Femoral venous oxygen saturation and central venous oxygen saturation in critically ill patients

ObjectivesTo investigate the relationship between central venous oxygen saturation (ScvO2) and femoral venous oxygen saturation (SfvO2) in a large group of critically ill patients.DesignObservational study.PatientsA group of unselected critically ill patients with central line placed into superior vena cava were included.SettingA 26-bed intensive care unit in a tertiary referral hospital.InterventionsNone.Measurements and main resultsVenous blood samples of superior vena cava and femoral vein were collected within an interval of 5 to 15 minutes and analyzed with blood gas/electrolyte analyzer immediately. Although SfvO2 was significantly correlated with ScvO2 (r = 0.493, P < .001; Pearson correlation, 2 tailed), the limits of agreement were wide (up to 61% to − 41%) between the 731 pairs of blood samples collected from 357 patients. The fit line of scatter diagram ScvO2 vs SfvO2 had a large intercept (48.68%) and a low slope (0.2978); ScvO2 was still around 50% while SfvO2 was nearing 0%. The distribution of blood flow, measured with Doppler ultrasound, had a similar trend in 237 patients and 412 measurements. The ratio of femoral artery flow over common carotid artery flow varied widely (from 0 to 7.13). Blood flow was not distributed in a fixed ratio to the superior vena cava–drained organs and tissues.ConclusionsCentral venous oxygen saturation was not representative of the whole systemic circulation in critically ill patients. Central venous oxygen saturation alone might be misleading in goal-directed therapy.