Effect of Hospital Volume on In-Hospital Mortality for Renal Artery Bypass

Background: A recent report determined that the nationwide mortality for renal artery bypass (RAB) is surprisingly high--10%. This observation raises an important question: What distinguishes centers with acceptably low mortality rates for RAB from other centers? We hypothesized that operative mortality for RAB is related to the volume of such operations performed in each center.
Methods: The Nationwide Inpatient Sample was analyzed to identify patients undergoing RAB for the years 2000-2005. Hospitals were classified into tertiles of low-, medium-, and high-volume hospitals according to the number of RAB operations performed annually. In-hospital mortality for RAB was compared between groups. To adjust for differences in the patient risk profile between hospitals, a composite risk score for each patient was calculated from the coefficients of the regression model used to identify independent risk factors for in-hospital mortality after RAB. These risk factors included advanced age, female gender, and a history of chronic renal failure, congestive heart failure, or chronic lung disease. Patients were stratified into quartiles of expected surgical risk using the composite risk score. Chi square, Fisher’s exact, and Cochran-Armitage trend tests were used to assess for differences in categorical variables between groups.
Results: During the study period, RAB was performed on 7,413 patients with an overall in-hospital mortality of 9.6%. There were no significant differences in in-hospital mortality between low-, medium-, and high-volume hospitals (9.9% vs. 10.5 vs. 8.2%; P=0.436). After adjusting for patient risk, however, differences in in-hospital mortality became apparent among patients in the lowest quartile of expected surgical risk (Table). Within the lowest stratum of risk, in-hospital mortality for RAB was more than four-fold higher in low-volume hospitals (6.35%), compared to high-volume hospitals (1.45%; *P=0.05 by Fisher’s exact test). In the remaining quartiles of patient risk there were no significant differences in in-hospital mortality based between low-, medium-, and high-volume hospitals. Across all hospitals there was a significant increase in in-hospital mortality for RAB with rising composite risk score (P<0.0001 by Cochran-Armitage trend test).

Conclusions: Differences in in-hospital mortality for RAB between low-, medium-, and high-volume hospitals are only detectable for patients in the lowest quartile of expected surgical risk. Within this risk stratum, higher hospital volume is associated with improved mortality rates for RAB. For the remaining patients, however, the risk of in-hospital mortality appears to be related primarily to patient comorbidities, as reflected by the composite risk score. These data suggest that low-risk patients may benefit from referral to a high-volume center for RAB, whereas higher risk patients experience relatively high mortality with RAB in most centers and may be appropriate candidates for lower risk alternatives, such as renal artery stenting.