How this instrument works
Qp/Qs applies the Fick principle — flow multiplied by an oxygen content change equals the oxygen picked up or given up — to compare two circuits without measuring flow directly in either. The systemic step, arterial saturation minus mixed venous saturation, reflects oxygen the body extracts on one full circuit. The pulmonary step, pulmonary venous saturation minus pulmonary arterial saturation, reflects oxygen picked up crossing the lungs. Divide one step by the other and the terms describing how much oxygen each unit of blood carries cancel out, leaving Qp/Qs: pulmonary flow over systemic flow.
Cardiac catheterization labs use this ratio to size and characterize shunts — an atrial septal defect, a ventricular septal defect, or a patent ductus arteriosus all let blood cross between the two circuits, pulling the ratio away from 1. A result at or near 1.0 means the circuits carry essentially equal flow, with no significant shunt. A result meaningfully above 1 means more blood is passing through the lungs than through the body — a left-to-right shunt recirculating oxygenated blood back into the pulmonary side. A result below 1 means the opposite: a right-to-left shunt, sending under-oxygenated blood into the systemic circuit without a detour through the lungs, the pattern behind cyanotic heart disease.
Pulmonary venous saturation is often assumed at 98%, standing in for fully-oxygenated blood, rather than sampled directly — threading a catheter into a pulmonary vein during a routine study is technically difficult and rarely attempted. That assumption holds reasonably well for a patient breathing room air with otherwise healthy lungs, but it introduces real error whenever lung disease itself is limiting oxygenation, or supplemental oxygen is changing the numbers being compared.
- Enter Systemic arterial saturation (%) — typically drawn from the aorta.
- Enter Mixed venous saturation (%) — usually from the pulmonary artery before oxygenation.
- Enter Pulmonary venous saturation (%) — often assumed near 98% if not sampled directly.
- Enter Pulmonary arterial saturation (%), then read the Qp/Qs ratio.
Worked example — three saturation sets
Systemic arterial 95%, mixed venous 75%, pulmonary venous 98%, pulmonary arterial 90%. The systemic step is 95 − 75 = 20; the pulmonary step is 98 − 90 = 8. Divide: 20 ÷ 8 = 2.5 — pulmonary flow running two and a half times systemic flow, a sizeable left-to-right shunt.
Change the readings to 96, 70, 98, and 94: the systemic step widens to 96 − 70 = 26, while the pulmonary step narrows to 98 − 94 = 4. The ratio climbs to 26 ÷ 4 = 6.5 — an even larger shunt, since shrinking the denominator moves the result just as much as growing the numerator would.
A third set — 94, 72, 97, 90 — gives a systemic step of 22 and a pulmonary step of 7, for a ratio of about 3.14.
Questions
What does a Qp/Qs of exactly 1 mean?
Pulmonary and systemic flow are equal — no shunt large enough to move the ratio measurably. Clinicians typically treat values roughly between 0.9 and 1.2 as within normal scatter for the measurement, read as no hemodynamically significant shunt.
Why use oxygen saturation instead of measuring flow directly?
Placing flow probes directly inside the heart's chambers isn't practical during a routine study, but saturation is easy to sample from catheters already threaded through those chambers for other reasons. The Fick principle converts those saturation differences into a flow ratio without ever measuring absolute flow in either circuit.
Why is pulmonary venous saturation often assumed rather than measured?
Sampling blood directly from a pulmonary vein means threading a catheter into a location that is technically difficult and rarely accessed during a standard catheterization. Assuming a value like 98% is shorthand for fully-oxygenated blood, reasonable for a patient with healthy lungs on room air, though it adds error if oxygenation is impaired for some other reason.
What does a ratio below 1 indicate?
A right-to-left shunt — blood is bypassing the lungs and reaching the systemic circuit without picking up oxygen there, the physiology behind cyanotic congenital heart disease. Values well below 1 flag a shunt significant enough to warrant closer imaging and characterization.
Is this ratio enough on its own to decide whether a shunt needs closure?
No — it's a major input, not the only one. A ratio of roughly 1.5 to 2.0 or higher is commonly cited as a point where closing a left-to-right shunt gets seriously considered, but the decision also weighs symptoms, chamber size on imaging, pulmonary artery pressures, and the specific defect's anatomy alongside this figure.
References
- Intracardiac Shunts — StatPearls, NCBI Bookshelf
- What radiologists need to know about the pulmonary-systemic flow ratio (Qp/Qs) — Radiología (English Edition)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.