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{{Short description|Measure of the transfer of gas from the lung to red blood cells}}
{{Infobox diagnostic
| Name = Diffusing capacity
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| OtherCodes = CPT: 94720
}}
'''Diffusing capacity''' of the lung (D<sub>L</sub>) (also known as
In [[respiratory physiology]], the diffusing capacity has a long history of great utility, representing [[Electrical resistance and conductance|conductance]] of gas across the alveolar-capillary membrane and also takes into account factors affecting the behaviour of a given gas with hemoglobin.{{Citation needed|reason=uncited definition|date=March 2014}}
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== Interpretation ==
In general, a healthy individual has a value of <math>D_{L_{CO}}</math> between 75% and 125% of the average.<ref name=uppsala>LUNGFUNKTION - Practice compendium for semester 6. Department of Medical Sciences, Clinical Physiology, Academic Hospital, Uppsala, Sweden. Retrieved 2010.</ref> However, individuals vary according to age, sex, height and a variety of other parameters. For this reason, reference values have been published, based on populations of healthy subjects<ref>{{cite journal |vauthors=Miller A, Thornton JC, Warshaw R, Anderson H, Teirstein AS, Selikoff IJ | year = 1983 | title = Single breath diffusing capacity in a representative sample of the population of Michigan, a large industrial state. Predicted values, lower limits of normal, and frequencies of abnormality by smoking history | journal = Am Rev Respir Dis | volume = 127 | issue = 3| pages = 270–7 | pmid = 6830050 | doi = 10.1164/arrd.1983.127.3.270 | doi-broken-date =
===Blood CO levels may not be negligible===
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In heavy smokers, blood CO is great enough to influence the measurement of <math>D_{L_{CO}}</math>, and requires an adjustment of the calculation when COHb is greater than 2% of the whole.
While <math>(D_L)</math> is of great practical importance, being the overall measure of gas transport, the interpretation of this measurement is complicated by the fact that it does not measure any one part of a multi-step process. So as a conceptual aid in interpreting the results of this test, the time needed to transfer CO from the air to the blood can be divided into two parts. First CO crosses the alveolar capillary membrane (represented by <math>D_M</math> ) and then CO combines with the hemoglobin in capillary red blood cells at a rate <math>\theta</math> times the volume of capillary blood present (<math>V_c</math>).<ref>{{cite journal |vauthors=Roughton FJ, Forster RE | year = 1957 | title = Relative importance of diffusion and chemical reaction rates in determining rate of exchange of gases in the human lung, with special reference to true diffusing capacity of pulmonary membrane and volume of blood in the lung capillaries | journal = J Appl Physiol | volume = 11 | issue = 2| pages = 290–302 | pmid = 13475180 | doi = 10.1152/jappl.1957.11.2.290 }}</ref> Since the steps are in series, the conductances add as the sum of the reciprocals:
{{NumBlk|::|<math>\frac {1} {D_{L_{CO}}} =\frac {1} {D_M} + \frac {1} {\theta * V_c}</math> . | {{EquationRef|3}} }}
The volume of blood in the lung capillaries, <math>V_c</math>, changes appreciably during ordinary activities such as [[Physical exercise|exercise]]. Simply breathing in brings some additional blood ''into'' the lung because of the negative intrathoracic pressure required for inspiration. At the extreme, inspiring against a closed glottis, the [[Müller's maneuver]], pulls blood ''into'' the chest. The opposite is also true, as exhaling increases the pressure within the thorax and so tends to push blood out; the [[Valsalva maneuver]] is an exhalation against a closed airway which can move blood ''out'' of the lung. So breathing hard during exercise will bring extra blood into the lung during inspiration and push blood out during expiration. But during exercise (or more rarely when there is a [[Atrioventricular septal defect|structural defect]] in the heart that allows blood to be shunted from the high pressure, systemic circulation to the low pressure, pulmonary circulation) there is also increased blood flow throughout the body, and the lung adapts by recruiting extra capillaries to carry the increased output of the heart, further increasing the quantity of blood in the lung. Thus <math>D_{L_{CO}}</math> will appear to increase when the subject is not at rest, particularly during inspiration.
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Finally, <math>V_c</math> is increased in '''[[obesity]]''' and when the subject lies down, both of which increase the blood in the lung by compression and by gravity and thus both increase <math>D_{L_{CO}}</math>.
The rate of CO uptake into the blood, <math>\theta</math>, depends on the concentration of hemoglobin in that blood, abbreviated [[Hemoglobin|Hb]] in the CBC ([[Complete Blood Count]]). More hemoglobin is present in [[polycythemia]], and so <math>D_{L_{CO}}</math> is elevated. In [[anemia]], the opposite is true. In environments with high levels of CO in the inhaled air (such as [[smoking]]), a fraction of the blood's hemoglobin is rendered ineffective by its tight binding to CO, and so is analogous to anemia. It is recommended that <math>D_{L_{CO}}</math> be adjusted when blood CO is high.<ref name="multiple" />
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Varying the ambient concentration of oxygen also alters <math>\theta</math>. At high altitude, inspired oxygen is low and more of the blood's hemoglobin is free to bind CO; thus <math>\theta</math> is increased and <math>D_{L_{CO}}</math> appears to be increased. Conversely, supplemental oxygen increases Hb saturation, decreasing <math>\theta</math> and <math>D_{L_{CO}}</math>.
Diseases that alter lung tissue reduce both <math>D_M</math> and <math>\theta * V_c</math> to a variable extent, and so decrease <math>D_{L_{CO}}</math>.
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# Diseases of the blood vessels in the lung, either inflammatory ([[Vasculitis|pulmonary vasculitis]]) or hypertrophic ([[pulmonary hypertension]]).
# Alveolar hemorrhage [[Goodpasture's syndrome]],<ref>{{cite journal|last=Greening|first=AP|author2=Hughes, JM|title=Serial estimations of carbon monoxide diffusing capacity in intrapulmonary haemorrhage.|journal=Clinical Science|date=May 1981|volume=60|issue=5|pages=507–12|pmid=7249536|doi=10.1042/cs0600507}}</ref> [[polycythemia]],<ref>{{cite journal|last=Burgess|first=J. H.|author2=Bishop, J. M.|journal=Journal of Clinical Investigation|volume=42|issue=7|pages=997–1006|doi=10.1172/JCI104804|pmc=289367|pmid=14016987|title=Pulmonary Diffusing Capacity and ITS Subdivisions in Polycythemia Vera|year=1963}}</ref> left to right [[Cardiac shunt|intracardiac shunts]],<ref>{{cite journal|last=AUCHINCLOSS JH|first=Jr|author2=GILBERT, R |author3=EICH, RH |title=The pulmonary diffusing capacity in congenital and rheumatic heart disease.|journal=Circulation|date=February 1959|volume=19|issue=2|pages=232–41|pmid=13629784|doi=10.1161/01.cir.19.2.232|s2cid=27264342|doi-access=free}}</ref> 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.<ref>{{cite journal|last=Collard|first=P|author2=Njinou, B |author3=Nejadnik, B |author4=Keyeux, A |author5= Frans, A |title=Single breath diffusing capacity for carbon monoxide in stable asthma.|journal=Chest|date=May 1994|volume=105|issue=5|pages=1426–9|pmid=8181330|doi=10.1378/chest.105.5.1426}}</ref>
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==History==
In one sense, it is remarkable that DL<sub>CO</sub> 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.<ref>{{cite journal | author = Gjedde A | year = 2010 | title = Diffusive insights: on the disagreement of Christian Bohr and August Krogh | journal = Adv Physiol Educ | volume = 34 | issue = 4| pages = 174–185 | doi = 10.1152/advan.00092.2010 | pmid = 21098384 | s2cid = 31010852 }}</ref> 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 [[August Krogh|Marie]], and convincingly demonstrated that gasses diffuse passively,<ref>Krogh A. 1910 On the oxygen metabolism of the blood. Skand Arch Physiol 23: 193–199</ref><ref>Krogh A. 1910 On the mechanism of the gas-exchange in the lungs of the tortoise. Skand Arch Physiol 23: 200–216.</ref><ref>Krogh A. 1910 On the combination of hæmoglobin with mixtures of oxygen and carbonic acid. Skand Arch Physiol 23: 217–223.</ref><ref>Krogh A. 1910 Some experiments on the invasion of oxygen and carbonic oxide into water. Skand Arch Physiol 23: 224–235</ref><ref>Krogh A. 1910 On the mechanism of gas exchange in the lungs. Skand Arch Physiol 23: 248–278</ref><ref>Krogh A, Krogh M. 1910 On the tensions of gases in arterial blood. Skand Arch Physiol 23: 179–192.</ref><ref>Krogh A, Krogh M. 1910 Rate of diffusion into lungs of man. Skand Arch Physiol 23: 236–247</ref> a finding that led to the demonstration that capillaries in the blood were recruited into use as needed – a Nobel Prize–winning idea.<ref>{{Cite web | url=https://www.nobelprize.org/nobel_prizes/medicine/laureates/1920/krogh-bio.html | title=The Nobel Prize in Physiology or Medicine 1920}}</ref>
==See also==
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