ObjectiveTo determine the validity of predictive equations in calculating oxygen consumption (Vo2) in ventilated patients with congenital heart disease.DesignProspective study.SettingCardiac catheterization laboratories and intensive care units of two university teaching hospitals.PatientsA total of 126 patients with congenital heart disease were studied. Of these, 75 patients received anesthesia in the pediatric cardiac catheterization laboratory, and 51 were deeply sedated in the intensive care unit after open heart surgery.Measurements and Main ResultsVo2was measured directly in all patients using respiratory mass spectrometry. Estimated values for absolute Vo2(mL/min) and indexed Vo2(mL·min−1·m−2) were calculated from the four predictive equations published by LaFarge and Miettinen, Lundell et al., Wessel et al., and Lindahl. The agreement between measured and estimated Vo2was evaluated by calculating their bias and limits of agreement. A failure of agreement between measured and estimated Vo2was noted in both groups of patients, irrespective the equation used, and the agreement was poorer in patients in the intensive care unit. The equation by LaFarge and Miettinen produced the closest estimation in patients at cardiac catheterization with a bias of 4.5 mL/min for absolute Vo2and 6.9 mL·min−1·m−2for indexed Vo2. A systematic error of overestimating lower and underestimating higher indexed Vo2mL·min−1·m−2was introduced in both groupsConclusionPredictive equations do not accurately estimate Vo2in ventilated patients with congenital heart disease.