What is diffusing capacity and why is it important?
Lung carbon monoxide diffusing capacity (DLCO) is a good indicator of the physiological health of the lungs—a value below the predicted range becomes a clue to the presence of a physiological problem.
However, there are a number of normal physiological factors that can affect the measurement obtained.
Changes to DLCO? Check these physiological factors first
There are six normal physiological factors that can affect DLCO measurements:
When changes to DLCO indicate pathology
DLCO declines in chronic obstructive pulmonary disease (COPD), with greater changes in emphysema than chronic bronchitis because of greater loss of alveolar surface area in emphysema.
Restrictive disorders that include pulmonary parenchymal disease (e.g., pulmonary fibrosis) are also accompanied by declines in DLCO.
Acute alveolar hemorrhage is associated with an increase in DLCO. This is secondary to hemoglobin in the alveoli serving as a sink for carbon monoxide used during measurement. When alveolar hemorrhage is chronic or recurrent, the resulting lung fibrosis reduces DLCO.
Through an increase in capillary red blood cells, polycythemia results in an increase in DLCO. Anemia, with reduced red cell mass, reduces DLCO.
Changes to carbon dioxide diffusion
Mild abnormalities of the pulmonary parenchyma are usually not associated with an increased arterial-alveolar gradient for carbon dioxide. With more advanced thickening of the alveolar-capillary membrane and a reduction in DLCO to about 25% of normal, an arterial-alveolar gradient for carbon dioxide may arise.
Become a great clinician with our video courses and workshops
- Grippi, MA. 1995. “Gas exchange in the lung”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 137–149)
- Grippi, MA. 1995. “Clinical presentations: gas exchange and transport”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 171–176)
- Grippi, MA and Tino, G. 2015. “Pulmonary function testing”. In: Fishman's Pulmonary Diseases and Disorders, edited by MA, Grippi (editor-in-chief), JA, Elias, JA, Fishman, RM, Kotloff, AI, Pack, RM, Senior (editors). 5th edition. New York: McGraw-Hill Education. (Grippi and Tino 2015, 502–536)
- Tino, G and Grippi, MA. 1995. “Gas transport to and from peripheral tissues”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Tino and Grippi 1995, 151–170)
- Wagner, PD. 2015. The physiologic basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases. Eur Respir J. 45: 227–243. PMID: 25323225