Dead Space Fraction (VD/VT)
Calculate physiologic dead space using the Bohr equation with arterial and mixed expired CO₂ to evaluate ventilation-perfusion efficiency.
From volumetric capnography or Douglas bag measurement.
Used for contextual info (conversion to °C).
Results
Dead space fraction
30.0%
Dead space volume
150 mL
Mildly increased dead space—consider V/Q mismatch or early pulmonary pathology.
Temperature
37.0 °C
Clinical notes:
- Elevated dead space fraction indicates wasted ventilation relative to perfusion.
- Consider causes such as pulmonary embolism, emphysema, or excessive PEEP.
- For ventilated patients, trending VD/VT helps assess response to therapy.
How to Use This Calculator
Obtain arterial and expired CO₂
Use arterial blood gas for PaCO₂ and volumetric capnography for mixed expired CO₂.
Enter tidal volume
Use measured exhaled tidal volume (ventilator or spirometer reading).
Interpret in context
Combine VD/VT with clinical assessment, imaging, and hemodynamics to guide therapy.
Formula
Bohr equation: VD/VT = (PaCO₂ − PECO₂) ÷ PaCO₂.
From Bohr’s physiologic dead space equation. Ensure PECO₂ reflects mixed expired gas.
Frequently Asked Questions
How do I obtain PECO₂?
Measured via volumetric capnography (end-tidal CO₂ integrated over exhalation) or Douglas bag collection.
Is end-tidal CO₂ (PETCO₂) the same?
No. PETCO₂ approximates alveolar CO₂ only; use mixed expired values for accurate Bohr calculations.
What is a normal VD/VT?
Roughly 0.2–0.3 in healthy individuals. Higher values indicate increased dead space or V/Q mismatch.
Can I use this in ARDS management?
Yes—dead space fraction helps gauge severity and predict outcomes in ARDS, guiding ventilator strategy.
Does body temperature affect calculation?
Temperature doesn’t change the equation but can influence gas solubility; included for clinical documentation.