Association between early postoperative fraction of inspired oxygen and pulmonary complications after coronary artery bypass grafting: a retrospective cohort study using public databases

Patients undergoing coronary artery bypass grafting (CABG) frequently require postoperative mechanical ventilation and oxygen therapy. However, whether higher early postoperative fraction of inspired oxygen (FiO₂) exposure is associated with an increased risk of postoperative pulmonary complications (PPCs) remains uncertain. This study evaluated the association between early postoperative FiO₂ exposure and PPCs after CABG using the MIMIC-IV database and externally validated the findings in the eICU database. We conducted a retrospective cohort study using MIMIC-IV 3.1 as the derivation cohort and the eICU Collaborative Research Database as an independent external cohort for consistency assessment.The MIMIC-IV cohort included 5,457 postoperative CABG patients, and the eICU validation cohort included 4,521 patients. Early postoperative FiO₂ exposure was defined as the mean FiO₂ during the first 6 h after ICU admission and was categorized as ≤ 0.60, 0.61–0.80, and > 0.80. The primary outcome was a composite of PPCs, including prolonged mechanical ventilation, reintubation, and pneumonia. Multivariable logistic regression, continuous exposure analysis per 0.1 increase in FiO₂, restricted cubic spline (RCS) analysis, propensity score weighting, and subgroup analyses were performed. In the MIMIC-IV cohort, the incidence of PPCs increased across FiO₂ categories: 5.51% (45/817) in the ≤ 0.60 group, 8.10% (261/3,223) in the 0.61–0.80 group, and 14.61% (207/1,417) in the > 0.80 group ( P < 0.001). In the eICU cohort, the corresponding rates were 9.41% (320/3,400), 14.25% (113/793), and 26.52% (87/328), respectively ( P < 0.001). In fully adjusted logistic regression, FiO₂ >0.80 remained associated with higher odds of PPCs in MIMIC-IV (OR 1.69, 95% CI 1.16–2.48; P = 0.007). In eICU, after further adjustment for peri-ICU interventions, FiO₂ 0.61–0.80 and > 0.80 were both associated with higher PPC risk (OR 1.51, 95% CI 1.13–2.03; P = 0.005 and OR 2.39, 95% CI 1.66–3.44; P < 0.001, respectively). Continuous exposure analysis showed that each 0.1 increase in FiO₂ was associated with increased PPC risk in MIMIC-IV (fully adjusted OR 1.13, 95% CI 1.05–1.21; P = 0.001) and eICU (Model 4 OR 1.25, 95% CI 1.16–1.35; P < 0.001). RCS analyses demonstrated significant overall associations in both cohorts without significant nonlinearity. Among postoperative ICU patients undergoing CABG, higher early postoperative FiO₂ exposure was associated with an increased risk of PPCs. This association was consistent across multivariable adjustment, continuous exposure analysis, RCS analysis, propensity score weighting, and evaluation in an independent eICU cohort. These findings suggest that early postoperative FiO₂ requirements may serve as a marker for identifying CABG patients at increased risk of pulmonary complications. Prospective studies are needed to determine whether FiO₂ optimization strategies can improve postoperative outcomes.