Carbon monoxide transfer coefficient

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Carbon monoxide transfer coefficient (often abbreviated as KCO) is a parameter often performed as part of pulmonary function tests. It is also often written as DLCO/VA (diffusing capacity per liter of lung volume) and is an index of the efficiency of alveolar transfer of carbon monoxide.

Interpretation of KCO depends on other parameters such as

  • VA: accessible alveolar volume
  • TLC: total lung capacity

Interpretation

In the context of normal VA, a low KCO (provided there is no anemia or recent smoking) could suggest:

In the context of a low VA, the next step is to look at the VA/TLC ratio.

  • Low VA/TLC ratio (< 0.8)

    • a normal KCO (not able to be interpreted): this could imply obstruction with ventilation distribution abnormalities, the KCO might turn “normal”. In this scenario, no further valid inferences can be made regarding KCO

    • however, if KCO is low despite those caveats this could imply extensive impairment in pulmonary gas exchange efficiency, e.g. severe emphysema

  • Normal VA/TLC ratio (≥ 0.8)

    • a high KCO indicates a predominance of VC over VA due to

      • incomplete alveolar expansion but preserved gas exchange i.e. extra-parenchymal restriction such as pleural, chest wall or neuromuscular disease)

      • an increase in pulmonary blood flow from areas of diffuse (pneumonectomy) or localized (local destructive lesions/atelectasis) loss of gas exchange units to areas with preserved parenchyma; this frequently leads to more modest increases in KCO (although a high KCO can also be seen with normal VA when there is "increased pulmonary blood flow" or redistribution (e.g. left-to-right shunt and asthma)

      • extra-vascular hemoglobin (e.g. alveolar hemorrhage)

    • a low KCO: could suggest intra-parenchymal restriction with impaired gas exchange efficiency as in some interstitial lung diseases (ILD)

    • a normal KCO: could suggest intra-parenchymal restriction with preserved KCO (can be a common finding in patients with HRCT abnormalities showing a pattern consistent with idiopathic interstitial pneumonia); normal KCO, therefore, should not be misinterpreted as “no ILD”

See also