How is dead space fraction calculated and what is normal?

Vd/Vt = (PaCO2 − PECO2) / PaCO2 (Enghoff modification), where PECO2 is the mixed expired CO2. Normal Vd/Vt is 0.25–0.35 in healthy adults. In ARDS, pulmonary vascular obstruction, regional hypoperfusion, and alveolar overdistension increase dead space — values above 0.60 are common in severe ARDS and are an independent predictor of mortality (Nuckton et al., NEJM 2002).

Why does dead space increase in ARDS?

In ARDS, dead space increases through three mechanisms: anatomical dead space is relatively constant but increases as a fraction of the reduced functional residual capacity; alveolar dead space increases due to pulmonary microvascular obstruction (microthrombi, vasoconstriction) creating ventilated but underperfused lung units; and high-pressure ventilation overdistends alveoli, compressing adjacent capillaries and creating additional alveolar dead space.

Dead Space Fraction (Vd/Vt)

Calculate the physiological dead space fraction (Vd/Vt) using the Enghoff modification of the Bohr equation. Elevated dead space >0.60 in ARDS is associated with increased mortality.

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Frequently Asked Questions

How is dead space fraction calculated and what is normal?

Why does dead space increase in ARDS?