MeSH D011653 | Other codes CPT: 94720 | |
Diffusing capacity of the lung (DL) 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. DL, especially DLCO, is reduced in certain diseases of the lung and heart. DLCO measurement has been standardized according to a position paper by a task force of the European Respiratory and American Thoracic Societies.
Contents
- Calculation
- Test Performance
- Interpretation
- Blood CO levels may not be negligible
- The two components of D L C O displaystyle DLCO
- Any change in V c displaystyle Vc alters D L C O displaystyle DLCO
- Reasons why displaystyle heta varies
- Lung diseases that reduce D M displaystyle DM and V c displaystyle heta Vc
- Lung conditions that increase D L C O displaystyle DLCO
- History
- References
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:
Calculation
The diffusion capacity for oxygen
Thus, the higher the diffusing capacity
Sampling the oxygen concentration in the pulmonary artery is a highly invasive procedure, but fortunately another similar gas can be used instead that obviates this need (DLCO). Carbon monoxide (CO) is tightly and rapidly bound to hemoglobin in the blood, so the partial pressure of CO in the capillaries is negligible and the second term in the denominator can be ignored. For this reason, CO is generally the test gas used to measure the diffusing capacity and the
Test Performance
The single-breath diffusing capacity test is the most common way to determine
The anatomy of the airways brings with it complications, since the inspired air must pass through the mouth, trachea, bronchi and bronchioles before it gets to the alveoli where gas exchange will occur; on exhalation, alveolar gas must return along the same path, and so the exhaled sample will be purely alveolar only after a 500 to 1,000 ml of gas has left the subject. While it is algebraically possible to approximate the effects of anatomy (the three-equation method), disease states introduce considerable uncertainty to this approach. Instead, the first 500 to 1,000 ml of the expired gas is disregarded and the next portion which contain gas that has been in the alveoli is analyzed. By analyzing the concentrations of carbon monoxide and inert gas in the inspired gas and in the exhaled gas, it is possible to calculate
Similarly,
where
Other methods that are not so widely used at present can measure the diffusing capacity. These include the steady state diffusing capacity that is performed during regular tidal breathing, or the rebreathing method that requires rebreathing from a reservoir of gas mixtures.
Interpretation
In general, a healthy individual has a value of
Blood CO levels may not be negligible
In heavy smokers, blood CO is great enough to influence the measurement of
The two components of D L C O {displaystyle D_{L_{CO}}}
While
Any change in V c {displaystyle V_{c}} alters D L C O {displaystyle D_{L_{CO}}}
The volume of blood in the lung capillaries,
In disease, hemorrhage into the lung will increase the number of haemoglobin molecules in contact with air, and so measured
Finally,
Reasons why θ {displaystyle heta } varies
The rate of CO uptake into the blood,
The lung blood volume is also reduced when blood flow is interrupted by blood clots (pulmonary emboli) or reduced by bone deformities of the thorax, for instance scoliosis and kyphosis.
Varying the ambient concentration of oxygen also alters
Lung diseases that reduce D M {displaystyle D_{M}} and θ ∗ V c {displaystyle heta *V_{c}}
Diseases that alter lung tissue reduce both
- Loss of lung parenchyma in diseases like emphysema.
- Diseases that scar the lung (the interstitial lung disease), such as idiopathic pulmonary fibrosis, or sarcoidosis
- Swelling of lung tissue (pulmonary edema) due to heart failure, or due to an acute inflammatory response to allergens (acute interstitial pneumonitis).
- Diseases of the blood vessels in the lung, either inflammatory (pulmonary vasculitis) or hypertrophic (pulmonary hypertension).
Lung conditions that increase D L C O {displaystyle D_{L_{CO}}} .
- Alveolar hemorrhage Goodpasture's syndrome, polycythemia, left to right intracardiac shunts, due increase in volume of blood exposed to inspired gas.
- Asthma due to better perfusion of apices of lung. This is caused by increase in pulmonary arterial pressure and/or due to more negative pleural pressure generated during inspiration due to bronchial narrowing.
History
In one sense, it is remarkable that DLCO has retained such clinical utility. The technique was invented to settle one of the great controversies of pulmonary physiology a century ago, namely the question of whether oxygen and the other gases were actively transported into and out of the blood by the lung, or whether gas molecules diffused passively. Remarkable too is the fact that both sides used the technique to gain evidence for their respective hypotheses. To begin with, Christian Bohr invented the technique, using a protocol analogous to the steady state diffusion capacity for carbon monoxide, and concluded that oxygen was actively transported into the lung. His student, August Krogh developed the single breath diffusion capacity technique along with his wife Marie, and convincingly demonstrated that gasses diffuse passively, a finding that led to the demonstration that capillaries in the blood were recruited into use as needed – a Nobel Prize–winning idea.