PhysiologyTier 1Medical Sciences concept

Interpreting DLCO and DLCO/VA in restrictive lung disease patterns

Core concept

DLCO measures the whole gas-transfer pathway - and it is the product of two things
- DLCO = KCO x VA
  - VA (alveolar volume) = accessible gas volume the test gas reaches (normally within ~10% of TLC)
  - KCO (= DLCO/VA) = rate constant of CO uptake per litre of accessible lung
CO is diffusion-limited, not perfusion-limited - it binds Hb so avidly that capillary partial pressure stays near zero, so uptake depends only on the membrane and the blood volume available
- O2 is perfusion-limited at rest and only becomes diffusion-limited in disease or at altitude - the classic paired fact
KCO reflects membrane integrity + pulmonary capillary blood volume; VA reflects how much lung the gas reached
- -> the KCO/VA pattern tells you WHY the DLCO is low

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