Clinical application of cardiac risk indices: how to avoid misleading numbers.

Perioperative cardiac complications are associated with a high mortality rate. During the preoperative assessment the anaesthetist must determine if the patient is at risk so that appropriate interventions can be made to ensure the best possible outcome. Over the past three decades several indices have been proposed to identify the surgical patient at risk of cardiac morbidity or mortality. These indices can be thought of as diagnostic tests which determine the likelihood of perioperative cardiac complication. In this article the authors review the epidemiological basis of diagnostic tests and apply this theory to three currently available indices of perioperative risk: the American Society of Anesthesiologists physical status classification, the Goldman multifactorial cardiac risk index and a modified version devised by Detsky. A case history describing a surgical patient with unstable angina is first presented to show how different indices can lead to different predictions. These indices are then analysed as if they were diagnostic tests to predict cardiac complications after surgery. The characteristics of diagnostic tests such as sensitivity, specificity, positive and negative predictive values, and receiver operating characteristic curves are described and applied to the indices. Basic principles such as odds ratio, likelihood ratio and their relation to prevalence and post-test probability are also presented. This overview will help the anaesthetist to understand the limitations of risk indices as predictors of adverse cardiac outcome and to apply these scales more effectively to individual patients at different institutions.