Respiratory
Shunt Fraction (Qs/Qt) Calculator
Fraction of cardiac output that passes the lungs without picking up oxygen, as in ARDS, pneumonia and atelectasis.
Formula
((CcO2 - CaO2) / (CcO2 - CvO2)) * 100
How it is used
- CcO₂ is the end-capillary content, worked from the alveolar PO₂ with hemoglobin taken as fully saturated; CvO₂ needs a mixed venous sample.
- Shunted blood never meets ventilated alveoli, so hypoxemia from shunt barely improves with more oxygen; PEEP and proning recruit the flooded or collapsed units instead.
- Causes: ARDS, pneumonia, atelectasis, pulmonary edema and intracardiac right-to-left shunts.
Related one-pagers
- IABP ManagementMechanical complications of MI (papillary muscle or septal rupture): afterload reduction cuts the regurgitant or shunt fraction until surgery; ESC 2023 ACS IIa
- CapnographyThe Bohr equation gives the dead-space fraction: VD/VT = (PACO₂ − PĒCO₂) ÷ PACO₂, where PĒCO₂ is mixed expired CO₂ collected over a minute or measured by volumetric capnography. Enghoff substituted PaCO₂ for the alveolar value, VD/VT = (PaCO₂ − PĒCO₂) ÷ PaCO₂, which is what bedside calculation uses. Because PaCO₂ also carries the effect of shunt and V/Q mismatch, Enghoff dead space is larger than true Bohr dead space. Normal VD/VT is about 0.3 breathing spontaneously; equipment adds to it on the ventilator.
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Related calculators
- A-a Gradient
- Alveolar Gas Equation (PAO₂)
- Arterial Oxygen Content (CaO₂)
- Dead Space Fraction (Bohr)
- Driving Pressure
- Dynamic Compliance
- Minute Ventilation
- O₂ Extraction Ratio
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For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.