Calculate physiologic dead space using the Bohr equation. Includes VD/VT ratio, alveolar ventilation, and dead space composition breakdown.
Physiologic dead space represents the portion of each tidal breath that does not participate in gas exchange. It comprises two components: **anatomic dead space** (the volume of the conducting airways from the nose/mouth to the terminal bronchioles, approximately 150 mL or 2 mL/kg in adults) and **alveolar dead space** (alveoli that are ventilated but not adequately perfused, which should be negligible in healthy lungs).
The **Bohr equation** quantifies the dead space fraction (VD/VT) by comparing arterial CO₂ (PaCO₂) with mixed expired CO₂ (PĒCO₂): VD/VT = (PaCO₂ − PĒCO₂) / PaCO₂. A normal VD/VT ratio is 0.20–0.35, meaning 20–35% of each breath is wasted in dead space. Values above 0.50 are considered pathologic and indicate significant ventilation-perfusion mismatch.
Elevated dead space is seen in pulmonary embolism (where blood flow to ventilated regions is obstructed), COPD (emphysematous destruction and bullae), ARDS (microvascular thrombosis), and other conditions causing V/Q mismatch. In mechanically ventilated patients, following the dead-space fraction over time can add physiologic context to blood-gas and capnography data, but the result still has to be interpreted alongside the rest of the clinical picture.
Dead space assessment helps turn routine blood gas and ventilator data into a clearer picture of ventilation-perfusion mismatch. This calculator keeps PaCO₂, mixed expired CO₂, tidal volume, and respiratory rate together so the dead-space fraction and alveolar ventilation can be reviewed as one physiology problem.
Bohr Equation: VD/VT = (PaCO₂ − PĒCO₂) / PaCO₂. Physiologic Dead Space: VD = VD/VT × VT. Alveolar Dead Space = VD(physiologic) − VD(anatomic). Alveolar Ventilation: VA = (VT − VD) × RR. Minute Ventilation: VE = VT × RR.
Result: VD/VT = 0.30
With PaCO₂ 40 and PĒCO₂ 28: VD/VT = (40−28)/40 = 0.30, indicating 30% dead space ventilation (normal). Physiologic dead space = 150 mL, alveolar ventilation = 4.9 L/min.
Dead space increases when ventilation is preserved but perfusion falls, or when parts of the lung are ventilated inefficiently. That is why pulmonary embolism, emphysema, ARDS, and excessive ventilator pressures can all produce a similar pattern even though the underlying cause differs.
A mild rise in VD/VT may simply reflect age, while larger increases usually point to clinically important V/Q mismatch. In ventilated patients, the trend often matters as much as the absolute value because worsening dead space can track perfusion or overdistension problems before oxygenation changes become obvious.
The Bohr calculation is simple, but it is easy to lose track of which values came from the blood gas, which came from capnography, and which came from ventilator settings. Keeping them together makes the result easier to review at the bedside and easier to compare across repeated measurements.
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This worksheet applies the Bohr/Enghoff dead-space fraction using PaCO₂ and mixed expired CO₂, then multiplies the ratio by tidal volume to estimate physiologic dead space and subtracts anatomic dead space for the displayed alveolar component. It is a planning and interpretation aid, not a diagnosis by itself, because the exact result depends on the measurement method, ventilator setup, and the patient's underlying physiology.
Normal VD/VT is 0.20–0.35 (20–35% of each breath is dead space). Values > 0.50 are pathologic and suggest significant V/Q mismatch.
Pulmonary embolism, COPD/emphysema, ARDS, high PEEP ventilation, hypovolemia, and any condition reducing pulmonary perfusion to ventilated regions. The shared feature is underperfused ventilation, which raises the dead-space fraction.
Mixed expired CO₂ is collected from a mixing chamber attached to the expiratory limb of the ventilator circuit, or estimated from volumetric capnography. It represents the average CO₂ content of the entire exhaled breath.
A rising VD/VT ratio can accompany worsening V/Q mismatch in ARDS. Serial measurements can add context when reviewed alongside the rest of the ventilator and blood-gas data.
Excessive PEEP can overdistend alveoli and compress adjacent capillaries, increasing alveolar dead space. Optimal PEEP minimizes both atelectasis and overdistension.
Anatomic dead space is the fixed volume of the conducting airways (~150 mL). Alveolar dead space represents ventilated but unperfused alveoli and is near zero in healthy lungs.