Diffusing capacity

Diffusion capacity
Medical diagnostics
MeSH	D011653
Other codes	CPT: 94720
[]

Diffusing capacity of the lung (D_L) measures the transfer of gas from air in the lung, to the red blood cells in lung blood vessels. It is part of a comprehensive series of tests (pulmonary function testing) to determine the overall ability of the lung to transport gas into and out of the blood. D_L, especially D_LCO, is reduced in certain diseases of the lung and heart. D_LCO measurement has been standardized according to a position paper by a task force of the European Respiratory and American Thoracic Societies.

In respiratory physiology, the diffusing capacity has a long history of great utility, representing conductance of gas across the alveolar-capillary membrane and also takes into account factors affecting the behaviour of a given gas with hemoglobin.

The term may be considered a misnomer as it represents neither diffusion nor a capacity (as it is typically measured under submaximal conditions) nor capacitance. In addition, gas transport is only diffusion limited in extreme cases, such as for oxygen uptake at very low ambient oxygen or very high pulmonary blood flow.

The diffusing capacity does not directly measure the primary cause of hypoxemia, or low blood oxygen, namely mismatch of ventilation to perfusion:

The diffusion capacity for oxygen ${\displaystyle (D_{L_{O_{2}}})}$ is the proportionality factor relating the rate of oxygen uptake into the lung to the oxygen gradient between the capillary blood and the alveoli (per Fick's laws of diffusion). In respiratory physiology, it is convenient to express the transport of gas molecules as changes in volume, since ${\displaystyle {V_{O_{2}}}\propto {n_{O_{2}}}}$ (i.e., in a gas, a volume is proportional to the number of molecules in it). Further, the oxygen concentration (partial pressure) in the pulmonary artery is taken to be representative of capillary blood. Thus, ${\displaystyle (D_{L_{O_{2}}})}$ can be calculated as the rate that oxygen is taken up by the lung ${\displaystyle ({\dot {V}}_{O_{2}})}$ divided by the oxygen gradient between the alveoli ("A") and the pulmonary artery ("a").

...
Wikipedia