Measuring Mixed Venous Saturation in Glenn Physiology
In patients with a bidirectional Glenn shunt, mixed venous oxygen saturation is measured from the inferior vena cava (IVC), as this represents the true mixed venous blood that has not yet entered the pulmonary circulation. 1
Understanding the Altered Circulation
After a bidirectional Glenn procedure, the circulatory anatomy fundamentally changes:
- Superior vena cava (SVC) blood bypasses the heart entirely and flows directly into the pulmonary arteries, connecting to both right and left lungs through the intact pulmonary artery confluence 2
- The SVC blood is therefore NOT mixed venous blood because it never mixes with IVC return in the heart 1
- Only IVC blood returns to the single ventricle, mixes with pulmonary venous return, and gets ejected systemically 1
The Correct Sampling Site
Sample from the IVC to obtain true mixed venous saturation in Glenn patients. 1 This is fundamentally different from normal two-ventricle physiology where pulmonary artery sampling represents mixed venous blood 3.
Why IVC Sampling is Correct:
- The IVC carries the only venous blood that actually "mixes" with oxygenated blood in the functional single ventricle 1
- SVC blood has already been arterialized by passing through the lungs via the Glenn connection 2
- Measuring SVC saturation would give you post-pulmonary (partially oxygenated) blood, not mixed venous blood 4, 5
Critical Technical Considerations
Challenges in Accurate Measurement:
The guidelines explicitly warn that "many problems persist in estimating pulmonary blood flow and PVR after cavopulmonary anastomoses" including difficulties with "sampling a true pulmonary artery saturation in the presence of aortopulmonary collaterals or residual forward flow from the ventricles" 1. These same challenges apply to venous sampling.
Specific Pitfalls to Avoid:
- Do not sample from the SVC - this represents post-Glenn oxygenated blood, not mixed venous blood 4, 5
- Do not use traditional mixed venous formulas that average SVC and IVC saturations, as these assume normal cardiac anatomy where both cavae drain to the right atrium 3
- Account for systemic-to-pulmonary collaterals that can develop and affect oxygen saturations, creating volume load on the ventricle despite the Glenn physiology 1
- Be aware of venous collaterals or arteriovenous malformations that can "steal" blood flow and distort saturation measurements 1
Clinical Monitoring Applications
For Hemodynamic Assessment:
When calculating transpulmonary gradient and pulmonary vascular resistance index (PVRI) before Fontan completion, use IVC saturation as your mixed venous value since measurements of these parameters "are important in the selection of patients with a single ventricle for cavopulmonary surgery" 1.
For Continuous Monitoring:
If using continuous central venous oxygen saturation (ScvO2) monitoring during or after Glenn procedures, position the catheter in the IVC or right atrium (where IVC blood enters) rather than the SVC 5, 6. Research shows that right atrial sampling (which receives IVC blood in Glenn physiology) may provide closer estimates than SVC sampling 6.
Optimizing Tissue Oxygenation:
In Glenn patients, a novel approach suggests monitoring the lower of SVC or IVC saturations (SO2min) provides the best assessment of overall tissue oxygenation, as maximizing this value "always gives physiologically sensible results" across varying metabolic distributions 4.
Practical Algorithm for Measurement
- Position catheter in the IVC (or right atrium if IVC access is difficult) 5, 6
- Obtain blood sample or continuous monitoring from this location 5
- This IVC saturation represents your mixed venous saturation for Glenn physiology 1
- Do NOT average with SVC values - the SVC is post-pulmonary blood in this circulation 4
- Consider complementary monitoring with near-infrared spectroscopy (rSO2) for regional brain oxygenation, as changes parallel ScvO2 but with different sensitivity 5