SUMMARYThe ratio of arterial oxygen tension to inspired oxygen fraction (PaO 2 / FiO 2 ) is a useful indicator for weaning patients from mechanical ventilation and a reliable predictor of pulmonary dysfunction after cardiac surgery. The aim of this study was to elucidate the patient characteristics and variables that affect the PaO 2 / FiO 2 ratio.Between 1994-1998, 167 patients who underwent coronary artery bypass grafting (CABG) were examined retrospectively. Spearman's correlation coefficients were calculated between the PaO 2 / FiO 2 ratio and intubation period, and length of ICU stay. Patients were then divided into two groups with a PaO 2 / FiO 2 ratio < 350 and PaO 2 / FiO 2 ratio ≥ 350. Univariate analysis of the putative risk factors was performed. A logistic regression model was developed to evaluate factors that would influence the PaO 2 / FiO 2 ratio.A significant correlation was observed between the PaO 2 / FiO 2 ratio and intubation period, and length of ICU stay. Univariate predictors of a PaO 2 / FiO 2 ratio < 350 were low body weight, low preoperative PaO 2 , long operation time, high FiO 2 , low postoperative PaO 2 , history of smoking, hypertension and opening of pleura (p < 0.05). Excellent prediction was found with a model consisting of preoperative PaO 2 and hypertension.Conclusion: The results of this study suggest that patients with a low preoperative PaO 2 or hypertension may need more careful peri-and postoperative management since these factors are closely associated with the PaO 2 / FiO 2 ratio. (Jpn Heart J 2001; 42: 143-153)