Examination of the carbon monoxide diffusing capacity (DL(CO)) in relation to its KCO and VA components

Am J Respir Crit Care Med. 2012 Jul 15;186(2):132-9. doi: 10.1164/rccm.201112-2160CI. Epub 2012 Apr 26.


The single-breath carbon monoxide diffusing capacity (DL(CO)) is the product of two measurements during breath holding at full inflation: (1) the rate constant for carbon monoxide uptake from alveolar gas (kco [minute(-1)]) and (2) the "accessible" alveolar volume (Va). kco expressed per mm Hg alveolar dry gas pressure (Pb*) as kco/Pb*, and then multiplied by Va, equals Dl(CO); thus, Dl(CO) divided by Va (DL(CO)/Va, also called Kco) is only kco/Pb* in different units, remaining, essentially, a rate constant. The notion that DL(CO)/Va "corrects" DL(CO) for reduced Va is physiologically incorrect, because DL(CO)/Va is not constant as Va changes; thus, the term Kco reflects the physiology more appropriately. Crucially, the same DL(CO) may occur with various combinations of Kco and Va, each suggesting different pathologies. Decreased Kco occurs in alveolar-capillary damage, microvascular pathology, or anemia. Increased Kco occurs with (1) failure to expand normal lungs to predicted full inflation (extrapulmonary restriction); or (2) increased capillary volume and flow, either globally (left-to-right intracardiac shunting) or from flow and volume diversion from lost or damaged units to surviving normal units (e.g., pneumonectomy). Decreased Va occurs in (1) reduced alveolar expansion, (2) alveolar damage or loss, or (3) maldistribution of inspired gases with airflow obstruction. Kco will be greater than 120% predicted in case 1, 100-120% in case 2, and 40-120% in case 3, depending on pathology. Kco and Va values should be available to clinicians, as fundamental to understanding the clinical implications of DL(CO). The diffusing capacity for nitric oxide (DL(NO)), and the DL(NO)/DL(CO) ratio, provide additional insights.

Publication types

  • Review

MeSH terms

  • Carbon Monoxide / pharmacokinetics*
  • Exercise / physiology
  • Humans
  • Lung / metabolism
  • Lung / physiology
  • Lung Diseases / metabolism
  • Lung Diseases / physiopathology
  • Lung Volume Measurements
  • Pulmonary Alveoli / metabolism
  • Pulmonary Alveoli / physiology
  • Pulmonary Diffusing Capacity / physiology*
  • Respiratory Function Tests / methods


  • Carbon Monoxide