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CARDIOVASCULAR ANESTHESIA

Evaluating Surrogate Measures of Renal Dysfunction After Cardiac Surgery

Duminda N. Wijeysundera, MD^*, Vivek Rao, MD PhD, FRCSC, W. Scott Beattie, MD PhD, FRCPC, Joan Ivanov, RN MSc, PhD, and Keyvan Karkouti, MD MSc, FRCPC^,

*Department of Anaesthesia, University of Toronto; Division of Cardiac Surgery, Toronto General Hospital, University Health Network; Department of Anaesthesia, University Health Network, University of Toronto; and Department of Health Policy, Management and Evaluation, University of Toronto, Toronto, Ontario, Canada

Address correspondence and reprint requests to Vivek Rao, MD, PhD, FRCSC, Division of Cardiac Surgery, Department of Surgery, University of Toronto, EN 14-222, Toronto General Hospital, University Health Network, 200 Elizabeth St., Toronto, Ontario, M5G 2C4, Canada. Address e-mail to vivek.rao{at}uhn.on.ca

Renal insufficiency after cardiac surgery is associated with increased mortality, morbidity, and length of intensive care unit stay. A convenient surrogate measure would facilitate the evaluation of renal-protective therapies. We evaluated two measures: the 72-h change in serum creatinine (Cr) (Cr_72h) and the percentage 72-h change in calculated (Cockcroft-Gault equation) Cr clearance (%CrCl_72h). We randomly selected 2000 individuals who underwent aortocoronary bypass, valve surgery, or both at the Toronto General Hospital between May 1999 and August 2000. The variables were analyzed with frequency histograms and normal probability plots. Their association with dialysis, mortality, and prolonged intensive care unit stay was determined by using receiver operating characteristic (ROC) curves. Cr_72h was skewed to the right, whereas %CrCl_72h was normally distributed. ROC curve areas showed Cr_72h to be a good predictor of dialysis (0.98), death (0.83), and prolonged hospitalization (0.74). %CrCl_72h had similar ROC curve areas for predicting dialysis (0.97), death (0.82), and prolonged hospitalization (0.74). ROC curve areas did not differ significantly with respect to mortality (P = 0.89), dialysis (P = 0.49), or prolonged hospitalization (P = 0.85). Both variables were correlated with patient-relevant outcomes. Mathematical transformation of Cr_72h to %CrCl_72h results in a normal distribution that is amenable to parametric statistical tests. Cr_72h and %CrCl_72h may be used as surrogate outcomes in future trials.

IMPLICATIONS: A convenient surrogate measure of renal function is needed for evaluating renal-protective therapies in cardiac surgery. We evaluated the performance of serum creatinine concentration and calculated creatinine clearance for predicting dialysis, mortality, and prolonged hospitalization. Both measures were correlated with clinical outcomes. Creatinine clearance had the advantage of a distribution suitable for parametric statistical tests.

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