when left atrial pressure is high. These properties include the position of the LAA high in the body of the left atrium; the increased distensibility of the LAA compared with the left atrium proper; the high concentration of atrial natriuretic factor (ANF) granules contained within the LAA; and the neuronal configuration of the LAA. Thrombus has a predilection to form in the LAA in patients with atrial fibrillation, mitral valve disease, and other conditions. The pathogenesis has not been fully elucidated; however, relative stasis which occurs in the appendage owing to its shape and the trabeculations within it is thought to play a major role. Obliteration or amputation of the LAA may help to reduce the risk of thromboembolism, but this may result in undesirable physiological sequelae such as reduced atrial compliance and a reduced capacity for ANF secretion in response to pressure and volume overload.

Figure 1

A coronal section of the heart showing trabeculated appendages in contrast to the smooth walled atrial bodies.

Figure 2

Picture of a gross specimen of the heart with the pericardium. The position of the appendage between the left ventricle and pulmonary artery and in close relation to the pericardium can be appreciated. The operator's hand is emphasising a prominent branch, which is narrow and has an irregular edge.

Figure 3

Flow in and out of the appendage during sinus rhythm. A complex quadriphasic pattern can be appreciated. Two inflow (below line) and two outflow (above line) phases are present. During atrial systole (after the P wave) a clear period of flow out of the appendage is seen. This is followed by flow into the appendage during ventricular systole (QRS complex). Two further waves are seen: an outflow wave (ventricular relaxation), and an inflow wave (probably related to appendageal elasticity).

Figure 4

Left atrial appendage thrombi in a patient in atrial fibrillation. A small thrombus is present within the lumen of the appendage, and a larger one appears to be more fixed, and is present at the orifice.

Figure 5

Prominent trabeculations within the left atrial appendage can easily be misinterpreted as thrombi.

Figure 6

Left atrial appendage flow during atrial fibrillation, as determined by pulsed Doppler during transoesophageal echocardiography. Both the velocity and the pattern of flow have been related to the formation of thrombi and to the development of subsequent thromboembolic events. In this example, a well defined flow pattern is present and the maximum inflow velocity is approaching 0.4 m/s, a relatively high velocity. Thus the risk of thrombus formation in this patient would be relatively low.