Dead Space Calculator
Calculate physiological dead space volume using the Bohr equation from alveolar CO2, expired CO2, and tidal volume.
What Is Physiological Dead Space?
Physiological dead space is the portion of each tidal breath that does not participate in gas exchange. It includes anatomical dead space in conducting airways plus alveolar dead space where ventilation exceeds perfusion. The Bohr equation estimates dead space from arterial and expired CO₂ partial pressures and tidal volume.
The Bohr Equation
Dead space volume equals tidal volume multiplied by the fraction (PaCO₂ minus PeCO₂) divided by PaCO₂. The same ratio gives the dead space fraction of tidal volume (VD/VT). Normal physiological dead space is roughly 20 to 30 percent of tidal volume in healthy adults at rest.
Clinical Context
Elevated dead space may appear in pulmonary embolism, severe COPD, or other ventilation-perfusion mismatch. This calculator supports educational and research use; clinical decisions require full arterial blood gas interpretation and patient context.
Frequently Asked Questions
What inputs does the Bohr equation need?
Alveolar CO₂ (PaCO₂), mixed expired CO₂ (PeCO₂), and tidal volume. CO₂ values are in mmHg; tidal volume can be entered in liters or milliliters.
Why must expired CO₂ be lower than alveolar CO₂?
Dead space air dilutes expired gas with lower CO₂. If PeCO₂ equals or exceeds PaCO₂, the Bohr fraction is zero or negative, which is not physiologic for this calculation.
What is a normal dead space fraction?
At rest, VD/VT is typically about 0.2 to 0.3, meaning 20 to 30 percent of each breath does not exchange gas effectively.
How is dead space volume reported?
This tool shows dead space in milliliters and liters, plus the percentage of tidal volume represented by dead space.
Can I use end-tidal CO₂ instead of PeCO₂?
End-tidal CO₂ approximates alveolar CO₂, not mixed expired CO₂. For accurate Bohr dead space, use measured mixed expired CO₂ from a metabolic cart when available.
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