Physiologic Shunt (Qs/Qt) Calculator
The fraction of cardiac output reaching the arterial side without being oxygenated — the classic three-content shunt equation, or the estimated form when no mixed-venous sample is available, with every oxygen content shown.
Written by Apex Respiratory Editorial Team
Most reliable at a high FiO₂.
Default 760 (sea level).
3.5 mL/dL for critically ill patients; 5.0 for normal extraction.
Enter the arterial gas and hemoglobin to calculate the shunt fraction.
Reading the shunt fraction
Qs/Qt compares three oxygen contents: end-pulmonary-capillary (CcO₂, assumed 100% saturated at the alveolar PO₂), arterial (CaO₂), and mixed venous (CvO₂). This tool derives the alveolar PO₂ internally from the alveolar gas equation, so you never enter it.
Estimated mode(default) is for when there is no PA catheter: it substitutes an assumed arterial–venous content difference, C(a-v)O₂ — 3.5 mL/dL for the critically ill (high output narrows the true gap) or 5.0 for normal extraction. The assumption can cross a severity band, so the value used is always shown. Classic mode uses a measured mixed-venous SvO₂ and PvO₂.
Bands: < 10% clinically insignificant (anatomic shunt ~2–5%), 10–19% mild, 20–29% significant, ≥ 30% severe. A true shunt responds poorly to a rising FiO₂ — the number is most meaningful at a high FiO₂, where V/Q mismatch contributes less.
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2020.