π« Qp/Qs Ratio Calculator
Quantify cardiac shunts from saturation data.
For informational purposes only, not medical advice. Consult a healthcare professional.
The Qp/Qs ratio compares pulmonary blood flow with systemic blood flow, and it is the number that decides whether a septal defect or other shunt matters hemodynamically. A ratio of 1 means balanced flows; 2 means twice as much blood crosses the lungs as the body, the classic significant left-to-right shunt. Ratios above 1.5 generally push toward closure of defects like ASD and VSD, while ratios approaching 1 with high pulmonary resistance raise the specter of Eisenmenger physiology. This calculator runs the saturation-based Fick equation for teaching and cath-lab reference.
How to Use This Calculator
Enter the four oxygen saturations from the catheterization oximetry run. The calculator applies the Fick-based shunt equation.
Take systemic arterial and mixed venous saturations plus the pulmonary artery and pulmonary vein saturations from the cath oximetry series.
Enter all four as percentages. Pulmonary vein saturation defaults to 98; adjust only if directly measured.
Read the Qp/Qs ratio. Above 1.5 generally marks a hemodynamically significant left-to-right shunt.
How the Math Works
Qp/Qs equals systemic arterial saturation minus mixed venous saturation, divided by pulmonary vein saturation minus pulmonary artery saturation. The numerator reflects systemic oxygen extraction and the denominator pulmonary extraction; a step-up in PA saturation from shunted blood shrinks the denominator and raises the ratio. With arterial 98, mixed venous 70, PA 85, and PV 98, the ratio is 28 divided by 13, about 2.15. The simplified saturation form assumes similar hemoglobin and dissolved oxygen across circuits, which holds well enough for clinical use. Significance bands: 1.0 normal, 1.0 to 1.5 small, 1.5 to 2.0 moderate and usually significant, above 2.0 large.
Worked Example
Oximetry shows arterial saturation 98, mixed venous 70, PA 85, and pulmonary vein 98 percent. The numerator is 28 and the denominator is 13, giving a Qp/Qs of 2.15. The calculator classifies this as a Large left-to-right shunt, since 2.15 clears the 2.0 line. The 15-point step-up from mixed venous to PA saturation is the fingerprint of the left-to-right shunt feeding oxygenated blood into the right heart.
Reading Your Result
A ratio of 1 with normal saturations means no significant shunt. The 1.0 to 1.5 band is a small shunt, usually watched. From 1.5 to 2.0, most guidelines consider the shunt hemodynamically significant and closure is discussed for suitable defects. Above 2.0 is a large shunt where closure is generally favored if pulmonary vascular resistance allows. A ratio under 1 suggests right-to-left shunting, and a ratio near 1 with elevated pulmonary pressures needs resistance calculations before any closure decision.
| Qp/Qs | Shunt | Typical stance |
|---|---|---|
| 1.0 | None | No shunt |
| 1.0-1.5 | Small | Watch |
| 1.5-2.0 | Moderate | Closure discussed |
| Above 2.0 | Large | Closure favored if PVR allows |
Limitations
The saturation method assumes steady state and accurate sampling; streaming, sedation changes, and supplemental oxygen distort it. It quantifies net shunt, not anatomy: the location and number of defects need imaging. Pulmonary vein saturation is usually assumed, not measured, which introduces error when the assumption fails. Closure decisions integrate pressures, resistances, symptoms, and anatomy, never the ratio alone.
Related Calculators
Related tools: pulmonary vascular resistance calculator PISA regurgitation calculator rate pressure product calculator.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What does Qp/Qs measure?
The ratio of blood flow through the lungs to blood flow through the body. In a normal heart both are equal, so the ratio is 1. A left-to-right shunt recirculates oxygenated blood through the lungs, raising pulmonary flow above systemic and pushing the ratio above 1.
What Qp/Qs is considered significant?
Above 1.5 is the widely used threshold for a hemodynamically significant left-to-right shunt, the point where defect closure is generally discussed. Above 2.0 is a large shunt. Below 1.5, small shunts are often simply monitored.
How is Qp/Qs measured?
By oximetry during cardiac catheterization: saturations sampled from the vena cavae, right atrium, right ventricle, pulmonary artery, pulmonary vein wedge position, and systemic artery. A saturation step-up between right-sided chambers localizes and quantifies the shunt.
What causes a high Qp/Qs?
Left-to-right shunts: atrial septal defect, ventricular septal defect, patent ductus arteriosus, and partial anomalous pulmonary venous return. All let oxygenated blood re-enter the right heart or pulmonary artery, creating the saturation step-up the formula detects.
What does Qp/Qs under 1 mean?
Net right-to-left shunting: deoxygenated blood reaching the systemic circulation, as in Eisenmenger syndrome or complex cyanotic lesions. It implies pulmonary vascular disease may have reversed the shunt, which changes management completely.
What is Eisenmenger syndrome?
The end stage of a large unrepaired left-to-right shunt: pulmonary vascular disease raises resistance until the shunt reverses right-to-left, causing cyanosis. At that point closing the defect is dangerous because the shunt acts as a relief valve. PVR measurement, not Qp/Qs alone, makes this call.
Can echo estimate Qp/Qs?
Doppler echo can estimate pulmonary versus systemic stroke volumes as a screening tool, but catheterization oximetry remains the reference for quantification. Echo finds the anatomy; cath quantifies the physiology.
Why assume pulmonary vein saturation?
Because sampling a pulmonary vein directly is rarely done. In the absence of lung disease it runs 95 to 100 percent, and 98 is the standard assumption. Significant lung disease or a right-to-left shunt elsewhere invalidates the assumption.
Does oxygen during the cath affect the result?
Yes. Supplemental oxygen raises saturations unevenly and can mask or distort step-ups. Oximetry runs are ideally done on room air or a noted stable FiO2, and the conditions must be recorded with the numbers.
When is a shunt closed?
Generally for Qp/Qs above 1.5 with suitable anatomy, evidence of right-heart volume overload, and pulmonary vascular resistance low enough to be safe, typically under about a third of systemic. Symptoms, arrhythmia risk, and paradoxical embolism history also weigh in. The heart team decides.