Prediction of rate and severity of adverse perioperative outcomes: "normal accidents" revisited.

The American Society of Anesthesiologists Physical Status classification system has been shown to predict the frequency of perioperative morbidity and mortality despite known subjectivity, inconsistent application, and exclusion of many perioperative confounding variables. The authors examined the relationship between the American Society of Anesthesiologists Physical Status and both the frequency and the severity of adverse events over a 10-year period in an academic anesthesiology practice. The American Society of Anesthesiologists Physical Status is predictive of not only the frequency of adverse perioperative events, but also the severity of adverse events. These nonlinear mathematical relationships can provide meaningful information on performance and risk. Calculated odds ratios allow discussion about individualized anesthesia risks based on the American Society of Anesthesiologists Physical Status because the added complexity of the surgical or diagnostic procedure, and other perioperative confounding variables, is indirectly factored into the Physical Status classification. The ability of the American Society of Anesthesiologists Physical Status to predict adverse outcome frequency and severity in a nonlinear relationship can be fully explained by applying the Normal Accident Theory, a well-known theory of system failure that relates the interactive complexity of system components to the frequency and the severity of system failures or adverse events.