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 a hallmark of 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, monitoring dead space helps guide ventilator settings and assess disease severity. An increasing VD/VT ratio is an independent predictor of mortality in ARDS and can help identify patients who may benefit from more aggressive interventions.
Dead space assessment is essential for ventilator management, diagnosing V/Q mismatch, and prognosticating in ARDS. This calculator provides a complete dead space analysis from readily available clinical data. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation. Align this note with review checkpoints.
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.
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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. Use this as a practical reminder before finalizing the result.
Mixed expired CO₂ is collected from a mixing chamber attached to the expiratory limb of the ventilator circuit, or estimated from volumetric capnography. Keep this note short and outcome-focused for reuse.
Elevated dead space (VD/VT > 0.57) is an independent predictor of mortality in ARDS. Serial monitoring helps assess disease progression and response to therapy.
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.