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 probably decreased in some other situations, but there aren’t easy measurements to assess function. Thus, the focus is on the size of the atria.

As with other aspects of cardiac POCUS, 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 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.


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 systole (maximal LA size) at the level of, and parallel to, the aortic valve annulus 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, the LA area is the most robust LA size measurement. Get the best view of the LA in 2D, getting an idea of the shape and boundaries. Freeze the image and find the maximum size, choose CALIPER, and then the option for MANUAL. Move the cursor to the base of the septal mitral leaflet and hit SELECT. Then, carefully and slowly move on the track pad to trace the outline of the LA (excluding the pulmonary veins). Hitting Select or Set at the top right will display the area and circumference. 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 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. If the JVP is normal, there is no TR, and the RV doesn’t appear large, be cautious calling RA enlargement.

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


A normal RA should be modestly smaller and certainly not larger than a normal 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