COPD: CO DIFFUSION
COPD: CO DIFFUSION — why is DLCO reduced in emphysema?
DLCO measures the lung's ability to transfer carbon monoxide (CO) from alveolar gas into pulmonary capillary blood.
CO is used because it binds avidly to hemoglobin, keeping its pulmonary capillary partial pressure extremely low—making its uptake a useful measure of gas-transfer capacity.
What determines diffusion?Think:
Large surface area + thin membrane + adequate pulmonary capillary blood/Hb → good DLCO
In emphysema:
Alveolar wall destruction → ↓ alveolar-capillary surface area → ↓ pulmonary capillary bed → ↓ CO transfer
Therefore: DLCO ↓
COPD phenotype mattersA low DLCO is not simply caused by airflow obstruction.
Emphysema → DLCO typically ↓
Chronic bronchitis without significant emphysema → DLCO may be relatively preserved
This makes DLCO useful when interpreting obstructive spirometry.
Important confoundersDLCO can also decrease with:
Anemia → pulmonary vascular disease → interstitial lung disease → loss of functioning lung tissue
Conversely, polycythemia or alveolar hemorrhage can increase measured DLCO.
Always interpret DLCO with hemoglobin and clinical context.
Postgraduate PearlA patient has:
FEV₁/FVC ↓ + DLCO markedly ↓
Think strongly about:
EMPHYSEMA / LOSS OF GAS-EXCHANGE SURFACE AREA
Whereas:
FEV₁/FVC ↓ + DLCO preserved
may fit airway-predominant disease, such as chronic bronchitis or asthma, depending on the clinical picture.
Take-home messageEMPHYSEMA → alveolar destruction → ↓ surface area + ↓ capillary bed → ↓ DLCO
Obstruction tells you about airflow; DLCO tells you about gas transfer.
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