From the Guidelines
Falsely low mixed venous oxygen saturation (SvO2) can be caused by improper positioning of the pulmonary artery catheter, technical issues with the measurement device, blood sample contamination, timing errors, or catheter wedging, which can lead to inaccurate measurements and unnecessary interventions. When considering the causes of falsely low mixed venous oxygen saturation, it is essential to evaluate the potential sources of error in measurement. According to the study by Rivers and colleagues 1, the use of central venous oxygen saturation (ScvO2) as a therapeutic goal may not be reliable in septic patients due to the potential for low oxygen extraction ratios. Some key factors to consider include:
- Improper positioning of the pulmonary artery catheter, which can lead to inaccurate sampling 1
- Technical issues with the measurement device, such as calibration errors or optical interference
- Blood sample contamination with room air or improper handling
- Timing errors, such as drawing blood during a period of transient hypoxemia or increased oxygen consumption
- Catheter wedging, which can cause regional sampling that doesn't reflect the true mixed venous blood. In clinical practice, it is crucial to verify proper catheter position and measurement technique before initiating treatments based on SvO2 values, especially in critically ill patients where low oxygen extraction ratios may be prevalent 1.
From the Research
Causes of Falsely Low Mixed Venous Oxygen Saturation
- Falsely low mixed venous oxygen saturation can be caused by several factors, including:
- Central venous pulsations, which can lead to underestimation of oxygen saturation measured by pulse oximetry 2
- Incorrect measurement techniques, such as rapid blood withdrawal, although one study found that rapid blood sampling does not falsely elevate measurements of mixed venous oxygen saturation 3
- Differences in measurement locations, such as central venous oxygen saturation (ScvO2) versus mixed venous oxygen saturation (SvO2), which may not always be interchangeable 4
- It is essential to consider these factors when interpreting mixed venous oxygen saturation values to ensure accurate assessment of cardiac output and tissue oxygenation 5, 6
Measurement Considerations
- Mixed venous oxygen saturation should be measured from the distal port of a pulmonary artery catheter, with slow aspiration of blood to avoid contamination with oxygenated blood from the pulmonary capillary bed 5, 3
- Central venous oxygen saturation can be used as a surrogate for mixed venous oxygen saturation, but its measurement should be taken from the tip of a central venous catheter placed close to or within the right atrium to reduce measurement error 6