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 (DO₂) achieved by increasing the fraction of inspired oxygen (FiO₂) is often considered clinically insignificant. In this study, we evaluated the change in DO₂, interrogated by mixed venous oxygen saturation (SvO₂), in response to a change in FiO₂ of 0.5 during cardiac surgery. When patients were hemodynamically stable, FiO₂ 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 SvO₂. The observed change in SvO₂ (ΔSvO₂) was compared to the expected ΔSvO₂ calculated using Fick's equation. A total 106 changes in FiO₂ (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 FiO₂ was 0.3 and 99.9% [0.2] when FiO₂ was 0.8), SvO₂ changed significantly as FiO₂ 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 ΔSvO₂ after the FiO₂ 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 ΔSvO₂ (the first changes, r =  - 0.06, P = 0.677; the second changes, r =  - 0.21, P = 0.138). The mean (SD) residual ΔSvO₂ (observed - expected ΔSvO₂) was 0%p (4). Residual ΔSvO₂ was more than 5%p in 14 (13.2%) changes and exceeded 10%p in 2 (1.9%) changes. Residual ΔSvO₂ 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). DO₂, interrogated by SvO₂, may increase to a clinically significant degree as FiO₂ is increased during cardiac surgery, and the increase of SvO₂ is not related to Hb concentration. SvO₂ increases more than expected in patients with chronic kidney disease. Increasing FiO₂ can be used to increase DO₂ during cardiac surgery.