What this calculator does
Not all of a breath reaches tissue where gas is exchanged. Some fills the conducting airways and some reaches alveoli with no blood flow, and both are dead space. The Bohr equation measures the combined share.
Carbon dioxide is what reveals it. Blood arriving at the lung carries carbon dioxide, so any exhaled gas that came from a properly perfused alveolus contains it. The gap between arterial and mixed expired carbon dioxide is therefore a direct measure of dilution by dead space.
The formula
The fraction is arterial carbon dioxide less mixed expired carbon dioxide, divided by the arterial figure. Multiplying by tidal volume gives the dead space in millilitres.
| Term | Meaning |
|---|---|
| VD/VT | The dead space fraction: the share of each breath that is wasted. |
| Mixed expired CO2 | Average carbon dioxide across the whole exhaled breath, including the dead space portion. |
| Enghoff modification | Using arterial rather than alveolar carbon dioxide, which is what makes the equation measurable. |
The inputs explained
| Field | What to enter |
|---|---|
| Arterial PaCO₂ (mmHg) | Arterial PaCO2 in mmHg from a blood gas. |
| Mixed expired CO₂ (PECO2) (mmHg) | Mixed expired CO2 in mmHg, measured from collected exhaled gas. |
| Tidal volume (mL) | Tidal volume in mL, used to convert the fraction into a volume. |
When to use it
Assessing ventilation efficiency
A rising dead space fraction means more of each breath is wasted.
Monitoring in critical care
Dead space fraction has prognostic value in acute respiratory distress syndrome.
Understanding why breathing harder does not always help
If dead space is high, extra ventilation is partly wasted.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does expired CO2 reveal dead space?
The same arterial CO2 at three mixed expired levels.
| Mixed expired CO2 | Dead space fraction (VD/VT) | Physiologic dead space volume |
|---|---|---|
| 20 mmHg | 50.0% | 250 mL |
| 28 mmHg | 30.0% | 150 mL |
| 34 mmHg | 15.0% | 75 mL |
Questions
What is the difference between anatomical and physiologic dead space?
Anatomical dead space is the conducting airways, roughly 150 mL in an adult. Physiologic dead space adds alveoli that are ventilated but not perfused, so it is the larger figure and the one that changes with disease.
Why does the equation use carbon dioxide?
Because there is essentially none in inspired air, so all exhaled carbon dioxide must have come from perfused alveoli. Dead space gas dilutes it, and the size of that dilution is the measurement.
What raises dead space?
Anything that ventilates lung without perfusing it: pulmonary embolism, low cardiac output, high airway pressures and emphysema among others. A sudden rise is clinically significant.
How is mixed expired CO2 measured?
By collecting exhaled gas over several breaths and averaging, typically with volumetric capnography. It is not the same as end-tidal CO2, which samples only the last portion of a breath.
For total air moved per minute, see the minute ventilation calculator. For lung-protective settings, see the ARDSnet tidal volume calculator.