Annals of Vascular Surgery
Volume 22, Issue 6 , Pages 730-735, November 2008

Influence of Age, Aneurysm Size, and Patient Fitness on Suitability for Endovascular Aortic Aneurysm Repair

Division of Vascular and Endovascular Surgery, Department of Surgery, University of Texas Southwestern Medical Center, and Dallas Veterans Affairs Medical Center, Dallas, Texas

published online 01 October 2008.

Article Outline

Prior to approval by the U.S. Food and Drug Administration of larger endografts (main body diameters up to 36mm), small abdominal aortic aneurysms (AAAs, <5.5cm) were shown to be more suitable for endovascular repair (EVAR) than large AAAs (≥5.5cm). The purpose of this study was to assess changes in EVAR suitability with the potential use of larger endografts in unselected consecutive patients. The influence of age, aneurysm size, and patient fitness on EVAR suitability was also assessed. We studied 186 male patients referred for evaluation of nonruptured AAAs who underwent contrast-enhanced computed tomographic scans with three-dimensional reconstructions. Morphologicall AAA features and neck characteristics were measured according to Society for Vascular Surgery reporting standards to determine EVAR suitability. Patient fitness for repair was assessed using the customized probability index, a validated fitness score for vascular surgery procedures. Suitability for EVAR was determined by neck anatomy, iliac artery morphology, and total aortic aneurysm angulation and tortuosity according to the clinicians' experience and current practice. The median age of the study cohort was 72 years (interquartile range [IQR] 65-79 years). The median maximum AAA diameter was 5.4cm (IQR 4.1-5.9). Median fitness score was +7 (IQR -7 to +14). EVAR suitability for large AAAs significantly increased with larger endografts (35-63%, p<0.001). Changes in EVAR suitability for small AAAs were not significant (69-75%, p=0.06). Maximum AAA diameter was not an independent predictor for EVAR suitability with larger endografts after adjusting for neck anatomy. Aortic neck length (odds ratio [OR]=1.2, 95% confidence interval [CI] 1.1-1.2) and diameter (OR=0.78, 95% CI 0.63-0.96) were the only independent predictors for EVAR suitability with larger endografts. Age, AAA size, and fitness did not differ between patients suitable and unsuitable for EVAR with larger endografts. In conclusion, introduction of larger endografts (up to 36mm in main body diameter) in the United States has resulted in significantly increased anatomic suitability for EVAR for large AAAs. Conversely, suitability has not significantly changed for small AAAs. Overall, EVAR suitability is not influenced by age, aneurysm size, or patient fitness.

 

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Introduction 

Age, abdominal aortic aneurysm (AAA) size, and gender have not been found to influence endovascular aneurysm repair (EVAR) suitability in observational studies.1, 2 Conversely, in a previous study from our institution in which the manufacturers' instructions for use were strictly followed to determine EVAR suitability, 64% of patients with small AAAs were candidates for EVAR compared with 39% of patients with large AAAs.3 Small AAAs had less complex anatomy with longer aortic necks, less neck angulation, and less tortuosity. Other morphological studies have also demonstrated that AAA maximum diameter is inversely related to the length of the aortic neck.4 Moreover, the diameter of the aneurysm has been shown to be the most useful surrogate determinant of feasibility for EVAR.4, 5 In a subsequent study, however, we demonstrated that morphological changes associated with aneurysm growth during surveillance of small AAAs were not clinically significant and had minimal effect on overall suitability for EVAR.6 In fact, EVAR suitability did not significantly change during mid-term follow-up (74% vs. 69%), suggesting that suitability is not affected by AAA size up to the threshold for repair.

In October 2004, the U.S. Food and Drug Administration approved the use of larger endografts (main body diameters up to 36mm) for EVAR. Although few European studies have assessed EVAR suitability with the use of these larger endografts,2, 7 most American studies have used selection criteria for EVAR using endografts with main body diameters up to 28 or 32mm.3, 8

Patient fitness has been identified as a major determinant of the need and outcome of AAA repair, including EVAR.9, 10 A customized probability index (CPI) based on comorbidities and the use of statins and beta-blockers has been proven to be a valid predictor for perioperative morbidity and mortality after vascular surgery procedures, including EVAR.9, 11 To date and to our knowledge, patient fitness and its association with EVAR suitability have not been assessed.

The purpose of this study was to determine changes in EVAR suitability with the potential use of larger endografts in consecutive unselective patients. The influence of age, aneurysm size, and patient fitness on EVAR suitability was also assessed.

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Methods 

During a 3-year period, 186 consecutive patients referred for AAA evaluation underwent helical computed tomographic (CT) scans at the Dallas Veterans Affairs Medical Center. All CT scans were performed using the helical Hi Speed I from GE Medical Systems (Milwaukee, WI) with collimation set at 3mm and a 2.0 pitch. All CT scans were evaluated using three-dimensional reconstruction on a Vitrea workstation (Vital Images, Plymouth, MN). Morphological AAA characteristics were determined from each CT scan according to the Society for Vascular Surgery (SVS) reporting standards.12, 13 These included angle-corrected aortic neck length and diameter, suprarenal and infrarenal aortic neck angle, amount of aortic neck thrombus and calcification, maximum orthogonal aneurysm diameter, aneurysm tortuosity index (median luminal centerline/straight line distance),8 amount of aneurysm thrombus, amount of iliac artery thrombus and calcification, common iliac artery diameter and length, iliac artery tortuosity (median luminal centerline/straight line distance), and external iliac artery diameter. The measurements obtained from all CT scans were entered into a database designed for this study, which was approved by the local institutional review board.

Suitability for EVAR was determined according to neck anatomy (diameter, length, and angulation), iliac artery morphology, and total aortic aneurysm angulation and tortuosity, which were defined according to the SVS reporting standards.12, 13 EVAR anatomic suitability was primarily determined according to clinicians' consensus, experience, and current practice with the endografts commercially available in the United States, i.e., AneuRx (Medtronic, Minneapolis, MN), Excluder (W. L. Gore, Flagstaff, AZ), PowerLink (Endologix, Irvine, CA), and Zenith (Cook, Bloomington, IN) devices, including the larger 36mm main body endografts. Specifically, EVAR suitability was defined as the clinician's expectation of delivering the potential endograft and achieving its secure fixation at the proximal and distal landing zones of the aortic neck and iliac arteries. Although aortic aneurysm proximal necks could be suitable for EVAR when the proximal neck landing zone was less than 15mm, neck angulation had to be minimal in these cases. The minimum aortic neck length when no angulation was present was 10mm, whereas maximum neck diameter was 32mm. Iliac artery characteristics were assessed in terms of access and the possibility of delivering both the main body and contralateral limb extensions of the endograft. If EVAR was not feasible with one device, the possibility of EVAR was assessed with each of the remaining devices.

Patient fitness for AAA repair was determined using the CPI, a previously validated and reported scoring system developed to assess perioperative mortality in patients undergoing vascular surgery.9 The CPI uses seven clinical characteristics: renal dysfunction (defined as a serum creatinine level of 2mg/dL or greater) adds 16 points, uncontrolled heart failure adds 14 points, ischemic heart disease adds 13 points, hypertension adds 7 points, and chronic pulmonary disease (defined as a forced expiratory volume of <60% of predicted) also adds 7 points. Treatment with beta-blockers and/or statins is considered to reduce the risk of perioperative mortality and, therefore, subtracts points from the overall score. Treatment with a beta-blocker subtracts 15 points, and treatment with a statin subtracts 10 points. This approach to fitness assessment using the CPI has also been tested and validated by Greenhalgh et al.11 using data collected on patients enrolled in EVAR trial 1 and EVAR trial 2 as the best indicator of patient fitness.

Descriptive statistics for categorical variables are presented as relative frequencies (percents), which were compared with the χ2 test (χ2 for independent groups, two-tailed p value). Morphological changes in aneurysm dimension were classified according to the reporting standards for EVAR.12, 13 Continuous variables were expressed as medians and interquartile ranges (IQRs); these were analyzed with the Mann-Whitney U-test. Multivariate stepwise logistic regression analysis was used to identify independent predictors of EVAR suitability. Variables that had a p value less than 0.25 in the univariate analysis and those known to be important or possible confounding factors were entered into the regression model and considered significant by forward stepwise selection at p<0.05 in the final regression equation. Findings were considered statistically significant for the primary end point, i.e., EVAR suitability, at p<0.05. Multivariate odds ratios (ORs) are reported with 95% confidence intervals (95% CIs). Changes in EVAR suitability with the introduction of larger endografts were assessed with matched and paired comparisons of EVAR suitability with the potential use of smaller and larger endografts (≤32mm and ≤36mm in main body endograft diameter, respectively) using the McNemar χ2 test (χ2 for paired groups, two-tailed p value). SPSS version 14.0 (SPSS Inc., Chicago, IL) and MedCalc for Windows version 8.1.0.0 (MedCalc Software, Mariakerke, Belgium) were used for data analyses.

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Results 

The median age of the study population was 72 years (IQR 65-79). All patients were male. The median maximum AAA diameter was 5.4cm (IQR 4.1-5.9). Median fitness (CPI) score was +7 (IQR -7 to +14). Among 186 patients undergoing CT angiography, 136 (73%) had AAAs suitable for EVAR, whereas 50 (27%) were noncandidates for EVAR. One hundred twenty-nine patients (69%) had small AAAs (maximum diameter <5.5cm), whereas 57 (31%) had large AAAs (≥5.5cm in maximum diameter). Median ages of patients suitable and unsuitable for EVAR were the same (72 years [IQR 65-80] vs. 72 years [IQR 67-77], respectively; p=0.74). Patient fitness was not significantly different between patients suitable and unsuitable for EVAR (median CPI scores, +7 [IQR, -3 to +14] vs. +5 [-3 to +14], respectively; p=0.28).

Median maximum aortic aneurysm diameter of patients suitable for EVAR (4.7cm, IQR 3.8-5.6) was significantly lower compared with patients unsuitable for EVAR (5.1cm, IQR 4.7-6.6; p=0.002). Median aortic neck diameter of patients suitable for EVAR was significantly lower (25mm, IQR 24-27, vs. 29mm, IQR 26-33; p<0.001) compared with patients unsuitable for EVAR. Median aortic neck length of patients unsuitable for EVAR was significantly shorter (6mm, IQR 0-13, vs. 35mm, IQR 25-39; p<0.001) compared with patients suitable for EVAR. Iliac artery morphological characteristics were not significantly different among patient suitable and unsuitable for EVAR (Table I).

Table I. Anatomic measurements of AAAs suitable and unsuitable for EVAR (n=186)
Suitable for EVARUnsuitable for EVARp
Aneurysm maximum diameter (cm), median (IQR)4.7 (3.8-5.6)5.1 (4.7-6.6)0.002a
Proximal neck diameter (mm), median (IQR)25 (24-27)29 (26-33)<0.001a
Proximal neck length (mm), median (IQR)35 (25-39)6 (0-13)<0.001a
Proximal neck suprarenal angle (α), median (IQR)165° (147°–174°)171° (145°–177°)NS
Proximal neck infrarenal angle (β), median (IQR)159° (144°–168°)149° (141°–165°)NS
Aortic neck calcification/thrombosis25-50%25-50%NS
Aortic angle (Ф), median (IQR)148° (141°–163°)143° (136°–156°)NS
Aortic tortuosity index, median (IQR)1.07 (1.02-1.14)1.11 (1.07-1.16)NS
Aortic aneurysm thrombus25-50%>50%0.006a
Iliac artery sealing zone diameter (left/right, mm, median)12/1312/13.5NS
Iliac artery sealing zone length (left/right, mm, median)50/5247/51NS
Iliac tortuosity index (left/right)1.221.24NS
Iliac angle (left/right, median)125/121123/120NS
Iliac calcification/thrombosis25-50%25-50%NS

aMann-Whitney U-test.NS, nonsignificant.

Multivariate logistic regression analyses revealed that median aortic neck length and diameter were the only independent determinants of EVAR suitability. Age, patient fitness (CPI score), and aneurysm size (median maximum diameter) were not independently associated with EVAR suitability (Table II).

Table II. Multivariate analysis of factors associated with EVAR suitabilitya
CoefficientORb95% CIp
Age−0.0150.980.92-1.040.61
Patient fitness (CPI)0.0151.010.97-10.60.51
Aneurysm maximum diameter (cm)0.1121.120.77-1.630.55
Proximal neck diameter (mm)−0.2440.780.63-0.960.02
Proximal neck length (mm)0.1531.171.10-1.23<0.001

aVariables with p<0.25 in the univariate analysis and those known to be important and possible confounding factors were entered into the multivariate logistic regression models and selected by forward stepwise selection if p<0.05 (p<0.001 for model).

bAdjusted ORs.

The introduction of larger endografts resulted in increased suitability for EVAR for all AAAs assessed in this series (59-73%, p<0.001). Stratified analyses according to AAA size further revealed that EVAR suitability significantly increased with the potential use of larger endografts compared to smaller ones in patients with large AAAs (35-63%, p<0.001). Changes in EVAR suitability for small AAAs were not significant (69-75%, p=0.06). Stratified multivariate logistic regression analyses revealed that maximum AAA diameter was not an independent predictor for EVAR suitability with either smaller or larger endografts after adjusting for neck anatomy.

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Discussion 

The results of our study indicate that the introduction of larger endografts (up to 36mm in main body diameter) in the United States has resulted in significantly increased anatomic suitability for EVAR for large AAAs. Conversely, suitability has not significantly changed for small aneurysms. Our findings, therefore, suggest that larger endografts have expanded the window of opportunity for EVAR for large AAAs. The effectiveness and durability of EVAR with the use of such large endografts, however, remain to be established. Our data also revealed that anatomic suitability for EVAR is not influenced by age, patient fitness, or aneurysm size. Instead, diameter and length of the aortic aneurysm proximal neck are the main factors that determine EVAR suitability.

Although in a previous study from our institution AAA size was found to be an independent predictor for EVAR suitability, other studies and our current series have failed to demonstrate an independent association between AAA maximum diameter and anatomic suitability for EVAR.1, 2 Several reasons may account for these different findings. First, in the original study by Welborn et al.,3 EVAR suitability was assessed using conservative criteria based on the instructions for use issued by the manufacturers using endografts with a main body diameter up to 32mm. In our series, EVAR suitability was determined with the potential use of larger endografts now available in the United States. A more liberal approach to assess EVAR suitability was also used in the current study, which is based on the current treatment of AAAs by more experienced operators. Second, aneurysm neck anatomic features were excluded from multivariate analysis by Welborn et al.3 because these variables were used to determine EVAR suitability. Conversely, our current data and other previous studies did include neck length, diameter, and angulation in multivariate logistic regression models as the assessment of their independent association with EVAR suitability is critical when considering other anatomic, demographic, and clinical variables in the analysis.2

As in our previous study, in which we assessed the morphological changes that occurred in an unselected cohort of patients with small aneurysms under surveillance,6 our current data demonstrate that the proximal aortic neck diameter and length are the most significant factors that predict EVAR suitability in aneurysms of different AAA size. Previous studies have suggested this relationship.1, 2, 14, 15 Bayle et al.4 found that large aneurysms had shorter proximal necks and increased iliac tortuosity. Armon et al.14 found that aneurysms >7cm in maximum diameter had shorter and wider aortic necks. Arko et al.1 similarly observed that as aneurysm size increased, aortic neck length decreased by 27%. These previous studies, like our current study, included an unselected group of small and large aneurysms. Our study is unique in its approach as EVAR suitability was assessed using modified criteria based not on the instructions for use of the manufacturers but on the current approach of experienced operators to determine EVAR suitability, taking into account all endografts available in the United States.

Our results revealed that age and patient fitness did not influence the suitability rates for EVAR. This suggests that older patients and those unfit for open repair do not have more unfavorable anatomy that may preclude EVAR compared with younger patients and those fit for AAA repair. Although similar findings have been reported in terms of age and its association with EVAR suitability,2 our study is the first, to our knowledge, to suggest a lack of association between patient fitness and the presence of unfavorable anatomic features for EVAR.

In our previous study, a wide proximal aortic neck diameter was the main contraindication for EVAR (57%), whereas inadequate neck length was the main limiting factor in 37% of the aneurysms.6 Conversely, in our current series, 41% of AAAs were unsuitable for EVAR due to inadequate length of the proximal neck, whereas a proximal aortic neck diameter >32mm, which is the largest neck accommodated by the latest commercially available 36mm Zenith endograft, was the limiting factor for EVAR suitability in 31% of patients. In both series, unfavorable proximal aortic neck anatomy was the only factor to account for exclusion of patients from EVAR.

Because improved outcomes after EVAR for small AAAs have been reported compared with large AAAs, it has been suggested that the AAA size threshold for repair should be lowered.5, 16 Another large observational study, however, found that mid-term results of early EVAR for small AAAs were suboptimal given the frequent need for reinterventions and continued aortic sac enlargement, which argues against expanding EVAR for the treatment of small AAAs.17 Until ongoing randomized trials comparing early EVAR vs. surveillance for the management of small AAAs are completed and provided that adequate sample size and power can be demonstrated, early EVAR for small AAAs remains unsubstantiated.

Although our data suggest a relationship between AAA size with shorter and wider proximal aortic necks, aneurysm maximum diameter per se does not significantly affect anatomic suitability for EVAR. In fact, small AAAs occasionally had short and wide proximal necks. Moreover, it appears that the suitability of an aneurysm for endovascular repair is predetermined early in the morphological life of the aneurysm, likely before maximum aortic diameter reaches 5cm. Because the aneurysm rupture risk of small AAAs is minimal and up to one-quarter of patients with small AAAs undergoing surveillance never require repair,18, 19 the need for early EVAR for aneurysms with a maximum diameter <5cm is probably not justifiable. A more sensible approach to the management of small AAAs should focus on the aneurysm proximal neck anatomy. Should the infrarenal aortic neck length and diameter be marginal, early EVAR may be justified as continued surveillance may result in AAA growth with associated loss of EVAR suitability. Conversely, small AAAs with long infrarenal aortic necks with suitable neck diameters may be safely observed as their growth would be unlikely to result in EVAR unsuitability. In this regard, proximal aortic neck length is probably the most important factor determining the need of repair as the availability of larger endografts has resulted in increased EVAR suitability. It is unlikely that intervention earlier in the life of an aneurysm may result in an improved window of opportunity for EVAR when repaired prior to 5cm in size, unless the proximal neck length is limited.

Although the present study includes a large series of patients referred for evaluation of nonruptured AAAs who underwent contrast-enhanced CT scans with three-dimensional reconstructions, several important limitations should be acknowledged. First, the majority of AAAs (69%) referred for evaluation were small. Second, all patients in this study were men; thus, our results do not apply to women with AAAs. Third, although all components of the CPI were assessed on all patients, complete cardiac and pulmonary evaluations were not routinely obtained as EVAR or open AAA repair was not offered to all patients. Finally, specific anatomic and other patient characteristics that could be explored as possible criteria to determine suitability for EVAR could not always be strictly defined as EVAR suitability continues to be subjective in most circumstances and related to clinicians' experience and current practice.

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References 

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 Presented at the 18th Annual Winter Meeting of the Peripheral Vascular Surgery Society, Snowmass Village, CO, February 1-3, 2008.

PII: S0890-5096(08)00335-X

doi:10.1016/j.avsg.2008.08.034

Annals of Vascular Surgery
Volume 22, Issue 6 , Pages 730-735, November 2008

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