Relationship between central venous oxygen saturation measured in the inferior and superior vena cava

Introduction

Central venous saturation ScvO₂ proved a useful alternative to mixed venous saturation, and also a reliable parameter to evaluate the oxygen debt in emergency and intensive care [1–3]. Under special circumstances of anatomical or coagulation disorders, however, canulation of the jugular or subclavian veins are not recommended. In these cases the femoral vein has to be catheterised. The aim of our prospective study was to investigate the relationship between the ScvO₂ measured in the superior vena cava (ScvsO₂) and in the inferior vena cava (ScviO₂).

Methods

After local ethics committee approval every ICU patient with two central venous catheters (one subclavian/internal jugular and one femoral) entered the study. Parallel blood gas analyses were performed in random, whenever ScvO₂ was requested by the attending physician. Vital parameters (heart rate, mean arterial pressure, Glasgow Coma Scale, respiratory parameters) and the dose of vasopressor and sedative drugs were also recorded. Results were compared using Wilcoxon test, Pearson correlation and Bland–Altman plots.

Results

In 13 patients 47 matched pairs were compared. Although ScvsO₂ = median 79% (range: 50–85) was significantly higher than ScviO₂ = 71% (45–87), P < 0.001, there was significant correlation between the two variables (r = 0.690, P < 0.001). Bland–Altman plots showed a mean bias of 7.6% with lower and upper levels of agreement of -5.6 and 20.7, respectively. The dose of vasopressor (norepinephrine) and dose of sedative (propofol) had a significant influence on the measured difference between the investigated variables (r = 0.562, P < 0.001; r = 0.538, P < 0.001, respectively).

Conclusion

The preliminary results of this study show that ScviO₂ underestimates ScvsO₂ with a low level of agreement and that this difference is affected by vasopressor support and sedation. ScviO₂ may be useful, however, for monitoring the trend of ScvsO₂.