The Qp/Qs ratio compares blood flow through the lungs to blood flow through the rest of the body, used to detect and quantify heart shunts where blood passes between the two circulations abnormally.
Aortic and mixed-venous saturations over pulmonary-vein and pulmonary-artery saturations.
No significant shunt.
Left-to-right shunt.
Large left-to-right shunt — often prompts closure.
Right-to-left shunt.
A shunt makes pulmonary and systemic flow unequal — the ratio quantifies it.
Classic examples include atrial septal defect, ventricular septal defect, and patent ductus arteriosus — all of which allow extra blood flow to the lungs (left-to-right shunt).
Qp/Qs of 1 means no significant shunt. Greater than 1 indicates a left-to-right shunt (more flow to the lungs); less than 1 indicates a right-to-left shunt.
A Qp/Qs above roughly 1.5–2.0 is generally considered hemodynamically significant and may prompt closure of the defect.
From oxygen saturations at several points using the Fick principle, or directly during cardiac catheterization.