Effect of changes in inspired oxygen fraction on oxygen delivery during cardiac surgery: a substudy of the CARROT trial.

When hemoglobin (Hb) is fully saturated with oxygen, the additional gain in oxygen delivery (DO) achieved by increasing the fraction of inspired oxygen (FiO) is often considered clinically insignificant. In this study, we evaluated the change in DO, interrogated by mixed venous oxygen saturation (SvO), in response to a change in FiO of 0.5 during cardiac surgery. When patients were hemodynamically stable, FiO 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. The observed change in SvO (ΔSvO) was compared to the expected ΔSvO calculated using Fick's equation. A total 106 changes in FiO (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] = 98.1% [1.5] when FiO was 0.3 and 99.9% [0.2] when FiO was 0.8), SvO changed significantly as FiO was changed (the first and second changes in on-pump, 7.7%p [3.8] and 7.6%p [3.5], respectively; off-pump, 7.9%p [4.9] and 6.2%p [3.9]; all P < 0.001). As a total, regardless of the surgery type, the observed ΔSvO after the FiO change of 0.5 was ≥ 5%p in 82 (77.4%) changes and ≥ 10%p in 31 (29.2%) changes (mean [SD], 7.5%p [3.9]). Hb concentration was not correlated with the observed ΔSvO (the first changes, r =  - 0.06, P = 0.677; the second changes, r =  - 0.21, P = 0.138). The mean (SD) residual ΔSvO (observed - expected ΔSvO) was 0%p (4). Residual ΔSvO was more than 5%p in 14 (13.2%) changes and exceeded 10%p in 2 (1.9%) changes. Residual ΔSvO was greater in patients with chronic kidney disease than in those without (median [IQR], 5%p [0 to 7] vs. 0%p [- 3 to 2]; P = 0.049). DO, interrogated by SvO, may increase to a clinically significant degree as FiO is increased during cardiac surgery, and the increase of SvO is not related to Hb concentration. SvO increases more than expected in patients with chronic kidney disease. Increasing FiO can be used to increase DO during cardiac surgery.