How this calculator works
The Bohr relationship treats inspired CO2 as negligible and estimates the fraction of each breath that does not exchange CO2. It requires alveolar and mixed expired CO2. Substituting arterial CO2 produces the Enghoff modification, which also reflects gas-exchange inequality; that is a different interpretation. Dead space cannot exceed the tidal volume in this model. The result is an estimate for information only and is not medical advice.
Formula
VD/VT = (PACO2 - PECO2)/PACO2. VD = VT x (1 - PECO2/PACO2). VT = VD/(1 - PECO2/PACO2). PACO2 = PECO2/(1 - VD/VT). PECO2 = PACO2 x (1 - VD/VT).
Worked example
Alveolar CO2 40 mmHg and mixed expired CO2 28 mmHg give VD/VT = 12/40 = 30%. With tidal volume 500 mL, dead space is 150 mL.
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Frequently asked questions
Can I use end-tidal CO2 as mixed expired CO2?
No. Mixed expired CO2 is averaged over the entire exhaled breath. End-tidal CO2 samples the end of expiration.
What happens if both CO2 pressures are equal?
Calculated dead space is zero. The inverse tidal-volume problem is then not uniquely determined from zero dead space.
Does arterial CO2 give the same physiological measurement?
Substitution gives the Enghoff index, which can increase with shunt and ventilation-perfusion inequality as well as dead space.