Mixed venous oxygen saturation cannot be estimated by central venous oxygen saturation in septic shock: reply to Dr. Bourdeaux

Abstract

Sir: We thank Dr. Bourdeaux for his interest in our study. We showed that during the first day in the intensive care unit (ICU), the average difference between ScvO2 and SvO2 values in patients with septic shock was 4%, but the individual differences varied from –8% to +17% [1]. Dr. Bourdeaux highlighted two issues that we would like to discuss: first, the correlation and the clinical interchangeability of SvO2 and ScvO2, and second, the importance of the sampling site. As Dr. Bourdeaux pointed out, SvO2 and ScvO2 values correlate well, which is not a surprise, because SvO2 is a mixture of ScvO2, saturation of inferior vena cava, and saturation of coronary sinus, blended according to the proportional blood flow. In addition, SvO2 and ScvO2 share the same determinants (SaO2, cardiac output, and haemoglobin concentration). Inevitably, therefore, there exists a clear causality between these two parameters. However, we would like to question the clinical relevance of correlation between these two parameters. We re-evaluated our data concerning possible clinical interventions according to saturation values. In a previous study we had shown that SvO2 values < 70% increased the risk of mortality in septic shock [2]. According to the present data, 16% of SvO2 values were below 70% while ScvO2 was above 70%. If we then assume that SvO2 over 65% and ScvO2 over 70% are clinically acceptable, treatment decisions would still have been different in 14% of measurements (either SvO2 below 65% with ScvO2 over 70% or SvO2 over 65% with ScvO2 below 70%). Another important issue in ScvO2 monitoring is where the tip of the central venous pressure catheter is located. The saturation values in shock are lower in right atrium and in inferior vena cava than in superior vena cava [3, 4]. Dr. Bourdeaux considered that as an advantage, but we feel that the uncertainty in ScvO2 monitoring may lead to false interpretation of the results and to overtreatment by unnecessary pushing of haemodynamics with vasoactive medication. We conclude that ScvO2 and SvO2 are neither equal nor interchangeable. However, is ScvO2 clinically useful “per se” in monitoring of global tissue oxygenation in sepsis or septic shock during ICU stay? Unfortunately, to the best of our knowledge the value of ScvO2 during the ICU stay as a target of haemodynamic treatment, or as a predictor of hospital mortality, has not been investigated. The generalisation of the results of the landmark study by Rivers et al. [5] from the emergency department to the ICU is, in any case, not appropriate. Although low ScvO2 is always a sign of globally insufficient tissue oxygenation, the problem still exists with normal ScvO2, by which we cannot rule out inadequate tissue oxygenation in septic shock. SvO2 is more sensitive for that purpose. References