The CABG Surgery Volume–Outcome Relationship: Temporal Trends and Selection Effects in California, 1998–2004

OBJECTIVE: To investigate the temporal trends in the volume-outcome relationship in coronary artery bypass graft (CABG) surgery in California from 1998 to 2004, and to assess the selection effects on this relationship by using data from periods of voluntary and mandatory hospital reporting. DATA SOURCES: We used patient-level clinical data collected for the California CABG Mortality Reporting Program (CCMRP, a voluntary reporting program with between 68 and 81 hospitals) from 1998 to 2002 and the California CABG Outcomes Reporting Program (CCORP, a mandatory reporting program with 121 and 120 hospitals) from 2003 to 2004. STUDY DESIGN: The patient was the primary unit of analysis, and in-hospital mortality was the primary outcome. We used hierarchical logistic regression models (generalized linear mixed models) to assess the association of hospital annual volume with hospital mortality while controlling for detailed patient-level covariates in each of the 7 years. DATA COLLECTION METHODS: All data were systematically collected, reviewed for accuracy, and validated by the State of California's Office of Statewide Health Planning and Development (OSHPD). PRINCIPAL FINDINGS: We found that during the period of voluntary hospital reporting (1998-2002), with the exception of 1998, higher volume hospitals had significantly lower risk-adjusted in-hospital mortality rates, on average, than lower volume hospitals (1998 odds ratio OR per 100 operations performed = 0.962, 95 percent confidence interval CI: 0.912-1.015; 1999 OR=0.955, 95 percent CI: 0.920-0.991; 2000 OR=0.942, 95 percent CI: 0.897-0.989; 2001 OR=0.935, 95 percent CI: 0.887-0.986; 2002 OR=0.946, 95 percent CI: 0.899-0.997). We also found that in the period of mandatory reporting (2003 and 2004) there was no volume-outcome relationship (2003 OR=0.997, 95 percent CI: 0.939-1.058; 2004 OR=0.984, 95 percent CI: 0.915-1.058) and that this lack of association was not due to a reporting bias from the addition of data from hospitals that did not originally contribute during the voluntary program. CONCLUSIONS: In California, where no state regulations support regionalization of CABG surgeries, a weak volume-outcome relationship was present from 1998 to 2002, but was absent in 2003 and 2004. The disappearance of the volume-outcome association was temporally related to the implementation of a statewide mandatory CABG surgery reporting program.