How this instrument works
Every breath moves air, but not all of it reaches alveoli that are actually exchanging gas with blood. Some air never gets past the conducting airways — the nose, trachea, and bronchi — before the next exhale; this is anatomic dead space. Some air does reach alveoli, but those alveoli aren't being perfused with blood at that moment, so no exchange happens there either; added to anatomic dead space, this combined total is called physiologic dead space. The Bohr equation estimates physiologic dead space as a fraction of each breath, Vd/Vt, by comparing the CO2 in arterial blood against the CO2 actually measured in a person's exhaled breath: since dead-space air carries no CO2 picked up from circulation, it dilutes the exhaled reading down from the arterial value, and the size of that dilution is the size of the wasted fraction.
A physiologic dead space fraction of roughly 0.2 to 0.4 — 20% to 40% of each breath — is generally considered a normal range in a resting adult, reflecting the ordinary anatomic dead space of the airways plus a small, usually unperfused margin of alveoli.
A higher fraction shows up in conditions where blood flow to ventilated lung regions is disrupted even though air keeps reaching them — pulmonary embolism is the textbook example, where a clot blocks perfusion to part of the lung while that part keeps filling with air on every breath, wasting an outsized share of each breath's ventilation. A lower fraction describes efficient gas exchange, with little ventilation going to waste.
Running the equation well depends on two numbers that aren't always easy to get at the bedside: PaCO2 from an arterial blood gas, and PeCO2 from mixed expired air, which needs a collection bag or a capnograph capable of averaging CO2 across a full breath rather than just reading its end-tidal value. The ratio itself is only as good as those two measurements, and it describes one snapshot in time rather than a fixed property of a person's lungs.
- Enter Arterial CO₂ (PaCO₂, mmHg) from a blood gas result.
- Enter Mixed expired CO₂ (PeCO₂, mmHg) from collected or capnograph-averaged exhaled air.
- Read Physiologic dead space fraction (Vd/Vt), computed automatically from the two values.
- Compare the result against the roughly 0.2-0.4 range typically considered normal at rest.
Worked example — PaCO₂ 40, PeCO₂ 28
An arterial CO2 of 40 mmHg against a mixed expired CO2 of 28 mmHg gives (40 − 28) ÷ 40 = 12 ÷ 40 = 0.3 — a dead space fraction of 30%, comfortably inside the roughly 0.2-0.4 range considered normal at rest.
Lower the mixed expired reading while the arterial value stays the same, and the estimated waste rises: PaCO2 40 against PeCO2 20 gives (40 − 20) ÷ 40 = 0.5, meaning half of each breath, by this estimate, isn't reaching perfused alveoli — a pattern seen in pulmonary embolism, where clots cut off blood flow to ventilated lung regions. Move the expired reading closer to the arterial value instead, and the fraction shrinks: PaCO2 40 against PeCO2 36 gives (40 − 36) ÷ 40 = 0.1, a low dead-space fraction describing efficient gas exchange with little ventilation wasted.
Questions
What does a Vd/Vt of 0.3 mean?
It means about 30% of each breath, by this estimate, isn't reaching alveoli that are both ventilated and perfused with blood — comfortably inside the roughly 0.2 to 0.4 range usually considered normal at rest. It's a single ratio at one point in time, drawn from an arterial CO2 and a mixed expired CO2 reading, not a permanent measure of lung function.
What counts as a normal physiologic dead space fraction?
Roughly 0.2 to 0.4 in a resting adult is the range most often cited as normal, reflecting ordinary anatomic dead space in the airways plus a small share of alveoli that aren't perfused at any given moment. Values climbing well above that range point toward wasted ventilation from conditions affecting blood flow to the lungs.
Why does pulmonary embolism raise the dead space fraction?
Because a clot blocks blood flow to part of the lung while that part keeps filling with air on every breath. Air reaches those alveoli, but with no blood arriving to pick up CO2 or drop off for oxygen, none of that ventilation counts toward actual gas exchange — raising the wasted share the Bohr equation reports as Vd/Vt.
What is the difference between anatomic and physiologic dead space?
Anatomic dead space is the air sitting in the conducting airways — nose, trachea, bronchi — that never reaches an alveolus before the next exhale. Physiologic dead space adds to that any alveoli that are ventilated but not perfused with blood at that moment. The Bohr equation, using arterial and exhaled CO2, estimates the physiologic total, the broader of the two.
Where do the PaCO2 and PeCO2 values come from?
PaCO2 comes from an arterial blood gas, a direct blood draw analyzed in a lab or point-of-care device. PeCO2, mixed expired CO2, needs exhaled air collected over one or more full breaths and averaged — either with a collection bag or a capnograph built to report that average rather than just the end-tidal reading at the end of exhalation.
Why is exhaled CO2 lower than arterial CO2?
Because exhaled air is a mixture of gas that came from perfused alveoli, carrying CO2 picked up from the blood, and dead-space air that never picked up any CO2 at all. Blending the two brings the measured exhaled value down below the arterial figure, and the size of that gap is exactly what the Bohr equation uses to estimate the wasted fraction.
References
- StatPearls — Physiology, Lung Dead Space (NCBI Bookshelf)
- StatPearls — Anatomy, Anatomic Dead Space (NCBI Bookshelf)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.