# Qp/Qs Ratio Calculator (Shunt Fraction)

URL: https://speedycalculator.com/calculators/health/qp-qs-ratio/
Last updated: 2026-10-30

> The Qp/Qs calculator estimates the ratio of pulmonary to systemic blood flow from oxygen saturations measured during cardiac catheterisation, to quantify the size of an intracardiac shunt such as an atrial or ventricular septal defect.

## Inputs

- Systemic arterial saturation (%)
- Mixed venous saturation (SVC/IVC) (%)
- Pulmonary venous saturation (%)
- Pulmonary arterial saturation (%)

## Results

- Qp/Qs

## Formula

```
Qp/Qs = (SaO₂ − SvO₂) ÷ (SpvO₂ − SpaO₂)
SaO₂ systemic arterial · SvO₂ mixed venous · SpvO₂ pulmonary venous (≈ SaO₂ or 98% if not measured) · SpaO₂ pulmonary arterial
```

## Explanation

In a left-to-right shunt, oxygenated blood recirculates through the lungs, so pulmonary flow exceeds systemic flow. A Qp/Qs above 1.5 usually indicates a significant shunt that may need closure; below 1 indicates net right-to-left flow.

### How to use the Qp/Qs Ratio Calculator

- **Systemic arterial saturation**
- **Mixed venous saturation (SVC/IVC)**
- **Pulmonary venous saturation**
- **Pulmonary arterial saturation**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the four oxygen saturations from the catheter study.

### Inputs explained

FieldWhat to enterSystemic arterial saturationIn %Mixed venous saturation (SVC/IVC)In %Pulmonary venous saturationIn %Pulmonary arterial saturationIn %
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — Systemic arterial saturation: 96 %; Mixed venous saturation (SVC/IVC): 70 %; Pulmonary venous saturation: 98 %; Pulmonary arterial saturation: 82 %.

Results — **Qp/Qs = 1.63**.

## Key terms

- **Qp/Qs**: Pulmonary-to-systemic flow ratio.

## FAQ

**What Qp/Qs is significant?**
Above 1.5:1, especially with right heart enlargement.

**Why measure mixed venous saturation carefully?**
It is often calculated as (3 × SVC + IVC) ÷ 4 because of streaming.

**Can echo estimate Qp/Qs?**
Yes – from flow in the pulmonary and aortic outflow tracts.


## Sources

- [Mathew R et al. Shunt evaluation by cardiac catheterization. StatPearls](https://www.ncbi.nlm.nih.gov/books/)