The Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO) test measures how efficiently gases transfer from the air sacs into the bloodstream. While many lung diseases cause this measurement to decrease, a surprising observation in some individuals with asthma is that their DLCO can actually be increased.
Understanding DLCO
The DLCO test, also known as the transfer factor for carbon monoxide (TLCO), quantifies the amount of carbon monoxide that moves from the air sacs (alveoli) into the red blood cells within the pulmonary capillaries each minute. During the test, a person inhales a small, harmless amount of carbon monoxide mixed with other gases, holds their breath for about 10 seconds, and then exhales. A machine analyzes the exhaled air to determine how much carbon monoxide was absorbed.
The efficiency of gas transfer, as measured by DLCO, depends on several factors. These include the total surface area of the alveoli available for gas exchange, the thickness of the alveolar-capillary membrane, and the volume of blood in the capillaries surrounding the alveoli.
The Unique Nature of DLCO in Asthma
In individuals with asthma, the observation that DLCO can be increased is notable, as it appears counter-intuitive for a lung condition. This finding sets asthma apart from many other obstructive lung diseases. For instance, in conditions like emphysema, which involves the destruction of alveolar walls and associated capillaries, the DLCO is typically decreased due to a reduction in the surface area available for gas exchange.
The preserved or even elevated DLCO in asthma highlights a fundamental difference in the underlying lung pathology. Unlike emphysema, asthma generally does not involve permanent damage or destruction of the alveolar walls, meaning the actual gas exchange surface remains largely intact. This distinction makes DLCO a useful tool in differentiating asthma from other conditions that might present with similar symptoms of airflow obstruction.
Key Mechanisms Behind Increased DLCO
Several physiological factors contribute to the increased DLCO sometimes seen in asthma. One primary reason is an increase in pulmonary capillary blood volume. The intermittent narrowing of airways in asthma can lead to a redistribution of blood flow within the lungs, potentially directing more blood to less constricted areas. This results in a greater volume of blood in the pulmonary capillaries available to pick up carbon monoxide, thus increasing the DLCO.
Asthma patients often undergo DLCO testing after receiving bronchodilator medications. These medications relax the muscles around the airways, widening them and improving air flow. By opening previously constricted airways, bronchodilators can enhance the matching of ventilation to perfusion and increase the effective surface area for gas diffusion, leading to a higher DLCO measurement.
Chronic inflammation, a hallmark of asthma, can also play a role through increased airway vascularity. Persistent inflammation in the airway walls can stimulate the formation of new blood vessels, a process known as angiogenesis. This increased network of capillaries in the airway walls may contribute to a larger effective diffusing surface. In some cases of severe asthma, hyperinflation, where air becomes trapped in the lungs, might lead to more alveoli being stretched and available for gas exchange, rather than collapsing.
Factors Influencing DLCO in Asthma and Clinical Implications
The DLCO value in individuals with asthma is not consistently elevated; it can show variability based on several influencing factors. Asthma severity and control, for example, can impact the measurement. The presence of other conditions, such as a history of smoking, might also alter the DLCO, potentially leading to a decreased value if smoking has caused emphysema.
The pattern of DLCO in asthma holds significant diagnostic value, particularly in distinguishing it from other lung diseases like chronic obstructive pulmonary disease (COPD), especially emphysema. A normal or elevated DLCO in the presence of airflow obstruction strongly suggests asthma, whereas a reduced DLCO is characteristic of emphysema. While DLCO is not a primary tool for routine asthma monitoring compared to spirometry, it can be useful in assessing the overall gas exchange function. Other factors unrelated to asthma can also influence DLCO, including anemia, which can lower it due to reduced hemoglobin levels, or body position, as lying down can increase pulmonary capillary blood volume.