Pulmonology
Physiological Dead Space Fraction (VD/VT - Bohr-Enghoff Equation)
Calculates physiological dead space ratio (VD/VT) and PaCO2-PetCO2 gradient in mechanical ventilation and ARDS.
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Parameters
Physiological Dead Space Fraction (VD/VT - Bohr-Enghoff Equation) Result
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Formula
Bohr-Enghoff Equation:
VD/VT = (PaCO₂ - PĒCO₂) ÷ PaCO₂
Where PaCO₂ is arterial CO₂ from ABG and PĒCO₂ is mixed expired CO₂.Clinical Pearls
The gold standard quantitative physiological index for unperfused ventilated lung zones (V/Q = infinity), correlating directly with ARDS mortality.
Limitations
Requires true mixed expired CO2 collection or dedicated volumetric capnography sensors; simple single-point end-tidal PetCO2 is an approximation.