This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
What dead space is
Not all the air we breathe takes part in gas exchange. The air that fills the nose, throat, trachea and bronchi never reaches the alveoli: this is the anatomical dead space, about 2 mL per kilo of ideal body weight, or roughly 150 mL in an adult. Alveoli that are ventilated but poorly perfused add alveolar dead space. Together they form the physiological dead space.
The Bohr-Enghoff equation
Christian Bohr described the dead space in 1891, and Henrik Enghoff replaced alveolar CO₂ with arterial CO₂ in 1938, which gives the version used in intensive care:
Vd/Vt = (PaCO₂ − PĒCO₂) ÷ PaCO₂
PaCO₂ comes from an arterial blood gas and PĒCO₂, the mixed expired CO₂, from collecting expired gas or from volumetric capnography. The calculator then gives:
- Dead space volume = Vd/Vt × tidal volume
- Minute ventilation = tidal volume × breathing rate
- Alveolar ventilation = (tidal volume − dead space) × breathing rate
Interpretation
| Vd/Vt | Meaning |
|---|---|
| about 0.2 to 0.3 | normal at rest |
| 0.35 to 0.6 | increased, as in COPD, pulmonary embolism, low cardiac output or ARDS |
| over 0.6 | severely increased |
In acute respiratory distress syndrome, a study by Nuckton and colleagues in 2002 found that each 0.05 rise in dead space fraction increased the odds of death, and values above about 0.57 were linked with high mortality.
Example
With a PaCO₂ of 40 mmHg, a mixed expired CO₂ of 28 mmHg, a tidal volume of 500 mL and 14 breaths a minute, Vd/Vt is 0.30, the dead space is 150 mL, minute ventilation 7 L/min and alveolar ventilation 4.9 L/min.
Practical points
- A large dead space means more ventilation is needed to clear the same CO₂, which explains rising PaCO₂ despite a high minute ventilation.
- The ventilator circuit adds instrument dead space between the Y-piece and the patient, such as filters and connectors.
- Without mixed expired CO₂, the end-tidal CO₂ gives a rough proxy, but it underestimates dead space.