The term “AO” stands for “aortic root,” which is the part of the aorta closest to the heart. When you measure an ascending aortic aneurysm, you include the aortic root (AO) in your assessment. This means that the measurement covers the section of the aorta right at the heart as well as the bulge caused by the aneurysm.

So, in simple terms, when you talk about the measurement of an ascending aortic aneurysm, you’re looking at the entire width of the aorta, including any swelling or bulging at the aortic root.