It is part of a comprehensive series of pulmonary function tests to determine the overall ability of the lung to transport gas into and out of the blood.
DLCO measurement has been standardized according to a position paper[1] 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.
[citation needed] 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.
[citation needed] The diffusing capacity does not directly measure the primary cause of hypoxemia, or low blood oxygen, namely mismatch of ventilation to perfusion:[2] The single-breath diffusing capacity test is the most common way to determine
[1] The test is performed by having the subject blow out all of the air that they can, leaving only the residual lung volume of gas.
The person then inhales a test gas mixture rapidly and completely, reaching the total lung capacity as nearly as possible.
The anatomy of the airways means inspired air must pass through the mouth, trachea, bronchi and bronchioles (anatomical dead space) 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 been breathed out.
[citation needed] While it is algebraically possible to approximate the effects of anatomy (the three-equation method[3]), disease states introduce considerable uncertainty to this approach.
First, the rate at which CO is taken up by the lung is calculated according to: Similarly, where Other methods that are not so widely used at present can measure the diffusing capacity.
In respiratory physiology, it is convenient to express the transport of gas molecules as changes in volume, since
Further, the oxygen concentration (partial pressure) in the pulmonary artery is taken to be representative of capillary blood.
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.
[4] However, individuals vary according to age, sex, height and a variety of other parameters.
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.
) and then CO combines with the hemoglobin in capillary red blood cells at a rate
[13] Since the steps are in series, the conductances add as the sum of the reciprocals: The volume of blood in the lung capillaries,
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.
But during exercise (or more rarely when there is a 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.
In disease, hemorrhage into the lung will increase the number of haemoglobin molecules in contact with air, and so measured
In this case, the carbon monoxide used in the test will bind to haemoglobin that has bled into the lung.
This does not reflect an increase in diffusing capacity of the lung to transfer oxygen to the systemic circulation.
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.
At high altitude, inspired oxygen is low and more of the blood's hemoglobin is free to bind CO; thus
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.
[18] 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,[19][20][21][22][23][24][25] a finding that led to the demonstration that capillaries in the blood were recruited into use as needed – a Nobel Prize–winning idea.