SvO₂ response to red blood cell transfusion in cardiovascular surgical icu patients: a retrospective observational study

BACKGROUND: Red blood cell (RBC) transfusion decisions after cardiovascular surgery require integration of hemoglobin (Hb), hemodynamics, bleeding status, and oxygen supply-demand balance. Mixed venous oxygen saturation (SvO₂) reflects the global relationship between oxygen delivery and oxygen consumption, but an increase in SvO₂ does not necessarily indicate improved tissue oxygenation or clinical benefit. We evaluated acute SvO₂ changes after RBC transfusion in cardiovascular surgical ICU patients. METHODS: We conducted a single-center retrospective cohort study of adult cardiovascular surgical ICU patients who received one RBC-equivalent transfusion with pre-transfusion Hb ≥ 7.5 g/dL and paired pre- and post-transfusion SvO₂ measurements. The primary outcome was an individual-level SvO₂ response, defined a priori as ΔSvO₂ ≥5% points from baseline to approximately 60 min after transfusion initiation. Multivariable regression was used to evaluate predictors selected a priori based on physiologic relevance to oxygen delivery and consumption. Receiver operating characteristic analysis was used to evaluate the discriminative ability of pre-transfusion SvO₂. RESULTS: 1,352 unique patients met the final inclusion criteria. Mean Hb increased from 9.82 ± 0.92 to 10.24 ± 0.98 g/dL, corresponding to a paired mean difference of 0.42 g/dL (95% CI, 0.38 to 0.46; P < 0.001). Mean SvO₂ changed minimally at the cohort level, from 73.79 ± 9.91% to 73.86 ± 9.37%, corresponding to a paired mean difference of 0.08% points (95% CI, -0.28 to 0.43; P = 0.671). Lower pre-transfusion SvO₂ was associated with a higher probability of an individual-level SvO₂ response. In multivariable logistic regression, pre-transfusion SvO₂ was associated with SvO₂ response (adjusted odds ratio, 0.89 per 1% increase; 95% confidence interval, 0.86-0.91; P < 0.001). Pre-transfusion SvO₂ showed moderate discrimination for SvO₂ response, with an area under the curve of 0.778 (95% confidence interval, 0.740-0.816). The Youden-derived cutoff was 69%, which should be interpreted as an exploratory predictor of acute SvO₂ rise rather than a transfusion trigger. CONCLUSIONS: Lower pre-transfusion SvO₂ was associated with acute individual-level SvO₂ increase after transfusion, but mean SvO₂ change was minimal. The association did not establish tissue oxygenation or clinical benefit and does not support using SvO₂, including the 69% cutoff, as a transfusion indication or trigger.