Mixed and central venous oxygen saturation in patients with septic shock: is there a difference?

Introduction

The mixed venous oxygen saturation (mSVO₂) is an important marker of global perfusion in patients with septic shock. There is some evidence that central venous oxygen saturation (cSVO₂) can replace mSVO₂. However, little is known about where it should be located: the superior vena cava (ccSVO₂) or the right atrium (acSVO₂). This study aimed at evaluating the differences between mSVO₂ and cSVO₂ (either from the cava or from the atrium) and the impact of them in patient management.

Methods

We included patients admitted to a tertiary universitary ICU with septic shock that had a Swan–Ganz catheter and a central venous catheter in place. Each patient was submitted to three sets of hemodynamic and respiratory monitoring, with a minimal interval of 4 hours. Each set included a blood gas analysis of samples collected from the proximal (acSVO₂) and distal port of the Swan–Ganz catheter (mSVO₂) and also from the central line catheter (ccSVO₂). Each of these samples was analyzed by a blinded critical care physician who decided the hypothetical management for the patient. Statistical analysis was done using a paired Student t test. Results were considered significant if P ≤ 0.005.

Results

We studied 22 sets of measures in seven patients (five female and two male) with a mean age of 60.57 ± 23.25 years. The mean values were 76.47 ± 8.02, 75.54 ± 11.96 and 70.90 ± 8.53 for ccSVO₂, acSVO₂ and mSVO₂, respectively. There was a significant difference between ccSVO₂ and mSVO₂ (P = 0.009) and acSVO₂ and mSVO₂ (P = 0.01), but not between ccSVO₂ and acSVO₂ (P = 0.60). The concordances in patient management were 63.2%, 68.2% and 78.9% between ccSVO₂ and mSVO₂, acSVO₂ and mSVO₂ and ccSVO₂ and acSVO₂. When only sets with a ccSVO₂ below 70 were considered, the concordance between ccSVO₂ and mSVO₂ was 75%.

Conclusion

Our results suggest that blood samples derived from a central catheter, even if it is located in the right atrium, may be not accurate enough to be used as a measure of tissue oxygenation and may lead to improper management of the patient, mainly when the values are above 70%.