Objective: To evaluate the association between annual hospital coronary artery bypass graft (CABG) surgery volume and in-hospital mortality.
Summary Background Data: The Leapfrog Group recommends health care purchasers contract for CABG services only with hospitals that perform ≥500 CABGs annually to reduce mortality; it is unclear whether this standard applies to current practice.
Methods: We conducted a retrospective analysis of the National Inpatient Sample database for patients who underwent CABG in 1998-2000 (n = 228,738) at low (12-249 cases/year), medium (250-499 cases/year), and high (≥500 cases/year) CABG volume hospitals. Crude in-hospital mortality rates were 4.21% in low-volume hospitals, 3.74% in medium-volume hospitals, and 3.54% in high-volume hospitals (trend P < 0.001). Compared with patients at high-volume hospitals (odds ratio 1.00, referent), patients at low-volume hospitals remained at increased risk of mortality after multivariable adjustment (odds ratio 1.26, 95% confidence interval = 1.15-1.39). The mortality risk for patients at medium-volume hospitals was of borderline significance (odds ratio 1.11, 95% confidence interval = 1.01-1.21). However, 207 of 243 (85%) of low-volume and 151 of 169 (89%) of medium-volume hospital-years had risk-standardized mortality rates that were statistically lower or comparable to those expected. In contrast, only 11 of 169 (6%) of high-volume hospital-years had outcomes that were statistically better than expected.
Conclusions: Patients at high-volume CABG hospitals were, on average, at a lower mortality risk than patients at lower-volume hospitals. However, the small size of the volume-associated mortality difference and the heterogeneity in outcomes within all CABG volume groups suggest individual hospital CABG volume is not a reliable marker of hospital CABG quality.
Patients who undergo coronary artery bypass graft (CABG) surgery at higher-volume hospitals have been found to have better outcomes than patients treated at lower-volume hospitals.1-13 Based on this evidence, popular press reports14-16 and several advocacy groups have endorsed CABG volume as a proxy for hospital CABG quality.17-19 In addition, the Leapfrog Group recommends health care purchasers consider hospital volume when contracting for CABG. The Leapfrog Group explicitly frames the use of a hospital CABG volume minimum as a matter of patient safety20 and estimates that 1486 deaths may be averted by referring CABG patients to hospitals that perform ≥500 procedures annually.21,22 This specific volume threshold is derived from an evaluation of the association of hospital CABG volume and in-hospital mortality based on patents hospitalized in New York in 1989.7 The past decade, however, has witnessed notable changes in CABG practice. Technical modifications, including the increased use of internal mammary artery grafts, warm cardioplegia, off-pump surgery,23,24 and the diffusion of CABG to higher-risk patient populations,25,26 raise the possibility that a hospital CABG volume minimum of 500 cases may no longer be appropriate. No study, to our knowledge, has assessed the association of hospital CABG volume and patient outcomes in an unselected, nationally representative patient population.
To address this issue, we evaluated the association between annual hospital CABG volume and in-hospital mortality in a national cohort of patients who underwent CABG between 1998 and 2000. To assess the potential effectiveness of the Leapfrog Group's hospital CABG volume minimum, we examined whether patients treated at hospitals with at least 500 cases had lower mortality rates than those at hospitals with lower CABG volumes, and measured heterogeneity in hospital risk-standardized outcomes within CABG volume groups.