2003B12 Explain the difference between perfusion limitation and diffusion limitation
in the transfer of gas between alveolus and pulmonary capillary.
Outline the factors that determine whether gas transfer is perfusion or diffusion limited.

Fick’s law

Diffusion capacity

·   Inverses are additive, since in parallel

·   L: lung

·   M: membrane

·   Θ: rate of reaction of O₂ with haemoglobin

·   V_c: volume in pulmonary capillaries, normally ~70mL

High rate of transfer

·   High gradient (e.g. 70% N₂O = 532mmHg gradient)

·   Normal area 80m²

·   Normal thickness 0.5μm

·   Normal V/Q matching

·   High lipid membrane solubility (e.g. CO₂:O₂ is 30:1)

·   Low MW (e.g. O₂ 16g/mol < CO₂ 44g/mol)

Low capacity for uptake

·   Low blood solubility (e.g. N₂O blood-gas partition coefficient 0.47)

·   Low protein binding

·   Low pulmonary capillary blood volume

Slow transit

·   At rest: 0.75 seconds (slower if heart failure)

Examples

·   N₂O

·   CO₂

·   O₂: If 1) resting 2) sea level 3) healthy

Low rate of transfer

·   Low gradient (e.g. 5000m altitude: pO₂ only 80mmHg)

·   Low area

o Tissue loss: (e.g. lobectomy, pneumonectomy, smoking -> destruction)

o Shunt (e.g. atelectasis)

o Dead space (e.g. pulmonary embolus)

·   High thickness (e.g. interstitial oedema, fibrosis)

·   Low lipid membrane solubility

·   High MW

High capacity for uptake

·   High blood solubility (e.g. isoflurane blood-gas partition coefficient 1.2)

·   High protein binding (e.g. CO 250x higher affinity for Hb than O₂)

·   High pulmonary capillary blood volume (e.g. LV failure)

Fast transit

Peak exercise: 0.25 seconds

Examples

·   CO:

o Solely diffusion-limited

o High affinity for Hb ensures P₂ near zero

o Allows simpler calculation of diffusing capacity

·   O₂: if

o Exercise

o Altitude

o Lung disease (especially if multiple)