Diffusion capacity
Encyclopedia
In biology
, diffusion capacity is a measurement of the lung
's ability to transfer gases. Oxygen uptake may be limited by diffusion in circumstances of low ambient oxygen or high pulmonary blood flow. Carbon dioxide is not limited by diffusion under most circumstances.
The "diffusing capacity" or "diffusion capacity" is part of a comprehensive test series of lung function called pulmonary function testing
. In Europe, this is often called the "transfer factor". Critics of the term "diffusion capacity" argue that it may be misleading, and point out two problems with the term. The first is that the test measures not just diffusion across the alveolar-capillary membrane, but also takes into account factors affecting the chemical combination of a given gas with hemoglobin. The second criticism is that the test is typically measured under submaximal conditions and doesn't truly reflect a functional capacity. For these reasons the term "transfer factor" better reflects the physiological process being measured.
), and the diffusion capacity (DLOO2) can be calculated as the ratio of oxygen flux (J^*) across the alveolar-capillary membrane over the oxygen partial pressure
gradient between the alveoli ("A") and the alveolar capillaries ("a")
(The asterisk should be read as a dot over the letter, which is used to denote rate
in respiratory physiology
.)
J*O2 is the uptake of oxygen (ml/min). PAO2 is the alveolar partial pressure of oxygen. PaO2 is the partial pressure
of oxygen in the alveolar capillary. Thus, the higher the diffusing capacity DL, the more gas will be transferred across the alveolar-capillary membrane for a given gradient in partial pressure (or concentration) of the gas.
Because it is difficult to measure the capillary partial pressure of oxygen, carbon monoxide
(CO) is used as the test gas to measure the diffusing capacity (Dlco). Because CO is so tightly bound to hemoglobin in the red blood cells, the partial pressure of CO in the capillaries is assumed to be zero. Thus,
inhalation of a test gas mixture that contains a small amount of carbon monoxide (usually 0.3%) and some helium or other non-absorbed tracer gas. The test gas is held in the lung for about 10 seconds and then is exhaled from the lung. The first part of the expired gas is discarded and the next portion which represents gas from the alveoli is collected. By analyzing the concentrations of carbon monoxide and helium in the inspired gas and in the exhaled gas, it is possible to calculate how much carbon monoxide was taken up during the breath hold, and what the partial pressure of carbon monoxide was during the breath hold. This method is known as the single-breath diffusing capacity test.
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. A more sophisticated "three-equation method" which involves fast responding analysers for the carbon monoxide and a tracer gas such as helium has been developed. It monitors all parameters continuously and solves some of the technical problems of the traditional method but is more expensive to implement.
The diffusing capacity (DLCO) is a test of the integrity of the alveolar-capillary surface area for gas transfer. It may be reduced in disorders that damage the alveolar walls (septa) such as emphysema, which leads to a loss of effective surface area. The DLCO is also reduced in disorders that thicken or damage the alveolar walls such as pulmonary fibrosis
. The DLCO is also reduced in interstitial lung diseases such as hypersensitivity pneumonitis
(e.g. bird fancier's lung
, farmer's lung
) or dust-inhalation diseases such as asbestosis
. Because the transfer of CO depends upon how much blood is present in the lung capillaries, the DLCO can be reduced due to anemia
and raised in polycythemia
. Some laboratories adjust or correct the DLCO for a serum hemoglobin to take into account the effect of anemia. Disorders of the pulmonary blood vessels such as chronic thromboembolic disease or pulmonary hypertension
also reduce the diffusing capacity, as can any condition/disease which results in reduction in pulmonary capillary blood volume. The DLCO is not, however, a sensitive test for acute pulmonary embolism. Cigarette smokers tend to have lower DLCO, which is partly due to damage to the lungs (mild emphysema) and partly due to the retention of carbon monoxide from the cigarettes, which causes back pressure (decreasing the CO gradient). For these reasons, the DLCO is a sensitive test for disorders of the lung, but does not point toward a specific diagnosis. By calculating the dilution of the tracer gas, the test can also be used to estimate the total lung capacity. In the presence of poor gas mixing from obstructive lung disease it is used to measure the single breath total lung capacity.
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, diffusion capacity is a measurement of the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
's ability to transfer gases. Oxygen uptake may be limited by diffusion in circumstances of low ambient oxygen or high pulmonary blood flow. Carbon dioxide is not limited by diffusion under most circumstances.
The "diffusing capacity" or "diffusion capacity" is part of a comprehensive test series of lung function called pulmonary function testing
Pulmonary function testing
Pulmonary Function Testing is a complete evaluation of the respiratory system including patient history, physical examinations, chest x-ray examinations, arterial blood gas analysis, and tests of pulmonary function. The primary purpose of pulmonary function testing is to identify the severity of...
. In Europe, this is often called the "transfer factor". Critics of the term "diffusion capacity" argue that it may be misleading, and point out two problems with the term. The first is that the test measures not just diffusion across the alveolar-capillary membrane, but also takes into account factors affecting the chemical combination of a given gas with hemoglobin. The second criticism is that the test is typically measured under submaximal conditions and doesn't truly reflect a functional capacity. For these reasons the term "transfer factor" better reflects the physiological process being measured.
Calculation
The rate of flow is directly proportional to the oxygen gradient (per Fick's law of diffusionFick's law of diffusion
Fick's laws of diffusion describe diffusion and can be used to solve for the diffusion coefficient, D. They were derived by Adolf Fick in the year 1855.- Fick's first law :...
), and the diffusion capacity (DLOO2) can be calculated as the ratio of oxygen flux (J^*) across the alveolar-capillary membrane over the oxygen partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
gradient between the alveoli ("A") and the alveolar capillaries ("a")
(The asterisk should be read as a dot over the letter, which is used to denote rate
Rate
In mathematics, a rate is a ratio between two measurements, often with different units. If the unit or quantity in respect of which something is changing is not specified, usually the rate is per unit time. However, a rate of change can be specified per unit time, or per unit of length or mass or...
in respiratory physiology
Respiratory physiology
Respiratory physiology is the branch of human physiology focusing upon respiration.Topics include:-Volumes:* lung volumes* vital capacity* functional residual capacity* dead space* spirometry* body plethysmography* peak flow meter-Mechanics:...
.)
J*O2 is the uptake of oxygen (ml/min). PAO2 is the alveolar partial pressure of oxygen. PaO2 is the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of oxygen in the alveolar capillary. Thus, the higher the diffusing capacity DL, the more gas will be transferred across the alveolar-capillary membrane for a given gradient in partial pressure (or concentration) of the gas.
Because it is difficult to measure the capillary partial pressure of oxygen, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO) is used as the test gas to measure the diffusing capacity (Dlco). Because CO is so tightly bound to hemoglobin in the red blood cells, the partial pressure of CO in the capillaries is assumed to be zero. Thus,
Test Performance
In practice, the test is performed by having the test subject blow out all of the air that they can to reach residual volume. The person then takes a full vital capacityVital capacity
Vital capacity is the maximum amount of air a person can expel from the lungs after a maximum inspiration. It is equal to the inspiratory reserve volume plus the tidal volume plus the expiratory reserve volume....
inhalation of a test gas mixture that contains a small amount of carbon monoxide (usually 0.3%) and some helium or other non-absorbed tracer gas. The test gas is held in the lung for about 10 seconds and then is exhaled from the lung. The first part of the expired gas is discarded and the next portion which represents gas from the alveoli is collected. By analyzing the concentrations of carbon monoxide and helium in the inspired gas and in the exhaled gas, it is possible to calculate how much carbon monoxide was taken up during the breath hold, and what the partial pressure of carbon monoxide was during the breath hold. This method is known as the single-breath diffusing capacity test.
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. A more sophisticated "three-equation method" which involves fast responding analysers for the carbon monoxide and a tracer gas such as helium has been developed. It monitors all parameters continuously and solves some of the technical problems of the traditional method but is more expensive to implement.
Interpretation
Values between 75% and 125% of average diffusion capacity in the healthy population are considered normal.The diffusing capacity (DLCO) is a test of the integrity of the alveolar-capillary surface area for gas transfer. It may be reduced in disorders that damage the alveolar walls (septa) such as emphysema, which leads to a loss of effective surface area. The DLCO is also reduced in disorders that thicken or damage the alveolar walls such as pulmonary fibrosis
Pulmonary fibrosis
Pulmonary fibrosis is the formation or development of excess fibrous connective tissue in the lungs. It is also described as "scarring of the lung".-Symptoms:Symptoms of pulmonary fibrosis are mainly:...
. The DLCO is also reduced in interstitial lung diseases such as hypersensitivity pneumonitis
Hypersensitivity pneumonitis
Hypersensitivity pneumonitis is an inflammation of the alveoli within the lung caused by hypersensitivity to inhaled organic dusts. Sufferers are commonly exposed to the dust by their occupation or hobbies.-Pathophysiology:Hypersensitivity pneumonitis involves inhalation of an antigen...
(e.g. bird fancier's lung
Bird fancier's lung
Bird fancier's lung is a type of hypersensitivity pneumonitis caused by bird droppings. The lungs become inflamed with granuloma formation.Bird Fancier's Lung , also called bird-breeder's lung and pigeon-breeder's lung, is a subset of Hypersensitivity pneumonitis...
, farmer's lung
Farmer's lung
Farmer's lung is a hypersensitivity pneumonitis induced by the inhalation of biologic dusts coming from hay dust or mould spores or other agricultural products. It results in a type III hypersensitivity inflammatory response and can progress to become a chronic condition which is considered...
) or dust-inhalation diseases such as asbestosis
Asbestosis
Asbestosis is a chronic inflammatory and fibrotic medical condition affecting the parenchymal tissue of the lungs caused by the inhalation and retention of asbestos fibers...
. Because the transfer of CO depends upon how much blood is present in the lung capillaries, the DLCO can be reduced due to anemia
Anemia
Anemia is a decrease in number of red blood cells or less than the normal quantity of hemoglobin in the blood. However, it can include decreased oxygen-binding ability of each hemoglobin molecule due to deformity or lack in numerical development as in some other types of hemoglobin...
and raised in polycythemia
Polycythemia
Polycythemia is a disease state in which the proportion of blood volume that is occupied by red blood cells increases...
. Some laboratories adjust or correct the DLCO for a serum hemoglobin to take into account the effect of anemia. Disorders of the pulmonary blood vessels such as chronic thromboembolic disease or pulmonary hypertension
Pulmonary hypertension
In medicine, pulmonary hypertension is an increase in blood pressure in the pulmonary artery, pulmonary vein, or pulmonary capillaries, together known as the lung vasculature, leading to shortness of breath, dizziness, fainting, and other symptoms, all of which are exacerbated by exertion...
also reduce the diffusing capacity, as can any condition/disease which results in reduction in pulmonary capillary blood volume. The DLCO is not, however, a sensitive test for acute pulmonary embolism. Cigarette smokers tend to have lower DLCO, which is partly due to damage to the lungs (mild emphysema) and partly due to the retention of carbon monoxide from the cigarettes, which causes back pressure (decreasing the CO gradient). For these reasons, the DLCO is a sensitive test for disorders of the lung, but does not point toward a specific diagnosis. By calculating the dilution of the tracer gas, the test can also be used to estimate the total lung capacity. In the presence of poor gas mixing from obstructive lung disease it is used to measure the single breath total lung capacity.