Methods

Patients were recruited from a vascular surgery clinic at the Townsville Hospital (Queensland, Australia) from May 2003 to July 2008. Inclusion criteria were (1) verbal and written informed consent, (2) presentation for investigation of abdominal aortic aneurysm (AAA) or symptomatic PAD, (3) no contraindications to abdominal aortic and lower limb CTA, and (4) no current plans for interventional therapy for PAD or AAA at entry. Exclusion criteria were (1) refusal to participate, (2) previous surgical repair of the abdominal aorta, and (3) contraindication to CTA, such as abnormal serum creatinine or contrast allergy. Intermittent claudication was diagnosed by a consultant vascular physician on the basis of an appropriate history along with clinical signs of lower limb ischemia and evidence of occlusive or stenotic PAD on CTA. AAA was defined as maximal infrarenal abdominal aortic diameter ≥30 mm from axial slices of the CTA. Risk factor definitions have been given in previous studies. Ethics approval for this study was provided by Human Research Ethics Committees of the Townsville Health Service District and James Cook University.

Contrast-enhanced computed tomographic images were obtained using a 64-slice multiscanner (Philips Electronics Australia, North Ryde, Australia) under a set acquisition protocol as previously described. The images were recorded at 3-mm intervals, with a slice thickness of 3 mm, to construct 3-mm contiguous image slices for analysis. One hundred milliliters of contrast agent (Ultravist 300), delivered by an automatic computed tomographic injection driver system (MEDRAD, Warrendale, Pennsylvania), was given intravenously.

Aortic calcific deposits were measured using a previously validated protocol with an interobserver coefficient of variation of about 1%. Images from the origin of the lowest main renal artery (excluding accessory arteries) to the bifurcation of the aorta were transferred to MxView Visualization Workstation software (Philips Electronics Australia) for analysis. Calcific deposits were thresholded using previously defined Hounsfield units (center level 1,400 Hounsfield units, window width 2,000 Hounsfield units). Using the volume-of-interest tool, an encircling line was drawn around the aorta to form a region of interest ensuring that only the aorta was included. The selected images were then saved to the workstation and reloaded into the 3-dimensional mode. The software program computed the volume of calcific deposits in cubic millimeters. Maximal axial infrarenal aortic diameter was assessed using the CTA viewer function on the Philips workstation. The region was scouted to find the area of maximal diameter, taking many measurements with electronic calipers. Maximal diameter was recorded in millimeters (to the nearest 0.1 mm).

At entry into the study, patient risk factors were obtained and CTA arranged. Patients were followed up according to the presenting complaint. Patients with intermittent claudication were seen 6 months after their initial assessments and then yearly unless symptoms changed. Patients with small AAAs measuring 30 to 39 mm were followed up yearly, and those with AAAs ≥40 mm every 6 months. Primary outcomes recorded during follow-up included cardiovascular events (nonfatal stroke, nonfatal myocardial infarction, coronary revascularization, amputations, and cardiovascular death). Outcome data were recorded during follow-up visits and subsequently checked via review of patients’ charts by 2 independent researchers (for all patients) and by additional phone calls to selected patients (no chart entry in previous 2 years; primary residence outside Townsville).

Data were prospectively entered into a spreadsheet (Microsoft Excel; Microsoft Corporation, Redmond, Washington) and later transferred to SPSS version 17.0 for Windows (SPSS, Inc., Chicago, Illinois) for further analysis. To investigate the characteristics of patients in relation to abdominal aortic calcium, 3 groups were defined: mild (calcific deposit volume <400 mm ³ ), intermediate (calcific deposit volume 400 to 1,700 mm ³ ), and severe (calcific deposit volume >1,700 mm ³ ). Severity was gauged relative to the 33rd (488 mm ³ ) and 66th (1,768 mm ³ ) percentiles of abdominal aortic calcific deposit volume. To define groups, these numbers were rounded down to the nearest 100 mm ³ .

Initially continuous variables were assessed using Kolmogorov-Smirnov test, histograms, and normal quantile-quantile plots, which demonstrated that they were not normally distributed. Continuous variables obtained at recruitment were compared using Kruskal-Wallis tests. Nominal variables were compared using chi-square test. Kaplan-Meier analysis was used to estimate the incidence of cardiovascular events. Cox proportional-hazards analysis was used to assess the independent effects of risk factors on cardiovascular events. Categorical variables were dummy coded. Known disease risk factors, including age, gender, smoking status, diabetes mellitus, hypertension, dyslipidemia, coronary artery disease, maximal orthogonal infrarenal aortic diameter, and intermittent claudication were assessed for their associations with cardiovascular events using stepwise selection procedures (backward and forward likelihood ratios). Definitions of diabetes, hypertension, dyslipidemia and coronary artery disease included history of or medical treatment (medication or intervention) for these conditions. Variables not in the stable model were then included individually in the model to assess confounding. Risk factors that led to changes in the coefficients >10% were retained in the final model as confounders. Power analysis on the basis of the parameter specifications of the Cox model estimated that the present sample size achieved 91% power at a 0.05 significance level to detect a relative risk of 2.8 when the event rate was about 20%.

Results

All 213 patients underwent CTA. The risk factors of patients in relation to the tertiles of infrarenal aortic calcific deposit volumes are listed in Table 1 . Aortic calcific deposits were associated with older age, larger aortic diameters, smoking, hypertension, lower body mass index (BMI), and reduced symptomatic intermittent claudication ( Table 1 ).

The following cardiovascular events occurred during follow-up: nonfatal myocardial infarction (n = 23), coronary revascularization (n = 6), stroke (n = 3), below-knee amputation (n = 2), and cardiovascular death (n = 11). Patients with more severe aortic calcific deposits had a higher incidence of cardiovascular events ( Figure 1 ). The incidence of cardiovascular events was 21.7% (95% confidence interval [CI] 13.0% to 30.4%), 33.0% (95% CI 26.2% to 39.8%) and 36.9% (95% CI 28.7% to 45.1%) for patients with mild, intermediate, and severe abdominal aortic calcific deposits, respectively. Given these findings, we compared outcomes for patients with severe calcific deposits to those with more mild (intermediate and mild) calcific deposits in the Cox proportional-hazards analysis. Calcific deposit volume >400 mm ³ and coronary artery disease were associated with increased incidence of new cardiovascular events ( Table 2 ).

Kaplan-Meier analysis depicting freedom from cardiovascular events in patients with mild (<400 mm ³ ), intermediate (400 to 1,700 mm ³ ), and severe (>1,700 mm ³ ) abdominal aortic calcific deposits. *Cardiovascular event rate at 4 years.

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