Qp/Qs Calculator (shunt fraction from oxygen saturations, Fick method)

Understanding the Qp/Qs calculator — concept and principle

Qp/Qs compares blood flow through the lungs (Qp) with blood flow to the body (Qs). This calculator works it out from four oxygen saturations measured during cardiac catheterisation, to judge the size and direction of a shunt such as an atrial or ventricular septal defect.

How it is calculated

Qp/Qs = (SaO₂ − SvO₂) ÷ (SpvO₂ − SpaO₂)

• SaO₂: systemic arterial saturation.
• SvO₂: mixed venous saturation, in shunt studies usually from the superior vena cava, upstream of an atrial shunt.
• SpvO₂: pulmonary venous saturation; if not sampled, often assumed to be 95–98%.
• SpaO₂: pulmonary artery saturation.

Pulmonary flow compared with systemic = (Qp/Qs − 1) × 100%.

The principle

By the Fick principle, flow through a circuit equals oxygen uptake divided by the oxygen content difference across it. The body's oxygen uptake is the same in both equations, and oxygen content is proportional to haemoglobin × saturation, so uptake and haemoglobin cancel. What remains is a ratio of saturation differences: a large difference across the body and a small one across the lungs means more blood is going through the lungs.

Worked example

Default inputs: SaO₂ 95%, SvO₂ 70%, SpvO₂ 98%, SpaO₂ 85%.

1. Systemic difference = 95 − 70 = 25.
2. Pulmonary difference = 98 − 85 = 13.
3. Qp/Qs = 25 ÷ 13 = 1.92 → Net left-to-right shunt.
4. Pulmonary flow is 92% higher than systemic; 1.92 ≥ 1.5 → usually called significant.

How to read the result

A ratio above 1 means a net left-to-right shunt; below 1 means net right-to-left. A ratio of 1.5 or more is commonly regarded as haemodynamically significant and may support closing a defect, but the decision also uses imaging, chamber sizes and pulmonary pressures.

Common mistakes

• Taking samples at different times or with the patient on high inspired oxygen, where dissolved oxygen matters.
• Using an inferior vena cava sample alone for mixed venous saturation.
• Entering saturations as fractions instead of percentages.

Frequently asked questions

What does Qp/Qs 2:1 mean?

Twice as much blood flows through the lungs as to the body.

Can echo estimate Qp/Qs?

Yes, from Doppler flow in the outflow tracts, but catheter oximetry is the classic method.

Why use the superior vena cava sample?

It is upstream of an atrial shunt, so it reflects true mixed venous blood.

Sources: Fick principle applied to the pulmonary and systemic circulations (standard cardiac catheterisation formula) · Baim DS, Grossman W, Grossman's Cardiac Catheterization, Angiography, and Intervention (shunt detection and measurement)