# Dead Space Fraction (VD/VT, Bohr & Enghoff) Calculate VD/VT using both the Bohr equation (PACO₂-based) and the Enghoff modification (PaCO₂-based). Normal ~0.20–0.40; >0.60 signals severe dead space in ARDS/PE. Derived from Bohr 1891, Enghoff 1938, Tusman 2012, LITFL, StatPearls. Not medical advice. > Canonical page: https://elysiatools.com/en/tools/dead-space-fraction - **Category:** Health - **Keywords:** dead space, vd vt, bohr equation, enghoff, physiological dead space, anatomical dead space, capnography, mixed expired co2, ards, pulmonary embolism, critical care, anesthesiology ## Overview Calculate the dead space fraction (VD/VT) with both the Bohr equation and the Enghoff modification. Enter mixed expired CO₂ and arterial PaCO₂, and optionally provide alveolar or end-tidal PACO₂ for the Bohr calculation. The result includes fractions, percentages, and a reference category; it is not medical advice. ## Inputs - **Mixed Expired CO₂ PĒCO₂ (mmHg)** (number): Mixed expired CO₂ (mmHg), collected over several breaths. - **Arterial PaCO₂ (mmHg)** (number): Arterial PaCO₂ (mmHg) from ABG — used by the Enghoff equation. - **Alveolar PACO₂ (mmHg, optional)** (number): Alveolar PACO₂ (mmHg) for the Bohr equation. Typically end-tidal CO₂ from capnography. If omitted, PaCO₂ is used as an approximation. - **Decimal Places** (number) ## When to use - Estimate physiological dead space from mixed expired CO₂ and arterial blood gas values. - Compare Bohr and Enghoff VD/VT when alveolar or end-tidal PACO₂ is available. - Review dead space values in critical care, anesthesiology, ARDS, or pulmonary embolism assessment. ## How it works - Enter mixed expired CO₂ (PĒCO₂) and arterial PaCO₂ in mmHg. - Optionally enter alveolar or end-tidal PACO₂ for the Bohr calculation; if omitted, PaCO₂ is used as an approximation. - The calculator applies Bohr: (PACO₂ − PĒCO₂) / PACO₂, and Enghoff: (PaCO₂ − PĒCO₂) / PaCO₂. - The JSON result reports both fractions and percentages, along with a reference category and the selected decimal precision. ## Use cases - Critical care teams can compare Enghoff and Bohr estimates when evaluating impaired gas exchange. - Anesthesiology workflows can use end-tidal PACO₂ with mixed expired CO₂ to estimate Bohr dead space. - Clinical education and research can demonstrate how different CO₂ measurements change VD/VT results. ## Frequently asked questions ### What does VD/VT measure? VD/VT is the fraction of each breath that does not participate effectively in CO₂ exchange, expressed as a ratio or percentage. ### What is the difference between Bohr and Enghoff? Bohr uses alveolar PACO₂, while Enghoff uses arterial PaCO₂ and may also reflect shunt and ventilation-perfusion mismatch. ### What happens if PACO₂ is not entered? The calculator uses arterial PaCO₂ as an approximation for PACO₂ in the Bohr calculation. ### What is a typical adult VD/VT range? A commonly referenced normal adult range is approximately 0.20–0.40, or 20–40%. ### What does a value above 0.60 indicate? A value above 0.60 is considered severely elevated and may be associated with severe dead space, including in ARDS or pulmonary embolism. Interpret it clinically. ## 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. 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