Dead Space / VD-VT Calculator
Calculate physiologic dead space fraction using the Bohr equation. Dead space helps explain why a patient can have a normal or high minute ventilation but still retain CO₂.
Calculator
Calculate VD/VT
Formula: VD/VT = (PaCO₂ − PeCO₂) ÷ PaCO₂. Enter PaCO₂ and mixed expired CO₂ in mmHg.
VD/VT
—
dead space fraction
Enter PaCO₂ and PeCO₂ to calculate VD/VT.
Formula
Dead Space Fraction Formula
VD/VT = (PaCO₂ − PeCO₂) ÷ PaCO₂
Example: PaCO₂ 40 and PeCO₂ 28
VD/VT = (40 − 28) ÷ 40
VD/VT = 12 ÷ 40 = 0.30 or 30%
Example: PaCO₂ 40 and PeCO₂ 28
VD/VT = (40 − 28) ÷ 40
VD/VT = 12 ÷ 40 = 0.30 or 30%
RT Memory Trick
Dead Space Dilutes Exhaled CO₂
Alveoli that are ventilated but not perfused do not add much CO₂ to exhaled gas. As dead space increases, mixed expired CO₂ falls compared with PaCO₂.
Remember:
The bigger the gap between PaCO₂ and PeCO₂, the larger the dead space fraction.
The bigger the gap between PaCO₂ and PeCO₂, the larger the dead space fraction.
Interpretation
How to Interpret VD/VT
| VD/VT | Interpretation | Clinical Connection |
|---|---|---|
| About 0.20–0.35 | Common adult range | Some dead space is normal due to conducting airways. |
| 0.36–0.50 | Elevated dead space | May indicate V/Q mismatch or increased wasted ventilation. |
| > 0.50 | Markedly elevated dead space | May be seen with severe lung disease, low pulmonary perfusion, or significant dead space ventilation. |
Clinical Pearl
High Dead Space Wastes Ventilation
Dead space ventilation does not participate in effective gas exchange. When dead space is high, the patient may need a higher minute ventilation to maintain the same PaCO₂.
High V/Q units
Ventilation is present but perfusion is low.
Ventilation is present but perfusion is low.
Examples
Pulmonary embolism, emphysema, low cardiac output, overdistension.
Pulmonary embolism, emphysema, low cardiac output, overdistension.
ABG connection
High dead space can contribute to CO₂ retention if ventilation cannot compensate.
High dead space can contribute to CO₂ retention if ventilation cannot compensate.
Clinical connection
Interpret with PaCO₂, ETCO₂, minute ventilation, disease process, and perfusion.
Interpret with PaCO₂, ETCO₂, minute ventilation, disease process, and perfusion.
Common Student Mistakes
Avoid These Errors
Confusing dead space with shunt
Dead space is ventilated but poorly perfused. Shunt is perfused but poorly ventilated.
Dead space is ventilated but poorly perfused. Shunt is perfused but poorly ventilated.
Using ETCO₂ as if it is always PeCO₂
ETCO₂ may approximate alveolar CO₂ but is not identical to mixed expired CO₂.
ETCO₂ may approximate alveolar CO₂ but is not identical to mixed expired CO₂.
Ignoring perfusion
Dead space can increase when pulmonary blood flow is reduced.
Dead space can increase when pulmonary blood flow is reduced.
Assuming high VE means good ventilation
If dead space is high, much of the VE may be wasted.
If dead space is high, much of the VE may be wasted.
Continue Learning
Connect Dead Space to Alveolar Ventilation
Dead space explains why total minute ventilation and effective alveolar ventilation are not the same thing.