Venous Oximetry – The concept of SvO2 and ScvO2

Abbreviations:

• SvO2 - True mixed venous oxygen saturation      
• ScvO2 - Central venous oxygen saturation  
• VO2 – Consumption of oxygen        
• DO2 – Delivery of oxygen

Determining the adequacy of tissue oxygenation in critically ill patients is central to ascertain the health of the patient. Unfortunately, normal values in blood pressure, central venous pressure, heart rate, and blood gases do not rule out tissue hypoxia or imbalances between whole-body oxygen supply and demand. This discrepancy has led to increased interest in more direct indicators of adequacy of tissue oxygenation such as mixed and central venous oxygen saturations.

The normal cardiovascular response of increasing VO2 is to increase O2 extraction and cardiac output. Usually VO2 is independent of DO2 since tissues can maintain O2 needs by increasing O2 extraction when DO2 decreases. However, this mechanism has its limits. Below a so-called critical DO2 compensatory increase in O2 extraction is exhausted, and VO2 becomes dependent on DO2. In this case tissue hypoxia occurs, and a rise in serum lactate levels may be observed.

A decrease in SvO2 and ScvO2 represents an increased metabolic stress, either DO2 does not increase in such a way to
cover an increased VO2, or DO2 drops because of decrease in either arterial O2 content, cardiac output, or both. The magnitude of the decrease indicates the extent to which the physiological reserves are stressed.

The cardiocirculatory system may be challenged by two different conditions. Firstly, a drop in DO2 can be induced by anemia, hypoxia, hypovolemia, or heart failure. Secondly, fever, pain, stress etc. may also decrease SvO2 or ScvO2 by increasing whole-body VO2.

Pulmonary artery catheterization allows obtaining true mixed venous oxygen saturation (SvO2) while measuring central venous oxygen saturation (ScvO2) via central venous catheter reflects principally the degree of oxygen extraction from the brain and the upper part of the body. SvO2 reflects the relationship between whole-body O2 consumption and cardiac output. Indeed, it has been shown that the SvO2 is well correlated with the ratio of O2 supply to demand.

The central venous catheter sampling site usually resides in the superior vena cava. Thus central venous blood sampling reflects the venous blood of the upper body but neglects venous blood from the lower body (i.e., intra-abdominal organs). As shown below, venous O2 saturations differ among several organ systems since they extract different amounts of O2. ScvO2 is usually less than SvO2 by about 2–3% because the lower body extracts less O2 than the upper body making inferior vena caval O2 saturation higher. The primary cause of the lower O2 extraction is that many of the vascular circuits that drain into the inferior vena cava use blood flow for nonoxidative phosphorylation needs (e.g., renal blood flow, portal flow, hepatic blood flow). However, SvO2 and ScvO2 change in parallel when the whole body ratio of O2 supply to demand is altered.

The difference between the absolute value of ScvO2 and SvO2 changes under conditions of shock. In septic shock, ScvO2 often exceeds SvO2 by about 8%. During cardiogenic or hypovolemic shock mesenteric and renal blood flow decreases followed by an increase in O2 extraction in these organs. In septic shock, regional O2 consumption of the gastrointestinal tract and hence regional O2 extraction increases despite elevated regional blood flows. On the other hand, cerebral blood flow is maintained over some period in shock. This would cause a delayed drop of ScvO2 in comparison to SvO2, and the
correlation between these two parameters would worsen. Some authors therefore argued that ScvO2 cannot be used as surrogate for SvO2 under conditions of circulatory shock.

Limitations:

Venous oximetry can reflect the adequacy of tissue oxygenation only if the tissue is still capable of extracting O2. In the case of arteriovenous shunting on the microcirculatory level or cell death, SvO2 and ScvO2 may not decrease or even show elevated values despite severe tissue hypoxia. As demonstrated in patients after prolonged cardiac arrest, venous hyperoxia with an ScvO2 higher than 80% is indicative of impaired oxygen use.

Conclusion:

While measurement of SvO2 requires the insertion of a pulmonary artery catheter, measurement of ScvO2 requires only central venous catheterization. ScvO2 directed early goal-directed therapy improves survival in patients with septic shock
who are treated in an emergency department. However, ScvO2 values may differ from SvO2 values, and this difference varies in direction and magnitude with cardiovascular insufficiency. ScvO2 should not be used alone in the assessment of the cardiocirculatory system but combined with other cardio-circulatory parameters and indicators of organ perfusion such as serum lactate concentration and urine output.

Ref: M. R. Pinsky · L. Brochard · J. Mancebo; Applied Physiology in Intensive Care Medicine, Springer-Verlag Berlin Heidelberg 2006