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* Not all pulmonary arterial blood goes to areas of the lung where gas exchange can occur (the anatomic or physiologic shunts), and this poorly oxygenated blood rejoins the well oxygenated blood from healthy lung in the pulmonary vein. Together, the mixture has less oxygen than that blood from the healthy lung alone, and so is hypoxemic.
* Similarly, not all inspired air goes to areas of the lung where gas exchange can occur (the [[Dead space (physiology)|anatomic and the physiological dead spaces]]), and so is wasted.
== Testing==
The '''single-breath diffusing capacity test''' is the most common way to determine <math>D_L</math>.<ref name="multiple" /> The test is performed by having the subject blow out all of the air that he/she can, leaving only the [[Lung volumes|residual lung volume]] of gas. The person then inhales a test gas mixture rapidly and completely, reaching the [[Lung volumes|total lung capacity]] as nearly as possible. This test gas mixture contains a small amount of carbon monoxide (usually 0.3%) and a ''tracer gas'' that is freely distributed throughout the alveolar space but which doesn't cross the alveolar-capillary membrane. [[Helium]] and [[methane]] are two such gasses. The test gas is held in the lung for about 10 seconds during which time the CO (but ''not'' the tracer gas) continuously moves from the alveoli into the blood. Then the subject exhales.▼
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''<ref>{{cite journal |vauthors=Graham BL, Mink JT, Cotton DJ | year = 1981 | title = Improved accuracy and precision of single-breath CO diffusing capacity measurements | url = | journal = J Appl Physiol | volume = 51 | issue = 5| pages = 1306–13 | pmid = 7298468 }}</ref>), 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.<ref name="multiple" /> By analyzing the concentrations of carbon monoxide and inert gas in the inspired gas and in the exhaled gas, it is possible to calculate <math>(D_{L_{CO}})</math> according to Equation {{EquationRef|2}}. First, the ''rate'' at which CO is taken up by the lung is calculated according to:▼
{{NumBlk|::|<math>\dot{V}_{CO} =\frac {\Delta{[CO]} * V_A} {\Delta{t}} </math> . | {{EquationRef|4}} }}▼
::::The pulmonary function equipment monitors the change in the concentration of CO that occurred during the breath hold, <math>\Delta{[CO]}</math>, and also records the time <math>\Delta{t}</math>.▼
::::The volume of the alveoli, <math>V_A</math>, is determined by the degree to which the tracer gas has been diluted by inhaling it into the lung.▼
Similarly,▼
{{NumBlk|::|<math>P_{A_{CO}} = V_B * F_{A_{CO_{O}}} </math> . | {{EquationRef|5}} }}▼
where▼
::::<math>F_{A_{CO_{O}}}</math> is the initial alveolar fractional CO concentration, as calculated by the dilution of the tracer gas.▼
::::<math>V_B</math> is the barometric pressure▼
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.▼
== Calculation ==
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{{NumBlk|::|<math>D_{L_{CO}} = \frac {\dot{V}_{CO}} {P_{A_{CO}} }</math>. | {{EquationRef|2}} }}
▲The '''single-breath diffusing capacity test''' is the most common way to determine <math>D_L</math>.<ref name="multiple" /> The test is performed by having the subject blow out all of the air that he/she can, leaving only the [[Lung volumes|residual lung volume]] of gas. The person then inhales a test gas mixture rapidly and completely, reaching the [[Lung volumes|total lung capacity]] as nearly as possible. This test gas mixture contains a small amount of carbon monoxide (usually 0.3%) and a ''tracer gas'' that is freely distributed throughout the alveolar space but which doesn't cross the alveolar-capillary membrane. [[Helium]] and [[methane]] are two such gasses. The test gas is held in the lung for about 10 seconds during which time the CO (but ''not'' the tracer gas) continuously moves from the alveoli into the blood. Then the subject exhales.
▲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''<ref>{{cite journal |vauthors=Graham BL, Mink JT, Cotton DJ | year = 1981 | title = Improved accuracy and precision of single-breath CO diffusing capacity measurements | url = | journal = J Appl Physiol | volume = 51 | issue = 5| pages = 1306–13 | pmid = 7298468 }}</ref>), 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.<ref name="multiple" /> By analyzing the concentrations of carbon monoxide and inert gas in the inspired gas and in the exhaled gas, it is possible to calculate <math>(D_{L_{CO}})</math> according to Equation {{EquationRef|2}}. First, the ''rate'' at which CO is taken up by the lung is calculated according to:
▲{{NumBlk|::|<math>\dot{V}_{CO} =\frac {\Delta{[CO]} * V_A} {\Delta{t}} </math> . | {{EquationRef|4}} }}
▲::::The pulmonary function equipment monitors the change in the concentration of CO that occurred during the breath hold, <math>\Delta{[CO]}</math>, and also records the time <math>\Delta{t}</math>.
▲::::The volume of the alveoli, <math>V_A</math>, is determined by the degree to which the tracer gas has been diluted by inhaling it into the lung.
▲Similarly,
▲{{NumBlk|::|<math>P_{A_{CO}} = V_B * F_{A_{CO_{O}}} </math> . | {{EquationRef|5}} }}
▲where
▲::::<math>F_{A_{CO_{O}}}</math> is the initial alveolar fractional CO concentration, as calculated by the dilution of the tracer gas.
▲::::<math>V_B</math> is the barometric pressure
▲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 ==
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