The CABG surgery volume-outcome relationship: temporal trends and selection effects in California, 1998-2004.

The association between the quantity of care that a physician or hospital provides and the quality of care that patients receive has been rigorously studied by clinicians and health services researchers (Luft, Bunker, and Enthoven 1979; Birkmeyer et al. 2003; Gandjour, Bannenberg, and Lauterbach 2003). This “volume–outcome” relationship has been documented for a wide variety of medical conditions and surgical procedures at the physician, clinical team, and hospital level of care (Halm, Lee, and Chassin 2002; Elixhauser, Steiner, and Fraser 2003). In a report reviewing the volume–outcome relationship published by the Institute of Medicine, the authors noted that 77 percent of the published volume–outcome studies had demonstrated a significant relationship between higher physician and hospital volumes and better health outcomes (Hewitt et al. 2001). The volume–outcome relationship has been most intensely studied for patients receiving coronary artery bypass grafting (CABG) surgery, perhaps because regionalization is a particularly promising policy strategy to improve care for high-risk elective surgery. Although many studies have found that hospitals performing relatively more CABG surgeries have better outcomes (Luft, Bunker, and Enthoven 1979; Birkmeyer et al. 2002, 2003; Hannan et al. 2003; Peterson et al. 2004), the importance of this relationship for health policy remains controversial. First, several recent studies using robust statistical methods and careful risk adjustment have found either minor or insignificant hospital CABG volume–outcome relationships, or no relationship at all (Christiansen and Morris 1997; Shahian et al. 2001; Kalant and Shrier 2004; Peterson et al. 2004). Second, studies of regionalization policies have been inconclusive regarding their impact on improving care (Vaughan-Sarrazin et al. 2002; Nobilio and Ugolini 2003). Third, there is some evidence that the observed volume–outcome association may reflect selective referral of patients to high-quality surgeons and hospitals, rather than greater experience leading to improved skills (Luft et al. 1990; Escarce et al. 1999). In addition, the effect of public reporting of CABG surgery performance on the volume–outcome relationship is unknown. In several studies, the publication of CABG surgery quality reports has been associated with reduced CABG mortality (Hannan et al. 1995; Rosenthal, Quinn, and Harper 1997; Romano and Zhou 2004) and enhanced quality improvement efforts, particularly for low-quality and low-volume outliers (Bentley and Nash 1998; Rainwater, Romano, and Antonius 1998; Chassin 2002). Therefore, public performance data may help stimulate standardization of procedures, improved training, acquisition of better equipment, and diffusion of innovation, thereby improving outcomes among low-quality and low-volume outliers and attenuating the volume–outcome relationship. Further, as some of the studies that have reported a significant volume–outcome relationship used data from voluntary reporting programs, selection bias could have contributed to misestimation of the volume effect (Clark 1996; Christian et al. 2003; Peterson et al. 2004). For example, the volume–outcome relationship could be overestimated if nonparticipating low-volume hospitals had better than average outcomes or if nonparticipating high-volume hospitals had worse than average outcomes. The goal of this study was to investigate the volume–outcome relationship for CABG surgery in California from 1998 to 2004, a period during which public reporting of CABG outcomes transitioned from a voluntary basis to a state-mandated program. We analyze data from the California CABG Mortality Reporting Program (CCMRP), a voluntary reporting program with between 68 and 81 participating nonfederal hospitals from 1998 to 2002, and from the California CABG Outcomes Reporting Program (CCORP), a mandatory reporting program with 121 and 120 nonfederal hospitals from 2003 to 2004, respectively. Our objectives were (1) to use hierarchical logistic regression to determine whether hospitals performing more CABG surgeries have lower risk-adjusted mortality than hospitals performing fewer cases, and (2) to assess potential explanations for any change in the magnitude of the volume–outcome association between the voluntary reporting period (1998–2002) and the mandatory reporting period (2003 and 2004).