Qp/Qs Ratio Calculator
Use oxygen saturations to estimate pulmonary-to-systemic blood flow ratio:
Qp/Qs = (SaO2 − SvO2) / (SpvO2 − SpaO2)
For educational use only. Always interpret with full hemodynamic and clinical context.
What Is the Qp/Qs Ratio?
The Qp/Qs ratio compares blood flow through the lungs (Qp, pulmonary flow) to blood flow through the body (Qs, systemic flow). It is commonly used in cardiology, especially in congenital heart disease, to estimate whether a shunt is present and how large it may be.
In simple terms, this number helps answer a practical question: Is extra blood being recirculated through the lungs, bypassing normal forward systemic flow, or vice versa?
How the Formula Works
This calculator uses oxygen saturation values:
- SaO2: systemic arterial oxygen saturation
- SvO2: mixed venous oxygen saturation
- SpvO2: pulmonary venous oxygen saturation
- SpaO2: pulmonary arterial oxygen saturation
The ratio is calculated as:
Qp/Qs = (SaO2 − SvO2) / (SpvO2 − SpaO2)
If the pulmonary flow and systemic flow are equal, the ratio is about 1.0.
How to Interpret Results
Typical interpretation ranges
- ~1.0: No significant shunt (or balanced flow)
- >1.0: Suggests left-to-right shunting (more blood to lungs)
- 1.5 or higher: Often considered hemodynamically significant in many scenarios
- <1.0: May suggest right-to-left shunting or measurement inconsistency
Interpretation depends on the clinical context, chamber sampling accuracy, oxygen content assumptions, and whether measurements were taken under stable conditions.
Worked Example
Suppose you have:
- SaO2 = 96%
- SvO2 = 66%
- SpvO2 = 98%
- SpaO2 = 78%
Then:
(96 − 66) / (98 − 78) = 30 / 20 = 1.50
A Qp/Qs of 1.50 suggests a meaningful left-to-right shunt and usually warrants formal specialist review in the broader hemodynamic picture.
Important Clinical Notes
Why this number is useful
- Helps assess shunt severity in conditions like ASD, VSD, or PDA
- Supports decisions on follow-up, imaging, and intervention timing
- Provides a quantitative checkpoint for serial comparisons
Limitations
- Saturation-based formulas are approximations of oxygen content methods
- Sampling errors can significantly affect results
- Unusual pulmonary venous saturation values can distort the denominator
- Never use Qp/Qs in isolation to make treatment decisions
Frequently Asked Questions
Can Qp/Qs be negative?
A negative or zero result usually indicates invalid input combinations or sampling inconsistency, not a physiologic endpoint for routine interpretation.
What value of Qp/Qs is “normal”?
Around 1.0 is generally considered normal flow balance.
Is a high Qp/Qs always an emergency?
Not always. Urgency depends on symptoms, ventricular loading, pulmonary pressures, and the underlying lesion.
Bottom Line
A Qp/Qs calculator is a fast way to estimate pulmonary vs systemic blood flow. It is especially helpful in shunt evaluation, but the number must be integrated with full cardiology assessment, imaging, and invasive data when needed.