The Left Atrium

The left atrium (LA) can be thought of as having three key haemodynamic functions. During left ventricular systole and its subsequent isovolumic relaxation phase, the LA acts as a reservoir for oxygenated blood returning to the heart from the lungs. During early diastole, the LA then acts as a passive conduit for this blood to enter the left ventricle. In late diastole, atrial contraction actively boosts left ventricular filling (unless the patient is in atrial fibrillation).

ECHO ASSESSMENT OF THE LEFT ATRIUM

The left atrium (LA) can be seen in several views:

left parasternal window
- parasternal long axis view
- parasternal short axis view (aortic valve level)
apical window
- apical 4-chamber view
- apical 2-chamber view
- apical 3-chamber (long axis) view
subcostal window
- subcostal long axis view.

An echo evaluation of the LA includes assessment of:

LA morphology
LA dimensions
LA function.

LEFT ATRIAL MORPHOLOGY

Inspect the overall size and shape of the LA, and check for the presence of any masses (e.g. tumour, thrombus). Be alert too for the presence of spontaneous echo contrast, particularly in presence of atrial fibrillation and/or mitral stenosis. Although best seen on transoesophageal echo, spontaneous echo contrast may also be evident during transthoracic imaging. LA masses and spontaneous echo contrast are discussed further in Chapter 27.

The left atrial appendage is not easily seen on transthoracic echo, but may be visible in the apical 2-chamber view. The pulmonary veins can sometimes also be hard to spot, but are generally best seen in the apical 4-chamber view (especially the right upper pulmonary vein, Fig. 17.3).

LEFT ATRIAL DIMENSIONS

The causes of LA dilatation (Fig. 18.1) include:

mitral valve disease
dilated cardiomyopathy
restrictive cardiomyopathy
LV diastolic dysfunction
atrial fibrillation or flutter
high-output states (e.g. anaemia)
‘athlete’s heart’ (p. 245).

LA diameter is measured at end-systole in the parasternal long axis view, using either 2D or M-mode imaging (Fig. 18.2).

LA volume is measured using the modified Simpson’s rule method:

1. In the apical 4-chamber view obtain the best view you can of the LA, paying particular attention to avoidance of foreshortening.

2. Freeze a loop and find the end-systolic image. Now trace the endocardial border around the LA to obtain an area measurement in cm². Ignore any pulmonary veins that may be visible.

3. Measure the length of the LA long axis in cm from the mid-point of the mitral annulus to the superior border (back wall) of the LA.

4. Repeat the measurements in the apical 2-chamber view.