Dead Space Fraction (VD/VT) Calculator
The fraction of each breath that does not take part in gas exchange — by the Bohr-Enghoff equation, with what drives it up and why an elevated value matters in ARDS.
Written by Apex Respiratory Editorial Team
From volumetric capnography or a metabolic cart — not end-tidal CO₂.
Enter arterial PaCO₂ and mixed expired CO₂ (P̄ECO₂) to calculate the dead space fraction.
Reading dead space
Dead space is ventilation without perfusion. The Bohr-Enghoff equation uses arterial CO₂ as the ideal alveolar value, so VD/VT = (PaCO₂ − P̄ECO₂) ÷ PaCO₂; P̄ECO₂ is the mixed expired CO₂ from volumetric capnography or a metabolic cart, not end-tidal CO₂.
Normal is ~0.20–0.40 breathing spontaneously and up to ~0.40–0.50 on the ventilator (apparatus + physiologic dead space).
A markedly elevated VD/VT (> 0.60) reflects massive wasted ventilation — pulmonary embolism, severe ARDS, or a low cardiac output — and an elevated dead-space fraction in ARDS is independently associated with mortality.
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- Nuckton TJ, Alonso JA, Kallet RH, et al. Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome. N Engl J Med. 2002;346(17):1281-1286.