Qp:Qs Ratio Calculator — Cardiac Shunt Quantification
Apply the Fick principle to cardiac catheterisation oxygen saturation data. Enter systemic arterial, mixed venous, pulmonary venous, and pulmonary arterial saturations to compute Qp/Qs and classify the shunt magnitude.
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Pulmonary-to-systemic flow ratio | 1.0 = no shunt
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Systemic O₂ difference (numerator)
97 − 70 = 27 - 2
Pulmonary O₂ difference (denominator)
97 − 70 = 27Pulmonary venous minus pulmonary arterial saturation. - 3
Qp : Qs ratio
27 ÷ 27 = 1
How does this calculator work?
Qp/Qs = (SaO₂ − SvO₂) ÷ (SpvO₂ − SpaO₂) via the Fick principle applied to cardiac catheterisation O₂ saturation data. Ratio ≈ 1.0 = no shunt; ≥ 1.5 = clinically significant left-to-right shunt; < 1.0 = right-to-left; ≥ 2.0 = large shunt often warranting intervention. Sampling site accuracy is essential.
Formula
How this is calculated
The Fick principle relates oxygen consumption (VO₂) to blood flow and the arteriovenous oxygen content difference. For the systemic circulation: VO₂ = Qs × (SaO₂ − SvO₂) × [O₂ capacity]; for the pulmonary circulation: VO₂ = Qp × (SpvO₂ − SpaO₂) × [O₂ capacity]. Since VO₂ and O₂ capacity cancel, Qp/Qs = (SaO₂ − SvO₂) / (SpvO₂ − SpaO₂).
In a normal heart with no shunt, Qp = Qs (ratio ≈ 1.0) and SpaO₂ equals the mixed venous saturation (~70%). A left-to-right shunt (ASD, VSD, PDA) adds oxygenated blood to the right heart, raising SpaO₂ and shrinking the denominator, so the ratio exceeds 1. A Qp/Qs ≥ 1.5 is the conventional threshold for a haemodynamically significant shunt; ≥ 2.0 is generally considered large. A ratio < 1 indicates right-to-left flow (cyanotic shunt), where deoxygenated blood enters the systemic circulation and lowers SaO₂.
Data accuracy is critical: SpvO₂ should be sampled from the pulmonary capillary wedge or left atrium; SpaO₂ from the main pulmonary artery distal to any mixing from the shunt; SvO₂ from a 'mixed' sampling run (proximal PA or right atrium mean). Errors in sampling site or oxygen saturation measurement directly affect the ratio. This calculation is used to guide intervention decisions for structural heart defects.
Frequently asked questions
A Qp/Qs ≥ 1.5 suggests a haemodynamically significant left-to-right shunt. Most guidelines recommend considering intervention for ASD or VSD when Qp/Qs ≥ 1.5 with symptoms, or ≥ 2.0 even asymptomatically, provided pulmonary vascular resistance is not prohibitively elevated. The overall clinical picture — right ventricular size, symptoms, and PVR — guides the final decision.
In a left-to-right shunt (e.g. ASD, VSD, PDA), oxygenated blood from the left heart enters the right heart or pulmonary artery. This raises SpaO₂ above the normal mixed-venous level (~70%), shrinks the denominator of the Fick ratio, and produces a Qp/Qs > 1. The 'step-up' in O₂ saturation between right-heart chambers localises the shunt level.
Normal: SaO₂ (aorta) ≈ 97%; SvO₂ (right atrium) ≈ 65–75%; SpvO₂ (left atrium/pulmonary vein) ≈ 97%; SpaO₂ (main pulmonary artery) ≈ 65–75%. With no shunt these give Qp/Qs = 1.0. Any step-up in O₂ saturation moving from the right atrium toward the pulmonary artery indicates a left-to-right shunt at that level.
Also known as
TG we-Calculate Editorial Team. (2026). Qp:Qs Ratio Calculator — Cardiac Shunt Quantification [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/qp-qs-ratio-calculator
TG we-Calculate Editorial Team. "Qp:Qs Ratio Calculator — Cardiac Shunt Quantification." TG we-Calculate. 2026. https://we-calculate.com/calculator/qp-qs-ratio-calculator.
TG we-Calculate Editorial Team, "Qp:Qs Ratio Calculator — Cardiac Shunt Quantification," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/qp-qs-ratio-calculator
@misc{wecalculate_qp_qs_ratio_calculator, title = {Qp:Qs Ratio Calculator — Cardiac Shunt Quantification}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/qp-qs-ratio-calculator}}, year = {2026}, note = {TG we-Calculate} }
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