Chapter: Atrial Size

Anatomy LA Size RA Size Enlargement Criteria

The Atria

An overview of the often-overlooked atrial anatomy and measurements important for Internal Medicine POCUS

The left and right atria have function, which is lost in atrial fibrillation and is decreased in some other situations. A pressure load on the atria appears to be more harmful to function than a volume load, but various causes of HFPEF, including infiltrative diseases, can reduce atrial function. Atrial ejection fraction has been measured by cardiac MR and atrial longitudinal function is being studied with speckle tracking, which measures longitudinal strain. Various publications have noted that as left atrial function deteriorates with preserved sinus rhythm, the velocities of both the A and a’ waves fall.  This becomes an issue in the assessment of diastology that is discussed in a future chapter.

The LV size/function chapter contains a table that shows the weighted means for A waves as people age (they increase). An A wave that is abnormally low for age should raise the suspicion of LA dysfunction. Similarly, an abnormally low a’ can raise this suspicion.

The focus in the rest of this chapter is the size of the atria. As with other aspects of cardiac IMBUS, eyeballing of atrial size is an acquired skill that takes many repetitions during which the eye is correlated with some measurements.

A quick overview of the tributaries to the LA and RA can be helpful in interpreting color flow Doppler for MR and TR as well as defining where the atria ends and its tributaries begin for area measurements. The video below is a quick overview of the anatomic entrance of the 4 pulmonary veins into the left atrium and then the entrance locales of the IVC and SVC into the RA.

While rotating from an apical4 to an apical2 view, it is possible to see the left atrial appendage in the same location where the left pulmonary veins and the coronary sinus are seen in the apical4. However, these latter two structures are not usually well seen as the view is moving toward the apical2, so a visible hypoechoic structure very close to the posterior/lateral mitral valve is probably the atrial appendage.


LEFT ATRIA

Left atrial size has been called the “hemoglobin A1c of left heart function”. The importance of LA size in the assessment of heart failure and valve disease requires careful and often repeated measurements. It is impractical for us to measure the gold-standard LA volume, but several valid LA size measurements are fairly quick to perform.

In the PLAX view, the LA width at end LV systole (maximal LA size) at the level of, and parallel to, the aortic valve is a screening test. This dimension varies with patient height, but will be < 4.0 cm in most normal adults. Here is an image of the PLAX LA width in a normal patient.

If the LA width in the PLAX view is easily normal (and not being “squished” in the measured dimension by an aortic aneurysm or severe RV enlargement, resulting in LA enlargement in the longitudinal plane instead) no other measurement may be needed (except for practice). However, the apical4 view can give a very important view of the atria. If the LA looks large, or certainly if the PLAX width was borderline or enlarged, the LA should be measured in the apical4 view.

When the apical4 LA view is fairly clear during an IMBUS exam, the LA area is the most robust size measurement. Get the best view of the LA. Freeze the image, find the maximum size, and then use the machine caliper method for measuring area. Normal area in an average size adult will be under 20 cm2.  Here is an example of LA area measurement. The table at the end gives the size ranges for increasing degrees of LA enlargement.


RIGHT ATRIAL SIZE

There is less research and clinical data about RA size. The RA is normally a little smaller than the LA (about 2/3) and also varies with patient height. The RA will dilate with pulmonary hypertension, tricuspid valve disease, right ventricular failure, and atrial fibrillation. It may require an RV enhanced apical view to best see the RA.

Often, if the LA has been measured, the difference between the RA and LA can be eyeballed. If the RA is definitely smaller than a normal LA, measurement isn’t needed and the RA is normal. If the LA is large or if it is normal but the RA appears larger than the LA, the RA needs to be measured. Here is a clip from a patient with an enlarged RV and RA.

A confounding factor is that the RA length may be stretched when the LA expands, even though there is no particular right-sided problem. This might cause overcalling of RA enlargement by eyeball method but the area should not be increased in this case.

RA volume is the current best standard for RA size, but this is impractical for IMBUS and often isn’t reported by formal labs. The IMBUS exam should measure RA area with the same method used for the LA


Atrial Size Reference

Normal

Mild

Moderate

Severe

LA Width, PLAX (cm)

2.7 - 4.0

3.9 - 4.6

4.3 - 5.2

≥ 4.7-5.3

LA Area, apical4 (cm2)

< 20

20 - 30

30 - 40

> 40