Venous oxygen saturation and lactate gradient from the superior vena cava to the pulmonary artery in ICU patients with septic shock

Introduction

Central venous oxygen saturation (ScvO₂) is considered comparable with mixed venous oxygen saturation (SvO₂) in the initial resuscitation phase of septic shock [1]. Our aim was to assess their agreement in septic shock in the ICU setting and the effect of a potential difference in a computed parameter, namely oxygen consumption. In addition, we sought for a central venous to pulmonary artery (PA) lactate gradient.

Methods

We enrolled 37 patients with septic shock who were receiving noradrenaline infusions and whose attending physicians had placed a PA catheter for fluid management. Blood samples were drawn in succession from the superior vena cava (CV), right atrium (RA), right ventricle and PA. Hemodynamic and treatment parameters were monitored and data were compared by correlation and Bland–Altman analysis.

Results

SvO₂ was lower than ScvO₂ (70.2 ± 11.4% vs. 78.6 ± 10.2%; P < 0.001), with a bias of -8.45% and 95% limits of agreement ranging from -20.23 to 3.33%. This difference correlated significantly to the noradrenaline infusion rate and the oxygen consumption and extraction ratio. These lower SvO₂ values resulted in a computed oxygen consumption calculated with oxygen saturation of pulmonary artery blood higher than the oxygen consumption calculated with oxygen saturation of central venous blood (P < 0.001), with a bias of 104.97 ml/min and 95% limits of agreement from -4.12 to 214.07 ml/min. Finally, lactate concentration was higher in the CV and RA than in the PA (2.42 ± 3.15 and 2.35 ± 3.16 vs. 2.17 ± 3.19 mmol/l, P < 0.01 for both comparisons).

Conclusion

Our data suggest that ScvO₂ and SvO₂ are not equivalent in ICU patients with septic shock. Additionally, the substitution of ScvO₂ for SvO₂ in the calculation of oxygen consumption produces unacceptably large errors. Finally, the decrease in lactate between RA and PA may support the hypothesis that the mixing of RA and coronary sinus blood is at least partially responsible for the difference between ScvO₂ and SvO₂.