Shock is defined as global tissue hypoxia secondary to an imbalance between systemic oxygen delivery (DO2) and oxygen demand (VO2), reflected by mixed venous oxygen saturation (SvO2). Intervention based on markers of tissue hypoperfusion may improve outcome. Central venous oxygen saturation (ScvO2) has been used as a surrogate marker for SvO2. In order to monitor ScvO2 during resuscitation, an internal jugular or subclavian line must be inserted. However, sometimes the femoral vein is the preferred or only possible site for access. The purpose of our study is to determine whether ScvO2 and femoral venous oxygen saturation (SfvO2) can be used interchangeably.
A single-center, prospective, controlled, observational study was conducted at the Gelre Hospitals Apeldoorn. One hundred stable cardiac patients who underwent elective right heart catherization in daycare served as a control group. In the study group (high-risk surgery, ASA >2, n = 30) we determined SfvO2 and ScvO2 simultaneously at the start (T = 0) and at the end (T = 1) of the procedure. For each time point we calculated the agreement and difference between both values.
Control group: ScvO2 and SfvO2 correlated significantly (r = 0.67, 95% CI = 0.50 to 0.80; P < 0.0001) with large limits of agreement (bias 2.0 ± 7.1; -11.8 to 15.9). In the surgical patients at T = 0, mean values were similar (SfvO2 82.5 ± 6.6% vs. ScvO2 81.1 ± 8.1; P = 0.28). According to Bland-Altman analysis, the mean bias between ScvO2 and SfvO2 was 2.7 ± 7.9% and 95% limits of agreement were large (-12.9% to 18.2%), while correlation between ScvO2 and SfvO2 was significant (r2 = 0.35; P < 0.01). At both time points SfvO2 and ScvO2 did not correlate significantly (P = 0.26 and P = 0.66 respectively) with similar negligible r2. Univariate analysis did not show any parameter (including dosages of dopamine or norepinephrine, total infusion, fluid balance, FiO2, type of surgery, lactate, and haemoglobin level) affecting either SfvO2 or ScvO2. Results were similar for changes in SfvO2 and changes in ScvO2.
Absolute values of SfvO2 are unsuitable as surrogate for absolute values of ScvO2. Also, the trends of both values are not interchangeable. Further studies should investigate the effects of treatment on SfvO2.