From the Guidelines
Central venous oxygen saturation (ScvO2) and arterial oxygen saturation (SaO2) are two distinct measurements that differ in their sampling locations and what they represent, with ScvO2 reflecting oxygen saturation in venous blood from the upper body and SaO2 representing oxygen saturation in arterial blood. The key difference between central Sat and venous Sat lies in their measurement locations and the blood components they represent 1. Central Sat, or ScvO2, is measured from the superior vena cava or right atrium via a central venous catheter, reflecting oxygen saturation from the upper body, whereas arterial oxygen saturation, or SaO2, is measured from arterial blood, representing the oxygen saturation of blood being delivered to the tissues 1. Some of the main points to consider when differentiating between central Sat and venous Sat include:
- ScvO2 values are typically higher than SvO2 (mixed venous oxygen saturation) because lower body organs like the kidneys extract more oxygen than upper body tissues 1.
- Normal ScvO2 is around 70-75%, while normal SaO2 is within a narrow range of about 95–98% in healthy adults at sea level 1.
- Both measurements help assess tissue oxygen delivery and consumption, with decreasing ScvO2 values indicating either increased oxygen extraction or decreased delivery, and low SaO2 levels (below about 80%) causing impaired mental functioning even in healthy participants 1.
- The brain is the most sensitive organ to the adverse effects of hypoxia, but other organs in patients with critical illness may be vulnerable to the risk of hypoxic tissue injury at oxygen levels above this range 1. In clinical practice, maintaining a desirable target saturation range of 94–98% for SaO2 is crucial, while ScvO2 is more commonly used due to the less invasive nature of central venous catheters 1.
From the Research
Central Saturation and Venous Saturation: Key Differences
- Central saturation (ScvO₂) and venous saturation (SvO₂) are two related but distinct measures of oxygen saturation in the blood.
- ScvO₂ is measured from the tip of a central venous catheter placed close to, or within, the right atrium, while SvO₂ is measured from a pulmonary artery catheter 2.
- SvO₂ is more directly related to tissue oxygenation and is less prone to error compared to cardiac output measurements 2.
Clinical Significance of Central and Venous Saturation
- ScvO₂ can be used as a surrogate for SvO₂ measurement, which is useful in guiding therapy, particularly in resuscitating patients using validated early goal-directed therapy treatment protocols 2.
- Correct clinical interpretation of SvO₂ or ScvO₂ can help estimate cardiac output, understand whether a patient's oxygen delivery is adequate, and guide clinical practice 2.
- The difference between central and venous saturation can provide valuable information about the patient's oxygenation status and guide treatment decisions 3, 2.
Measurement and Interpretation of Central and Venous Saturation
- ScvO₂ should be measured from the tip of a central venous catheter placed close to, or within, the right atrium to reduce measurement error 2.
- The measurement of SvO₂ is more invasive, requiring a pulmonary artery catheter, but provides a more direct assessment of tissue oxygenation 2.
- The interpretation of ScvO₂ and SvO₂ requires consideration of the patient's clinical context, including their oxygenation status, cardiac output, and hemoglobin level 3, 2.