摘要：When hemoglobin (Hb) is fully saturated with oxygen, the additional gain in oxygen delivery (DO
2) achieved by increasing the fraction of inspired oxygen (FiO
2) is often considered clinically insignificant. In this study, we evaluated the change in DO
2, interrogated by mixed venous oxygen saturation (SvO
2), in response to a change in FiO
2 of 0.5 during cardiac surgery. When patients were hemodynamically stable, FiO
2 was alternated between 0.5 and 1.0 in on-pump cardiac surgery patients (pilot study), and between 0.3 and 0.8 in off-pump coronary artery bypass grafting patients (substudy of the CARROT trial). After the patient had stabilized, a blood gas analysis was performed to measure SvO
2. The observed change in SvO
2 (ΔSvO
2) was compared to the expected ΔSvO
2 calculated using Fick’s equation. A total 106 changes in FiO
2 (two changes per patient; total 53 patients; on-pump, n = 36; off-pump, n = 17) were finally analyzed. While Hb saturation remained near 100% (on-pump, 100%; off-pump, mean [SD
