Formula for Qp/Qs Calculation Using Oxygen Saturations
The Qp/Qs ratio is calculated using the Fick principle with oxygen saturations (or content) measured from pulmonary and systemic arterial and venous systems, where Qp/Qs = (SaO₂ - SvO₂) / (SpvO₂ - SpaO₂).
The Standard Oximetry Formula
The reference standard method for calculating Qp/Qs uses invasive oximetry with the following formula 1, 2:
Qp/Qs = (SaO₂ - SvO₂) / (SpvO₂ - SpaO₂)
Where:
- SaO₂ = systemic arterial oxygen saturation
- SvO₂ = central/mixed venous oxygen saturation (systemic venous)
- SpvO₂ = pulmonary venous oxygen saturation
- SpaO₂ = pulmonary arterial oxygen saturation
This can also be expressed using oxygen content instead of saturations 2.
Interpretation of Results
Qp/Qs > 1.0: Indicates a left-to-right shunt (more blood flowing to pulmonary circulation)
- Higher ratios indicate larger shunts
- Qp/Qs > 1.1 is generally considered clinically significant 2
Qp/Qs < 1.0: Indicates a right-to-left shunt (more blood flowing to systemic circulation)
- The closer to zero, the larger the shunt 1
Qp/Qs = 1.0: Balanced circulation with no net shunt
Critical Measurement Requirements
The accuracy of this calculation depends on obtaining oxygen saturations from the correct anatomic locations 1:
- Sampling must be performed distal to the shunt for accurate quantification
- Mixed venous saturation requires adequate sampling (problematic in certain anatomies like single ventricle after bidirectional Glenn where SVC and IVC circuits are separated)
- Measurements must be obtained during steady state to minimize error propagation 1
Important Limitations and Pitfalls
Multiple sampling site requirements during steady state can result in significant error propagation and inaccurate Qp/Qs quantification 1. Additional limitations include:
- Cannot be used in extracardiac lesions where sampling distal to the shunt is impossible (e.g., systemic-pulmonary arterial collaterals) 1
- Assumes pulmonary venous saturation is available - estimating Qp/Qs without this value may result in significant errors 3
- Invasive nature carries morbidity risk and high cost 1
Alternative Non-Invasive Methods
While the oximetry formula remains the reference standard when pulmonary artery pressure and vascular resistance measurements are needed (e.g., Eisenmenger physiology) 1, cardiac MRI using phase-contrast imaging (PC-CMR) has become the non-invasive reference standard for Qp/Qs measurement, offering highly accurate and reproducible quantification without the limitations of invasive oximetry 1.