Two Long-Term Mortality Risk Models For Coronary Artery Bypass Graft Surgery Produced in American Populations Validated in an Australian Population

Abstract

Short-term mortality prediction models have an important role in current cardiac surgical practice. There has been much less attention paid to prediction of long-term outcomes which are probably an equal marker both of surgeon performance and appropriateness of surgical treatment. The aim of this study was to assess the performance of the New York State Cardiac Surgery Reporting System (NYSCSRS) risk model and the Northern New England Cardiovascular Disease Study Group (NNECDSG) risk model on the Australian and New Zealand Society of Cardiac and Thoracic Surgeons (ANZSCTS) patient database. The NYSCSRS and the NNECDSG risk models were applied to all patients undergoing isolated coronary artery bypass graft (CABG) surgery that had complete data, were over the age of 18 and had a body mass index between 12 and 78 kg/m2. Predicted mortality was calculated using the published risk model formulae and compared with observed mortality, obtained via linkage with the National Death Index, at four time-points (one, three, five and seven years following surgery). Model discrimination and model calibration were tested at all four time points by determining the C-statistics for receiver operator characteristic (ROC) curves, and studying the Hosmer-Lemeshow chi-square tests, respectively. The NYSCSRS and NNECDSG risk models were applied to 34,961 and 34,998 patients, respectively. The NYSCSRS risk model over-predicted mortality by between 130% and 216% at all four time-points while the NNECDSG risk model under-predicted mortality at one year by 4.3% but over-predicted mortality at three, five and seven years by between 42.5% and 145.7%. The C-statistics obtained fell between 0.779 and 0.741 for the NYSCSRS risk model and between 0.785 and 0.752 for the NNECDSG risk model at all four time-points. Hosmer-Lemeshow chi-square tests returned p-values <0.001 at all four time-points for both risk models. The NYSCSRS and NNECDSG risk models do not accurately predict long-term mortality following isolated CABG surgery in the ANZSCTS patient population. The use of either of these risk models is not appropriate in Australia.