What Qp/Qs ratio indicates a significant shunt?

A Qp/Qs > 1.5:1 is generally considered hemodynamically significant for left-to-right shunts. For ASD closure, most guidelines recommend intervention at Qp/Qs ≥ 1.5:1 with evidence of right heart volume overload.

Why use the Flamm formula for mixed venous oxygen?

The SVC and IVC have different oxygen saturations due to varying tissue oxygen extraction. The Flamm formula (3×SVC + 1×IVC)/4 weights the SVC more heavily because it better represents myocardial oxygen saturation and avoids falsely elevated values from renal blood return in the IVC.

Can Qp/Qs be calculated non-invasively?

Yes, cardiac MRI and echocardiography can estimate Qp/Qs. MRI uses phase-contrast velocity mapping of the aorta and pulmonary artery. Echo uses Doppler-derived stroke volumes. However, catheterization remains the gold standard.

What if Qp/Qs is less than 1?

A ratio below 1.0 indicates predominant right-to-left shunting, meaning deoxygenated blood is bypassing the lungs and entering the systemic circulation. This causes cyanosis and is seen in conditions like Tetralogy of Fallot and Eisenmenger syndrome.

How does oxygen consumption affect the calculation?

VO₂ is needed for absolute Fick flow calculations (Qp and Qs in L/min) but does NOT affect the Qp/Qs ratio itself, which depends only on oxygen saturation differences. However, inaccurate VO₂ will affect the absolute flow values.

What is Eisenmenger syndrome?

Eisenmenger syndrome occurs when a long-standing left-to-right shunt causes irreversible pulmonary hypertension, eventually reversing the shunt direction to right-to-left. At this stage, shunt closure is contraindicated because the shunt serves as a pressure relief valve.

Qp/Qs Ratio Calculator — Pulmonary-to-Systemic Flow

Calculate the Qp/Qs ratio from cardiac-catheterization oxygen saturations to review intracardiac shunt magnitude in ASD, VSD, PDA, and related congenital-heart contexts.

⚠️ Medical Disclaimer: The Qp/Qs ratio requires cardiac catheterization data. Interpretation must be performed by cardiologists in the full clinical context. This tool is for educational and decision-support purposes.

Oxygen Saturations (%)

Pulmonary Vein O₂ Sat (%)Or systemic arterial if no PV sample

Pulmonary Artery O₂ Sat (%)

Aortic O₂ Sat (%)

SVC O₂ Sat (%)Superior vena cava

IVC O₂ Sat (%)Inferior vena cava

Additional Parameters

Heart Rate (bpm)

Hemoglobin (g/dL)

O₂ Consumption (mL/min)VO₂ — estimated or measured

Qp/Qs Ratio

1.02

Normal — no significant shunt

Pulmonary Flow (Qp)

5.71 L/min

Fick-derived

Systemic Flow (Qs)

5.59 L/min

Fick-derived

Mixed Venous O₂

71.5%

Flamm formula: (3×SVC + IVC) / 4

Shunt Fraction

2.1%

121 mL/min shunt volume

Clinical Significance

Not significant

May follow conservatively

Normal — no significant shunt

Qp/Qs = 1.02 | Qp = 5.71 L/min | Qs = 5.59 L/min

Qp/Qs Interpretation Guide

Qp/Qs Range	Interpretation	Clinical Action
< 1.0	Right-to-left shunt	Cyanotic heart disease evaluation
1.0–1.3	Normal / trivial shunt	No intervention needed
1.3–1.5	Small left-to-right shunt	Follow-up, may not require closure
1.5–2.0	Moderate L-to-R shunt	Consider closure if symptomatic
> 2.0	Large L-to-R shunt	Closure generally indicated
> 3.0	Very large shunt	Assess for Eisenmenger physiology

Common Shunt Lesions

Lesion	Shunt Direction	Typical Qp/Qs
ASD (atrial septal defect)	Left → Right	1.5–3.0
VSD (ventricular septal defect)	Left → Right	1.5–4.0+
PDA (patent ductus arteriosus)	Left → Right	1.5–3.0
Tetralogy of Fallot	Right → Left	< 1.0
Eisenmenger syndrome	Right → Left	< 1.0

Planning notes, formulas, and examples

About the Qp/Qs Ratio Calculator — Pulmonary-to-Systemic Flow

The pulmonary-to-systemic flow ratio (Qp/Qs) is a standard catheterization measure for summarizing shunt magnitude. Using oxygen saturation data from multiple chambers, it helps show whether pulmonary blood flow is roughly equal to, greater than, or less than systemic flow in congenital-heart disease review.

A Qp/Qs of 1.0 indicates equal pulmonary and systemic blood flow (no shunt). Ratios above 1.5 are generally considered hemodynamically significant for left-to-right shunts (ASD, VSD, PDA), while ratios below 1.0 indicate right-to-left shunting (as in cyanotic congenital heart disease).

This calculator uses the Fick principle with oxygen saturation data from the pulmonary veins, pulmonary artery, aorta, SVC, and IVC. It computes mixed venous oxygen saturation using the Flamm formula, derives the Qp/Qs ratio, and estimates absolute pulmonary and systemic flows when hemoglobin and oxygen consumption are provided.

When This Page Helps

Qp/Qs is useful because it turns several catheterization oxygen saturations into one compact shunt summary. Keeping the mixed-venous estimate, the final ratio, and the direction-of-shunt context together makes catheterization data easier to review consistently before it is interpreted with imaging, chamber size, and pulmonary-pressure data.

How to Use the Inputs

Enter oxygen saturations obtained during cardiac catheterization: pulmonary venous, pulmonary arterial, aortic, SVC, and IVC.
Enter the patient heart rate and hemoglobin for Fick cardiac output calculation.
Enter oxygen consumption (VO₂) — either measured (preferred) or estimated (125 mL/min/m² × BSA).
Review the Qp/Qs ratio, shunt direction and magnitude, and absolute flow calculations.
Compare results against the interpretation guide to assess need for shunt closure.

Formula used

Mixed Venous O₂ = (3 × SVC O₂ + IVC O₂) / 4 (Flamm formula)
Qp/Qs = (Aortic O₂ − Mixed Venous O₂) / (Pulmonary Vein O₂ − PA O₂)
Qp = VO₂ / (1.36 × Hb × 10 × (PV O₂ − PA O₂) / 100) L/min
Qs = VO₂ / (1.36 × Hb × 10 × (Ao O₂ − MV O₂) / 100) L/min

Example Calculation

Result: Qp/Qs = 1.02 — No significant shunt

Mixed venous O₂ = (3×70 + 76)/4 = 71.5%. Qp/Qs = (95 − 71.5) / (98 − 75) = 23.5/23 = 1.02. With a ratio near 1.0, there is no hemodynamically significant intracardiac shunt.

Tips & Best Practices

Always verify oxygen saturation step-ups across chambers to localize the shunt level before calculating Qp/Qs.
Measured VO₂ is preferred over estimated values — metabolic variability can introduce significant error.
In patients with atrial fibrillation, average saturations over multiple beats for more reliable results.
If pulmonary vein sampling is not available, use systemic arterial saturation as a surrogate (assuming no right-to-left shunt).
Serial Qp/Qs measurements help track shunt progression and response to medical therapy.

Practical Guidance

Use consistent catheterization sources for each saturation value and document whether VO2 was measured or estimated.

Common Pitfalls

Most errors come from mixing venous sampling sites, using an inconsistent mixed-venous formula, or rounding too early. Recheck the chamber labels and the direction of the shunt before acting on the result.

Interpreting Borderline Ratios

Borderline Qp/Qs values near 1.0 to 1.2 often need the full clinical picture, including chamber enlargement, symptoms, pulmonary pressures, and imaging findings. The ratio is one input to decision-making, not the only one.

Sources & Methodology

Last updated: March 28, 2026

Methodology

This calculator estimates mixed venous saturation with the standard weighted SVC/IVC worksheet, then applies saturation-based indirect Fick relationships to show Qp/Qs and, when hemoglobin and oxygen consumption are supplied, estimated pulmonary and systemic flows. The ratio itself depends on the saturation differences; the absolute flow estimates depend on the additional Fick inputs.

The output is meant to summarize catheterization data, not to replace the full congenital-heart review. Shunt anatomy, chamber enlargement, pulmonary vascular resistance, and the broader cath findings still determine how the ratio should be used.

Sources

2018 AHA/ACC Guideline for the Management of Adults With Congenital Heart Disease (American Heart Association / American College of Cardiology) — Guideline context for Qp:Qs thresholds in adult congenital heart disease.

Frequently Asked Questions

A Qp/Qs > 1.5:1 is generally considered hemodynamically significant for left-to-right shunts. For ASD closure, most guidelines recommend intervention at Qp/Qs ≥ 1.5:1 with evidence of right heart volume overload.