Association of low mixed venous oxygen saturations during early ICU stay with increased 30-day and 1-year mortality after cardiac surgery: a single-center retrospective study

Background Low postoperative mixed venous oxygen saturation (SvO 2 ) values have been linked to poor outcomes after cardiac surgery. The present study was designed to assess whether SvO 2 values of < 60% at intensive care unit (ICU) admission and 4 h after admission are associated with increased mortality after cardiac surgery. Methods During the years 2007–2020, 7046 patients (74.4% male; median age, 68 years [interquartile range, 60–74]) underwent cardiac surgery at an academic medical center in Finland. All patients were monitored with a pulmonary artery catheter. SvO 2 values were obtained at ICU admission and 4 h later. Patients were divided into four groups for analyses: SvO 2 ≥ 60% at ICU admission and 4 h later; SvO 2 ≥ 60% at admission but < 60% at 4 h; SvO 2 < 60% at admission but ≥ 60% at 4 h; and SvO 2 < 60% at both ICU admission and 4 h later. Kaplan–Meier survival curves, Cox regression models, and receiver operating characteristic curve analysis were used to assess differences among groups in 30-day and 1-year mortality. Results In the overall cohort, 52.9% underwent coronary artery bypass grafting (CABG), 29.1% valvular surgery, 12.1% combined CABG and valvular procedures, 3.5% surgery of the ascending aorta or aortic dissection, and 2.4% other cardiac surgery. The 1-year crude mortality was 4.3%. The best outcomes were associated with SvO 2 ≥ 60% at both ICU admission and 4 h later. Hazard ratios for 1-year mortality were highest among patients with SvO 2 < 60% at both ICU admission and 4 h later, regardless of surgical subgroup. Conclusion SvO 2 values < 60% at ICU admission and 4 h after admission are associated with increased 30-day and 1-year mortality after cardiac surgery. Goal-directed therapy protocols targeting SvO 2 ≥ 60% may be beneficial. Prospective studies are needed to confirm these observational findings.