INTRODUCTION
The left atrium is an oval chamber with thin, muscular walls, located between the aorta and the esophagus. Because of this location, it is easily recognized not only by transthoracic but also by transesophageal echocardiography. The left atrium collects blood from the pulmonary veins and ejects it into the left ventricle. The left atrial (LA) chamber changes its size during a cardiac cycle, being largest at ventricular systole and smallest at atrial systole. This phasic change involves mainly the anteroposterior and supero-inferior diameters, while the mediolateral diameter does not change significantly. The left atrium has three functions in a cardiac phase (see Chapter 4 ). During ventricular systole, the left atrium functions as a reservoir that collects pulmonary venous flow; and during early diastole, following passive atrial emptying, it functions as a conduit allowing the passage of stored blood from the left atrium to the left ventricle. During atrial contraction, the left atrium acts as a contractile pump that delivers as much as one-third of the left ventricular (LV) filling. This atrial contraction makes a significant contribution to maintaining cardiac output, especially in patients with LV dysfunction.
Because the left atrium is connected to the left ventricle, enlargement of LA size suggests the presence of elevated filling pressure and diastolic dysfunction. Thus, LA size has been noted to be a good prognosticator in various cardiovascular diseases. For example, increased LA size is associated with atrial fibrillation, stroke, and adverse cardiovascular outcomes. LA size has also been known to correlate with overall mortality after myocardial infarction and risk of death and hospitalization in patients with dilated cardiomyopathy.
PATHOPHYSIOLOGY
Quantification of Left Atrial Size
Measurement of Left Atrial Diameter
LA size is usually measured at end systole, when the left atrium is most dilated. Traditionally, it is measured at the parasternal long-axis view using M-mode or B-mode echocardiography ( Fig. 13-1 ). When the M-mode technique is used, the cursor should pass through the aortic valve. The largest anteroposterior diameter is measured from the trailing edge of the posterior aortic wall to the leading edge of the posterior LA wall. Although the convention for M-mode is to measure from the leading edge to the leading edge, the trailing edge of the posterior aortic root is recommended by the American Society of Echocardiography, to avoid the variable extent of space between the left atrium and the aortic root. When the M-mode cursor is not perpendicular to the posterior aortic wall or the posterior LA wall, measurement based on B-mode echocardiography should be done.
Measurement of Left Atrial Area by Echocardiography
The left atrium can be observed at multiple views besides the parasternal long axis. It can also be observed at apical two- and four-chamber views and a parasternal short-axis view, and a diameter can be measured at each. Anteroposterior and supero-inferior diameters can be measured at the parasternal long-axis view. Anteroposterior and mediolateral diameters can be measured at the parasternal short-axis view, and supero-inferior and mediolateral diameters can be measured at the four-chamber view ( Fig. 13-2 ). The normal values for each diameter are shown in Table 13-1 .
| MEAN ± SD (cm) | RANGE (cm) | |
|---|---|---|
| Parasternal long-axis view | ||
| Anteroposterior diameter | 3.0 ± 0.3 | 2.3-3.8 |
| Supero-inferior diameter | 4.8 ± 0.8 | 3.1-6.8 |
| Parasternal short-axis view | ||
| Anteroposterior diameter | 2.9 ± 0.4 | 2.2-4.1 |
| Mediolateral diameter | 4.2 ± 0.6 | 3.1-6.0 |
| Apical 4-chamber view | ||
| Supero-inferior diameter | 4.1 ± 0.6 | 2.9-5.3 |
| Mediolateral diameter | 3.8 ± 0.4 | 2.9-4.9 |
Measurement of Left Atrial Volume by Echocardiography
Linear measurements have been reported to correlate with the angiographically determined LA size. However, the left atrium has an oval shape that changes asymmetrically according to disease state and loading conditions. Therefore, it is not always the case that the anteroposterior diameter represents LA size. For example, patients with mitral stenosis have an enlarged anteroposterior diameter, but patients with mitral regurgitation due to mitral valve prolapse often have an enlarged supero-inferior diameter. Thus, the measurement of LA anteroposterior diameter may be misleading, and LA volume, instead of diameter, should be determined in both clinical practice and research.
LA volume can be calculated using either an ellipsoid method or Simpson’s method. In the ellipsoid method, the left atrium is assumed to be represented as a prolate ellipse, and the volume can be calculated by
Volume = 4 π / 3 ( L / 2 ) ( D 1 / 2 ) ( D 2 / 2 ) ,
Volume = 8 ( A 1 ) ( A 2 ) / 3 π ( L ) ,
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