Measurement Location for Mixed Venous Oxygen Saturation
Mixed venous oxygen saturation (SvO₂) is best measured from the pulmonary artery via a pulmonary artery catheter (PAC), as this represents true mixed venous blood from the entire body. 1
Optimal Measurement Site: Pulmonary Artery
The pulmonary artery catheter provides the gold standard for SvO₂ measurement, as it samples blood that has mixed from the superior vena cava, inferior vena cava, and coronary sinus, representing the complete venous return from all body tissues. 1, 2
Modern PACs can measure SvO₂ continuously using fiberoptic oximetry technology, providing real-time monitoring of the balance between oxygen delivery and consumption. 1, 2
The PAC allows simultaneous measurement of pulmonary artery pressures, cardiac output, and mixed venous oxygen saturation, making it particularly valuable in hemodynamically unstable patients not responding predictably to traditional treatments. 1
Alternative Site: Central Venous Oxygen Saturation (ScvO₂)
Central venous oxygen saturation (ScvO₂) measured from the superior vena cava or right atrium serves as a clinically acceptable surrogate for SvO₂ when pulmonary artery catheterization is not feasible or desired. 1, 3
For accurate ScvO₂ measurement, the central venous catheter tip must be positioned close to or within the right atrium to minimize measurement error. 3
ScvO₂ typically runs approximately 5% higher than true SvO₂ (ScvO₂ ≥70% corresponds to SvO₂ ≥65%), and the two values are closely correlated (r = 0.945) with differences <5% in 90% of patients. 4, 5
Clinical Context for Site Selection
When to Use Pulmonary Artery Catheter:
Hemodynamically unstable patients with combined congestion and hypoperfusion who are not responding predictably to standard therapy require PAC insertion for optimal fluid loading and vasoactive/inotropic agent guidance. 1
Patients with severe ARDS or sepsis-associated disorders not responding to initial therapy should be considered for advanced hemodynamic monitoring with PAC. 1
The PAC should be inserted when specific hemodynamic data are needed and removed as soon as it is no longer helpful, as complications increase with duration of use. 1
When Central Venous Measurement is Sufficient:
Central venous lines provide adequate SvO₂ estimation (as ScvO₂) for initial evaluation and resuscitation protocols in most critically ill patients, offering a less invasive alternative with lower risk and cost. 1, 3, 5
ScvO₂ is particularly useful in early goal-directed therapy protocols for septic shock, where the target is ScvO₂ ≥70%. 4, 3
Important Measurement Considerations
The superior vena cava or right atrium provides ScvO₂ values, while only the pulmonary artery provides true mixed venous saturation (SvO₂) that includes blood from all venous drainage including the inferior vena cava and coronary sinus. 1, 3
Continuous fiberoptic oximetry via PAC shows excellent correlation (r = 0.95) with laboratory co-oximeter measurements and provides valuable real-time warning of cardiopulmonary deterioration. 2
Normal SvO₂ values are 70-75%, while normal ScvO₂ is approximately 75%, reflecting the typical 5% difference between these measurement sites. 4