Keywords
abdominal aortic aneurysm - AAA - obesity - mortality - elective
Introduction
The “obesity paradox” has been widely studied in vascular surgery, specifically on
abdominal aortic aneurysm (AAA) repair.[1]
[2]
[3]
[4] The paradox's main dictum is that the body mass index (BMI; mass (kg)/height (kg)[2])-mortality curve is typically “U-shaped” with patients who are at the extremes of
BMI being more likely to die.[5]
[6] Other studies, however, have shown no correlation between obesity (i.e., BMI > 30)
and all-cause mortality; and a third group has shown decreased mortality in individuals
with a raised BMI.[7]
[8]
[9]
[10]
[11]
One explanation for the variance in results is the reliance on BMI to measure obesity.
Data suggests individuals with central obesity defined as “normal” based on BMI had the worst long-term survival even compared with overweight and
obese counterparts[12]; owing to this and other discrepancies, there have been suggestions that BMI is
suboptimal for defining obesity.[5]
[13]
Furthermore, the World Health Organization have suggested that waist-hip ratio (WHR)
may provide a more practical correlate of abdominal fat distribution and associated
ill health.[14] The INTERHEART study[15] also showed that WHR was predictive of myocardial infarction whereas BMI was not.
WHR has been shown to predict adverse events after elective colorectal surgery[16]; and indeed, when compared with BMI, only measures of abdominal obesity such as
WHR were found to have a significant positive association with AAA presence.[17]
Given the inherent issues with BMI as a measure of central obesity, yet overreliance
upon this measure widely across the literature, a comparison of BMI versus WHR in
terms of predicting outcomes is required. The primary aim of this study was to compare
BMI and WHR as predictors of mortality postelective AAA repair. Secondary aims looked
at the independent variables ability to predict all-cause readmission and length of
stay (LOS).
Materials and Methods
Data Collection
A review of a prospectively held local registry, the Health Quality Improvement Partnership
National Vascular Registry, was undertaken to identify consecutive patients undergoing
AAA repair at Leeds Vascular Institute between January 2006 and December 2016. Exclusions
were made based on BMI (i.e., BMI < 18.5) owing to the scarcity of the data and on
nonelective repair (i.e., urgent and emergency intervention).
Patient Demographics and Outcome Measures
Patient demographic data collected included: age, AAA size, sex, repair type, American
Society of Anesthesiologists Physical Status Classification System grade (ASA),[18] smoking history, diabetes, hypertension, ischemic heart disease (IHD), congestive
heart failure (CHF), chronic kidney disease (CKD), cerebrovascular disease (CVD),
and chronic obstructive pulmonary disease (COPD).
The date of death was retrieved from local electronic patient health records. Mortality
was assessed at 30 days and 4 years. Similarly, the LOS and 30-day readmission were
identified.
Measurements of Adiposity
Patient BMI was measured during patient preassessment prior to intervention. WHR was
obtained by analyzing preoperative routine computerized tomography (CT) imaging. The
“waist” measurement was calculated as the abdominal skin circumference measured at
the third lumbar vertebra at the upper end plate and the “hip” measurement was calculated
using the skin circumference at the level of the tip of the greater trochanters.
CT measurements were made using a free-hand measuring tool on Picture Archiving and
Communication System image viewer XERO (Agfa HealthCare, Belgium). Example measurements
are shown in [Fig. 1].
Fig. 1 An example measurement of skin circumference (orange) at the level of the L3 upper
end plate (L3UEP), and the greater trochanter (GT). Abdominal aortic aneurysm (AAA)
for illustration.
Image Analysis
All analyzed measurements were made by observer 1 (O1). O1 later repeated measurements
on a random third of the cohort (O1R) and a second observer (O2) repeated measurements
on a recalculated random third of the cohort. The intra- (O1 vs. O1R) and inter- (O1
vs. O2) observer differences were analyzed as mean difference ± standard deviation
(SD) and a t-test was used to check for significant differences in measurements using techniques
well described.[19] Random numbers were generated by www.random.org.
Statistical Analysis
SPSS version 26 was used for all data analysis.
Patient demographics were described as mean ± SD for continuous data and absolute
values and percentages (%) for categorical and binary data.
Adjustments were made for age, AAA size, sex, repair type, ASA, smoking history, diabetes,
hypertension, IHD, CHF, CKD, CVD, and COPD. Unadjusted and adjusted binary logistic
regression was used to calculate odds ratios with 95% confidence intervals describing
the association between BMI and WHR in relation to 30-day and 4-year mortality and
all-cause 30-day readmission. Linear regression was used to quantify the association
between the independent variables and LOS.
A Bonferroni correction[20] was used to derive a p-value associated with statistical significance. Owing to the eight comparisons performed,
a Bonferroni correction was calculated, meaning a p-value of less than 0.00625 (0.05/8) was required for significance.[20]
Results
A total of 681 AAA repairs were undertaken at Leeds Vascular Institute during the
specified time frame. After exclusions for underweight BMI (n = 5) and no urgency recorded (n = 6), 670 AAA repairs remained for analysis.
A subset of the data was identified, excluding a further 238 urgent repairs on basis
of nonelective urgency level. Analysis was undertaken on the remaining 432 elective
AAA repairs ([Fig. 2]).
Fig. 2 A flow diagram detailing exclusions from the study. BMI, body mass index; WHR, waist-hip
ratio.
Baseline demographics are detailed in [Table 1]. There were 5 (1.2%) deaths within 30 days, and 87 (20.1%) within 4 years. The 30-day
readmission rate was 3.9% and the average LOS was 7.33 (SD 18.525) days.
Table 1
Baseline demographics
|
Variables
|
Mean (SD)
|
Missing
|
%
|
|
Age (y)
|
75 (11)
|
0
|
(0.0)
|
|
AAA size (mm)
|
62.37 (8.332)
|
31
|
(11.0)
|
|
BMI
|
28.03 (6.305)
|
157
|
(36.3)
|
|
WHR
|
1.025 (0.067)
|
77
|
(17.8)
|
|
LOS (d)
|
7.33 (18.53)
|
54
|
(12.5)
|
|
Count
|
%
|
|
Gender
|
Men
|
379
|
87.7
|
|
Women
|
53
|
12.2
|
|
Missing
|
0
|
0.0
|
|
Repair type
|
EVAR
|
281
|
65.0
|
|
OSR
|
151
|
35.0
|
|
Missing
|
0
|
0.0
|
|
ASA
|
I
|
4
|
0.9
|
|
II
|
126
|
29.2
|
|
III
|
248
|
57.4
|
|
IV
|
12
|
2.8
|
|
V
|
0
|
0.0
|
|
Missing
|
42
|
9.7
|
|
Smoker
|
Yes
|
288
|
52.8
|
|
No
|
162
|
37.5
|
|
Missing
|
42
|
9.7
|
|
Diabetes
|
Yes
|
48
|
11.1
|
|
No
|
384
|
88.9
|
|
Missing
|
0
|
0.0
|
|
Hypertension
|
Yes
|
150
|
34.7
|
|
No
|
282
|
65.3
|
|
Missing
|
0
|
0.0
|
|
IHD
|
Yes
|
116
|
26.9
|
|
No
|
316
|
73.1
|
|
Missing
|
0
|
0.0
|
|
CHF
|
Yes
|
13
|
3.0
|
|
No
|
419
|
97.0
|
|
Missing
|
0
|
0.0
|
|
CKD
|
Yes
|
30
|
6.9
|
|
No
|
402
|
93.1
|
|
Missing
|
0
|
0.0
|
|
CVD
|
Yes
|
16
|
3.7
|
|
No
|
416
|
96.3
|
|
Missing
|
0
|
0.0
|
|
COPD
|
Yes
|
59
|
13.7
|
|
No
|
373
|
86.3
|
|
Missing
|
0
|
0.0
|
|
30-d mortality
|
Yes
|
5
|
1.2
|
|
No
|
427
|
98.8
|
|
Missing
|
0
|
0.0
|
|
4-y mortality
|
Yes
|
87
|
20.1
|
|
No
|
345
|
79.9
|
|
Missing
|
0
|
0.0
|
|
30-d readmission
|
Yes
|
17
|
3.9
|
|
No
|
415
|
96.1
|
|
Missing
|
0
|
0.0
|
Abbreviations: AAA, abdominal aortic aneurysm; ASA, American Society of Anesthesiologists;
BMI, body mass index; CHF, congestive heart failure; CKD, chronic kidney disease;
COPD, chronic obstructive pulmonary disease; CVD, cerebrovascular disease; IHD, ischemic
heart disease; LOS, length of stay; SD, standard deviation; WHR, waist-hip ratio.
Note: Mean and SD are used to describe continuous data and count and % are used for
categorical and binary data.
Observer Variation
BMI was recorded at admission for 275 (63.7%) patients and preoperative CT scans allowed
for the calculation of WHR for 355 (82.2%) patients. Operator 1 calculated WHR for
all 355 patients which were used in the study.
Intraobserver Error
Prior to excluding nonelective AAA repairs, Operator 1 remeasured WHRs for a random
third (284/670) of the data set. Operator 2 remeasured 183. The repeat assessments
by Observer 1 did not show any significant intraobserver differences in WHR (mean
difference = 0.00025, SD = 0.017362, t-test p-value = 0.845), nor did the repeat measurements made by Observer 2 (mean difference = 0.00144,
SD = 0.019339, t-test p-value = 0.317).
Patient Outcomes
The results of the unadjusted and adjusted outcomes are detailed in [Table 2].
Table 2
Results of the unadjusted and adjusted binary logistic regression quantifying the
effects of body mass index and waist-hip ratio on 30-day mortality, 4-year mortality,
and 30-day all-cause readmission with associated odds ratios, 95% CIs, and p-value
|
Unadjusted
|
Adjusted
|
|
30-d mortality
|
|
OR
|
95% CIs
|
p-Value
|
OR
|
95% CIs
|
p-Value
|
|
BMI
|
0.764
|
0.485–1.203
|
0.246
|
0.548
|
0.000–> 10
|
0.999
|
|
WHR
|
0.001
|
0.000–417.199
|
0.308
|
0.000
|
0.000–> 10
|
0.976
|
|
4-y mortality
|
|
BMI
|
1.002
|
0.953–1.053
|
0.943
|
1.017
|
0.967–1.070
|
0.514
|
|
WHR
|
0.087
|
0.002–3.724
|
0.203
|
0.170
|
0.001–22.979
|
0.479
|
|
30-d readmission
|
|
BMI
|
1.010
|
0.941–1.083
|
0.789
|
1.009
|
0.925–1.101
|
0.833
|
|
WHR
|
0.002
|
0.000–4.341
|
0.113
|
0.000
|
0.000–13.149
|
0.129
|
|
LOS
|
|
BMI
|
0.121
|
0.001–0.241
|
0.047
|
0.111
|
0.020–0.202
|
0.017
|
|
WHR
|
–22.414
|
–55.265 to 10.438
|
0.180
|
0.940
|
–8.472 to 10.352
|
0.844
|
Abbreviations: BMI, body mass index; CI, confidence interval; LOS, length of stay;
OR, odds ratio; WHR, waist-hip ratio.
Note: Results of the unadjusted and adjusted linear regression quantifying the effects
of BMI and WHR on LOS with associated ORs, 95% CIs, and p-value. A p-value of < 0.00625 was used for significance.
After Bonferroni correction, neither BMI nor WHR was found to predict mortality, readmission,
or LOS in patients undergoing elective AAA repair.
Discussion
The primary aim of determining if 30-day and 4-year mortality were associated with
BMI or WHR suggested that there was no correlation. This is in keeping with much of
the literature on the subject,[21]
[22]
[23]
[24]
[25] though our study is unique in utilizing both BMI and WHR as a measure of obesity
in regard to outcomes after AAA repair.
LOS and 30-day readmission were not found to be correlated with BMI or WHR. This is
in keeping with findings from other authors[22] and is important considering that LOS is often the largest contributor to overall
cost of intervention.[19]
Study Limitations
The findings are based from experience at a single center and with significant exclusions,
though the multiple measures with consistent results lend confidence to the outcome.
Of note, exclusion of underweight patients may have biased the outcome described.
Underweight patients often have poorer outcomes after vascular intervention and commonly
have higher rates of COPD and smoking, among other variables.[1]
[3]
[6]
[26] Given the small number excluded (n = 5) it is unlikely that this would have significantly changed the results, and exclusion
allowed for comparison targeted toward obese patients who make up a majority of the
caseload.
Second, though large for a single center, the number of patients is limited in comparison
with national data. Third, several patients had variables missing, in particular BMI
measurements were absent for 36.3% of patients included in the study.
The relatively large number of variables (13) used in modeling, in conjunction with
rare outcomes (5 deaths at 30 days) may have led to overfitting and we would welcome
studies from other centers to validate the results described here.
Though we accept inherent differences in open standard repair (OSR) and endovascular
repair, we considered them together owing to all patients being considered for either
at presentation and the limited number of patients in the study.
Clinical Implications
Other authors have concentrated on the effects of BMI on either OSR or endovascular
repair.[2]
[3]
[10]
[11]
[21]
[23]
[25]
[26] In this study, we have pooled data from OSR and endovascular repair as the decision
to employ either technique should be made based on numerous variables by a multidisciplinary
team on a case-by-case basis. Although the results from such studies may be more useful
in considering the effects of obesity on a single repair type, pooling results allows
for a more robust, real-world conclusion.
The lack of negative outcomes for patients with larger BMIs or WHRs suggests that
obese patients should not be excluded on the grounds of obesity alone, which is an
important consideration given the propensity for obese individuals to develop AAAs.[17] Though the results published here could be accounted for by selection bias, where
only obese patients who are fitter than their nonobese counterparts are considered
for intervention, a larger study would be required to tease out this conclusion.
Further, better powered studies are encouraged to further quantify the link or lack
thereof between central obesity as quantified by WHR and the outcomes after AAA repair.
The results demonstrated here, along with the plethora of literature suggesting WHR
as a superior measurer to BMI, should give confidence to potential authors.
Conclusion
This study suggests that there is no significant correlation between obesity as measured
by BMI or WHR and mortality after AAA repair in an elective setting. Furthermore,
LOS and 30-day readmission were also shown to not correlate with obesity. These results,
alongside much of the literature, should give confidence to obese patients who are
considered good candidates for intervention and to their clinicians.
