U01.21.035 Physiologic Dead Space (VD)

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Understand and calculate physiologic dead space, which represents the portion of tidal volume that does not participate in gas exchange.

Formula:

VD = VT \times \frac{PaCO_2 - PECO_2}{PaCO_2}

Explanation:

  • VD = Physiologic dead space (mL)
  • VT = Tidal volume (mL)
  • PaCO_2 = Arterial CO₂ pressure (mm Hg)
  • PECO_2 = Mixed expired CO₂ pressure (mm Hg)

This is based on the Bohr equation, which quantifies the fraction of tidal volume that does not participate in alveolar gas exchange (anatomic + alveolar dead space).

Interpretation:

  • VD/VT ratio:
    • Normal ≈ 0.2–0.35
    • ↑ in pulmonary embolism, emphysema, or ventilation-perfusion mismatch

Example:

Given:
VT = 500 \text{ mL}
PaCO_2 = 40 \text{ mm Hg}
PECO_2 = 30 \text{ mm Hg}

VD = 500 \times \frac{40 - 30}{40} = 500 \times \frac{10}{40} = 500 \times 0.25 = 125 \text{ mL}

✅ Answer: Physiologic Dead Space = 125 mL

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