Rupture of an abdominal aortic aneurysm has high morbidity and mortality rates. Many patients die before reaching the operating room. Surgeons often electively repair abdominal aortic aneurysms that measure 4 to 5.5 cm in diameter even though the long-term survival benefit of early elective surgery is uncertain. The decision is made more difficult because of uncertainty about the rupture risk of abdominal aortic aneurysms of different sizes. Abdominal aortic aneurysm diameter is probably a determinant of rupture, but measurements are poorly reproducible. The rupture rate of abdominal aortic aneurysms that measure less than 5 cm in diameter vary widely, from zero to 1 percent per year to as high as 6 percent per year. Modeling studies have suggested that the rupture risk for abdominal aortic aneurysms measuring 6.5 and 7.5 cm in diameter is 9 percent and 12.5 percent per year, respectively. Knowledge of other risk factors would help management decisions.

The U.K. Small Aneurysm Trial and the additional follow-up of patients who were ineligible or who refused randomization provided a large cohort for evaluating factors that influence the rupture risk of abdominal aortic aneurysms. Patients who had abdominal aortic aneurysms that measured 4 to 5.5 cm in diameter were randomized to undergo serial ultrasound surveillance or surgery. Patients who did not meet the enrollment qualifications were also followed. The primary end point of the study was rupture of the abdominal aortic aneurysm.

Among the 1,090 randomized patients, 25 abdominal aortic aneurysm ruptures occurred. In the 1,167 nonrandomized patients, abdominal aortic aneurysm rupture occurred in 78 patients. Of the 103 ruptures, 26 patients died without ever reaching the hospital, 53 patients died in the hospital without undergoing surgery, 13 patients died within 30 days of surgery and 11 patients died more than 30 days following surgery. Factors significantly and independently associated with abdominal aortic aneurysm rupture included initial aneurysm diameter (much higher risk of rupture in patients when the diameter of the abdominal aortic aneurysm measured greater than 6 cm), female gender, higher mean blood pressure reading and current smoker as measured by baseline plasma cotinine level. Although women typically have smaller abdominal aortic aneurysm measurements than men (mean diameter at rupture: 5 versus 6 cm), a surprising finding of this study was a rate of aneurysm rupture three times higher in women than in men. This might suggest that the ratio of infrarenal/suprarenal diameter is an important determinant of abdominal aortic aneurysm stability or rupture, although suprarenal diameters are not reproducible by ultrasonography. This study sheds new light on the risk factors associated with the rupture of abdominal aortic aneurysms, particularly smaller ones.

The authors conclude that patients with abdominal aortic aneurysm who are at marginal fitness levels, refuse surgery or are awaiting surgery should maintain adequate control of blood pressure and receive counseling and support to stop smoking. Although small abdominal aortic aneurysms do rupture, the risk of rupture of an abdominal aortic aneurysm that measures smaller than 5 cm in diameter is notably low; the risk of rupture of abdominal aortic aneurysms that measure 5 to 5.9 cm is also low but escalates sharply for abdominal aortic aneurysms that measure 6 cm or more in diameter. Different thresholds for abdominal aortic aneurysm repair should apply to women than to men because of the greater tendency for smaller aneurysms to rupture.