Preoperative functional status predicts perioperative outcomes after infrainguinal bypass surgery

Article Outline

• Abstract
• Methods
  • Statistical analysis
• Results
• Discussion
• Author contributions
• References
• Copyright

Objective

Infrainguinal surgical bypass (BPG) is a durable method for lower extremity revascularization, but is accompanied by significant 30-day morbidity and mortality (MM). The goal of this study is to relate preoperative functional status, a defined metric in the National Surgical Quality Improvement Program (NSQIP) database, to perioperative MM.

Methods

Between January 1, 2005 and December 31, 2007, all patients who underwent BPG from the NSQIP private sector database were reviewed. The primary end-point was 30-day MM. Patients were stratified by preoperative functional status: independent (IND) vs dependent (DEP). Associated patient demographic/clinical data were analyzed using univariate and multivariate methods. Composite odds ratios were constructed with clusters of high-risk comorbidities.

Results

There were 5639 BPG patients (4600 [81.6%] IND and 1039 [18.4%]) DEP. DEP patients were significantly older (71.6 ± 11.8 vs 66.8 ± 11.8 years; P < .0001), had more chronic obstructive pulmonary disease (COPD) (16.7% vs 11.4%; P < .0001), diabetes (54.2% vs 40.7%; P < .0001), dialysis dependence (16.4% vs 5.6%; P < .0001), and critical limb ischemia (64.6% vs 44.0%; P < .0001). DEP patients had a higher incidence of death (6.1% vs 1.5%; P < .0001) and major complications (30.3% vs 14.2%; P < .0001). DEP was an independent predictor of major complications (odds ratio [OR]: 2.0; 95% confidence interval [CI]: [1.7-2.4]; P < .0001) major systemic complications (2.5 [1.9-3.2]; P < .0001), major operative site complications (1.6 [1.4-1.9]; P < .0001) and death (2.3[1.6-3.4]; P < .0001). The combination of DEP with emergency surgery, Cr > 1.8, or rest pain increased the odds of major complications by five, seven, or 11-fold, respectively. The combination of DEP with hemodialysis, emergency surgery, or age ≥80 years increased the odds of death by 13, 38, or 87-fold, respectively.

Conclusion

Preoperative DEP is significantly correlated with all adverse 30-day outcomes in BPG patients. Furthermore, when combined in high-risk composites with specific preoperative clinical variables, DEP is associated with prohibitive MM, thereby identifying patient cohorts that may be unsuitable for BPG.

Peripheral arterial disease (PAD) affects at least 10 million Americans.¹, ² With the increasing population of older Americans, PAD will further place a strain on health care resources. Behavior modification, medical therapy, exercise regimens, endovascular therapy, and open surgery all play a role in PAD management. Many factors including anatomic disease extent and clinical presentation, resource availability, operator expertise, patient preference, and comorbidities will influence treatment options for individual patients.³, ⁴

Although endovascular solutions have provided viable alternatives to bypass graft (BPG) in many patients with PAD,⁴, ⁵, ⁶ patients still require surgical lower extremity BPG.⁶, ⁷ Up to this point, prospective studies have perhaps surprisingly not shown that angioplasty is the preferred method of revascularization.⁸ A previous report from our group has defined the significant morbidity and mortality associations with contemporary BPG.⁹ Considerations to provide alternative therapies to patients identified as being at high risk for periprocedural complications (ie, percutaneous endovascular treatment, conservative management, or primary amputation), although attractive, should be tempered by the reality that these procedures have not been shown to have superior patient outcomes. In fact, one can argue that for certain patients with critical limb ischemia, bypass surgery remains the most appropriate management option.

In an era of strained health care resources,¹⁰ the ability to accurately identify patient cohorts at increased risk for complications after BPG is of paramount importance. From a purely economic point of view, major surgical complications are associated with a significant increase in cost even after adjusting for differences in patient characteristics.¹¹ From a therapeutic point of view, the ability to identify which patients are at “high risk” for substantial morbidity and mortality (MM) after BPG would considerably aid decision making.

Finally, given the evident limitations on life expectancy imposed by, in particular, critical limb ischemia (CLI),¹² minimizing complications and patient recovery after intervention for PAD is of paramount importance.

Various reports have documented low perioperative morbidity, mortality, and good BPG patency outcomes for various groups of patients considered “high risk” for BPG such as the elderly,¹³ patients with renal insufficiency,¹⁴ diabetes,⁷ and other preoperative predictors of poor operative outcome;¹⁵ however, quantitative stratification of MM for patients with a combination of these predictors has not been done. Stratification of patients based on preprocedural functional status has been previously described in elderly patients (≥80 years old) as an accurate predictor of postoperative functional outcomes.¹⁶ The purpose of our study is to quantify the predictive ability of a preoperative determination of functional status as a sensitive predictor of postoperative morbidity and mortality in a large population of patients undergoing infrainguinal BPG from the National Surgical Quality Improvement Program (NSQIP). Furthermore, preoperative functional status was then combined with other clinical variables known to be associated with increased perioperative MM to provide additional information for risk stratification of patients undergoing BPG.

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Methods 

The private sector NSQIP is a validated,¹⁷ independently adjudicated, prospective database of a systematic sample of cases which provides detailed patient demographics, procedural data, and information regarding 30-day mortality and major and minor morbidity. Utilizing procedure CPT codes for infrainguinal BPG, all data from patients between January 1, 2005 and December 31, 2007 were retrieved from the NSQIP database. Preoperative patient characteristics, clinical risk factors, and postoperative outcomes up to 30 days are collected prospectively from private and academic medical centers at each hospital by a risk-assessment nurse. Definitions for variables collected in the NSQIP database have been described in previous reports.¹⁸ Patient demographics and clinical comorbid conditions were obtained for each of the patients and were catalogued as previously described.¹⁷ All nominal variables were extracted directly from the NSQIP database except for race (Black and Hispanic patients were compared vs Caucasian patients), American Society Anesthesiology (ASA) category (ASA category 4/5 was compared vs ASA category 1/2/3), and dependent status (see next paragraph). Continuous variables used in our final analysis were dichotomized into clinically relevant categories (creatinine: Cr ≥ 1.8; Age: age ≥ 80 years old; body mass index: BMI ≥ 35).

Patients were stratified by preoperative functional status: independent (IND) vs dependent (DEP). This determination was performed at the time the patient was being considered as a candidate for surgery (no longer than 30 days prior to surgery). IND was defined as a patient who does not require assistance from another person for activities of daily living. This includes a person who is able to function independently with prosthetics, equipments, or devices. DEP is defined as someone who requires some (partially dependent) or total (totally dependent) assistance from another person for activities of daily living. This data was taken directly from the NSQIP database. The primary study end-point was 30-day MM. This included minor complications, operative site complications, major systemic complications, major complications, and death. Major systemic complications were limited to ventilator requirement for >48 hours, unplanned intubations, pneumonia, acute renal failure (dialysis requirement in a patient not requiring dialysis preoperatively), pulmonary embolism, stroke, coma >24 hours, cardiac arrest, and myocardial infarction. Major operative site complications included wound dehiscence, organ space infection (ie, abscess), deep wound infection, sepsis, bleeding requiring transfusion, and graft thrombosis. Overall major complications were a combination of major systemic and major operative site complications. Minor complications included superficial wound infections, urinary tract infections, deep venous thrombosis or thrombophlebitis, and peripheral nerve injuries.

Statistical analysis 

Baseline characteristics were compared across groups using χ², Fisher exact test, and t tests where appropriate. Multivariate analysis was performed using stepwise logistic regression and included univariate associations with a P < .05 of nonconfounding variables. Those variables with P value <.0005 from the logistic regression were then selected for inclusion in clusters to determine the effect of various combinations of comorbidities on short-term outcomes. For each possible combination of comorbidities, we used the individual regression parameter estimates to obtain a composite odds ratio that integrates the effects of each comorbidity in a cluster.¹⁹ All statistical calculations were performed with Statview 5.0.1 software.

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Results 

As detailed in Table I, functionally dependent patients were older (71.6 vs 67.5 years), had more congestive heart failure (7.9% vs 1.8%), a higher incidence of myocardial infarction in the prior 6 months (5.9% vs 1.4%), CLI (64.6% vs 44.0%), dialysis dependency (16.4% vs 5.6%) and were more likely to be categorized as either an ASA IV/V prior to the operation (33.1% vs 14.2%). Patients with DEP prior to their operation had an increased incidence of all study end-points. Death occurred in 6.1% of DEP, vs 1.5% of IND (P < .0001; Table II). Patients with DEP also had an increased incidence of major complications, major systemic complications, major operative site complications, and minor complications. By multivariate regression analysis for death in patients undergoing BPG, DEP increases the odds of death by greater than twofold as does a variety of other clinical variables (Table III). Dialysis was the most powerful independent correlate of death, with almost a sixfold increase compared with patients not on dialysis.

Table I. Demographic and clinical variables for functionally independent vs dependent patients

Demographics	Independent n = 4600 (81.6%)	Dependent n = 1039 (18.4%)	P value
Gender: male	67.5%(3107)	59.0%(613)	<.0001
Age (years)	66.8±11.8	71.6±11.8	<.0001
BMI kg/m2	27.8±6.3	26.5±6.3	<.0001
Respiratory
Ventilator dependent	0.04%(2)	1.8%(19)	<.0001
Smoke	42.5%(1957)	29.1%(302)	<.0001
COPD	11.4%(526)	16.7%(174)	<.0001
Dyspnea	18.0%(824)	24.1%(250)	<.0001
Cardiovascular
Congestive heart failure	1.8%(85)	7.9%(82)	<.0001
Myocardial infarction	1.4%(66)	5.9%(61)	<.0001
Previous coronary intervention	18.0%(829)	18.1%(188)	1.0
Hypertension medication	82.9%(3813)	87.1%(905)	.0009
Critical limb ischemia	44.0%(2022)	64.6%(671)	<.0001
Renal/metabolic
Diabetes	40.7%(1872)	54.2%(563)	<.0001
Dialysis	5.6%(258)	16.4%(170)	<.0001
Creatinine ≥ 1.8	12.6%(555)	24.4%(249)	<.0001
Neurologic
Impaired sensorium	0.7%(34)	4.2%(44)	<.0001
Hemiplegia	1.7%(78)	6.4%(67)	<.0001
TIA	6.3%(290)	7.5%(78)	.16
CVA with deficit	5.7%(261)	13.7%(142)	<.0001
Preoperative
ASA category 4/5	14.2%(655)	33.1%(344)	<.0001
Race
White	71.8%(3304)	66.9%(695)	<.0001
Black/Hispanic	20.0%(872)	24.6%(256)	<.0001

BMI, Body mass index; COPD, chronic obstructive pulmonary disease.

Table II. Incidence of primary end-points in functionally dependent vs independent patients undergoing lower extremity revascularization procedures

Outcomes	Independent n = 4600 (81.6%)	Dependent n = 1039 (18.4%)	P value
Death	1.5%(68)	6.1%(63)	<.0001
Major complications	14.2%(654)	30.3%(315)	<.0001
Major systemic complications	3.7%(170)	13.0%(135)	<.0001
Major operative site complications	12.0%(553)	21.2%(220)	<.0001
Minor complications	10.0%(458)	73.3%(762)	<.0001

Table III. Multivariate regression analysis of death in patients undergoing lower extremity revascularizations

OR, Odds ratio; CI, confidence interval.

Table IV shows the results of multivariate regression analysis for major complications, major systemic complications, operative site, and minor complications for patients undergoing BPG. DEP was a significant predictor of all complication categories with odds ratios ranging from 1.6 to 2.5. Emergency surgery was a significant predictor of major systemic and operative site complications, while patients at high risk for surgery per ASA classification (Class IV/V), high baseline creatinine, or previous ventilator dependency were the strongest predictors for major systemic complications. Tibial outflow was a significant predictor of major operative site complications. Table V shows the “high-risk composite” analysis for predictors of death after lower extremity bypass operations. Combined with dependent status, dialysis dependency predicted the highest odds of death (>13-fold). The addition of rest pain to the preceding combination increased the odds of death from BPG >28-fold. Finally, patients with DEP who are hemodialysis dependent, ≥80 years old, and present with rest pain as an emergency had greater than 200-fold increased odds of death after BPG.

Table IV. Multivariate regression analysis of major and minor complications in patients undergoing lower extremity revascularizations

End-points	OR	95% CI	P value
Major complications
Dependent functional status	2.0	1.7-2.4	<.0001
Critical limb ischemia	1.5	1.3-1.7	<.0001
Emergency surgery	2.5	1.9-3.2	<.0001
ASA 4/5	1.4	1.2-1.7	.0001
Creatinine ≥ 1.8	1.5	1.2-1.7	.0001
Major systemic complications
Dependent functional status	2.5	1.9-3.2	<.0001
Age ≥ 80 years	1.7	1.3-2.2	.0003
Ventilator dependency	7.2	2.6-19.8	.0001
Emergency surgery	2.3	1.6-3.3	<.0001
ASA 4/5	2.1	1.6-2.7	<.0001
Creatinine ≥ 1.8	2.0	1.5-2.7	<.0001
Major operative site complications
Dependent functional status	1.6	1.4-1.9	<.0001
Tibial outflow	1.6	1.4-1.9	<.0001
Critical limb ischemia	1.5	1.3-1.7	<.0001
Emergency surgery	2.1	1.6-2.8	<.0001
Minor complications
Dependent functional status	1.7	1.4-2.0	<.0001
Gender: female	1.4	1.2-1.7	.0001
BMI ≥ 35	1.8	1.4-2.2	<.0001

OR, Odds ratio; CI, confidence interval; BMI, body mass index.

Table V. Results of combined odds ratios for death for patients undergoing lower extremity bypass operations

OR, Odds ratio; CI, confidence interval.

The results of high-risk composite calculations for major complications (Table VI), major systemic complications (Table VII), major operative site complications, and minor complications (Table VIII) for patients undergoing lower extremity bypass operations are shown. DEP, a high baseline creatinine, high operative risk (ASA IV/V), and emergent presentation increased the odds of major complications greater than 10-fold. If the patient presents with critical limb ischemia, the odds increases by greater than 15-fold above baseline. Operative site complications are increased greater than fivefold in patients with DEP presenting with critical limb ischemia as an emergency, or if alternatively their outflow was to a tibial vessel. Minor complications were increased greater than fourfold in DEP females with a BMI > 35. Ventilator dependency prior to operation strongly influenced the incidence of major systemic complications. Combined with DEP, major systemic complications are increased greater than 18-fold. The successive addition of emergency status, high baseline creatinine and age ≥ 80 increased the odds of major systemic complications greater than 40-fold, greater than 76-fold, and greater than 138-fold, respectively.

Table VI. Results of combined odds ratios for major complications for patients undergoing lower extremity bypass operations

OR, Odds ratio; CI, confidence interval.

Table VII. Results of combined odds ratios for major systemic complications for patients undergoing lower extremity bypass operations

OR, Odds ratio; CI, confidence interval.

Table VIII. Results of combined odds ratios for major operative site and minor complications for patients undergoing lower extremity bypass operations

OR, Odds ratio; CI, confidence interval; BMI, body mass index.

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Discussion 

Our results show the presence of total or partial preoperative dependent functional status is a sensitive predictor for all major negative 30-day outcomes in patients undergoing lower extremity bypass operations. Furthermore, when combined in high-risk composites with specific preoperative clinical variables, dependent functional status can be associated with prohibitive morbidity and mortality.

Until recently, therapies for patients presenting with critical limb ischemia and disabling claudication were limited to distal revascularization, exercise regimens with or without pharmacological therapy, conservative wound management, and primary amputation when appropriate. Because of the limited therapeutic options, clinical research focused primarily on patency and outcomes. The coming of age of endovascular therapies for the treatment of lower extremity ischemia, especially in the last decade, has provided an effective substitute therapy to traditional open surgery.⁴, ⁵, ⁶, ²⁰ While the long-term patency of lower extremity endovascular revascularizations may not rival that of traditional open surgery, perioperative outcomes, including death, major, and operative site complications are generally reduced with this less invasive approach.⁴, ²⁰ Results from the multicenter, randomized bypass vs angioplasty in severe ischemia of the leg (BASIL) trial showed at 1 year that in patients presenting with severe limb ischemia who are suitable for both bypass and angioplasty, major outcomes are similar and surgery more expensive than angioplasty.⁸ Because of this, a renewed focus to improve our ability to identify those patients at increased risk for perioperative death and complications is warranted.

It is known that infrainguinal bypass operations are accompanied by significant morbidity and mortality.⁷ In a prior report of 2404 patients from the NSQIP database, we demonstrated an overall mortality of 2.7%. Major complications occurred in 18.7% and major systemic complications in 5.9% of patients. Major operative site complications occurred in 15.1% of patients. Final analysis revealed that the combination of dialysis and age >80 years was the most powerful high-risk composite for both death (13.3-fold) and major complications (2.2-fold).⁹ This mortality and complication rate is in the range of other NSQIP²¹ and large non-NSQIP reports²², ²³ of lower extremity revascularization.

A number of stratification systems have been used to preoperatively evaluate potential morbidity and mortality in surgical patients,²⁴, ²⁵, ²⁶ although not specific to vascular surgical patients. Recently, a variation of the ASA's Physical Status Classification was used to risk stratify patients undergoing leg revascularization for disabling claudication or critical limb ischemia. This report by Dosluoglu and colleagues, separated patients classified as ASA III (moderately severe systemic disorder) into two categories based on their ability to perform less than four metabolic exercise equivalents (METS). Multivariate analysis showed that being ASA IIIA (<4 METS) or IIIB (≥4 METS) was an independent predictor of survival for this cohort. These finding are limited by several factors. First, findings are confined to patients placed in ASA category III. Second, the predictive abilities of ASA classification were limited to mortality, with no further ability as a predictor of complications. Finally, although widely used, ASA categorization can be performed inconsistently and inaccurately, a problem that could be improved by the inclusion of preoperative risk factors commonly recorded in the current NSQIP database into preoperative risk assessment schemes.²⁷

Functional status prior to lower extremity bypass has been previously used as an indicator of postoperative functional outcome in patients undergoing lower extremity bypass operations. Abou-Zamzam and colleagues examined the records of over 500 cases to determine the relationship between preoperative and postoperative living situation (independent or dependent) and ambulatory status for patients undergoing infrainguinal bypass for limb salvage indications. Multivariate analysis showed that preoperative living situation and ambulatory status was predictive of outcome at 6 months, ie, 99% of survivors who lived independently before the operation, continued to do so after the surgery, whereas only 4% of survivors not living independently before the surgery achieved independence after the procedure.²⁸

A more recent report by Taylor et al reviewed the influence of preoperative medical and functional status on postoperative outcomes in patients undergoing infrainguinal revascularization for critical limb ischemia in patients 80 years or older.¹⁶ This single institution report of 122 patients (57 open surgery; 65 angioplasty) categorized patients as either functionally ambulatory, homebound ambulatory or transfer only ambulatory. No bedridden or nonambulatory patients were offered revascularization. Analysis based on open vs endovascular intervention showed a significantly worse overall and amputation free survival for patients undergoing endovascular procedures at 36 months. It is important to note that only 38% of functional ambulators were offered an endovascular option, whereas 80% of transfer-only ambulators had angioplasty as their revascularization procedure, indicating significant selection bias. No attempt was made to control for baseline comorbidities between the two groups. When looking at the results stratified for preoperative functional status, there was a statistically significant deterioration in outcomes (survival, amputation-free survival, and secondary patency) with declining functional status at 36 months. Finally, using a Cox proportional hazard model, patients who were homebound ambulators prior to their procedures were two times more likely to experience death by 36 months, and patients who were transfer-only ambulators five times more likely. Although preoperative functional status is clearly shown to be predictive of outcome, the single institution nature and the restrictive cohort (patients >80 years old) does not lend itself to broad generalization of the findings of this study.

Based partly on a previous report of the cost associated with excessive morbidity, complications were categorized as major, minor, and operative site complications.¹¹ Our study, based solely on 30-day outcomes from the NSQIP database, identified dependent functional status as a predictor of death even after controlling for preprocedural variables. The combined death rate for our study was 2.3%, which is in the range of earlier published large series.⁷, ⁸, ²² Other variables that independently predicted death in our cohort (age ≥80 years, rest pain at presentation, emergency surgery, and dialysis dependency) have been previously identified as predictors of death in other large studies.⁸, ²², ²⁹, ³⁰, ³¹ When combined in a high-risk composite with dependent functional status, dialysis dependency and emergent presentation for surgery produced the strongest predictors of perioperative death (13 and 6.6-fold increase). When all three were present, there was a 38-fold increase in the incidence of death. Attempting as yet, less durable therapeutic options such as endovascular procedures, or foregoing revascularization altogether in favor of primary amputation should be given consideration given the prohibitive risk of death incurred by patients presenting with these risk-factor combinations, although these therapeutic options are not without significant morbidity and mortality.

The impact of complications on costs, long-term disability and functional outcome in patients undergoing lower extremity revascularizations cannot be underestimated. In an era of significant economic pressure, curbing health care expenses has assumed prominence in the national spotlight.¹⁰ A report by Dimick et al showed that the occurrence of major complications was associated with a significant increase in hospital costs.¹¹ As a sensitive indicator of such, dependent functional status should immediately raise a major concern for the possibility of poor outcomes. This uncomplicated preoperative determination of a patient's ability to perform activities of daily living independently predicted the occurrence of major, major systemic, operative site, and minor complications in patients undergoing lower extremity bypass operations.

Our data shows a 19.1% incidence of major complications, of which 13.7% were operative site and 5.4% major systemic complications. This data is in line with the data we reported previously from a more limited data-set (18.7% incidence of major complications).⁹ Not surprisingly, ventilator dependence prior to the procedure had by far the greatest impact when combined with dependent status on the incidence of major systemic complications. Major complications have been shown to have a significant impact by increasing hospital costs,¹⁶ and in this report occurred more than twice as often in our dependent cohort. Major complications include myocardial infarctions which occurred in 0.8% of patients. This perioperative rate is lower than that reported in other large series.⁸, ³² Dependent patients had a 1.4% incidence of perioperative myocardial infarctions; double that of independent patients (0.7%).

Operative site complications are an important contributor to higher hospital cost and a source of significant morbidity for patients undergoing lower extremity bypass operations. Our report includes graft thrombosis as part of our operative site complications. It is, therefore, not surprising that more distal tibial outflow predicted the occurrence of such complications and was the second highest predictor after emergency presentation. This finding is consistent with other NSQIP reports on infrainguinal bypass graft.³³ Graft patency rates have been an important component in the outcome analysis for lower extremity bypass operations, if perhaps more so for the analysis of long-term patency, which is not possible with the current NSQIP data collection protocol. Previous reports have documented successful outcomes after distal bypass operations with low (4.2%) 30-day thrombosis rates, severe wound infection rates (2.0%), and a 6.6% rate of early re-operation for bleeding, infection or thrombosis.¹³ This study reported on the incidence infections requiring reintervention. Accordingly, it is not surprising that our reported rate of operative site complications in the nondependent cohort (12%) is somewhat higher, as it includes all infections and transfusions for bleeding not necessarily requiring reintervention. In dependent patients, the rate of operative site complications increased to 21%, again showing dependent status is a sensitive indicator of poor outcomes in our cohort.

Dependent patients had a staggering (73%) rate of minor complications, compared with only 10% in the nondependent cohort. Female sex predicted increased complications, a finding that has been detailed in other reports of infrainguinal bypass.³⁴ Although individually, minor complications may not appear to add significantly to the increase in hospital costs,¹¹ as an aggregate they present a major burden and reflect the susceptible nature of this patient cohort.

There are several limitations in our study. First, our analysis categorizes complications based partly on established criteria. While this categorization makes intuitive sense; however, it is somewhat arbitrary and different categorizations could be made which may affect our results. A second shortcoming relates to the subjective nature of the determination of dependent status. This is done by a trained research nurse who is part of the NSQIP at each institution. Patients are categorized based on their ability to perform activities of daily living. As such, considerable variability could be expected in the determination of dependent status. Perhaps, a more objective scoring system could be applied in the future to prospectively validate these results and make it easier for them to be applied by other non-NSQIP institutions. Our intent in this study was to identify those variables that would have the highest predictive value for the occurrence of complications when combined with dependent functional status. Because of this, the comorbidities used to generate our high-risk composites were limited to those variables that on multivariate analysis had P < .0005. This excluded some traditional, well known variables that on multivariate analysis were significant predictors of complications, but focused our analysis on the variables that were the strongest predictors of morbidity and mortality.

In conclusion, this study details the utility of functional status as a predictor of morbidity and mortality for patients undergoing lower extremity bypass graft. Prospective validation of functional dependence, alone, or in combination with other commonly measured preoperative variables should be obtained.

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Author contributions 

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References 

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