Quantitative Evaluation of Abdominal Aortic Aneurysm

Rupture of an abdominal aortic aneurysm (AAA) is usually predicted on the basis of measuring the maximum diameter. However, since the size of a normal aorta varies depending on each individual, this indicator does not seem to be ideal. The authors propose a new indicator for the enlargement of aorta and have started clinical application of this technique. The abdominal aortic diameter was measured by using two-dimensional ultrasonography in 261 adults (aged 20 and over). The control group consisted of subjects with no evident aortic dilatation. Patients with AAA were divided into the nonruptured group and the ruptured group. Analysis of the data from the control group revealed that the abdominal aortic diameter could be defined by the following equation: Diameter (mm) = 0.147 x age (years) + 0.169 x height (cm) - 15.9. The baseline diameters of the abdominal aorta were calculated by applying the age and height of individual subjects in this equation. The measured diameter was divided by the baseline diameter to obtain the aortic expansion index (AEI). If a cut-off value of the aneurysm diameter and the AEI (calculated by using age 20) for the prediction of significant risk of rupture is set at 44.2 mm (= mean - 2SD of the aneurysm diameter in the ruptured group) and at 2.7 (= mean - 2SD of the AEI in the ruptured group), the risk of rupture based on the diameter in the nonruptured group is underestimated in 11% of cases. However, when the risk of rupture is based on the AEI, the risk is not underestimated in any case in the nonruptured group. The authors conclude that the AEI is a useful indicator for quantitatively evaluating the risk of rupture of an AAA.