Normal Aortic Root: When One Size Does not Fit All
In a metanalysis, WASE Normal Values Study investigated several echocardiographic parameters, the most recently published data presents the results for normal echocardiography values of aortic root size according to age, gender, and race. The study included 1,585 adults from 15 countries. All subjects were free of cardiac, pulmonary, or renal disease. The distribution of subjects was even among gender and age: young (18–40 years), middle-aged (41–65 years), and old (>65 years). As seen with prior investigations, all aortic root measurements (annulus, SoV, sinotubular junction (STJ)) were greater in men compared to women. When indexing to BSA, all measured dimensions were greater in women. However, when indexing to subject height, all measurements were found to be higher in men. Also consistent with prior reports, aortic dimensions increased for both genders as age increased. The most impactful findings of the WASE Study for aortic root size were regarding the impact of race on aortic dimensions. While Asians had the lowest BSA and the smallest absolute aortic dimensions, the aortic diameters when indexed to BSA demonstrated that Asians had the largest values (for both men and women). These differences could potentially result in incorrectly categorizing a patient's aortic dilatation if current guideline reference ranges that do not account for race are used in clinical decision-making.

Another significant finding of the WASE Normal Values Study for aortic root size is that the upper limit of normal for the aortic valve annulus, the SoV, and the STJ were all lower than those included in the current aortic guidelines. This may be the result of improved imaging as well as the more diverse population as compared to the decades-old studies that have served as benchmarks for so long. These additional findings of a ‘new normal’ should be incorporated into aortic guideline recommendations, as the authors suggest. Along with updated reference ranges for men and women, ranges that feature age and race should also be included.

While the WASE Normal Values Study provides clinicians with a better understanding of the variations in different patient populations, there remains much work to be done in clearly defining reference ranges for aortic dimensions. It has been noted that current reference values for the aortic root may be derived from CT imaging or 2D echocardiography, depending on the source. While echocardiography remains the most utilized tool, it is unclear if certain at-risk patients should undergo CT imaging with multiplanar reformatting, given that echocardiography will underestimate aortic measurements. With the complexity of the aortic root, the value of 3D transesophageal echocardiography (TEE) has been described to improve accuracy; however, recent evidence would suggest that it may not be as accurate as once thought. Given that aortic root and ascending aorta dimensions are critical in transcatheter aortic valve replacement (TAVR), an investigation of 185 TAVR patients compared both 2D and 3D TEE to CT angiography. Both methods of echocardiography imaging underestimated the aortic annulus, and discrepancies >10% between CT and 3D TEE still occurred, leading to a decrease in post-TAVR survival. If 2D echocardiography is to be used given that it is more readily available, another area that requires further investigation is the approach to cursor placement during analyses. Echocardiography typically relies on the leading edge-to-leading edge method for the aortic root and the aorta, while CT imaging or magnetic resonance imaging (MRI) will often utilize the inner edge-to-inner edge method. Without a standardized approach, reference ranges will continue to have discrepancies depending on the imaging modality. To further complicate the normal values, timing in the cardiac cycle must be considered. While adult echocardiography guidelines suggest making SoV and ascending aorta measurements in end-diastole, uniform guidelines for CT and MRI are not available. Furthermore, pediatric guidelines suggest acquisition in mid-systole using the inner edge-to-inner edge convention. While the impact on measured values can be debated, these differences between the pediatric and adult world must be considered when redefining new normals.