Operative mortality for renal artery bypass in the United States: Results from the National Inpatient Sample

Article Outline

• Abstract
• Material and methods
  • Database
  • Statistical analysis
• Results
• Discussion
• Conclusions
• Author contributions
• Table I (online only). 
• References
• Copyright

Background

The mortality rate for renal artery bypass grafting (RABG) is reported to be 0% to 4% for patients with renovascular hypertension and 4% to 7% for patients with ischemic nephropathy. However, these data come from high-volume referral centers known for their expertise in treating these conditions. Because of the relative infrequency of these operations in most vascular surgery practices, the nationwide outcomes for RABG are not known. The purpose of this study was to define the operative mortality rate for RABG in the United States and to identify risk factors for perioperative mortality.

Methods

The National Inpatient Sample was analyzed to identify patients undergoing RABG for the years 2000 to 2004. Categoric data were analyzed using χ² and the Cochran-Armitage trend tests. Multivariate logistic regression analyses were performed to identify risk factors for perioperative mortality after RABG.

Results

During the study period, 6608 patients underwent RABG, representing a frequency of 3.51 operations per 100,000 discharges. More than two-thirds were performed at teaching hospitals (4564 vs 2,044; P < .0001). The frequency of RABG decreased by 30.7% between 2000 and 2004 (4.28 vs 2.96 RABGs per 100,000 discharges; P for trend < .0001). The in-hospital mortality for RABG was 10.0%. On univariate analysis, in-hospital mortality after RABG varied with increasing age, race, region of the country, and a preoperative history of chronic renal failure, congestive heart failure, or chronic lung disease. Logistic regression models identified advanced age (odds ratio [OR] 1.57; 95% confidence interval [CI], 1.44-1.72], female gender (OR, 1.20; 95% CI, 1.02-1.41), and a history of chronic renal failure (OR, 2.21; 95% CI, 1.75-2.78), congestive heart failure (OR, 1.94; 95% CI, 1.44-2.62), or chronic lung disease (OR, 1.40; 95% CI, 1.18-1.67) as independent markers of risk-adjusted, in-hospital mortality (P < .0001 for each of these five variables).

Conclusions

Nationwide in-hospital mortality after RABG is higher than predicted by prior reports from high-volume referral centers. Advanced age, female gender, and a history of chronic renal failure, congestive heart failure, or chronic lung disease were predictive of perioperative death. For the typical vascular practice, these data may provide a rationale for lower risk alternatives, such as renal artery stenting or referral to high-volume referral centers for RABG.

For several decades, renal artery bypass grafting (RABG) has been a mainstay of treatment for renovascular hypertension (RVH) and ischemic nephropathy. Treatment benefits have generally outweighed the operative risk associated with these procedures. RABG has been associated with durable improvement or cure of hypertension in >82% of patients with RVH¹, ², ³, ⁴, ⁵, ⁶ and with a salutary effect on renal function in 43% to 80% of patients with ischemic nephropathy.⁷, ⁸, ⁹, ¹⁰ In published series, operative mortality has ranged from 0% to 4.6% for RVH and 0% to 7.3% for ischemic nephropathy.¹, ², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁹, ¹⁰ However, these mortality rates may not reflect the national experience because they were reported by large referral centers with documented expertise in the surgical treatment of renovascular disease. Furthermore, the rapid increase in the number of percutaneous interventions for renovascular disease in the past several years has led to a drastic reduction in surgical RABG volume.¹¹ Because of the relative infrequency of RABG in most vascular surgery practices today, the nationwide outcomes for RABG are not known. The purpose of this study was to define the operative mortality rate for RABG in the United States and to identify predictors of perioperative mortality.

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Material and methods 

Database 

The Nationwide Inpatient Sample (NIS) from the Healthcare Cost and Utilization Project (HCUP) was used to identify all patients undergoing RABG for the years 2000 to 2004. This database represents the largest all-payer inpatient database in the United States.¹² The NIS consists of a 20% stratified sampling of inpatient admissions to US acute care hospitals. The database represents >1000 hospitals with >38 million discharges annually from a variable number of states, ranging from 32 states in 2000 to 37 states in 2004. Patient demographics, primary and secondary diagnoses and procedures reported by International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis and procedure codes, and clinical outcomes are reported in the database. The database lists 29 comorbidities defined by the HCUP (Table I, on-line only based on the presence of secondary diagnoses after excluding postoperative complications and diagnoses directly related to the primary diagnosis. To generate national estimates of operation frequencies and outcome events, both hospital and discharge weights are provided, including data to calculate the variance of estimates. The NIS databases are available to the public as aggregate data without personal identifiers, making this study exempt from review by the Institutional Review Board.

Renal artery bypasses were identified using a combination of ICD-9-CM procedure and diagnosis codes. The ICD-9-CM procedure codes for aortorenal bypass (39.24) and “other abdominal bypass” (39.26) were merged with the diagnosis codes for renal atherosclerosis (440.1) and fibromuscular dysplasia (447.3) to identify patients undergoing RABG for these diagnoses. This strategy captured a variety of open surgical RA reconstructions, including aortorenal bypass, hepatorenal bypass, splenorenal bypass, and iliorenal bypass.

Comorbidities were itemized and used to calculate a modified Charlson comorbidity index (CCI).¹³, ¹⁴ The CCI is a global measure of comorbidities that is calculated for individual patients according to the presence of the four atherosclerotic comorbidities of peripheral arterial disease, myocardial infarction, cerebrovascular disease, and congestive heart failure (CHF), and 13 nonatherosclerotic comorbid conditions, including diabetes mellitus with and without complications, chronic lung disease, gastrointestinal ulcer, arthritis, paraplegia, renal failure, malignancy with and without metastasis, acquired immunodeficiency syndrome, dementia, liver disease, and liver failure.¹³, ¹⁴ The CCI has been validated for administrative databases and correlated with operative morbidity and mortality for major abdominal vascular operations, including RABG.¹³, ¹⁵

Hospitals were classified as teaching or nonteaching hospitals by the presence of any residency approved by the Accreditation Council for Graduate Medical Education or membership in the Council of Teaching Hospitals. Designation as an urban or rural hospital was according to 2000 Census definitions of urban population (≥50,000) or rural population (<50,000).¹⁶

Statistical analysis 

The primary end point of the study was in-hospital mortality after RABG. Categoric data were analyzed using χ² and the Cochran-Armitage trend tests. Continuous data were reported as medians and interquartile ranges (IQR). Stepwise multivariate logistic regression analyses were performed to identify predictors of perioperative death after RABG. The variables included in the model were those with a P < .10 and other variables hypothesized as being related to RABG mortality on theoretic grounds, as prescribed by Katz.¹⁷

The analyses were performed taking into account the sampling design of the NIS. NIS stratum was used as the stratification variable. This variable is a four-digit stratum identifier used to poststratify hospitals for the calculation of universe and frame weights and contains information about the NIS sampling strata. The PROC SURVEYFREQ and PROC SURVEYLOGISTIC procedures of the SAS software (SAS Inst, Cary, NC) were used to perform two-way table and logistic regression analyses, respectively. To produce national estimates, all analyses were weighted using the discharge-level weight variable provided in the NIS dataset. For all statistical analyses, the threshold for significance was .05. Statistical analysis was performed using SAS 9.13 software.

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Results 

During the study period (2000 to 2004), RABG procedures were performed on 6608 patients nationwide, in whom 1.6% were performed for fibromuscular dysplasia and atherosclerosis was the indication for operation in the rest. In a subset of patients, the RABG was performed in combination with open abdominal aortic aneurysm (AAA) repair (0.6% of all RABGs), thoracoabdominal aortic aneurysm (TAAA) repair (0.3% of all RABGs), or aortobifemoral bypass (29.5% of all RABGs). RABG operations are relatively rare, performed at a frequency of 3.51 operations per 100,000 discharges. The frequency of RABG decreased by 30.7% between 2000 and 2004 (P for trend < .0001).

The demographics and comorbid conditions of the study population are outlined in Table II. The patient population was typically male and white, with a median age of 68 years (IRQ, 60-74 years). The extent of preoperative comorbidity was scored using a modified CCI (Table II). The median CCI was 1 (IQR, 1-2). The hospitals performing RABGs are characterized in Table III. More than two-thirds of RABGs were performed at teaching hospitals, even though teaching hospitals represented 17.4% of the hospitals in the NIS database. Urban hospitals and hospitals in the southern United States were disproportionately represented as sites for RABG.

Table II. Patient demographics and comorbidities

Characteristic	Patients, No. (%)	Pa
Sex		<.0001
Male	3494(52.9)
Female	3114(47.1)
Age, years		<.0001
1-17	73(1.1)
18-44	266(4.0)
45-64	2073(31.4)
65-84	4039(61.1)
≥85	104(1.6)
Unknown age	52(0.8)
Race		<.0001
White	4248(64.3)
African American	289(4.4)
Hispanic	141(2.1)
Asian/Pacific Islander/other	177(2.7)
Unknown	1754(26.5)
Primary expected payer		<.0001
Medicare/Medicaid	4299(65.4)
Private insurance	2094(31.9)
Self-pay/other	183(2.7)
Comorbidity
Congestive heart failure	327(4.9)
Peripheral arterial disease	4089(61.9)
Diabetes	801(12.1)
Chronic pulmonary disease	1914(29.0)
Chronic renal failure	598(9.1)
Myocardial infarction	745(11.3)
Cerebrovascular disease	139(2.1)
Obesity	110(1.7)
Charlson comorbidity index score		<.0001
0	1091(16.5)
1	2575(39.0)
2	1737(26.3)
≥3	1205(18.2)

Table III. Hospital characteristics

Characteristic	Percentage	Pa
Hospital type		<.0001
Teaching	69.1
Nonteaching	30.9
Hospital location		<.0001
Urban	96.0
Rural	4.0
Hospital region		<.0001
South	42.8
Midwest	23.6
Northeast	22.0
West	11.6

The crude in-hospital mortality rate for RABG was 10.0% nationwide during the study period. Subset analysis showed the mortality rate for RABG differed according to the indication for surgery. No deaths occurred in 105 RABGs performed for fibromuscular dysplasia, but the mortality rate was 10.1% (P = .0011) for those performed for atherosclerosis. The in-hospital mortality rate was not significantly different for RABG combined with AAA repair (11.5%) or aortobifemoral bypass (9.9%) compared with RABG alone (9.9%; P = .95). The mortality rate for RABG performed with TAAA repair was 0% (P = .27 vs RABG alone), although only 20 cases were reported to the NIS database during the 5-year study period.

On univariate analysis, mortality rates varied significantly according to age, race, region of the country, and primary expected payer (Table IV). Preoperative comorbidities, including chronic renal failure, CHF, and chronic lung disease, had a substantial effect on in-hospital mortality (Table IV). Mortality also varied according to the CCI (Table IV). Mortality was not affected significantly by sex, type of hospital (teaching vs nonteaching), hospital location (urban vs rural), or the presence of obesity (Table IV).

Table IV. In-hospital mortality rates

Variable	Mortality rate (%)	Pa
Sex		.13
Male	9.4
Female	10.6
Age, years		<.0001
1-17	0
18-44	3.6
45-64	6.5
65-84	12.0
≥85	24.6
Race		<.0001
White	10.2
African American	17.3
Hispanic	3.2
Asian/Pacific Islander/other	10.8
Hospital type		.21
Teaching	9.7
Nonteaching	10.7
Hospital location		.55
Urban	10.0
Rural	8.9
Hospital region		<.0001
South	9.5
Midwest	12.6
Northeast	9.6
West	7.0
Primary expected payer		<.0001
Medicare/Medicaid	12.1
Private insurance	6.2
Self-pay/other	5.8
Indication for surgery		.0011b
Atherosclerosis	10.1
Fibromuscular dysplasia	0.0
Chronic renal failure		<.0001
Absent	9.2
Present	18.1
Congestive heart failure		<.0001
Absent	9.5
Present	18.5
Chronic lung disease		.0003
Absent	9.1
Present	12.1
Obesity		.37
Absent	9.9
Present	12.5
Charlson comorbidity index score		.0001c
0	9.1
1	9.0
2	9.2
≥3	14.1

To identify independent risk factors for in-hospital mortality, 10 variables were entered into a stepwise logistic regression model: age, sex, hospital teaching status (teaching vs nonteaching), hospital region, hospital location (urban vs rural), primary expected payer, chronic renal failure, CHF, chronic lung disease, and obesity. Multivariate analysis identified age, female gender, chronic renal failure, CHF, and chronic lung disease as independent predictors of risk-adjusted in-hospital mortality (Table V). In fact, the odds of in-hospital mortality were increased 57% for every 10-year increase in age. The presence of chronic renal failure increased the risk of in-hospital mortality by 120%, whereas CHF increased the mortality risk by 94%. Chronic lung disease produced a 40% increase in the risk of in-hospital mortality. Female gender had the least affect on mortality among these variables, increasing the risk by 20%.

Table V. Independent predictors of in-hospital mortality

Predictor	Odds ratio⁎ (95% Confidence interval)	P Value
Age (per 10 year increase)	1.57(1.44-1.71)	P<.0001
Female gender	1.20(1.02-1.42)	P<.0001
Chronic renal failure	2.21(1.75-2.78)	P<.0001
Congestive heart failure	1.94(1.44-2.62)	P<.0001
Chronic lung disease	1.40(1.18-1.67)	P<.0001

To assess the impact of the entire constellation of comorbidities on mortality, a secondary stepwise logistic regression analysis was performed with the CCI score rather than with individual comorbidities. This model confirmed that age (odd ratio [OR], 1.57; 95% confidence interval (CI), 1.44-1.72; P < .0001), female gender (OR, 1.20; 95% CI, 1.12-1.28; P < .0001), and CCI (OR, 1.20; 95% CI, 1.02-1.41; P < .0001) were independent predictors of in-hospital mortality. These data demonstrated that operative mortality increased by 20% for every 1-point increase in CCI.

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Discussion 

RABG is a relatively rare operation. During the 5-year study period (2000 to 2004), RABG was performed at a frequency of 3.5 operations per 100,000 discharges. Our data indicate that the frequency of RABGs has declined 30% from 2000 to 2004. A prior study from the University of Michigan documented a 56% drop in RABG operations between 1988 and 2001, coinciding with a 173% increase in the frequency of catheter-based treatments of renovascular disease.¹¹ Whether this trend in the frequency of RABG operations is a direct consequence of the proliferation of percutaneous RA stenting would be conjecture. Nonetheless a potential consequence of this trend in the frequency of RABG is the risk of diluting the experience with RABG among individual surgeons and hospitals to such an extent that it adversely impacts outcomes. To address this question, we hypothesized that the nationwide operative mortality rate for RABG is significantly higher than previously reported by individual high-volume referral centers. Indeed, using the largest all-payer inpatient database in the United States, the NIS database, we found that the crude in-hospital mortality rate for RABG was 10.0% during the study period.

A 10% nationwide mortality rate for RABG stands in stark contrast to those reported in single-center studies published during the past several years. Several studies have documented 30-day mortality rates of 0% to 4.6% for RVH and 0% to 7.3% for ischemic nephropathy.¹, ², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁹, ¹⁰ In perhaps the largest series of RA reconstructions reported to date, the group at Wake Forest University reported a series of 626 patients undergoing RA reconstruction during a 13-year period.¹ Within this cohort, 397 patients underwent RABG. Although the mortality rate for RABGs was not reported separately, the 30-day mortality for the entire cohort was 4.6%. Considering the burden of cardiovascular disease in this subset of patients,¹⁸, ¹⁹ this mortality rate is laudable.

It should be recognized, however, that this mortality rate is the product of a high-volume referral center that performed a minimum of 30 RABG operations per year from 1987 to 1999. Our data on the nationwide frequency of RABG operations (3.5 operations per 100,000 discharges) suggest that RABGs are rare operations in most vascular practices. Given this disparity in the frequency of RABG operations between practices, it is perhaps not surprising that nationwide mortality rates for RABG are higher than those reported by centers that frequently perform these operations.

The gap in mortality rates between large referral centers and nationwide data is recapitulated for patients operated on for presumed ischemic nephropathy. Hansen et al⁷ from Wake Forest University reported a series of 232 patients undergoing renal revascularization for ischemic nephropathy during an 11-year period.⁷ Two-thirds of patients underwent RABG, but this subset was not analyzed apart from the larger study population. The operative mortality rate for the entire cohort was 7.3%. Marone and the Massachusetts General Hospital group reported a series of 96 patients with ischemic nephropathy,⁸ of whom 76% underwent RABG. Again, the RABG procedures were not analyzed separately, but the overall mortality was 4.1%. The current study, in contrast, found that patients with chronic renal failure had an in-hospital mortality rate of 18.1%, whereas patients without renal failure had a 9.2% mortality rate (Table IV). These data demonstrate once again that the nationwide mortality rates are more than twofold higher than those reported by high-volume referral centers.

An additional goal of this study was to define the variables that may increase the risk of in-hospital mortality for RABG surgery. Our data analyses identified advanced age, female gender, chronic renal failure, CHF, and chronic lung disease as independent predictors of in-hospital mortality after RABG (Table V). These data reinforce prior studies that identified advanced age, chronic renal failure, and CHF as risk factors for operative mortality after RABG.¹, ⁷ Although chronic lung disease had not been previously associated with an increased risk of death after RABG, it has been associated with increased mortality for AAA repair.²⁰ It is not clear why women are predisposed to a higher risk of in-hospital mortality after RABG. This finding deserves further investigation.

The mortality data reported in the current study are sobering. A central question raised by these data is: What distinguishes centers with acceptably low mortality rates for RABG from other centers? We hypothesize that improved results reported in single-center studies are related to the volume of RABG operations performed in those centers. It is likely that both surgeons and ancillary personnel in these centers develop experience and expertise in patient selection, operative management, and perioperative care that translates into improved outcomes. The concept of volume-related improvements in outcomes has received significant attention in recent years. Using large administrative databases, several groups have documented that outcomes for carotid endarterectomy and AAA repair are related to hospital and provider volume for the index procedure.²¹, ²², ²³, ²⁴, ²⁵, ²⁶ We surmise that RABG is another example of this phenomenon, although further investigation is required to address this hypothesis.

The implications of this study for clinical practice are significant. First, an operative mortality rate of 10% for RABG casts an unflattering light on an operation with documented ability to improve long-term survival among patients with a favorable response of blood pressure or renal function.¹, ⁷ Factoring these data into the risk-benefit analysis for RABG may mitigate against surgical revascularization in many patients. In an era of unprecedented scrutiny by federal agencies, third-party payers, and the public sector, it is incumbent upon surgeons to heed these unfavorable results and collectively strive to improve outcomes. It would be reasonable, for instance, for surgeons and hospitals to prospectively monitor outcomes for RABG analogous to other surgical quality-improvement initiatives, such as the National Veterans Administration Surgical Quality Improvement Program and the Northern New England Cardiovascular Disease Study Group, that have improved surgical outcomes.²⁷, ²⁸

These data also have profound implications on patient selection for RABG. We identified five predictors of a higher risk of in-hospital mortality after RABG: advanced age, female gender, and preoperative chronic renal failure, CHF, or chronic lung disease. The presence of multiple predictors in a given patient may warrant consideration of lower-risk alternatives, including medical therapy, extra-anatomic bypass, or RA stenting. Despite the potential benefits of renal revascularization in survivors, high operative mortality rates for RABG may strengthen the argument for medical management rather than surgery in patients with RVH or ischemic nephropathy. Several studies have documented a lower risk of operative mortality for hepatorenal and splenorenal bypasses, making this an attractive surgical option in some patients.², ²⁹, ³⁰ Percutaneous RA stenting offers the obvious advantages a lower risk of morbidity and mortality and more rapid return to full functional status compared with surgery. Unfortunately, percutaneous renal revascularization has also yielded less favorable outcomes for blood pressure and renal function.³¹, ³² Finally, for surgeons who do not perform RABG regularly, these data may provide a rationale for referral to high-volume centers with documented expertise and acceptable outcomes for RABG.

The current study has some limitations that deserve comment. First, the use of administrative databases for population-based outcome studies has been criticized by some authors.³³, ³⁴ These investigators contend that data derived from administrative databases may be contaminated by inaccurate coding, undercoding of comorbidities, nonstandardized mortality end points, and use of the data for analyses for which they were not intended. These authors have proposed the use of clinical databases that have extensively been audited for accuracy, rather than administrative databases. The shortcoming of this approach lies in the relative lack of statistical power provided by the small volume of RABG operations performed in each center. Moreover, the largest clinical databases would be derived from high-volume referral centers with expertise particular expertise in treating renovascular disease, which may introduce another source of bias.

Concerns about the end point of the study, in-hospital mortality, are valid because in-hospital mortality is not necessarily equivalent to 30-day mortality. This discrepancy may actually underestimate the true 30-day mortality rate for RABG, which would not alter the conclusions of the current study. In addition, it is beyond the scope of this study to assess the impact of extra-anatomic bypasses on mortality for RABG.

Perhaps the most significant shortcoming of these data is the inability to pinpoint what distinguishes centers with acceptably low mortality rates for RABG from other centers. Future studies will need to address this issue.

A final issue relates to the true mortality rate in centers that do not regularly perform RABG operations. Because a disproportionate volume of RABGs are performed by a few centers, it is possible that the excellent results from these centers actually lower the nationwide mortality rate. This logic would suggest that the mortality rate for other centers is actually higher than 10%, which would pose an even greater concern.

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Conclusions 

The current study found that nationwide in-hospital mortality after RABG is significantly higher than predicted by prior reports from individual high-volume referral centers. Several predictors of operative mortality were identified that may assist with patient selection for RABG. For surgeons who do not regularly perform RABG operations, these data may provide a rationale for lower-risk alternatives, such as RA stenting, or referral to high-volume centers for RABG.

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Author contributions 

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Table I (online only) 

Table I (online only). Comorbidities defined by Healthcare Cost and Utilization Project

Comorbidity

Acquired immune deficiency syndrome

Alcohol abuse

Deficiency anemias

Rheumatoid arthritis/collagen vascular diseases

Chronic blood loss anemia

Congestive heart failure

Chronic pulmonary disease

Coagulopathy

Depression

Diabetes, uncomplicated

Diabetes with chronic complications

Drug abuse

Hypertension

Hypothyroidism

Liver disease

Lymphoma

Fluid and electrolyte disorders

Metastatic cancer

Other neurologic disorders

Obesity

Paralysis

Peripheral vascular disorders

Psychoses

Pulmonary circulation disorders

Renal failure

Solid tumor without metastasis

Peptic ulcer, excluding bleeding

Valvular heart disease

Weight loss

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