Length of intensive care unit stay following cardiac surgery: is it impossible to find a universal prediction model?

OBJECTIVES: Accurate models for prediction of a prolonged intensive care unit (ICU) stay following cardiac surgery may be developed using Cox proportional hazards regression. Our aims were to develop a preoperative and intraoperative model to predict the length of the ICU stay and to compare our models with published risk models, including the EuroSCORE II.

METHODS: Models were developed using data from all patients undergoing cardiac surgery at St. Olavs Hospital, Trondheim, Norway from 2000-2007 (n = 4994). Internal validation and calibration were performed by bootstrapping. Discrimination was assessed by areas under the receiver operating characteristics curves and calibration for the published logistic regression models with the Hosmer-Lemeshow test.

RESULTS: Despite a diverse risk profile, 93.7% of the patients had an ICU stay <2 days, in keeping with our fast-track regimen. Our models showed good calibration and excellent discrimination for prediction of a prolonged stay of more than 2, 5 or 7 days. Discrimination by the EuroSCORE II and other published models was good, but calibration was poor (Hosmer-Lemeshow test: P < 0.0001), probably due to the short ICU stays of almost all our patients. None of the models were useful for prediction of ICU stay in individual patients because most patients in all risk categories of all models had short ICU stays (75th percentiles: 1 day).

CONCLUSIONS: A universal model for prediction of ICU stay may be difficult to develop, as the distribution of length of stay may depend on both medical factors and institutional policies governing ICU discharge.