Respiratory

Dead Space Fraction (Bohr) Calculator

Share of each breath that is wasted ventilation, from PaCO2 and end-tidal CO2; rises with PE and overdistension.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

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Formula

((PaCO2 - PetCO2) / PaCO2) * 100

Reference: PaCO₂–EtCO₂ gap normally 2–5 mmHg

How it is used

• This is the alveolar dead-space fraction, (PaCO₂ − EtCO₂) ÷ PaCO₂, a bedside stand-in for the Enghoff VD/VT, which uses mixed expired CO₂.
• The PaCO₂–EtCO₂ gap is normally 2–5 mm Hg and widens with dead space: pulmonary embolism, low cardiac output, overdistension or auto-PEEP.
• In ARDS a higher dead-space fraction carried a higher risk of death: about 45% higher odds for each 0.05 rise in VD/VT (Nuckton 2002).

Same formula as the app. Open it in the web app.

Related one-pagers

• ABG Interpretation in 6 StepsAbove the knee, large PaO₂ changes barely move saturation; below it, saturation falls steeply. A right shift lowers affinity and unloads O₂ to tissue: acidosis (the Bohr effect), hypercapnia, fever, raised 2,3-DPG (chronic hypoxemia, anemia, altitude). A left shift holds O₂: alkalosis, hypothermia, low 2,3-DPG (banked blood), CO, methemoglobin, fetal hemoglobin.
• 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.
• Ventilator Modes and Settings

Related calculators

• A-a Gradient
• Alveolar Gas Equation (PAO₂)
• Arterial Oxygen Content (CaO₂)
• Driving Pressure
• Dynamic Compliance
• Minute Ventilation
• O₂ Extraction Ratio
• Oxygen Consumption (VO₂)

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.