Acute Aortic Disease.. - Index of

180 Coady and Elefteriades
aneurysms may be reflective of the difference in embryologic origin between the
ascending aorta, originating from ectoderm, as opposed to the abdominal aorta,
arising from mesoderm.
Risk Factors for Aneurysm Growth
Size
The natural history of TAA, as in any aneurysm, is related to size. Since the initial
studies in 1966 by Szilagyi et al. (30), size has been shown to be a significant risk
factor for aortic rupture. Survival expectancy in untreated small abdominal aortic
aneurysms (6 cm). For
nonsurgical aneurysms over a 10-year period, the risk of rupture was 19.5% for
small aneurysms and 43% for larger aneurysms. Removal of the aortic aneurysm
was shown to double the patient’s survival expectancy (31).
Size has also been considered a major risk factor for complications (dissection
and rupture) and is perhaps the single most important factor in the decision
to intervene surgically on a nonemergent basis. The empirical evidence on the
influence of size on rate of growth, however, is mixed. Dapunt et al. (32) note, for
example, that a higher rate of expansion was found in those patients with a larger
aortic diameter (>5 cm) at diagnosis. In contrast, Hirose et al. (33) find no significant
effect of size on aortic growth.
Table 1 shows the estimated aneurysm growth rates from our own institutional
data in relation to initial aneurysm size, chronic dissection, and location.
Annual growth varied from 0.10 cm/yr for small (4.0 cm) aneurysms to 0.19 cm/yr
for large (8.0 cm) aneurysms. This relationship is depicted graphically in Figure 4.
Also seen in Table 1, dissected aortas grow more rapidly than non dissected aortas,
and descending aneurysms grow more rapidly than ascending aneurysms physicians
must remember to compare to the patient’s earliest study—not the most recent prior
study—in order to asses growth rate properly in a particular patient (34).
Females are at higher risk than males at relatively similar aortic sizes for the
combined endpoint of rupture or dissection (35). We hypothesize that this may be
due in part to differences in mean body size between males and females, with a
given aortic size representing a proportionally greater diameter in smaller women.
In a recent manuscript, we demonstrated that, at any given aortic size, lower body
surface area (BSA) is associated with a higher incidence of complications—
including rupture, dissection, and death. In order to more accurately assess the
risk of aneurysm complications, we developed a new measurement, aortic size
index (ASI), which takes into account both aortic diameter and BSA. ASI is
simply aortic diameter divided by BSA. Throughout all methods of analysis,
ASI was a better predictor of complications than maximal aortic diameter.
In particular, we found that using ASI, patients could be stratified into three
categories of risk (Fig. 5). Those with ASI