The diffusing capacity of the lung for carbon monoxide (DLCO) measures how efficiently your lungs move oxygen into your bloodstream. This test assesses gas transfer between the air you inhale and the red blood cells in your pulmonary capillaries. By using a small, safe amount of carbon monoxide, which binds strongly to hemoglobin, the test determines the functional capacity of the lung tissue responsible for gas exchange. A low DLCO score indicates that this transfer process is impaired, suggesting a physiological problem that impacts your ability to oxygenate your body. This impairment often serves as an early indicator of developing lung conditions, sometimes before other lung function tests show changes.
Understanding the DLCO Measurement
A low DLCO score signifies a structural or functional problem at the alveolar-capillary membrane, the delicate interface where gas exchange takes place. This membrane consists of the thin walls of the air sacs (alveoli) and the walls of the surrounding tiny blood vessels (capillaries). A reduced DLCO means that oxygen transfer from the air sacs into the blood is not happening efficiently.
A low result is primarily caused by issues related to two components of gas transfer: the surface area available for diffusion and the thickness of the membrane itself. A low DLCO can indicate that the total surface area of the alveoli is reduced, lessening the available space for oxygen to cross into the blood. Alternatively, the score can be due to a thickening of the alveolar-capillary membrane, creating a longer distance for gas molecules to travel.
The result is typically expressed as a percentage compared to a predicted value, calculated based on factors like age, height, and sex. A result below the expected range suggests an underlying pathology hindering gas transfer. For example, a severely reduced DLCO (less than 40% of the predicted value) is associated with a greater risk of adverse health outcomes.
Recognizing the Physical Signs
The primary physical manifestation of a low DLCO is hypoxemia, the state of the body not receiving enough oxygen. The most common symptom experienced by individuals with impaired gas exchange is dyspnea, or shortness of breath. Initially, this breathlessness may only be noticeable with physical exertion, such as walking up stairs or performing moderate exercise.
As the DLCO impairment becomes more pronounced, the effort needed to breathe increases, and dyspnea may begin to occur even while resting. This persistent lack of oxygen leads to chronic fatigue and general weakness, as the body’s tissues are deprived of necessary fuel. The body attempts to compensate for low oxygen levels by increasing the heart rate, which can be perceived as heart palpitations or a rapid pulse.
In cases of severe gas exchange impairment, where blood oxygen levels drop significantly, a visible change may occur in the skin and mucous membranes. This can manifest as cyanosis, a bluish tint to the lips, fingers, or nail beds, indicating a high concentration of deoxygenated hemoglobin. Persistent oxygen deficiency can also affect brain function, potentially leading to signs like confusion, restlessness, or an unusual headache upon waking.
Underlying Causes of Impaired Gas Exchange
A low DLCO is not a diagnosis but a physiological finding pointing toward several categories of underlying diseases that impact the lung’s gas-exchange machinery. These causes are broadly grouped based on how they damage the alveolar-capillary unit.
Parenchymal Destruction
This category involves the physical damage and permanent loss of the delicate walls of the air sacs. Emphysema is the most common example, characterized by the breakdown of the alveolar walls. This drastically reduces the total surface area available for gas transfer, causing the efficiency of oxygen uptake to fall directly, leading to a low DLCO.
Interstitial Thickening
A second mechanism involves interstitial or alveolar thickening, where the space between the alveoli and capillaries becomes scarred or inflamed. Conditions such as pulmonary fibrosis and various interstitial lung diseases cause this membrane to become thick and stiff. This creates a barrier that slows the diffusion of oxygen into the blood.
Vascular Issues
The third major category involves vascular issues that affect the blood supply within the lungs. Disorders like pulmonary hypertension or chronic pulmonary embolism reduce the volume of blood flowing through the capillaries surrounding the alveoli. This reduction in pulmonary capillary blood volume means fewer red blood cells are available to pick up oxygen, resulting in a reduced DLCO measurement.