How to Properly Measure the Left Atrium Using Echocardiography
This is normally a pediatric blog, but in this case I am going to primarily address the measurement of left atrial size in adults, but the concepts can still easily apply to pediatrics.
The first step is to measure left atrial diameter. I have worked in many echo labs over my 30-year career, and the measurement for the left atrial diameter is done many ways. Some cardiologists are very rigid in that they only want it to be measured in the parasternal long axis or only in the apical view, however most cardiologists are pretty flexible.
It has been my experience that it is really easy to measure the left atrial diameter improperly. For instance, many techs will run an m-line through the aorta and the left atrium (for those who still use m-mode), or if one is using 2-D measuring, will measure in a straight line through the aorta and left atrium. The problem with this approach is, that the left atrium is at a 45Ëš angle to the aorta. If the left atrium were perfectly spherical, that would not necessarily be a problem, but the atrium is actually an ellipsoid shaped structure.
If one measures the atrium with a straight line through the aorta and atrium, the line will usually crosscut through the atrium at an oblique angle, causing the measurement to be larger than it should. The measurement should be at the widest part of the atrium, parallel to the mitral annulus. The same holds true when measuring the atrium in the apical view. Note the illustration below. Measurement (1) cut the atriums at an oblique angle. While measurement (2) is the appropriate way to get an accurate left atrial diameter. The second illustration demonstrates the proper way to measure the atrium in the apical view
As as a result of the sternum and the spine, there is mild compression of the atrium causing the anterior posterior diameter measurement (PLAX view) to be smaller than a measurement lateral-to-lateral (side-to-side) as in an apical view. If you have ever wondered why the left atrium looks larger in an apical view than the PLAX view, this is why.
Atrial fibrillation is a disease that can significantly cause malformations of the left atrium. In this, one will see atria that may be twice the length of the atria as they are wide. I think it is appropriate to measure both the length and width of the atrium in these cases. Left atrial enlargement can be correlated to body size, particularly in obesity. Mitral regurgitation will cause left atrial enlargement, as will as mitral stenosis.
Atrial size varies from person to person, disease to disease, and gender; but the gold standard for the measurement of the left atrium is the left atrial index. Left atrial index correlates left atrial size with body surface area in order to come up with a number (ratio) that is appropriate for the population as a whole, as opposed to a diameter measurement that can be very misleading.
As you can see from the chart above (for adults), diameter and volumes vary significantly between men and women. As you look at the volume/BSA index, the numbers are the same for both men and women. In other words, this ratio compensates for all types of atrial size and volumes and reduces this to a single number that can be representative as normal, mild, moderate and severely enlarged.
In order to accurately measure the LA index, measure the volume of the LA in both the apical and two chamber views. Your machine should average these two measurements and then divide the result by BSA. As the volume and diameter of the LA increases, the LA index number increases. It now becomes very easy categorize the size of the atrium.
As a side note, this is how to accurately measure the aorta: The aorta should be measured just distal to the coronary sinuses (those little bumps you see just distal to the aortic valve cusps.) The left ventricular outflow tract (LVOT) should be measured just inferiorly to the aortic valve cusps at the membranous level (where muscle meets the aortic annulus).
Ken Heiden RDCS