Normally about 30% of each breath stays in the airways. Dead space rises in pulmonary embolism, emphysema, ARDS and with high ventilator pressures; a high dead space fraction in ARDS predicts higher mortality.
How to use the Dead Space Calculator
- PaCO₂
- Mixed expired PCO₂
- Tidal volume
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter arterial PaCO₂, mixed expired PCO₂ (from volumetric capnography or a collected sample) and tidal volume.
Inputs explained
| Field | What to enter |
|---|---|
| PaCO₂ | In mmHg |
| Mixed expired PCO₂ | In mmHg |
| Tidal volume | Units: mL, L, fl oz |
What each result means
- Dead space fraction (VD/VT) = ([PaCO₂] – [Mixed expired PCO₂])÷[PaCO₂]×100
- Dead space volume = [Dead space fraction (VD÷VT)]÷100×[Tidal volume]
- Alveolar volume per breath = [Tidal volume] – [Dead space volume]
Example values
How the main result changes when PaCO₂ changes and everything else stays at the example values above:
| PaCO₂ (mmHg) | Dead space fraction (VD/VT) (%) |
|---|---|
| 22.5 | -24 |
| 33.75 | 17 |
| 45 | 38 |
| 56.25 | 50 |
| 67.5 | 59 |
| 90 | 69 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
VD/VT = (PaCO₂ − PECO₂) ÷ PaCO₂
Dead space volume = VD/VT × tidal volume
Worked example
Inputs — PaCO₂: 45 mmHg; Mixed expired PCO₂: 28 mmHg; Tidal volume: 450 mL.
Results — Dead space fraction (VD/VT) = 38 %; Dead space volume = 170 mL; Alveolar volume per breath = 280 mL.
Key terms
- VD/VT
- Dead space to tidal volume ratio.
Frequently asked questions
What is a normal dead space fraction?
About 0.2–0.35 in healthy adults.
What raises dead space?
Pulmonary embolism, emphysema, ARDS, low cardiac output and high PEEP.
What is the difference between Bohr and Enghoff?
Bohr uses alveolar PCO₂; Enghoff substitutes arterial PaCO₂.