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

To investigate the relationship between central venous oxygen saturation (ScvO(2)) and femoral venous oxygen saturation (SfvO(2)) in a large group of critically ill patients.

Observational study.

A group of unselected critically ill patients with central line placed into superior vena cava were included.

A 26-bed intensive care unit in a tertiary referral hospital.

None.

Venous 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 SfvO(2) was significantly correlated with ScvO(2) (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 ScvO(2) vs SfvO(2) had a large intercept (48.68%) and a low slope (0.2978); ScvO(2) was still around 50% while SfvO(2) 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.

Central 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.