Gas Exchange, Diffusion and V/Q Matching
Key equations, concepts and clinical rules for alveolar gas exchange, diffusion limitation, dead space and shunt.
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Questions Covered in This Set
10 cards to master
Alveolar gas equation
P_AO₂ = FiO₂(P_atm − 47) − PaCO₂/R; with room air and PaCO₂ 40, P_AO₂ ≈ 150 − 40/0.8 = 100 mmHg.
Why is inspired PO₂ ~150 mmHg rather than 159?
Inspired air is humidified at 37 °C; water vapor exerts 47 mmHg, so PiO₂ = 0.21 × (760 − 47) ≈ 150 mmHg.
Normal A–a gradient and its rule of thumb
P_AO₂ − PaO₂ ≈ 5–15 mmHg in a young adult; estimate = age/4 + 4.
Hypoxemia with a NORMAL A–a gradient — causes?
Hypoventilation (high PaCO₂) or low inspired PO₂ (high altitude). The lung parenchyma itself is fine.
Hypoxemia with a WIDENED A–a gradient — causes?
A problem inside the lung: diffusion limitation, shunt, or V/Q mismatch.
Fick's law of diffusion
V̇gas = A × D × (P1 − P2)/T, where D ∝ solubility/√MW; area ~70 m², thickness ~0.3 µm.
Why is CO₂ retention never a diffusion problem?
CO₂ is ~20× more diffusible than O₂ due to high solubility, so hypercapnia reflects inadequate ventilation, not the membrane.
Perfusion-limited vs diffusion-limited gases
Perfusion-limited: O₂ at rest, N₂O — equilibrates fast, transfer depends on blood flow. Diffusion-limited: CO always (basis of DLCO), and O₂ in fibrosis + exercise.
Dead space (V/Q = ∞)
Ventilation without perfusion; gas approaches inspired air (PO₂ 150, PCO₂ 0). Anatomic ~150 mL; alveolar from PE, low cardiac output, high PEEP.
Shunt (V/Q = 0) and Bohr equation
Shunt = perfusion without ventilation; blood keeps mixed venous values (PO₂ 40, PCO₂ 46) — pneumonia, edema, atelectasis, R→L shunt. Bohr: V_D/V_T = (PaCO₂ − P_ECO₂)/PaCO₂.
