# Pulmonary Shunt Fraction (Qs/Qt) Calculate Qs/Qt = (CcO₂ − CaO₂)/(CcO₂ − CvO₂). PAO₂ from alveolar gas equation. Normal <5%; >20% severe shunt refractory to O₂. Defaults SvO₂=0.75, PvO₂=40 when mixed-venous values unavailable. Derived from Berggren 1942, StatPearls, LITFL, West. Not medical advice. > Canonical page: https://elysiatools.com/en/tools/shunt-equation - **Category:** Health - **Keywords:** shunt fraction, qs qt, pulmonary shunt, oxygen content, cao2, cvo2, ards, hypoxemia, right to left shunt, oxygenation, critical care, pulmonology ## Overview The Pulmonary Shunt Fraction (Qs/Qt) calculator estimates the proportion of blood bypassing effective pulmonary gas exchange from hemoglobin, arterial blood gas, oxygen saturation, inspired oxygen, and optional mixed-venous values. It calculates PAO₂ with the alveolar gas equation and reports Qs/Qt, oxygen contents, and a shunt category. This tool is for education and clinical reference, not medical advice. ## Inputs - **Hemoglobin (g/dL)** (number): Hemoglobin concentration (g/dL). - **PaO₂ (mmHg)** (number): Arterial PO₂ (mmHg) from ABG. - **SaO₂ (arterial O₂ sat, fraction)** (number): Arterial oxygen saturation as a fraction (0–1, e.g. 0.92 = 92%). - **PvO₂ (mixed venous PO₂, mmHg)** (number): Mixed venous PO₂ (mmHg) from PA catheter. Default 40 if not measured. - **SvO₂ (mixed venous O₂ sat, fraction)** (number): Mixed venous oxygen saturation as a fraction (0–1). Default 0.75 if not measured. - **PaCO₂ (mmHg)** (number): Arterial PCO₂ (mmHg) from ABG, used to derive PAO₂. - **FiO₂ (inspired oxygen fraction)** (number): Inspired oxygen fraction. Use 1.0 for the 100%-O₂ shunt test; 0.21 for room-air estimate. - **Atmospheric Pressure (mmHg, optional)** (number): Atmospheric pressure (mmHg). Default 760 at sea level. - **Decimal Places** (number) ## When to use - Estimate pulmonary shunt fraction from arterial blood gas and oxygenation data. - Assess oxygenation during a room-air estimate or a 100%-oxygen shunt test. - Review how hemoglobin, arterial saturation, and mixed-venous values affect oxygen content and Qs/Qt. ## How it works - Enter hemoglobin, PaO₂, SaO₂, PaCO₂, and FiO₂; FiO₂ defaults to 1.0. - The calculator derives alveolar oxygen pressure using PAO₂ = FiO₂ × (Patm − 47) − PaCO₂/0.8, with atmospheric pressure defaulting to 760 mmHg. - It calculates capillary, arterial, and venous oxygen content using the provided values. - It applies Qs/Qt = (CcO₂ − CaO₂)/(CcO₂ − CvO₂) and returns the fraction, percentage, category, and whether mixed-venous defaults were used. ## Use cases - Critical care review of hypoxemia and suspected pulmonary shunt. - Respiratory physiology teaching using arterial and mixed-venous oxygen content. - Comparison of room-air estimates with values from a 100%-oxygen shunt test. ## Frequently asked questions ### What inputs are required? Hemoglobin, PaO₂, SaO₂, PaCO₂, and FiO₂ are required. Patm, PvO₂, and SvO₂ are optional. ### What happens if PvO₂ or SvO₂ is missing? The calculator uses PvO₂ of 40 mmHg and SvO₂ of 0.75, and identifies the venous source as default values. ### What FiO₂ should I enter for a 100%-oxygen shunt test? Enter 1.0. For a room-air estimate, enter 0.21. ### What does a Qs/Qt above 20% indicate? It is categorized as a severe shunt in this calculator. Interpret the result with the clinical context. ### Is this calculator medical advice? No. It provides an estimate for education and clinical reference and does not replace professional assessment. ## Related tools - [A-a Gradient (Alveolar-Arterial O₂)](https://elysiatools.com/en/tools/a-a-gradient-calculator): Calculate the alveolar-arterial O₂ gradient to distinguish hypoxemia causes. PAO₂ = FiO₂×(P_atm−47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂. Normal ≈ (Age+10)/4. Elevated → V/Q mismatch, shunt, diffusion impairment. Derived from StatPearls, LITFL, Cornell PICU, and Martin 1999. Not medical advice. - [Alveolar Gas Equation (PAO₂ Calculator)](https://elysiatools.com/en/tools/alveolar-gas-equation): Calculate PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. Defaults: P_atm 760 mmHg (sea level), RQ 0.8. Gives a directional hint vs expected ~100 mmHg. For full A-a gradient interpretation use a-a-gradient-calculator. Derived from StatPearls, LITFL, Cornell PICU, and West. Not medical advice. - [Alveolar Ventilation Calculator (VA)](https://elysiatools.com/en/tools/alveolar-ventilation-calculator): Calculate VA = (VT − VD) × RR in L/min. VD may be supplied or estimated as ~2 mL/kg body weight. Normal resting ~4–6 L/min; low → hypercapnia, high → hypocapnia. Derived from West, LITFL, StatPearls, and Radford 1955. Not medical advice. - [Apgar Score (Newborn 1 & 5 min)](https://elysiatools.com/en/tools/apgar-score): Calculate the Apgar score to assess newborn condition at 1 minute and 5 minutes after birth. Five signs are each scored 0–2 at both time points: Appearance (skin color), Pulse (heart rate), Grimace (reflex irritability), Activity (muscle tone), and Respiration. Each time point totals 0–10. Interpretation: 7–10 reassuring; 4–6 moderately abnormal (may need assistance); 0–3 critically low (immediate resuscitation). The tool reports both the 1-min and 5-min scores, the change (delta), and per-time-point interpretation. A 5-min score, and especially the change from 1 to 5 min, indicates response to resuscitation; persistent low 5-min scores warrant continued resuscitation and reassessment at 10 min. Derived from Apgar 1953, reaffirmed by ACOG/AAP. The Apgar is a physiologic snapshot at a moment in time and is not, by itself, a marker of asphyxia or long-term neurologic outcome. 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