Calculate the Qp/Qs ratio from cardiac-catheterization oxygen saturations to review intracardiac shunt magnitude in ASD, VSD, PDA, and related congenital-heart contexts.
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.
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.
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
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.
Use consistent catheterization sources for each saturation value and document whether VO2 was measured or estimated.
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.
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.
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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.
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.
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.
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.
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.
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.
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.