About the Qp/Qs ratio
Qp/Qs is the ratio of pulmonary blood flow (Qp, the volume of blood passing through the lungs) to systemic blood flow (Qs, the volume passing through the rest of the body). In a structurally normal heart these two flows are equal, so Qp/Qs is about 1.0. When a shunt lets blood cross between the pulmonary and systemic circulations - through an atrial septal defect, ventricular septal defect, patent ductus arteriosus, or similar lesion - the two flows diverge, and the ratio quantifies how much.
The saturation-based formula
Measuring Qp and Qs directly requires oxygen consumption and hemoglobin, which are hard to obtain reliably at the bedside. Because those terms appear in both the Qp and Qs equations of the Fick principle, they cancel out algebraically, leaving a shortcut that uses only oxygen saturations measured during cardiac catheterization:
- Qp/Qs = (SaO2 - SmvO2) / (SpvO2 - SpaO2)
- SaO2: systemic arterial oxygen saturation.
- SmvO2: mixed venous oxygen saturation, commonly estimated as (3 x SVC saturation + IVC saturation) / 4 because superior and inferior vena cava blood does not mix evenly in the right atrium.
- SpvO2: pulmonary vein oxygen saturation (often assumed near 95-98% when not directly sampled).
- SpaO2: pulmonary artery oxygen saturation.
Interpreting the ratio
A Qp/Qs near 1.0 means no significant net shunt. A ratio above 1.0 indicates a left-to-right shunt: extra blood is recirculating through the lungs before returning to the systemic circulation. As a general reference, 1.0-1.5 is often described as a small, hemodynamically insignificant shunt, 1.5-2.0 as moderate, and above 2.0 as large - large left-to-right shunts are more likely to be considered for surgical or catheter closure. A ratio below 1.0 indicates a right-to-left shunt, where some deoxygenated systemic venous blood bypasses the lungs entirely; this pattern is always clinically significant and can cause cyanosis.
When to consult a professional
This tool performs the standard saturation-based Qp/Qs arithmetic for education and reference. It does not diagnose a shunt lesion or determine treatment. Any decision about catheterization, medication, or surgical closure should be made by a cardiologist using the full clinical picture, imaging, and hemodynamic data.